Basal Plane Stacking Fault Density Research Articles

Effects of Buffer Layer on Structural Properties of Nonpolar (112¯0)-Plane GaN Film

Nonpolar (112¯0) a-plane GaN films were grown on semipolar (11¯02) r-plane sapphire substrates using various buffer layers within a low-pressure metal organic chemical vapor deposition system. The structural properties of nonpolar a-plane GaN films were intensively investigated by X-ray diffraction and Raman spectra measurements. A set of buffer layers were adopted from a GaN layer to a composite layer containing a multiple AlN layers and a gradually varied-Al-content AlGaN layer, the full width at half maximum of the X-ray rocking curves measured along the [0001] and [101¯0] directions of a-plane GaN were reduced by 35% and 37%, respectively. It was also found that the basal-plane stacking faults (BSFs) density can be effectively reduced by the heterogeneous interface introduced together with the composite buffer layer. An order of magnitude reduction in BSFs density, as low as 2.95 × 104 cm−1, and a pit-free surface morphology were achieved for the a-plane GaN film grown with the composite buffer layer, which is promising for the development of nonpolar GaN-based devices in the future.

Improved crystallinity and surface morphology of a-plane AlN grown on high temperature annealed AlN/sapphire template by pulsed-flow mode metal-organic vapor phase epitaxy

Preparing high quality non-polar aluminum nitride (AlN) templates is the key to improving the performance of non-polar deep-ultraviolet light-emitting diodes. In this study, we investigated the effect of buffer layer on the crystallinity and surface morphology of a-plane AlN films regrown by pulsed-flow mode (PM) metal-organic vapor phase epitaxy (MOVPE). Three buffer layers were compared including low-temperature AlN buffer layer grown by MOVPE (MO-buffer), sputtered AlN buffer layer (SP-buffer), and high-temperature annealed sputtered AlN buffer layer (HTA-buffer). It is found that the (11-20) plane x-ray rocking curve-full width at half maximum (XRC-FWHM) values of a-plane AlN films are significantly reduced after the regrowth process. Thanks to the high crystalline quality of HTA-buffer, AlN regrown on HTA-buffer exhibits the smallest (11-20) plane XRC-FWHM values of 1260/1440 arcsec along the [0001]/[1-100] direction. The mosaic tilt and basal plane stacking fault density are estimated to be 0.41° and 1.76 106 cm−1, respectively. The surface shows a uniform stripe-like pattern with the lowest root mean square value of 0.82 nm. Furthermore, the a-plane AlN epilayer regrown on HTA-buffer displays a weak in-plane stress anisotropy and high optical transmittance in the ultraviolet-visible region. Our work suggests that combining the PM regrowth with HTA-AlN buffer layer is a promising route to prepare high-quality a-plane AlN templates for efficient non-polar deep-ultraviolet light emitters.

Improved Optical Properties of Nonpolar AlGaN-Based Multiple Quantum Wells Emitting at 280 nm

The optical properties of nonpolar AlGaN multiple quantum wells (MQWs) emitting at 280 nm were investigated intensively using temperature-dependent and time-resolved photoluminescence spectra associated with the characterization of structural properties. The densities of superficial pits and basal-plane stacking faults (BSFs) were reduced by 33.8% and 35.9%, respectively, for nonpolar AlGaN MQWs due to the carefully optimized dual nitridation. It was found that the nonpolar MQWs emission can be significantly improved by reducing the BSFs density as the BSFs emission was the main competing channels. Moreover, an internal quantum efficiency of 39% for nonpolar Al0.43Ga0.57N MQWs at emission wavelength of 279 nm was achieved even the full width at half maximum values of X-ray rocking curves were 0.565° for c-direction and 0.797° for m-direction. This fact means that a highly efficient deep ultraviolet light sources can be expected by means of nonpolar AlGaN due to the elimination of quantum confined Stark effect induced the decrease in radiative lifetime.

Growth of GaN on a three-dimensional SCAATTM bulk seed by tri-halide vapor phase epitaxy using GaCl3

GaN with a film thickness of 200–600 μm was grown on the as-grown three-dimensional supercritical acidic ammonia technology (SCAATTM) bulk seed that comprised only semipolar planes at a temperature as high as 1390 °C by tri-halide vapor phase epitaxy; further, the GaN film was also characterized. The FWHM value of the X-ray rocking curves was ∼40″, which was almost similar to the value of the used seed. The curvature radii were as large as 40–64 m. Further, the carrier concentrations were observed to be as small as 5.1 × 1017–9.1 × 1017 cm−3. However, the basal plane stacking fault densities were observed to be 3.4 × 101–5.4 × 101 cm−1 and were observed to increase during the growth process because of the as-grown SCAATTM seed surface condition.

