Organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE) are the most common methods for the growth of thin films of A1N and GaN. Sapphire is the most common substrate; however, a host of materials have been used with varying degrees of success. Both growth techniques have been employed by the authors to grow AIN, GaN and AlxGa1−xN thin films primarily on 6H-SiC(0001). The mismatch in atomic layer stacking sequences along the growth direction produces double positioning boundaries in A1N and the alloys at the SiC steps; this sequence problem appears to discourage the two-dimensional nucleation of GaN. Films of these materials grown by MBE at 650°C are textured; monocrystalline films are achieved between 850°C (pure GaN) and 1050°C (pure A1N) by this technique and OMVPE. Donor and acceptor doping of GaN has been achieved via MBE without post growth annealing. Acceptor doping in CVD material requires annealing to displace the H from the Mg and eventually remove it from the material. High brightness light emitting diodes are commercially available; however, numerous concerns regarding metal and nitrogen sources, heteroepitaxial nucleation, the role of buffer layers, surface migration rates as a function of temperature, substantial defect densities and their effect on film and device properties, ohmic and rectifying contacts, wet and dry etching and suitable gate and field insulators must and are being addressed. Selected issues surrounding the growth of these materials with particular examples drawn from the authors' research are presented herein.

In this paper we review recent developments in the growth of bulk GaN crystals by a high-pressure, high-temperature method. We also provide information on various physical properties of bulk GaN material. Then, some preliminary results on the homoepitaxial growth of GaN are given. In the second part of this paper we discuss the following problems: the possible origin of the large free electron concentration in undoped GaN material, the parasitic effect of yellow luminescence and the nature of Zn- and Mg-acceptors.

We present a study of the growth of cubic GaN films on (001) GaAs by molecular beam epitaxy. Our investigations focus on the nucleation stage as well as on the subsequent growth of GaN. The phenomenon of epitaxial growth at this extreme mismatch (20%) is demonstrated to arise from a coincidence lattice between GaAs and GaN. The presence of a high-density of stacking faults in the GaN layer is explained within this understanding as being a natural consequence of the coalescence of perfectly relaxed nuclei. We furthermore analyze the growth kinetics of GaN via the surface reconstruction transitions observed upon an impinging Ga flux, from which we obtain both the desorption rate of Ga as well as the diffusion coefficient of Ga adatoms on the Ga-stabilized GaN surface. The diffusivity of Ga is found to be very low at the growth temperatures commonly used during molecular beam epitaxy, which provides an explanation for the microscopic surface roughness observed on our samples.

The development and device applications of the InGaAIN system have progressed dramatically with improvements in crystalline quality by achieved through a buffer layer, the realization of p-type doping, and the growth of ternary alloys. As a substrate, sapphire is mainly used for epitaxial growth because of the lack of a GaN bulk crystal. However, many cracks in GaN film can still be observed and its X-ray rocking curve width is less than 100 arc seconds. This is are thought to be due to the lattice constants and thermal expansion coefficients of GaN and sapphire differ by 13.8% and by -34.2%, respectively. These values are extremely large in comparison with the corresponding values for InP and GaAs. Lattice-matching growth thus remains a basic problem in growing the high-quality epitaxial films necessary for high-performance devices.

This paper reviews attempts at lattice-matching growth. Lattice-matching growth of InGaN on a house-made ZnO substrate and near-lattice-matching growth of GaN on SiC and NdGaO3 substrates have been proposed and performed, and the effects of lattice-matching have been confirmed. Various types of surface planes commercially available sapphire substrates are also discussed.

