Photoluminescence study of Si-doped a-plane GaN grown by MOVPE

Si-doped a-plane GaN films with different doping concentrations were grown by metal-organic vapor phase epitaxy. A mirrorlike surface without pits or anisotropic stripes was observed by optical microscopy. Detailed optical properties of the samples were characterized by temperature- and excitation-intensity-dependent PL measurements. A series of emission peaks at 3.487, 3.440, 3.375–3.350, 3.290 and 3.197 eV were observed in the low-temperature PL spectra of all samples. The origin of these emissions is discussed in detail.     

Optical properties of MOVPE-grown a-plane GaN and AlGaN

a-Plane GaN and AlGaN were grown on r-plane sapphire by low-pressure metal-organic vapor epitaxy (LP-MOVPE), and the effects of reactor pressure (from 40 to 500 Torr) and growth temperature (from 1020 to 1100 °C) on the crystalline quality and surface morphology of a-plane GaN were studied. The a-plane GaN grown under 40 Torr had a smooth-surface morphology but a poor crystalline quality; however, the a-plane GaN grown under 500 Torr had higher crystalline quality and optical properties, whose full-width at half-maximum of the X-ray rocking curve (XRC-FWHM) and intensity of yellow luminescence (YL) were smaller. Furthermore, the optical properties of a-plane GaN were investigated by photoluminescence (PL) in detail. We also studied the emission properties of a-plane Al_0.35Ga_0.65N grown at room temperature.     

One-sidewall-seeded epitaxial lateral overgrowth of a-plane GaN by metalorganic vapor-phase epitaxy

The selective regrowth of GaN during sidewall-seeded epitaxial lateral overgrowth was performed. In addition to adjusting the V/III ratio, control of offset angle of the sidewall was found to be effective for realizing one-sidewall-seeded a-plane (1 1 2¯ 0) GaN on r-plane (1 1¯ 0 2) sapphire. The number of coalescence regions on the grooves was reduced, and threading-dislocation and stacking-fault densities as low as 10⁶–10⁷ cm⁻² and 10³–10⁴ cm⁻¹, respectively, were successfully realized.     

Improvements in a-plane GaN crystal quality by AlN/AlGaN superlattices layers

Non-polar (1 1 2¯ 0) a-plane GaN films have been grown by low-pressure metal-organic vapor deposition on r-plane (1 1¯ 0 2) sapphire substrate. We report on an approach of using AlN/AlGaN superlattices (SLs) for crystal quality improvement of a-plane GaN on r-plane sapphire. Using X-ray diffraction and atomic force microscopy measurements, we show that the insertion of AlN/AlGaN SLs improves crystal quality, reduces surface roughness effectively and eliminates triangular pits on the surface completely.     

Effects of initial stages on the crystal quality of nonpolar a-plane AlN on r-plane sapphire by low-pressure HVPE

A 4–6 μm thick a-plane (1 1 2¯ 0) AlN was grown on r-plane sapphire substrate by low-pressure hydride vapor phase epitaxy (LP-HVPE), using a direct growth without any nitridation and buffer layer, a single-step nitridation growth, a two-step nitridation growth and a two-step buffer growth method. For the two-step buffer growth procedure, smoother surface is observed with the lower full widths at half maximum (FWHM) of X-ray rocking curves (XRC) compared with the other two kinds of nitridation procedures. A smaller FWHM of in-plane XRC peak anisotropy features are reversed, which is consistent with the smaller in-plane stress anisotropic distribution in a-plane AlN, when the two-step nitridation or buffer growth method is used. In four kinds of initial growth procedures, the two-step buffer method is the suitable method for the growth of a-plane AlN by HVPE with the high crystal quality and more isotropic distribution.     

Influence of off-cut angle of r-plane sapphire on the crystal quality of nonpolar a-plane AlN by LP-HVPE

The effect of the off-cut angle of an r-plane sapphire substrate has been investigated on the growth of a-plane AlN thick layer by low-pressure hydride vapor phase epitaxy (LP-HVPE). The off-cut angle (θ) was changed from +5.0° (close to c-axis) to −5.0° (close to m-axis). Results show that the crystalline quality and surface morphology are very sensitive to the sign of θ off-angle. The plus θ off-angle is found to be dramatically reduce the full-widths at half-maximum (FWHM) of X-ray rocking curves (XRC), compared with the minus θ off-angle. In-plane FWHM anisotropic feature marked as M- or W-shape dependence on azimuth angle was observed for a-plane AlN. The shape and degree of anisotropy depend on the sign of θ off-angle, while the plus of θ off-angle will leads to the W-shape and the decreased anisotropy. The minimum crystal tilts and twists of the films are observed for the vicinal sapphires with the plus off-angles of +0.2° to +1.0°.     

