Likely contamination of GaN films by impurities emanating from Al2O3, SiC and ZnO substrates during growth has been studied by secondary ion mass spectrometry analysis. The highly defective interfacial region allows impurities to incorporate more readily as comparedjo the equilibrium solubility in a perfect crystal at a given temperature as evidenced by increased impurity levels in that region as detected by SIMS. The SIMS measurements in GaN layers grown on SiC and sapphire showed large amounts of Si and O, respectively, within a region wider than the highly disordered interfacial region pointing to the possibility of impurity diffusion at growth temperatures. The qualitative trend observed is fairly clear and significant. These observations underscores the necessity for developing GaN and/or AlN substrates.

In this paper we present a class of MOCVD reactors with loading capacities up to seven 2″ wafers, designed for the mass production of LED structures.

Our processes yield device quality GaN with excellent PL uniformities better than 1 nm across a 2″ wafer and thickness uniformities typically better than 2%.

We also present full 2″ wafer mapping data, High Resolution Photoluminescence Wafer Scanning and sheet resistivity mapping, revealing the excellent composition uniformity of the nitride compounds InGaN and AlGaN. As well we will show sheet resistivity uniformity for Si-doped GaN and Mg-doped GaN.

We report on the growth and characterization of InGaN bulk films and AlGaN/InGaN/AlGaN double heterostructures (DHs). Good quality bulk InGaN films have been grown by metalorganic chemical vapor deposition (MOCVD) with up to 40% InN as characterized by x-ray diffraction. The effect of hydrogen in the growth ambient on the lnN% incorporation in the InGaN films is presented. Photoluminescence (PL) spectra of AlGaN/InGaN/AlGaN DHs exhibit emission wavelengths from the violet through yellow depending on the growth conditions of the active InGaN layer. The PL spectra are fairly broad both at room temperature and 20 K, and could be a result of native defects or impurity related transitions. We also observed a linear dependence between the PL intensity and excitation power density in the 0.001 W/cm2 to 10 MW/cm2 range. Time resolved PL of one of these DHs suggest a recombination lifetime on the order of 520 ps.

AlGaN is an important material for the realization of nitride heterostructures, involved in most device designs. We have studied the growth of this alloy using low pressure MOVPE (76 Torr), and using triethyl-gallium (TEGa), trimethyl-aluminum (TMA1) and ammonia (NH3) as precursors. First the solid -gas aluminum segregation was studied in order to calibrate the incorporation of Al in the solid phase. We found that aluminum is more readily incorporated than gallium in the solid, leading to an apparent Al segregation coefficient greater than unity. A simple kinetic model is used to fit the experimental data. Scanning electron microscopy has been used to investigate the morphology of the samples through the whole range of Al content (x = 0 to 1), and we observe a clear evolution of the surface features versus aluminum concentration: at low Al contents, small (below 1 pm) hexagonal holes are observed while at high Al, acicular features are observed, with a sudden transition between those morphologies around x = 0.5. Transmission electron microscopy was used to analyze the crystalline structure of these samples. Finally, the samples were studied by low temperature (2K) reflectivity and Raman spectroscopy. We report the evolution of the optical quality of samples (x < 0.4) versus Al content, as evaluated from the broadening of the observed excitonic transitions in the 2K reflectivity.

Growth of pure crystalline carbon nitride (c-CN) with crystal sizes large enough to enable measurement of its properties has not been achieved so far. We report here that incorporation of silicon in the growth of CN can promote formation of large, well faceted crystallites. Crystalline thin films of SiCN have been grown by microwave plasma-enhanced chemical vapor deposition using CH4, N2, and SiH4 gases. Auger electron spectroscopy, scanning electron microscopies, and X-ray diffraction spectroscopy have been employed to characterize the composition, the morphology and the structure of the films. The new crystalline ternary compound (C; Si)xNy exhibits hexagonal structure and consists of a network wherein the Si and C are believed to be substitutional elements. While the N content of the compound is about 35%, the extent of Si substitution varies from crystal to crystal. In some crystals, the Si content can be as low as 10%. Optical properties of the SiCN compounds have been studied by photoluminescence (PL) and piezoreflectance (PzR) spectroscopies. From the PzR measurement, we determine the band gap of the new crystals to be around 3.8 eV at room temperature. From the PL measurement, it is found that the compounds have a strong subband-gap emission centered around 2.8 eV at room temperature, which can be attributed to the effect of defects containing in the crystals.

