The growth of undoped and doped GaN and AlGaN films on off-axis 6H SiC substrates was investigated using plasma-assisted molecular beam epitaxy (MBE). Smooth and crack-free GaN and AlGaN films were obtained; the best results occurred at the highest growth temperature studied (800°C) and with a 40 to 50 nm A1N buffer layer grown at the same temperature. Carrier concentrations of up to n = 4 × 1020 cm−3 were accomplished with silicon, with a 40 to 50% activation rate as determined by secondary ion mass spectrometry (SIMS). Unintentionally doped AlxGa,.xN (x≈0.1) was n-type with a carrier concentration of 7 × 1018 cm−3. N-type AlGaN (x≈0.1)/p-type 6H SiC (0001) heterostructures showed excellent junction characteristics with leakage currents of less than 0.1 nA at 5 V reverse bias at room temperature and 0.5 nA at 200°C operating temperature.

We report the growth of InGaN alloys over practically the entire composition range at the growth temperature of GaN (700–800 °C) by MBE. We found that when the grown films are thick (> 0.3 μm), incorporation of more than about 30% indium results in phase separation of InN, which is consistent with spinodal decomposition. On the other hand we discovered that such phase separation is absent in thin InGaN films ( < 600Å) grown as GaN/InGaN/GaN heterostructures. In such configurations we were able to incorporate up to 81% In, which is the highest yet reported.

The electrical and luminescent properties of Mg-doped GaN films grown by molecular beam epitaxy (MBE) using ammonia as the nitrogen source have been investigated. Due to their different growth environments, the Mg-doped GaN films grown by MBE using ammonia exhibited properties that were different from similar films grown by metal-organic chemical vapor deposition (MOCVD). It has been found that the introduction of positive charges during growth is important in the achievement of p-type Mg-doped GaN grown by MBE using ammonia. With the introduction of a moderate nitrogen plasma, we have achieved p-type Mg-doped GaN films with a hole density of 4×1017 cm−3 and a mobility of 15 cm2/V-s at room temperature.

Thin films of wurtzite GaN have been grown by molecular beam epitaxy on 6H SiC (basal plane), Si {111} and sapphire (c-plane) substrates with and without various buffer layers. The defect microstructure of the films and the substrate/buffer/GaN interfacial quality have been characterized by cross-sectional transmission electron microscopy. The morphology was dominated by threading defects that originated at the substrate/buffer and/or buffer/film interfaces. Typical defect densities dropped rapidly with distance from the substrate but remained ∼108–109/cm2, depending on the particular substrate, for film thicknesses approaching one micron or more. The best quality films were grown at 770°C on sapphire with A1N buffer layers, and had X-ray rocking curve full-width at half-maximum values of ∼ 55arc-sec.

We report the growth of high-quality III-V nitride heteroepitaxial films on (0001) sapphire substrates by low-pressure metalorganic chemical vapor deposition (MOCVD). These films have exhibited narrow X-ray diffraction rocking curves with full-width-at-half-maximum values as low as ΔΘ∼37 arc sec. Photoluminescence and transmission electron microscopy analysis further indicate the samples to be of high quality.

Gallium nitride (GaN) films have been grown by hydride vapor phase epitaxy (HVPE) in a vertical reactor design. We report on GaN growth directly on sapphire using a GaCl surface pretreatment. The electrical properties of these films compare favorably with the highest values reported in the literature for GaN. Specifically, a room temperature Hall mobility as high as 540 cm2 /V-s, with a corresponding carrier concentration of 2×1017 cm−3, have been attained. Additionally, the vesical reactor design has assisted in reducing nonuniformities in both film thickness as well as in transport properties due to depletion effects, as compared with horizontal designs. The dislocation density in these films has been determined by plan-view transmission electron microscopy to be ∼3×l08 cm−2 .

Photoluminescence spectra obtained at 2 K show intense, sharp, near-bandedge emission with minimal deep level emissions. Stimulated emission has been observed in these films, utilizing a nitrogen laser pump source (λ=337.1 nm) with a threshold pump power of ∼0.5 MW/cm2 . These results suggest that HVPE is viable for the growth of high-quality nitride films, particularly for the subsequent homoepitaxial overgrowth of device structures by other growth methods such as OMVPE and MBE.

Monocrystalline AlxGa1−xN(0001) (0.05 ≤ x ≤ 0.70) thin films, void of oriented domain structures and associated low-angle grain boundaries, have been grown at high temperatures via OMVPE directly on vicinal and on-axis α(6H)-SiC(0001) wafers using TEG, TEA and ammonia in a cold-wall, vertical, pancake-style reactor. The surface morphologies were smooth and die densities and distributions of dislocations were comparable to that observed in GaN(0001) films grown on high temperature A1N buffer layers. Double-crystal XRC measurements showed a FWHM value as low as 186 arc sec for the (0002) reflection. Spectra obtained via CL showed strong near band-edge emissions with FWHM values as low as 31 meV. The compositions of the AlxGa1−xN films were determined using EDX, AES and RBS and compared to the values of the bandgap as measured by spectral ellipsometry and CL emissions. A negative bowing parameter was found. Controlled n-type, Si-doping of AlxGa1−xN for x ≤ 0.4 has been achieved with net carrier concentrations ranging from ≈ 2 × 1017 cm−3 to 2 × 1019 cm−3. Acceptor doping with Mg for x < 0.13 was also successful.

