The thermal expansion of different GaN samples is studied by high-resolution Xray 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.

GaN decomposition is studied as a function of pressure and temperature in mixed NH3 and H2 flows more characteristic of the MOVPE growth environment. As NH3 is substituted for the 6 SLM H2 flow, the GaN decomposition rate at 1000 °C is reduced from 1x1016 cm-2s-1 (i.e. 9 monolayers/s) in pure H2 to a minimum of 1x1014 cm-2s-1 at an NH3 density of 1x1019 cm-3. Further increases of the NH3 density above 1x1019 cm-3 result in an increase in the GaN decomposition rate. The measured activation energy, EA, for GaN decomposition in mixed H2 and NH3 flows is less than the EA measured in vacuum and in N2 environments. As the growth pressure is increased under the same H2 and NH3 flow conditions, the decomposition rate increases and the growth rate decreases with the addition of trimethylgallium to the flow. The decomposition in mixed NH3 and H2 and in pure H2 flows behave similarly, suggesting that surface H plays a similar role in the decomposition and growth of GaN in NH3.

Carrier concentration and mobility were measured for intrinsic cubic InN and GaN, and for Si-doped cubic GaN as a function of temperature. Metallic n-type conductivity was found for the InN, while background p-type conductivity was observed for the intrinsic GaN layer. Doping the cubic GaN with Si two regimes were observed. For low Si-doping concentrations, the samples remain p-type. Increasing the Si-doping level, the background acceptors are compensated and the samples became highly degenerated n-type. From the carrier concentration dependence on temperature, the activation energy of the donor and acceptor levels was determined. Attempts were made to determine the scattering mechanisms responsible for the behavior of the mobility as a function of temperature.

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 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 20) IDB and the (10 10) SMB and consequently alter the electronic structure of these boundaries.

InGaN multi-quantum-well laser diodes have been fabricated on fully-coalesced laterally epitaxially overgrown (LEO) GaN on sapphire. The laterally overgrown ‘wing’ regions as well as the coalescence fronts contained few or no threading dislocations. Laser diodes fabricated on the low-dislocation-density wing regions showed a reduction in threshold current density from 8 kA/cm2 to 3.7 kA/cm2 compared the those on the high-dislocation ‘window’ regions. Laser diodes also showed a twofold reduction in threshold current density when comparing those on the wing regions to those fabricated on conventional planar GaN on sapphire. The internal quantum efficiency also improved from 3% for laser diodes on conventional GaN on sapphire to 22% for laser diodes on LEO GaN on sapphire.

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 fr 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.

High quality zinc oxide (ZnO) films were epitaxially grown on R-plane sapphire substrates by metalorganic chemical vapor deposition at temperatures in the range 350-600°C. In-situ nitrogen compensation doping was performed using NH3. The metalsemiconductor-metal ultraviolet-sensitive photodetectors were fabricated on nitrogencompensation-doped epitaxial ZnO films. The photoresponsivity of these devices exhibits a linear dependence upon bias voltage up to 10 V, with a photoresponsivity of 400 A/W at 5 V. The rise and fall times are 1 and 1.5 μs, respectively.

InGaN thin films were grown by low-pressure metalorganic-vapor-phase-epitaxy (MOVPE) and characterized by cathodoluminescence (CL) and scanning electron microscopy (SEM). SEM images showed that InGaN samples have inverted hexagonal pits which are formed by the In segregation on the (1011) surfaces. Room temperature CL at the wavelengths corresponding to the GaN band edge, the In-poor and In-rich regions showed that the In–rich regions formed at the periphery of the hexagonal pits.

Silicon doped layers GaN were grown with MOCVD on sapphire substrates using silane as silicon precursor. The influence of the silicon doping concentration on the physical and optical properties is investigated. A linear relationship is found between the silane-input molfraction and the free carrier concentration in the GaN layers. The morphology of the samples is drastically changed at high silicon concentrations. Photoluminescence was used to probe bandgap variations as function of the silicon concentration. Increasing of the doping concentration led to a continuous shift of the exciton related PL to lower energies, while the intensity of the UV emission was found to increase up to a carrier concentration of n=2.5×1018 cm−3.

Ti/Pt/Au metallization on p-type GaN/AlxGa1-xN (x=0.10 and 0.20) superlattices (SL) were investigated as ohmic contacts. Current-voltage and specific contact resistance measurements indicate enhanced p-type doping in the superlattice structures compared to that in GaN. Ti/Pt/Au is shown to be an effective ohmic metallization scheme on p-type GaN/AlxGa1-xN superlattices. A specific contact resistance of Rc = 4.6×10-4 Ω-cm2 is achieved for unalloyed Ti/Pt/Au on GaN/Al0.2Ga0.8N SL. This is reduced to 1.3×10-4 Ω-cm2 after annealing for 5 minutes at 300 °C.