Two precursor routes culminating in bulk nanocrystalline gallium nitride materials are reported, with emphasis on the materials' XRD/crystalline features and photoluminescence (PL). First, the new polymeric gallium imide, {Ga(NH)3/2}n, can be converted to nanocrystalline, cubic/hexagonal GaN ranging in color from yellow to light gray. Second, a new route to gallazane, [H2GaNH2]x, from the combination of LiGaH4 and NH4X (X = Cl, Br) in Et2O is shown to result in a material that slowly converts to a polymeric solid via H2 and NH3 elimination-condensation pathways. Both the gallazane and the polymeric solid are pyrolyzed to dark gray nanocrystalline, phase-inhomogeneous GaN as above. Specific variations in the pyrolysis conditions enable some control over the particle nanosize and a degree of crystalline phase-inhomogeneity of the materials. These nanophase GaN materials have also been characterized by room temperature photoluminescence (PL) measurements. In general, the observed emission spectra are strongly dependent on pyrolysis temperature and typically exhibit weak defect yellow-green emission. While the as-prepared GaN does not exhibit band-edge PL, a brief hydrofluoric acid etch yields nanophase GaN exhibiting an intense blue-emitting PL spectrum with an emission maximum near 420 nm.

The present work reports on the formation of GaN-containing clusters from metalorganic precursors by combining pulsed laser photolysis and pulsed nozzle methods. Ammonia (NH3) and triethylgallium (C2H5)3Ga (TEG) or trimethylgallium (CH3)3Ga (TMG) with He, Ar, or N2 as the carrier gas are introduced into a high vacuum chamber via a specialized dual pulsed nozzle source. The light from an ArF excimer laser (193 nm, 23 ns FWHM) is focused into the mixing and reaction region of the nozzle source, and the products are then mass analyzed with a quadrupole mass spectrometer. Efficient laser-assisted growth of (GaN)x-containing clusters is shown with this technique.

In-situ cathodoluminescence (CL) is presented as a technique to optimize GaN, and AlGaN films deposited by MBE using an RF plasma as a source of reactive nitrogen. Excitation of the MBE grown nitride films is conveniently achieved in the preparation chamber using an Auger electron gun. The photoemission is monitored through a side port and dispersed with a 1/8 m monochromator with a typical resolution of 3 nm. The in-situ CL spectra of AlGaN and GaN films provides quick determination of both material composition, doping, and quality from the position and width of the band edge emission. The use of CL for the assessment of material composition in the growth of nitride materials is extremely beneficial since the complementary technique of RHEED oscillations is not routinely observed for these systems. The determination of material quality using CL has been used to optimize growth conditions for GaN PIN junction photovoltaic detectors on (0001) sapphire. Detectors having peak responsivity of 0.175 AAV at the GaN band edge of 365 nm and a UV to visible rejection ratio of greater than 105 have been fabricated. The high rejection ratio is accredited to the reduction of the yellow defect levels in the MBE grown material. Material optimization using in-situ CL for growth of AlGaN MODFETs having drain currents of 425 ma/mm and gm of 66 mS/mm is discussed.

In this work, we present growth of InxGa(1-x)N films by CVD technique and their optical characterization. Experimental results indicate that these films are promising materials for semiconductor device applications. We focus on solar cells and present preliminary experimental data on prototype devices.

