This PDF file contains the front matter associated with SPIE Proceedings Volume 10533, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

The unique properties of Ga2O3 and related oxides enable applications as transparent conductors and in power electronics. Ga2O3 has a large band gap (4.8 eV) but can also be highly n-type doped. A thorough understanding of its properties, combined with knowledge of how to control them, is crucial to improving materials quality and enabling further applications. I will show how first-principles modeling, using advanced hybrid functional calculations within density functional theory, can accurately predict band structure [1], properties of point defects [2,3] and impurities [4], and transport [5]. Combining Ga2O3 with In2O3 [6] or Al2O3 allows tuning the atomic and electronic structure. We determine the preferential crystal structures as a function of alloy composition, along with values for band gaps and band alignment. These results provide guidance for incorporating Ga2O3 into devices. Work performed in collaboration with H. Peelaers, J. B. Varley and Y. Kang. [1] H. Peelaers and C.G. Van de Walle, Phys. Status Solidi B 252 (2015), 828. [2] J. B. Varley et al., Appl. Phys. Lett. 97 (2010), 142106. [3] J. B. Varley et al., J. Phys. Condens. Matter 23 (2011), 334212. [4] H. Peelaers and C. G. Van de Walle, Phys. Rev. B 94 (2016), 195203. [5] Y. Kang et al., J. Phys.: Condens. Matter 29 (2017), 234001. [6] H. Peelaers et al., Phys. Rev. B 92 (2015), 85206.

𝛽-Ga2O3 has recently received attention as an ultra-wide-band-gap material, promising as transparent conductor and for high-power device applications. In this talk, we review some recent computational work on the electronic band structure and it defects. The electronic band structure was calculated using the quasiparticle self-consistent GW method including a correction of the screened Coulomb interaction W for electron-hole interaction and lattice-polarization effect. The selection rules of the optical transition help understand the anisotropy of the optical absorption.1 Among the point defects, which have already been discussed in various papers,2 we here focus on the two types of Ga–vacancies, which can occur on both the tetrahedral and octahedral Ga site. Electron paramagnetic resonance (EPR) related to the Ga-vacancy has been reported.3,4 Here we present first-principles calculations of the g-factor and hyperfine structure, which allow us to identify the EPR spectrum observed after high energy particle irradiation, with the tetrahedral Ga vacancy in the 2- charge state. Modification of the EPR spectrum under low temperature photoexcitation are explained by the following model. The EPR spectrum of the tetrahedral Ga vacancy shows superhyperfine interaction with two Ga atoms which are nearest neighbors to the oxygen on which the defect localizes in the q=2- charge state. The g-tensor has its largest deviation from the free-electron value for the crystallographic b-direction in which the defect Ga-O-Ga complex is oriented. According to Ref. 2, the tetrahedral Ga vacancy has its 2-/3- transition level closer to the conduction band minimum than the octahedral one. Upon illumination, this defect may become inactivated while the octahedral one is activated. The latter has a different orientation of its main largest g-tensor component but similar superhyperfine splitting with two Ga atoms. These computational findings explain the corresponding experimental observations. 1 Amol Ratnaparkhe and Walter R. L. Lambrecht, Appl. Phys. Lett.  110, 132103 (2017) 2 Peter Déak, Quoc Duy Ho, Florian Seemann, Bálint Aradi, Michael Lorke, and Thomas Frauenheim, Phys. Rev. B 95, 075208 (2017) and references therein. 3 B. E. Kananen , L. E. Halliburton , K. T. Stevens , G. K. Foundos , and N. C. Giles, Appl. Phys. Lett. 110, 202104 (2017) 4 H.Jürgen von Bardeleben, unpublished

First-principles simulations of excited states, using density-functional and many-body perturbation theory, are now capable of accurately predicting electronic and optical properties of complex oxides, enabling unprecedented understanding and computational materials design. After briefly discussing these techniques and their numerical implementation, I will focus on their application to Ga2O3. This material is interesting for transparent electronics in the semiconductor industry since it conducts electrical current while being transparent at the same time. For Ga2O3 I will provide an overview of recent efforts by several groups to understand the optical absorption in terms of quasiparticle electronic structure and optical transition-matrix elements. These studies provide a clear quantitative picture of the optical anisotropy. I will then show how excitonic effects influence the spectrum close to the absorption onset and at high photon energies. In order to achieve an accurate computational description of excitons, the electron-hole interaction needs to be taken into account. To this end it is mandatory to understand the influence of dielectric screening, which is a long-standing problem in computational materials science. I will explain how the presence of free carriers and of lattice polarization contributes to dielectric screening, impacting the electron-hole interaction, with consequences for optical spectra. Finally, I will also allude to the computational infrastructure needed to compute these highly accurate theoretical spectra for Ga2O3, since, more generally, combining numerical approaches with cutting-edge computation allows to further develop computational materials science and to perform highly accurate theoretical spectroscopy for modern, complex materials that drive societal progress.

The monoclinic beta-Ga2O3 has a wide bandgap of ~4.9 eV and can be grown from melt, making it attractive for electronics applications. In order to use its potential for devices various aspects related to doping have to be understood. This talk will focus first on using first-principle calculations to obtain information on formation energies and electronic structure of group-IV dopants and transition metal compensation doping in beta-Ga2O3. Particular aspects like the contribution of antisites, electron trapping to the activation energies seen in Hall measurements, and magnetism will be discussed. The second part of the talk will present modeling of polarons based on first-principle electron-phonon coupling.

