This PDF file contains the front matter associated with SPIE Proceedings Volume 7216, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

Epitaxial growth technology for GaN devices on large-diameter Si substrate has been studied in this paper, which is essential for device cost reduction. In an effort to improve the buffer breakdown voltage for increasing the breakdown voltage of the device, carbon concentration in the GaN layer was controlled to find that the carbon concentration significantly contributed to buffer breakdown voltage improvements. Device performance was evaluated for the devices with an AlGaN/GaN HFET structure on Si substrate, and it was shown that the performance was equivalent to that of the device on sapphire substrate. A large-area device having this structure was fabricated in order to confirm its potential as a power device, and a current capacity of 120 A or more and a breakdown voltage of 1.8 kV were achieved. On the other hand, with respect to the problematical issue of current collapse in GaN HFETs, the HFET structure on Si substrate has resulted in a significant improvement compared with the structure on sapphire substrate, thus realizing a highperformance device that does not show a salient current collapse up to 1 kV.

We have quantitatively modeled the conduction current and charge storage of an HFET in terms its physical dimensions and material properties. For DC or small-signal RF operation, no adjustable parameters are necessary to predict the terminal characteristics of the device. Linear performance measures such as small-signal gain and input admittance can be predicted directly from the geometric structure and material properties assumed for the device design. We have validated our model at low-frequency against experimental I-V measurements and against two-dimensional device simulations. We discuss our recent extension of our model to include a larger class of electron velocity-field curves. We also discuss the recent reformulation of our model to facilitate its implementation in commercial large-signal high-frequency circuit simulators. Large signal RF operation is more complex. First, the highest CW microwave power is fundamentally bounded by a brief, reversible channel breakdown in each RF cycle. Second, the highest experimental measurements of efficiency, power, or linearity always require harmonic load pull and possibly also harmonic source pull. Presently, our model accounts for these facts with an adjustable breakdown voltage and with adjustable load impedances and source impedances for the fundamental frequency and its harmonics. This has allowed us to validate our model for large signal RF conditions by simultaneously fitting experimental measurements of output power, gain, and power added efficiency of real devices. We show that the resulting model can be used to compare alternative device designs in terms of their large signal performance, such as their output power at 1dB gain compression or their third order intercept points. In addition, the model provides insight into new device physics features enabled by the unprecedented current and voltage levels of AlGaN/GaN HFETs, including non-ohmic resistance in the source access regions and partial depletion of the 2DEG in the drain access region.

To improve the device performance of GaN-based Metal-Insulator-Semiconductor (MIS) heterostructure field-effect transistors (HFETs), the layer design of MIS AlGaN/GaN HFETs has been investigated by considering the insulator/AlGaN structures as the total barrier layers of MIS HFETs. Since deposition of insulators on AlGaN has proven to change electrical properties of the heterostructures as the increase in AlGaN thickness does, considering the insulator/AlGaN structures as the total barrier layers is effective and indispensable in designing the MIS AlGaN/GaN HFETs. With an increased insulator thickness and decreased AlGaN thickness, where the gate capacitance is kept constant, a high transconductance was successfully obtained that was equivalent to that of HFET, in addition to the reduced gate leakage current in MIS HFETs. Moreover, Al₂O₃-based MIS HFETs were revealed to exhibit superior DC characteristics over conventional Si₃N₄ MIS HFETs, which can be ascribed to larger bandgap energy of Al₂O₃ than Si₃N₄. Channel doping design has also been proposed to incorporate into MIS HFETs to modulate the threshold voltage aiming at the same application of depression mode as conventional HFETs. In a MIS HFET with channel doping design, a significantly reduced gate leakage current was obtained with a transconductance and threshold voltage that are kept close to typical values of HFETs, indicating that the device has a potential to replace conventional HFETs. Thus, design of insulator/AlGaN structures and incorporation of channel doping design are the key, with which MIS HFETs exhibiting a low gate leakage current and high transconductance with a desired threshold voltage can be realized.

We review our recent advances of GaN-based high voltage power transistors. These are promising owing to low on-state resistance and high breakdown voltage taking advantages of superior material properties. However, there still remain a couple of technical issues to be solved for the GaN devices to replace the existing Si-based power devices. The most critical issue is to achieve normally-off operation which is strongly desired for the safety operation, however, it has been very difficult because of the built-in polarization electric field. Our new device called GIT (Gate Injection Transistor) utilizing conductivity modulation successfully achieves the normally-off operation keeping low on-state resistance. The fabricated GIT on a Si substrate exhibits threshold voltage of +1.0V. The obtained on-state resistance and off-state breakdown voltage were 2.6mΩ•cm2 and 800V, respectively. Remaining technical issue is to further increase the breakdown voltage. So far, the reported highest off-state breakdown voltage of AlGaN/GaN HFETs has been 1900V. Overcoming these issues by a novel device structure, we have demonstrated the world highest breakdown voltages of 10400V using thick poly-crystalline AlN as a passivation film and Via-holes through sapphire which enable very efficient layout of the lateral HFET array avoiding any undesired breakdown of passivation films. Since conventional wet or dry etching cannot be used for chemically stable sapphire, high power pulsed laser is used to form the via-holes. The presented GaN power devices demonstrate that GaN is advantageous for high voltage power switching applications replacing currently used Si-based power MOSFETs and IGBTs.

Non-equilibrium (hot) occupancy of longitudinal-optical (LO) phonon modes is considered for InAlN/AlN/GaN and related structures with two-dimensional electron gas (2DEG) channels located in GaN and subjected to high electric fields. The channel operation is found to depend critically on the LO-mode occupancy controlled by hot-phonon lifetime. The lifetime dependences on the occupancy and the 2DEG density are presented and discussed in terms of plasmon-assisted decay of hot-phonons. The optimal 2DEG density for the fastest removal of the LO-mode heat is estimated.

