We report the growth, fabrication and characterization of back-illuminated AlGaN p-i-n photodetectors operating in the solar-blind wavelength range. A peak external quantum efficiency (EQE) of 45.5% (0.1A/W) was obtained at a wavelength of 270 nm. This EQE was obtained at zero volt bias and without the use of any anti-reflection coating on the back of the substrate. To our knowledge, this is the highest EQE reported to date for p-i-n photodetectors operating in the solar-blind wavelength range.

Transmission electron microscopy of plan-view and cross-section samples of pendeo-epitaxial layers is described. Samples grown with and without silicon nitride masks are compared. A large misorientation of the GaN grown above the mask was observed, with 2-3° tilt between wing and seed areas, caused by additional nucleation on the mask layer. Some misorientation was also observed between wing/wing areas of the sample. Samples grown without silicon nitride masks show much smaller misorientations and contain different types of defects.

In this paper we review studies aiming at elucidation of the mechanisms responsible for anomalously low pressure coefficients of the light emission energy, dEE/dP, observed in quantum structures of InGaN/GaN and GaN/AlGaN. We have established that in hexagonal InGaN/GaN and GaN/AlGaN structures the main mechanism involved is related to the pressure induced increase of the piezoelectric field which determines also the strong red shift of the emission energy with thickness of the quantum well. To reproduce the experimental findings in InGaN/GaN case, it is necessary to take into account the dependence of the piezoelectric constants on the volume-conserving strain. Whereas the experimental results on a decrease of dEE/dP in GaN/AlGaNstructures can be fully accounted for within the linear elasticity theory. In contrast to these findings, dEE/dP magnitude measured in cubic InGaN/GaN quantum structures shows value close to changes of the InGaN bangap with pressure obtained from first principle calculations. The latter result is consistent with the absence of the built-in electric fields in the cubic nitride structures.

With the solid phase reaction between the ZnO film and -Al2O3 substrate, the ZnAl2O4/-Al2O3 complex substrate have been prepared. GaN films were then directly grown on this new kind of substrate using light-radiation heating low-pressure metalorganic chemical vapor deposition (LRH-LP-MOCVD) without any nitride buffer layer. The structure and surface morphology of the ZnAl2O4/-Al2O3 substrates and GaN epilayers have been characterized by employing X-ray diffraction (XRD) and scanning electron microscope (SEM). The result show that as the thickness of ZnAl2O4 layer is increased, the film changes from a (111)-oriented single crystal to a poly-crystal, together with the surface morphology transforms from uniform islandsa a to the bulgy-line structure, leading to GaN films grown on ZnAl2O4/ -Al2O3 substrates varying from c-axis oriented single-crystal to poly-crystal.

The chemical, electrical, and microstructural properties of Au/Pd contacts on p-type GaN(0001) surfaces previously cleaned either ex situ or ex situ and in situ have been investigated. The in situ process involved a high temperature, NH3-based chemical vapor clean (CVC); it produced ordered, stoichiometric, p-type GaN surfaces with no detectable C and an O constituent which was subsequently reduced from 15 at% on the ex situ treated surface to 2 ± 1 at% following the CVC process. The Pd contacting layer grew epitaxially in a layer-by-layer mode on the CVC surface and formed an abrupt, unreacted metal-semiconductor interface. The Au capping layer also grew epitaxially on the Pd. Au/Pd contacts on both HCl and CVC treated surfaces exhibited identical surface roughness values (RMS) in the as-deposited state and following a 500°C anneal. Contact structures on CVC treated surfaces demonstrated excellent high-temperature microstructural stability as evidenced by the absence of significant change in the surface roughness (RMS) with successive annealing at 600 and 700°C. Identical contact structures on ex situ cleaned surfaces exhibited poor high temperature microstructural stability, as indicated by a significant increase in the surface roughness (RMS) following successive anneals at 600 and 700°C. There was a significant degradation in the morphology of both surfaces following the 800°C anneal as evidenced by the formation of large voids in the contact metallization and the exposure of the underlying p-type GaN substrate. The lowest resistance contact structures with uniform metal coverage were obtained for Au/Pd contacts on a CVC treated surface annealed at 700°C.

