This PDF file contains the front matter associated with SPIE Proceedings Volume 10919, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Quantum technologies are seen to be within reach for ripe applications with recent developments in quantum optics and photonics resulting into better control of quantum systems [1]. Many platforms, such as photonic crystals and cavities involving complex fabrications as well as many materials such as diamond or III-V semiconductors have been proposed for quantum photonics. We will report on a different material approach and in particular with the use of ZnO as a promising quantum photonics materials. Several aspects will be covered where we will show some results on using ZnO as a photonics material for guiding light and have a strong light-matter interaction with other quantum emitters. We will also show how ZnO nanowires (NWs) can be used as photodetectors and as such being integrated on optical circuits. Finally, we will develop why ZnO is interesting intrinsically for quantum technologies and in particular by manipulating and using defects within its band gap.

Undoped ZnO nanowires typically grow along (0001) C-axis and exhibit (10-10) M-facets with hexagonal morphology. However, literature shows that growth direction, morphology and facet orientation can be tuned by adding a dopant in the source material (in CVD processes) or metal ions in the solution (e.g. hydrothermal growth). This work focuses on the nanowire facet transformations induced by the addition of trimethylgallium in the gas phase. For spontaneously grown ZnO nanowires, the morphology evolves rapidly towards Christmas-tree like and hierarchical structures upon intentional Ga doping. Regarding ZnO/ZnO:Ga core-shell structures, a change of the smooth initial M-oriented facets occurs, with the development of {20-21} surfaces, and further {10-11} and {0001} surfaces. Interestingly, a similar evolution of the lateral roughness is observed in Au-catalyzed doped nanowires. SIMS measurements reveal high Ga concentrations from 1E19 up to 2.E21 at/cm3 in the doped ZnO reference layers. In addition, photoluminescence spectra show an increase of the donor bound exciton emission at 3.358 eV, assigned to Ga impurities. The influence of Ga doping on the facet transformations and the occurrence of unexpected {0001} polar surfaces are discussed. The results can be mainly understood by a Ga surfactant effect (at least partial) responsible for the modification of the surface energies and kinetics. In particular, density functional calculations support the floating behaviour of the negatively charged Ga- ion on the growing surface. Finally, first photoacoustic measurements show an optical absorption at 6 µm, evidencing that the degenerate material is suitable for plasmonic applications in the IR range.

In this paper we report on systematic studies conducted for the improvement in both the device structure and the materials quality of perovskite based solar cells (PSCs). We have incorporated TiO₂ nanorods, of length around 350-400 nm, in the device structure. Such structures were grown by solvothermal technique directly on the glass/FTO substrates. Characterization by femtosecond transient absorption (fs-TA) spectroscopy indicates that the incorporation of TiO₂- nanorod array (NA) greatly enhances the collection efficiency of the photo-generated carriers due to substantial reduction in carrier diffusion distance. To improve the crystallinity of the perovskite films we performed systematic studies on cryoassisted growth of the material. The technique eliminates the need for environmentally harmful anti-solvents and enables decoupling of the nucleation and crystallization phases by inhibiting chemical reactions in the precursor films rapidly cooled by immersion in liquid nitrogen. Furthermore, the technique leads to uniform precipitation of precursors due to the supersaturation condition in the residual solvents at cryogenic temperature resulting in highly uniform coverage of the films. Systematic characterization of the films by low-frequency noise and photothermal deflection technique indicate significant in the trap density of the films which is attributed as the main underlying reason for the observed improvement in the power conversion efficiency of the device. A high efficiency of 21.4% is achieved for our champion device.

The epitaxial growth of beta-aluminum-gallium oxide alloys on beta-gallium oxide substrates is limited by the difference in lattice constants and lattice symmetries of beta-gallium oxide (monoclinic) and aluminum oxide (corundum). The monoclinic and corundum phases of the alloys have been compared mostly on the grounds of their total energies at zero Kelvin, considering that the vibrational entropy contribution is relatively small. Here we show that the lattice mismatch-induced strain can lead to zone edge softening, providing a path for phase transitions in epitaxial growth of aluminum-gallium oxide alloys. This mechanism depends on both the alloy concentration ratio and on the direction of growth. Detailed insights are obtained from first-principles calculations of total energies and vibrational spectra throughout the Brillouin zone.

The accuracy of DFT-based methods rely on the quality of the underlying functional. To test the quality of a functional, both the band gap and the generalized Koopmans theorem can be employed. As the fullfillment of the Koopmans theorem garantees correct localization, we can identify traped holes in (small) polaron states and assign the corresponding vertical transition levels as well as the atoms where these states are localized upon. We present a theoretical study of intrinsic defects in beta-Ga2O3, using an optimized, Koopmans-complient, hybrid functional as an example. It is shown that all observed photoluminescence bands of beta-Ga2O3 can be explained by electron recombination at trapped holes, with different intrinsic defects or nitrogen acting as hole traps.

The rising momentum of research and developments on Ga2O3 has broaden the area of material exploration even for metastable phases. In the field of metastable Ga2O3, we have focused on defective-spinel-structured γ-phase and established some milestones: epitaxial stabilization of single crystalline films on MgAl2O4 [T. Oshima,et al., J. Cryst. Growth 359, 60 (2012).], carrier generation by impurity doping [T. Oshima et al., J. Cryst. Growth 421, 23 (2015).], and band-gap engineering by alloying γ-Al2O3 [T. Oshima et al., Appl. Phys. Express 10, 051104 (2017).]. We consider their results endorse further semiconductor engineering studies on γ-Ga2O3-related materials. Therefore, as a successive study, we have attempted to fabricate first γ-(AlxGa1−x)2O3-based heterostructures, particularly the superlattices (SLs) comprised with the end members of the alloy, to consider the possibility of obtaining coherent heterojunctions for future heterojunction device applications. 10-period γ-Al2O3/Ga2O3 SLs on (001) MgAl2O4 substrate were fabricated by plasma-assisted molecular beam epitaxy. By controlling the each layer thickness, we tuned the average Al composition (x_ave) of the coherent SLs from 0.26 to 0.86, and obtained nearly-lattice-matched SLs to the substrate at x_ave ~ 0.5. The lattice-matched SLs maintained coherent interfaces up to a period length of 7.2 nm (3.2/4.0 nm for γ-Al2O3/Ga2O3 layers) in spite of a large lattice mismatch between the end members (−3.6%). These successful fabrication of γ-Al2O3/Ga2O3 SLs means wide flexibility in designing γ-(AlxGa1−x)2O3-based heterostructures including superlattices for future development of functional heterojunction devices.

Ultrawide bandgap (UWBG) gallium oxide (Ga2O3) represents an emerging semiconductor material with excellent chemical and thermal stability. It has a band gap of 4.5-4.9 eV, much higher than that of the GaN (3.4 eV) and 4H-SiC (3.2 eV). The monoclinic beta-phase Ga2O3 represents the thermodynamically stable crystal among the known five phases . The breakdown field of beta-Ga2O3 is estimated to be 6-8 MV/cm, which is much larger than that of the 4H-SiC and GaN. These unique properties make beta-Ga2O3 a promising candidate for high power electronic device and solar blind photodetector applications. More advantageously, single crystal beta-Ga2O3 substrates can be synthesized by scalable and low cost melting based growth techniques. Different from the molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) growth techniques, we have developed a low pressure chemical vapor deposition (LPCVD) method to grow high quality beta-Ga2O3 thin films on both native Ga2O3 and c-sapphire substrates with controllable doping and fast growth rates up to 10 um/hr. In this talk, we present the growth, material characterization and device demonstration of beta-Ga2O3 thin films grown via LPCVD. The beta-Ga2O3 thin films were grown on native beta-Ga2O3 (010), (001) and (-201) substrates and sapphire substrates using high purity gallium and oxygen as the precursors, and argon (Ar) as the carrier gas. The growth temperature ranged between 850 ˚C and 950 ˚C. Fundamental material properties including temperature dependent Hall measurements and device demonstration based on vertical Schottky barrier diodes will be discussed.

Ultra-wide bandgap zinc gallium oxide (ZGO) and GO films were prepared on c-plane sapphire by conventional radio-frequency magnetron sputtering. In the current sputtered oxide studies, target composition or growth temperature is usually the main deposition variable, and the other growth conditions are fixed. This would make it difficult to fully understand the theory and characterization of ZGO films. In this study, several growth parameters as well as the post-thermal treatment were all modulated to realize and optimize the ZGO growth. From x-ray and TEM analyses, stabilization of stoichiometry and control of crystallinity transformation were confirmed to be important factors in determining the film quality. The optical bandgap of ZGO can reach 5.0-5.1 eV with a maximum responsivity peak at 240 nm. A metal-semiconductor-metal photodetector is demonstrated with a maximum responsivity over 2 A/W under a 5-V biased voltage. Furthermore, the photo/dark current ratio can be improved to be over ten thousand. As compared with those of the sputtered GO photodetector, the spectral response peak of ZGO showed a blue shift to 240 nm with higher responsivity. The data presented exhibit the ZGO material will become another potential candidate for ultra-wide bandgap semiconductor applications.