We have been developing the technology that enables us to grow high quality and large GaN single crystals for producing high quality GaN single crystal substrates using Na flux method. In 2005, we have succeeded in the growth of 2-inch GaN substrates with low dislocation density (< 105 cm^-2) by applying the Liquid Phase Epitaxy to the Na flux method. Recently, we reported that dislocation density lower than the order of 10⁴ cm^-2 and the growth rate more than 20 um/h is achievable in the Na flux LPE. This achievement was due to introducing some new techniques; such as thermal convection, mechanical stirring, addition of carbon additive, development of new growth apparatus and so on, to the Na flux LPE.

The equations appropriate to calculate spontaneous and piezoelectric polarization fields at heterointerfaces of wurtzitic semiconductors grown on any (hk.ℓ) planes are presented in the context of a description that fulfils the prescriptions of crystallography and elasticity theory. Some specific crystal orientations may lead to a cancellation of the Quantum Confined Stark Effect in such heterostructures. The angle between the normal to these orientations and (0001) is not constant and depends on h, k and ℓ. These crystal orientations do not correspond to a minimization of the elastic energy stored in the strained layers. The impact of this reduction or cancellation of the QCSE is discussed, computed and compared with fresh experimental data collected in case of devices grown on non polar and semi polar substrates for the GaInN-GaN combination.

Aluminum nitride is a material of great potential for high power electronic devices, UV photonic devices as well as acoustic devices. However, the lack of a good crystal growth technology for bulk material and substrate hinders the development of these AlN-based devices. While AlN has been successfully grown on sapphire substrate for some time, the presence of a large number of dislocations in the material is still a major barrier to overcome [1]. In this work, we demonstrate a low-dislocation-density AlN template on sapphire by inserting an AlN interlayer by metal-organic chemical vapor deposition. The main idea of our approach is to change the growth mode in the course of the epitaxial growth by decreasing growth temperature and changing V/III ratio. As the growth mode changes, dislocations tend to be redirected and/or form dipole half loops via annihilation processes [2]. The etch-pit-density of the AlN templates is reduced from 3.6×10⁹ cm^-2 to 1.7×10⁹ cm^-2. Accordingly, the full width at half maximum of the (0002) x-ray rocking curve is reduced from 37 arcsec to 12 arcsec. The result indicates that the AlN template has low screw and mixed type dislocations. AlGaN/GaN Schottky diodes fabricated on this high quality AlN template exhibit very high breakdown voltage (> 2000 V), which sets a record-high figure of merit of 1.15 GW/cm².

The growths of droplet-free narrow-bandgap InN semiconductors on Ga-polar and N-polar GaN templates on c-plane sapphire substrates were performed by pulsed-MOVPE growth techniques. Under the optimum In-polar InN growth conditions, the carrier mobility and n-type carrier concentration were measured as 681 cm²/(V.sec) and 1.5×10¹⁹ cm^-3, respectively. The room-temperature photoluminescence measurements of optimized In-polar grown by pulsed-MOVPE technique resulted in peak wavelength at 0.76 eV. The growth of N-polar InN grown on the N-polar GaN template is discussed and compared to that of the In-polar InN.

Position controlled InN nanocolumns were fabricated by electron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE) on hole-patterned GaN template by focused ion beam (FIB). Dependences of morphological changes in InN nanocolumn on V/III ratio and hole size were investigated. It is found that growth with a higher V/III ratio and a larger hole size resulted in the multiple formation of InN nanocolumns in one hole. By varying the hole density on the GaN template, we succeeded to change the density of InN nanocolumn by one order of magnitude from about 100/μm² to 9/μm². InN nanocolumn showed luminescence with peak energy of 0.74 eV by cathodoluminescence measurement.

The QED quantum 1/f noise formulas have been recently refined for the case of AlGaN/GaN HFETs and FETs through a better definition of the coherence parameter s, with much better agreement with the experiment. Indeed, for a FET/HFET width w>>L>t, this yielded s≈πnrtLlog(w/2L) instead of the old s=2nrtw formula. Here we generalize this basic result for the first time to a finite piezoelectric case. Here L= source to drain length, t is the thickness (depth) of the channel, n is the concentration of carriers, π3.1416, and r=2.8•10-13 cm is the classical radius of the electron. In piezoelectric materials, particularly those also showing ferroelectric spontaneous polarization, transversal phonons are the massless quanta leading to large piezoelectric, or lattice-dynamic, quantum 1/f effects, conventional and coherent. As in the usual QED case, the parameter s' yields the observed 1/f noise as a weighted sum of conventional and coherent quantum 1/f effects. The HFET piezocoherence weighting parameter s', derived here is (gN'h/m*v_s)(v_s/u)³ F(u/vs)t/12w, with N'=nLt, vs the piezophonon speed, u the drift velocity, and F(x) is a function defined earlier, equal to (2/3)x³ for x<<1. This s' is increasing, ~t², important for reliability and device optimization. For HFET stability, a slower decrease of conductivity than of polarization is found to be needed for stability along the large device width, when the temperature increases.

Vertical double heterostructures based on GaN were prepared and investigated for their current voltage characteristics and compared to theory. In our quest to observe negative differential resistance (NDR) phenomenon based on quantum mechanical tunneling to, we fabricated resonant tunneling diode (RTD) like structures grown on low defect density and high quality templates prepared by metal organic chemical vapor deposition using in situ SiN nanonetwork induced epitaxial lateral overgrowth. The measured threading dislocation density of the template was in the range of 10⁷ cm^-2. Inductively coupled plasma reactive ion etching (ICP-RIE) where in enhanced chemical etching mode was used for reducing the detrimental surface defects on the mesa walls which otherwise contribute to current. Double barrier structures with varying barrier and quantum well thicknesses as well as doping profiles were tested for their I-V characteristics. The rectifying phenomenon occurred as a result of depletion region in GaN above the top Al(Ga)N layer and asymmetric barrier shape of GaN RTD-like structure due to polarization. With the aid of calculated band structure and resultant doping profile optimization, we now observe what appears to be resonant increase in current, the source of which is not yet clear, at quantum states of the well.

