Bulk single crystals of GaN with different degrees of Fe doping were studied using time-domain terahertz spectroscopy at high temperatures. Features due to free carriers were observed in the complex permittivity spectra with a pronounced dependence on both doping and temperature. Fitting the spectra using the Drude model made it possible to deduce a defect ionization energy of 16 meV in the undoped sample while the spectra of doped samples are consistent with an ionization energy of 60 meV. Also, the free carrier concentrations at temperatures from 300 to 900 K were estimated.

Systematic study of optical properties of undoped and Fe-doped substrates grown by hydride vapor phase epitaxy has revealed a strong dependence of the photoluminescence, transmission, reflectivity and ellipsometric spectra on the Fe-doping level. The changes of the near-band-gap transmission, reflectivity and photoluminescence has been observed and ascribed to the absorption introduced by the density of states tails, and the Fe 3+ -ions incorporated in the GaN-lattice. Several approaches towards quantifying the Fe-doping level are suggested.

Abstract  We report on the growth of Al0.25Ga0.75N/GaN heterostructures grown on low dislocation density vicinal surfaces of semi-insulating c-axis GaN substrates. Atomic force microscopy (AFM), photoluminescence (PL), cathodoluminescence (CL), high-resolution x-ray diffraction (HRXRD), secondary-ion mass spectroscopy (SIMS), Hall effect, and Raman spectroscopy have been used to assess structural and electrical properties as a function of substrate offcut. Bulk GaN substrates with vicinal offcut between 0.5° and 1.4° are optimal with respect to surface roughness and dopant incorporation. AFM, PL, and CL show decreasing Mg incorporation with increasing offcut angle. Raman spectroscopy, used to analyze biaxial strain, confirms essentially strain-free heterostructure growth on vicinal substrates with offcut angles between 0.5° and 1.4° off [0001] toward $$ [1overline1 00] $$. Aluminum (Al) incorporation in the Al x Ga1−x N barrier assessed by Raman vibration is in excellent agreement with trends found by HRXRD.

The effect of m -plane GaN substrate miscut on the growth of InGaN/GaN quantum wells (QWs) was investigated. It was found that the miscut toward [0 0 0 1] c + -axis resulted in an increase of In incorporation efficiency and in a green-shift of the QW emission, while the miscut toward [1 1  2  0] a -axis resulted in even higher In compositions but it also led to an increased epitaxial surface roughness and deterioration of the QW structures. The results indicated that miscut toward a -axis is undesirable while miscut toward c + -axis is beneficial for achieving longer wavelength emission in QWs grown on m -plane GaN substrates.

Despite notable progress in light-emitting and charge transport devices based on GaN heterostructures containing In, there is still controversy as to the light emission characteristics at high injection levels in InGaN-based light-emitting diodes (LEDs) and hot-carrier scattering in GaN-based field effect transistors (FETs) with AlGaN or AlInN barriers. For LEDs to be inserted into conventional lighting systems, reasonably high efficiencies would have to be retained at high injection levels to meet intensity requirements which is not yet borne out by many experiments but might be mitigated by use of nonpolar m-plane varieties according to the most recent data. The efficiency degradation at high injection levels, beyond that which is expected due to heating and current crowding, has been attributed by what is shaping up to be two camps to mainly Auger recombination and carrier spillover. The latter has been attributed to or helped by polarization-induced fields which is contrary to again recent experiments on LEDs. In terms of the FETs the conventional wisdom of increased carrier concentrations leading to better devices does not seem to hold beyond a certain point. This is due to strong electron LO phonon coupling in this highly ionic material and the resultant hot phonon population. Hot phonon lifetime decreases with increasing carrier concentration up to a point owing to plasmon–phonon interaction. But beyond the concentration at which the plasma frequency and phonon frequency match, the phonon lifetime begins to increase again. Increased phonon lifetimes lead to reduced carrier velocity and inefficient heat transfer, and thus performance degradation ensues. Another intriguing feature is that the aforementioned phenomenon is electric field dependent at least because increased field in FETs means widening of the channel and thus for the same volume density the resonance occurs at higher sheet densities. In this paper the details of carrier recombination in the context of InGaN LEDs both on polar c-plane and nonpolar m-plane GaN at high injection levels, and hot-carrier-scattering-related physics in the context of HFETs based on the GaN material family containing In will be elucidated.

