Influence of the carrier gas composition on metalorganic vapor phase epitaxy of gallium nitride

The influence of hydrogen and nitrogen as carrier gases on the rates of gallium nitride (GaN) growth and etching in the process of metalorganic vapor phase epitaxy (MOVPE) have been studied. Based on these data, the possible roles of hydrogen and nitrogen in the events on the surface of an epitaxial GaN layer are considered.     

Progress in III-nitrides: Process issue and purity perspective

The growth of good quality layers of gallium nitride (GaN) as suitable for epitaxial growth is of great technological importance. Chloride vapour phase epitaxy (Cl-VPE) has been employed to grow good quality layers of GaN. The grown layers have been extensively characterized for their structural and optical properties. MOVPE grown GaN layers have been used to address process issues on device structuring and fabrication. GaN samples with different transition metal dopants have been synthesized and their usefulness as semi-magnetic materials, which are also identified as dilute magnetic semiconductors (DMS), have been evaluated. Better results have been obtained on the magnetic characteristics of GaN with ruthenium as the dopant. Nano dimensional structures of GaN have been obtained with excellent control of the growth parameters.     

Optimization of growth conditions for undoped and doped GaAs layers using an empirical model of metalorganic vapor phase epitaxy

We present semiempirical relations that quantify the influence of metalorganic vapor phase epitaxy (MOVPE) conditions on the parameters of GaAs-based epitaxial layers. These relations have been used to create a MOVPE training simulator system.     

Semi-insulating GaN:C epilayers grown by metalorganic vapor phase epitaxy using propane as a carbon source

The influence of propane present in a reactor at various stages of GaN growth by metalorganic vapor phase epitaxy (MOVPE) on sapphire substrates on the character of epitaxial process and the properties of epilayers has been studied. Doped GaN epilayers with carbon concentration 5 × 10¹⁸ cm^–3 characterized by high crystalline perfection, an atomically smooth surface, and electric breakdown voltage above 500 V at a doped layer thickness of 4 μm have been obtained.

     

Interlayer relaxation in group-III-nitride-based heterostructures according to X-Ray diffraction data

The structure perfection in two samples of the InN-GaN bilayer heterosystem, grown by molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) on (0001)-oriented sapphire substrates, has been studied by the X-Ray diffraction techniques. Components of the microdistortion tensor were determined from an analysis of the broadening of diffraction peaks measured in various geometries. These data were used to evaluate the densities of various dislocation families in each layer of the heterosystem and to trace a change in the dislocated structure from the lower (GaN) to upper (InN) layer. A difference in the behavior of dislocations in the two samples grown by different methods (MBE versus MOVPE) suggests that different mechanisms of relaxation of the elastic stresses between InN and GaN layers are operative in these cases.     

Mechanism of misfit stress relaxation during epitaxial growth of GaN on porous SiC substrates

A decrease in the density of threading dislocations has been observed during the epitaxial growth of GaN layers on porous silicon carbide (PSC) substrates by means of chloride hydride vapor phase epitaxy. It is established that, in the early growth stage, the substrate is capable of redistributing stresses in the growing heterostructure, which leads to relaxation of the lattice misfit stresses via generation of a superlattice of planar defects. In the subsequent growth stage, these defects prevent the propagation of threading dislocations. Owing to this phenomenon, 1-μm-thick GaN layers on PSC can be obtained with a density of dislocations reduced by two orders of magnitude as compared to epilayers of the same thickness grown on nonporous substrates.     

X-ray diffraction reciprocal space and pole figure characterization of cubic GaN epitaxial layers grown on (0 0 1) GaAs by molecular beam epitaxy

X-ray diffraction reciprocal space maps and pole figures were used to analyse the cubic GaN epitaxial layers grown on (0 0 1) GaAs by r.f. plasma source MBE; the presence of hexagonal phase in cubic GaN layers was detected by high resolution x-ray analysis and the relationships among various crystal axes of cubic and hexagonal phase GaN were analysed with respect to V/III source-supply ratio. As for the growth conditions of the epitaxial layers, the V/III ratio was found to drastically affect the quality of the layers. High-temperature growth under near-stoichiometric conditions was necessary to obtain high quality epitaxial layers. It was found that inclusion of the hexagonal phase in the cubic GaN layers could be less than 0.4%, though previously reported typical c-GaN epitaxial layers included as much as 10–20% hexagonal phase GaN. On the basis of the measurements and analyses of reciprocal space maps and pole figures, it was revealed that the orientation of crystal axes of the hexagonal phase was unique in the present GaN epitaxial layers and they were different from those of previously reported c-GaN epitaxial layers.     