We report a comprehensive study on THz emission and spectroscopy of indium nitride (InN) films and its nanorod arrays grown by plasma-assisted molecular beam epitaxy technique. For the enhancement of THz emission from InN, we demonstrated two method; firstly using nanorod arrays, which have large surface area for optical absorption and THz emission, and secondly using nonpolar InN film, of which the electric field is along the sample surface. We propose that a "screened" photo-Dember effect due to narrow surface electron accumulation layer of InN is responsible for the nanorod-size-dependent enhancement from InN nanorods. The primary THz radiation mechanism of nonpolar InN is found to be due to the acceleration of photoexcited carriers under the polarization-induced in-plane electric field. THz time-domain spectroscopy has been used to investigate THz conductivity and dielectric response of InN nanorod arrays and epitaxial film. The complex THz conductivity of InN film is well fitted by the Drude model, while the negative imaginary conductivity of the InN nanorods can be described by using a non-Drude model, which includes a preferential backward scattering due to defects in InN nanorods, or a Coulombic restoring force from charged defects.

Surface characterization and control technologies were applied to GaN and AlGaN surfaces. It was found that a unique "air-gap CV" technique is effective in evaluating surface state density on free AlGaN surfaces. A photoelectrochemical process, utilizing a mixed solution of propylene glycol and tartaric acid, was employed to form a thin oxide layer on GaN and AlGaN. We observed an enhancement of drain current in the AlGaN/GaN HEMT having a narrow channel width of 200 nm after the oxidation of the channel walls by the electrochemical process. To improve the uniformity of the effective electric field in the channel, a multi-mesa-channel (MMC) AlGaN/GaN HEMT has been proposed and developed. With forming a periodic trench just under the gate region by an ECR-plasma assisted dry etching, the MMC HEMT has parallel mesa-shaped channels with 2-dimensional electron gas (2DEG) surrounded by the top- and side-gate electrodes.

High quality epitaxial growth and uniform current spreading are essential to III-nitride light emitting diodes (LEDs) for superior wall-plug efficiency and reliability. An analysis method of current spreading based on 3-dimensional circuit modeling is introduced. We have investigated influences of the current spreading in the lateral-electrode type blue LEDs of 320 × 320 μm² size theoretically and experimentally. It is known that the current spreading can be greatly reduced by careful design of electrode pattern. Uniform current spreading is very important to improve electrical and optical characteristics such as series resistance, efficiency droop, leakage current with operational time leakage current, and electrostatic discharge (ESD) voltage. A method improving ESD voltage is presented by inserting floating metal near the n-electrode. About 4 times larger ESD voltages are experimentally measured at LEDs with floating metal compared to conventional LEDs without one. The internal quantum efficiency (IQE) is the most important factor affecting overall LED performances. A measurement method of the IQE measurable just at room temperature is proposed and demonstrated. The method utilizes both the time responses of the time-resolved photoluminescence (TRPL) as a function of the excitation femtosecond laser pumping power and their theoretical analysis based on the carrier rate equation.

We demonstrate the possibility for controlled micro- and nanostructuring of GaN layers by low-dose focused-ion-beam (FIB) treatment with subsequent photoelectrochemical (PEC) etching. The proposed novel maskless approach based on ultra-fast direct writing of surface negative charge that shields the material against PEC etching allows one to fabricate GaN nanowalls and nanowires with lateral dimensions as small as 100 nm. Compared with commonly used lithography masks and/or FIB etching approaches for patterning GaN, the surface charge lithography enables one to fabricate high-aspect ratio micro- and nanostructures and mitigates the need for additional mask layers on the surface prior to etching, and is much faster than FIB etching alone reducing furthermore the ion exposure of material and therefore reducing ion beam damage. We show, in particular, the possibility to etch voids in between structures as narrow as 200 nm and to fabricate GaN suspended membranes and sub-micrometer hollow squares with the thickness defined by the main projection range of implanted ions. The obtained results demonstrate the feasibility of maskless device fabrication based on low-dose FIB direct writing with subsequent wet etching.

We report on the characterization of V-defects in GaN-based heterostructures via scanning force microscopy techniques. The diameter and density of the V-defects are found to strongly depend on growth thickness and temperature of the top layer, respectively, while no correlation between the V-defect formation and the type of doping could be identified. Kelvin probe force microscopy measurements revealed for both, n- and p-doped GaN top layers, a decrease of the Kelvin voltage within the V-defects, which indicates an enhanced work function of the facets of the V-defects with respect to the planar surface. Surprisingly, an increase of the current flow within the V-defects is found by conductive atomic force microscopy in case of the n-doped top layer, while current flow into the V-defect is suppressed for the p-doped top layer. For a consistent explanation of these results we suggest a model, which is based on an enhanced electron affinity of the {10-11}-surfaces within the V-defects as compared to the planar (0001)-surface.

The Pt/Ga₂O₃/GaN diodes were fabricated in which the Ga₂O₃ oxide layers were directly grown on GaN layer using a photoelectrochemical method. Then, the Ga₂O₃ oxide films were annealed in O₂ ambiance at 700 °C for 2 hours to perform the β-Ga₂O₃ crystalline phases. The hydrogen sensing characteristics of Pt/GaN (metal-semiconductor, MS) and Pt/β-Ga₂O₃/GaN (metal-insulator-semiconductor, MIS) diodes under hydrogen-containing ambiance were studied in an air atmosphere. Compared with the MS devices, the MIS devices exhibited better hydrogen sensing ability. The result demonstrates that the β-Ga₂O₃ layer plays an important role in the hydrogen sensing of the GaN based MIS diodes.

