InGaN/GaN multi-quantum well (MQW) laser diodes (LDs) were grown on c-plane sapphire substrates using a multi-wafer MOCVD system. The threshold current for pulsed lasing was 1.6 A for a gain-guided laser diode with a stripe of 10 × 800 μm2. The threshold current density was 20.3 kA cm−2 and the threshold voltage was 16.5 V. The optical power ratio of transverse electric mode to transverse magnetic mode was found to be greater than 50. The characteristic temperature measured from the plot of threshold current versus measurement temperature was between 130 and 150K.

InGaN quantum-well-structure blue LEDs were grown on epitaxially laterally overgrown GaN (ELOG) and sapphire substrates. The output power of both LEDs was as high as 6 mW at a current of 20 mA. The LED on sapphire had a considerable amount of leakage current in comparison with that on ELOG. These results indicate that In composition fluctuation is not caused by threading dislocations (TDs), free carriers are captured by radiative recombination centers before they are captured by nonradiative recombination centers in InGaN, and that the dislocations form the leakage current pathway in InGaN. Red LED with an emission peak wavelength of 650 nm was fabricated by increasing the In composition and thickness of InGaN well layer. When the laser diodes (LD) was formed on the GaN layer above the SiO2 mask region, the threshold current density was as low as 3 kAcm−2. When the LD was formed on the window region, the threshold current density was as high as 6 to 9 kAcm−2. There is a possibility that a leakage current due to a large number of TDs caused the high threshold current density on the window region. InGaN multi-quantum-well (MQW) structure LDs grown on the ELOG substrate showed an output power as high as 420 mW under RT-CW operation. The longest lifetime of 9,800 hours at a constant output power of 2 mW was achieved. The InGaN MQW LDs were fabricated on a GaN substrate. The fundamental transverse mode was observed up to an output power of 80 mW.

Lateral epitaxial overgrowth (LEO) on Si(111) substrates using an AlN buffer layer is demonstrated and characterized using scanning electron microscopy, atomic force microscopy, transmission electron microscopy, x-ray diffraction, photoluminescence spectroscopy, and cathodoluminescence imaging. The <100>-oriented LEO GaN stripes grown on silicon substrates are shown to have similar structural properties as LEO GaN grown on GaN/Al2O3 substrates: the surface topography is characterized by continuous crystallographic steps rather than by steps terminated by screw-component threading dislocations; the density of threading dislocations is <106 cm−2; the LEO regions exhibit crystallographic tilt (0.7-4.7°) relative to the seed region. The AlN buffer thickness affects the stripe morphology and, in turn, the microstructure of the LEO GaN. The issues of chemical compability and thermal expansion mismatch are discussed.

Wide bandgap nitride semiconductors have recently attracted a great level of attention owing to their direct bandgaps in the visible to ultraviolet regions of the spectrum as emitters and detectors. However, this material system with its favorable heterojunctions and transport properties began to produce very respectable power levels in microwave amplifiers. If and when the breakdown fields achieved experimentally approaches the predicted values, this material system may also be very attractive for switching power devices. In addition to the premature breakdown, a number of scientific challenges remain including a clear experimental investigation of polarization effects. In this paper, transport properties as pertained to electronic devices and potential switching devices, and polarization effects will be treated.

The current status of GaN crystallization under high nitrogen pressure will be presented. Both conductive and semi-insulating GaN crystals will be characterized.

In particular the influence of Mg on the growth mechanisms will be discussed. The influence of Mg doping on morphology of Mg-doped crystals grown under pressure and Mg-doped homoepitaxial layers will be shown. It will be also shown that the addition of about 1 at.% of Mg into the solution improves significantly the structural quality of crystals reducing dislocation density at least by 3 orders of magnitude comparing to the crystals grown without an intentional doping. As it was estimated by selective wet etching and transmission electron microscopy the dislocation densities in the Mg-doped GaN is as low as 10 cm−1. The introduction of Mg also lowers the optical absorption coefficients for energies below fundamental edge by 2 to 3 orders of magnitude what is explained by disappearance of defect related states in the gap.

The procedures for preparation of atomically flat epi-ready (000 1 ) surfaces without subsurface damage will be described. It will be shown that high quality homoepitaxial layers growing by monoatomic steps are possible on these substrates.

We report on the fabrication and characterization of GaN/4H-SiC n-p-n heterojunction bipolar transistors (HBTs). The device structure consists of an n-SiC collector, p-SiC base, and selectively grown n-GaN emitter. The HBTs were grown using metalorganic chemical vapor deposition on SiC substrates. Selective GaN growth through a SiO2 mask was used to avoid damage that would be caused by reactive ion etching. In this report, we demonstrate common base transistor operation with a modest dc current gain of 15 at room temperature and 3 at 300°C.