In this talk, we provide an overview overview of our work on beta-Ga2O3 growth, doping, heterostructures and devices. In plasma-assisted MBE growth of beta-Ga2O3 we demonstrate the differences in growth on cleavage (e.g., (100) vs. non-cleavage planes (e.g., (010) or (001)), the propensity for growth rate limitations by suboxide formation. We demonstrate unintentional doping concentration in the mid10E15 cm-3 range, and controllable intentional doping with electron concentrations from 10E16 to >10E20 cm-3. We review the development and characterization of coherent (Al,Ga)2O3/Ga2O3 heterostructures and demonstration of modulation doping and MODFETs.

When investigating Schottky contacts on heteroepitaxial β-Ga₂O₃ thin films, several non-idealities are observed in the current voltage characteristics, which cannot be accounted for with the standard diode current models. In this article, we therefore employed a model for the rigorous calculation of the diode currents in order to understand the origin of this non-idealities. Using the model and a few parameters determined from the measurements, we were able to simulate the characteristics with good agreement to the measured data for temperatures between 30 °C and 150 °C. Fitting of the simulated curves to the measured curves allows a deeper insight into the microscopic origins of said non-idealities.

This paper describes the bulk crystal growth of β-Ga₂O₃ using edge-defined film-fed growth (EFG) process. We first describe the method of the crystal growth and show that large-size crystal with width of up to 6 inch can be grown. Then, we discuss the way to control electrical properties. In the discussion, we give some experimental results of residual impurity measurement, intentional doping using Si and Sn for n-type doping and Fe for insulating doping.

Gallium oxide (Ga₂O₃) has emerged as a new competitor to SiC and GaN in the race toward next-generation power switching and harsh environment electronics by virtue of the excellent material properties and the relative ease of mass wafer production. In this proceedings paper, an overview of our recent development progress of Ga₂O₃ metal-oxide-semiconductor field-effect transistors and Schottky barrier diodes will be reported.

β-Ga₂O₃ is emerging as an interesting wide band gap semiconductor for solar blind photo detectors (SBPD) and high power field effect transistors (FET) because of its outstanding material properties including an extremely wide bandgap (Eg ~4.9eV) and a high breakdown field (8 MV/cm). This review summarizes recent trends and progress in the growth/doping of β-Ga₂O₃ thin films and then offers an overview of the state-of-the-art in SBPD and FET devices. The present challenges for β-Ga₂O₃ devices to penetrate the market in real-world applications are also considered, along with paths for future work.

The emergence of conductive gallium oxide single crystal substrates offers the potential for vertical Schottky detectors operating in the UV-C spectral region. We report here on our recent work in the development of Tin Gallium oxide (TGO) thin film metal-semiconductor-metal (MSM) and Schottky detectors using plasma-assisted molecular beam epitaxy on c plane sapphire and bulk Ga₂O₃ substrates. Tin alloying of gallium oxide thin films was found to systematically reduce the optical band gap of the compound, providing tunability in the UV-C spectral region. Tin concentration in the TGO epilayers was found to be highly dependent on growth conditions, and Ga flux in particular. First attempts to demonstrate vertical Schottky photodetectors using TGO epilayers on bulk n-type Ga₂O₃ substrates were successful. Resultant devices showed strong photoresponse to UV-C light with peak responsivities clearly red shifted in comparison to Ga₂O₃ homoepitaxial Schottky detectors due to TGO alloying.

Oxides provide various fascinating physical properties that could find use in future device applications. However, the physical properties of oxides are often affected by formation of oxygen vacancies during device fabrication processes. In this study, to develop a damage-free patterning process for oxides, we focus on a lift-off process using a sacrificial template layer, by which we can pattern oxide thin films without severe chemical treatment or plasma bombardment. As oxides need high thin-film growth temperature, a sacrificial template needs to be made of thermally stable and easily etchable materials. To meet these requirements, we develop a sacrificial template with a carefully designed bilayer structure. Combining a thermally and chemically stable LaAlO₃ and a water-soluble BaO_x, we fabricated a LaAlO₃/BaO_x sacrificial bilayer. The patterned LaAlO₃/BaO_x sacrificial bilayers were prepared on oxide substrates by room-temperature pulsed laser deposition and standard photolithography process. The structure of the sacrificial bilayer can be maintained even in rather tough conditions needed for oxide thin film growth: several hundred degrees Celsius under high oxygen pressure. Indeed, the LaAlO₃/BaO_x bilayer is easily removable by sonication in water. We applied the lift-off method using the LaAlO₃/BaO_x sacrificial bilayer to a representative oxide conductor SrRuO₃ and fabricated micron-scale Hall-bar devices. The SrRuO₃ channels with the narrowest line width of 5 μm exhibit an almost identical transport property to that of the pristine film, evidencing that the developed process is beneficial for patterning oxides. We show that the LaAlO₃/BaO_x lift-off process is applicable to various oxide substrates: SrTiO₃, MgO, and Al₂O₃. The new versatile patterning process will expand the range of application of oxide thin films in electronic and photonic devices.