We report the use of ScAlMgO4 as a substrate for the epitaxial growth of wurzitic GaN. The low misfit (+1.8%) allows coherent epitaxy of GaN, as observed by RHEED. The congruent melting of ScAlMgO4 makes Czochralski growth possible, suggesting that large, high quality substrates can be realized. Boules about 17mm in diameter are reported. We have used nitrogen-plasma molecular beam epitaxy to grow GaN epitaxial films onto ScAlMgO4 substrates. Band-gap photoluminescence has been observed from some of these films, depending primarily on the deposition conditions. A 3×3 superstructure has been observed by RHEED on the GaN surfaces. Structural analysis by x-ray diffraction indicates very good in-plane alignment of the GaN films. We also report thermal expansion measurements for ScAlMgO4.

A method of producing epitaxial GaN on single-crystal Hf has been developed. The metal substrate is formed by a strain-anneal process yielding macroscopic (5-mm) grain sizes, followed by polishing, chemical etching, and Ar ion sputtering at elevated temperature in ultrahigh vacuum. The growth is conducted by plasma-assisted molecular beam epitaxy using an initial passivation layer deposited at low temperature and subsequent growth at 700 °C. The resulting films are in registry with the hep substrate lattice as observed by reflection high-energy electron diffraction during growth and verified by plan-view transmission electron microscopy. High-resolution x-ray rocking curve linewidths of the GaN and Hf [1012] peaks are as narrow as 900 and 180 arc seconds, respectively. The [0002] peak separation confirms the approximately 2.7% mismatch in the c axis spacing. Initial photo]uminescence observations at 20 K of a (donor-bound exciton) peak at 3.467 eV are consistent with the assumption of a nearly strain-free film resulting from the exact basal-plane lattice match and close thermal coefficient match between GaN and Hf.

We propose a MgAl2O4 substrate for GaN-based laser diodes. We grew GaN epitaxial layers using metal-organic vapor phase epitaxy on a MgAl2O4 substrate. The GaN on the MgAl2O4 showed a narrow X-ray diffraction peak of 310 arcsec, a photoluminescence dominated by the band-edge emission, and good electronic properties. The optical cavity was fabricated by cleaving the GaN on the MgAl2O4, and the stimulated emission from the cavity using optical pumping was observed. We also fabricated a light emitting diode (LED) with a AlGaN/InGaN double-heterostructure. The electroluminescence was dominated by the band-edge emission of InGaN with a narrow FWHM of 13.5 nm. The characteristics of LED on the MgAl2O4 were comparable to that on a A12O3 substrate. MgAl2O4 is feasible as a substrate for laser applications.

Hexagonal GaN films grown on non-isomorphic substrates are usually characterized by numerous threading defects which are essentially boundaries between wurtzite GaN domains where the stacking sequences do not align. One origin of these defects is irregularities on the substrate surface such as surface steps. Using Si <111> substrates and a substrate preparation procedure that makes wide atomically flat terraces, we demonstrate that reduction of these irregularities greatly improves the crystalline and luminescent quality of GaN films grown by plasma-enhanced molecular beam epitaxy. X-ray rocking curve width decreases from over 1 degree to less than 20 minutes, while PL halfwidth decreases from over 15 meV to less than 10 meV.

Hydrazoic acid, HN3, was shown to be an extremely reactive molecule and an effective precursor species for 'active nitrogen'. At room temperature, hydrazoic acid dissociatively chemisorbed on GaAs (110) surfaces to form the NH radical and N2. At 473 K, the NH fragment reacted with the substrate, broke Ga-As bonds and formed GaN and AsN species. At 673 K, an anionic exchange mechanism commenced; the surface became completely depleted of As and a thin film of GaN formed. When the surface was dosed at 773 K, a 20 Å thick GaN film was grown. This evidence demonstrates that hydrazoic acid is highly reactive at comparatively low surface temperatures and offers a viable alternative to ammonia as a nitrogen precursor gas.

We have demonstrated growth of crystalline GaN on Si substrates by using, for the first time, a novel inorganic precursor Cl2GaN3 and ultra-high-vacuum chemical vapor deposition techniques. Cross-sectional electron microscopy of the highly conformal films showed columnar growth of wurtzite GaN while Auger and RBS oxygen- and carbon-resonance spectroscopies showed that the films were pure and highly homogeneous. In addition to the high growth rates of 70–500 Å per minute, the low deposition temperature of 550–700 °C, and the nearly perfect GaN stoichiometry that we obtain, another notable advantage of our method is that it provides a carbon-free growth environment which is compatible with p-doping processes.

