The influences of supersaturation on LPE growth of GaN single crystals using the Na flux method

The dependency of LPE growth rate and dislocation density on supersaturation in the growth of GaN single crystals in the Na flux was investigated. When the growth rate was low during the growth of GaN at a small value of supersaturation, the dislocation density was much lower compared with that of a substrate grown by the Metal Organic Chemical Vapor Deposition method (MOCVD). In contrast, when the growth rate of GaN was high at a large value of supersaturation, the crystal was hopper including a large number of dislocations. The relationship between the growth conditions and the crystal color in GaN single crystals grown in Na flux was also investigated. When at 800 °C the nitrogen concentration in Na–Ga melt was low, the grown crystals were always tinted black. When the nitrogen concentration at 850 °C was high, transparent crystals could be grown.     

Correlations between photoluminescence and Hall mobility in GaN/sapphire grown by metalorganic chemical vapor deposition

We have investigated photoluminescence (PL) and electron Hall mobility for unintentionally doped GaN epitaxial layers grown by low-pressure metalorganic chemical vapor deposition on c-plane Al₂O₃ substrates. Four GaN films having identical dislocation density but remarkably different electron Hall mobility were exploited. At low temperature (12 K), a PL line associated with a bound exciton was observed and strong correlations were found between the Hall mobility and the PL intensity of the exciton transition. That is, relative PL intensity of the bound exciton to a donor-bound exciton monotonously increased with decreasing the electron mobility of the GaN films. This correlation was interpreted in terms of electrical compensation. Efforts to find the chemical origin of the PL line led to the conclusion that the BE line originated neither from threading dislocations nor from extrinsic point defects. Intrinsic acceptors such as Ga vacancy and GaN anti-site were suspected as plausible origin.     

The effects of annealing on non-polar (1 1 2¯ 0) a-plane GaN films

Non-polar a-plane (1 1 2¯ 0) GaN films were grown on r-plane sapphire by metal–organic vapor phase epitaxy and were subsequently annealed for 90 min at 1070 °C. Most dislocations were partial dislocations, which terminated basal plane stacking faults. Prior to annealing, these dislocations were randomly distributed. After annealing, these dislocations moved into arrays oriented along the [0 0 0 1] direction and aligned perpendicular to the film–substrate interface throughout their length, although the total dislocation density remained unchanged. These changes were accompanied by broadening of the symmetric X-ray diffraction 1 1 2¯ 0 ω-scan widths. The mechanism of movement was identified as dislocation glide, occurring due to highly anisotropic stresses (confirmed by X-ray diffraction lattice parameter measurements) and evidenced by macroscopic slip bands observed on the sample surface. There was also an increase in the density of unintentionally n-type doped electrically conductive inclined features present at the film–substrate interface (as observed in cross-section using scanning capacitance microscopy), suggesting out-diffusion of impurities from the substrate along with prismatic stacking faults. These data suggest that annealing processes performed close to film growth temperatures can affect both the microstructure and the electrical properties of non-polar GaN films.     

The impact of an intermediate temperature buffer on the growth of GaN on an AlN template by hydride vapor phase epitaxy

The present study focused on the effect of an intermediate-temperature (IT; ∼900 °C) buffer layer on GaN films, grown on an AlN/sapphire template by hydride vapor phase epitaxy (HVPE). In this paper, the surface morphology, structural quality, residual strain, and luminescence properties are discussed in terms of the effect of the buffer layer. The GaN film with an IT-buffer revealed a relatively lower screw-dislocation density (3.29×10⁷ cm⁻²) and a higher edge-dislocation density (8.157×10⁹ cm⁻²) than the GaN film without an IT-buffer. Moreover, the IT-buffer reduced the residual strain and improved the luminescence. We found that the IT-buffer played an important role in the reduction of residual strain and screw-dislocation density in the overgrown layer through the generation of edge-type dislocations and the spontaneous treatment of the threading dislocation by interrupting the growth and increasing the temperature.     

Hydride vapor phase epitaxy of GaN boules using high growth rates

The boule-like growth of GaN in a vertical AIXTRON HVPE reactor was studied. Extrinsic factors like properties of the starting substrate and fundamental growth parameters especially the vapor gas composition at the surface have crucial impact on the formation of inverse pyramidal defects. The partial pressure of GaCl strongly affects defect formation, in-plane strain, and crystalline quality. Optimized growth conditions resulted in growth rates of 300–500 μm/h. GaN layers with thicknesses of 2.6 and of 5.8 mm were grown at rates above 300 μm/h. The threading dislocation density reduces with an inverse proportionality to the GaN layer thickness. Thus, it is demonstrated that growth rates above 300 μm/h are promising for GaN boule growth.     

