The band-gap of indium nitride has long been believed to be about 1.9eV with slight variations due to band-tailing in polycrystalline samples and degenerate doping. Recently, other values as low as 0.7 eV have apparently been observed. We have compared samples spanning this apparent range of band-gap using secondary ion mass spectroscopy (SIMS), X-ray Photoelectron Spectroscopy (XPS) and heavy ion elastic recoil detection analysis (ERDA), in conjunction with spectral optical density measurements. Once structural inhomogeneiteies are taken into account, we show that much of the conflicting data are compatible with direct photoionisation with a threshold energy of about 1.0eV. This feature was first reported in polycrystalline indium nitride over 15 years ago and attributed to a ∣p> like defect state. We ask whether the feature may instead be a direct band-gap.

AuTiAlTi, AuPdAlTi and AuAlTi ohmic contacts to AlGaN/GaN layers rapid thermal annealed at temperatures up to 950°C have been characterised using conventional and chemical transmission electron microscopy techniques. The relationship between the as-deposited metallic structure, annealing temperature, post-anneal interfacial microstructure and contact resistance is examined.

The presence of a TiN interfacial layer is found to correlate with the onset of ohmic behaviour. Ti and Pd barrier layers are found to be ineffective at stopping the diffusion of Au to the interface. Au is implicated in the development of the inclusions, which are associated with threading dislocations. Once activated, the presence of the inclusions has little influence on the ohmic behaviour of the sample.

The mechanism of Ti/Al reaction to n-GaN was studied to form ohmic contacts with low contact resistivity. N-GaN layers with a carrier concentration of 2.17×1018 cm−3 were deposited on sapphire substrates. Ti/Al metals were deposited by conventional electron-beam techniques. Contact resistivity decreased as the Ti thickness increased, and increased as the Al thickness increased. The lowest contact resistivity was measured at 1.20 × 10−6 Ωcm2 for 80 nm Ti /100 nm Al. After annealing at 900 °C, Al/AlTi/TiN layers on GaN were formed and Al Ti alloy thickness decreased as Ti thickness increased, from 1.5 MeV Rutherford Backscattering Spectroscopy(RBS) measurement. It was found that the contact resistivity was reduced as alloy metal thickness into GaN was increased.

Bulk GaN crystals up to several mm in size, grown by a Na/Ga flux method, have been characterized using room and low temperature photoluminescence (PL) and panchromatic cathodoluminescence (CL) imaging. Highly resolved excitonic PL spectra are obtained for material grown in a new, large-scale reactor. The crystal polarity affects the incorporation of residual Zn and Mg or Si acceptors and the deep level luminescence bands in c-oriented platelets. A Zn (A°,X) triplet structure with unusual thermalization properties and a highly resolved structural defect related PL peak are observed. Striations are found in some of the smaller platelets by CL imaging, but are absent in the prismatic crystals.

This paper described observation of cathodoluminescence (CL) of microcrystalline InGaN bulk crystals under a scanning electron microscope (SEM) with a high-spatial-resolution (HR) CL measuring apparatus. HR-CL spectra from facets of InGaN crystals vary from facet to facet and are single peaked. Histogram analysis of the CL peak positions of HR spectra from the facets of the crystals in the area scanned during a low-resolution CL measurement shows a two-peaked form with comparable peak wavelengths. The diffusion length of a generated electron- ho le pair or an exciton from the recombination centers with a higher-energy-level state to that with a lower state is estimated to be 500 nm at the longest by the comparison of two monochromatic HR-CL images of adjoining facets.

A broad band with a maximum at about 2.9 eV (blue band) is widely observed in the photoluminescence (PL) and cathodoluminescence (CL) spectra of unintentionally doped GaN grown by metalorganic chemical vapor deposition (MOCVD) or by hydride vapor phase epitaxy (HVPE). In some samples this band exhibits fine structure attributed to electron-phonon coupling, in others it appears featureless. Different defect origin and recombination mechanisms responsible for the blue band have been suggested in the past. The situation is complicated by the fact that bands similar in shape and position were observed also in Zn- and Mg-doped GaN, as well as in undoped GaN after dry or wet etching. We investigated PL in HVPE-grown undoped, Si- and Zn-doped GaN layers in wide temperature and excitation intensity ranges. We have found that the shape, temperature and excitation intensity dependencies of the blue band in undoped GaN are almost identical to those in GaN lightly doped with Zn. Moreover, in both undoped and Zn-doped samples we observed a distinctive set of peaks related to an exciton bound to the Zn acceptor. Although the exact structure of the Zn acceptor is still unknown, our experimental results unambiguously demonstrate that Zn impurity is responsible for the blue band in unintentionally doped GaN. The results of transient PL study are also consistent with the above attribution.

We report on the homoepitaxial growth of GaN on freestanding [1100] oriented GaN substrates using metalorganic chemical vapor deposition. A proper pretreatment of the substrates was found to be essential for the GaN homoepitaxy. The influence of growth conditions such as V/III molar-ratio and temperature on the surface morphology and optical properties of epilayers was investigated. Optimized pretreatment and growth conditions led to high quality [1100] oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Based on these GaN epilayer, AlGaN/GaN multiple quantum wells have been grown on the freestanding M-plane GaN. Photoluminescence data confirm that built-in electric field for M-plane structures is very weak, and this situation results in a stronger PL intensity in comparison with C-plane multiple quantum wells in tests at low excitation level.