Photoluminescence (PL) and surface photovoltage (SPV) of GaN layers were studied in vacuum and air ambient. SPV transients were measured with two set-ups: traditional Kelvin probe attached to an optical cryostat and atomic force microscope in contact potential mode. It is found that upward band bending in GaN decreases from its dark value of about 0.9 eV to about 0.3 eV under intense UV light and then gradually increases in air ambient (due to photoadsorption of oxygen) and decreases in vacuum (due to photodesorption of oxygen). Manifestations of such changes were observed as changes in PL intensity and in SPV (both increased in vacuum and decreased in air). The effects were sample-dependent.

One of the most desired features of the semiconductor blue/near UV laser diodes (LDs) is the possibility to obtain high output powers from the devices. This can be realized by means of multi emitter structures. We demonstrate the construction of violet blue multi-quantum-well (MQW) InGaN/GaN laser mini - bars, yet quite novel system in nitride-based devices. It consists of three laser stripes (3 μm wide), closely spaced with 40 μm pitch. The structures were fabricated on high pressure grown, low dislocation density substrates. Under cw operation the measured spectra demonstrate sharp, almost single line emission (FWHM around 1.43 Å at λ = 406 nm). Measurements of the optical far field pattern revealed that when operated below threshold the device was emitting light from all three stripes, whereas during lasing we observed the optical mode only in the area of the middle laser stripe. The behavior of optical mode suggests the formation of the so called supermode (coherent emission from all three devices) which was observed also in case of structures fabricated with wider ridges (of 7 μm with 20 μm pitch).

The authors report on room temperature (RT) lasing action in two different types of nitride-based microcavities (MCs): vertical cavity surface emitting lasers (VCSELs) and polariton lasers which operate in the weak and in the strong coupling regime, respectively. Following a brief description of these two operating regimes, an analysis of lasing action at RT is reported for a crack-free planar VCSEL structure based on a bottom lattice-matched AlInN/GaN distributed Bragg reflector (DBR) and a top dielectric DBR. The cavity region, formed by n- and p-type GaN layers surrounding only three InGaN/GaN quantum wells, corresponds to a typical active region suitable for an electrically driven VCSEL. Processing issues of such planar VCSEL structures and electroluminescence characteristics of processed devices are also reported. Then, an alternative approach relying on the realization of coherent GaN-MC light sources based on the spontaneous decay of a macroscopic polariton population, the so-called polariton laser, is described. It is shown that this kind of devices could work at RT with a potentially much lower threshold current density than VCSELs as it does not necessitate reaching population inversion conditions. As for VCSELs, we demonstrate laser-like properties above threshold, i.e. an intense polarized emission and a strong spectral narrowing. Differences of polariton lasers with conventional lasers are also highlighted.

In the paper, we describe the fabrication and performance characteristics of GaN-based vertical-cavity surface-emitting lasers (VCSELs) by optical pumping and current injection. According to the employment of high-quality and high-reflectivity AlN/GaN DBRs in the whole structure, the lasing action of optically pumped GaN-based VCSELs with hybrid mirrors has been observed at room temperature. Due to the excellent results of optically pumped GaN-based VCSELs with hybrid mirrors, we further demonstrated the lasing behavior of GaN-based VCSELs by continuous-wave current injection at 77 K. The laser has one dominated blue wavelength located at 462 nm with a linewidth of about 0.15 nm and the threshold injection current at 1.4 mA. The divergence angle and polarization ratio of the GaN-based VCSELs with hybrid mirrors are about 11.7° and 80%, respectively. A larger spontaneous coupling efficiency of about 7.5×10^-2 was also measured.

A microstructure in AlGaN with a whole compositional range on an AlN/sapphire template is systematically investigated. At the interface between AlGaN and AlN, misfit dislocation density increases with Ga composition in AlGaN. However, most misfit dislocations bend owing to the compressive stress and form loops, by which threading dislocation density is markedly reduced if the thickness exceeds several microns. The effective acceptor energy of Mg in Al_0.5Ga_0.5N is found to depend on Mg concentration with a negative one-third power. A SiO₂/AlN dielectric multilayer mirror is very effective for controlling the reflectivity of the Fabry-Perot resonator mirrors at the cleaved edge of the ultraviolet (UV) laser diodes (LDs). A UV LD with an emission wavelength of 358 nm shows a threshold current density as low as 3.9 KA/cm² at room temperature.

True blue lasers with wavelengths of ~450 nm are of great interest for full color laser projection. These kind of applications usually require high output power and, in particular, an excellent wall plug efficiency within a wide temperature range. In this paper we therefore present experimental and theoretical investigations of the temperature behavior of 60mW InGaN lasers in a range of -10 °C to 100 °C. The laser parameters threshold current density, slope efficiency and operating voltage describe the wall plug efficiency of the device. The slope efficiency does not show any significant temperature dependence which is due to an almost temperature independent injection efficiency in the temperature range that is of interest for most commercial applications. In contrast, the laser threshold current density increases with temperature and we determine a characteristic temperature T₀ of about 141K for our devices emitting at 445nm. This increasing threshold current density can be explained by lower gain of the quantum wells at higher temperature. Furthermore, Auger recombination influences the threshold as verified by simulations. The second electro-optical parameter is the electrical voltage, which is dominated by electrical barriers. The voltage decreases with increasing temperature and compensates the increasing threshold current resulting in a nearly constant high wall plug efficiency of 13% between -10°C and 100°C.

Light extraction efficiency is important to the brightness of LEDs. In this study, various texturing and roughing schemes were formed on the surface or interface of InGaN-based LED structure grown on sapphire substrate to investigate their effects. Throughout the research, temperature-dependent PL measurement was used to calculate the internal quantum efficiency so as to derive the light extraction efficiency. The light extraction efficiency is around 60 to 65% while the epitaxy and substrate are flat. On the other hand, the light extraction efficiency reaches an optimal value of around 85% while the p-GaN surface is textured and the substrate is patterned. However, for LED having only one-side surface texturing structure optimized on either p- or n-side, the light extraction efficiency can be already as high as 75 to 80%. Methods for further enhancement, such as use of ZnO nanorod on chip surface, were also discussed.