Nanopatterned epitaxy of non-polar Ga1-yInyN layers with caps and voids

Low-defect density non-polar a-plane Ga1-yInyN layers on r-plane sapphire substrates are reported by implementing self-assembling nanopatterning in metalorganic vapor phase epitaxy. Nanopillar capping and void formation in regrowth lead to a 90% defect reduction. An ex-situ Ni layer transforms into a nanoisland etch mask to pattern GaN templates. a-Plane GaN and Ga1-yInyN layers with an InN content in the range of y = 0.04–0.11 are then regrown. Both exhibit a low density of basal-plane stacking faults of (4.6 ± 1.3) × 104 cm−1 by transmission electron microscopy analysis. Growth parameters and the template pattern are discussed by help of an X-ray rocking curve analysis. We find pattern the fill factor and V/III ratio to dominate the defect reduction. Resulting layers should enable efficient long-wavelength light-emitting and solar cell devices.

Effect of nano‐porous SiNx interlayer on propagation of extended defects in semipolar ‐orientated GaN

Semipolar orientations of GaN have attracted considerable attention as potential platforms for high brightness light‐emitting devices. The heteroepitaxy of semipolar GaN layers results in high densities of threading dislocations (TDs) and basal‐plane stacking faults (BSFs). Various defect reduction methods have been employed to improve the crystal quality of the semipolar substrates, among which the in situ epitaxial lateral overgrowth (ELO) is of great importance due to its relatively simple procedure, low cost, and high effectiveness in reducing density of TDs. We have shown that in situ deposited nano‐porous SiNx could act as a blocking layer preventing both TDs and BSFs from penetrating to overgrown semipolar GaN layers. In this contribution, we present microscopic study of interaction of extended defects (TDs and BSFs) with nano‐porous SiNx in ‐oriented GaN layers grown by MOCVD on m‐sapphire substrates. Cross‐sectional scanning transmission electron microscopy‐cathodoluminescence (STEM‐CL) reveals reduction of BSF density by about 50% for dual insertion of SiNx interlayers. It is also shown that the integrated CL intensity is enhanced by about two orders of magnitude indicating that the majority of TDs are blocked by the interlayers. The interaction of TDs and BSFs with nano‐porous SiNx and defect distribution in GaN nuclei are briefly discussed.

Thick nonpolar m-plane and semipolar [formula omitted] GaN on an ammonothermal seed by tri-halide vapor-phase epitaxy using GaCl3

GaN layers of thickness 0.5–1.3mm were grown at 1280°C at a growth rate of 95–275µm/h by tri-halide vapor-phase epitaxy on nonpolar m-plane (101̅0) and semipolar (101̅1̅) ammonothermal GaN substrates. For nonpolar m-plane (101̅0) with a −5° off-angle, the full widths at half maximum (FWHMs) of X-ray rocking curves (XRCs) and the basal plane stacking fault (BSF) density increased from 50 to 178″ and from 4.8×101 to 1.0×103cm−1, respectively, upon increasing the growth rate from 115 to 245µm/h. On the other hand, the XRC-FWHM and the BSF density for semipolar (101̅1̅) grown at 275µm/h were as small as 28″ and 8.3×101cm−1, respectively.

Temperature dependent study of basal plane stacking faults in Ag:ZnO nanorods by Raman and photoluminescence spectroscopy

We report the specific features of basal plane stacking faults (BSFs) in ZnO nanorods (NRs), studied by temperature dependent photoluminescence and Raman spectroscopy. At low temperature (4K) the intense band of emission at 3.321eV is attributed to the presence of BSFs defects and Ag as an acceptor dopant in ZnO. This specific peak red-shifts with the temperature increase, occupying the position 3.210eV at RT. The nature of the emission is explained as exciton recombination of the electrons, confined in the homo-heterojunction QW, with the holes, localized near the Ag atoms close to SFs. Raman spectroscopy revealed that Ag:ZnO nanorods have slightly downshifted positions of the modes 330cm−1 and 440cm−1 by 4cm−1, which we explain as due to the presence of BSFs. It was also observed, that the longitudinal optical phonon mode ALO, which is common polar mode for ZnO, was not detected by Raman spectroscopy in the samples with high BSFs density. This feature can be explained as due to existence of the bound charge induced by the BSFs in the NRs.