Single source precursors which contain preformed gallium-nitrogen and aluminum-nitrogen bonds are being considered for the growth of gallium and aluminum nitride because of their potential for overcoming problems associated with conventional precursors. Presented is the evaluation of dimethylgallium azide, Me2GaN3(1), bisdimethylamidogallium azide, (Me2N)2GaN3(2), and bisdimethylamidoaluminum azide, (Me2N)2AlN3(3) as potential precursors for A1N and GaN film growth. The compounds were evaluated for stability, ease of transport, temperature of decomposition and quality of film deposited. Amorphous thin films of GaN with a band gap of 3.4 eV were deposited with 2 at 250 °C. Increasing the substrate temperature to 580 °C resulted in the deposition of epitaxial GaN films. Polycrystalline A1N films were grown with 3 at 600 °C.

The surface chemistry of Triallylamine (TAA), (C3H5)3N on Si(111) has been studied by adsorption under UHV conditions and in-situ characterization. High Resolution Electron Energy Loss Spectroscopy (HREELS) yields the spectrum of vibration modes at the surface, and X-ray Photoelectron Spectroscopy (XPS) yields the chemical bonding and the partial concentration of the different adsorbates in the near surface region. The tertiary amine TAA physisorbs at RT without dissociation. Successive annealing steps of the physisorbed phase induce the dissociation of the amine at 400 °C. At higher temperatures the allyl groups are partially desorbed and the rest fully dissociated at 600 °C, where the hydrogen leaves the surface and the nitrogen and carbon start to diffuse into the Si substrate. A very similar behaviour is observed for the adsorption of TAA on a heated Si substrate. The coadsorption with Triethylgallium (TEG) in the temperature range 500–800 °C does not induce significant changes in the reaction at the Si surface. A negligible quantity of Ga is detected at the surface after codeposition in the whole investigated temperature range. The growth of a GaN phase has not been observed, neither on Si(111) nor on Al2O3(0001) substrates.

Gas phase reactions between trimethylgallium (TMG) and ammonia were studied at high temperatures, characteristic to MOCVD of GaN reactors, by means of insitu mass spectroscopy in a flow tube reactor. It is shown, that a very fast adduct formation followed by elimination of methane occurs. The decomposition of TMG and the adduct - derived compounds are both first order and have similar apparent activation energy. The pre-exponential factor of the adduct decomposition is smaller, and hence is responsible for the higher full decomposition temperature of the adduct relative to that of TMG.

The parasitic reactions between ammonia and commonly used alkyls have been studied in a horizontal OMVPE reactor. The results indicate that parasitic reactions between TMA1 and NH3 is severe, leading to the necessity to grow A1N at low reactor pressure. On the other hand, parasitic reactions between TMGa+NH3 and TMIn+NH3 are not significant and it is possible to grow GaN and GaInN at any reactor pressure.

The epitaxial growth of wurtzite and zincblende GaN on (0001) sapphire and (001) Si by the Electron Cyclotron Resonance-assited Molecular Beam Epitaxy (ECR-MBE) method is discussed. We show that films can be grown in the layer-by-layer mode when growth occurs in Ga-rich regime. Surface roughening mechanisms are addressed. The similarity of photoluminescence data of Mg-doped wurtzite GaN films with those of undoped zincblende GaN films suggests that Mg doping facilitates the formation of stacking faults in the wurtzite structure which are nucleation sites for zincblende domains.