Thanks to their superior breakdown fields, both beta- and alpha-phase Ga2O3 are poised to achieve ultra-high-performance devices enabling highly efficient, high voltage power switching systems. To realize the thick films required of the highest voltage devices, a growth technique which can achieve high growth rates is desired. Kyma Technologies has developed a low-cost halide vapor phase epitaxy (HVPE) tool for the growth of both beta- phase and alpha- phase Ga2O3 films which boasts high growth rates and smoothness while simultaneously being able to be lightly and controllably doped with Si and free of carbon. We will outline our recent growth results including effects of substrate preparation and growth conditions on epilayer morphology and mobility.

With a high bandgap of 4.7 eV, β-Ga2O3 can be made semi-insulating by doping with Fe or Mg and thereby possesses a very high breakdown field necessary for high-powered switches. Somewhat surprisingly, β-Ga2O3 can also be made highly conductive by doping with Si, which leads to great potential for n+ohmic contacts and transparent current spreading layers. In the latter application, the goal is to achieve both high conductivity (high concentration n and mobility μ) and high transparency in the visible and UV regions. Recently we have achieved n = 2 x 1020 cm-3 in β-Ga2O3, using pulsed laser deposition (PLD) with a Ga2O3 target containing 1-wt%-SiO2. Although n is temperature-independent, µ is not, and by fitting µ vs T, we can determine donor ND and acceptor NA concentrations. However, at higher temperatures, µ is strongly affected by longitudinal optical (LO) phonon scattering, which is much more complicated to model in Ga2O3 (9 LO phonons) than in ZnO, GaN, and other binary semiconductors (1 LO phonon). Highly-doped samples have another complication, disorder in the dopant placement. Fortunately, this disorder leads to small quantum corrections delta sigma in the conductivity which are also affected by LO phonons. Indeed, the study of delta sigma vs T and vs magnetic field B at low temperatures is crucial in understanding mobility at 300 K. We demonstrate calculations of ND and NA in PLD-grown β-Ga2O3 under the assumption that the dominant acceptor is the Ga vacancy in various charge states.

We used temperature-resolved cathodoluminescence to determine the characteristics of luminescence bands and carrier dynamics in edge-defined film-fed grown (EFG) beta-Ga2O3 single crystals synthesized by Tamura Corporation. The crystal is nominally undoped and has a (-201) surface orientation. The main impurities are Si, Ir, Al and Fe, with [Fe] ~ 10^17 cm-3 verified by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The CL emission was found to be dominated by a broad UV emission peaked at 3.40 eV, which exhibits strong quenching with increasing temperature; however, its spectral shape and energy position remain virtually unchanged up to 500 K. Depth-resolved analysis reveals the luminescence spectrum is independent of sampling depth. We observed a super-linear increase of CL intensity with excitation density; this kinetics of carrier recombination can be explained in terms of carrier trapping and charge transfer at Fe3+/2+ centers. The temperature-dependent properties of this UV band were found to be consistent with weakly bound electrons in self-trapped excitons with an activation energy of 48 +/- 10 meV. In addition to the self-trapped exciton emission, a blue luminescence (BL) band is shown to be related to a donor-like defect, which increases significantly in concentration after remote hydrogen plasma treatment. The point defect responsible for the BL, likely an oxygen vacancy or a complex, is strongly coupled to the lattice with a Huang-Rhys factor S = 7.3.

Calcium and magnesium doped beta-phase gallium oxide (β-Ga₂O₃) single crystals were Czochralski grown in an iridium crucible and subsequently annealed in hydrogen at ~800° C. IR spectroscopy was used to investigate the roles of Mg and Ca dopants as well as H and Ir impurities. An IR peak at 3492 cm^-1 was previously assigned to an O-H bondstretching mode of a MgH complex in hydrogen-annealed Ga₂O₃:Mg and an IR peak at 3541 cm^-1 is tentatively assigned to a similar mode in a CaH complex for H annealed Ga₂O₃:Ca. An IR peak at 5148 cm^-1 is attributed to the presence of Ir⁴⁺ when magnesium is present. Excitation with several different wavelengths was used to confirm the position of the Ir level within the band gap. Our results suggest that isolated substitutional Ca dopants are less effective as deep acceptors than Mg. Polarization dependence of the iridium IR spectra is also discussed.

We have applied positron annihilation spectroscopy to study vacancy-type defects in unintentionally doped and Si and Sn doped β-Ga₂O₃ homoepitaxial thin films grown by metal-organic chemical vapor deposition (MOCVD). We detect Ga vacancy related defects at high concentrations in semi-insulating and highly resistive material, while conductive (ntype) material exhibits very low Ga vacancy concentrations. These findings show that Ga vacancies can act as efficient electrical compensators for n-type conductivity, but their concentrations can be suppressed by controlling the growth environment, leading to efficient n-type doping. We also note the strong anisotropy of the positron annihilation signals and give recommendation for presenting positron data obtained in β-Ga₂O₃.

Gallium oxide (β-Ga₂O₃) is a suitable material for next generation high power devices because of its huge critical electric field strength. However, most current device structures are not enough to take advantage of the full potential of Ga₂O₃ because these structures are optimized for material properties of silicon. To bring out the potential of Ga₂O₃, we propose a trench structure. First, we made Ga₂O₃ metal-oxide-semiconductor Schottky barrier diodes (MOSSBDs). The HfO₂ film was deposited on the trench bottom and sidewall. Ga₂O₃ MOSSBD had a small leakage current level, and had about a 40% lower forward voltage than that of the commercially available SiC SBDs. We thus successfully demonstrated that the performance of Ga₂O₃ devices can exceed that of SiC devices. Next, we made Ga₂O₃ junction barrier Schottky (JBS) diodes. p-type region was made by p-type NiO. The Ga₂O₃ JBS diode had several orders of magnitude smaller leakage current than that of the normal SBD. This result indicates that the electric field at the Schottky junction decreased as a result of using the JBS structure. Finally, we fabricated Ga₂O₃ trench MOS field effect transistors. We used a static induction transistor-type structure that can be made only with n-type semiconductors. Si-doped Ga₂O₃ n⁺ contact and n^-drift layers were grown on Sn-doped (001) Ga₂O₃ substrate with HVPE. The gate dielectric was HfO₂. The device showed clear current modulation characteristics and a maximum current density of 1.36 kA/cm². The device had a high on-off ratio of over 10⁷.

In this study, the defects analysis of different thicknesses of ZnGa2O4 thin-film transistors grown on the sapphire substrate had been investigated. The thickness of ZnGa2O4 epilayer is controlled by growth time. The electrical properties and physical characteristics are strongly related to the thickness, which is also dependent on both crystallinity and the amount of oxygen vacancies in thin-film and independent on thin-film surface roughness. The study shows that crystallinity gets better and the oxygen vacancies which can be served as defect center also increase rapidly when the thickness is increasing. On the contrary, when the epilayer is thin, the film would be influenced by dislocation between ZnGa2O4 epilayer and the sapphire substrate. The results suggested that source of defects may come from crystallinity, oxygen vacancies, and dislocation between ZnGa2O4 and sapphire substrate. However, the lower resistance in ZnGa2O4 thin-film is not only due to better crystallinity but the more amount of oxygen vacancies especially. It is a trade-off between the conductivity and defects in ZnGa2O4 epilayer. The results also show that the high conductivity in ZnGa2O4 epilayer is mainly due to the amount of existing oxygen vacancies especially instead of crystallinity.

Cubic ZnxMg1-xO have been proposed as wide bandgap semiconductors for short wavelength optoelectronic applications operating in the deep UV region. By combing MBE growth and HRTEM we were able to determine conditions in which ZnO and ZnxMg1-xO alloys in the rocksalt phase can be grown on MgO substrates. It was found that the maximum ZnxMg1-xO layer thickness strongly depends on Zn concentration, decreasing with x, which reflects the alloy phase instability. The band structures of rocksalt ZnxMg1-xO alloys were calculated in a supercell geometry by density functional theory in the Local Density Approximation (LDA). The atomic coordinates were determined using pseudopotentials implemented in the VASP Simulation Package. Then, the band structures were obtained by a Linear-Muffin-Tin-Orbital method in a full-potential version with a semi-empirical correction (LDA+C) for the band gaps. As MgO in the rocksalt structure has a direct band gap and ZnO has an indirect one, we expected transition: direct to the indirect gap for a certain content, x, of Zn. However, it is shown, that the ZnxMg1-xO band gaps depend strongly on the local arrangement of atoms in a 64 atoms supercell. For each concentration of Zn we obtained a set of the band gap values depending on the arrangement of atoms. Instead of two crossing lines illustrating the dependence of the direct and indirect gaps on composition, we got two crossing bands. The crossing of the two bands covers composition from 10% of Zn up to almost 70% of Zn. The results are compared with the experimental data.