A novel doping design has been proposed that yields high two-dimensional electron gas (2DEG) densities in the AlGaN/GaN heterostructure field-effect transistors (HFETs) even when the AlGaN barrier layers are designed to be very thin. In the novel doping design, an asymmetric double-heterostructure is employed, and donor atoms are doped not only in the surface-side AlGaN layer but also in the underlying AlGaN layer. In this structure, electrons are efficiently supplied also from the back-doped AlGaN barrier layer to the GaN channel, with the help of the negative polarization charges at the heterointerface between the GaN channel and the underlying AlGaN barrier layer. High 2DEG densities can thus be obtained. Moreover, relatively high 2DEG mobilities can be obtained for high 2DEG densities, because back-doped donor atoms are sufficiently remote from the position of the 2DEG so that the 2DEG is less subjected to the ionized impurity scattering due to the relevant donor atoms. By using this back-doping design, a very high 2DEG density of 2.8x1013 cm-2 (2DEG mobility is 850 cm2/Vs) has been obtained at 300 K in the Al0.3Ga0.7N/GaN HFET whose barrier layer (Al0.3Ga0.7N) is as thin as 120 Å. Thus, the back-doping design is effective to obtain high 2DEG densities in the HFETs with thin barrier layers, and promising for high-power applications.

The band-gap shift of GaN:Mg epilayers on (0001)-oriented sapphire was studied as a function of uniaxial strain compression along the c-axis using time-resolved, optical absorption measurements in shock wave experiments. For longitudinal stresses ranging from 4 to 14 GPa, the band gap shift is approximately 0.026 eV/GPa. Combining this result with the known behavior of wurtzite GaN under hydrostatic pressure and biaxial stress, a new set of deformation potentials has been estimated: acz-D1 = -10.2 eV, act-D2 = -7.9 eV, D3 = 1.33 eV and D4 = -0.74 eV. A slow band gap shift is also observed following the immediate band gap increase upon impact. This phenomenon can be explained by a time-dependent screening of the piezoelectric field.

InN films were grown on sapphire (0001) substrates by radio-frequency plasma-assisted molecular beam epitaxy. The InN buffer layers deposited at low temperature were either grown on a substrate with nitridation or on a substrate without nitridation. The InN buffer layers on the nitridated substrates were always single crystalline, whereas the buffer layers on non-nitridated substrates were always polycrystalline. However, even without nitridation process, single crystalline InN films could be grown on the polycrystalline InN buffer layers; in this case, the orientation was always [1120] InN//[1120] sapphire epitaxy, which differed from the [1010] InN//[1120] sapphire epitaxy in films grown with nitridation.

P-type active incorporation of magnesium into the gallium nitride lattice is still a challenge for the realization of wide band gap light emitters. This work presents a detailed microstructural study of phenomena connected with the problem of Mg doping of films grown by metal organic vapor phase epitaxy. Transmission Electron Microscopy both in conventional and in high resolution mode are used to obtain a better understanding of the Mg related defect formation processes. It is shown that the defect formation never occurs instantaneously at the beginning of the doping but a defect free zone always precedes the onset of the defect formation. This effect cannot solely be addressed to the memory effect of the reaction chamber since it is found that the defect density oscillates during growth. A more detailed investigation reveals that segregation of Mg plays a crucial role in the formation of defects. The observations emphasize the fact that a critical surface concentration is necessary for the defects to form. Beside the known processes of inversion boundary formation it is found that small nitrogen polar GaN grains in GaN films are gradually increasing on the expense of Ga polar grains if a sufficiently high Mg coverage and film thickness is realized. This process is found to occur in alternating steps along the {1123} and {0001}planes.