Carrier lifetime and efficiency droop of green light emitting InGaN QWs in LEDs and laser diodes (LD) are discussed in term of Shockley-Read-Hall, radiative, and Auger-like recombination. The carrier lifetime, spectral shift, and carrier density as function of current density was measured by time-resolved electroluminescence with a streak camera. The current density was varied from very low for the LEDs to threshold for the LD. The transition from spontaneous emission in the low carrier density regime to stimulated emission in the high carrier regime is continuous. A comparison with Hakki-Paoli optical gain spectra provides evidence for a significant contribution of stimulated emission already at intermediate current densities. Stimulated emission is discussed as possible contribution to efficiency droop. A wavelength dependency of the lifetime is certainly caused by stimulated emission. Slope efficiency of the laser diode provides a lower limit for the injection efficiency of the LD. Time-resolved electroluminescence spectroscopy with applied bias demonstrates the variation of radiative recombination and consequently carrier lifetime with applied bias. Under certain conditions for modulated operation more light is emitted during times of zero bias than during forward bias.

We have investigated the efficiency droop in InGaN based multiple-quantum light-emitting diodes (MQWs-LEDs) and double hetero-structure light-emitting diodes (DH-LEDs) by changing the barrier (both thickness and barrier height) within quantum wells. Our results show that for MQW-LEDs, with the decrease of barrier width from 12nm In_0.01Ga_0.99N to 3nm In_0.01Ga_0.99N, the external quantum efficiency (EQE) droop point is increased from 350 Acm^-2 to >1000 Acm^-2, and the slope of EQE drop is also greatly reduced. When the barrier height of the MQW-LEDs is decreased, i.e. barriers changed from In_0.01Ga_0.99N (3nm) to In_0.06Ga_0.94N (3nm), the EL intensity is reduced to half. In the case of DH-LEDs, 6nm DH-LED shows the highest EL intensity and no EQE droop up to 1000 Acm^-2. When the active region of the DHLED is increased from 6nm to 12nm, the electroluminescence (EL) intensity is reduced to 70% of that of the 6nm DHLED, and the EQE shows negligible droop compared to the 6nm DH-LED due to both enhanced hole injection and reduced electron overflow. These results suggest that heavy effective mass of holes and low hole injection efficiency (due to relatively lower p-doping) leading to severe electron leakage are responsible for the efficiency droop.

Employment of layered structures made of semiconductor materials with different optical absorption bands is a new way of realizing either broad band spectrum or selective multiple band photodetectors. A new concept of structures fabricated using stacked semiconducting layers to obtain a multi band spectral response is reported. Based on this approach, fabrication of a Solar-blind dual-band UV/IR photodetectors is demonstrated. Optimization of the device was carried out by modeling of the electric field distribution and developing tunneling barriers. The optimized Solar-blind UV/IR photodiode UV spectral response turns-on approximately around 265 nm (solar-blind) and peaks at 230 nm with a responsivity of approximately 0.0018 A/W. The IR diode response peaks at 1000nm with a responsivity of approximately 0.01 A/W.

We fabricated the high efficiency white LEDs. The white LEDs, the yellow YAG-phosphors-coated small-size (290µm × 500µm) blue LED, designed for minimizing forward voltage. At a forward current of 20mA, the luminous flux, the forward voltage (Vf), the correlated color temperature (Tcp), the luminous efficiency, and the wall-plug efficiency (WPE) are 9.5lm, 2.8V, 5193K, 169Lm/W, and 50.8%, respectively. The high-power white LEDs were fabricated from the larger-size (1mm × 1mm) blue LED chips with the output power of 651mW at 350mA. Flux, Vf, Tcp, luminous efficiency, and WPE of the high-power white LED are 145Lm, 3.09V, 4988K, 134Lm/W, and 39.5%, respectively, at 350mA. This power white LEDs showed total flux of 361Lm at 1A. Moreover, we succeeded in developing high-power and high-efficiency blue laser diodes (LDs) with an emission wavelength at 445nm range by using GaN-based materials. This achievement leads to the full-color laser display applications. We fabricated multi-transverse mode LDs by a single emitter, and adopting φ9mm packages for the reduction of the thermal resistance. The typical optical-output power, voltage and wall-plug efficiency of the LDs at forward current of 1.0A at a temperature of 25ºC was 1.17W, 4.81V and 24.3%, respectively. The catastrophic optical damage at the facets of these LDs did not appear up to 3W in the optical output power. The estimated lifetime of the LDs at a temperature of 25ºC under continuous-wave operation 1.0A in automatic current control condition was over 30,000 hours.

We demonstrate 222-282 nm AlGaN and InAlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) fabricated on low threading dislocation density (TDD) AlN template. Low TDD AlN templates were realized by using ammonia (NH3) pulse-flow multilayer (ML) growth technique. The edge- and screw-type dislocation densities of AlN layer were reduced to 7.5×10⁸ and 3.8×10⁷, respectively. We obtained significant increase of an AlGaN quantum well (QW) emission (by more than 50 times) by fabricating them on a low TDD ML-AlN template. We fabricated AlGaN multi (M)QW DUV-LEDs with emission range of 222-273 nm on ML-AlN templates. Single-peaked electroluminescence (EL) was obtained for AlGaN DUV-LEDs. We obtained the maximum output power of 1.1, 2.4 and 3.3 mW for the AlGaN LEDs with wavelengths of 241, 253 and 273 nm, respectively, under RT CW operation. The maximum output power of 227 and 222 nm AlGaN-QW were 0.15mW and 0.014mW, respectively, under RT pulsed operation. The maximum external quantum efficiency (EQE) of the 227 and 250 nm AlGaN LEDs were 0.2% and 0.43 %, respectively. We also fabricated 280 nm-band quaternary InAlGaN-MQW DUV-LEDs with p-type InAlGaN layers on low TDD ML-AlN templates. We obtained significant increase of photoluminescence (PL) intensity by introducing Si-doped InAlGaN buffer and barrier layers and undoped InAlGaN interlayer. We then demonstrated high internal quantum efficiency (IQE) of 284 nm InAlGaN-QW emission, which was confirmed by the fact that the ratio of the integrated intensity of the RT-PL against the 77K-PL was 86%. The maximum output power and EQE of the 282 nm InAlGaN LED were 10.6 mW and 1.2%, respectively, under RT CW operation.