The group-III nitrides have become versatile semiconductors for short wavelength emitters, high temperature microwave transistors, photodetectors, and field emission tips. The processing of these materials is significant due to the unusually high bond energies that they possess. The dry and wet etching methods developed for these materials over the last few years are reviewed. High etch rates and highly anisotropic profiles obtained by inductively-coupled-plasma reactive ion etching are presented. Photoenhanced wet etching provides an alternative path to obtaining high etch rates without ion-induced damage. This method is shown to be suitable for device fabrication as well as for the estimation of dislocation densities in n-GaN. This has the potential of developing into a method for rapid evaluation of materials.

Recent studies revealed specific features of chemical processes occurring on the surface of growing group-III nitrides – extremely low sticking probability of molecular nitrogen, low sticking coefficient and incomplete decomposition of ammonia frequently used as the nitrogen precursor. These features (kinetic by nature) result in the growth process going on under conditions remarkably deviated from the gas-solid heterogeneous equilibrium. In this paper we propose a novel approach to modeling of group-III nitride growth by MOVPE taking into account these features. In the model the sticking/evaporation coefficients of N2 and NH3 extracted from independent experiments are used allowing adequate description of the kinetic effects. The model is applied to analysis of growth of binary (GaN) and ternary (InGaN) compounds in a horizontal tube reactor. The growth rate and the solid phase composition are predicted theoretically and compared with available experimental data. The modeling results reveal lower ammonia decomposition ratio on the surface of the crystal as compared to thermodynamic expectations. The developed model can be used for optimization of growth process conditions.

In this paper, we discuss the issue of fabricating reliable and reproducible ohmic contacts on AlGaN HFET structures. During the course of our investigation of fabricating contacts to HFETs, we found that the contact properties could vary significantly from one sample to another, even though they were nominally the same. This problem was prominently manifested in the ohmic contact behavior. The origin of this problem was traced back to the variation of the HFET structure during growth. In this paper, we report an attempt to fabricate reproducible ohmic contacts of these structures.

Multiwafer Planetary Reactor is a promising system for large-scale production of heterostructures for LED's based on III-group nitrides. Analysis of chemical processes occurring in the reactor allows one to get insight into specific mechanisms governing growth of nitride based heterostructures. In the present paper results of modeling analysis of MOVPE of InxGa1−xN layers in AIX-200 Reactor and AIX 2000 HT Planetary Reactor are reported. The model used for MOVPE process analysis accounts for gas flow, heat transfer, and multicomponent mass transport along with gas phase and surface chemical reactions. Results of the modeling analysis of In transport and incorporation into the solid phase are compared with experimental data. It is shown that the model predicts reasonably well the In incorporation during MOVPE of InGaN under In/(In+Ga) ratio in the gas phase less than 20%.

The growth rate evolution versus V/III ratio and substrate temperature was studied by means of optical reflectivity during MBE of GaN layers using NH3 as nitrogen source. The GaN desorption becomes observable at temperatures above 800°C and causes the reduction of growth rate accompanied with the surface roughening at temperatures above 850-870°C. Unlike GaAs, which evaporates in accordance with the action mass law, the desorption rate of GaN is found to be almost independent of V/III ratio within the N-rich growth conditions. The activation energy for GaN desorption during the growth is found to be (3.2±0.1)eV. This value is very close to the activation energy for free evaporation. At V/III ratio values exceeding 200 the GaN growth rate reduction caused by violation of the molecular flow regime is observed. The Mg-doped samples grown under these extreme conditions tend to have improved acceptor activation and thus p-type conductivity.

We review pyroelectric and piezoelectric properties of GaN-based materials. Pyroelectric effects in GaN have been studied in two different regimes: (i) uniform sample heating regime and (ii) under applied temperature gradient along the sample. The modeling results show that the pyroelectric coefficient, Pv, in GaN (for c-axis along the contacts) can reach 7×105 V/m-K (compared to Pv = 5×105 V/m-K for the best-known high temperature pyroelectric/piezoelectric material LiTaO3). This points to a high potential of GaN-based sensors for high temperature pyroelectronics. Piezoelectric effects strongly affect the performance of electronic and light-emitting devices based on III-N materials. Piezoelectrically induced charge in heterostructures can be as large as 3 to 4×1013 cm−2. Hence, strong lattice polarization effects provide unique possibilities for utilizing GaN-based materials in high temperature piezoelectronics and for their applications in pyroelectric detectors.

Photoluminescence spectra were measured for 100 Å wurtzite GaN AlGaN modulation doped quantum wells. Three well-resolved peaks originate from the quantum well. The theoretically calculated confinement energies have been compared to the experimental energy positions and found to be in good agreement with the data, assuming that the piezoelectric field is largely screened by the electron gas. The highest energy transition may originate from the Fermi edge, consistent with the temperature dependence of the photoluminescence. Decay times for the different transitions indicate that the photoexcited holes are localized.

We present a device fabrication technology and measurement results of both optically pumped and electrically injected InGaN/GaN-based distributed feedback (DFB) lasers operated at room temperature. For the optically pumped DFB laser, we demonstrate a complex coupling scheme for the first time, whereas the electrically injected device is based on normal index coupling. Threshold currents as low as 1.1 A were observed in 500 μm long and 10 μm wide devices. The 3rd order grating providing feedback was defined holographically and dry-etched into the upper waveguiding layer by chemically-assisted ion beam etching. Even when operating these lasers considerably above threshold, a spectrally narrow emission (3.5 Å) at wavelengths around 400 nm was seen.

