Crystalline Quality Of GaN Films Research Articles

Role of film thickness on the structural and optical properties of GaN on Si (100) grown by hollow-cathode plasma-assisted atomic layer deposition

In this study, the authors report on the evolution of crystallinity, chemical composition, surface morphology, and optical properties of highly oriented (002) GaN films as a function of film thickness, ranging from 5.37 to 81.40 nm deposited on Si substrates via low-temperature hollow-cathode plasma-assisted atomic layer deposition (HCPA-ALD). GaN thin films were prepared using a sequential injection of triethyl gallium and N2/H2 plasma within the self-limited growth regime, i.e., ALD window at 200 °C. The grazing-incidence x-ray diffraction (GIXRD) analysis reveals that GaN films have a (002) preferential growth direction and the crystalline quality of GaN films was improved with the increase in thickness. GIXRD and atomic force microscopy confirmed the presence of GaN grains, and the grain size increases when the thickness of the GaN layer increases from 5.37 to 48.65 nm. From the x-ray photoelectron spectra, it was shown that the amount of oxygen incorporated in the GaN film decreases as the thickness increases. The spectroscopic ellipsometry analysis reveals that the optical film density and local crystallinity was improved with increasing film thickness to 48.65 nm, but further increase in the film thickness does not seem to improve these features. The optical band edge results suggested that bandgap widening is valid for all HCPA-ALD grown GaN samples. The overall results suggested that GaN films with thicknesses above 48.65 nm have different behavior compared to the thinner GaN films.

Structural evolution of AlN buffer and crystal quality of GaN films on a‐ and c‐sapphire grown by metalorganic vapor phase epitaxy

AbstractLow‐temperature‐deposited (LT‐) AlN layers and the initial growth of GaN on a‐ and c‐sapphire have been investigated and compared. The as‐deposited LT‐AlN layers on both a‐ and c‐sapphire are composed of high‐density islands. By annealing, the islands become large. The density of islands on a‐ and c‐sapphire are 5.6 × 1010 and 7.9 × 1010 cm‐2, respectively, indicating that the evolution of the solid phase epitaxy of AlN islands induced by annealing is different on a‐ and c‐sapphire. The moiré spacings obtained by plan‐view electron microscopy reveal that the strain of the AlN islands on a‐sapphire is larger than that of islands on c‐sapphire. These results imply that development of the solid phase epitaxy is influenced by the strain. The coalescence of GaN islands grown on a‐sapphire, which has larger AlN islands, is advanced compared with that of islands on c‐sapphire. The relationship between the annealed AlN island density and the threading dislocation (TD) density in the GaN films is investigated on a‐ and c‐sapphire. The TD density of the GaN films is small when the density of annealed AlN islands is low. These results indicate that decreasing the island density of the annealed AlN layers is essential for improving the crystalline quality of GaN films. It is therefore important to control the solid phase epitaxy of LT‐AlN layers by annealing. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Properties comparison of GaN epilayers deposited under different growth temperatures

AbstractUndoped GaN epilayers were grown on c-plane sapphire substrates under different growth temperatures by metalorganic chemical vapour deposition (MOCVD). The optical and structural characteristics of these grown samples were studied and compared. It was found that the crystalline quality of GaN film deposited at 1050°C was better that of other samples. Photoluminescence spectra showed that the intensities of yellow luminescence band of the samples decreased as the growth temperature increased. All above test results demonstrate that high temperature deposition can serve as a good method for high-quality GaN epilayer growth and there exists an optimal growth temperature.

Structural characterization of cubic and hexagonal GaN thin films grown by IBA–MBE on SiC/Si

Buried ion beam synthesized 3C-SiC layers were revealed to the surface of silicon wafers to provide lattice matched substrates for GaN thin film epitaxy. Both epitaxial SiC/Si(1 1 1) and SiC/Si(0 0 1) growth surfaces with either six- or four-fold crystal symmetry, respectively, were formed. GaN thin film growth was achieved by ion beam assisted molecular beam epitaxy (IBA–MBE) which – owing to the energy and momentum transfer of the ions – allows to deposit epitaxial thin films at particularly low growth temperatures where both the stable hexagonal and the metastable cubic polytype of GaN can be formed. It is shown by X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (XTEM) that using appropriate fluxes of Ga atoms both purely hexagonal and purely cubic GaN films can be grown epitaxially on SiC/Si(1 1 1) and SiC/Si(0 0 1), respectively. Thereby Ga rich growth conditions seem to stabilize the formation of the cubic polytype. It is obvious from XTEM studies that the high density of crystal defects in the SiC layer is not transferred onto the growing GaN films and that the crystalline quality of GaN films improves with increasing film thickness. The influence of surface roughness and wettability, interfacial cavities and the nucleation of twin-type defects at the GaN/SiC interface on the crystalline quality of the GaN thin films is discussed.