Reverse breakdown voltages larger than 1 kV have been reported for both unterminated Ga₂O₃ vertical rectifiers (1000- 1600 V) and field-plated Schottky diodes (1076-2300 V) with an epi thickness of 8-20 μm. If the doping is in the 10¹⁶ cm^-3 range, the breakdown is usually in the 500-800V regime. Furthermore, the switching characteristics of discrete Ga₂O₃ vertical Schottky rectifiers exhibited reverse recovery times in the range of 20 to 30 ns. Large area (up to 0.2 cm² ) Ga₂O₃ rectifiers were fabricated on a Si-doped n-Ga₂O₃ drift layer grown by halide vapor phase epitaxy on a Sn-doped n+ Ga₂O₃ (001) substrate. A forward current of 2.2 A was achieved in single-sweep voltage mode, a record for Ga₂O₃ rectifiers. The on-state resistance was 0.26 Ω·cm² for these largest diodes, decreasing to 5.9 × 10^-4 Ω·cm² for 40x40 μm² devices. We detail the design and fabrication of these devices. In addition, an inductive load test circuit was used to measure the switching performance of field-plated, edge-terminated Schottky rectifiers with a reverse breakdown voltage of 760 V (0.1 cm diameter, 7.85x10^-3 cm² area) and an absolute forward current of 1 A on 8 Μm thick epitaxial β-Ga₂O₃ drift layers. These devices were switched from 0.225 A to -700 V with t_rr of 82 ns, and from 1 A to -300 V with t_rr of 64 ns and no significant temperature dependence up to 125°C. There was no significant temperature dependence of t_rr up to 150°C.

Accumulation of non-equilibrium hot longitudinal optical (LO) phonons limits the electron drift velocity for electronic devices operating under high electric field. Ultrafast decay of hot phonons can take place via plasmon-LO phonon resonance, which leads to fast electron energy relaxation and hence high electron drift velocity and optimum operation of the devices. This need motivates us to create heterostructures with 2DEG density close to the plasmon-LO phonon resonance region. Through incorporating a few percent of Be into the BeMgZnO barrier to switch the strain sign in the barrier from compressive to tensile, we have achieved 2DEG densities over a wide range in Zn-polar BeMgZnO/ZnO heterostructures with moderate Mg content (below 30%) grown by molecular beam epitaxy. We have obtained electron mobility of 250 cm²/Vs at room temperature (293 K) and 1800 cm²/Vs at 13 K in Be_0.02Mg_0.26ZnO/ZnO heterostructures. Via capacitance-voltage (CV) spectroscopy, we have explored the depth profiles of the apparent carrier density of samples grown under different conditions. The correlations between electrical properties and MBE growth parameters of Zn-polar BeMgZnO/ZnO heterostructures are discussed.

We report on the first demonstration of quantum cascade detectors based on ZnO/ZnMgO quantum wells grown by molecular beam epitaxy on an m-plane ZnO substrate. The sample is processed in the form of square mesas with special attention paid to the passivation of the side facets. Photocurrent spectroscopy reveals a resonance at 2.8 μm wavelength slightly blue-shifted with respect to the intersubband absorption peak at 3 μm wavelength. The photocurrent persists up to room temperature. The peak responsivity amounts to 0.15 mA/W under irradiation at Brewster’s angle of incidence of the top surface of the mesas.

Two dimensional electron gases (2DEGs) formed at interfaces of two oxides have been drawing growing attentions for their intriguing magnetic, 2D superconducting and optical properties. To investigate optically 2DEG formed at LiNbO₃/indium-tin-oxide interface, the power of the very first reflection beam was monitored under illumination of one (two) laser beam(s). It was found the very first reflection can be reduced to as low as 1.13% from the original 12.9%, pointing unambiguously to a subwavelength coupling and corresponding to conservatively estimated exponential gain coefficient of -78525 cm^-1 by taking half a wavelength as the coupling range, since the 1^st reflection is dictated by what occurs in that range. Such high exponential gain coefficient, far beyond the reach of conventional photorefractive theoretical framework, is consistent with a physical picture of 2DEG supported interface plasmon polaritons. Such dramatic reflection reduction and corresponding high exponential gain coefficient are highly valuable in designing nanometric photonic devices, such as waveguides, attenuators (amplifiers), modulators and sensors, which are compatible to photonic circuits nowadays. In addition, such a material system is promising for nonlinear plasmonic applications.

Due to the strong demand for photonic computing, on-chip optical communication, medical imaging, and biosensing at the nanoscale, interest in nanolasers has grown dramatically in recent years. Plasmonic lasers are promising as nanoscale laser sources and have been widely studied using semiconductor nanowires on metal surfaces grown by bottom-up techniques. However, these nanowire plasmonic lasers require transfer and positioning after fabrication, making their use in practical on-chip devices difficult. In this study, we demonstrate a monolithically fabricated plasmonic-waveguide nanolaser. This is the first report showing a non-transfer plasmonic-waveguide nanolaser with a structure size (not only the mode size) in the sub-wavelength regime. A plasmonic waveguided mode capable of sustaining lasing is carefully designed so that top-down fabrication techniques can be used (no need of nanostructure transfer) to simultaneous fabricate the nanolasers together with waveguides for an optical circuit. Moreover, the design supports a lasing mode with a large effective area and confines the absorption of the pump light to the area in which the plasmonic-waveguide mode is most intense, reducing the lasing threshold. Lasing up to room temperature with a low threshold intensity of 0.20 mJ/cm^2 is demonstrated.

Nanophotonics profits enormously from the unique optical response shown by metamaterials and multilayers based in oxides. The functional optical response is achieved by either embedding non-oxide nano-resonators to enable a plasmonic behavior, or by modifying the oxide composition/structure to tune its optical, electronic and magnetic properties. In this contribution first, we review the non-conventional plasmonics based on elements of the p-block. The plasmonic response has its origin on the interband transitions of these elements in the infrared [1,2]. Especial emphasis will be made on bismuth nano-resonator structures because Bi shows the strongest interband transitions reported so far [3]. Second, we will show the key role of oxide layers in the design of plasmonic metamaterials to enable a strong coupling between plasmonic and photonic modes. Such coupling has demonstrated to be instrumental for development of high resolution optical thermometry sensors [4]. Finally, we show how the fine control of the composition of europium monoxide (EuO) nanocrystalline films induces a large tuning of its band-gap and of the ferroelectric response, which is necessary for its integration in optical and spintronics platforms [5]. [1]. J. Toudert, and R. Serna, Opt. Mat. Expr. 7, 2434 (2016); [2]. J. Toudert, and R. Serna, Opt. Mater. Express 7, 2299 (2017); [3]. J. Toudert, R. Serna, I. Camps, J. Wojcik, P. Mascher, E. Rebollar, T. Ezquerra, J. Phys. Chem. C 121, 3511 ( 2017); [4] G. Baraldi, M.García Pardo, J. Gonzalo, R. Serna and J. Toudert, Adv. Mater. Interfaces 5, 1870058 (2018); [5] A. Mariscal, et al., Appl. Surf. Science 456,980 (2018).

Transparent conducting oxides (TCOs) are extensively investigated because of their applications as transparent electrodes in solar cells and light-emitting devices. TCOs of interest include indium-tin oxide, aluminum-doped zinc oxide, nickel oxide (NiO), and their combinations. There is strong interest in NiO because no heteroatoms are required to “dope” it at high transparency levels. It has been speculated that paramagnetic defects due to Ni³⁺ centers and O interstitials are responsible for the electrical conductivity of otherwise insulating and antiferromagnetic NiO, but direct investigation of such defects has been limited. Here, the electrical conductivity in nanostructured NiO thin films is investigated and correlated to the paramagnetic defect density extracted from electron spin resonance (ESR). Two types of ESR-active centers are identified. Our work points at defect engineering as a necessary step to optimize NiO thin films for their applications as TCOs.