A remarkable new electronic ground-state of a high-temperature superconductor oxide (YBa₂Cu₃O_7−δ) is found when it is grown in-between layers of a specific manganite (Pr_0.5La_0.2Ca_0.3MnO₃). The superconductor in these ‘superconductor sandwiches’ apparently adopts an exotic granular-state due to an interaction with the manganite. Uniquely, a strong magnetic field recovers a more ‘customary’ superconducting state. Here we show how Raman spectroscopy, state-of-the-art THz ellipsometry, and transport measurements are being used to reveal the nature of this new ground-state. These measurements are shedding light on how the manganite and superconductor layers interact to cause such novel behaviour, however the exact mechanism remains unknown.

An oscillating electron model for iron of selenide FeSe, niobium of germanate Nb3Ge, magnesium diboride MgB2, cuprate YBa2Cu3O6.5+χ, mercury cuprate HgBa2Ca2Cu3O8+ χ superconductors was considered. The superconducting phase transition is accompanied by the destruction of valence bonds, phase separation and the formation of molecules takes place. For the case of a binary crystal, this process can be represented in the form 2AB = A2 + B2. The square plasma energies ħω_pv ² and the interaction parameters q = 2ω² [ab] / (ω² [a2] + ω² [b2]) were calculated. The dependence of the superconducting phase transition temperature Tc on the interaction parameter q was obtained. The equation has the form Tc = ─ 188.66537q + 181.40836. Equations for the isotope effect of iron selenide FeSe, magnesium diboride MgB2, cuprate YBa2Cu3O6.5 + χ have been obtained.

A thorough investigation of copper oxide, specifically cupric oxide (CuO), is performed in the following work with a focus on CuO’s ultrafast free-carrier dynamics and bandstructure. An above-bandgap control beam and below-bandgap signal beam are utilized in transient absorption spectroscopy to gain insight on CuO nanocrystals’ recombination and relaxation dynamics at varying control beam fluences. The authors witnessed three distinct time constants, the first of which changed with control beam fluence between 330 and 630 fs, while the second and third remained constant at 2 ps and 50 ps, respectively. The first time constant is attributed to momentum relaxation from valence band carrier-carrier scattering and exciton-exciton annihilation. The second time constant is attributed to energy relaxation from valence band carrier-phonon scattering. The third time constant is attributed to trapping and recombination as a result of the CuO nanocrystals’ increased trap state density. The findings of this work provide a basis for future research on this emerging CuO nanocrystal system.

InGaZnO (IGZO) is a promising semiconductor material for thin-film transistors (TFTs) used in DC and RF switching applications, especially since it can be grown at low temperatures on a wide variety of substrates. Enhancement-mode TFTs based on IGZO thin films grown by pulsed laser deposition (PLD) have been recently fabricated and these transistors show excellent performance; however, compositional variations and defects can adversely affect film quality, especially in regard to electrical properties. In this study, we use thermally stimulated current (TSC) spectroscopy to characterize the electrical properties and the deep traps in PLD-grown IGZO thin films. It was found that the as-grown sample has a DC activation energy of 0.62 eV, and two major traps with activation energies at ~ 0.16-0.26 eV and at ~ 0.90 eV. However, a strong persistent photocurrent (PPC) sometimes exists in the as-grown sample, so we carry out post-growth annealing in an attempt to mitigate the effect. It was found that annealing in argon increases the conduction, produces more PPC and also makes more traps observable. Annealing in air makes the film more resistive, and removes PPC and all traps but one. This work demonstrates that current-based trap emission, such as that associated with the TSC, can effectively reveal electronic defects in highlyresistive semiconductor materials, especially those are not amenable to capacitance-based techniques, such as deeplevel transient spectroscopy (DLTS).

The goal is to gain additional insight into physical mechanisms and the role of microstructure on the formation of ohmic contacts and the reduction of contact resistance. We have measured a decreasing film resistivity in the vertical direction with increasing thickness of pulsed-laser deposited ZnO and IGZO. As the ZnO thickness increases from 122 nm to 441 nm, a reduction in resistivity from 3.29 Ω-cm to 0.364 Ω-cm occurred. The IGZO resistivity changes from 72.4 Ω-cm to 0.642 Ω-cm as the film is increased from 108nm to 219 nm. In the ZnO, the size of nanocolumnar grains increase with thickness resulting in fewer grain boundaries, and in the amorphous IGZO, the thicker region exhibits tunnel-like artifacts which may contribute to the reduced resistivity.

Hybrid perovskite solar cells with different crystalline grain size have been characterized by conventional techniques and additionally by low-frequency electrical noise spectroscopy. A clear correlation between the morphological structure of the perovskite grains, the energy disorder of the defect states, and the device performance has been demonstrated. In addition, the analysis of the temperature dependence of the noise amplitude has been also used to clearly identify the low-temperature transition between the orthorhombic and the tetragonal phases in perovskite solar cells and to extract important electronic parameters for both crystalline configurations.

