Electrical contacts to both n and p-type GaN films have been investigated using electron-beam evaporated and sputtered films of metals such as Al, Au, Cr, Cu, Ni, Pt, and Ti. Films deposited by electron-beam evaporation for the n-type films with doping levels of 1 × 1018/cm3 and lower showed rectifying characteristics with all the metals studied with the exception of Al. Aluminum contact diodes were ohmic in the as-deposited state. The Pt rectifying contact was near-ideal with an ideality factor close to 1.0. Ideality factors for the other metals were much greater than 1. This deviation from thermionic behavior was interpreted as space charge limited current conduction in the presence of deep-level states. Sputtered films showed very similar characteristics to electron-beam deposited films, with the exception of Ti. The Ti contact was ohmic in the as-deposited state. Non-linear Cu contacts to n-type films became ohmic on annealing. However, for p-type films, Ar ion sputter-cleaning prior to metal deposition by sputtering created ohmic contacts with Cu and Pt. Low resistance ohmic contacts were achieved by ion implantation and anneal of Si in n-type and Mg in p-type films, prior to metallization. The implant parameters and anneal temperatures are currently being optimized.

We investigate implantation of high purity HVPE GaN with Mg, Be, C, Zn, Cd, Ca, N, O, P, As, Ne, and Ar. After annealing at 1300 °C, the material is characterized using low temperature photoluminescence (PL). The Mg acceptors exhibit much better optical activation than Be, C, Zn, Cd, or Ca acceptors implanted and annealed under the same conditions. Acceptor-bound exciton peaks and well-resolved donor-acceptor pair bands are observed for both Mg and Zn. A broad peak centered near 2.78 eV is obtained for Cd, confirming that it is deeper than Zn. Isoelectronic As or P exhibit sharp no-phonon bound exciton lines at 2.952 and 3.200 eV, respectively. Defect-related bands centered at 2.2 and 2.35 eV are studied. Both Be and C strongly enhance the yellow (2.2 eV) PL band, but no other impurities do so, including O.

In this study, InN films with thickness up to 7.5 micron were prepared by molecular beam epitaxy (MBE) on (0001) sapphire and quasi-bulk GaN templates. Previously it has been challenging to grow InN film much beyond 1 micron because the growing surface tended to become rough. Techniques to overcome this limit have been developed. Various buffer techniques were used and compared to optimize the epitaxial growth. It was found that with increasing film thickness, Hall mobility will monotonically increase, while carrier concentration decreases. Hall mobility beyond 2100 cm2/Vs with carrier concentration close to 3×1017 cm−3 was obtained at room temperature. Compared with the lowest carrier concentration ∼2×1018 cm−3 obtained on thin InN films grown at the same condition, the conclusion is that impurities from the growth environment are not responsible for the high background doping of InN. Instead, some structural defects or substrate/buffer impurities may be the major source of the unintentional doping, which can be reduced by growing thicker films.

Some results on Mg and Be doping of InN will be reported as well. To date, all Mg and Be doping attempts have resulted in n-type material.

We report flip-chip 325 nm emission light emitting diodes over sapphire with dc powers as high as 0.84 mW at 180mA and pulse powers as high as 6.68 mW at 1A. These values to date are the highest reported powers for such short wavelength emitters. Our data shows the device output power under dc operation to be limited by the package heat dissipation. A study is presented to determine the role of thermal management in controlling the power output for the reported 325 nm ultraviolet light emitting diodes.

Radiative recombination processes in bulk InGaN grown by molecular beam epitaxy (MBE) on lithium gallate (LGO or LiGaO2) substrates were investigated using microscopic PL and time-resolved photoluminescence (TRPL). The improved structural quality resulting from a better lattice match of the LGO substrate to III-V nitride materials simplifies these investigations because well-defined composition phases can be analyzed for both homogeneous and phased separated InGaN samples. Epilayers of InGaN intentionally grown with and without indium segregation were studied. X-ray diffraction measurements showed that the homogeneous epilayer was high quality In0.208Ga0.702N and the segregated epilayer exhibited peaks corresponding to both In0.289Ga0.711N and In0.443Ga0.557N indicating the presence of higher In concentration regions in this sample. Spatially resolved photoluminescence spectra confirm the existence of these regions. The photoluminescence intensity decay is non-exponential for both samples and a stretched exponential fit to the decay data confirms the existence of local potential fluctuations in which carriers are localized before recombination.