Staggered back-gated Field Effect Transistor (FET) structures were made by growing Li-doped NiO on S_i3N₄/SiO₂/Si (111) using room temperature pulsed laser deposition. Optical studies showed over 80% transmission for the NiO:Li channel at wavelengths > 500nm. The MISFET revealed rectifying transfer characteristics, with a V_ON close to zero, a channel mobility of ~ 1 cm²/Vs, a gate leakage current (at +5V) of 0.8 mA and an I_ON/I_OFF ratio (at a Vgs of -15V) of ~ 10³. The transistors showed enhancement-mode output characteristics indicative of a p-type channel with sharp pinchoff, hard saturation, a comparatively high (milliampere range) Id and a relatively low on-resistance of ~11 kΩ. Hence the adoption of Li doping in NiO channels would appear to be a promising approach to obtain p-type TFTs with superior transparency, speed and energy efficiency.

By incorporating low weight percentages of graphene in our precursor solution and using a technique derived from spray pyrolysis, we obtained a low cost TCO with excellent properties, characterized by AFM, UV-VIS and Hall Effect. The film roughness was calculated to be ~7.49-12.7 nm, while band gap was determined to be ~4.0-4.2 eV. The material’s optical transparency ranges from 84-86% and its resistivity was measured to be ~5.5-8.5 x10^-3 Ω cm. With these results, we suggest that the obtained material is a proper candidate for use in photovoltaic applications, such as Grätzel solar cells, which will be our main focus.

Perovskite solar cells have been rapidly developed owning to their simple fabrication procedures, low cost and high efficiency. Currently, record efficiency of perovskite solar cells on rigid substrates exceeds 20%. In general, solution process is used to synthesis the perovskite layer, i.e. MAPbI3. Although solution process is direct and simple, significant usage of organic solvent like chlorobenzene, which is used as anti-solvent during one-step solution process, is not environmentally friendly. In addition, solution processing at low temperatures results in small average grain sizes and poor crystallinity. Furthermore, the film morphology and properties are strongly dependent on fabrication conditions, and the presence of solvent vapor in the glovebox can significantly affect the repeatability and the reproducibility of the perovskite film properties, and consequently the solar cell performance. Thus, it is desirable to develop solvent-free process for the preparation of the perovskite film, different from vapour-deposition processes which would increase perovskite fabrication cost. Such a solvent-free processed perovskite film may lead to not only better efficiency with better film quality, but also more reproducible and greener fabrication process. Approaches of using solid state reaction between PbI2 and MAI will be focused to fabricate MAPbI3 films. After optimization of the solvent-free fabrication process, solar devices for both solvent-free and solution processed were fabricated for efficiency comparison.

Semiconductor quantum wells and superlattices, which are usually fabricated through metal-organic chemical vapor deposition or molecular beam epitaxy, are key building blocks in modern optoelectronics. The ability to simultaneously realize defect-free epitaxial growth and to individually fine-tune the chemical composition and band structure of each layer is essential for achieving the desired performance. Such structures are challenging to realize using organic or hybrid materials because of the difficulty of controlling the materials growth. In this talk, I will present a molecular approach to the synthesis of high-quality organic-inorganic hybrid perovskite quantum wells through incorporating widely tunable organic semiconducting building blocks. By introducing sterically tailored groups into the molecular motif, the strong self-aggregation of the conjugated organic molecules can be suppressed, and single crystalline organic-perovskite hybrid quantum wells (down to one mono-layer thick) and superlattices can be easily obtained via one-step solution-processing. Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically-flat interface and ultra-small interlayer distance. The 2D hybrid perovskite superlattices are surprisingly stable, due to the protection of the bulky hydrophobic organic groups. Finally, we demonstrate the applications of these materials in high performance solar cells and field effect transistors.

Over the past few years, zinc oxide nanorods (ZnO NRs) have started emerging as a promising candidate in the area of optoelectronics and various sensor applications due to their structural advantages over the thin film. Enhancing near band edge emission (NBE) with suppression in the defect state emission (DBE) is a challenging problem for utilizing hydrothermally grown ZnO NRs in device application. In this work, we are reporting improvement in NBE and decrement in the DBE peak intensities with post growth UV-Ozone (UV-O) treatment. Hydrothermal bath process was used to fabricate nanorods on the annealed ZnO seed layer followed by UV-O treatment for 20 minutes. Growth of the nanorods was confirmed using field emission gun scanning electron microscopy (FEGSEM). Room temperature photoluminescence (PL) spectra of sample B shows 1.6 times enhancement of NBE/DBE ratio as compared to as grown sample A. Reduction in the oxygen vacancies was confirmed using high resolution x-ray photoelectron spectroscopy (HRXPS), where it was observed to reduce from 25% for as-deposited sample to 7% for UVO annealed sample, leading to increase of NBE/DBE ratio, as observed from PL spectra. High resolution x-ray diffraction (HRXRD) pattern exhibited dominant (002) peak from both samples. A slight right shift was observed in HRXRD peak which suggest improvement in stoichiometric ratio for UV-O treated sample.

On one hand, interest on the tunability of the optical microcavities has increased in the last few years due to the need for selective nano- and microscale light sources to be used as photonic building blocks in several applications. On the other, transparent conductive oxide (TCO) β-Ga₂O₃ is attracting attention in the optoelectronics area due to its ultra wide band gap and high breakdown field. However, at the micro- and nanoscale there are still some challenges to face up, namely the control and tuning of the optical and electrical properties of this oxide. In this work, Cr doped Ga₂O₃ elongated microwires are grown using the vapor-solid (VS) mechanism. Focused Ion Beam (FIB) etching forms Distributed Bragg Reflector (DBR)-based resonant microcavities. Room temperature microphotoluminescence (μ-PL) spectra show strong modulations in the red-NIR range on five cavities with different lengths. Selectivity of the peak wavelengths is obtained, proving the tunability of this kind of optical systems. The confined modes are analyzed experimentally, analytically and via finite difference time domain (FDTD) simulations. Experimental reflectivities up to 78% are observed.

Transparent Conducting Oxide (TCO) materials are degenerately-doped, wide-bandgap semiconductors which exhibit simultaneous high-conductivity and visible transparency. These unique properties are well known and frequently exploited for technologies such as touch-screen devices. In recent years, TCOs have been recognized as a promising material platform for nanophotonic devices, namely because of their simple, compatible fabrication, low-losses, dynamic modulation, and novel low-index properties. In this talk, I will highlight recent progress in the field of TCO-based nanophotonics, share our ongoing results and observations, and discuss future research challenges and directions. In particular, I will discuss our progress in developing metal-dielectric hybrid metasurfaces which incorporate TCOs for all-optical, ultrafast switching. Here, we incorporate defect-rich zinc oxide with a refractory titanium nitride metasurface for efficient light modulation at near-terahertz switching frequencies. My talk will also focus on TCO films for studying and observing low-index phenomena. Our recent work with aluminum-doped zinc oxide films demonstrates the ability for low-index materials to both enhance negative refraction and engender strongly coupled plasmonic systems with large room-temperature Rabi frequencies. Our work signifies the strong potential for incorporating transparent conducting oxides into plasmonic and nanophotonic devices to provide advances toward practical technologies and depth in scientific understanding.

Tunable photonic circuits were demonstrated by using ferroelectrics that had large electro-optic effect. The photonic circuits were fabricated by complementary metal–oxide–semiconductor (CMOS) technology thus enabling their integration with the present microelectronics for high speed signal modulation. From the scanning electron microscopy and energy-dispersive X-ray spectroscopy, we showed the photonic devices, including the optical waveguides, had sharp waveguide edges and interfaces. A sharp fundamental waveguide mode was observed over a broad spectral range. Tunability using Pockels effect were experimental results. Our device paves the way for ultra broadband integrated photonics critical for optical computing.

Transparent and flexible conductive materials are critical components in many optoelectronic devices, such as wearable electronics, biosensors, displays, etc. Conventional transparent electrodes made of a single material, such as indium tin oxide (ITO), ultrathin metals, graphene and poly-(3, 4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) have limitations and hardly possess the desired combination of broadband transmittance, low electrical resistivity, mechanical flexibility, and biocompatibility. Herein, we designed and constructed an ultra-flexible, conductive, transparent thin film using a PEDOT:PSS/ITO/Ag/ITO multilayer structure on Parylene C. The multilayer assembly was optimized to achieve the lowest theoretical reflectance by simulating the coatings admittance loci under the preferred reference wavelength. ITO and Ag were deposited consecutively using RF magnetron sputtering at room temperature, followed by spin-coating of PEDOT:PSS. The sputtering deposition temperatures were tuned to achieve the optimal optical and electrical properties. Compared to a single-layer ITO film of equivalent thickness, the multilayer films exhibited significantly decreased sheet resistance, reduced electrochemical impedance, remarkable transmittance, large Young’s modulus values, and superior stability in air and saline. The multilayer films also showed strong adhesion to the Parylene C substrate and subsequently excellent bending tolerance. Moreover, the peak transmittance of our multilayer flexible thin films could be tailored to a specific wavelength for particular applications, such as optogenetics that utilizes light of different wavelengths to excite or inhibit the activity of genetically targeted neurons or intracellular signaling pathways.