We demonstrate that the low temperature synthesis chemical route can be utilized to control the functionality of zinc oxide (ZnO) nanoparticles (NPs) and nanorods (NRs) for optical and magneto-optical performance. Different structural, optical, electro- and magneto-optical results will be displayed and analyzed. In the first part, we show how high quality ZnO NPs can be efficient for photodegradation using ultra-violet radiation. In the second part we will present our recent results on the control of the core defects in cobalt doped ZnO NR. Here and by using electron paramagnetic resonance (EPR) measurements, the substitution of Co²⁺ ions in the ZnO NRs crystal is shown. The relation between the incorporation and core defects concentration will be discussed. The findings give access to the magnetic anisotropy of ZnO NRs grown by the low temperature chemical route and can lead to demonstrate room temperature ferromagnetism in nanostructures with potential for different device applications.

Surface-Enhanced Raman spectroscopy (SERS) is a widely used technique adopted in both academia and industry for the detection of trace quantities of Raman active molecules. This is usually accomplished by functionalizing distributions of plasmonic metal nanoparticles with the analyte molecules. Recently metal-coated nanostructures have been investigated as alternatives to dispersions of metal nanoparticles in order to avoid clustering and homogeneity/reproducibility issues. In this paper, several samples of Au-coated ZnO nanoarrays are adopted as SERS substrates in order to investigate the molecular sensing capacity for methylene blue (MB) molecules. Self-forming ZnO nanoarrays were grown on both c-sapphire and silicon substrates by pulsed laser deposition. The nanoarrays were then coated with 30 nm of gold using thermal evaporation and the SERS signals of MB functionalized samples were obtained with a Raman microspectrometer. The ratio of SERS intensity to that of an MB functionalized glass substrate (_ISERS/I_Raman) was calculated based on the averaged SERS signals. A relatively good within-wafer homogeneity of the enhancement effect was found with I_SERS/I_Raman values as high as 64.2 for Au-coated nano ZnO grown on silicon substrates. The experimental results show that the Au-coated ZnO nanoarrays can be excellent SERS substrates for molecular/chemical analyte sensing.

Indium-doped ZnO bulk crystals grown by the hydrothermal method are highly-conductive, with resistivity at 0.01 Ωcm at room temperature as revealed by Hall-effect measurement. In this paper we report on structural and optical properties of these crystals. The grown In:ZnO crystals have been studied by high resolution X-ray diffraction, micro-Raman scattering and low-temperature photoluminescence and cathodoluminescence. It was found that the c lattice parameter of the grown In:ZnO crystal expanded 0.06% with respect to the lithium-doped ZnO crystal seed, and the In-doped ZnO overgrew the seed crystal pseudomorphically but with high quality crystallinity; the X-ray rocking curves show the FWHM of the Zn face and O faces are only 0.05° and 0.1° ; and the indium concentration in the crystal reaches the solubility limit. Raman spectra show strain relaxation gradually from the regrowth interface as well as a weak spectral feature at 723 cm^-1. The peak at 312 cm^-1 noticed in hydrothermally grown In:ZnO nanostructures does not appear in our In-doped crystals, indicating that this peak may be associated with specific defects (e.g. surface related) of the nanostructures. Photoluminescence measurements show that an indium donor bound exciton peak I₉ (In⁰X) is the dominant peak in the PL spectrum, located at 3.3586 eV on the zinc face and 3.3577 eV on the oxygen face. Both of them deviated from the consensus literature value of 3.3567 eV, probably due to strain in the crystal induced by impurities.

The advantages of additive manufacturing for electronic devices have led to the demand of printing functional material in search of a replacement for the conventional subtractive fabrication process. Zinc oxide (ZnO), thanks to its interesting properties for the electronic and photonic applications, has gathered many attentions in the effort to fabricate functional devices additively. Although many potential methods have been proposed, most of them focus on the lowtemperature processing of the printed material to be compatible with the polymer substrate. These low-temperature fabrication processes could establish a high concentration of defects in printed ZnO which significantly affect the performance of the device. In this study, ZnO thin film for UV photodetector application was prepared by inkjet printing of zinc acetate dihydrate solution following by different heat treatment schemes. The effects of annealing to the intrinsic defect of printed ZnO and photoresponse characteristics under UV illumination were investigated. A longer response/decay time and higher photocurrent were observed after the annealing at 350°C for 30 minutes. X-ray photoelectron spectroscopy (XPS) analysis suggests that the reducing of defect concentration, such as oxygen vacancy, and excess oxygen species in printed ZnO is the main mechanism for the variation in photoresponse. The result provides a better understanding on the defect of inkjet-printed ZnO and could be applied in engineering the properties of the printed oxide-based semiconductor.

In this work we show the potential of the ZnO/ZnMgO material system for intersubband (ISB)-based devices. This family of alloys presents a unique set of properties that makes it highly attractive for THz emission as well as strong coupling regimes: it has a very large longitudinal optical phonon energy of 72 meV, it can be doped up to ~10²¹ cm^-3, it is very ionic with a large difference between the static and high frequency dielectric constants, and it can be grown homoepitaxially on native substrates with low defect densities. The films analyzed here are grown by molecular beam epitaxy (MBE) on a non-polar orientation, the m-plane, with varying QW thicknesses and 30% Mg concentrations in the barrier, and are examined with polarization-dependent IR absorption spectroscopy. The QW band structure and the intersubband transitions energies are modeled considering many body effects, which are key to predict correctly the measured values.

