Optical and structural studies of high-quality bulk-like GaN grown by HVPE on a MOVPE AlN buffer layer (original) (raw)
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Journal of Applied Physics, 2004
Crack-free bulk-like GaN with high crystalline quality has been obtained by hydride-vapor-phase-epitaxy ͑HVPE͒ growth on a two-step epitaxial lateral overgrown GaN template on sapphire. During the cooling down stage, the as-grown 270-m-thick GaN layer was self-separated from the sapphire substrate. Plan-view transmission electron microscopy images show the dislocation density of the free-standing HVPE-GaN to be ϳ2.5ϫ10 7 cm Ϫ2 on the Ga-polar face. A low Ga vacancy related defect concentration of about 8ϫ10 15 cm Ϫ3 is extracted from positron annihilation spectroscopy data. The residual stress and the crystalline quality of the material are studied by two complementary techniques. Low-temperature photoluminescence spectra show the main neutral donor bound exciton line to be composed of a doublet structure at 3.4715 ͑3.4712͒ eV and 3.4721 ͑3.4718͒ eV for the Ga-͑N-͒ polar face with the higher-energy component dominating. These line positions suggest virtually strain-free material on both surfaces with high crystalline quality as indicated by the small full width at half maximum values of the donor bound exciton lines. The E 1 (TO) phonon mode position measured at 558.52 cm Ϫ1 ͑Ga face͒ by infrared spectroscopic ellipsometry confirms the small residual stress in the material, which is hence well suited to act as a lattice-constant and thermal-expansion-coefficient matched substrate for further homoepitaxy, as needed for high-quality III-nitride device applications.
Japanese Journal of Applied Physics, 2005
High-quality 2 00 crack-free free-standing GaN has been attained by hydride vapour phase epitaxial growth on a Si-doped MOVPE GaN template with a low dislocation density and subsequent laser-induced lift-off process. A low value of dislocation density of $2:0 Â 10 7 cm À2 on the Ga-polar face was determined from cathodoluminescence images. X-ray diffraction (XRD) and low-temperature photoluminescence (PL) were exploited to investigate the structural and optical properties of the GaN material. The full width at half maximum value of XRD !-scan of the free-standing GaN is 248 arcsec for the (1 0 1 1 4) reflection. The XRD and low-temperature PL mapping measurements consistently proved the high crystalline quality as well as the lateral homogeneity and the small residual stress of the material. Hence, the bulk-like free-standing GaN studied here is highly advantageous for being used as a lattice-constant and thermal-expansion-coefficient matched substrate for additional strain-free homoepitaxy of III-nitrides-based device heterostructures. The strain-free homoepitaxy will significantly reduce the defect density and thus, an improvement of the device performance and lifetime could be achieved.
Optical and Structural Characteristics of Virtually Unstrained Bulk-Like GaN
Japanese Journal of Applied Physics, 2004
Bulk-like GaN with high structural and optical quality has been attained by hydride vapor-phase epitaxy (HVPE). The asgrown 330 mm-thick GaN layer was separated from the sapphire substrate by a laser-induced lift-off process. The full width at half maximum values of the X-ray diffraction (XRD) !-scans of the free-standing material are 96 and 129 arcsec for the (1 0 À1 4) and (0 0 0 2) reflection, respectively, which rank among the smallest values published so far for free-standing HVPE-GaN. The dislocation density determined by plan-view TEM images is 1{2 Â 10 7 cm À2 . Positron annihilation spectroscopy studies show that the concentration of Ga vacancy related defects is about 1:5 Â 10 16 cm À3 . The high-resolution XRD, photoluminescence, -Raman, and infrared spectroscopic ellipsometry measurements consistently prove that the free-standing material is of high crystalline quality and virtually strain-free. Therefore it is suitable to serve as a substrate for stress-free growth of high-quality III-nitrides based device heterostructures.
Properties of nonpolar a-plane GaN films grown by HVPE with AlN buffers
Journal of Crystal Growth, 2005
The influence of high temperature AlN buffer layers on the morphology, structural and optical characteristics of aplane GaN grown by hydride vapour phase epitaxy on r-plane sapphire was investigated. While the morphology of the a-GaN was found to be significantly improved by using a-plane AlN buffer layer similarly to the effect observed in cplane hydride vapour phase epitaxy GaN growth, the microstructure ensemble was revealed to be more complicated in comparison to that of the c-plane GaN. Higher dislocation density and prismatic stacking faults were observed. Moreover, in-plane anisotropic structural characteristics were revealed by high resolution X-ray diffraction employing azimuthal dependent and edge X-ray measurement symmetric geometry. In addition, the near band edge photoluminescence peaks, red-shifted with respect to that in c-plane GaN were observed. The latter were explained by the influence of the higher defect density and more complex strain distribution.
