We report on the interaction of native point defects with commonly observed planar defects in GaN. Using a pair potential model we find a positive binding energy for all native defects to the three boundary structures investigated indicating a preference for native defects to form in these interfaces. The binding energy is highest for the Ga interstitial and lowest for vacancies. Interstitials, which are not thought to occur in significant concentrations in bulk GaN, should form in the (11 0) IDB and the (10 0) SMB and consequently alter the electronic structure of these boundaries.

AlxGa1−xN {x=30% (doped and undoped), 45% (doped)} thin films were grown by MOCVD on ∼2 μm thick GaN layer using Al2O3 substrate. These films were designed to be the active parts of HFETs with nsμ product of about 1016(Vs)-1. The layers were then studied by means of transmission electron microscopy (TEM) techniques. In this paper, it is shown that the AlxGa1−xN layer thickness was non-uniform due to the presence of V-shaped defects within the AlxGa1−xN films. The nucleation of these V-shaped defects has taken place about 20 nm above the AlxGa1−xN/GaN interface. Many of these V-shaped defects were associated with the presence of the threading dislocations propagating from the GaN/Al2O3 interface. We show that the density of these V-shaped defects increases with the doping level and also with the Al mole fraction in the films. The formation mechanism of the V-shaped defects seems to be related to the concentration of dopants or other impurities at the ledges of the growing film. This suggestion is supported by high resolution TEM analysis. The growth front between the V-shaped defects in the lower Al concentration thin films was planar as compared with the three-dimensional growth in the doped, higher Al concentration film. This interpretation of the origin of the V-shaped defects is consistent with the observed lowering of the Schottky barrier height in n-doped AlGaN/Ni Schottky diodes.

Deliberate oxygen doping of GaN grown by MOVPE has been studied. The electron concentration increased as the square root of the oxygen partial pressure. Oxygen is a shallow donor with a thermal ionization energy of 27 ±2 meV. A compensation ratio of Θ = 0.3-0.4 was determined from Hall effect measurements. The formation energy of ON of EF = 1.3 eV, determined from the experimental data, is lower than the theoretically predicted value.

Si-doping of cubic GaN epilayers grown by an rf plasma-assisted molecular beam epitaxy on semi-insulating GaAs (001) substrates is investigated by secondary ion mass spectroscopy (SIMS), photoluminescence (PL) and by Hall-effect measurements. SIMS measurements show a homogeneous incorporation of Si in cubic GaN epilayers up to concentrations of 5*1019 cm−3. PL shows a clear shift of the donor-acceptor emission to higher energies with increasing Si-doping. Above a Si-flux of 1*1011cm−2s−1 the near band edge lines merge to one broad band due to band gap renormalization and conduction band filling effects. The influence of the high dislocation density (≈1011cm−2) in c-GaN:Si on the electrical properties is reflected in the dependence of the electron mobility on the free carrier concentration. We find that dislocations in cubic GaN act as acceptors and are electrically active.

For the first time, p-type doping through beryllium implantation in gallium nitride was achieved by using a new annealing process, in which the sample was first annealed in forming gas (12% H2 and 88% N2), followed by annealing in pure nitrogen. Variable temperature Hall measurements showed that sheet hole concentrations of the annealed samples were about 1×1013 cm−2 with low hole mobilities. An ionization energy of 127 meV was estimated with a corresponding activation efficiency of ∼ 100%. SIMS results revealed a relationship between the enhanced diffusion of Be and activation of the acceptors.

We investigated the doping characteristics of Mg doped, Mg-Si co-doped, and Mg-Zn co-doped GaN films grown by metalorganic chemical vapor deposition. We have grown p-GaN film with a resistivity of 1.26 Ωcm and a hole density of 4.3 × 1017 cm−3 by means of Mg-Si co-doping technique. The Mg-Si co-doping characteristic was also explained effectively by taking advantage of the concept of competitive adsorption between Mg and Si during the growth. For Mg-Zn co-doping, p-GaN showing a low electrical resistivity (0.7 Ωcm) and a high hole concentration (8.5 × 1017 cm−3) was successfully grown without the degradation of structural quality of the film. Besides, the measured specific contact resistance for Mg-Zn co-doped GaN film is 5.0 × 10−4 cm2, which is lower value by one order of magnitude than that for only Mg doped GaN film (1.9 × 10−3Ωcm2).

Mg-doped superlattices consisting of uniformly doped AlxGa1−xN and GaN layers are analyzed by Hall-effect measurements. Acceptor activation energies of 70 meV and 58 meV are obtained for superlattice structures with an Al mole fraction of x = 0.10 and 0.20 in the barrier layers, respectively. These energies are significantly lower than the activation energy measured for Mg-doped GaN thin films. At room temperature, the doped superlattices have free hole concentrations of 2 × 1018 cm−3 and 4 × 1018 cm−3 for x = 0.10 and 0.20, respectively. The increase in hole concentration with Al content of the superlattice is consistent with theory. The room temperature conductivity measured for the superlattice structures are 0.27 S/cm and 0.64 S/cm for an Al mole fraction of x = 0.10 and 0.20, respectively.