We have fabricated AlxGa1−xN/GaN heterostructures with high two-dimensional electron gas (2DEG) mobilities and high sheet carrier densities by metalorganic vapor phase epitaxy (MOVPE). The 2DEG sheet density and mobility exhibit a compositional dependence on the Al fraction of the electron donor layer. The highest mobility (5750 cm2/Vs at 16K) was measured in a sample with x=0.15 that had a sheet carrier density of 8.5×1012 cm−2. The undoped AlxGa1−xN layers have low background carrier concentrations and can be intentionally doped n-type using SiH4. The effect of intentional n-type doping of the AlxGa1−xN donor layer on the electrical properties of the 2DEG was studied in structures that included an undoped AlxGa1−xN spacer layer of varying thickness. Higher 2DEG mobilities were obtained when a 100Å thick undoped layer was included in the structure due to spatial separation of the 2DEG from ionized impurities in the doped AlxGa1−xN. These initial results demonstrate that the electrical properties of AlxGa1−xN/GaN heterostructures can be controlled by intentional doping and appropriate layer design.

GaN epilayers have been grown using low pressure MOVPE (76 Torr) onto (0001) sapphire substrates. Triethylgallium (TEGa) and Ammonia (NH3) were used as precursors. The growth rate has been determined versus growth parameters. The growth rate versus NH3 flow displays a rather unusual behavior, in particular at low growth temperature (buffer layer), where the growth rate decreases strongly when the NH3 flow increases. A growth mechanism involving a competitive adsorption step onto the surface is proposed, and results in a strong dependence of the growth rate on the V/III ratio.

The structural quality of the layers has been assessed by X-ray diffraction versus the growth parameters, and will be reported here. A substrate nitridation step, prior to buffer layer deposition has been introduced in our growth process. The effect of substrate nitridation on the structural and optical properties of the epilayers is studied and we propose an optimized pre-treatment for the MOVPE growth of GaN onto (0001) sapphire substrates.

We report on the low temperature epitaxial growth of InxGa1−xN with 0≤x≤0.27 by Atomic Layer Epitaxy (ALE). GaN and InGaN single crystal films have been grown by ALE in the temperature range between 600 and 700 °C using the rotating substrate approach. Films were deposited on sapphire substrates using TMG, EdMIn, and NH3 as precursors. Up to 27% indium content has been achieved in the InGaN films. The FWHM of the (0002) InGaN peak by double crystal X-ray diffraction of these films was as small as 5 minutes. Room-temperature photoluminescence (PL) from these films was dominated by band edge emission between 365 nm and 446 nm. AlGaN/InGaN double heterostructures were grown in a hybrid reactor, in which the AlGaN barrier layers were grown by MOCVD and the InGaN active layer by ALE. The structures showed good crystal quality, and sharp PL emission with peak intensity at 410 nm.

AlGaInN quaternary alloy based devices can cover the emission wavelength from deep UV to red. This Quaternary alloy also offers lattice matched heterostructures for both optical and microwave devices. We will report on the MOCVD growth of AlxGa1−x-yInyN (0<x<0.12), (0<y<0.15) at 750 °C on sapphire substrates, using TMG, EDMIn, TMAl and NH3 precursors. Chemical composition, lattice constants and bandgaps of the grown films were determined by EDS, X-ray diffraction and room temperature PL. Data indicates that the lattice constants can also be deduced using Vegard's law, indicating a solid solution of this alloy. PL showed band edge emission, however emission from deep levels was also observed. Optimized growth conditions and heterostructures using this quaternary alloy will be presented.

Using atomic force microscopy (AFM) and X-ray diffraction (XRD) we have determined that on [0001] oriented sapphire, the GaN buffer layer shows a degree of crystallinity that is dependent on growth rate. Annealing studies show evolution of the crystallinity and the emergence of a preferred orientation. Also, substrate orientation is found to influence the buffer layer crystallinity. Based on this work and previous results, we propose that the GaN buffer layer growth can be described by the Stranski-Krastanov growth process.

In this paper GaN films are examined, which are grown on basal plane (0001) sapphire substrates. Growth is performed in a novel type of vertical rotating disk reactor. Results on the effect of a GaN nucleation layer on the properties of the overgrown GaN epilayer are presented. Characterisation includes surface morphology studies, DC X-ray diffraction and optical characterisation. Best film quality so far has a double crystal X-ray half width of 85 arcsec at approximately 1 μm thickness.

Preliminary results of a study of GaN nucleation and growth by molecular beam epitaxy using a nitrogen rf plasma source are presented. Nucleation layers and 3000 Å thick layers were investigated by atomic force microscopy and x-ray diffraction. Growth under gallium-rich conditions both increased nucleation island size and promoted two-dimensional growth.