The creation of symmetrical pairs of inclined dislocations was observed in the GaInN/GaN quantum wells (QWs) of c -axis grown green light-emitting diodes (LEDs) on low-defect density bulk GaN substrate, but not in green LEDs on sapphire substrate with high threading dislocation (TD) density. Pairs of dislocations start within 20 nm of the same QW and incline 18°–23° toward two opposite ⟨11̅ 00⟩ directions or in a 120° pattern. We propose that in the absence of TDs, partial strain relaxation of the QWs drives the defect formation by removal of lattice points between the two dislocation cores. In spite of those inclined dislocation pairs, the light output power of such green LEDs on GaN is about 25% higher than in LEDs of similar wavelength on sapphire.

We report microwave characteristics of field effect transistors employing InAlN/AlN/GaN heterostructures grown on low-defect-density bulk Fe-doped GaN substrates. We achieved unity current gain cutoff frequencies of 14.3 and 23.7 GHz for devices with gate lengths of 1 and 0.65 μm, respectively. Measurements as a function of applied bias allow us to estimate the average carrier velocity in the channel to be ∼ 1.0×107 cm/sec for a 1 μm device. Additionally, we found nearly no gate lag in the devices, which is considered a precondition for good performance under large signal operation.

Impurity-based p-type doping in wide-band-gap semiconductors is inefficient at room temperature for applications such as lasers because the positive-charge carriers (holes) have a large thermal activation energy. We demonstrate high-efficiency p-type doping by ionizing acceptor dopants using the built-in electronic polarization in bulk uniaxial semiconductor crystals. Because the mobile hole gases are field-ionized, they are robust to thermal freezeout effects and lead to major improvements in p-type electrical conductivity. The new doping technique results in improved optical emission efficiency in prototype ultraviolet light-emitting-diode structures. Polarization-induced doping provides an attractive solution to both p- and n-type doping problems in wide-band-gap semiconductors and offers an unconventional path for the development of solid-state deep-ultraviolet optoelectronic devices and wide-band-gap bipolar electronic devices of the future. 10.1126/science.1183226

We report the development of 480 nm cyan and 520 nm green light emitting diodes (LEDs) with a highly stable emission wavelength. The shift is less than 3 nm when the drive current density is changed from 0.1 to 38 A/cm2. LEDs have been obtained in GaInN-based homoepitaxy on nonpolar m-plane GaN bulk substrates. For increasing emission wavelength we find a large number of additional dislocations generated within the quantum wells (2×108 to ∼ 1010 cm2) and a decrease in the electroluminescence efficiency. This suggests that the strain induced generation of defects plays a significant role in the performance limitations.

High brightness InGaN light emitting diodes (LEDs) require high quantum efficiency and its retention at high injection levels. The efficiency drop at a high injection levels in InGaN light emitting diodes (LEDs) has been attributed, e.g. to polarization field on polar c-plane InGaN and the heavy effective hole mass which impedes high hole densities and transport in the active quantum wells. In this study, we carried out a comparative investigation of the internal quantum efficiency (IQE) of InGaN active region in LED structures using resonant optical excitation for layers with polar (0001) orientation on c-plane sapphire, and nonpolar (1-100) m-plane orientation, the latter on specially patterned Si and on m-plane bulk GaN. Analysis of the resonant photoluminescence (PL) intensity as a function of the excitation power indicate that at comparable generated carrier concentrations the IQE of the m-plane InGaN on Si is approximately a factor of 2 higher than that of the highly optimized c-plane layer. At the highest laser excitation level employed (corresponding carrier concentration n ~ 1.2 x 1018 cm-3), the m-plane LED structure on Si has an IQE value of approximately 65%. We believe that the m-plane would remain inherently advantageous, particularly at high electrical injection levels, even with respect to highly optimized c-plane varieties. The observations could be attributed to the lack of polarization induced field and the predicted increased optical matrix elements.

Ballistic and quasiballistic electron transport across the active InGaN layer are shown to be responsible for electron overflow and electroluminescence efficiency droop at high current levels in InGaN light emitting diodes both experimentally and by first-order calculations. An InGaN staircase electron injector with step-like increased In composition, an “electron cooler,” is proposed for an enhanced thermalization of the injected hot electrons to reduce the overflow and mitigate the efficiency droop. The experimental data show that the staircase electron injector results in essentially the same electroluminescence performance for the diodes with and without an electron blocking layer, confirming substantial electron thermalization. On the other hand, if no InGaN staircase electron injector is employed, the diodes without the electron blocking layer have shown significantly lower (three to five times) electroluminescence intensity than the diodes with the blocking layer. These results demonstrate a feasible method for the elimination of electron overflow across the active region, and therefore, the efficiency droop in InGaN light emitting diodes.