We have measured the doping concentration dependence of the room temperature thermal conductivity (κ) of two series of n-GaN/sapphire (0001) fabricated by hydride vapor phase epitaxy (HVPE). In both sets κ decreased linearly with log n, the variation being about a factor two decrease in κ for every decade increase in n. κ ≈ 1.95 W/cm-K was obtained for one of the most lightly doped samples (n = 6.9×1016 cm−3), higher than the previously reported κ ≈ 1.7-1.8 W/cm-K on lateral epitaxial overgrown material [V.A. Asnin et al, Appl. Phys. Lett. 75, 1240 (1999)] and κ ≈ 1.3 W/cm-K on a thick HVPE sample [E.K. Sichel and J.I. Pankove, J. Phys. Chem. Solids 38, 330 (1977)]. The decrease in the lattice component of κ due to increased phonon scattering from both the impurities and free electrons outweighs the increase in the electronic contribution to κ.

The hardness of single crystal GaN (gallium nitride) at elevated temperature is measured for the first time and compared with other materials. A Vickers indentation method was used to determine the hardness of crack-free GaN samples under an applied load of 0.5N in the temperature range 20 - 1200°C. The hardness is 10.8 GPa at room temperature, which is comparable to that of Si. At elevated temperatures GaN shows higher hardness than Si and GaAs. A high mechanical stability for GaN at high temperature is deduced.

We report on the effect of strain induced polarization fields in AlGaN/GaN heterostructures due to the incorporation of Si dopant ions in the lattice. By Si-doping (Al)GaN, a contraction of the wurtzite unit cell can occur leading to strain in doped AlGaN/GaN heterostructures such as high electron mobility transistors (HEMTs). In a typical modulation doped AlGaN/GaN HEMT structure, the Si-doped AlGaN supply layer is separated from the two-dimensional electron gas channel by an undoped AlGaN spacer layer. This dopant-induced strain, which is tensile, can create an additional source of charge at the AlGaN:Si/AlGaN interface. The magnitude of this strain increases as the Si doping concentration increases and the AlN mole fraction in the AlGaN decreases. Consideration of this strain should be given in AlGaN/GaN HEMT structure design.

We report on non-thermal negative differential conductivity (NDC) in AlGaN/GaN HEMTs grown on sapphire substrates by low-pressure MOCVD. The sheet electron density was on the order of few times 1012cm−2 and the Hall mobility was 1,000 cm2/V.s. The HEMTs had threshold voltage close to zero and could operate at high positive gate bias up to 3 to 3.5 Volts, with a very low gate leakage current. NDC was observed at the gate bias larger than 1.5V and at the drain biases between approximately 0.5Vg and Vg. We excluded the possibility of self-heating as the cause, since the NDC occurs at relatively small power levels where self-heating effects are negligible.

An explanation we provided for the NDC effect is the new mechanism of real space charge transfer from 2D to 3D GaN states, which leads to a decrease in the channel mobility at large 2D electron gas densities. The observed low leakage can be explained by an enhanced molar fraction of aluminum at the heterointerface that results in a larger conduction band discontinuity. Our model that accounts for the piezoelectric and pyroelectric effects is consistent with the observed NDC effect. The Hall mobility dependence on the gate bias and sheet carrier concentration [1] is consistent with the real space transfer mechanism.

This NDC effect in GaN/AlGaN HEMTs may find applications in high-performance digital circuits at elevated temperatures.

Two-dimensional electron gas transport properties have been investigated in nitride double-heterostructures. A striking effect has been observed that the two-dimensional electron gas mobility has been drastically enhanced in the AlGaN/GaN/AlGaN double-heterostructure, compared with that in the conventional AlGaN/GaN single-heterostructure. The observed mobility enhancement has been shown to be mainly due to the enhanced polarization-induced electron confinement in the double-heterostructure, and additionally due to the improvement of the interface roughness in the structure. Device operation of an AlGaN/GaN/AlGaN double-heterostructure field effect transistor has been demonstrated: a maximum transconductance of 180 mS/mm has been obtained for a 0.4 μm-gate-length device. In the double-heterostructure using InGaN channel, the increased capacity for the two-dimensional electron gas has been observed. The AlGaN/(In)GaN/AlGaN double-heterostructures are effective for improving the electron transport properties.