Optical spectroscopy is a powerful approach for detecting defects and impurities in ZnO, an important electronic material. However, knowledge of how common optical signals are linked with defects and impurities is still limited. The Cu-related green luminescence is among the best understood luminescence signals, but theoretical descriptions of Cu-related optical processes have not agreed with experiment. Regarding native defects, assigning observed lines to specific defects has proven very difficult. Using first-principles calculations, we calculate the properties of native defects and impurities in ZnO and their associated optical signals. Oxygen vacancies are predicted to give luminescence peaks lower than 1 eV; while related zinc dangling bonds can lead to luminescence near 2.4 eV. Zinc vacancies lead to luminescence peaks below 2 eV, as do the related oxygen dangling bonds. However, when complexed with hydrogen impurities, zinc vacancies can cause higher-energy transitions, up to 2.3 eV. We also find that the Cu-related green luminescence is related to a (+/0) deep donor transition level.

The bandgap of wurzite ZnO layers grown on 2 inch diameter c-Al₂O₃ substrates by pulsed laser deposition was engineered from 3.7 to 4.8 eV by alloying with Mg. Above this Mg content the layers transformed from single phase hcp to mixed hcp/fcc phase before becoming single phase fcc above a bandgap of about 5.5 eV. Metal-Semiconductor-Metal (MSM) photodetectors based on gold Inter-Digitated-Transducer structures were fabricated from the single phase hcp layers by single step negative photolithography and then packaged in TO5 cans. The devices gave over 6 orders of magnitude of separation between dark and light signal with solar rejection ratios (I₂₇₀ : I₃₅₀) of over 3 × 10⁵ and dark signals of 300 pA (at a bias of -5V). Spectral responsivities were engineered to fit the “Deutscher Verein des Gas- und Wasserfaches” industry standard form and gave over two decade higher responsivities (14 A/W, peaked at 270 nm) than commercial SiC based devices. Homogeneous Ga₂O₃ layers were also grown on 2 inch diameter c-Al₂O₃ substrates by PLD. Optical transmission spectra were coherent with a bandgap that increased from 4.9 to 5.4 eV when film thickness was decreased from 825 to 145 nm. X-ray diffraction revealed that the films were of the β-Ga₂O₃ (monoclinic) polytype with strong (-201) orientation. β-Ga₂O₃ MSM photodetectors gave over 4 orders of magnitude of separation between dark and light signal (at -5V bias) with dark currents of 250 pA and spectral responsivities of up to 40 A/W (at -0.75V bias). It was found that the spectral responsivity peak position could be decreased from 250 to 230 nm by reducing film thickness from 825 to 145 nm. This shift in peak responsivity wavelength with film thickness (a) was coherent with the apparent bandgap shift that was observed in transmission spectroscopy for the same layers and (b) conveniently provides a coverage of the spectral region in which MgZnO layers show fcc/hcp phase mixing.

Oxides represent the largest family of wide bandgap (WBG) semiconductors and also offer a huge potential range of complementary magnetic and electronic properties, such as ferromagnetism, ferroelectricity, antiferroelectricity and high-temperature superconductivity. Here, we review our integration of WBG and ultra WBG semiconductor oxides into different solar cells architectures where they have the role of transparent conductive electrodes and/or barriers bringing unique functionalities into the structure such above bandgap voltages or switchable interfaces. We also give an overview of the state-of-the-art and perspectives for the emerging semiconductor β- Ga₂O₃, which is widely forecast to herald the next generation of power electronic converters because of the combination of an UWBG with the capacity to conduct electricity. This opens unprecedented possibilities for the monolithic integration in solar cells of both self-powered logic and power electronics functionalities. Therefore, WBG and UWBG oxides have enormous promise to become key enabling technologies for the zero emissions smart integration of the internet of things.

Zinc oxide (ZnO) is an earth abundant wide bandgap semiconductor of great interest in the recent years. ZnO has many unique properties, such as non-toxic, large direct bandgap, high exciton binding energy, high transparency in visible and infrared spectrum, large Seebeck coefficient, high thermal stability, high electron diffusivity, high electron mobility, and availability of various nanostructures, making it a promising material for many applications. The growth techniques of ZnO is reviewed in this work, including sputtering, PLD, MOCVD and MBE techniques, focusing on the crystalline quality, electrical and optical properties. The problem with p-type doping ZnO is also discussed, and the method to improve p-type doping efficiency is reviewed. This paper also summarizes the current state of art of ZnO in thermoelectric and photovoltaic applications, including the key parameters, different device structures, and future development.

We present investigations on the growth of high quality CH₃NH₃PbI₃ (MAPI) thin films using both vapor and solution techniques. Recent work on perovskite film growth indicates critical dependencies of the film quality on the nucleation and crystallization steps requiring: i.) uniform distribution of nucleation sites; and ii.) optimal crystallization rate that facilitates the growth of a compact, continuous film with low density of pinholes. Our work shows that the hybrid chemical vapor deposition technique (HCVD) technique is well suited for the deposition of evenly distributed nucleation sites and the optimization of the crystallization rate of the film through detailed monitoring of the thermal profile of the growth process. Signficant reduction in the defect states is recorded by annealing the perovskite films in O₂. The results are consistent with theoretical studies by Yin et al. 1 on O and Cl passivation of the shallow states at the grain boundary of MAPI. Their work provides the theoretical basis for our experimental observations on the passivation of shallow states by oxygen annealing. High quality films were achieved through detailed management of the carrier gas composition and the thermal profile of the nucleation and crystallization steps.