In-situ sensing in high temperature and chemically reactive environments – i.e., within solid oxide fuel cells or power plant boiler systems – is inherently challenging due to the rapid degradation of most traditional sensor materials within this regime. Although optical fiber based sensors provide clear advantages in this context, progress in this area of application has hinged on the development of (a) optical fiber materials and (b) thin film materials with strong optical response to gas environment, both of which must resist degradation under such conditions. Conducting metal oxide thin films have been examined in the literature as a candidate to solve the latter problem, due to a free-carrier governed optical response in the NIR (1-2 μm), that can be strongly dependent upon gas environment. In this work, we present the impact of incorporating gold nanoparticles in one such metal oxide, lanthanum-doped strontium titanite (LSTO), on the gas sensing response, both in the NIR and UV-VIS range. Via optical transmission measurements performed at high temperature (up to 800 C), the intertwined free-carrier response of the film and the localized surface plasmon response of the nanoparticles are examined in the presence of hydrogen of varying concentration. Measurements are presented for films coated both on planar substrates and on optical fibers.

The effects of annealing on gas sensing properties of NOx gas sensor based on novel material ZnGa2O4 epi-layer grown by MOCVD were studied. The metal-semiconductor-metal (MSM) structure sensor with Ti/Al/Ni (50/75/25 nm) electrode in the multilayers which were deposited by an E-gun evaporator and patterned by a lift-off process. The devices were annealed at 700 ºC in N2 ambient for 1hr, and the sensing area is 30um x 250um. The results show that the sensitivity of the ZnGa2O4 gas sensor increases and the response time reduces after annealing. The sensitivity is defined as Rg/Ra, where Rg is the resistance with analyzed gases, and Ra is the resistance with the dry air. At the operation temperature 300ºC, the sensitivity of sensors without thermal treatment are 1.026, 1.015, 1.009, 1.003, and 1 when exposed to NO concentration 6.25ppm, 1ppm, 500ppb, 250ppb, and 125ppb, respectively. After 700ºC annealing for 1hr, the sensitivity remarkably increases to 52.108, 10.491, 7.744, 4.961, and 3.942 with the same NO concentration as mentioned above. Not only the sensitivity increases more than 10 times but thin-film can detect extremely low NO concentration (125ppb) after thermal treatment. The sensitivity is linear dependent on the NO concentration. Besides, the response time improved all under 30s with the concentration range from 1ppm to 125ppb. Most important of all, the sensors show excellent selectivity which means the sensitivity were all below 1.02 when exposed to CO, CO2, SO2 gases with 1ppm. The results point out that the ZnGa2O4 gas sensors after annealing exhibit the better NO sensing properties, shorter response time and outstanding selectivity.

The last several years has been one of rapid adoption of new technologies that use the convergence of super resolution optical microscopy and Artificial intelligence, not only for research but in factory settings. Detection and classification of defects and devices on semiconductors has commonly been used for process control and quality control. Now, utilizing advances in deep learning and other forms of Artificial intelligence, defect inspection framework is extended to be used not only as a reliable tool for identifying problems, but assigning casualty. This presentation will focus on a new convergence of high throughput inspection with Photoluminescence Imaging. Photoluminescence Imaging allows quantification and colocalization of different types of defects by combining multiple spectral snapshots of the sample collected with different output filters. These defects cannot be detected or classified under conventional brightfield microscopy. This method will provide a way to test functionality on the same wafer or product, and potentially at the same time as routine classification, giving a robust single station for solving production problems and leading faster design iteration. Data will be presented that shows this functionality in its early stages.

Alumina ceramic is widely used as a thermocouple protector or engineering ceramic in harsh environments due to its high-temperature stability, chemical inertness, and low thermal conductivity. Because alumina is prevalent in high-temperature equipment, it is useful to understand its Raman properties to enable temperature measurement via Raman spectroscopy in these harsh-environment systems. In this paper, we report temperature mapping on the surface of high-temperature alumina ceramics via Raman spectroscopy. The ceramics studied were as cast (unpolished) to approximate use in high-temperature industrial applications. Temperature calibration equations were generated covering the range from room temperature to ~1000°C. The most accurate mathematical temperature relation used the Raman line shift of the most intense Stokes A_1g peak at 418cm^-1. The average standard deviation of the Raman temperature measurement was less than 5°C over the entire experimental temperature range. This method will provide accurate non-contact surface temperature measurments in harsh environments where thermal radiation and variable surface emissivity contributes significant error to simpler infrared thermometry.

In this work, we propose a simple and time-saving method for the characterization of the ZnO-NWs. The method is based on the measurement of the spectral reflection of the ZnO-NWs in the UV-VIS-NIR ranges. Then the ZnO-NWs effective refractive index, and subsequently the density, and the length are obtained making use of the interference pattern contrast and periodicity in the reflection response versus wavelength. The extracted NWs length and density using the proposed method show good agreement with the SEM results. This characterization method opens the door for easy and cheap monitoring of the growth within microfluidic environment.

Multiferroic oxide thin films continue to receive intensive research interest due to their excellent combination of properties that are related to the ferroic orders. In this paper, BiFeO₃ thin films are fabricated on (001) Si substrates at varying ad-atom energy to establish the role of microstructure and stress evolution on their elastic properties. Stress evolution in the films is accomplished through incorporation of argon ions during in-situ film growth. The dynamics of the propagation of surface acoustic waves on BiFeO₃ thin films of varying microstructure and stress is investigated using surface Brillouin scattering (SBS). The film substrate configuration is of a slow on fast configuration that supports Rayleigh surface acoustic waves (RSAW) and Sezawa waves. Hence the phonon velocity dispersion curves were determined from the frequency shifts of both sets of the waves in the discrete region at constant incident angle of 71 deg. Using the inverse problem the elastic constant for isotropic film systems have been determined with the results showing that Ar⁺ incorporation decreases the bulk moduli and increases the shear moduli by 50%.

We report the energetic 8 MeV electron beam induced modification on linear and nonlinear optical process in Mn doped ZnO (MZO) thin films at different irradiation dosage. The modifications incorporated on third order nonlinear optical absorption were studied using Z-Scan technique in both continuous and pulsed laser regime. Open aperture Z-scan measurement indicates that pristine and film treated at 15 kGy electron beam dosages reveals reverse saturable absorption (RSA) mechanism and films treated at 5 kGy, 10 kGy and 20 kGy exhibits saturable absorption (SA) phenomena. The irradiation resulted in a high βeff value of 12.1 ×10^-2 cm/W in continuous wave excitation and 5.6×10^-4 cm/W for pulsed excitation as compared pristine films. Gaussian deconvolution fitting on room temperature PL spectra shows a quenching of defect centers upon electron beam irradiation. The observed decrement in PL emission intensity for the films treated with energetic electron beam can be probably due to recombination of defect centers and enhanced non radiative defects. The decrease in the energy band gap and increase in the urbach energy of the MZO thin films was observed due to creation of deep energy levels into the band gap. The irradiation treatment resulted in significant changes on the crucial parameters of optical sensing such as limiting threshold and optical clamping. The present study indicates that nonlinear parameters of MZO thin films can be tuned by choosing appropriate electron beam dosage for photonics applications.

We here demonstrate the synthesis of different nanostructures, including nanoparticles, nanorods, core-shell structures, and compound metal oxide nanostructures all synthesized by a low temperature chemical process. We further investigated their photocatalytic properties for degradation of toxic waste and their photochemical efficiency for water splitting. All the photocatalytic properties as well as the photochemical properties were utilized using sun radiation. The results presented indicate huge potential for the investigated processes with positive impact to energy consumption and benefits for the environment.