Assessment of defect reduction methods for nonpolar a-plane GaN grown on r-plane sapphire

This work assesses the relative effectiveness of four techniques to reduce the defect density in heteroepitaxial nonpolar a-plane GaN films grown on r-plane sapphire by metalorganic vapour phase epitaxy (MOVPE). The defect reduction techniques studied were: 3D–2D growth, SiN_x interlayers, ScN interlayers and epitaxial lateral overgrowth (ELOG). Plan-view transmission electron microscopy (TEM) showed that the GaN layer grown in a 2D fashion had a dislocation and basal-plane stacking fault (BSF) density of (1.9±0.2)×10¹¹ cm⁻² and (1.1±0.9)×10⁶ cm⁻¹, respectively. The dislocation and BSF densities were reduced by all methods compared to this 2D-grown layer (used as a seed layer for the interlayer and ELOG methods). The greatest reduction was achieved in the (0 0 0 1) wing of the ELOG sample, where the dislocation density was <1×10⁶ cm⁻² and BSF density was (2.0±0.7)×10⁴ cm⁻¹. Of the in-situ techniques, SiN_x interlayers were most effective: the interlayer with the highest surface coverage that was studied reduced the BSF density to (4.0±0.2)×10⁵ cm⁻¹ and the dislocation density was lowered by over two orders of magnitude to (3.5±0.2)×10⁸ cm⁻².     

High-resolution X-ray diffraction analysis on HVPE-grown thick GaN layers

The threading dislocation density of hydride vapor phase epitaxy (HVPE)-grown thick GaN layers was measured by high-resolution X-ray diffraction (HR-XRD). Three models were compared, namely mosaic model, Kaganer model and modified Kaganer model. X-ray rocking curves (XRC) of (0 0 0 2), (1 0 1¯ 5), (1 0 1¯ 4), (1 0 1¯ 3), (1 0 1¯ 2), (1 0 1¯ 1) and (1 0 1¯ 0) planes were recorded for quantitative analysis. The screw-, edge-, and mixed-type threading dislocation densities were simulated from the XRD line profile by using the three models. The dislocation density was also measured by atomic force microscopy (AFM), wet chemical etching and cathodoluminescence (CL). The results showed that the Kaganer model was more physically precise and well explained the rocking curve broadening for HVPE-grown high-quality GaN compared with the mosaic model. Assuming a randomly distributed threading dislocation configuration, we modified the Kaganer model. Based on the modified Kaganer model, the edge and screw threading dislocation densities in HVPE-grown GaN thick films ranging from 20 μm up to 700 μm were analyzed. It was shown that screw-type dislocation density decreased more rapidly than edge-type dislocation with increase in film thickness.     

Hydride vapor phase epitaxial growth of thick GaN layers with in-situ optical monitoring

An in-situ optical monitoring system made in our laboratory is set up on the horizontal hydride vapor phase epitaxy (HVPE) equipment. From the growth rate information provided by this system, some basic growth parameters are optimized and high-quality GaN layers are grown. The growth stress of the HVPE GaN layer grown on different templates is also examined through the in-situ optical measuring.     

Growth of high quality a-plane GaN epi-layer on r-plane sapphire substrates with optimization of multi-buffer layer

Nonpolar (1 1–2 0) a-plane GaN films have been grown using the multi-buffer layer technique on (1–1 0 2) r-plane sapphire substrates. In order to obtain epitaxial a-plane GaN films, optimized growth condition of the multi-buffer layer was investigated using atomic force microscopy, high resolution X-ray diffraction, and transmission electron microscopy measurements. The experimental results showed that the growth conditions of nucleation layer and three-dimensional growth layer significantly affect the crystal quality of subsequently grown a-plane GaN films. At the optimized growth conditions, omega full-width at half maximum values of (11–20) X-ray rocking curve along c- and m-axes were 430 and 530 arcsec, respectively. From the results of transmission electron microscopy, it was suggested that the high crystal quality of the a-plane GaN film can be obtained from dislocation bending and annihilation by controlling of the island growth mode.     