Reduction of threading dislocations in migration enhanced epitaxy grown GaN with N-polarity by use of AlN multiple interlayer

High quality GaN layer was obtained by insertion of high temperature grown AlN multiple intermediate layers with migration enhanced epitaxy method by the RF-plasma assisted molecular beam epitaxy on (0 0 01) sapphire substrates. The propagating behaviors of dislocations were studied, using a transmission electron microscope. The results show that the edge dislocations were filtered at the AlN/GaN interfaces. The bending propagation of threading dislocations in GaN above AlN interlayers was confirmed. Thereby, further reduction of dislocations was achieved. Dislocation density being reduced, the drastic increase of electron mobility to 668 cm²/V s was obtained at the carrier density of 9.5×10¹⁶ cm⁻³ in Si doped GaN layer.     

Transmission electron microscopy study of an AlN nucleation layer for the growth of GaN on a 7-degree off-oriented (0 0 1) Si substrate by metalorganic vapor phase epitaxy

We have investigated the morphology of the high-temperature-grown AlN nucleation layer and its role in the early stage of GaN growth, by means of transmission electron microscopy. The nitride was selectively grown on a 7-degree off-oriented (0 0 1) patterned Si substrate by metalorganic vapor phase epitaxy. AlN was deposited on the inclined unmasked (1 1 1) facet in the form of islands. The size of the islands varied along the slope, which is attributable to the diffusion of the growth species in the vapor phase. The GaN nucleation occurred at the region where rounded AlN islands formed densely. The threading dislocations were observed to generate in the GaN nucleated region.     

Approach for dislocation free GaN epitaxy

The characteristics of confined epitaxial growth are investigated with the goal of determining the contributing effects of mask attributes (spacing, feature size) and growth conditions (V/III ratio, pressure, temperature) on the efficiency of the approach for dislocation density reduction of GaN. In addition to standard (secondary electron and atomic force) microscopy, electron channeling contrast imaging (ECCI) is employed to identify extended defects over large (tens of microns) areas. Using this method, it is illustrated that by confining the epitaxial growth, high quality GaN can be grown with dislocation densities approaching zero.     

Control of the morphology of InGaN/GaN quantum wells grown by metalorganic chemical vapor deposition

Various techniques for morphological evolution of InGaN/GaN multiple quantum well (MQW) structures grown by metalorganic chemical vapor deposition have been evaluated. Atomic force microscopy, photoluminescence (PL) and X-ray diffraction measurements have been used for characterization. It is shown that inclusions, that are generated into the V-defects in the InGaN quantum wells (QW), can be removed by introducing a small amount of hydrogen during the growth of GaN barriers. This hydrogen treatment results in partial loss of indium from the QWs, but smooth surface morphology of the MQW structure and improved optical quality of InGaN wells are obtained. The density of the V-defects could be reduced by reducing the dislocation density of the underlying GaN buffer.     

MOCVD growth of GaN on Si(1 1 1) substrates using an ALD-grown Al2O3 interlayer

GaN thin films have been grown on Si(1 1 1) substrates using an atomic layer deposition (ALD)-grown Al₂O₃ interlayer. This thin Al₂O₃ layer reduces strain in the subsequent GaN layer, leading to lower defect densities and improved material quality compared to GaN thin films grown by the same process on bare Si. XRD ω-scans showed a full width at half maximum (FWHM) of 549 arcsec for GaN grown on bare Si and a FWHM as low as 378 arcsec for GaN grown on Si using the ALD-grown Al₂O₃ interlayer. Raman spectroscopy was used to study the strain in these films in more detail, with the shift of the E₂(high) mode showing a clear dependence of strain on Al₂O₃ interlayer thickness. This dependence of strain on Al₂O₃ thickness was also observed via the redshift of the near bandedge emission in room temperature photoluminescence (RT-PL) spectroscopy. The reduction in strain results in a significant reduction in both crack density and screw dislocation density compared to similar films grown on bare Si. Screw dislocation density of the films grown on Al₂O₃/Si substrates approaches that of typical GaN layers on sapphire. This work shows great promise for the use of oxide interlayers for growth of GaN-based LEDs on Si.     