We report on electron velocities deduced from current gain cutoff frequency measurements on GaN heterostructure field effect transistors (HFETs) with InAlN barriers on Fe-doped semi-insulating bulk GaN substrates. The intrinsic transit time is a strong function of the applied gate bias, and a minimum intrinsic transit time occurs for gate biases corresponding to two-dimensional electron gas densities near 9.3×1012 cm−2. This value correlates with the independently observed density giving the minimum longitudinal optical phonon lifetime. We expect the velocity, which is inversely proportional to the intrinsic transit time, to be limited by scattering with non equilibrium (hot) phonons at the high fields present in the HFET channel, and thus, we interpret the minimum intrinsic transit time in terms of the hot phonon decay. At the gate bias associated with the minimum transit time, we determined the average electron velocity for a 1.1 μm gate length device to be 1.75±0.1×107 cm/sec.

Cation vacancies like V Ga , V Al and their complexes with oxygen are predicted to be abundant in III-nitrides and to play an important role in nonradiative recombination. Appearing in triple or double negatively charged states, they are not paramagnetic and have not so far been detected by magnetic resonance even under illumination. In this Brief Report, we demonstrate an efficient way to make cation vacancy defects in GaN detectable by electron paramagnetic resonance and present our identification of the V Ga O N pair in GaN which is the model material for the III-nitrides and their alloys.

Recent advances in the research, development, and commercial production of native GaN substrates with low defect density and high structural and optical quality have attracted a renewed interest in development of nitride devices based on native substrates. The still low yet rapidly increasing availability of native GaN substrates opens the full potential of GaN devices and has accelerated progress in the development of several electronic and optoelectronic devices. In this paper, progress in the primary competing growth techniques for producing native GaN substrates will be reviewed. The technological issues pertaining to faster scalability of GaN substrate production will be discussed. The current state-of-the-art substrate material properties and the future prospects for the growth approaches and substrate quality will be presented.

Thick free-standing GaN grown by hydride vapour phase epitaxy and epi-ready substrates with c-, a-, m-plane surfaces are examined by variable-temperature photoluminescence (PL), polarized PL and spatially resolved micro-PL. Both as-grown samples and polished substrates exhibit linewidth of the donor- bound exciton emission below 0.7 meV at 2 K indicative of a high structural quality of the material. For as-grown samples the relative intensity of green (2.4 eV) and red (1.8 eV) deep-level-defect emissions are found to decrease with increasing sample thickness. Based on plentiful two-electron transition spectra measured in the samples the electronic fine structure of the donors and their bound-exciton complexes was examined and discussed.

Polar and nonpolar bulk GaN substrates with low defect density and high structural and optical quality are demonstrated. The effect of doping by silicon, oxygen and iron within moderate doping levels on the properties of the polar GaN substrates was found uncompromised, as confirmed by high resolution X-ray diffraction and low temperature photoluminescence spectroscopy. In contrast, the lattice parameters were affected significantly, which has to be considered in the subsequent homoepitaxial device growth. The boule growth and respectively the nonpolar substrate homogeneity were found to be hampered by the doping, due to surface microcracking and higher impurity incorporation, while n-type undoped nonpolar substrates were demonstrated of superior quality.

Lai, MAL Johnson, T Paskova, AD Hanser, K Udwary, EA Preble, KR Evans

Grenko, CW Ebert, CL Reynolds, MAL Johnson, AD Hanser, EA Preble, T Paskova, KR Evans

We demonstrate homoepitaxial growth of GaInN/GaN-based green (500–560 nm) light emitting diodes (LEDs) on a -plane and m -plane quasi-bulk GaN prepared by hydride vapor phase epitaxy (HVPE). We find that in order to achieve an emission peak wavelength beyond 500 nm, a minimum InN-fraction of 14% is needed for both, a - and m -plane quantum wells (QWs), while 8% are enough for c -plane-oriented QWs. Besides increasing the InN-fraction in these non-polar QWs, widening the QW also proves to effectively shift the emission to longer wavelengths without loosing efficiency with the benefit of maintaining a low InN-fraction.

An enhancement of radiative recombination in GaInN/GaN heterostructures is being pursued by a reduction of defects associated with threading dislocations and a structural control of piezoelectric polarization in the active light-emitting regions. First, in conventional heteroepitaxy on sapphire substrate along the polar c -axis of GaN, green and deep green emitting light-emitting diode (LED) wafers are being developed. By means of photoluminescence at variable low temperature and excitation density, internal quantum efficiencies of 0.18 for LEDs emitting at 530 nm and 0.08 for those emitting at 555 nm are determined. Those values hold for the high current density of 50 A/cm 2 of high-power LED lamps. In bare epi dies, we obtain efficacies of 16 lm/W. At 780 A/cm 2 we obtain 22 lm when measured through the substrate only. The 555 nm LED epi material under pulsed photoexcitation shows stimulated emission up to a wavelength of 485 nm. This strong blue shift of the emission wavelength can be avoided in homoepitaxial multiple quantum well (MQW) and LED structures grown along the non-polar a- and m- axes of low-dislocation-density bulk GaN. Here, wavelength-stable emission is obtained at 500 and 488 nm, respectively, independent on excitation power density opening perspectives for visible laser diodes.