Most silicon based detectors are sensitive predominantly in the visible region. To extend their sensitivity further into the UV and IR regions, surface modification techniques can be employed. In addition, there is always a need for biocompatible, eco-friendly, and cost effective devices for bio-medical applications. Therefore, devices fabricated using silicon and titanium can enable the fabrication of affordable health care devices. Another critical factor that plays a major role in controlling the device performance is its surface wettability. Tuning the surface energy and thus its wettability can improve the stability, passivation, self-cleaning, and anti-aging properties of device surfaces. In this paper, we present a method for tuning the surface wettability with enhanced optical absorption in the UV-Vis-NIR regions. We optimize the titanium coatings on silicon substrates and demonstrate how the controlled deposition of film thickness and annealing temperature or time can affect the self-cleaning behavior and light localization with anti-reflection properties of surfaces. We take the combined advantage of the anti-reflective properties of anatase or rutile phases of oxides of titanium (TiO_x) and the nature of opto-semiconductor Schottky barrier Ti-O-Si formed at the titanium and silicon interface for the improved absorption in a wide spectral range of radiation ranging from 200 nm (UV) to 3000 nm (short-wavelength infrared) wavelengths. An average optical reflectance tuned to less than 10% and absorbance greater than 70% can be obtained. Further, these coatings act as a protective encapsulation by possessing a hydrophobic surface with water contact angles < 100°.

Controlling the flow of light is fundamental to optical applications. With the recent advances in nanofabrication capabilities and new theoretical concepts, ground breaking platforms for the nanoscale manipulation of light have been demonstrated in recent years. These include metasurface and epsilon-near-zero (ENZ) materials and structures, which offer unique optical features such as sub-wavelength field confinement, unusual optical nonlinear/quantum properties and advanced wavefront shaping. This talk will review our recent development on an electrically tunable conducting oxide metasurface that can tune the optical phase and amplitude and a broadband, tunable, and ultrathin conducting oxide epsilon-near-zero for meta-devices. I will present our recent development on the use of gate-tunable materials, transparent conducting oxides, to demonstrate an electrically tunable metasurfaces that can tune the optical phase and amplitude for ultrathin beam steering devices [1]. In addition, a broadband, field-effect tunable, and ultrathin ENZ perfect absorber enabled by the excitation of ENZ modes will be discussed [2,3]. 1. Y. W. Huang, H. W. Lee, R. Sokhoyan, R. Pala, K. Thyagarajan, S. Han, D. P. Tsai, and H. A. Atwater, “Gate-tunable conducting oxide metasurfaces,” Nano Lett. 16, 5319-5325 (2016). 2. A. Anopchenko, L. Tao, C. Arndt, H. W. Lee, “Gate tunable and broadband Epsilon-near-zero perfect absorbers with deep subwavelength thickness,” ACS Photonics (accepted, 2018). 3. A. Anopchenko, S. Gurung, L. Tao, C. Arndt, H. W. Lee, “Atomic Layer Deposition of Ultra-thin and smooth Al-doped ZnO for Zero-Index Photonics”, Materials Research Express, 5, 014012 (2018).

In this work, the electron-carrier-selectivity of ALD deposited TiO2 contact on n-type and p-type c-Si wafers is presented. The optical, compositional, and diode quality dependence of TiO2 on the ALD deposition temperature were analyzed using spectroscopic ellipsometry, AFM, XPS, GI-XRD, and CV measurements. By optimizing the ALD process parameters, an impressive effective minority carrier lifetime of up to 2.3 milliseconds corresponding to an iVoc of ~700 mV was obtained from wet chemical oxide-SiO2/TiO2 passivation stack layers. Finally, the asymmetry in C-V and J-V measurements betweenTiO2/n-type and TiO2/p-type c-Si heterojunctions was examined and the electron transport selectivity of TiO2 was revealed.

In this work, we report on the tunability of carrier concentration and epsilon-near-zero (ENZ) wavelength (i.e. the region where the real dielectric permittivity of a material approaches zero) in ultrathin (<100 nm) Al-doped ZnO (AZO) nano-layers fabricated through the atomic layer deposition (ALD) technique. ALD is a variation of chemical vapor deposition in which a substrate is exposed to only one self-limiting reactant at a time, allowing for ultra-smooth, conformal deposition and precise control over film-thickness at the nanometer scale. To create the AZO meta-films, fused silica substrates are exposed to alternating cycles of Diethylzinc (DEZ) and water vapor, with periodic dopant cycles of Trimethylaluminium (TMA). Optical and material properties of the meta-films are determined using spectroscopic ellipsometry. Using the Drude model and regression analysis with measured values, properties such as film thickness, ENZ wavelength, and complex refractive index are then determined. Furthermore, excitation of ENZ modes in the fabricated films has been demonstrated experimentally using the Kretschmann-Raether configuration. It was found that by varying the deposition temperature, Al:ZnO doping ratio, and film thickness, the ENZ wavelength of AZO thin films could be precisely tuned in the near infrared region from 1520 to 1700 nm. The results of this work allow for the precise engineering of optical properties of AZO films for zero-index photonic applications.

Performance of adaptive infrared camouflage is usually parameterized in terms of cycle-ability, response time, actuation mechanism, stability etc., however, one of the key components that has not been addressed so far is the spatial density of infrared information that can be encoded and actively manipulated for camouflaging. We report an adaptive infrared camouflage system that can be engineered to operate at any desired wavelength in the technologically relevant, infrared transparent 3 – 5 µm and 8 – 12 µm bands. We exploit the metal-insulator phase transition in VO2 to design an optical cavity coupled infrared absorber where the cavity length can be altered by controlling the VO2 phase. Cavity tuning is done by strategically placing the VO2 layer inside the optical cavity composed of a tri-layer architecture. In its insulating state VO2 is transparent to infrared such that incident light couples to the entire cavity length, however in the metallic state, VO2 behaves like a mirror and shortens the cavity length by reflecting ~80% of incident light. The Maxwell Garnett EMT describes the phase transition dependent optical response of the absorber better than the Bruggeman EMT when compared to the experimental results. We tailor the device parameters to demonstrate adaptive thermal camouflage of multispectral encoded infrared information on a pixelated designer surface with a pixel resolution (~20 µm) and density comparable to the industry standard for infrared sensors. We envision this work will pave the way for novel tunable optical devices for technological advancements in infrared tagging, camouflaging and anti-counterfeiting efforts.

The crystal is represented in the form of individual molecule sublattices. Within oscillation electronic model, a superconducting phase transition in an ABCD crystal is possible when the square plasma energy of the resulting molecular sublattices is equal (or larger) to the square plasma energy of the initial crystal. Lanthanum cuprate crystal is La2CuO4 considered. An elementary cell with two molecules z = 2 is considered, number of sublattices is n = 4. The square plasma energies of the initial crystal A B C D and its individual molecules A2, B2, C2, D2 were calculated. The phase transition curve of a superconductor is described by the quadratic function equation, where Tc is the temperature of the superconducting phase transition, q is the interaction parameter. From the equation of the phase transition curve in pure ideal mixed oxides of lanthanum and copper, a superconducting phase transition is not detected. When Lanthanum La is replaced by Strontium Sr, the superconducting transition temperature Tc reaches a value of up to 40K according to the literature data. Interaction parameter and order parameter are given. The influence of zinc and nickel impurities on the superconductivity of mixed copper and lanthanum oxides is calculated. The superconducting phase transition in a crystal proceeds as follows antiferromagnetic state - spin glass - phase separation - superconducting state. It should be noted that when the transition to the superconducting state of the base crystal, the dopants pass into the antiferromagnetic state. Superconducting - antiferromagnetic - paramagnetic state La, Superconducting - antiferromagnetic - paramagnetic - diamagnetic state Cu, Superconducting - antiferromagnetic - paramagnetic state Sr, Superconducting - antiferromagnetic - paramagnetic - diamagnetic state Zn, Superconducting - antiferromagnetic - paramagnetic - ferromagnetic state Nι. When calculating the temperature of the superconducting transition Tc for a crystal, it is necessary to consider the balance criterion of the square plasma energies, established for each temperature with allowance for thermodynamic equilibrium. Doping causes a violation of thermodynamic equilibrium at a certain temperature. This leads to the formation of valence bonds ―Cu ― О― with the release of energy with decreasing temperature, then a balance of energies is observed at a lower temperature, valence bonds are formed ―Cu ― Cu―, the released energy is spent on breaking the bonds ―Cu ― O― and so on.

Intersubband absorption at normal incidence is forbidden by the selection rules and requires oblique incidence operation or texturing of the surface of intersubband-based devices such as focal plane arrays, adding additional processing steps to their fabrication and therefore increasing complexity and costs. Here we demonstrate normal-incidence, polarization sensitive intersubband absorption by wurtzite ZnO/MgZnO quantum wells grown on an m-plane orientation. When grown in this non-polar plane, the ZnO/MgZnO quantum wells spontaneously assemble forming a V-groove profile in the direction perpendicular to the c-axis, i.e. along the a-direction. A stack of quantum wells featuring this morphology acts as a metamaterial that allows for intersubband absorption at normal incidence whenever the electric field of the light is polarized in the direction perpendicular to the c axis. This phenomenon occurs because when the electric field is perpendicular to the c-axis it is no longer contained in the plane of the quantum wells therefore allowing for a small intersubband absorption. On the contrary, if the electric field is parallel to the c-axis, the usual normal-incidence conditions are recovered and no absorption is observed.