InGaN multiple quantum wells (MQWs) with green light emission have been grown on GaN stripes oriented along the [11-20] direction by selective metal-organic vapor phase epitaxy (MOVPE). Several different window widths were designed in the SiO₂ mask. Completed pyramidal InGaN stripes with flat and smooth {1-101} sidewall were produced on 2-μm windows while trapezoidal stripes with both {1-101} sidewall and (0001) top surface were obtained on the 5-μm windows. The former has uniform CL emissions at 500 nm on the {1-101} sidewall and at 550 nm on the narrow ridge. The latter exhibits similar CL emissions at 500 nm on the sidewall and at 570 nm on the top surface. These wavelength shifts relative to the CL spectrum peak (450 nm) from the reference region are attributed to thickness enhancement and indium enrichment in selective MOVPE. The short-wavelength shoulder near 500 nm in the spectrum from the ridge of the completed pyramidal strip is attributed to overlapping excitation of the sidewall by the SEM incident beam.

We report our experimental studies on the time-resolved dynamics of conduction electrons in high quality Al_xGa_1-xN single crystals. The Al_xGa_1-xN synthesis with the Al content of up to 90% was performed using a solution growth technique in a high nitrogen pressure system, maintained for several days under the high (up to 15 kBar) gas pressure and at very high (up to 1800°C) temperatures. The resulting crystals were colorless (fully transparent to visible light) platelets up to ~1 mm² in size, and exhibited a hexagonal Würtzite structure. Our optical measurements were performed at room temperature, using a two-color (ultraviolet and near-infrared), femtosecond, pump-probe spectroscopy, by invoking a two-photon, volume absorption and scanning the normalized transient differential transmitivitty (ΔT/T) signal of the probe beam. The observed ΔT/T signal was a superposition of a femtosecond, two-photon-absorption (TPA) correlation transient and a conventional optical pump-probe photoresponse signal with a rise time of ~1 ps and decay of ~75 ps. We have found that the TPA process was quite efficient with the absorption coefficient β ≈ 0.337 cm/GW at 380 nm, and enabled us to directly determine that the optical bandgap of Al_xGa_1-xN (x = 0.86) was 5.81 +/-0.01 eV. The latter value is in the good agreement with the one deduced from the x-ray diffraction measurements.

Monolithic multi-color light-emitting diodes (LEDs) based on micro-structured InGaN/GaN quantum wells are demonstrated. The microstructure is created through regrowth on SiO2 mask stripes along the [1¯100] direction and consists of (0001) and {11¯22} facets. The LEDs exhibit polychromatic emission, including white, due to the additive color mixture of facet-dependent emission colors. Altering the growth conditions and mask geometry easily controls the apparent emission color. Simulations predict high light extraction efficiencies due to their three-dimensional structures. Furthermore, we demonstrate that the apparent emission colors can externally be controlled over a wide spectral range that encompasses green to blue or white at a color temperature of 4000 K to blue along the Planckian locus. The controllability relies on the facet-dependent polychromatic emissions; the pulsed current operation with the appropriate duties varies their relative intensities and the consequent apparent colors without seriously affecting the total number of emitted photons, particularly for the blue to green variation. The proposed LEDs can be fabricated through a simple process and, therefore, may be a key device for advanced solid-state lighting.

We have demonstrated surface normal detecting/filtering/emitting multiple functional ultraviolet (UV) optoelectronic devices based on InGaN/GaN, InGaN/AlGaN and Al_xGa_1-xN/AlyGa_1-yN multiple quantum well (MQW) structures with operation wavelengths ranging from 270 nm to 450 nm. Utilizing MQW structure as device active layer offers a flexibility to tune its long cut-off wavelength in a wide UV range from solar-blind to visible by adjusting the well width, well composition and barrier height. Similarly, its short cut-off wavelength can be adjusted by using a GaN or AlGaN block layer on a sapphire substrate when the device is illuminated from its backside, which further provides an optical filtering effect. When a current injects into the device under forward bias the device acts as an UV light emitter, whereas the device performs as a typical photodetector under reverse biases. With applying an alternating external bias the device might be used as electroabsorption modulator due to quantum confined Stark effect. In present work fabricated devices have been characterized by transmission/absorption spectra, photoresponsivity, electroluminescence, and photoluminescence measurements under various forward and reverse biases. The piezoelectric effect, alloy broadening and Stokes shift between the emission and absorption spectra in different InGaN- and AlGaN-based QW structures have been investigated and compared. Possibilities of monolithic or hybrid integration using such multiple functional devices for biological warfare agents sensing application have also be discussed.

To realize high-efficiency light-emitting diodes (LEDs), it is indispensable to increase light extraction efficiency. We propose the moth-eye structure on the surface of an LED chip, which consists of periodic cones with a pitch of optical wavelength scale, and enables the significantly enhancement of light extraction efficiency. We have developed a new technique for moth-eye structure fabrication, on the basis of low-energy electron-beam projection lithography (LEEPL), which can be applied to the mass production of LEDs. The moth-eye structure formed at the bottom of a SiC wafer has periodically arranged cones with a 300 nm pitch and a 750 nm height. We also present blue LEDs fabricated on SiC substrates with and without the moth-eye structure, and discuss the effect of the moth-eye structure on the performance of LEDs.