Role of substrate quality on the performance of semipolar (112¯2) InGaN light-emitting diodes

We compare the optical properties and device performance of unpackaged InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) emitting at ∼430 nm grown simultaneously on a high-cost small-size bulk semipolar (112¯2) GaN substrate (Bulk-GaN) and a low-cost large-size (112¯2) GaN template created on patterned (101¯2) r-plane sapphire substrate (PSS-GaN). The Bulk-GaN substrate has the threading dislocation density (TDD) of ∼105 cm−2–106 cm−2 and basal-plane stacking fault (BSF) density of 0 cm−1, while the PSS-GaN substrate has the TDD of ∼2 × 108 cm−2 and BSF density of ∼1 × 103 cm−1. Despite an enhanced light extraction efficiency, the LED grown on PSS-GaN has two-times lower internal quantum efficiency than the LED grown on Bulk-GaN as determined by photoluminescence measurements. The LED grown on PSS-GaN substrate also has about two-times lower output power compared to the LED grown on Bulk-GaN substrate. This lower output power was attributed to the higher TDD and BSF density.

Improved Semipolar (112̄2) GaN Quality Grown on m-Plane Sapphire Substrates by Metal Organic Chemical Vapor Deposition Using Self-Organized SiNx Interlayer**Supported by the National Natural Science Foundation of China under Grant Nos 61204006 and 61574108, the Fundamental Research Funds for the Central Universities under Grant No JB141101, and the Foundation of Key Laboratory of Nanodevices and Applications of Suzhou Institute of

The effect of a self-organized SiNx interlayer on the defect density of (112̄2) semipolar GaN grown on m-plane sapphire is studied by transmission electron microscopy, atomic force microscopy and high resolution x-ray diffraction. The SiNx interlayer reduces the c-type dislocation density from 2.5 × 1010 cm−2 to 5 × 108 cm−2. The SiNx interlayer produces regions that are free from basal plane stacking faults (BSFs) and dislocations. The overall BSF density is reduced from 2.1 × 105 cm−1 to 1.3 × 104 cm−1. The large dislocations and BSF reduction in semipolar (112̄2) GaN with the SiNx interlayer result from two primary mechanisms. The first mechanism is the direct dislocation blocking by the SiNx interlayer, and the second mechanism is associated with the unique structure character of (112̄2) semipolar GaN.

Molecular beam epitaxy of single crystalline GaN nanowires on a flexible Ti foil

We demonstrate the self-assembled growth of vertically aligned GaN nanowire ensembles on a flexible Ti foil by plasma-assisted molecular beam epitaxy. The analysis of single nanowires by transmission electron microscopy reveals that they are single crystalline. Low-temperature photoluminescence spectroscopy demonstrates that in comparison to standard GaN nanowires grown on Si, the nanowires prepared on the Ti foil exhibit an equivalent crystalline perfection, a higher density of basal-plane stacking faults, but a reduced density of inversion domain boundaries. The room-temperature photoluminescence spectrum of the nanowire ensemble is not influenced or degraded by the bending of the substrate. The present results pave the way for the fabrication of flexible optoelectronic devices based on GaN nanowires on metal foils.

Self-assembled Multilayers of Silica Nanospheres for Defect Reduction in Non- and Semipolar Gallium Nitride Epitaxial Layers.

Non- and semipolar GaN have great potential to improve the efficiency of light emitting devices due to much reduced internal electric fields. However, heteroepitaxial GaN growth in these crystal orientations suffers from very high dislocation and stacking faults densities. Here, we report a facile method to obtain low defect density non- and semipolar heteroepitaxial GaN via selective area epitaxy using self-assembled multilayers of silica nanospheres (MSN). Nonpolar (11–20) and semipolar (11–22) GaN layers with high crystal quality have been achieved by epitaxial integration of the MSN and a simple one-step overgrowth process, by which both dislocation and basal plane stacking fault densities can be significantly reduced. The underlying defect reduction mechanisms include epitaxial growth through the MSN covered template, island nucleation via nanogaps in the MSN, and lateral overgrowth and coalescence above the MSN. InGaN/GaN multiple quantum wells structures grown on a nonpolar GaN/MSN template show more than 30-fold increase in the luminescence intensity compared to a control sample without the MSN. This self-assembled MSN technique provides a new platform for epitaxial growth of nitride semiconductors and offers unique opportunities for improving the material quality of GaN grown on other orientations and foreign substrates or heteroepitaxial growth of other lattice-mismatched materials.