GaN buffer layers and thick GaN layers were grown on (001) GaAs substrates by hydride vapor phase epitaxy. The ratio of cubic to hexagonal components in the grown layer was estimated from the ratio of the integrated X-ray diffraction intensities of the cubic (002) plane and hexagonal (1011) planes measured by w scan. The optimum growth conditions were thermal cleaning at 600°C, growth temperature of 500°C and thickness of 30 nm for the buffer layer, and the Will ratio of 300 for thick GaN growth at 800°C. Cubic component in the layer grown with those conditions was more than 85% and strong cubic photoluminescence emission was observed at 377 nm (3.28 eV).

High quality GaN films are grown on sapphire(0001) substrates by low pressure MOCVD using TMG and NH3 as source materials. Effects of surface nitridation and buffer layer thickness on the quality of over-grown GaN films are investigated. It is revealed by atomic force microscope (AFM) observations that surface roughness of the annealed buffer layers strongly depends on the nitridation time. Dislocation density and surface morphology of the high temperature GaN layer depend on the buffer layer thickness. It is found that sufficient surface nitridation of sapphire makes the buffer layer just prior to the high temperature growth very smooth, which is essential to obtain flat thick-GaN on it. It is also found that thickness of the buffer layer largely influences the dislocation density in the over-grown thick GaN. In order to obtain good surface morphology and low dislocation density at the same time, both nitridation time and buffer layer thickness must be optimized.

GaN films were grown on sapphire substrates at temperatures below 725 °C utilizing a Constricted Glow Discharge plasma source. A three dimensional growth mode is observed at such low growth temperatures resulting in films that are composed of individual but oriented grains. The strain that originates from the growth on the lattice mismatched substrate with a different thermal expansion coefficient is utilized to influence the thin film growth. The strain can be largely altered by the growth of suitable buffer layers. Thereby, optical and structural film properties can be engineered. It is argued that the surface diffusion of Ga ad-atoms is affected by engineering the strain. Alternatively, surface diffusion can be influenced by surfactants. It is demonstrated that the use of bismuth as a surfactant allows to modify the surface morphology of the GaN films that reflects the size of the grains in the films. The results suggest that a substantial increase of the oriented grain sizes in the films is possible while maintaining a low growth temperature.

A thermodynamic analysis of GaN grown by MOVPE has been proposed based on quasi-thermodynamic equilibrium established on the solid-vapor interface. Phase diagrams for the MOVPE growth of GaN using TEGa and NH3 has been calculated. The phase diagram is consists of four phases regions: the region for single condensed phase of GaN, the region for double condensed phase of GaN (s) +Ga(l), the etching region with Ga droplets and the etching region without Ga droplets. The effect of growth temperature, reactor pressure, content of carrier gas, deposition ratio of NH3 and V/III ratio upon growth of GaN using MOVPE has been studied.

The smooth and highly oriented AlN films were obtained using pulsed laser deposition from sintered AlN target in a nitrogen ambient. The XRD investigation revealed that highly oriented AlN thin films along the c-axis (AlN (0002)) normal to the substrate were obtained both on Si(111) and on Si(100) substrates. The (0002) x-ray peak width became narrower with increasing substrate temperature. The CL investigation showed that AlN films at high laser energy density (Ed) indicated CL peak at shorter wavelength (306nm) than that at low Ed (394nm). N/Al atomic ratio in AlN films grown at high Ed also increased as comparison with the films grown at low Ed.

Gallium nitride (GaN) was prepared by the pyrolytic conversion of both [Ga(NEt2)3]2 and its ammonolysis product at 600 °C for 4 h under Ar. The pyrolyzed residues were analyzed by X-ray powder diffraction and scanning electron microscopy, and the pyrolysis processes of the precursors under He were investigated by thermogravimetry-mass spectrometry. The XRD pattern of the pyrolyzed residue of [Ga(NEt2)3]2 showed well-resolved peaks due to a mixture of cubic and hexagonal close-packed layers of GaN. The broad XRD pattern of the pyrolyzed residue of the ammonolysis product was also attributed to the mixture of cubic and hexagonal close-packed layers of GaN. For the pyrolysis of [Ga(NEt2)3]2, the evolution of hydrocarbons was extensively observed at relatively high temperature, but a large amount of carbon (11 mass%) was still detected in the pyrolyzed residue. On the other hand, the amount of carbon was only 1.1 mass% in the pyrolyzed residue of the ammonolysis product. The pyrolysis results of the ammonolysis product under Ar were very similar to those of [Ga(NEt2)3]2 under NH3.