Epitaxial layers of GaN contain a very high density of threading dislocations. In the first stage of growth they may form low and high angle grain boundaries. Energetic calculations of <0001> tilt grain boundaries have been performed with the Stillinger-Weber potential modified to take into account the wrong bonds Ga-Ga and N-N. The variation of the energy has been calculated as a function of the rotation angle. Two minima exist with special atomic structures based on a limited number of structural units. They are used to describe the other misorientations in terms of dislocation cores.

The comparative study of photoluminescence (PL) dynamics of wurtzite-type GaN/AlGaN multiple quantum wells (MQWs) fabricated using low-pressure metalorganic chemical vapor deposition technique over GaN coated [0001]-sapphire (C-plane) and single crystalline [1100]-oriented freestanding GaN (M-plane) substrates is presented. The MQWs on C-plane sapphire at low excitation exhibited much lower (∼30 times) PL intensity in comparison with M-plane samples. The C-plane MQWs showed a strong excitation intensity-induced PL spectrum line blueshift (up to 140 meV). Meanwhile identical MQW structures on M-plane substrates demonstrated no PL peak shifts indicating an absence of polarization fields. At higher excitation (>50 kW/cm2) the PL intensity and spectra peak positions for both the C- and the M-plane MQWs become nearly the same and do not differ with subsequent increase of pumping. Theoretical analysis and comparison with PL experimental data revealed strong (up to ∼1.2–1.3 MV/cm) built-in electrostatic fields in the C-plane structures whereas M-plane structures are almost non-polar.

Photoluminescence of the GaN layers grown both on N-face and Al-face bulk AlN is studied under CW and pulsed laser excitation in the temperature range from 8 K to 300 K. We compare localization of excitons, residual strain, and activation energies for thermally activated transfer of carriers to nonradiative recombination. At high excitation intensities, conditions for carrier heating, which is important for the threshold of stimulated emission, are also investigated.

The effect of oxygen pressure on MgO grown by RF plasma assisted gas-source molecular beam epitaxy was investigated. Increasing oxygen pressure was found to decrease the growth rate, improve the morphology and reduce the Mg/O ratio to near that obtained from bulk single crystal MgO. By contrast, the electrical characteristics of MgO/GaN diodes showed continual improvement in breakdown field and interface state density as the pressure was decreased. The lowest pressure tested, 1×10−5Torr, produced the lowest Dit, 3×1011 eV−1cm−2, and the highest VBD, 4.4 MV/cm. Cross sectional transmission electron microscopy of the MgO grown at the lowest pressure showed the initial 40 monolayers to be epitaxial, with the remainder of the layer appearing to be fine grained poly-crystal. Comparisons with films grown using an electron cyclotron resonance (ECR) plasma suggest that higher ion energies are desirable for obtaining the best electrical characteristics.

Growth of pendeo-epitaxial (PE) layers introduces misorientation between the seed layers and the overgrown wing layers. The origin of this misorientation has been studied by Transmission Electron Microscopy (TEM) using a set of samples in which subsequent procedures utilized in PE were applied, i.e. growth of GaN template, stripe etching, annealing at the growth temperature of the PE layers and final PE growth. It was shown that etching of seed-stripes did not change the type of defects or their distribution. However, heating to the PE growth temperature drastically modified the surface and V-shaped pits were formed. The surface became smooth again after the PE growth took place. Overgrowth of the V-shaped pits resulted in formation of edge threading dislocations over a seed-stripe region with a dislocation density of 8.0×108 cm−2. Formation of new edge dislocations over the seed can have an influence on the misorientation between the PE grown regions.

Herein, we discuss the use of a novel new substrate for III-Nitride epitaxy, Lithium Niobate. It is shown that Lithium Niobate (LN) has a smaller lattice mismatch to III-Nitrides than sapphire and can be used to control the polarity of III-Nitride films grown by plasma assisted molecular beam epitaxy. Results from initial growth studies are reported including using various nitridation/buffer conditions along with structural and optical characterization. Comparisons of data obtained from GaN and AlN buffer layers are offered and details of the film adhesion dependence on buffer layer conditions is presented. Lateral polarization heterostructures grown on periodically poled LN are also demonstrated. While work is still required to establish the limits of the methods proposed herein, these initial studies offer the promise for mixing III-Nitride semiconductor materials with lithium niobate allowing wide bandgap semiconductors to utilize the acoustic, pyroelectric/ferroelectric, electro-optic, and nonlinear optical properties of this new substrate material as well as the ability to engineer various polarization structures for future devices.