A major limitation of the current technology for GaN epitaxy is the availability of suitable substrates matched in both lattice constant and thermal expansion coefficient. One alternative for the development of GaN substrates rests in the application of halide vapor phase epitaxy (HVPE) to produce GaN films at high growth rates. In this paper, we describe the growth of thick GaN films via the HVPE technique on (0001) sapphire and (111) Si substrates. At a temperature of 1030°C, films are grown at rates between 70 and 90 μm/hr, yielding total thicknesses exceeding 200 μm on sapphire. DCXRD measurements of GaN/sapphire indicate FWHM values less than 220 arcsec on 180 μm thick films. Room temperature PL measurements of GaN/sapphire indicate strong emission at 3.41 eV, with a FWHM value of 65 meV. Moreover, no detectable deep level emission was found in room temperature PL measurement. Under optimized conditions, films are morphologically smooth and optically clear. The GaN morphology appears to be a strong function of the initial nucleation conditions, which in turn are strongly affected by the partial pressure of GaCl. HVPE growth on (111) Si substrates is accomplished using an AlN MOVPE buffer layer.

We have investigated the deposition of AlN thin films on Si(100), Al2O3(0001), and Al2O3(0112) substrates at lower temperatures (523–723 K) using a novel aluminum source, dimethylethylamine:alane (DMEAA), with ammonia as a nitrogen source in a low-pressure MOCVD atomic layer growth process. At reactor pressures of 25 and 50 Torr a four-step sequence of reactant flow steps separated by flush steps was cycled. We observed a tendency toward a self-limiting growth rate as the DMEAA step flow time was increased. The deposition uniformity was observed to be dependent on temperature and non-uniform deposition occurred at higher temperatures. The microstructure and crystalline orientation were examined using x-ray diffraction and crystalline A1N films were deposited at temperatures as low as 573 K. Crystallite size decreased with substrate temperature and at 523 K. amorphous films were deposited. At T > 650 K preferentially oriented crystalline films were deposited with orientations of Si(100)//AIN(0001), Al2O3(0001)//AIN(0001), Al2O3(0112)//AIN(11 20).

A model for the growth of gallium nitride in a vertical metalorganic vapor phase epitaxy reactor is presented. For a mixture of non-dilute gases, the flow temperature and concentration profiles are predicted. The results show that the growth of GaN epilayers is through an intermediate adduct of TMG and ammonia. Growth rates are predicted based on simple reaction mechanisms and compared with those obtained experimentally. Loss of adduct species due to polymerization leads to lowering in growth rate. An attempt to quantify loss of reacting species is made based on experimentally observed growth rates.

Low-pressure metal-organic chemical vapor deposition (MOCVD) has been used to deposit unnucleated and self-nucleated GaN thin films on (00.1) sapphire substrates. For the self-nucleated films, initial layers were grown at 540°C using trimethylgallium and ammonia as elemental sources and either nitrogen or hydrogen as the carrier gas. Using these same gas phase conditions, overlayers on native (00.1) sapphire substrates or the GaN-nucleated (00.1) sapphire substrates were deposited at 1025°C. The surface morphology and mosaic dispersion of these unnucleated and self-nucleated GaN thin films have been surveyed by a combination of real space images from atomic force microscopy and reciprocal space intensity data from X-ray scattering measurements. As expected, the unnucleated GaN films show a large-grained hexagonal relief, typical of three-dimensional island growth. However, the self-nucleated films are shown to be dense mosaics of highly oriented islands, emblematic of a more two-dimensional growth.

Three-dimensional GaN pyramids have been successfully obtained on dot-patterned GaN(0001)/sapphire substrates by using the selective MOVPE technique. The dot-pattern is a hexagon arranged with a 5μm width and a 10μm spacing. The GaN structure comprises a hexagonal pyramid covered with six {11ȃ01} pyramidal facets on the side or a frustum of a hexagonal pyramid having a (0001) facet on the top. The facet formation mechanism has been investigated by observing the facet structure with the growth time. The {11ȃ01} facets are very stable during the growth. The (0001) facet growth is dominant at the initial growth but almost stops at a certain growth time and then the facet structure is maintained. The appearance of the self-limited (0001) facet is attributed to the balance of flux between incoming Ga atoms from the vapor phase to the (0001) surface and outgoing Ga atoms from the (0001) surface to the {11ȃ01} surface via migration. The longer the diffusion length of the Ga atoms on the (0001) surface is, the more the surface migration is enhanced, resulting in the appearance of the wider (0001) facet on the top.

InGaN based optical devices can cover from the violet through orange regions of the visible spectrum. Difficulties in the growth of this alloy, which have impeded its applications, include problems such as the high vapor pressure of In, weak In-N bonds and lack of sufficient nitrogen during growth. We report on the MOCVD growth of InxGa1−xN (0 < x < 0.4) on sapphire substrates in the 750 - 800 °C temperature range. X-ray diffraction data show full width at half maximum line widths as narrow as 250 arcsec for low values of x, while films with higher lnN% exhibit broader line widths. Room temperature photoluminescence spectra exhibit band edge emission, with emission from deep levels increasing with x. Preliminary investigations of AlGaN/lnGaN/AlGaN double heterostructures have been conducted.