In this research, we demonstrate array of transverse-junction (TJ) blue light-emitting-diodes (LEDs), which are specified as a horizontal carrier flow instead of side-by-side injection, with a consequence of InxGa1-xN/GaN multiple-quantumwells (MQWs) as the active region. The demonstrated devices were carried out by the re-growth of n-type GaN on the sidewall of p-type GaN. Regarding the transverse carrier flow of injected carriers, these TJ-LEDs, as compared to the control related to traditional vertical junction structure, can effectively spread injected currents more uniformly, minimize the problem of nonuniform carrier-distribution and current crowding effect, and achieve 35% improvement of power performance.

Chemical sensors can be used to analyze a wide variety of environmental and biological gases and liquids and may need to be able to selectively detect a target analyte. Different methods, including gas chromatography (GC), chemiluminescence, selected ion flow tube (SIFT), and mass spectroscopy (MS) have been used to measure biomarkers. These methods show variable results in terms of sensitivity for some applications and may not meet the requirements for a handheld biosensor. A promising sensing technology utilizes AlGaN/GaN high electron mobility transistors (HEMTs). HEMT structures have been developed for use in microwave power amplifiers due to their high two dimensional electron gas (2DEG) mobility and saturation velocity. The conducting 2DEG channel of GaN/AlGaN HEMTs is very close to the surface and extremely sensitive to adsorption of analytes. HEMT sensors can be used for detecting gases, ions, pH values, proteins, and DNA. In this paper we review recent progress on functionalizing the surface of HEMTs for specific detection of glucose, kidney marker injury molecules, prostate cancer and other common substances of interest in the biomedical field.

Isotopically enriched gallium nitride films, Ga¹⁴N and Ga¹⁵N, have been fabricated by molecular-beam epitaxy to study the effect of nitrogen atomic mass on structures and properties of GaN. Due to the isotopic substitution, the phonon frequency shift has been clearly observed by using Raman spectroscopy. The lattice constants of Ga¹⁵N differed from Ga¹⁴N, and, actually, the unit cell volume of Ga¹⁵N was approximately 0.07% less than that of Ga¹⁴N. Temperature-dependent photoluminescence measurements revealed that recombination mechanism in Ga¹⁴N and Ga¹⁵N was the same to each other in the temperature range of 4-50 K, and the band gap energy difference between them was found to be E_g15-E_g14=6.0±0.1 meV. This E_g difference is discussed in terms of volume shrinkage and change in phonon-electron interaction due to isotopic substitution.

We report on GaN-based vertical cavities on highly reflective and crack-free 40.5 pair of AlGaN/GaN distributed Bragg reflectors (DBRs) by using a selective growth method to avoid wafer cracking that is commonly observed in conventional planar Al(Ga)N/GaN DBRs. An Al_0.46Ga_0.54N/GaN DBR with ~ 98% reflectivity was selectively grown with square patterns of up to 150 × 150 μm² in size, which were separated from each other by 10 μm wide SiO₂ mask stripes. Vertical cavity structures employing InGaN/InGaN multiple quantum wells (MQWs) were grown on these crackfree patterned DBRs and capped with 13 pair SiO₂/SiNx DBRs to complete the full cavity structure. A cavity mode at ~ 442 nm in 150 × 150 μm² area was observed, having a quality factor of ~300. The selective growth technique to eliminate crack formation is very promising for the fabrication of nitride-based vertical-cavity surface emitting laser devices.

Carrier-screening effect in an AlGaN quantum-well avalanche photodiode was investigated. The avalanche photodiode is a p-i-n diode consisting of three periods of Al_0.1Ga_0.9N/Al_0.15Ga_0.85N multiple quantum wells (MQWs) as the active region. Avalanche electroluminescence (EL) spectra were measured at different reverse bias voltages. By measuring the quantum Stark induced red-shift of the EL peak at different bias values , it was found that carrier screening decreases the local electric field across the AlGaN quantum wells, resulting in a reduced red shift of the EL peak, while enhancing the EL intensity. The carrier screening was found to be strong at the onset of avalanche breakdown of the diode and become weaker with increasing of the applied electric field. This is explained by considering the strong polarization-induced internal field in the barriers of the quantum wells. The polarization-induced electric field in the barriers has a direction opposite to the applied field thus producing a potential barrier to block the carrier transport. This leads to accumulation of carriers in the quantum wells, thus producing the screening effect. The direction of electric field in the barriers inverts when the applied bias increases to be larger than an inverse threshold. Carriers can then be smoothly transported and carrier screening disappears. Our study shows that carrier-screening effect can play an important role in III-Nitride heterojunction devices due to the existence of strong polarization fields.

Nitride-based materials have been widely used for photodetectors, however, Nitride-based p-i-n photodetectors with the use of InGaN as the intrinsic layer to modify the cut-off wavelength from 360 to 500nm was few reported. In this study, the p-i-n structure photodetectors with a p-GaN layer, an InGaN active layer and an n-type GaN layer were fabricated successfully. The Ni catalysts technology was applied in p-GaN layer to enhance ohmic characteristics of ptype layer. It was found that the photo-to-dark contrast ratios at 0.5V bias are 51.83. The cut-off wavelength was around 375nm.

Employment of layered structures made of semiconductor materials with different optical absorption bands is a new way of realizing either broad band spectrum or selective multiple band photodetectors. A new concept of structures fabricated using stacked semiconducting layers to obtain a multi band spectral response is reported. Based on this approach, fabrication of a Solar-blind dual-band UV/IR photodetectors is demonstrated. Optimization of the device was carried out by modeling of the electric field distribution and developing tunneling barriers. The optimized Solar-blind UV/IR photodiode UV spectral response turns-on approximately around 265 nm (solar-blind) and peaks at 230 nm with a responsivity of approximately 0.0018 A/W. The IR diode response peaks at 1000nm with a responsivity of approximately 0.01 A/W.