Crystallization of free standing bulk GaN by HVPE
physica status solidi (c), 2006
Gallium nitride was crystallized on 2 inch MOVPE GaN/sapphire substrates by Hydride Vapor Phase Epitaxy. A stable growth has been achieved in long duration (>10 h) processes at growth rates bigger than 100 µm/h. As a result, entirely transparent and colorless bulk crystals with thickness exceeding 2 mm were obtained. The cracks in the thick HVPE GaN layers deposited on the MOVPE GaN/sapphire substrates can appear especially during cooling of the system after crystallization. It is shown that the formation of cracks at cooling is dependent on the gradients in the layer thickness The relaxation of strains in the resulting crystal coupled to the substrate leads to the self separation of GaN from sapphire. (At present 30 x 30 x 2 mm free standing bulk GaN crystals are obtained). The GaN crystals are characterized by defect selective etching (DSE) and X-ray diffraction. The density of threading dislocations (measured by DSE of (0001) surface) decreases with the thickness of the HVPE layer and becomes lower than 10 7 cm -2 in the layers thicker than app. 1 mm. The X-ray rocking curves for (0002) reflection (slit 0.5 x 0.1 mm) are in the range of 80-95 arcsec. However, larger scans reveal bending of crystallographic {0001} planes. The behavior of these "deformed" free standing crystals used as substrates for HVPE re-growth is also analyzed.
MOVPE homoepitaxy of high-quality GaN: Crystal growth and devices
Progress in Crystal Growth and Characterization of Materials, 2000
Epitaxial growth on GaN bulk single crystal substrates sets new standards in GaN material quality. The outstanding properties provide insights into fundamental material parameters (e.g. lattice constants, exciton binding energies, etc.) with a precision not obtainable from heteroepitaxial growth on sapphire or Sic. With metdorgunic vapor phase epituxy (MOVPE) we realized unstrained GaN layers with dislocation densities about six orders of magnitude lower than in heteroepitaxy. By the use of dry etching techniques for surface preparation, an important improvement of crystal quality is achieved. Those layers reveal an exceptional optical quality as determined by a reduction of the low-temperature photoluminescence (PL) linewidth from 5 meV to 0.1 meV and a reduced X-ray diffraction (XRD) rocking curve width from 400 to 20 arcsec. As a consequence of the narrow PL linewidths, new features as, e. g. a fivefold fine structure of the donor-bound exciton line at 3.471 eV was detected Additionally, all three free excitons as well as their excited states are visible in PL at 2K. Dry etching techniques for surface preparation allow morphologies of the layers suitable for device applications. We report on InGaN/GaN multiquantum-well (MQW) structures as well as GaN pn-and InGaN/GaN double heterostructum light emitting diodes (LIDS) on GaN bulk single crystal substrates. Those LEDs are twice as bright as 0960-8974/01/S -see front matter 0 2001 Published by Elsevier Science Ltd. PII: SO960-8974(00)00044-9 58 C. Kirchener et al. /Prog. Crystal Growth and Charact. 41 (2000) their counterparts grown on sapphire. In addition they reveal an improved high power characteristics, which is attributed to an enhanced crystal quality and an increased doping.
Growth and structural, optical and electrical properties study of bulk GaN
physica status solidi (c), 2012
High-quality GaN substrates with a thickness of 0.5 to 1.2 mm were prepared by halide vapor-phase epitaxy (HVPE) on c-plane sapphire and by means of a post-growth laser-induced lift-off or natural stress-induced (self-) separation process. Structural, optical and electrical properties of the grown material were studied in detail. Resistivity and doping concentrations dependence on temperature were investigated and the results revealed a low donor concentration (2x10 15 cm-3 at 273 K) and a moderate resistivity (10-48 Ωcm) of the samples. Several deep levels observed in the DLTS spectra were identified. The study shows that the GaN material is selfcompensated and a strong influence of point defects related to dislocations on electronic transport is observed. The HVPE growth on InGaN/GaN multi-quantum wells buffer layers and subsequent self-separation method was seen as advantageous, in comparison to the laser-induced lift-off one, in respect to a lower cost and better crystalline quality of the GaN material obtained.
Applied Physics Letters, 2000
A free-standing 300-μm-thick GaN template grown by hydride vapor phase epitaxy has been characterized for its structural and optical properties using x-ray diffraction, defect delineation etch followed by imaging with atomic force microscopy, and variable temperature photoluminescence. The Ga face and the N face of the c-plane GaN exhibited a wide variation in terms of the defect density. The defect concentrations on Ga and N faces were about 5×105 cm−2 for the former and about 1×107 cm−2 for the latter. The full width at half maximum of the symmetric (0002) x-ray diffraction peak was 69 and 160 arc sec for the Ga and N faces, respectively. That for the asymmetric (10–14) peak was 103 and 140 arc sec for Ga and N faces, respectively. The donor bound exciton linewidth as measured on the Ga and N faces (after a chemical etching to remove the damage) is about 1 meV each at 10 K. Instead of the commonly observed yellow band, this sample displayed a green band, which is centered at about...