Hexagonal GaN Films Research Articles

Low-temperature GaN growth on silicon substrates by single gas-source epitaxy and photo-excitation

We report a unique low-temperature growth method for epitaxial GaN on Si(111) substrates via a ZrB2(0001) buffer layer. The method utilizes the decomposition of a single gas-source precursor (D2GaN3)3 on the substrate surface to form GaN. The film growth process is further promoted by irradiation of ultraviolet light to enhance the growth rate and ordering of the film. The best epitaxial film quality is achieved at a growth temperature of 550°C with a growth rate of 3nm∕min. The films exhibit intense photoluminescence emission at 10K with a single peak at 3.48eV, indicative of band-edge emission for a single-phase hexagonal GaN film. The growth process achieved in this study is compatible with low Si processing temperatures and also enables direct epitaxy of GaN on ZrB2 in contrast to conventional metalorganic chemical vapor deposition based approaches.

Growth of crack-free GaN on Si(1 1 1) with graded AlGaN buffer layers

Hexagonal GaN films on Si(1 1 1) substrates have been grown by metalorganic vapor phase epitaxy (MOVPE). High-temperature AlN buffers provided a reliable diffusion barrier to avoid meltback etching. By introducing a thick, graded AlGaN buffer layer, the critical thickness for cracking has been increased to at least 2 μ m . The films have been characterized by optical microscopy, transmission electron microscopy (TEM) and X-ray diffraction.

Growth mechanism and structural characterization of hexagonal GaN films grown on cubic GaN (1 1 1)/GaAs (1 1 1)B substrates by MOVPE

Hexagonal phase GaN layers were grown by metalorganic vapor phase epitaxy (MOVPE) on GaAs (1 1 1)B substrates using the cubic phase GaN (c-GaN) as an intermediate layer. High-resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM) measurements showed fairly good crystalline qualities of the h-GaN layers. Dislocation densities estimated from cross-sectional TEM micrographs were found to be as low as 10 7–10 8 cm −2. Flat growth of the c-GaN intermediate layer on GaAs (1 1 1)B substrate was achieved at a low-growth temperature (600 °C). The interface between the GaN layer and the GaAs (1 1 1)B substrate is very smooth without any voids, in spite of the high-growth temperatures of 880–960 °C. It is interesting that the lattice-matched h-GaN layer to the c-GaN intermediate layer can be obtained by controlling the cubic-to-hexagonal structural transition during the MOVPE growth process at appropriate growth temperatures. The growth mechanism and characteristics of the grown h-GaN layers are discussed.

Growth and characterization of GaN films on Si(111) substrate using high-temperature AlN buffer layer

Hexagonal GaN films exceeding 1 μm have been prepared on Si(111) substrates using high-temperature AlN buffer layers, and no cracks were observed. The FWHM of X-ray rocking curve for GaN (0002) was 560 arcsec. Strong band-edge photoluminescence (PL) was present in PL spectra. Micro-Raman spectra using shifts of E 2 phonon showed that GaN films were in compressive stress, which agreed with the characterization result of X-ray lattice parameters method.

Electronic properties of deep defects in n-type GaN

The recurrent presence of deep levels in n-type hexagonal GaN films grown by metal organic chemical vapour deposition (MOCVD) is testified by Fourier transform deep level transient spectroscopy (FTDLTS) studies performed in various samples. Some of these electron traps are systematically detected with emission rates showing activation energies in the range 0.94–1.30 eV and apparent capture cross sections from 10 −15 to 10 −12 cm 2. These properties are deduced from an Arrhenius diagram where data coming from samples of different origins are close to the same straight line, assessing a common physical identity for the corresponding defects. However, isothermal spectroscopy and trap filling kinetics characteristics contradict these capture cross section values and even show nonmonotonic variations which must be ascribed to the emission and capture rates of interactive multiple deep levels. Fine structure of the energy levels, although it is never resolved by standard FTDLTS, is indeed revealed by a high resolution DLTS method (HRDLTS) which relies on a time domain treatment of the transients recorded over a large range of time. Multiple levels are well evidenced in this way and both their ionisation energy and capture cross sections turn out to be much less than those measured by standard FTDLTS, respectively in the ranges 0.26–0.90 eV and near 10 −17 cm 2 on the one hand or less than 10 −20 cm 2 on the other hand, suggesting that these defects can capture two electrons, leading to three charge states and negative charges at least when filled with 2 electrons. Correlation of the concentration of these defects both with the shallow donor concentration and dislocation density suggest that they are due to the doping impurities in dislocation sites.