The influence of sapphire substrate orientation on crystalline quality of GaN films grown by hydride vapor phase epitaxy

Abstract GaN films were grown by hydride vapor phase epitaxy on sapphire substrates with orientations c -(0 0 0 1), a -(1 1 2¯ 0), m -(1 0 1¯ 0) and r -(1 0 1¯ 2) using N 2 as a carrier gas. The crystalline perfection of the grown films was studied by X-ray diffraction, by optical microscopy and scanning electron microscopy and by microcathodoluminescence (MCL). It was found that, for c - and a -oriented sapphire substrates, the GaN films showed (0 0 0 1) orientation, for m -oriented sapphire the films showed semi-polar (1 0 1¯ 3) orientation, while for r -sapphire substrates GaN layers with non-polar a (1 1 2¯ 0) orientation could be grown. The surface morphology of the GaN films and their crystalline structure strongly depended on the substrate orientation. With increasing the layer thickness the halfwidth of the X-ray rocking curves monotonically decreased which points to improvement of the crystalline quality. The best quality films were grown for the c - and r -oriented substrates: respectively, 460 arcseconds at the thickness of 400 mm, 600 arcseconds at the thickness of 300 μm. For m -oriented substrates the halfwidth was 1300 arcseconds at comparable thickness. The density of stacking faults for semi-polar and non-polar films was determined by MCL imaging. The linear density of stacking faults was found to be 5×10 4 cm −1 for films grown on m -oriented sapphire substrates and 5103 cm −1 for a -GaN films grown on r -sapphire.

Effect of reactor pressure on the growth rate and structural properties of GaN films

The effect of reactor pressure on the growth rate, surface morphology and crystalline quality of GaN films grown on sapphire by metalorganic chemical vapor deposition is studied. The results show that as the reactor pressure increases from 2500 to 20000 Pa, the GaN surface becomes rough and the growth rate of GaN films decreases. The rough surface morphology is associated with the initial high temperature GaN islands, which are large with low density due to low adatom surface diffusion under high reactor pressure. These islands prolong the occurrence of 2D growth mode and decrease the growth rate of GaN film. Meanwhile, the large GaN islands with low density lead to the reduction of threading dislocation density during subsequent island growth and coalescence, and consequently decrease the full width at half maximum of X-ray rocking curve of the GaN film.

The influence of nitridation time on the structural properties of GaN grown on Si (111) substrate

In the present paper, the effect of in-situ substrate nitridation time on crystalline quality of GaN films grown on Si (111) substrates by metal organic chemical vapor deposition (MOCVD) were investigated. A thin buffer layer of silicon nitride (SiN x ) with various thicknesses was achieved through the nitridation of substrate at different nitiridation times ranging from 0 to 660 s. The structural characteristics, such as dislocation densities, correlation lengths of columnar crystallites, the tilt and twist of the mosaic structure, and the angles of rotational disorder, were all studied in detail by using a planar and cross-sectional view of high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (HRXRD) performed at different scattering geometries. It was found that the dislocation densities, lateral coherence lengths, vertical coherence lengths, and the tilt and twist of mosaic blocks in GaN films monotonically varies with the nitridation time. The experimental findings showed that the nitridation times had more influence on edge dislocation densities than the screw type.

Annealing of GaN/ZnO/Si Films Deposited by Pulsed Laser Deposition

ZnO buffer layers were used in the fabrication of GaN films by pulsed laser deposition (PLD) in order to improve the initial nucleation and growth of GaN films when they are annealed in ammonia ambience at different temperatures. The crystalline quality, composition and surface morphology of the films were characterized by X-ray diffraction (XRD) analysis, Fourier transform infrared (FTIR) analysis and atomic force microscopy (AFM). Through the analyses of measured results, a conclusion can be drawn that the crystalline quality of GaN films deposited on ZnO buffer layers is much higher than that of those deposited directly on Si substrates with the volatilization of ZnO. The volatilization of ZnO buffer layers gives the grains of GaN more chances to move and form Ga–N bonds at high temperatures. The annealing temperature markedly affects the preparation of GaN films and the optimum annealing temperature is 950 °C under our experimental conditions.