Undoped, 4µm thick GaN layers grown by Metal Organic Chemical Vapor Deposition were used for fabrication of high stand off voltage (356 V) Schottky diode rectifiers. The figure of merit VRB2/RON, where VRB is the reverse breakdown voltage and RON is the on-resistance, was ~ 4.53 MW-cm−2 at 25°C. The reverse breakdown voltage displayed a negative temperature coefficient, due to an increase in carrier concentration with increasing temperature. Secondary Ion Mass Spectrometry measurements showed that Si and O were the most predominant electrically active impurities present in the GaN.

The epitaxial growth of zinc-blende (cubic) GaN and InGaN on GaAs with a common cleavage plane and readily high-quality, low-cost wafers may be considered as an alternative approach for the future realization of cleaved laser cavities. To obtain detailed information about the potential of cubic GaN and InGaN for device applications we performed optical gain spectroscopy accompanied by time-integrated and time-dependent photoluminescence measurements at 2 K and 300 K. From intensity-dependent gain measurements, the identification of the gain processes was possible. For moderate excitation levels, the biexciton decay is likely to be responsible for a gain structure at 3.265 eV in cubic GaN [10]. For the highest pump intensities, the electron- hole-plasma is the dominant gain process, providing gain values up to 200 cm −1. Furthermore cubic GaN samples with different cavity lengths from 250 to 600 µm were cleaved to investigate the influence of the sample geometry on the gain mechanisms. In these samples increased gain values up to 150 cm −1 as well as lower threshold excitation densities were observed, indicating the potential of cubic GaN for device applications. The results of GaN will be compared with intensity-dependent gain measurements on InGaN samples, grown on GaAs with varying In-content. The observed gain mechanisms in cubic InGaN will be discussed in detail.

Temperature-dependent photoluminescence (PL) studies have been performed on InGaN epilayers and InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition. We observed anomalous temperature dependent emission behavior (specifically an S-shaped decrease-increase-decrease) of the peak energy (EPL) of the InGaN-related PL emission with increasing temperature. In the case of the InGaN epilayer, EPL decreases in the temperature range of 10 – 50 K, increases for 50 – 110 K, and decreases again for 110 – 300 K with increasing temperature. For the InGaN/GaN MQWs, EPL decreases from 10 – 70 K, increases from 70 – 150 K, then decreases again for 150 – 300 K. The actual temperature dependence of the PL emission was estimated with respect to the bandgap energy determined by photoreflectance spectra. We observed that the PL peak emission shift has an excellent correlation with a change in carrier lifetime with temperature. We demonstrate that the temperature-induced S-shaped PL shift is caused by the change in carrier recombination dynamics with increasing temperature due to inhomogeneities in the InGaN structures.

A visible-blind UV camera based on a 32 × 32 array of backside-illuminated GaN/AlGaN p-i-n photodiodes has been successfully demonstrated. Each of the 1024 photodiodes in the array consists of a base n-type layer of AlGaN (~20%) onto which an undoped GaN layer followed by a p-type GaN layer is deposited by metallorganic vapor phase epitaxy. Double-side polished sapphire wafers are used as transparent substrates. Standard photolithographic, etching, and metallization procedures were employed to obtain fully-processed devices. The photodiode array was hybridized to a silicon readout integrated circuit using In bump bonds. Output from the UV camera was recorded at room temperature at a frame rate of 30 Hz. This new type of visible-blind digital camera is sensitive to radiation from 320 nm to 365 nm in the UV spectral region.

We present a study of the radiative recombination in In0.15Ga0.85N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied. The piezoelectric field as well as short range potential fluctuations are screened via different mechanisms by donor electrons and excited electron-hole pairs. These effects account for a large part of the spectral shift with donor doping (an upward shift of the photoluminescence (PL) peak up to 0.2 eV is observed for a Si donor density of 2 × 1018 cm−3 in the well), with excitation intensity and with delay time after pulsed excitation (also shifts up to 0.2 eV). It appears like 2-dimensional screening of short range potential fluctuations is needed to fully explain the data. We suggest that excitons as well as shallow donors are at least partly impact ionized by electrons in the rather strong lateral potential fluctuations.

A new process for synthesis and bulk crystal growth of GaN is described. GaN single crystal c-plane platelets up to 9mm by 2mm by 100μm thick have been grown by the Chemical Vapor Reaction Process (CVRP). The reaction between gallium and a nitrogen precursor is produced by sublimation of solid ammonium chloride in a carrier gas, which passes over gallium at a temperature of approximately 900°C at near atmospheric pressures. Growth rates for the platelets were 25-100 μm/hr in the hexagonal plane. Seeded growth in the c-direction was also accomplished by re-growth on previously grown c-plane platelets. The crystals were characterized by X-ray diffractometry, atomic force microscopy, secondary ion mass spectrometry, inert gas fusion, and room temperature Hall effect and resistivity measurements.