Growth of (1 0 1¯ 3¯ ) semipolar GaN on m-plane sapphire by hydride vapor phase epitaxy

The crystalline, surface, and optical properties of the (1 0 1¯ 3¯) semipolar GaN directly grown on m-plane sapphire substrates by hydride vapor phase epitaxy (HVPE) were investigated. It was found that the increase of V/III ratio led to high quality (1 0 1¯ 3¯) oriented GaN epilayers with a morphology that may have been produced by step-flow growth and with minor evidence of anisotropic crystalline structure. After etching in the mixed acids, the inclined pyramids dominated the GaN surface with a density of 2×10⁵ cm⁻², revealing the N-polarity characteristic. In the low-temperature PL spectra, weak BSF-related emission at 3.44 eV could be observed as a shoulder of donor-bound exciton lines for the epilayer at high V/III ratio, which was indicative of obvious reduction of BSFs density. In comparison with other defect related emissions, a different quenching behavior was found for the 3.29 eV emission, characterized by the temperature-dependent PL measurement.     

Fabrication of freestanding m-plane GaN wafer by using the HVPE technique on an aluminum carbide buffer layer on an m-plane sapphire substrate

A freestanding m-plane GaN wafer is fabricated by using the hydride vapor-phase epitaxy (HVPE) technique on an aluminum carbide buffer layer on an m-plane sapphire substrate. X-ray pole-figure measurements show a clear m-plane orientation of the GaN surface. The full-width at half-maximum (FWHM) of GaN (1 1¯ 0 0) X-ray rocking curve (XRC) with the scattering vector along the [1 1 2¯ 0] direction is approximately 800 arcsec; this indicates good crystallinity. On the other hand, the FWHM for the case in which the scattering vector is oriented along the [0 0 0 1] direction is broad; this suggests the influence of structural defects along this direction. In fact, basal plane stacking faults (BSF) with a density of approximately 3×10⁵ cm⁻¹ is observed by transmission electron microscopy (TEM). The preparation of a 45-mm-diameter m-plane GaN wafer due to spontaneous separation of the GaN layer from the sapphire substrate is demonstrated.     

Hydrothermal growth of ZnO nanorods on a-plane GaN/sapphire template

ZnO nanorod arrays are grown on a-plane GaN template/r-plane sapphire substrates by hydrothermal technique. Aqueous solutions of zinc nitrate hexahydrate and hexamethylenetetramine were employed as growth precursors. Electron microscopy and X-ray diffraction measurements were carried out for morphology, phase and growth orientation analysis. Single crystalline nanorods were found to have off-normal growth and showed well-defined in-plane epitaxial relationship with the GaN template. The 〈0 0 0 1〉 axis of the ZnO nanorods were observed to be parallel to the 〈1 0 1¯ 0〉 of the a-plane GaN layer. Optical property of the as-grown ZnO nanorods was analyzed by room temperature photoluminescence measurements.     

Epitaxial lateral overgrowth on (2 1¯ 1¯ 0) a-plane GaN with [0 1¯ 1 1]-oriented stripes

Epitaxial lateral overgrowth was applied to a-plane GaN on r-plane sapphire using SiO₂ stripe masks oriented parallel to [0 1¯ 1 1]. Coalescence and defect distribution was studied using scanning electron microscopy and cathodoluminescence. Defects, i.e., threading dislocations and basal plane stacking faults from the template propagate into the overgrown layer through the mask openings. Stacking faults spread into the whole overgrown layer, whereas threading dislocations are laterally confined in the region above the mask where a part of them is terminated at the inclined coalescence boundary. Lateral overgrowth and dislocation termination at the coalescence boundary leads to an improvement in luminescence intensity and crystal quality, in comparison to the template. The measured XRD rocking curve FWHM were 453″ with incidence along the [0 0 0 1] c-direction and 280″ with incidence along the [0 1 1¯ 0] m-direction.     

N-face GaN growth on c-plane sapphire by metalorganic chemical vapor deposition

In this work, we report the growth of smooth, high-quality N-face GaN on c-plane sapphire by metalorganic chemical vapor deposition. It is found that the nitridation temperature of sapphire has a critical effect on the surface morphology of N-face GaN. Sapphire after a severe nitridation gives rise to a high density of hexagonal hillocks during N-face GaN growth. Smooth N-face GaN has been grown on appropriately nitridized sapphire. The N-polarity of the GaN film has been confirmed with no inversion domain by convergent beam electron diffraction. Controlled growth interruption is carried out to study the nucleation evolution during N-face GaN growth, which is found distinctly different from the two-step growth of Ga-face GaN. Atomically smooth N-face GaN has been achieved with comparable structural quality to Ga-face GaN.     