Zn₂GeO₄ is a novel transparent conductive oxide material, with an ultra-wide band gap of 4.5 eV, and rather good electrical conductivity. Zn₂GeO₄ nano- and microwires grown by a thermal technique show two intense emission bands centered in the UV and VIS region, respectively. In this work, we have studied the correlation between luminescence and structural properties in undoped and Mg doped samples. The waveguiding behavior of the microrods has been assessed by the excitation with a 325 nm laser and analysis of the transmitted light along the structure. The intense white luminescence of Zn₂GeO₄ nanomaterials makes them of interest as efficient phosphors for field emission displays.

ZnO can be used as a phosphor that has several applications and uses in the lighting and solar cell fields. ZnO as a host material in combination with various dopant ions with the correct valence state can be used to obtain emissions from the Ultra violet (UV) to the infra-red (IR) wavelength ranges. The major problem that limits solar cells’ efficiency is their insensitivity to the whole solar spectrum which is the so-called spectral mismatch. Therefore, several mechanisms have been explored based on photoluminescence to convert the solar cell spectrum where the spectral response of the solar cell is low to regions where the spectral response of the solar cell is high. For single crystalline silicon (Si) photovoltaic (PV) cells with a rather small semiconductor band-gap (Eg: 1.12 eV, corresponding to a wavelength of ~1100 nm), the transmission loss of the sub-band-gap photons can still amount to about 20% of the sun’s energy irradiated onto the Earth’s surface. For PV cells with a larger band-gap, such as amorphous Si (Eg: 1.75 eV) solar cells, which are limited to absorb sunlight with wavelengths below 708 nm, manifest even higher near infrared transmission losses. Downconversion, up-conversion and downshifting are some of the mechanisms that may be applied to improve the spectral response. Doped ZnO can be used for both down shifting and up-conversion applications, especially for the improvement of solar cell efficiency. ZnO thin films prepared with different methods such as Pulsed laser deposition, chemical bath deposition and spin coating are compared with each other for possible uses in lighting and solar cell applications.

Zinc oxide (ZnO), a wide bandgap (3.37 eV) semiconductor with large exciton binding energy (60 meV), is a promising candidate for optoelectronics application. The bottleneck to harness its capability is linked to the default ntype nature imposed on the material by native defects. Thus, controlling its conductivity (p, i or n) with post growth processes is a strenuous task. Phosphorus is a preferred p-type dopant because of its large solubility. In this report, we have studied the effect of variation in phosphorus implantation time to 40, 60 and 70s on the optical and structural properties of ZnO thin films. Plasma immersion ion implantation technique was carried out to dope the thin film deposited by RF sputter technique and samples were further annealed at 900oC in oxygen ambience for 10s. Low temperature photoluminescence (PL) spectra showed improvement in acceptor behaviour with increase in doping time. Sample doped for 70s exhibited maximum number of acceptor based excitonic peaks at around 3.24, 3.31 and 3.35 eV corresponding to donor-acceptor pair (DAP), free acceptor (FA) and acceptor-bound (AoX) excitons, respectively. High resolution x-ray diffraction showed dominant (002) peak from all samples and increase in phosphorus implantation time shifted the peak towards higher 2θ angle. X-ray photoelectron spectroscopy further suggested increment in phosphorus concentration with implantation time as the number of peaks corresponding to P-O bond observed from P 2p spectra was improved. Scanning electron microscopy images revealed better annihilation of implantation defects post annealing.

Real time gas sensing in high temperature energy conversion devices can enable optimal and efficient operation at both component and system levels, and the optical fiber based sensing platform shows significant advantages for harsh environment applications. In this research, (La_0.8Sr_0.2)_0.95MnO_3-δ (LSM), (La_0.8Sr_0.2)_0.95CoO_3-δ (LSC) and (La_0.8Sr_0.2)_0.95Co_0.2Fe_0.8O_3-δ (LSCF) films with thicknesses of several tens of nm are integrated with the optical fiber sensing platform as a functional sensor layer using a finely tuned-RF sputtering system designed for the fiber substrate deposition. Oxygen sensitivities, stabilities and overall feasibilities of these representative perovskite materials on the optical fiber platform are evaluated in the solid oxide fuel cell operational temperature regime at the oxygen concentration up to 19%, relevant for in-cell cathode stream gas composition sensing through optical transmission measurement which covers visible and near infrared wavelength ranges. Various sensitivity comparisons are carried out as a function of thickness, oxide composition, and deposition conditions. In general, the LSM sensor shows a stepwise absorption response to increasing levels of O₂ in a N₂ background, but also exhibits relatively slow kinetics including a continuous baseline drift. In contrast, LSCF based sensors exhibited enhanced transmittance responses in O₂ containing gas and a more rapid recovery and response, presumably due to the enhanced oxygen ion diffusion kinetics as compared to LSM. The results presented here are promising for the broad application areas of high temperature O₂ sensor research and a concomitantly wide range of energy related applications including combustion, solid oxide fuel cells, and others.

Inherent properties of wide bandgap (3.37 eV) and high exciton binding energy (60 meV) have helped zinc oxide (ZnO) to claim its potential in the area short-wavelength optoelectronic devices. Furthermore, it exhibits n-type conductivity due to presence of native defects which has restricted its effective utilization in junction devices. Doping ZnO to achieve p-type conductivity has been an area of interest over the last couple of decades. Taking into consideration the limitations imposed by mono-dopant on the p-type behaviour achieved, co-doping has emerged out to be promising technique with the advantage of increasing dopant solubility and reducing ionization energy. In this report we have studied the enhancement in properties of phosphorus doped ZnO thin film with boron as a co-dopant. Doping was done using plasma immersion ion implantation (PIII) technique where phosphorus was implanted for 70 s and subsequently boron for 10s followed by annealing at 800oC for 10 s in oxygen ambient. Low temperature photoluminescence (PL) spectra showed improvement in the acceptor behaviour with donor-acceptor pair (DAP) and free acceptor (FA) peaks observed at around 3.24 and 3.31 eV, respectively for co-doped sample as compared to phosphorus doped sample which did not show these peaks. High resolution x-ray diffraction (HRXRD) showed c-axis (<002<) orientation of the film with increase in peak intensity and angle for the co-doped sample. For co-doped sample, a blue-shift was observed for E_2H peaks in Raman spectra with increase in peak intensities suggesting an improvement in the film crystallinity.

Semiconductor industry thrives on the principle of continuous improvement and it has come a long way relying on that. Zinc oxide (ZnO) semiconductors is one such candidate whose bandgap can be tuned by assimilation of Mg thus making it a promising candidate for various optoelectronic applications. But as in case with ZnO, zinc magnesium oxide (ZnMgO) too has difficulty in achieving p-type conductivity due to native donor defects. For achieving p-type conductivity in such materials, co-doping technique seems to be the most viable solution as it improves the acceptor solubility and also lowers acceptor energy levels. In this report, we have studied the effect of boron doping on the optical and structural properties of phosphorus doped Zn_0.85Mg_0.15O thin film. Plasma immersion ion implantation (PIII) technique was used to dope RF sputtered Zn_0.85Mg_0.15O film with phosphorus for 70 s followed by boron doping for 5 s. The sample was further annealed at 1000oC in oxygen ambience for 10s. Low temperature photoluminescence (PL) spectra exhibited improvement in acceptor type behaviour with free acceptor (FA) peak at around 3.55 eV and near band edge (NBE) emission was further improved with the presence of free exciton (FX) peak at around 3.65 eV. These peaks were absent in phosphorus doped sample. High resolution x-ray diffraction (HRXRD) showed <002< orientation for codoped samples. X-ray photoelectron spectroscopy (XPS) confirmed the presence of boron and increment in phosphorus concentration with co-doping.