Gallium nitride (GaN) has emerged as one of the most important semiconductors in modern technology. Its future shines even brighter as wee see the advances towards solid state lighting and high-powered electronics. What was mainly pushing, actually creating this entirely new sector of GaN-based device technology, was the success in achieving reliable p-type doping and consequently, the ability for fabrication of light emitter devices (LEDs and LDs). A pioneer in the field, Shuji Nakamura, has summarized this work in S. Nakamura, G. Fasol, The Blue Laser Diode, Springer-Verlag, 1st edition, 1997.Much has been done, since then, in the development of better and more efficient GaN-based devices, already creating a multibillion dollar market. This is even more astounding in the light of the relatively underdeveloped technology for lattice and thermally-matched substrates for GaN-based devices. Only since around the year 2000, has the crystal growth technology of GaN been developed to a now widely recognized field in academia and industry.This book is designed to bring to the readership for the first time, a comprehensive overview of the state-of-the-art GaN crystal growth technology, reflecting the tremendous progress made particularly over the last decade, drawing the possible path we still have to cover to realize our common goal: large-size, dislocation-free, GaN crystals to fabricate non-polar, semi-polar, and polar GaN wafers in sufficient quantity and at a reasonable price.

Despite the rapid commercialization of III-nitride semiconductor devices for applications in visible and ultraviolet optoelectronics and in high-power and high-frequency electronics, their full potential is limited by two primary obstacles: i) a high defect density and biaxial strain due to the heteroepitaxial growth on foreign substrates, which result in lower performance and shortened device lifetime, and ii) a strong built-in electric field due to spontaneous and piezoelectric polarization in the wurtzite structures along the well-established [0001] growth direction for nitrides. Recent advances in the research, development, and commercial production of native GaN substrates with low defect density and high structural and optical quality have opened opportunities to overcome both of these obstacles and have led to significant progress in the development of several optoelectronic and high-power devices. In this paper, the recent achievements in bulk GaN growth development using different approaches are reviewed; comparison of the bulk materials grown in different directions is made; and the current achievements in device performance utilizing native GaN substrate material are summarized.

The internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) in InGaN light-emitting diodes (LEDs) emitting at 400 nm with and without electron blocking layers (EBLs) on c-plane GaN and m-plane GaN were investigated in order to shed some light on any effect of polarization charge induced field on efficiency killer carrier spillover. Without an EBL the EQE values suffered considerably (by 80%) for both orientations, which is clearly attributable to carrier spillover. Substantial carrier spillover in both polarities, therefore, suggests that the polarization charge is not the major factor in efficiency degradation observed, particularly at high injection levels. Furthermore, the m-plane variety with EBL did not show any discernable efficiency degradation up to a maximum current density of 2250 A cm−2 employed while that on c-plane showed a reduction by ∼ 40%. In addition, IQE of m-plane LED structure determined from excitation power dependent photoluminescence was ∼ 80% compared to 50% in c-plane LEDs under resonant and moderate excitation condition. This too is indicative of the superiority of m-plane LED structures, most probably due to relatively larger optical matrix elements for m-plane orientation.

We investigated the internal quantum efficiency (IQE) and the relative external quantum efficiency (EQE) of m-plane InGaN light emitting diodes (LEDs) grown on m-plane freestanding GaN emitting at ∼ 400 nm for current densities up to 2500 A/cm2. IQE values extracted from intensity and temperature dependent photoluminescence measurements were consistently higher, by some 30%, for the m-plane LEDs than for reference c-plane LEDs having the same structure, e.g., 80% versus 60% at an injected steady-state carrier concentration of 1.2×1018 cm−3. With increasing current injection up to 2500 A/cm2, the maximum EQE is nearly retained in m-plane LEDs, whereas c-plane LEDs exhibit approximately 25% droop. The negligible droop in m-plane LEDs is consistent with the reported enhanced hole carrier concentration and light holes in m-plane orientation, thereby enhanced hole transport throughout the active region, and lack of polarization induced field. A high quantum efficiency and in particular its retention at high injection levels bode well for m-plane LEDs as candidates for general lighting applications.