High-quality GaN was grown using gas-source molecular-beam epitaxy (GSMBE). The mobility of undoped GaN was 350 cm2/Vsec and the carrier concentration was 6×1016 cm−3 at room temperature. A GaN metal semiconductor field-effect transistor (MESFET) and an n-p-n GaN bipolar junction transistor (BJT) were fabricated for high-temperature operation. The high-temperature reliability of the GaN MESFET was also investigated. That is, the lifetime of the FET at 673 K was examined by continuous current injection at 673 K. We confirmed that the FET performance did not change at 673 K for over 1010 h. The aging performance of the BJT at 573 K was examined during continuous current injection at 573 K for over 850 h. The BJT performance did not change at 573 K. The current gain was about 10. No degradation of the metal-semiconductor interface was observed by secondary ion-mass spectrometry (SIMS) and transmission electron microscopy (TEM). It was also confirmed by using Si-ion implantation that the contact resistivity of the GaN surface and electrode materials could be lowered to 7×10−6 ohmcm2.

Junction field effect transistors (JFET) were fabricated on a GaN epitaxial structure grown by metal organic chemical vapor deposition. The DC and microwave characteristics, as well as the high temperature performance of the devices were studied. These devices exhibited excellent pinch-off and a breakdown voltage that agreed with theoretical predictions. An extrinsic transconductance (gm) of 48 mS/mm was obtained with a maximum drain current (ID) of 270 mA/mm. The microwave measurement showed an fT of 6 GHz and an fmax of 12 GHz. Both the ID and the gm were found to decrease with increasing temperature, possibly due to lower electron mobility at elevated temperatures. These JFETs exhibited a significant current reduction after a high drain bias was applied, which was attributed to a partially depleted channel caused by trapped electrons in the semi-insulating GaN buffer layer.

We are attempting to grow bulk AlN that would be suitable as a substrate for nitride film growth. Bulk AlN films were grown by physical vapor transport on 3.5° off-axis and on-axis 6H SiC seed crystals and characterized by TEM, x-ray-diffraction, Auger electron microscopy, and SEM. TEM images show that the bulk AlN does not have the columnar structure typically seen in AlN films grown by MOCVD. Although further optimization is required before the bulk AlN is suitable as a substrate, we find that the structural characteristics achieved thus far indicate that quality bulk AlN substrates may be obtained in the future.

The thermal expansion of different GaN samples is studied by high-resolution X-ray diffraction within the temperature range of 10 to 600 K. GaN bulk crystals, a homoepitaxial layer and different heteroepitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were investigated. Below 100 K the thermal expansion coefficients (TEC) were found to be nearly zero which has to be taken into account when estimating the thermal strain of GaN layers in optical experiments commonly performed at low temperatures. The homoepitaxial layer and the underlying GaN substrate with a lattice mismatch of −6·10−4 showed identical thermal expansion. The comparison between the temperature behavior of lattice parameters of heteroepitaxial layers and bulk GaN points to a superposition of thermally induced biaxial strain and compressive hydrostatic strain.

Successive growth of thick GaN layers separated by either LT-GaN or LT-AlN interlayers have been investigated by transmission electron microscopy techniques. One of the objectives of this growth method was to improve the quality of GaN layers by reducing the dislocation density at the intermediate buffer layers that act as barriers to dislocation propagation. While the use of LT-AlN results in the multiplication of dislocations in the subsequent GaN layers, the LT-GaN reduces dislocation density. Based upon Burgers vector analysis, the efficiency of the buffer layers for the propagation of the different type of dislocations is presented. LT-AlN layer favor the generation of edge dislocations, leading to a highly defective GaN layer. On the other hand, the use of LT-GaN as intermediate buffer layers appears as a promising method to obtain high quality GaN layer.

In this paper we illustrate the application of electron beam techniques to the measurement of strain, defect and alloy concentrations in nitride thin films. We present brief comparative studies of CL spectra of AlGaN and InGaN epilayers and EBSD patterns obtained from two silicon-doped 3 μm thick GaN epilayers grown on an on-axis (0001) sapphire substrate and a sapphire substrate misoriented by 10° toward the m-plane (10 0).

We report the growth and characterization of quaternary AlGaInN. A combination of photoluminescence (PL), high-resolution x-ray diffraction (XRD), and Rutherford backscattering spectrometry (RBS) characterizations enables us to explore the contours of constant- PL peak energy and lattice parameter as functions of the quaternary compositions. The observation of room temperature PL emission at 351nm (with 20% Al and 5% In) renders initial evidence that the quaternary could be used to provide confinement for GaInN (and possibly GaN). AlGaInN/GaInN MQW heterostructures have been grown; both XRD and PL measurements suggest the possibility of incorporating this quaternary into optoelectronic devices.

The N-side of GaN single crystals with off-angle orientations of 0°, 2°, and 4° towards the [100] direction was used as a substrate for homo-epitaxial MOCVD growth. The highest misorientation resulted in a reduction of the density of grown hillocks by almost two orders of magnitude as compared with homo-epitaxial films grown on the exact (000) surface. The features still found on the 4° misoriented sample after growth can be explained by a model involving the interaction of steps, introduced by the misorientation and the hexagonal hillocks during the growth process.