Band gap engineering of zinc oxide (ZnO) has gain substantial interest from past few years. Assimilation of Mg in ZnO increases band gap from 3.37 eV to 4.1 eV and it has versatile application in short wavelength UV devices. In this work, we have studied influence of growth ambient on RF sputtered Zn_1−xMg_xO (x = 15%) thin films. Here, we have deposited Zn_1−xMg_xO films at varying Ar/(O₂) gas ratio from 4:1, 3:2, 2:3, 1:4 and 0:5 in growth ambient which yielded sample A, B, C, D and E, respectively. As the O₂ content in chamber decreases, (i.e. sample A), the surface recombination of O₂ atoms in the horizontal direction reduces as compared to that of Zn²⁺ and Mg²⁺. Hence, we observed (100) orientation peak along with (002) in High resolution X-ray diffraction pattern. Reduction in full width half maximum values of the (002) peak were observed from sample A to E which suggests reduction in the Mg concentration. Photoluminescence measurement demonstrated the growth of films in Ar-rich ambient (A) lead to the formation of oxygen vacancies whereas reducing it results in grain size reduction, grain boundary–related defects formation and lower Mg concentration. X-ray photoelectron spectroscopy(XPS) spectra confirmed the reduction in oxygen vacancy peaks with increase in the oxygen concentration in the growth ambient. To summarize, Zn_1−xMg_xO film grown in 3:2 ratios of Ar/O₂ ambient yielded superior quality with minimal defect states using RF sputtering technique.

Since conventional TiO₂ electron transporting layer (ETL) can accelerate the degradation of organic lead halide perovskite solar cells, alternative ETL is required for PSCs with extended lifetime. In this work, SnO₂ nanoparticles, a potential ETL material, were synthesized via facile sol-gel method. Different dopants were introduced in an attempt to enhance conductivity and improve electronic properties of pristine SnO₂. The prepared nanoparticles were identified to be rutile SnO₂ phase with crystalline size less than 4 nm. The presence of the dopant in the doped samples was confirmed by energy dispersive X-ray spectroscopy, and it is also evident from the colour change of the doped samples compared to undoped one. While the dopant incorporation has been successful, nanoparticles exhibited pronounced aggregation in water dispersions, which prevented preparation of sufficiently smooth films for application in planar perovskite solar cells. Further optimization of the nanoparticle surfaces is needed to obtain dispersions suitable for spin-coating uniform thin films of doped SnO₂.

UV photodetector have been successfully implemented in various applications like ozone sensing, communication, astronomy and flame detection etc. Zn(Mg)O is a wide band gap material with high excitonic binding energy at room temperature thus making it a promising material in optoelectronic industries. In the present work, we are achieving increased photocurrent and high responsivity in UV-A regime with post growth UV-ozone treatment from photodetector fabricated using RF sputtered ZnMgO thin film. The thin film was deposited on semi-insulating silicon wafer at 400C for 20 min followed by 70 min UV-Ozone treatment. The final step was to fabricate an interdigitated electrode on the processed samples. More than two times enhancement of photocurrent was observed after UVO treatment. Noteworthy responsivity values of 22 A/W and 67 A/W were measured from as-deposited and UVO annealed photodetector, respectively at 380 nm with an applied bias of -5 V bias. However, the measured detectivity values for as grown and UV-O annealed sample was 1.3 × 10¹³ and 2.7 × 10¹³, respectively. Noise equivalent power in as-deposited sample and UVO treated sample was estimated to be 2.4 × 10^-12 W/√Hz and 1.1 × 10^-12 W/√Hz respectively. Photo detector fabricated with UV-O annealing exhibited good switching behaviors with 37 ms and 30 ms rise and fall time, respectively.

Owing to wide band gap and high exciton energy of ZnMgO can be used in UV based applications like laser diodes, LED and as transparent conducting oxide in solar cells. In present work, we study the effect of UVOzone (UVO) annealing on RF sputtered ZnMgO thin films. Here, we have deposited ZnMgO thin films on Si <100< substrate followed by UVO annealing treatment for 30 min. The as-deposited and UVO treated films were characterized using various optical, structural and elemental characterization techniques and compared with as-deposited ZnMgO thin films. Room Temperature PL results for as-deposited film exhibited dominant defects band emission (DBE) peak intensities in visible region with negligible emission from near band emission (NBE) peak. An increase in NBE emission peak at around ~ 360 nm (3.44eV) with minimal defect states emission was observed for UVO treated sample. NBE to DBE Intensity ratio (I_NBE/I_Defect) of 1.7 was observed from sample B which was almost zero for as-grown film. High resolution X-ray diffraction (HRXRD) results exhibited (002), (220) and (311) crystal orientation peak in as-deposited sample. Post UVO (002) peak shifted to higher angle side. X-ray photoelectron spectroscopy results of Zn-2p and O-1s shows increase in metal-oxide bonds and decrease in oxygen vacancies. Atomic force microscopy results show increase in film roughness from 2.3 to 2.46 nm for as-deposited and UVO treated sample respectively. Authors would like to acknowledge IITBNF for all its facilities at IIT Bombay.

Zinc magnesium oxide (ZnMgO) thin films has emerged as a promising material for optoelectronic applications in last decade. Its high electron mobility makes it a good candidate for thin film transistors applications used in active matrix of LCDs (AMLCDs). ZnMgO thin film have inherent n-type conductivity due to oxygen vacancies, oxygen interstitials and zinc vacancies. For thin film transistors (TFTs), control of doping and defects is very important to maintain carrier concentration and proper threshold voltage of device. Threshold voltage of device is greatly influenced by carrier concentrations and stoichiometry of the film. So defects need to be minimized/controlled for achieving best device performances. These defects can be controlled using reactive oxygen supplied by UV-Ozone treatment by varying the rate and time of oxygen supply. In this study we report effect of UV-Ozone treatment on bottom gate enhancement type ZnMgO TFT using air as source of oxygen for Ozone. ZnMgO thin film was deposited using RF sputter technique using ZnMgO target. Titanium (Ti)/Gold (Au) was deposited to fabricate source and drain contacts. The fabricated TFT was characterized for input characteristics. Post UVO treatment, an increase of two times in drain current is measured along with a decrease in the threshold voltage by 5.8V. The On-OFF (ION/IOFF) ratio for as-deposited film was 7.5 × 103 which increased to 2 × 104 for UVO annealed TFT.

Disordered one-dimensional photonic bandgap (PBG) structures could prove useful in designing broadband reflectors capable of filtering chosen polarizations of incoming light. By capitalizing on the similarities between defects and disorder, it is possible to construct a 1D PBG structure such that the layers are non-uniform but the structure can retain its most novel properties. This is done by allowing the thickness of the layers in the structure to deviate uniformly around an average thickness by a preselected amount of deviation. A mathematical model using the Transfer Matrix Method for simulation has been previously constructed by this group. This model has been verified using FDTD simulation as well. The PBG structure was then fabricated consisting of TiO₂ deposited by electron-beam physical vapor deposition (e-beam PVD) first at normal incidence and then at a 70o oblique angle. This pattern was repeated to create six bilayers of TiO₂ films. This alternating pattern gives rise to the novel structure of a PBG structure by creating a repeating pattern of amorphous and biaxial, columnar, birefringent TiO₂ ,which is analogous to using two different materials. Through testing using a polarizer, analyzer, and HeNe laser with a wavelength of 632.8 nm, it has been found that the sample does in fact match well with the expected theoretical results and acts as a broadband reflector for the TM polarization designed for a 70º incidence angle. The average layer thickness of the fabricated TiO₂ PBG is 22.7 nm.

In this study, nanoparticles of pure zinc oxide (ZnO) and ZnO doped with iron of various doping concentrations (Zn_{1- x}Fe_xO) are analyzed using fluorescence spectroscopy. Excitation and emission spectra using various operating wavelengths were collected. Individual spectra and excitation emission matrix were analyzed. Various peaks including strong ultraviolet (UV) emission peaks and strong blue emission peaks that are corresponding to the near-band-edge emission (NBE) and defect emission (DE) peaks were studied based on the peak intensities, peak wavelengths, and NBE peak to defect peak ratios. The Zn_1-xFe_xO materials were also analyzed using X-ray diffraction and optical absorption spectroscopy. The variation in the band gap energy and in the NBE emission energy with dopant concentration was analyzed. A red-shift was observed with the NBE emission peak. The NBE to DE ratio initially increases from pure ZnO to Zn_0.97Fe_0.03O and then decreases as the dopant concentration increases.

A wrinkle-network structure of Mn doped ZnO is presented in this paper. The undoped and Mn doped ZnO thin film samples have been prepared on ITO coated glass substrates by sol-gel spin coating technique as it is a simple and lowcost method to deposit semiconductor thin films. High resolution X-ray diffraction technique confirms the formation of hexagonal wurtzite structure with diffraction pattern corresponding to ZnO. Mn related phases have not been observed within the detection limit of HR-XRD. The incorporation of Mn dopant in the sample has been confirmed by energy dispersive X-ray spectroscopy (EDS). Both the undoped and Mn doped samples have high optical transmittance in the wavelength range of 300 nm – 800 nm, with a maximum of 88% as recorded by UV-VIS spectroscopy. There is an increase in the bandgap of ZnO thin films by the introduction of Mn dopants which has been calculated by Tauc plot.