Room‐temperature ferromagnetism in Cr‐doped GaN films grown by MOMBE on GaAs(111)A substrates

c-axis-oriented Cr-doped hexagonal GaN films with Cr of 0.3, 1 and 5 at.% were epitaxially grown on GaAs(111)A substrates by metal organic molecular beam epitaxy (MOMBE) with metal Ga and dimethylhydrazine (DMHy) as a nitrogen source. Clear hysteresis loop was observed in magnetization versus magnetic filed (M–H) curves for Cr-doped GaN films, and the ferromagnetic behaviour was confirmed even at 350 K. The saturation magnetization per Cr atom is estimated to be approximately 1 Bohr magnetron at 300 K. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

MOCVD of GaN Films on Si Substrates Using a New Single Precursor

Hexagonal GaN (h-GaN) films have been grown on Si(111) substrates by metal organic chemical vapor deposition using the azidodiethylgallium methylamine adduct, Et₂Ga(N₃)·NH₂Me, as a new single precursor. Deposition was carried out in the substrate temperature range 385-650 °C. The GaN films obtained were stoichiometric and did not contain any appreciable amounts of carbon impurities. It was also found that the GaN films deposited on Si(111) had the [0001] preferred orientation. The photoluminescence spectrum of a GaN film showed a band edge emission peak characteristic of h-GaN at 378 nm.

Transmission electron microscopy and atomic force microscopy studies of GaN films grown on AlAs/GaAs(001) substrates

Hexagonal GaN (H-GaN) films have been grown on GaAs(0 0 1) substrates with AlAs buffer layers in a molecular beam epitaxy system. Transmission electron microscopy observations show the tilt of the sub-grains of H-GaN film with respect to the substrate. Such mis-orientation is derived from the steps on AlAs surface due to nitridation. Atomic force microscopy observations show a large density of pinholes on the surface of GaN film grown this way. But when an AlN buffer layer is applied on AlAs/GaAs(0 0 1) substrate, the quality of the GaN film can be much improved with the elimination of mis-orientation and pinholes.

Evidence for a Type-II band alignment between cubic and hexagonal phases of GaN

The photoluminescence spectra of a series of thin, undoped, hexagonal GaN films containing cubic GaN inclusions grown by molecular-beam epitaxy on 6H-SiC have been studied as a function of temperature and excitation power. The dependence of the line shape and peak position of a peak at ∼3.17 eV on laser power suggests that it is associated with a spatially indirect Type-II transition between hexagonal and cubic GaN. The values of the band offsets extracted from our data are in good agreement with theoretical predictions.

GaN thin film growth on GaAs (0 0 1) by CBE and plasma-assisted MBE

GaN thin films were deposited on GaAs (0 0 1) substrates using chemical beam epitaxy (CBE) and plasma-assisted molecular beam epitaxy (MBE) methods. The effect of in situ substrate cleaning and pre-treatment on the interfacial structure was investigated. Time-of-flight mass spectroscopy of recoiled ions was utilized to measure in situ the surface composition. The microstructure of GaN films was studied with selected area electron diffraction, and both conventional and high-resolution transmission electron microscopy. It was found that simultaneous electron cyclotron resonance (ECR) plasma treatment using an Ar/N 2 mixture increases the yield of a cubic phase in GaN films during CBE growth. Resulting films are less defective than mixed phase GaN films grown after pure N 2 plasma nitridation. In the case of plasma-assisted MBE growth, a low-temperature annealing followed by N 2 ECR nitridation process yields cubic GaN thin films while N 2 ECR nitridation without a sample annealing to 600°C yields hexagonal phase GaN films.