We investigated internal quantum efficiency (IQE) of polar (0001) InGaN on c-sapphire, and (100) nonpolar m-plane InGaN on both m-plane GaN and specially patterned Si. The IQE values were extracted from the resonant photoluminescence intensity versus the excitation power. Data indicate that at comparable generated carrier concentrations the efficiency of the m-plane InGaN on patterned Si is approximately a factor of 2 higher than that of the highly optimized c-plane layer. At the highest laser excitation employed ( ∼ 1.2×1018 cm−3), the IQE of m-plane InGaN double heterostructure on Si is approximately 65%. We believe that the m-plane would remain inherently advantageous, particularly at high electrical injection levels, even with respect to highly optimized c-plane varieties. The observations could be attributed to the lack of polarization induced field and the predicted increased optical matrix elements in m-plane orientation.

GaInN/GaN multiple quantum well light-emitting diode structures in polar c -axis and non-polar m -axis growth have been compared in terms of luminescence properties. Grown under identical conditions, under low excitation density the c -axis structure has a luminescence maximum at 558 nm while the m -axis structure shows a maximum at 488 nm and shows superluminescence at 485 nm under high photoexcitation density. Under the same conditions, on increasing the excitation power, the peak intensity increases 40 fold in the m -axis structure without any variation of the emission wavelength. In similar but separately grown c -axis structures without a p-side, luminescence shifts from 555 nm at low excitation density to superluminescence at 485 nm under high excitation. The coincidence, of the superluminescence wavelength in the polar structure with the stable peak wavelength in the non-polar one, suggests that the wavelength shift in the polar structure is due to its piezoelectric polarization. The absence of such effects in the m -axis-grown structure therefore suggests a stronger dipole matrix element, potentially enabling higher quantum efficiencies and suitability for high efficiency light-emitting diode and laser diode designs in the green spectral region.

Thick c -plane unintentional doped and iron-doped GaN substrates were grown by hydride vapor phase epitaxial technique on sapphire substrates. The morphology and crystalline quality of the freestanding samples show no evident degradation due to iron doping. Low-temperature photoluminescence measurements show reduction of the exciton-bound to neutral impurities band intensities with iron doping increase. Near-infrared photoluminescence studies confirm the incorporation and activation of iron impurities. Variable temperature resistivity measurements verified that the iron-doped films are semi-insulating.

We report the development of 520–540 nm green light emitting diodes (LEDs) grown along the nonpolar a axis of GaN. GaInN/GaN-based quantum well structures were grown in homoepitaxy on both, a-plane bulk GaN and a-plane GaN on r-plane sapphire. LEDs on GaN show higher, virtually dislocation-free crystalline quality and three times higher light output power when compared to those on r-plane sapphire. Both structures show a much smaller wavelength blue shift for increasing current density (<10 nm for 0.1 to 12.7 A/cm2) than conventional LEDs grown along the polar c axis.

Visible blue light emitting diodes have been produced on freestanding nonpolar GaN (11-20) a-plane substrates by metal-organic chemical vapor deposition. The growth conditions have been optimized for smooth growth morphology of GaN nonpolar homoepitaxial layers without surface features, leading to light emitting diode epitaxial structures that are free of crystalline defects such as threading dislocations and stacking faults. Electroluminescence of light emitting diodes exhibit peak wavelengths of ∼ 450 nm and are independent of current level at low current densities before the heating effects are evidenced.

Wide-bandgap III-nitride-based avalanche photodiodes (APDs) are important for photodetectors operating in UV spectral region. For the growth of GaN-based heteroepitaxial layers on lattice-mismatched substrates such as sapphire and SiC, a high density of defects is introduced, thereby causing device failure by premature microplasma breakdown before the electric field reaches the level of the bulk avalanche breakdown field, which has hampered the development of III-nitride based APDs. In this study, we investigate the growth and characterization of GaN and AlGaN-based APDs on free-standing bulk GaN substrates. Epitaxial layers of GaN and AlxGa1-xN p-i-n ultraviolet avalanche photodiodes were grown by metalorganic chemical vapor deposition (MOCVD). Improved crystalline and structural quality of epitaxial layers was achieved by employing optimum growth parameters on low-dislocation-density bulk substrates in order to minimize the defect density in epitaxially grown materials. GaN and AlGaN APDs were fabricated into 30ìm- and 50ìm-diameter circular mesas and the electrical and optoelectronic characteristics were measured. APD epitaxial structure and device design, material growth optimization, material characterizations, device fabrication, and device performance characteristics are reported.