Room temperature time-resolved photoluminescence (TRPL) studies of multiple quantum well (MQW) structures of the binaries GaN and AlN grown by molecular beam epitaxy are reported. The eventual application of these structures is for GaN intersubband IR light emitters. However, as an initial study, the structures are evaluated at UV to investigate materials parameters relevant to IR light emission. The nominally 0.9, 1.3 and 1.5 nm GaN quantum wells are clad by 6nm of AlN on top of a thick AlN buffer grown on sapphire. All samples consisted of 20 quantum wells. The observed peak energy of the emission spectrum is in excellent agreement with a model that includes the strong confinement present in these structures and the existence of the large built-in piezoelectric field and spontaneous polarization present inside the wells. Furthermore, consistent with screening of the in-well field as carriers are injected in the well, a clear blue shift of the emission is observed at short times after carrier injection. Subsequently, as the carriers recombine, the peak emission red-shifts and the screening of the field is reduced. Moreover, the observed lifetimes were energy dependent as should be expected from field dependent elongation of lifetimes due to spatial separation of the injected carriers. Specifically, the decay time at high energies can be fitted by a stretched exponential with a beta value of 0.8 which is consistent with carrier spatial separation. The lifetimes obtained from the fitting are of the order of 1ns, longer than the reported recombination lifetimes in similar GaN/AlGaN MQW's. On the low energy side of the PL feature the intensity time decay becomes exponential with lifetimes ranging from 3 to 10ns. The strong UV emission at room temperature makes these structures promising for UV emitters.

For ohmic contact on p GaN, palladium is one of the best candidates showing ohmic characteristics already without annealing. To be realized in devices, it is necessary to know the behavior of the ohmic contacts at accelerated conditions, especially for high temperatures and power. We report on the structural evolution of palladium layers (30 nm) deposited on GaN (0001) by electron beam evaporation without intentional annealing. They were next cut into various pieces which were individually submitted to rapid thermal annealing at 400, 500, 600, 700 and 800°C for 10 sec. We investigate the differences in the microstructure and the location of interfacial phases and their relationships as determined by X-ray diffraction and transmission electron microscopy, we then suggest the formation mechanism based on the relationship. It is shown that the interface is disrupted at annealing above 600°C and by 800°C only very small patches of Pd are still present, however they area completely imbedded in a matrix of intermetallic phases (gallides) formed by the reaction with GaN.

Single crystalline InN films were grown on Si substrates by radio-frequency plasma-excited molecular beam epitaxy. Electrical property of InN/Si heterojunction was investigated. We obtained rectifying characteristics in n-InN/p-Si heterostructure for the first time. Forward I-V characteristics were affected by both the buffer layer deposition and the nitridation process. Strong photoluminescence peaks for both single crystalline and polycrystalline InN films grown on the Si substrates were observed at around 0.8 eV, which were smaller than the previous reported PL emission peak of around 1.9 eV.

We have characterized the surface morphology and luminescence properties of GaN/AlN/ SiC layers of various thicknesses using secondary electron imaging (SEI), panchromatic room temperature cathodoluminescence (CL), atomic force microscopy (AFM), optical Nomarski microscopy, and room and low temperature photoluminescence (PL). The nominally undoped GaN layers were grown by MOCVD on 0.1 m thick AlN buffer layers on commercial 6H-SiC(0001) substrates. The GaN layer thicknesses are 0.5, 1.0, 1.6, and 2.6 m. A second 1.0 m thick layer was grown by identical procedures on a 6H-SiC substrate that was first etched in H2 to remove scratches and damage due to mechanical polishing. Biaxial compressive lattice mismatch stress is present in all layers and decreases with increasing layer thickness, while PL linewidths decrease. The 1 m layer on the H-etched substrate is as relaxed as the 2.6 m layer on a non H-etched substrate, however. Pronounced surface structures, apparently corresponding to columnar subgrain boundaries, are observed on the samples on non H-etched SiC. Their typical sizes increase from about 3 to 10 m with increasing layer thickness. They are absent in the H-etched sample. These structures are generally nonradiative in CL images, although mottled contrast is also observed inside them. Similar layers doped with 3×1018 cm−3 Si do not show these features, suggesting a different microstructure.