Growth of lll-V nitrides by molecular beam epitaxy (MBE) is being studied at NCSU using an rf nitrogen plasma source. GaN/SiC substrates consisting of ∼3 μm thick GaN buffer layers grown on 6H-SiC wafers by MOVPE at Cree Research, Inc. are being used as substrates in the MBE film growth experiments. The MBE-grown GaN films exhibit excellent structural and optical properties — comparable to the best GaN films grown by MOVPE—as determined from photoluminescence, x-ray diffraction, and vertical-cross-section TEM micrographs. Mg and Si have been used as dopants for p-type and n-type layers, respectively. AlxGa1−xN films (x∼0.06-0.08) and AlxGa1−xN/GaN multi-quantum-well structures have been grown which display good optical properties. Light-emitting diodes (LEDs) based on double-heterostructures of AlxGa1−xN/GaN which emit violet light at ∼400 nm have also been demonstrated. Key issues that must be addressed before lll-V nitride laser diodes can be demonstrated and commercialized are discussed. New integrated heterostructures are proposed for the development of a variety of vertical-transport devices such as light-emitting diodes, laser diodes, photocathodes, electron emitters based on the negative-electron-affinity of AIN, and certain transistor structures.

We report on the growth of GaN in GSMBE using NH3 as nitrogen source. Special focus will be on the NH3 cracking, where we applied an On Surface Cracking technique (OSC). Using OSC we achieve photoluminescence linewidths as narrow as 5.5meV (5K) and mobilities of 220 cm2/Vs at room temperature.

The kinetics of Ga incorporation during gas-source molecular beam epitaxy of GaN are investigated for varying substrate temperature and incident ammonia flux. Incident Ga atoms eventually either: 1) react with NH3 to form GaN, 2) accumulate on the film surface, or 3) desorb. Low substrate temperatures lead to significant Ga surface accumulation due to the temperature-dependent reactivity of NH3 towards Ga. High substrate temperatures give rise to significant Ga desorption. Increasing NH3 flux retards both Ga surface accumulation and Ga desorption. The GaN formation rate variation with substrate temperature peaks near 750°C and increases with NH3 flux. The observation of two distinct and very low activation energies for Ga desorption suggests a relatively complex surface chemistry and a strong likelihood that hydrogen is playing an important role.

Electron Cyclotron Resonance Plasma Assisted Molecular Beam Epitaxy (ECR-MBE) and Gas Source Molecular Beam Epitaxy (GSMBE) have been used to grow hexagonal GaN on Si (111). In the ECR-MBE configuration high purity nitrogen has been used as nitrogen source. In GSMBE ammonia was supplied directly to the substrate to be thermally cracked in the presence of gallium.

By a combined application of in-situ reflection high-energy electron-diffraction (RHEED) and cross-sectional transmission electron microscopy (TEM) the growth mode and structure of GaN were determined. The growth mode strongly depends on growth conditions. Quasi two dimensional growth was observed in ECR-MBE configuration for a substrate temperature of 640°C while three dimensional growth occured in GSMBE configuration in the temperature range from 640 to 800°C.

Low temperature (9 K) photoluminescence spectra show that for samples grown by ECR-MBE and GSMBE a strong near band gap emission peak dominates while transitions due to deep level states are hardly detectable. The best optical results (the highest near band gap emission peak intensity) have been observed for samples grown by GSMBE at high temperature (800°C). This could be explained by the increase of grain dimensions (up to 0,3 – 0,5 μm) observed in samples grown by GSMBE at 800°C.

We report on methods for the growth of GaN by MBE directly on 6H-SiC substrates. The films were doped p-type by the incorporation of Mg and the samples were characterized by studying their structure and morphology by RHEED, XRD and SEM and their recombination properties by photoluminescence measurements. The undoped films were found to be atomically smooth with 2×2 surface reconstruction and have an x-ray rocking curve with a FWHM of 3.5 arcmin. The photoluminescence spectra indicate that recombination is dominated by transition across the gap. The p-type doped films have a rocking curve with FWHM of 6.5 arcmin. and the majority of recombination occurs through D-A transitions at 3.26 eV.