Surface characterization of GaN and AlGaN layers grown by MOVPE

Surface properties of GaN and Al_0.17Ga_0.83N materials grown by metal organic vapor phase epitaxy (MOVPE) were systematically investigated by X-ray photoelectron spectroscopy (XPS). Air-exposed samples showed highly non-stoichiometric surfaces, which included a large amount of natural oxides. Deposition of Al on the air-exposed GaN surface caused interfacial reactions, resulting in the formation of oxide layers including Al₂O₃ and Ga oxide at the interface. A natural oxide layer of AlGaN surface possessed a complicated composition distribution in depth where the Al-oxide component was dominant on the topmost layer. Such natural oxide layers were found to be removed from GaN and AlGaN surfaces after the treatment in an NH₄OH solution at 50°C for 10 min, resulting in oxide-free and well-ordered surfaces.     

Asymmetry of the Defect Structure of Semipolar GaN Grown on Si(001)

The defect structure of a thick (~15 μm) semipolar gallium nitride (GaN) layer grown by hydride–chloride vapor phase epitaxy on a Si(001) substrate with buffer layers has been studied by transmission electron microscopy. The asymmetry of the defect structure of GaN epilayer has been revealed and analyzed. The influence of this asymmetry on the rate of decrease in the density of threading dislocations in the growing epitaxial layer is discussed.

     

Thermal Oxidation of Silicon Carbide Substrates

Thermal oxidation was used to remove the subsurface damage of silicon carbide (SiC) surfaces. The anisotrow of oxidation and the composition of oxide layers on Si and C faces were analyzed. Regular pits were observed on the surface after the removal of the oxide layers, which were detrimental to the growth of high quality epitaxial layers. The thickness and composition of the oxide layers were characterized by Rutherford backscat-tering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS), respectively. Epitaxial growth was performed in a metal organic chemical vapor deposition (MOCVD) system. The substrate surface morphol-ogy after removing the oxide layer and gallium nitride (GaN) epilayer surface were observed by atomic force microscopy (AFM). The results showed that the GaN epilayer grown on the oxidized substrates was superior to that on the unoxidized substrates.     

Cubic GaN layers grown by metalorganic chemical vapor deposition on GaN templates obtained by nitridation of GaAs

Cubic gallium nitride epitaxial layers were grown by metalorganic chemical vapor deposition on GaN templates obtained by nitridation of GaAs substrates. The GaN structure can be peeled off the GaAs substrate and it can be handled separately. X-ray diffraction, Raman and photoluminescence measurements show that the epitaxial layers are cubic and monocrystalline.     

X-ray absorption and Raman study of GaN films grown on different substrates by different techniques

Raman scattering and polarization-dependent synchrotron radiation X-ray absorption, in combination, have been employed to examine the residual stress of undoped GaN epitaxial layers grown on Si by molecular beam epitaxy and Si-doped n-type GaN layers grown on sapphire by metalorganic chemical vapor deposition. Values of the lattice constant of different GaN films can be deduced from the interatomic distances in the second coordination shell around Ga by polarization-dependent extended X-ray absorption fine structure analysis and the strain of the films can be obtained. This result is further confirmed by Raman scattering spectra in which the phonon modes show a significant shift between different GaN epitaxial layers with different growth conditions.     

MOVPE of structures with aluminum nanocluster layers in a GaAs matrix

Metalorganic vapor phase epitaxy (MOVPE) was used to grow semiconductor structures comprising a GaAs single crystal matrix with incorporated layers of aluminum nanoclusters (Al-NCs). A new regime of GaAs overgrowth on Al-NC layers is proposed, which ensures planarization of the semiconductor layer surface at a thickness comparable with the height of Al-NCs.     

Effect of growth temperature on polytype transition of GaN from zincblende to wurtzite

We have investigated effect of growth temperature on the polytype conversion of cubic GaN (c-GaN) grown on GaAs (001) substrates by MOVPE. It was found that the polytype transition of GaN from zincblende (cubic) to wurtzite (hexagonal) structures is much dependent on the growth temperature. Transmission electron microscopy (TEM) observations demonstrate that the GaN grown layers have the cubic structure (c-GaN) and contain bands of stacking faults (SFs) parallels to {111} planes. For low growth temperatures (∼ 900 °C), XRD results demonstrate that the GaN grown layers with the cubic phase purity higher than 85% were obtained. No different types of single diffraction spots, indicating the incorporation of single-crystal h-GaN, on the selected area diffraction (SAD) pattern was observed. It is also found that a density of SFs decreases with the distance from the interface of c-GaN/GaAs. On the other hand, GaN layers exhibited a transition from cubic to mixed cubic/hexagonal phase under conditions of increasing growth temperature (∼ 960 °C) as determined using TEM-SAD technique with complementary XRD and PL observations. In addition, the optical characteristics of c-GaN layers are shown to be very sensitive to the presence of the single-crystal h-GaN.     