Ion implantation is used to investigate the spectroscopic properties of Mg, Be, and C acceptors in GaN. Activation of these species is studied using low temperature photoluminescence (PL). Low dose implants into high quality undoped hydride vapor phase epitaxial (HVPE) material are used in conjunction with high temperature (1300 °C) rapid thermal anneals to obtain high quality spectra. Dramatic, dose-dependent evidence of Mg acceptor activation is observed without any co-implants, including a strong, sharp neutral Mg acceptor-bound exciton and strong donor-acceptor pair peaks. Variable temperature measurements reveal a band-to-acceptor transition, whose energy yields an optical binding energy of 224 meV. Be and C implants yield only slight evidence of shallow acceptor-related features and produce dose-correlated 2.2 eV PL, attributed to residual implantation damage. Their poor optical activation is tentatively attributed to insufficient vacancy production by these lighter ions. Clear evidence is obtained for donor-Zn acceptor pair and acceptor-bound exciton peaks in Zn-doped HVPE material.

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.

Growth of GaN and AlxGa1−xN thin films on 6H-SiC(0001) and Si(111) substrates with low densities of defects using the PENDEO™ process and the characterization of the resulting materials are reported. The application of a mask on the GaN seed structures hinders the vertical propagation of threading dislocations of the seed material during regrowth, but introduces a misregistry in the overgrowing material resulting in low quality crystal growth. This misregistry has been eliminated due to advanced processing and the exclusion of the masking layer. The new generation of samples do not show any misregistry, as shown by transmission electron microscopy.

Photoluminescence (PL) enhancement due to the screening of piezoelectric field induced by Si-doping is systematically studied in GaN/AlGaN quantum wells (QWs) fabricated by metal organic vapor-phase-epitaxy (MOVPE). The PL enhancement ratio of QWs for Si-doped directly into the wells was much larger than that for doped only into the barrier layers. This result shows that the crystal quality of the quantum well is not so damaged by heavy Si-doping, which is different from the cases of GaAs or InP material systems. The PL intensity enhancement ratio was especially large for thick wells. The typical value of the enhancement ratio was 30 times for a 5 nm-thick single QW. The optimum Si-doping concentration was approximately 4×1018 cm-3. From the well width dependence of the PL enhancement ratio and PL peak shift under high excitation conditions, we determined that the dominant effect inducing the PL enhancement is screening of piezoelectric field in the QWs. These results indicate that Si-doping is very effective for the application of GaN/AlGaN QWs to optical devices.

Visible-blind UV cameras based on a 32 × 32 array of backside-illuminated GaN/AlGaN p-i-n photodiodes have been successfully demonstrated. The photodiode arrays were hybridized to silicon readout integrated circuits (ROICs) using In bump bonds. Output from the UV cameras were recorded at room temperature at frame rates of 30-240 Hz. These new visible-blind digital cameras are sensitive to radiation from 285-365 nm in the UV spectral region.

We have shown that Zr-based metallization can effectively remove hydrogen from the p-type GaN subsurface, which eventually leads to the formation of an ohmic contact. As the release of hydrogen starts at ∼900°C, the thermal stability of the contact system is of particular importance. The remarkable thermal behavior of the ZrN/ZrB2 metallization is associated to the microstructure of each individual Zr-based compound, as well as to the interfacial crystalline accommodation.

Exposure of wurtzite GaN films grown on Si-polar 6H-SiC(0001) to magnesium during molecular beam epitaxy (MBE) has been studied. In the nitrogen rich regime of MBE growth, GaN films are known to grow with rough morphology. We observe on GaN(0001) that small doses of Mg act as a surfactant, smoothing out this roughness. An interpretation of this surfactant behavior is given in terms of electron counting arguments for the surface reconstructions. Previously, we have reported that larger doses of Mg lead to inversion of the Ga-polar GaN film to produce N-polar GaN. Several Mg-related reconstructions of the resulting GaN(000 ) surface are reported.

A series of experiments were performed to explore the growth of InN by Molecular Beam Epitaxy (MBE). The growth conditions were optimized based on the study of RHEED during growth and InN dissociation experiments. Characterization of the InN thin films were performed by various techniques such as TEM and XRD.

An optical characterization technique is proposed for GaN based compounds deposited on sapphire. In AlGaN films grown by MOCVD, the film optical behavior and the substrate to layer interface are qualified from the measured optical data. The experimental and theoretical approach used for this purpose is described in detail. The results clearly show bending effects at the interface which may be related to structural defects; a good agreement with transmission electronic microscopy analysis is obtained.