Revealing extended defects in HVPE-grown GaN

In this communication we will summarize the results of a complementary study of structural and chemical non-homogeneities that are present in thick HVPE-grown GaN layers. It will be shown that complex extended defects are formed during HVPE growth, and are clearly visible after photo-etching on both Ga-polar surface and on any non-polar cleavage or section planes. Large chemical (electrically active) defects, such as growth striations, overgrown or empty pits (pinholes) and clustered irregular inclusions, are accompanied by non-uniform distribution of crystallographic defects (dislocations). Possible reasons of formation of these complex structures are discussed. The nature of defects revealed by selective etching was subsequently confirmed using TEM, orthodox etching and compared with the CL method. The non-homogeneities were studied in GaN crystals grown in different laboratories showing markedly different morphological characteristics.     

Investigation of characteristics of laterally overgrown GaN on striped sapphire substrates patterned by wet chemical etching

The new developed maskless lateral-epitaxial-overgrowth technique, in which the striped substrates are patterned by wet chemical etching, is systematically investigated using scanning electron microscopy, X-ray diffraction, and atomic force microscopy (AFM). Wing tilt is measured for the GaN films on patterned substrates with a range of “fill factor” (ratio of groove width to stripe period) and for the GaN in different growth time. It is found that changes in these parameters have a significant effect on the extent and distribution of wing tilt in the laterally overgrown regions relative to the GaN directly on the sapphire substrate. Increasing the thickness of GaN films is benefit to reduce wing tilt. The tilt is avoided in the GaN films with 4.5 μm thickness and fill factor for 0.46. The full-width at half-maximum of X-ray rocking curves of the asymmetric diffraction peaks and the image of AFM both show that the threading dislocations in the developed lateral epitaxial overgrowth of GaN films are reduced sharply. The GaN template could be used as an excellent substrate to fabricate high-performance optoelectronic devices.     

Influence of AlGaN/GaN/InGaN superlattice on the characteristics of LEDs grown by metalorganic chemical vapor deposition

This study examined the influence of strain-compensated triple AlGaN/GaN/InGaN superlattice structures (SLs) in n-GaN on the structural, electrical and optical characteristics of LEDs by analyzing the etch pits density (EPD), stress measurement, high-resolution X-ray diffraction (HRXRD), sheet resistance, photoluminescence (PL) and light–current–voltage (L–I–V). EPD, stress measurement and HRXRD studies showed that the insertion of AlGaN/GaN/InGaN SLs during the growth of n-GaN effectively distributed and compensated for the strong compressive stress, and decreased the dislocation density in n-GaN. The operating voltage at 20 mA for the LEDs grown with SLs decreased to 3.18 V from 3.4 V for the LEDs grown without SLs. In addition, a decrease in the spectral blue shift compared to the LEDs grown without SLs was observed in the LEDs grown with the SLs.     

Reducing dislocation density in GaN films using a cone-shaped patterned sapphire substrate

The threading dislocation (TD) density in GaN films grown directly on flat sapphire substrates is typically >10¹⁰/cm², which can deteriorate the properties of GaN-based LEDs significantly. This paper reports an approach to reducing the TD density in a GaN layer using a variety of patterned sapphire substrates (PSS). A cone-shaped PSS produced by metal organic chemical vapor deposition (MOVCD) was used for GaN deposition. Three types of GaN specimens were prepared at the initial nucleation stage, middle growth stage and final growth stage. The TDs generated on the cone-shaped PSS were analyzed by transmission electron microscopy (TEM) and a strain mapping simulation using HRTEM images, which evaluated the residual strain distribution. A large number of TDs were generated and the residual strain by the lattice distortions remained above the top of the cone-shaped regions. However, no TDs and residual strain were observed at the slope of the cone-shaped regions. This might be due to the formation of a GaN layer by lateral overgrowth at the slope of the cone-shaped regions, resulting in less lattice mismatch and incoherency between the GaN and sapphire. In conclusion, the TD density in the GaN layer could be reduced significantly, approximately 10⁷/cm², using the cone-shaped PSS.     