GaN-based heterojunction field effect transistors (HFETs) are strong contenders to replace vacuum tubebased devices in the high power, and high frequency arena. However, the piezoelectric stain, exploited to generate high density two-dimensional electron gases (2DEGs) in AlGaN/GaN devices is not necessarily desirable nor might it bode well in terms of device reliability. By using lattice-matched InAlN as the barrier, even higher densities of 2DEG and now respectable DC and RF performance can be achieved while at the same time avoiding the strain and subsequent reliability issues in the devices. However, little work has been done in identification of trapping mechanisms in the InAlN-based devices. The trapping is at the heart of the reduced RF performance of all the GaN-based devices, limiting the maximum attainable RF power. In this work, transient current spectroscopy, which observes the dynamics of the drain current during gate lag measurements, is utilized to ascertain information about the trapping levels, cross sections, and spatial locations of traps in the InAlN-based devices. Preliminary measurements indicate that one of the traps identified in this work (at 0.12eV) is similar to one measured by deep level transient spectroscopy (DLTS) in similar structures. Investigations of this type are imperative for the further development and implementation of this highly promising material system.

The ferroelectric effect has been demonstrated on an AlGaN/GaN heterostructure field effect transistor using a Pb(Zr_0.52Ti_0.48)O₃ layer deposited by radio-frequency magnetron sputtering. The device with a metal-ferroelectric-metal-semiconductor (MFMS) structure was fabricated with a Schottky contact placed between ferroelectric PZT and AlGaN/GaN 2-dimensional electron gas (2DEG) channel. The Schottky contact performs as a bottom electrode of the ferroelectric PZT and also as a barrier layer to prevent interaction at the interface between PZT and GaN. X-ray diffraction revealed the formation of (111)-oriented perovskite phase PZT on gate patterned AlGaN/GaN heterostructures. Transfer characteristics of the double-gate ferroelectric field effect transistor was determined by measuring its source-drain current as the gate bias applied on the top electrode was swept from -15 V to 15 V and then back to -15 V with a voltage step of 0.1 V. Ferroelectric behavior was observed in the plot of source-drain current versus gate voltage that showed a large counterclockwise hysteresis with a 50 % current modulation at zero gate bias.

Near lattice matched Al_0.81In_0.19N/GaN heterojunction structures are compared with conventional Al_0.3Ga_0.7N/GaN heterojunctions in terms of the sheet density and mobility and their dependence on barrier and spacer layer parameters. With the insertion of an AlN spacer, the mobility of both structures is improved dramatically. Self-consistent solution of Poisson-Schrödinger equations was developed in order to determine the band structure and carrier distribution in these GaN based heterostructures in an effort to gain insight into the experimental observations. Surface donor states were included to account for the origin of electrons in 2DEG, which is treated as charge neutralization conditions in the simulation. Also the change in the piezoelectric polarization due to the electromechanical coupling effect, and shift of band gap caused by uniaxial strain were both included in the calculations. The calculated sheet density is close to the measured values, especially for the AlGaN samples investigated, but a notable difference was noted in the AlInN cases. The discrepancy is confirmed to be caused by the existence of a Ga-rich layer on the top of AlN spacer during the growth interruption, which can split the 2DEG into two channels with different mobilities and lower the overall sheet density. When the modifications made necessary by this GaN layer are taken into account in our model for the AlInN barrier case, the calculations match with the experimental data. When the spacer thickness increase from 0.3 to 3 nm, the total sheet density was found to slightly increase experimentally, which agreed with the theoretical prediction.

We demonstrate that Ti/Al/Cr/Mo/Au ohmic contact has an extremely smooth surface morphology of 29.0 nm and a low specific contact resistivity (ρ_c) of 1.1×10^-6 ohm-cm2 on n-type AlGaN/GaN heterostructures. The use of Cr interlayer in Ti/Al/Cr/Mo/Au contacts leads to significantly improved contact morphology without any degradation on the contact resistance. This is attributed to the reduced inter-diffusion and reaction between the layers in the contact stack.

The Gallium Nitride (GaN) Light-Emitting-Diode (LED) bottom refection grating simulation and results are presented. A microstructure GaN bottom grating, either conical holes or cylindrical holes, was calculated and compared with the non-grating (flat) case. A time monitor was also placed just above the top of the LED to measure both time and power output from the top of the LED. Many different scenarios were simulated by sweeping three parameters that affected the structure of the micro-structure grating: unit cell period (Α) from 1 to 6 microns, unit cell width (w) from 1 to 6 microns, and unit cell grating height (d) from 50 to 200nm. The simulation results show that the cylindrical grating case has a 98% light extraction improvement, and the conical grating case has a 109% light extraction improvement compared to the flat plate case.

Compound semiconductors based on GaN have multiple functional applications. Useful compositions include GaN, and ternary and quaternary compositions of (AlGaIn)N. Defects arising from lattice mismatch, point defects, or impurities may act as electrical trapping centers and degrade device efficiency. Current-voltage, capacitance-voltage, thermal admittance spectroscopy (TAS), and deep level transient spectroscopy (DLTS) measurements are applied to characterize the defects in Al_0.40Ga_0.80N and In_0.18Al_0.82N in this report. Broad peaks with a shoulder at high temperature dominate the DLTS spectra in each of the materials. An acceptor trap associated with a dislocation appears at 340 K in AlGaN. The defect has an energy of 0.2 eV and capture cross section of 10^-21 cm². A second trap at 0.35 eV, 10^-14 cm² appears in the TAS measurements in addition to the trap at 0.2 eV. Defects in InAlN are dominated by a peak near 150 K. Two traps appear in the TAS measurements. Both traps in the InAlN are acceptors, based on a lack of field dependent emission rates using double pulse DLTS (DDLTS). The two energy levels in InAlN appear to be coupled, with only one state occupied at a time.