We report on the in-situ measurement of temperature, i.e., self-heating effects, in multi-finger AlGaN/GaN HFETs grown on SiC substrates. Optical micro-spectroscopy was used to measure temperature with 1m spatial resolution. Thermal resistance (temperature rise per W/mm) was measured as a function of device pitch and gate finger width. There is significant thermal cross talk in multi-finger AlGaN/GaN HFETs and this needs to be seriously considered for device performance and ultimately device reliability. A comparison with theoretical modeling is presented. Uncertainties in modeling parameters currently make modeling less reliable than experimental temperature assessment of devices.

The microstructural evolution of AlxGa1-xN films grown by metalorganic vapor phase epitaxy on 6H-SiC (0001) was studied by means of X-ray diffraction, atomic force microscopy and transmission electron microscopy in conjunction with energy dispersive X-ray spectroscopy. A significant spatial variation of composition was found in 100 nm thick layers the nature of which could be traced back to the initial stage of film formation. Upon nucleation two phases are formed: a wetting layer and isolated islands of high and low aluminum content, respectively. The observed results are discussed in terms of strain and growth rates.

We report on fabrication and measurement of a surface acoustic wave resonator prepared on ∼10m thick GaN(0001) films. The films were grown by metal-organic vapor phase epitaxy on a c-plane sapphire substrate. The surface morphology of the films were examined with scanning electron and atomic force microscopy. A metallic bilayer of Al/Ti was subsequently evaporated on the nitride film surface. Definition of the resonator interdigital transducers, designed for a wavelength of λ=7.76m, was accomplished with standard UV lithography and lift-off. S-parameter measurements showed a resonator center frequency f0=495MHz at room temperature, corresponding to a surface acoustic wave velocity of 3844m/s. The insertion loss at center frequency was measured at 8.2dB, and the loaded Q-factor was estimated at 2200. Finally, measurements of the resonator center frequency for temperatures in the range 25–155°C showed a temperature coefficient of -18ppm/°C. The intrinsic GaN SAW velocity and electromechanical coupling coefficient were estimated at νSAW=383 1m/s and K2=1.8±0.4·10−3.

GaN films were deposited on glass substrates using a compound-source molecular beam epitaxy technique. Electroluminescent devices with a double-insulator structure were also fabricated using the deposited films. When the devices were operated using a sine-wave voltage, one of the emission peaks was located in the UV spectral region. Introducing a small ammonia flow increased the deposition rate.

GaN layers were grown on a (001) rutile TiO2 substrate by electron cyclotron resonance plasma-excited molecular beam epitaxy. For the first time, c-GaN with a preferential growth orientation was obtained. Based on the results from electron diffraction and X-ray diffraction analysis, we found that c-GaN with the growth direction of [110] was grown on the TiO2 substrate. The formation of c-GaN was also confirmed by cathodoluminescence, in which a luminescence peak was observed at 3.24eV.

The existence of excitons of the structure in zinc-blende InxGa1-xN is reported in this paper. The LCAO electron band structure of zinc-blende InxGa1-xN is calculated as function of both the electron wave vector and the electron radius-vector. The observed optical absorption edge in In-rich regions in InxGa1-xN is explained on the basis of this electron band structure. The excitons of the structure are found on the basis of the electron band structure of zinc-blende InxGa1-xN. The binding energy and the hydrogen like energy levels of these excitons are determined. It is found that these excitons are localized. The observed photoluminescence spectrum in In-rich regions of InxGa1-xN is explained by the excitons of the structure. It is found that destroying of these excitons occurs in their interactions with hetero-junction and that the electrons and the holes of exciton origin penetrate in the semiconductor of wider energy band gap. This phenomenon is used for explanation of the observed spectral blue shift of the electroluminescence in the quantum well structures on InxGa1-xN.

Tunnel effects in luminescence spectra and electrical properties of LEDs based on InGaN/GaN-heterostructures made by different technological groups were studied. The tunnel radiation in a spectral region of 1.9 - 2.7 eV predominates at low currents (J<0.2 mA). The position of the tunnel luminescence maximum orħħωmax is approximately equal to the voltage U, orħħωmax = eU. The low energy spectral band is described by the theory of tunnel radiative recombination. Tunnel recombination mechanisms in GaN-based heterostructures are caused by high electric fields in the active InGaN/GaN - MQW layers. The energy diagram of the structures is analyzed. The probability of tunnel radiation is higher due to piezoelectric fields in InGaN quantum wells. The tunnel radiation spectral band was not observed in the more effective LEDs with modulated doped MQWs. The spectra of GaN-based LEDs are compared with tunnel radiation spectra of GaAs-, InP- and GaSb- based LEDs. The equation: orħħωmax = eU describes experimental data in various semiconductors in the range 0.5–2.7 eV.

Most of the work done on GaN has taken into account layers grown on the (0001) sapphire plane. However one would expect the growth on the (1120) plane to lead to different structural defects. As has been shown, in one direction, the mismatch is rather small. In this work, we have carried out structural analysis of nucleation layers grown at temperatures ranging from 600°C to 1100°C. It is shown that for many of the structural parameters, such as the orientation relationships, the layer morphology and the nucleation mechanism critically depend on the growth temperature. At the lowest temperatures, the growth is completely three dimensional with a mixture of the two traditional orientation relationships, but the coalescence thickness is small. In a next step, the A orientation relationship predominates and the layer roughness tends to slightly decrease. This orientation is never perfect, and there is always 1.5° misorientation to the same direction in sapphire, whereas the B orientation is always perfect. At an intermediate temperature, island growth is predominant, whereas towards the high temperature end the B orientation becomes predominant. For the highest growth temperatures, the nucleated layers are completely flat and with the B orientation, although they contain a quite large number of defects such as inversion domains.