Ever since the discovery of the astonishing properties of GaN, many research groups have been involved in the processing of the perfect GaN crystal. Iodine Vapor Phase Growth (IVPG) technique was employed to grow GaN epilayer on a MOCVD pre-deposited buffer layer. This new epitaxial system was characterized by TEM, AFM and EFM. A complete AFM study involved the polarity measurements and the etch pit density measurements. For the first time a systematic study was performed of the dislocation density changing as a function of distance from the substrate. TEM performed on the cross-section, as well as the plan view, of the samples showed a remarkable decrease in the dislocations in the current system, compared to the samples that were solely deposited by MOCVD. Advanced analytical methods of polarity and dislocation density measurements have been established to understand the relation between microstructure and electrical properties of the thick film GaN. Electrostatic Force Microscopy has been suggested as a potential tool for obtaining polarity information.

We report detailed investigations of generation-recombination (G-R) noise in GaN films grown by rf-plasma assisted molecular beam epitaxy on intermediate-temperature buffer layer (ITBL) in addition to conventional low-temperature buffer layer. To characterize the film quality affected by the use of ITBL, low-frequency noise measurements were performed. The voltage noise power spectra show a strong dependence on the thickness of the ITBL. A model has been presented to explain the observed G-R noise, which stipulates that the phenomenon arises from the thermally activated trapping and detrapping of carriers by traps. The process leads to the correlated fluctuations in both the carrier number and the coulombic scattering rate. Detailed numerical evaluation shows that number fluctuation dominates in our samples. The calculated trap densities show that the use of ITBL can effectively reduce defect density by over an order of magnitude.

Deep traps in a 300-m m-thick freestanding GaN sample were characterized by deep level transient spectroscopy (DLTS), using Schottky barrier diodes (SBDs) fabricated on the Ga polarity surface. Most of the SBDs show nearly ideal current-voltage characteristics, with both forward and reverse currents controlled by the thermionic emission mechanism. Five common traps, which include A1 (1.0 eV), A (0.66 eV), B (0.59 eV), C (0.35 eV), and D (0.25 eV), can be consistently observed in all SBDs. Two of them, A1 and C, are related to surface damage. Surprisingly, some new traps can be found in the DLTS spectra of some SBDs if higher reverse biases are used in the measurements. However, they cannot be fitted by DLTS simulations, and are likely associated with parasitic capacitance somewhere in the cryostat.

High-quality GaN layers grown by hydride-vapor phase epitaxy (HVPE) and homoepitaxial layers grown thereon by molecular beam epitaxy (MBE) are studied after fast proton irradiation. A radiation-induced decrease of the band edge luminescence is observed. Time-resolved photoluminescence (PL) reveals a significant reduction of the carrier lifetime. A lifetime degradation constant of Kt = 4×10-15cm2/ns is reported for very thick HVPE GaN, and large variations are observed for the different layers. Partial recovery of the carrier lifetime and PL intensity with annealing is observed for thin HVPE GaN but not for the MBE overgrowth layer.

Optical properties of fully-strained wurtzite and zincblende InxGa1-xN/GaN multiple quantum well (MQW) structures were compared to discuss the origin of exciton localization. In contrast to the hexagonal InGaN MQWs, the photoluminescence (PL) peak energy of cubic InGaN MQWs showed a moderate blueshift with decreasing well thickness, L, and low-temperature PL decay time of the cubic MQWs did not depend strongly on L. The results imply that the wavefunction overlap in cubic InGaN MQWs was not reduced compared to the hexagonal ones, since they do not suffer from the electric field normal to the QW plane due either to spontaneous or piezoelectric polarization. Both MQWs exhibited a large and composition-dependent bandgap bowing, and time-resolved PL (TR-PL) signals showed a stretched-exponential decay even at room temperature. The exciton localization is considered to be an intrinsic property of InGaN.

The hardness of single-crystal aluminum nitride (AlN) 0.5-mm-thick wafers was measured at elevated temperatures and compared with that of other semiconductors. A Vickers indentation method was used to determine the hardness under an applied load of 0.5 – 5 N in the temperature range 20 - 1400°C. The average hardness was measured as 17.7 GPa at room temperature, harder than GaN and InN. The fracture toughness is 0.5 MPa•m1/2. AlN exhibits the hardness higher than that of GaN in the entire temperature range investigated. Up to about 1100°C, AlN maintains its hardness and thus, a high mechanical stability for AlN at elevated temperatures is deduced.