The hexagonal apatite single crystals have been investigated for their applications as laser host materials. Czochralksi and flux growth methods have been utilized to obtain single crystals. For low temperature processing (<100 ⁰C), several techniques for crystal growth have been developed. The hexagonal apatite structure (space group P⁶3/m) is characteristic of several compounds, some of which have extremely interesting and useful properties as laser hosts and bone materials. Calcium lanthanum silicate (Nd-doped) and lanthanum aluminate material systems were studied in detail. Nanoengineered calcium and lanthanum based silicates were synthesized by a solution method and their optical and morphological characteristics were compared with Czochralski grown bulk hydroxyapatite single crystals. Materials were evaluated by absorbance, fluorescence and Raman characteristics. Neodymium, iron and chromium doped crystals grown by a solution method showed weak but similar optical properties to that of Czochralski grown single crystals.

Persistent luminescence materials present many applications including security lighting and bio-imaging. Many progresses have been made in the elaboration of persistent luminescent nanoparticles suitable for the first NIR partial transparency window (650 - 950 nm). Moving to the second and third near-infrared partial transparency windows (1000 nm - 1800 nm) allows further reducing of scattering, absorption and tissue autofluorescence effects. In this work, we present the synthesis of Co²⁺ and Ni²⁺ doped zinc-gallate nanoparticles with broad emission covering the NIR-II range. Site occupancy, energy levels, optical features and persistent phenomena are presented.

The synthesis of garnet-type Gd₃Sc₂A_l3O₁₂ (GSAG) nanoparticles doped with Ce³⁺ ions under solvothermal conditions in 1,4-butanediol is reported in this work. X-ray diffraction with LeBail fitting, Electron microscopy images and photoluminescence spectra were used to characterize the samples. A detailed Transmission Electron Microscopy study shows strong modification of particle morphology upon reaction temperature. Performing the reaction at high temperature and high pressure leads to a clear increase of the particle crystallinity and allows the formation of 100-nm sized nanocrystals with a small size dispersion (± 20 nm). The formation mechanism of these particles is through the self-orientation of primary crystalline grains. Yellow-orange luminescence of Ce³⁺-doped GSAG nanoparticles is observed upon 457-nm excitation. Photoluminescence intensity drastically increases when the particles are synthesized at high temperature, which is directly correlated to their crystal quality.

Yttria-stabilized zirconia (YSZ: ZrO₂ + Y₂O₃) and alumina (Al₂O₃) are widely used in high-temperature applications due to their high-temperature stability, low thermal conductivity, and chemical inertness. Alumina is used extensively in engineered ceramic applications such as furnace tubes and thermocouple protection tubes, while YSZ is commonly used in thermal barrier coatings on turbine blades. Because they are already often found in high temperature and combustion applications, these two substances have been compared as candidates for Raman thermometry in high-temperature energy-related applications. Both ceramics were used with as-received rough surfaces, i.e., without polishing or modification. This closely approximates surface conditions in practical high-temperature situations. A single-line argon ion laser at 488nm was used to excite the materials inside a cylindrical furnace while measuring Raman spectra with a fixed-grating spectrometer. The shift in the peak positions of the most intense A_1g peak at 418cm^-1 (room temperature position) of alumina ceramic and relatively more symmetric E_g peak at 470cm^-1 (room temperature position) of YSZ were measured and reported along with a thermocouple-derived reference temperature up to about 1000°C. This study showed that alumina and YSZ ceramics can be used in high-temperature Raman thermometry with an accuracy of 4.54°C and 10.5°C average standard deviations respectively over the range of about 1000°C. We hope that this result will guide future researchers in selecting materials and utilizing Raman non-contact temperature measurements in harsh environments.

Using hybrid density functional theory, we investigate the influence on electronic structure of common defects and impurities in tungsten oxide (WO₃). As an easily reducible perovskite with the A-site atom missing, high concentrations of foreign dopants and oxygen deficiencies are possible. Our calculations show that both oxygen vacancies and alkali dopants are shallow donors, and we explore the physical origins for this behavior. In particular, we examine whether oxygen vacancies can give rise to localized states or small polarons. Our results show that in crystalline material no such charge localization occurs. We discuss how these results impact electrical conductivity and optical properties.

Thermal conductivity of undoped and Sn-doped β-Ga₂O₃ bulk and single-crystalline thin films have been measured by the 3ω technique. The bulk samples were grown by edge-defined film-field growth (EFG) method, while the thin films were grown on c-plane sapphire by pulsed-laser deposition (PLD). All samples were with (-201) surface orientation. Thermal conductivity of bulk samples was calculated along the in-plane and cross-plane crystallographic directions, yielding a maximum value of ~ 29 W/m-K in the [010] direction at room temperature. A slight thermal conductivity decrease was observed in the Sn-doped bulk samples, which was attributed to enhanced phonon-impurity scattering. The differential 3ω method was used for β-Ga₂O₃ thin film samples due to the small film thickness. Results show that both undoped and Sndoped films have a much lower thermal conductivity than that of the bulk samples, which is consistent with previous reports in the literature showing a linear relationship between thermal conductivity and film thickness. Similarly to bulk samples, Sn-doped thin films have exhibited a thermal conductivity decrease. However, this decrease was found to be much greater in thin film samples, and increased with Sn doping concentration. A correlation between thermal conductivity and defect/dislocation density was made for the undoped thin films.