Control of the crystalline quality of wurtzitic GaN films deposited on γ-LiAlO2 by ion-beam assisted molecular-beam epitaxy

Ion-beam assisted molecular-beam epitaxy was applied to deposit m-plane oriented wurtzitic gallium nitride films on γ-LiAlO2(100) substrates. The crystalline quality of the films was characterized by high resolution X-ray diffraction and transmission electron microscopy. The results show that in particular the nitrogen ion-to-gallium atom flux ratio is a crucial parameter influencing the growth direction and the crystalline quality of the GaN films on the (100) plane of γ-LiAlO2. Films deposited in the N-rich regime are exclusively m-plane oriented and exhibit a columnar morphology with a high mosaic spread and a rough surface. Films deposited in the Ga-rich regime are also exclusively m-plane oriented, but in contrast, they are of high crystalline quality and the surfaces are relatively smooth. Furthermore, extremely N-rich deposition conditions were found to lead to the formation of c-plane oriented wurtzitic GaN domains.

Relationship between structure characteristic and blue luminescence in unintentional doped GaN layers

Properties of some GaN layers with different blue luminescence relative intensity were investigated by photoluminescence, Rutherford backscattering/channeling and double crystal X-ray diffraction, respectively. The surface minimum yields χ min of Rutherford backscattering/channeling and the FWHM of double crystal X-ray diffraction obviously increase with the increase of the intensity ratio of the blue luminescence to the band-edge emission. The blue luminescence is due to some intrinsic defects that can largely deteriorate the crystalline quality of GaN films. The luminescence mechanism of the blue luminescence in unintentional doped GaN films is different from that of the blue luminescence about 2.9 eV in GaN:Mg films.

Epitaxial tilting of GaN grown on vicinal surfaces of sapphire

The epitaxial tilting effect of GaN films grown on vicinal (0001) surfaces of sapphire and its relationship with the offcut angles and the substrate surface steps have been revealed using synchrotron Laue method and high-resolution x-ray diffraction. This effect is a general consequence of the large out-of-plane lattice mismatch between GaN and sapphire and can be explained by the extended Nagai theory based on the step configurations. The large lattice tilts and their formation mechanism indicate that the substrate surface morphology may be a very important factor that influences the epitaxy process and the crystalline quality of GaN films in vicinal surface epitaxy.

Synthesis of epitaxial GaN single-crystalline film by ultra high vacuum r.f. magnetron sputtering method

GaN films were prepared by an ultra high vacuum r.f. magnetron sputtering method, and their crystalline structures depending on total gas pressure, growth temperature, substrate material and film thickness were examined. In X-ray diffraction measurements, GaN film grown at room temperature and gas pressures less than 0.53 Pa, indicated a higher c-axis preferred orientation. When the growth temperature was increased up to 800 °C, crystalline quality of the GaN film was dramatically improved as compared with those prepared at temperatures less than 800 °C. For GaN films on α-Al 2O 3 (0001) substrates, full width at half maximum values of both X-ray rocking curve for α-GaN (0002) plane and E 2 (high) phonon peak at Raman spectra, became narrower than those for GaN films on Si (100) substrates. Moreover, the crystalline quality of GaN film was improved with an increasing film thickness, in the case of α-Al 2O 3 substrate. A scanning electron microscope image of 1.5 μm thick GaN film surface on α-Al 2O 3 substrate indicated a number of hexagonal pyramids with six {10–11} facets. The X-ray pole-figure indicated that the GaN film on α-Al 2O 3 substrate was epitaxially grown single crystal.

Residual stress in GaN epilayers grown on silicon substrates

GaN films on AlN buffer layers were grown on (111) silicon substrates in a vertical rotating disk metal–organic chemical-vapor deposition reactor (CVD, Inc.). The dependence of residual stress in GaN films on the V/III molar flow ratio was studied. The crystalline quality of GaN films was analyzed by x-ray diffractometry, and the x-ray lattice parameter method was used to determine the residual stress in the films by measuring the c-axis and a-axis strain separately. The x-ray results show that the residual stress decreases with an increasing V/III molar flow ratio if all other growth parameters are kept constant.

A comparative study of GaN epilayers grown on sapphire and SiC substrates by plasma-assisted molecular-beam epitaxy

We report structural, electrical, and optical data for GaN samples grown on both 6H-SiC and sapphire substrates. A two-stage substrate preparation procedure was employed for removing oxygen from 6H-SiC and c-plane sapphire substrates without the need for elaborate high-temperature thermal degassing. Both sapphire and SiC substrates were treated with hydrogen plasma to reduce the surface contamination as evidenced by the observation of sharp (1×1) reconstruction RHEED (reflected high-energy electrons diffraction) patterns. Thin AlN buffer layers were employed and the crystalline quality of GaN films was studied by temperature-dependent Hall measurements, photoluminescence, and x-ray diffraction. Layers with room-temperature mobilities as high as 580 cm2/V s on SiC substrates were obtained.