We have fabricated high quality single crystalline GaN films on sapphire (0001) substrates using pulsed laser deposition. Our best GaN films on sapphire (0001) featured the FWHM of the GaN (002) peak rocking curve of 7 arcmin, the RBS minimum yield of only 3%, and the energy gap width of 3.4 eV. The effect of the deposition temperature on the crystalline quality of the films is discussed.

We propose a new valence control method, the “codoping method (using both n- and p-type dopants at the same time)”, for the fabrication of low-resistivity p-type GaN crystals based on the ab-initio electronic band structure calculations. We have clarified that while doping of acceptor dopants, BeGa and MgGa, leads to destabilization of the ionic charge distributions in p-type GaN crystals, doping of Sica or ON give rise to p-type doped GaN with high doping levels due to a large decrease in the Madelung energy. The codoping of the n- and p-type dopants (the ratio of their concentrations is 1:2) leads to stabilization of the ionic charge distribution inp-type GaN crystals due to a decrease in the Madelung energy, to result in an increase in the net carrier densities.

Molecular dynamics simulations are employed to study native defects and dopants in AlN. We use local basis density functional theory within the local density approximation where charge transfer between the ions is included in a self-consistent fashion. Employing this code we find reasonable agreement for the band structure compared to other recent calculations, suggesting the suitability of our method to adequately describe AlN.

Wurtzite and zincblende 96 atom AlN cells are used to study the relaxations and electronic properties of common defects in the crystal structure, including vacancies and antisites. We investigate the electronic signatures of these defects. The local topology of column-IV impurities in anion and cation sites is studied. We analyze the lattice relaxations and electronic consequences of these impurities and identify midgap defect, donor and acceptor levels.

Mg-doped GaN films grown by metalorganic chemical vapor deposition were characterized by variable-temperature Hall-effect measurements and Fourier-transform infrared absorption spectroscopy. As-grown, thermally activated, and deuterated Mg-doped GaN samples were investigated. The existence of Mg-H complexes in GaN is demonstrated with the observation of a local vibrational mode (LVM) at 3125 cm-1 (8 K). At 300 K this absorption line shifts to 3122 cm-1. The intensity of the LVM line is strongest in absorption spectra of as-grown GaN. Mg which is semi-insulating. Upon thermal activation, the intensity of the LVM line significantly decreases and an acceptor concentration of 2×1019cm-3 is derived from the Hall-effect data. After deuteration at 600°C the resistivity of the Mg-doped GaN increased by four orders of magnitude. A LVM line at 2321 cm-1 (8 K) appears in the absorption spectra which is consistent with the isotopie shift of the vibrational frequency when D is substituted for H.

Metalorganic molecular beam epitaxy has been utilized to incorporate Er into AlGaN materials during growth utilizing elemental and metalorganic sources. Room temperature 1.54 μm photoluminescence was observed from AlN:Er and GaN:Er. Photoluminescence from AlN:Er doped during growth using the elemental source was several times more intense than that observed from implanted material. For the first time, strong room temperature 1.54 μm PL was observed in GaN:Er grown on Si. Temperature-dependent photoluminescence experiments indicated the 1.54 μm intensities were reduced to 60% and 40% for AlN:Er and GaN:Er, respectively, between 15 K and 300 K. The low volatility of Er(III) tris (2,2,6,6 - tetramethyl heptanedionate) and temperature limitations imposed by transport considerations limited maximum doping levels to ∼1017 cm-3 indicating that this precursor is unsuitable for UHV.