In this study, we characterized the structural defects in blue and green GaInN/GaN LEDs grown on c-plane bulk GaN and sapphire substrates. Low density large V-defects with diameters around 600 nm were found in the blue LEDs on bulk GaN. They were initiated by edge-type threading dislocations (TDs) around the homoepitaxial growth interface. On the other hand, a high density 7E9 cm-2 of smaller V-defects with sidewalls on 1-101 facets was observed in the active region of green LEDs on sapphire. Their diameter ranges from 150 to 200 nm. Misfit dislocations (MDs) generated in the quantum wells are found to initiate these V-defects. With optimizing the epitaxial growth conditions, the generation of MDs and their smaller V-defects was largely suppressed. As a result, the light output power improved by one order of magnitude. For green LEDs on bulk GaN, another unique type of defect was found in the active region: an inclined dislocation pair (IDP). In it a pair of dislocations propagate at a tilt angle of 18 to 23º from the [0001] growth direction towards <1-100>. This defect seems to be a path of strain relief in the high indium composition quantum wells.

ya Paskova

Large gallium nitride (GaN) crystals were grown using a hydride vapor phase epitaxy (HVPE) technique and were processed into substrates for device applications. Polishing procedures were developed for GaN substrates to produce surfaces prepared for epitaxial growth. Surface preparation of (0 0 0 1) and (0 0 0 1¯) substrates was examined, along with preparation of (1 1 2¯ 0) and (1 1¯ 0 0) non-polar surfaces. For all surfaces, chemical mechanical polishing (CMP) resulted in an average root mean square (RMS) surface roughness on a 5 μm×5 μm scanning probe microscope (SPM) image of <0.2 nm. Characterization of the surfaces of polished substrates by cross-sectional transmission electron microscope (TEM) showed no sub-surface damage and no epitaxial defects generated at the substrate/epi interface during homoepitaxial growth. Cathodoluminescence (CL) imaging was used to verify the defect density and that no defects related to polishing were present. The average dislocation density of the substrates was <5×10 6  cm −2 .

We investigate the role of substrate temperature and gallium flux on the DC and microwave properties of AlGaN/GaN high electron mobility transistors grown by molecular-beam epitaxy on free standing, hydride vapor phase epitaxy grown GaN substrates. The free-standing substrates have threading dislocation densities below 10 7  cm −2 . We find that AlGaN/GaN heterostructures with excellent properties may be grown within a wide range of substrate temperatures and fluxes. Electron Hall mobilities above 1700 cm 2 /V s and sheet resistances below 370 Ω/□ are typical. We are able to obtain high saturated drain currents with low gate leakage. Off-state breakdown voltages as high as 200 V with low drain and gate leakage currents have been measured. Further, we have measured microwave output power densities above 5 W/mm at 4 GHz with a power-added efficiency of 46% and an associated gain of 13.4 dB. We attribute improved electrical properties in these devices to the reduced threading dislocation density compared to those grown on non-native substrates.

Bulk GaN sliced in bars along (11-20) and (1-100) planes from a boule grown in the [0001] direction by HVPE was confirmed as strain free material with a low dislocation density by using several characterization techniques. The high-structural quality of the material allows photoluminescence studies of free excitons, principal donor bound excitons and their two-electron satellites with regard to the optical selection rules. Raman scattering study of the bulk GaN with nonpolar orientations allows a direct access to the active phonon modes and a direct determination of their strain-free positions.

The authors present the fabrication and characterization of vertical-geometry Schottky-type ultraviolet (UV) photodetectors based on a bulk n-GaN substrate. By using low temperature rapid thermal annealing of the semitransparent Schottky contacts (nickel with 7% vanadium), they obtained an ultralow dark current of 0.56 pA at −10 V reverse bias. A responsivity of ∼ 0.09 A/W at zero bias was measured for wavelength shorter than the absorption edge of GaN, and it was found to be independent of the incident power in the range measured (50 mW/m2–2.2 kW/m2). The devices are visible blind, with an UV/visible contrast of over six orders of magnitude. An open-circuit voltage of 0.3 V was also obtained under a broadband UV illumination.

The authors have studied the effects of Fe doping at the interface between GaN epitaxial layers for heterostructure field-effect transistors grown by metal-organic chemical vapor deposition and the corresponding impact on the device characteristics. The epitaxial structures were grown with different Fe-doped GaN layers at the layer-template interface. Analysis of the measured electron and interface charge distributions in the heterostructures demonstrated the important role of Fe doping at the regrowth interface. No charge at the regrowth interface was observed in transistor structures with a thick Fe-doped layer. Characterization of the electrical properties of the transistor structures revealed the presence of high sheet carrier concentrations and improved mobilities with increasing thickness of the Fe-doped GaN layer at the regrowth interface.