Reduction of dislocations in a (1 1 2¯ 2)GaN grown by selective MOVPE on (1 1 3)Si

Two-step selective epitaxy (SAG/ELO) of (1 1 2¯ 2)GaN on (1 1 3)Si substrate is studied to reduce the defect density in the epitaxial lateral overgrowth. The first SAG/ELO is to prepare a (1 1 2¯ 2)GaN template on a (1 1 3)Si and the second SAG/ELO is to get a uniform (1 1 2¯ 2)GaN. It is found that the reduction of the defect density is improved by optimizing the mask configuration in the second SAG/ELO. The minimum dark spot density obtained is 3×10⁷/cm², which is two orders of magnitude lower than that found in a (0 0 0 1)GaN grown on (1 1 1)Si.     

Strain-reduced GaN thick-film grown by hydride vapor phase epitaxy utilizing dot air-bridged structure

A 300 μm GaN thick-film, in diameter 1.5 in, was demonstrated without any crack by hydride vapor phase epitaxy (HVPE) growth. The technique used in relaxing the residual stress caused by differences of thermal expansion coefficients (TEC) and lattice constants between GaN and sapphire substrate to prevent GaN film from crack is called a dot air-bridged structure. After the laser lift-off process, 300-μm-thick freestanding GaN wafer, in diameter 1.5 in, could be fabricated. The compressive stress in the dot air-bridged structure was measured by micro-Raman spectroscopy with the E₂(high) phonon mode. The compressive stress could be reduced to as small as 0.04 GPa, which could prevent the crack during the epitaxial process for GaN growth by HVPE. It is important to obtain a large-area crack-free GaN thick-film, which can be used for fabricating freestanding GaN wafer.     

MOVPE growth and properties of GaN on (1 1 1)Si using an AlInN intermediate layer

Using an AlInN intermediate layer, GaN was grown on (1 1 1)Si substrate by selective metalorganic vapor phase epitaxy. The variation of the surface morphology was investigated as a function of the In composition and thickness of the AlInN layer. It was found that the In composition in the AlInN layer was a function of the growth temperature and thickness. Because of the small band offset at the AlInN/Si hetero-interface, we have achieved a low series resistance of the order of 9 Ω (0.0036 Ω cm²) across the GaN/AlInN/AlN/Si layer structure.     

Optical characteristics of m-plane InGaN/GaN multiple quantum well grown on LiAlO2 (1 0 0) by MOVPE

We investigate the optical properties of m-plane InGaN/GaN multiple quantum well grown on LiAlO₂ substrate by metal organic vapor phase epitaxy. Polarization-dependent photoluminescence and polarization-dependent photoluminescence excitation measurements have been performed at low temperature to study the optical absorption and emission characteristics. The main emission band possesses large polarization anisotropy which may be attributed to the anisotropic biaxial strain. We found the optical emission is not influenced by the polarization-induced electric field from the excitation-dependent photoluminescence measurements. From our results, we attribute the low-temperature emission band around 3.2 eV to interband transition in the quantum well. Besides, the mechanism of the main emission band is associated with interband transition and subsequent carrier localization. The realization of good-quality non-polar GaN-based devices can then be expected in near future.     

Maskless selective growth of semi-polar (1 12¯ 2) GaN on Si (3 1 1) substrate by metal organic vapor phase epitaxy

Semi-polar (1 1 2¯ 2) GaN layers were selectively grown by metal organic chemical vapor phase epitaxy on patterned Si (3 1 1) substrates without SiO₂ amorphous mask. The (1 1 2¯ 2) GaN layers could be selectively grown only on Si (1 1 1) facets when the stripe mask width was narrower than 1 μm even without SiO₂. Inhomogeneous spatial distribution of donor bound exciton (DBE) peak in low-temperature cathodoluminescence (CL) spectra was explained by the difference of growth mode before and after the coalescence of stripes. It was found that the emission intensity related crystal defects is drastically decreased in case of selective growth without SiO₂ masks as compared to that obtained with SiO₂ masks.