The influence of a weak magnetic field (H ~ 1 T) on the exciton radiation of the ZnO films covered with Ag island film was investigated at room temperature. It was found that there are some areas on these films where the several-percent enhancement of the exciton radiation intensity can be observed. The value of the enhancement slightly depends on the excitation level. At the same time there are other areas on the same films where the effect is negligible or absent. It is supposed that in the first case, magnetoexcitons are possibly formed due to the increase of the exciton momentum nearby an Ag nanoparticle. The probability of magnetoexciton recombination significantly increases even in a weak magnetic field due to the strong stretch of its wave function.

High band gap (3.34 eV) and large exciton binding energy (60 meV) at room temperature facilitates ZnO as a useful candidate for optoelectronics devices. Presence of zinc interstitial and oxygen vacancies results in n-type ZnO film. Phosphorus implantation was carried out using plasma immersion ion implantation technique (2kV, 900W) for constant duration (50 s) on RF sputtered ZnO thin films (Sample A). For dopant activation, sample A was subjected to Rapid Thermal Annealing (RTA) at 700, 800, 900 and 1000°C for 10 s in Oxygen ambient (Sample B, C, D, E). Low temperature (18 K) photoluminescence measurement demonstrated strong donor bound exciton peak for sample A. Dominant donor to acceptor pair peak (DAP) was observed for sample D at around 3.22 eV with linewidth of 131.3 meV. High resolution x-ray diffraction measurement demonstrated (001) and (002) peaks for sample A. (002) peak with high intensity was observed from all annealed samples. Incorporation of phosphorus in ZnO films leads to peak shift towards higher 2θ angle indicate tensile strain in implanted samples. Scanning electron microscopy images reveals improvement in grain size distribution along with reduction of implantation related defects. Raman spectra measured A1(LO) peak at around 576 cm^-1 for sample A. Low intensity E2 (high) peak was observed for sample D indicating formation of (P_Zn+2V_Zn) complexes. From room temperature Hall measurement, sample D measured 1.17 x 10¹⁸ cm ^-3 carrier concentration with low resistivity of 0.464 Ω.

ZnO has potential application in the field of short wavelength devices like LED’s, laser diodes, UV detectors etc, because of its wide band gap (3.34 eV) and high exciton binding energy (60 meV). ZnO possess N-type conductivity due to presence of defects arising from oxygen and zinc interstitial vacancies. In order to achieve P-type or intrinsic carrier concentration an implantation study is preferred. In this report, we have varied phosphorous implantation time and studied its effect on optical as well structural properties of RF sputtered ZnO thin-films. Implantation was carried out using Plasma Immersion ion implantation technique for 10 and 20 s. These films were further annealed at 900°C for 10 s in oxygen ambient to activate phosphorous dopants. Low temperature photoluminescence (PL) spectra measured two distinct peaks at 3.32 and 3.199 eV for 20 s implanted sample annealed at 900°C. Temperature dependent PL measurement shows slightly blue shift in peak position from 18 K to 300 K. 3.199 eV peak can be attributed to donoracceptor pair (DAP) emission and 3.32 eV peak corresponds to conduction-band-to-acceptor (eA⁰) transition. High resolution x-ray diffraction revels dominant (002) peak from all samples. Increasing implantation time resulted in low peak intensity suggesting a formation of implantation related defects. Compression in C-axis with implantation time indicates incorporation of phosphorus in the formed film. Improvement in surface quality was observed from 20 s implanted sample which annealed at 900°C.

Fabricated omnidirectional anti-reflection nanostructure films as a one of the promising alternative solar cell applications have attracted enormous scientific and industrial research benefits to their broadband, effective over a wide range of incident angles, lithography-free and high-throughput process. Recently, the nanostructure SiO₂ film was the most inclusive study on anti-reflection with omnidirectional and broadband characteristics. In this work, the three-dimensional silicon dioxide (SiO₂) nanostructured thin film with different morphologies including vertical align, slant, spiral and thin films were fabricated by electron beam evaporation with glancing angle deposition (GLAD) on the glass slide and silicon wafer substrate. The morphological of the prepared samples were characterized by field-emission scanning electron microscope (FE-SEM) and high-resolution transmission electron microscope (HRTEM). The transmission, omnidirectional and birefringence property of the nanostructure SiO₂ films were investigated by UV-Vis-NIR spectrophotometer and variable angle spectroscopic ellipsometer (VASE). The spectrophotometer measurement was performed at normal incident angle and a full spectral range of 200 – 2000 nm. The angle dependent transmission measurements were investigated by rotating the specimen, with incidence angle defined relative to the surface normal of the prepared samples. This study demonstrates that the obtained SiO2 nanostructure film coated on glass slide substrate exhibits a higher transmission was 93% at normal incident angle. In addition, transmission measurement in visible wavelength and wide incident angles -80 to 80 were increased in comparison with the SiO2 thin film and glass slide substrate due to the transition in the refractive index profile from air to the nanostructure layer that improve the antireflection characteristics. The results clearly showed the enhanced omnidirectional and broadband characteristic of the three dimensional SiO2 nanostructure film coating.