We demonstrate homoepitaxial growth of GaInN/GaN-based light emitting diodes (LED) on quasi-bulk GaN with an atomically flat polished surface. The threading dislocation densities of the epitaxial layers were 2–5×10 8  cm −2 which was one order of magnitude less than those grown on c -plane sapphire substrate. The growth defects introduced during the epitaxial process were also one order of magnitude smaller than those grown on the sapphire substrate. The crystalline quality and the optical properties of the epitaxial layer and device performance were much improved. The optical output power of the light emitting diode increased by more than one order of magnitude compared to those on sapphire substrate.

The performance of high power transistor devices is intimately connected to the substrate thermal conductivity. In this study, the relationship between thermal conductivity and dislocation density is examined using the 3 omega technique and free standing HVPE GaN substrates. Dislocation density is measured using imaging cathodoluminescence. In a low dislocation density regime below 10 5  cm −2 , the thermal conductivity appears to plateau out near 230 W/K m and can be altered by the presence of isotopic defects and point defects. For high dislocation densities the thermal conductivity is severely degraded due to phonon scattering from dislocations. These results are applied to the design of homoepitaxially and heteroepitaxially grown HEMT devices and the efficiency of heat extraction and the influence of lateral heat spreading on device performance are compared.

The performance of III-Nitride high power, high frequency transistors and laser diodes is intimately connected with the ability to dissipate heat from the junction to the substrate. The thermal conductivity was characterized by the three omega method for undoped and doped gallium nitride bulk substrates grown by HVPE from room temperature to 450 K. The thickness of the samples varied from thin film epilayers on sapphire to 2 millimeter thick free standing samples Dislocation density of the substrates was measured by imaging cathodoluminescence, SIMS was used to measure impurity levels of oxygen, hydrogen, silicon, and iron, while carrier concentrations and resistivity were determined from electrical measurements and EPR. A semi-insulating, 2 mm thick iron doped sample had the highest thermal conductivity of 230W/K-m at room temperature. Undoped samples had comparable, but lower thermal conductivities throughout the temperature range from 300-450 K. By comparing these results with previously reported experimental results including those on MOCVD grown GaN free of grain boundaries, we establish an empirical relationship in a compact formula that relates the thermal conductivity of GaN and the dislocation density with three different regimes of low, intermediate, and high dislocation densities. In the high dislocation regime, the thermal conductivity improves significantly with reduction of dislocation densities. As material quality continues to improve it remains to be seen if in the low dislocation density regime, thermal conductivities will approach 300 W/K-m or plateau out near 250 W/K-m. As point defects start to limit the thermal conductivity when dislocation density becomes very low, gallium vacancies are expected to play an increasing role. Iron is postulated to substitute on the gallium site. The indication from this study is that iron doping at concentration of 1018 cm-3 is not limiting the thermal conductivity in the 300-450 K range.

The authors experimentally find that the thermal conductivity of gallium nitride depends critically on dislocation density using the 3-omega technique. For GaN with dislocation densities lower than 106 cm−2, the thermal conductivity is independent with dislocation density. The thermal conductivity decreases with a logarithmic dependence for material with dislocation densities in the range of 107–1010 cm−2. These results are in agreement with theoretical predictions. This study indicates that the hydride vapor phase epitaxy method offers an attractive route for the formation of semi-insulating gallium nitride with optimal thermal conductivity values around 230 W/m K and very low dislocation density near 5×104 cm−2.

AlGaN/GaN high electron mobility transistors (HEMTs) by plasma-assisted molecular beam epitaxy on free-standing GaN substrates grown by hydride vapour phase epitaxy (HVPE) have been fabricated. Hall measurements yielded typical electron mobilities of 1750 cm2/Vs with sheet densities of 1.1×1013 cm–2. Off-state breakdown voltages as high as 200 V were measured on unpassivated devices. Output power density at 4 GHz was measured to be 5.1 W/mm at a power-added efficiency of 46% and an associated gain of 13.4 dB. This constitutes significant improvement of RF performance by MBE-grown AlGaN/GaN HEMTs on free-standing HVPE GaN.