M-plane GaN(1100) is grown by plasma assisted molecular beam epitaxy on ZnO(1100) substrates. A low-temperature GaN buffer layer is found to be necessary to obtain good structural quality of the films. Well oriented (1100) GaN films are obtained, with a slate like surface morphology. On the GaN(1100) surfaces, reconstructions with symmetry of c(2×2) and approximate “4×5” are found under N- and Ga-rich conditions, respectively. We propose a model for Ga-rich conditions with the “4×5” structure consisting of ≥ 2 monolayers of Ga terminating the GaN surface.

This paper deals with defects induced by proton irradiation in n-GaN. The samples were HVPE grown, irradiated with 24 GeV protons. DLTS was performed on both as-grown and irradiated samples with planar and normal collector configurations to evidence the dislocation effect. Two electron traps were identified in the as-grown material by both diode structures: trap EC1 (EC – 0.19eV) and trap EC2 (EC – 0.25eV), EC2 being the dominant one. Irradiation consistently affects the pre-existing levels in such a way so as their appearance strongly depends on the diode structure, hence on the region probed by DLTS.

This contribution focuses on the actual existence of two deep traps EC1 and EC2 emitting in GaN at low temperature and on the different nature of the deep levels associated to EC1 and EC2. Their filling kinetics was studied since we supposed that EC2 would be associated to extended defects, as already reported in literature. Indeed, the site density of EC2 logarithmically depends on the filling pulse width, demonstrating that this trap is definitely associated to extended defects. The trap EC1, on the contrary, exhibits the filling kinetics peculiar of point defects.

GaN growth on Si substrate is very attractive for realizing low cost electronic devices. We grew a thin GaN film on p-type Si(111) substrate using AlGaN high temperature buffer without using a conventional low temperature buffer. A homogeneous buffer layer was obtained at 1093 K and a homogenous 500 nm thick GaN layer was also obtained without any crack. Using a transmission electron microscopy (TEM), we observed that the cross-section of GaN and AlGaN buffer was very smooth and also the surface of GaN was flat although the threading dislocations were observed. Furthermore, we directly fabricated a metal semiconductor field effect transistor (MESFET) using a 500 nm-thick GaN/Si without any high resistive GaN layer. A Schottky electrode was Pt/Au and an ohmic electrode was Al/Ti/Au. A Schottky breakdown voltage was over 100 V. Also, we confirmed a high temperature operation of the MESFET using a thin GaN film on Si substrate at 573 K.

Temperature programmed desorption (TPD) was performed on deuterated GaN(0001) surfaces which had been exposed to various doses of 90-eV electrons. TPD of the deuterated surface without electron exposure shows a broad D2 desorption feature with a peak desorption temperature at ∼400 °C. Electron exposure results in a decrease in intensity of the desorption peak which is attributed to removal of surface deuterium by electron stimulated desorption (ESD). This removal of deuterium by ESD produces no change in the peak desorption temperature indicating that recombinative desorption is first order in deuterium coverage.

Annealing experiments were carried out on gallium nitride layers, which were grown on sapphire through Metal Organic Chemical Vapor Deposition (MOCVD). Rutherford Backscattering Spectrometry (RBS) was performed on as-grown and annealed GaN samples using a 2 MeV proton beam to study the stoichiometric changes in the near-surface region (750 nm) with depth resolution better than 50 nm. No decomposition was measured for temperatures o up to 800 °C. Decomposition in the near-surface region increased rapidly with a further increase o of temperature, resulting in a near-amorphous surface-region for annealing at 1100 °C. The depth profiles of nitrogen and incorporated oxygen in the decomposed GaN are extracted from the nanoscale RBS data for different annealing temperatures. The surface roughness of the GaN layers observed by atomic force microscopy (AFM) is consistent with RBS decomposition measurements. We describe the range of annealing conditions under which negligible decomposition of GaN is observed, which is important in assessing optimal thermal processing conditions of GaN for both conventional and nanoscale optoelectronic devices.