Growth of Eu-doped GaN by gas source molecular beam epitaxy and its optical properties

Eu-doped GaN with various Eu concentrations were grown by gas source molecular beam epitaxy, and their structural and optical properties were investigated. With increasing Eu concentration from 0.1 to 2.2 at%, deterioration of the structural quality was observed by reflection high-energy electron diffraction, atomic force microscopy and X-ray diffraction. Such a deterioration may be caused by an enhancement of island growth and formation of dislocations. On the other hand, room temperature photoluminescence spectra showed red emission at 622 nm due to an intra-atomic f–f transition of Eu³⁺ ion and Fourier transform infrared spectra indicated an absorption peak at about 0.37 eV, which may be due to a deep defect level. The intensity of the red luminescence and the defect-related absorption peak increased with increasing Eu concentration, and a close correlation in the increasing behavior was observed between them. These results suggest that the deep defect level plays an important role in the radiative transition of Eu³⁺ ion in GaN and the optical process for the luminescence at 622 nm was discussed with relation to the defect.     

The K2S2O8–KOH photoetching system for GaN

A recently developed photoetching system for n-type GaN, a KOH solution containing the strong oxidizing agent potassium peroxydisulphate (K₂S₂O₈), was studied in detail. By careful selection of the etching parameters, such as the ratio of components and the hydrodynamics, two distinct modes were defined: defect-selective etching (denoted by KSO-D) and polishing (KSO-P). Both photoetching methods can be used under open-circuit (electroless) conditions. Well-defined dislocation-related etch whiskers are formed during KSO-D etching. All types of dislocations are revealed, and this was confirmed by cross-sectional TEM examination of the etched samples. Extended electrically active defects are also clearly revealed. The known relationship between etch rate and carrier concentration for photoetching of GaN in KOH solutions was confirmed for KSO-D etch using Raman measurements. It is shown that during KSO-P etching diffusion is the rate-limiting step, i.e. this etch is suitable for polishing of GaN. Some constraints of the KSO etching system for GaN are discussed and peculiar etch features, so far not understood, are described.     

GaN single crystals of different habit grown from solution at near atmospheric pressure

Near atmospheric pressure solution growth is one of the many developing methods for growing bulk GaN from solution. Apart from other approaches, this method holds certain advantages, such as relatively low growth pressure and temperature, and the ability to grow high quality GaN crystals with different orientations by varying the solvent composition. GaN whiskers of millimeter scale size with exceptional mechanical and optical properties were grown from solution. Crystals of near isotropic shape were also grown from solution by manipulating additives in the basic solvent.     

Ion implantation effects on the microhardness and microstructure of GaN

We study the effect of N⁺ and O⁺ implantation on the microhardness and the microstructure of epitaxially grown GaN. The microhardness is measured using a Knoop diamond indenter while information on the effect of implantation on the surface morphology, microstructure and electronic structure is provided by atomic force microscopy, cross-section transmission electron microscopy and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. It is demonstrated that implantation increases the surface microhardness. A possible mechanism for the surface hardening effect is based on the formation of N interstitials that pin the dislocations and prohibit the plastic deformation. In addition to the hardening effect, the implantation induced N interstitials introduce a characteristic resonance in the NEXAFS spectra, at 1.4 eV below the absorption edge.     

Effect of the N/Al ratio of AlN buffer on the crystal properties and stress state of GaN film grown on Si(1 1 1) substrate

The effect of the N/Al ratio of AlN buffers on the optical and crystal quality of GaN films, grown by metalorganic chemical vapor deposition on Si(1 1 1) substrates, has been investigated. By optimizing the N/Al ratio during the AlN buffer, the threading dislocation density and the tensile stress have been decreased. High-resolution X-ray diffraction exhibited a (0 0 0 2) full-width at half-maximum as low as 396 acrsec. The variations of the tensile stress existing in the GaN films were approved by the redshifts of the donor bound exiton peaks in the low-temperature photoluminescence measurement at 77 K.     

Defect and stress relaxation in HVPE-GaN films using high temperature reactively sputtered AlN buffer

The influence of high temperature buffer layers on the structural characteristics of GaN grown by hydride vapour phase epitaxy on sapphire was investigated. Strain relaxation as well as mismatch-induced defect reduction in thick GaN layers grown on AlN buffer was microscopically identified using cathodoluminescence and micro-Raman spectroscopy in cross-section of the films. The results were correlated with photoluminescence and Hall-effect data of layers with different thicknesses. These relaxation processes were suggested to account for the specific defect distribution in the buffers revealed by high-resolution X-ray diffraction and transmission electron microscopy.