We report the performance of GaN p-i-n ultraviolet avalanche photodiodes grown on bulk GaN substrates by metal-organic chemical vapor deposition. The low dislocation density in the devices enables low reverse-bias dark currents prior to avalanche breakdown for ∼ 30 μm diameter mesa photodetectors. The photoresponse is relatively independent of the bias voltage prior to the onset of avalanche gain which occurs at an electric field of ∼ 2.8 MV/cm. The magnitude of the reverse-bias breakdown voltage shows a positive temperature coefficient of ∼ 0.05 V/K, confirming that the avalanche breakdown mechanism dominates. With ultraviolet illumination at λ ∼ 360 nm, devices with mesa diameters of ∼ 50 μm achieve stable maximum optical gains greater than 1000. To the best of our knowledge, this is the highest optical gain achieved for GaN-based avalanche photodiodes and the largest area III-N avalance photodetectors yet reported.

A vertical Schottky diode rectifier was fabricated using a bulk n−GaN wafer. Pt Schottky contacts were prepared on the Ga face and full backside ohmic contact was prepared on the N face by using Ti/Al. The root mean square surface roughnesses of the Ga and N faces are 0.61 and 4.7 nm, respectively. A relatively high breakdown field of 5.46 kV/cm was achieved with no additional edge termination. The breakdown field decreases as the size of the device increases. The background electron concentration of the bulk GaN wafer was low (5×1015 cm−3), which may lead to a relatively high breakdown field even with no special edge termination. The forward turn-on voltage was as low as 2.4 V at the current density of 100 A/cm2. The device exhibited an ultrafast reverse recovery characteristics (reverse recovery time <20 ns).

In this investigation, Schottky diodes with different device sizes (150um, 420um and 700um) were fabricated on the Ga-face of a free-standing n--GaN wafer produced by Kyma Technologies, Inc. Full area back side ohmic contact was prepared on the N-face of the bulk GaN using Ti/Al. Without any edge-termination scheme, a relatively high reverse breakdown voltage of 240V was achieved. The reverse breakdown voltage decreases as the device size increases. The forward turn-on voltage was as low as 2.4V at room temperature for the 150¥ìm diameter Schottky diodes. The best on-state resistance was 7.56 mOhm/cm2 for diodes with VB=240V, producing a figure-of-merit (VB2/RON) of 7.6 MWcm-2. The Schottky diode also showed an extremely short reverse recovery time (< 20 ns) switching from forward bias to reverse bias.

Thick GaN bars with [110] orientation have been sliced from GaN boules grown on freestanding films by hydride vapor phase epitaxy (HVPE) in the [0001] direction. High-resolution x-ray diffraction and transmission electron microscopy have been used to study the structural quality and defect distribution in the material in comparison to heteroepitaxially grown thick HVPE-GaN films grown in the [110] direction on (102)-plane sapphire. It is demonstrated that while the heteroepitaxial material possesses a high density of stacking faults and partial dislocations, leading to anisotropic structural characteristics, the (110)-plane bulk GaN, sliced from boules, exhibits low dislocation density and narrow rocking curves with isotropic in-plane character.

Thick films of hydride vapor phase epitaxy (HVPE) grown GaN were studied by various techniques. Time-integrated and time-resolved photoluminescence (PL) measurements were performed at room temperature and 77 K. The time-integrated PL spectrum has no observed deep-level transitions and a very narrow linewidth, which indicates good material quality. Time-resolved PL spectra are also presented and the temporal evolution of the PL around the band-gap exhibits a biexponential decay with a fast and a slow decay component. Cathodoluminescence, x-ray, and Raman spectroscopy were also used. The full width half maximum of the x-ray rocking curve for our sample is approximately 375 arcsec. The polarized Raman spectra exhibited only the allowed modes. The deposited GaN films were found to be relatively stress free. The x ray and Raman analysis also revealed that the HVPE-grown GaN films are of high crystal quality. The effect of thermal annealing on the sample was also investigated by time-integrated and time-resolved PL and Raman spectroscopy. No significant changes in the material were observed in either time-integrated or Raman spectroscopy. The film was thermally stable upon annealing up to 1000 °C in N2 ambient based on the results of these measurements. In time-resolve photoluminescence measurement, the temporal evolution of the band-edge transitions broadens after each annealing step and is significantly different after the 1000 °C anneal. ©2003 American Vacuum Society.

High quality, single crystal GaN substrates have been demonstrated using a boule growth process. Here we report on the crystalline boules that were formed during the growth process and their material characterization. Using hydride vapor phase epitaxy process, GaN crystals were grown at growth rates greater than 200 µm/hr. Boules greater than 3 mm thick were grown and processed into free-standing substrates. Rocking curve measurements using high-resolution X-ray diffraction were performed on the substrates with FWHM values of 92 and 146 arcsec for the (002) and (102) reflections, respectively. Atomic force microscope images, etch pit studies, and transmission electron micrographs of the GaN material show high quality material quality with a dislocation density in the range of 5×10^6 to 1×10^7 cm-2.