Oxygen-free nanosized InN was synthesized by pulse discharge. NH3/N2 mixture and bulk indium were used as nitrogen source and discharge cathode, respectively. Optical emission spectra show that N2 and NH3 were highly dissociated in the discharge and oxygen could be eliminated effectively. A comparison was made among samples produced in the discharge of N2, NH3 and N2/NH3, respectively. The XPS was used to determine the composition of the synthesized materials. Only In-N was produced when N2/NH3 was used as the discharge gas, whereas indium oxide was found in the case using N2 or NH3 as the discharge gas. The size of the produced InN was between 5 and 200 nm. A broad band with a blue shift of 150 nm from that of bulk InN was detected in the photoluminescence of the produced nanosized InN.

A comprehensive thermodynamic analysis has been performed for the Ga/N/H/C/O system. Apriori process-property relationships of the metal organic chemical vapor deposition (MOCVD) of Gallium Nitride (GaN) are thus obtained. The parameter space for pure GaN growth is studied for system water vapor levels of 0.1 ppb – 10 ppm, system pressure between 10-6- 106 Torr, N/Ga feed ratios of 1 - 100,000, C/Ga feed ratios of 0 – 100, and H2/Ga feed ratios of 100 – 10,000. Higher growth temperatures for pure GaN are predicted at high operating pressures (for pressures greater than 0.1 Torr), low C/Ga feed ratios, high carrier gas flow rates, and mostly at low N/Ga feed ratios. Because relative C/Ga, N/Ga and H2/Ga feed ratios have been considered, the predictions in this study are applicable to any multiple and single precursor systems. Such analyses can be easily extended to the molecular beam epitaxy of GaN, when the feed ratio C/Ga = 0. Experimental data reported on the growth of GaN are found to be in good agreement with our theoretical predictions, for many systems that have included different source species.

A physico-chemical model of the High Pressure Organometallic Chemical Vapor Deposition (HPOMCVD) process that describes three dimensional transport phenomena as well as gas-phase and surface reactions underlying the growth of compound semiconductors is presented. A reduced-order model of the Organometallic Chemical Vapor Deposition of indium nitride (InN) from trimethylindium In(CH3)3 or TMI and ammonia (NH3) at elevated pressures has been developed and tested using the computational fluid dynamics code, CFD-ACE+. The model describes the flow dynamics coupled to chemical reactions and transport in the flow channel of the Compact Hard Shell Reactor, as a function of substrate temperature, total pressure and centerline flow velocity.

In this study, the photoluminescence (PL) spectra of InGaN/GaN multiple quantum wells (MQWs) grown on sapphire substrates with different orientation at 720°C were investigated at room temperature. Four different substrates were used, which have different surface step structures. Their orientations are: c-plane, a-plane, c-plane with off-set of 2 degree and c-plane with off-set of 6 degree towards a-plane. PL spectra from InGaN/GaN MQWs grown on c-plane substrates showed double-peak emission, while those from the MQWs grown on the others substrates showed single-peak emission. The strongest emission was found on the substrate with off-set of 2 degree towards a-plane. A Shift of PL peak position to long wavelength was observed on the substrates with an off-set angle. Meantime, all emissions were also investigated at different excitation power density. Experimental results indicate that the regular step structure on the substrates can seriously affect the growth of InGaN/GaN MQWs. Segregation effect in InGaN will lead to the high In composition regions (quantum-wire like structure) in the substrates with an off-set angle due to the regular steps on them, as observed by atomic force microscopy. It can be concluded that the surface steps on substrate play an important role in the formation of the In-rich InGaN quantum-wire like structure.

Electron holography has been used to measure the electric potential around edge and screw dislocations in n-GaN viewed in a near end-on geometry. It is shown that the potential at edge dislocations is 2V below that in the bulk consistent with a negative charge of 2 electrons/c (c = 0.52nm). Preliminary results, which suggest that screw dislocations are also negatively charged, are discussed.