Investigations of selectively grown GaN/InGaN epitaxial layers

We have studied GaN/InGaN heterostructures grown by selective area low pressure metalorganic vapor phase epitaxy (LP-MOVPE). A GaN layer already grown on the c-face of sapphire has been used as substrate, partly masked by SiO₂. In a second epitaxial step a GaN/InGaN single heterostructure and GaN/InGaN/GaN double heterostructures were grown on the unmasked rectangular fields. We obtained good selectivity for GaN and for InGaN. A larger growth rate as compared to planar epitaxy and strong growth enhancement at the edges was observed. Spatially resolved measurements of the luminescence show an increase in indium incorporation of about 80% at the edges. Besides the larger indium offering at the edges, this is due to an enhanced growth rate. Very smooth facets are obtained. The influence of pressure on the surface morphology and growth enhancement was investigated.     

Fabrication and performance of GaN electronic devices

GaN and related materials (especially AlGaN) have recently attracted a lot of interest for applications in high power electronics capable of operation at elevated temperatures. Although the growth and processing technology for SiC, the other viable wide bandgap semiconductor material, is more mature, the AlGaInN system offers numerous advantages. These include wider bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy) for producing very thick layers or even quasi-substrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices, including heterostructure field effect transistors (HFETs), Schottky and p–i–n rectifiers, heterojunction bipolar transistors (HBTs), bipolar junction transistors (BJTs) and metal-oxide semiconductor field effect transistors (MOSFETs). This review focuses on the development of fabrication processes for these devices and the current state-of-the-art in device performance, for all of these structures. We also detail areas where more work is needed, such as reducing defect densities and purity of epitaxial layers, the need for substrates and improved oxides and insulators, improved p-type doping and contacts and an understanding of the basic growth mechanisms.     

The structure of GaN layers grown on SiC and sapphire by molecular beam epitaxy

A comparative study of the defects at the interfaces and inside the layers was carried out in GaN/AlN epitaxial layers on SiC and sapphire. Whereas surface cleaning of the sapphire substrates is rather standardised now, the SiC substrates cleaning is still to optimise conditions, as the high densities of defects inside the epitaxial layers cannot be explained solely by the 3.54% lattice mismatch. The investigated specimens were grown by molecular beam epitaxy (MBE), either assisted by electron cyclotron resonance or an NH₃ gas source system to provide atomic nitrogen. Assuming that MBE is a growth technique more or less close to equilibrium, the observed defects are interpreted and a growth mechanism, for GaN layers on the stepped (0001) SiC and sapphire surfaces, is proposed.     

Influence of AlN buffer layer thickness and deposition methods on GaN epitaxial growth

A gallium nitride (GaN) epitaxial layer was grown by metal-organic chemical vapor deposition (MOCVD) on Si (111) substrates with aluminum nitride (AlN) buffer layers at various thicknesses. The AlN buffer layers were deposited by two methods: radio frequency (RF) magnetron sputtering and MOCVD. The effect of the AlN deposition method and layer thickness on the morphological, structural and optical properties of the GaN layers was investigated. Field emission scanning electron microscopy showed that GaN did not coalesce on the sputtered AlN buffer layer. On the other hand, it coalesced with a single domain on the MOCVD-grown AlN buffer layer. Structural and optical analyses indicated that GaN on the MOCVD-grown AlN buffer layer had fewer defects and a better aligned lattice to the a- and c-axes than GaN on the sputtered AlN buffer layer.     

The effect of nitridation parameters and initial growth conditions on the polarity of GaN epitaxial layers grown by plasma-assisted molecular-beam epitaxy on Si(111) substrates

The dependence of the crystallographic polarity of GaN epitaxial layers produced by nitrogen plasma-enhanced molecular-beam epitaxy on Si(111) substrates on the nitridation parameters and initial growth conditions has been studied. A rapid procedure for determining the polarity of GaN epitaxial layers was developed. It was found experimentally that the nitridation parameters of the silicon substrate have no effect on the polarity of a GaN layer. It was shown that the substrate temperature in the stage of nucleation of a GaN epitaxial layer is one of the factors determining its polarity.     

New nanocrystalline powder substrates for nitrides layer epitaxy

GaN and related III-V nitride materials have been applied for fabrication of electronic and optical devices. The most important factor limiting the mass production of devices based on III-V nitride materials is the high cost of substrates and the elaborate growth techniques. The lack of large, bulk GaN substrates causes that the epitaxial layer of nitrides must be grown on heteroepitaxial substrates. The most widely applied are monocrystalline sapphire, SiC and silicon substrates; but the question of cheap and available substrates for nitrides growth is still open.In this paper, authors present some results of the growth of nitrides layer by the metal-organic vapor-phase epitaxy (MOVPE) technique on new nanocrystalline powder substrates (compressed Al₂O₃+SiC). The influence of substrate composition (the amount of SiC powder) on the properties of the GaN layer are presented. Also the impact of the conditions of epitaxial process on properties of the nitride layers are discussed.