Dislocation core structures in GaN grown on Si(111) substrate

The Ga-N bond extension and compression at the cores of different types of dislocation in hexagonal GaN films grown on Si(111) substrates have been studied by high-resolution transmission electron microscopy. The relative bond extension and compression are about 6 4 and 7 4% respectively around the cores of mixed dislocations and about 9 6 and 12 7% respectively around the cores of pure edge dislocations. The core structure of a pure edge dislocation is an eightfold atom-column ring.

Synthesis of wurtzite GaN films by reactive hot wall vapour deposition technique: fabrication of Au/GaN Schottky diode

Wurtzite GaN films were synthesised on p-Si(1 0 0) and n-Si(1 0 0) substrates by plasma assisted hot wall vapour deposition technique without using any buffer layer. The films were characterised by electrical and optical measurements while the microstructural information was obtained from atomic force microscopy (AFM) and X-ray diffraction (XRD) studies. The compositional study for the GaN film was carried out using SIMS. Photoluminescence (PL) measurement at room temperature exhibited near band edge (BE) emission at ∼363 nm (3.4 eV) along with a broad yellow band. The bonding environment in the film was revealed from FTIR studies. Schottky diode (Au/GaN), fabricated with hexagonal GaN film on n-Si substrate, was characterised by I– V and C– V analysis.

Initial growth of hexagonal GaN grown on an Si(1 1 1) substrate coated with an ultra-thin SiC buffer layer

The initial growth of hexagonal GaN films grown on Si(1 1 1) substrates coated with an ultra-thin SiC buffer layer was studied. It was found that a 2.5-nm-thick SiC layer is an effective buffer layer for GaN growth on an Si(1 1 1) substrate. Under Ga-rich growth conditions, Ga adatoms, in comparison to those under N-rich growth conditions, were highly mobile. Consequently, the GaN films had a flat surface and an almost stacking-fault-free microstructure. The initial GaN nucleations quickly coalesced laterally to submicron-sized grains. Under N-rich growth conditions, the initial GaN nucleations saturated at a diameter of about 50 nm (measured at the film surface). The grown GaN films showed statistical roughening of the surface and a characteristic columnar structure. The yellow-band luminescence (YL) was sensitive to the microstructures of the GaN films prepared under almost the same growth conditions, suggesting that the Ga-vacancy is not the sole source of YL.

Growth of crack-free hexagonal GaN films on Si(100)

Hexagonal GaN films have been grown on Si(100) substrates by employing a sputtered AlN buffer layer followed by another high-temperature metalorganic chemical vapor deposition (MOCVD) grown AlN buffer layer. The highly oriented structure of sputtered AlN provides a hexagonal template for subsequent AlN and GaN growth. The GaN films are evaluated by transmission electron microscopy, selected area electron diffraction, x-ray diffraction, and photoluminescence and exhibit a purely hexagonal columnar structure. The orientation of the GaN columns depends on the thickness of both the sputtered AlN buffer layer and the MOCVD grown AlN buffer layer. The surface of GaN films is shiny and crack free up to a thickness of 2 μm studied in this work.

Selective area metalorganic vapor phase epitaxy of thick crack-free GaN films on trenched SiC substrates

Crack-free GaN films up to 11-μm-thick have been grown by using trenched SiC substrates and selective area metalorganic vapor phase epitaxy. These crack-free GaN films have hexagonal shapes and are surrounded by trenches. 97% of the hexagonal GaN films with side lengths of 100 μm and thickness of 11 μm was crack-free. The GaN films do not crack because the lateral propagation of cracks stops at the trenches and strain is relaxed in the small-area hexagonal GaN. This strain relaxation is confirmed by micro-Raman scattering measurements and agrees well with theoretical predictions.