We examine the effects on the spatial and angular Goos-Hanchen (GH) beam shifts of spherical and cylindrical pores in a thin film. In our calculations, a p-polarized light is incident on a 1-μm thick porous silicon (Si) thin film on a Si substrate. The beam shifts are within the measurement range of usual optical detectors. Our results show that a technique based on GH shift can be used to determine the porosity and pore structure of thin films at a given thickness.

Perovskite solar cells are emerging photovoltaic technology with potential for low cost, high efficiency devices. Currently, flexible devices efficiencies over 15% have been achieved. Flexible devices are of significant interest for achieving very low production cost via roll-to-roll processing. However, the stability of perovskite devices remains a significant challenge. Unlike glass substrate which has negligible water vapor transmission rate (WVTR), polymeric flexible film substrates suffer from high moisture permeability. As PET and PEN flexible substrates exhibit higher water permeability then glass, transparent flexible backside encapsulation should be used to maximize light harvesting in perovskite layer while WVTR should be low enough. Wide band gap materials are transparent in the visible spectral range low temperature processable and can be a moisture barrier. For flexible substrates, approaches like atomic layer deposition (ALD) and low temperature solution processing could be used for metal oxide deposition. In this work, ALD SnO₂, TiO₂, Al₂O₃ and solution processed spin-on-glass was used as the barrier layer on the polymeric side of indium tin oxide (ITO) coated PEN substrates. The UV-Vis transmission spectra of the prepared substrates were investigated. Perovskite solar cells will be fabricated and stability of the devices were encapsulated with copolymer films on the top side and tested under standard ISOS-L-1 protocol and then compared to the commercial unmodified ITO/PET or ITO/PEN substrates. In addition, devices with copolymer films laminated on both sides successfully surviving more than 300 hours upon continuous AM1.5G illumination were demonstrated.

ZnO is considered a potential alternative of TiO₂ for electron transport materials of perovskite solar cells due to its relatively high electron mobility and the ease of low temperature solution-processing. Nevertheless, ZnO-based perovskite devices usually exhibit inferior device performance and stability compared to TiO₂ based devices due to the defect states at ZnO/perovskite interface. In this study, an ultrathin TiO₂ layer by ALD is applied to ZnO nanostructures, and its effect on device performance is investigated. The results indicate that TiO₂ ultrathin layer can effectively passivate the surface of ZnO nanostructures, resulting in enhanced device performance.

N-type wide bandgap oxide semiconductors are important components of perovskite solar cells (PSCs)). The present paper illustrates the various important key roles of oxides in PSCs. Since the perovskite layer is prepared on the oxide(s) sublayer(s) it has a great influence the absorber properties. For high efficiency, the oxide hole blocking layer must be well-crystallized, thin and well-covering. We show that the best technique to achieve such characteristics and get high efficiency PSSC is the spray pyrolysis. Moreover, the use of a mesoporous oxide layer improves the cell efficiency but using too thick mesoporous layers is detrimental for the cell performances. TiO₂ is the most popular oxide used for selective contact but SnO2 is also an alternative candidate providing good efficiencies. The oxide used must be adjusted to the absorber material properties. This is shown in the case of a silver iodobismuthate absorber (Ag₂Bi₃I₁₁) which works much better when combined with a SnO₂ selective contact compared to a TiO₂ selective contact.

The electrodeposition of silicon at room temperature in 1-Butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide and N-Propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquids containing SiCl₄ salt is shown. The electrodeposition window has been determined by cyclic voltammetry. Layers have been deposited in a three electrode cell placed in an inert atmosphere and at constant applied potential. The characterizations by x-ray diffraction and Raman spectroscopy showed the formation of a layer made of amorphous silicon. The scanning electron microscopy examination revealed that the layers were featureless and well-covering.

In this work, zinc oxide (ZnO) nanostructured films were grown using a simple synthesis from chemical solutions (SCS) approach from aqueous baths at relatively low temperatures (< 95 °C). The samples were doped with Pd (0.17 at% Pd) and functionalized with PdO nanoparticles (NPs) using the PdCl₂ aqueous solution and subsequent thermal annealing at 650 °C for 30 min. The morphological, micro-Raman and optical properties of Pd modified samples were investigated in detail and were demonstrated to have high crystallinity. Gas sensing studies unveiled that compared to pure ZnO films, the Pd-doped ZnO (ZnO:Pd) nanostructured films showed a decrease in ethanol vapor response and slight increase in H₂ response with low selectivity. However, the PdO-functionalized samples showed excellent H₂ gas sensing properties with possibility to detect H₂ gas even at room temperature (gas response of ~ 2). Up to 200 °C operating temperature the samples are highly selective to H₂ gas, with highest response of ~ 12 at 150 °C. This study demonstrates that surface functionalization of n-ZnO nanostructured films with p-type oxides is very important for improvement of gas sensing properties.