A study of cracking in GaN grown on silicon by molecular beam epitaxy

It is observed that GaN layers grown on silicon substrates often crack. The crack characteristics in hexagonal GaN films on Si(111) has been characterized using scanning electron microscopy and Nomarski optical microscopy. The effects of growth temperature, layer thickness, and V/III ratios on the cracking have been analyzed. The critical thickness for crack initiation was estimated using a simple theoretical model and is shown to have good agreement with experimental results. Crack-free GaN on Si(111) of thicknesses greater than one micron is possible by using low growth temperatures.

Polarity dependence of hexagonal GaN films on two opposite c faces of Al2O3 substrate

GaN films were grown in pairs on two opposite c faces of Al2O3 substrate by low-pressure metal-organic vapor phase epitaxy, and studied by scanning electron microscopy and converged beam electron diffraction. It is found that GaN film on the c-Al2O3 whose c face is forward to its crystal seed has [0001] polarity, and the other film on the c-Al2O3 whose c face is backward to its crystal seed has [0001̄] polarity.

Effective carrier mass and mobility versus carrier concentration in p- and n-type α-GaN determined by infrared ellipsometry and Hall resistivity measurements

We use infrared spectroscopic ellipsometry (IRSE) to obtain optical free-carrier parameters in p- and n-type hexagonal ( α-) GaN films. Si- and Mg-doped films (40 nm to 4 μm) were grown on sapphire by MOCVD and MBE, respectively. Results from electrical (Hall) measurements are combined with those obtained from the Drude model by lineshape analysis of the IRSE data. We derive effective electron ( m e) and hole ( m h) mass parameters for GaN, where m h depends on the free-hole concentration N e. Undoped but n-type conductive films grown under similar conditions with different thicknesses, d, show a double-logarithmic dependence over two orders of magnitude between N e and d: log( N e)∝ c elog( d) with c e<0. A similar behavior is observed for free hole concentrations in Mg-doped α-GaN. Inhomogeneously activated donors or acceptors due to a decrease in misfit dislocations along the growth direction could explain the thickness dependence. A low electron mobility and large lattice mode broadening were observed at the layer–substrate interface in n-type GaN films. For high N e values, the films reveal surface carrier depletion layers with thickness d SL. We find that d SL increases with decreasing N e.

Nucleation of hexagonal AlN in nitridized AlAs buffer on (0 0 1) GaAs substrate

Hexagonal GaN film has been grown on (0 0 1) GaAs substrate with nitridized AlAs buffer layer. The (0 0 0 1) plane of the hexagonal GaN(h-GaN) is parallel to the (0 0 1) plane of the GaAs substrate. The nitridized AlAs/GaAs(0 0 1) material was analyzed by high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). The HRTEM observations show that hexagonal AlN(h-AlN) can nucleate on the AlAs surface after nitridation with its (0 0 0 1) plane parallel to the (0 0 1) plane of the AlAs. Some cubic AlN(c-AlN) micro grains were also observed. The close-packed atom planes of the c-AlN grains are parallel to the (0 0 1) planes of the AlAs. The AFM result shows that the surface morphology of the nitridized AlAs is rough with high-density dots. Such AlN layer can be taken as a prelayer for growing h-GaN on the (0 0 1) GaAs substrate.

Fine Structure of the Blue Photoluminescence in High-Purity Hexagonal GaN Films

The photoluminescence of GaN films grown by molecular-beam epitaxy on 6H-SiC(0001) substrates was studied. Owing to the high structural perfection of the films, a series of narrow emission lines related to donor–acceptor recombination was detected in the blue spectral region. This finding lends support to the donor–acceptor origin of the blue emission in GaN. In the spectra of less perfect films, these lines overlap and merge into a broad band.