Optical and structural properties of GaN epitaxial layers on LiAlO2 substrates and their correlation with basal-plane stacking faults

The optical and structural properties of m-plane GaN layers grown by metal organic vapor phase epitaxy on LiAlO2 (100) substrates were investigated. Temperature-dependent and time-resolved photoluminescence (PL), X-ray diffraction and Raman scattering measurements were performed to analyze the correlation of the sample properties with the density of I1-type basal-plane stacking faults (BSFs). Electron channeling contrast imaging was used to reveal and calculate the density of BSFs reaching the surface of an m-plane GaN/LiAlO2 layer. It was shown that a local increase of BSF density in the investigated samples results in a rise of the total PL efficiency at low temperatures because of the localization of excitons at BSFs and, therefore, a suppression of their diffusion to nonradiative centers. Parameters of time decay and temperature quenching of the BSF-related PL band were determined. A correlation of both εxx and εzz strain components with the BSFs and crystal mosaicity was observed, and possible reasons of this correlation are discussed.

Optimizing GaN () hetero‐epitaxial templates grown on () sapphire

AbstractThe hetero‐epitaxy of () GaN on () sapphire was optimized in metal–organic vapor phase epitaxy. Best results were obtained from an AlN nucleation followed by AlN and AlGaN layers, and inserting low‐temperature AlN interlayers (ILs) as well as a SiNx IL. X‐ray diffraction (XRD) of scans of the symmetric () reflection yielded an FWHM along [] and along [] together with a m rms roughness below 10 nm as determined by atomic force microscopy. The lowest threading dislocation density achieved was cm while the basal plane stacking fault density was in the lower cm range as determined by transmission electron microscopy. The suppression of the unwanted () phase was lower than 1 in 10,000 as judged from XRD.

Growth and coalescence studies of oriented GaN on pre‐structured sapphire substrates using marker layers

In this article, the growth and coalescence of semi-polar oriented GaN layers, deposited on pre-structured r-plane sapphire substrates, is studied with the help of Si-doped marker layers. It has been found to be very important to adjust the shape of the initial GaN stripes by varying the growth temperature to obtain not only a smooth surface, but also a small density of basal plane stacking faults (BSFs) and threading dislocations (TDs) on the wafer surface. With the help of transmission electron microscopy (TEM) and cathodoluminescence measurements (CL), we can conclude that during growth, we need to achieve a compromise between small BSF density, small TD density, and perfect coalescence with smooth surface, free of fissures, and other growth artifacts. Also the formation of arrow-head-shaped surface artifacts called “chevrons” [1, 2] can be understood to be caused by imperfect coalescence. We observe with the help of the marker layers that the growth rate fluctuates between neighboring stripes. This effect strongly increases for higher growth temperature.

Optical Emission of Individual GaN Nanocolumns Analyzed with High Spatial Resolution.

Selective area growth has been applied to fabricate a homogeneous array of GaN nanocolumns (NC) with high crystal quality. The structural and optical properties of single NCs have been investigated at the nanometer-scale by transmission electron microscopy (TEM) and highly spatially resolved cathodoluminescence (CL) spectroscopy performed in a scanning transmission electron microscope (STEM) at liquid helium temperatures. TEM cross-section analysis reveals excellent structural properties of the GaN NCs. Sporadically, isolated basal plane stacking faults (BSF) can be found resulting in a remarkably low BSF density in the almost entire NC ensemble. Both, defect-free NCs and NCs with few BSFs have been investigated. The low defect density within the NCs allows the characterization of individual BSFs, which is of high interest for studying their optical properties. Direct nanometer-scale correlation of the CL and STEM data clearly exhibits a spatial correlation of the emission at 360.6 nm (3.438 eV) with the location of basal plane stacking faults of type I1.

Three-dimensional reciprocal space mapping with a two-dimensional detector as a low-latency tool for investigating the influence of growth parameters on defects in semipolar GaN

A rapid nondestructive defect assessment and quantification method based on X-ray diffraction and three-dimensional reciprocal-space mapping has been established. A fast read-out two-dimensional detector with a high dynamic range of 20 bits, in combination with a powerful data analysis software package, is set up to provide fast feedback to crystal growers with the goal of supporting the development of reduced defect density GaN growth techniques. This would contribute strongly to the improvement of the crystal quality of epitaxial structures and therefore of optoelectronic properties. The method of normalized three-dimensional reciprocal-space mapping is found to be a reliable tool which shows clearly the influence of the parameters of the metal–organic vapour phase epitaxial and hydride vapour phase epitaxial (HVPE) growth methods on the extent of the diffuse scattering streak. This method enables determination of the basal stacking faults and an exploration of the presence of other types of defect such as partial dislocations and prismatic stacking faults. Three-dimensional reciprocal-space mapping is specifically used in the manuscript to determine basal stacking faults quantitatively and to discuss the presence of partial dislocations. This newly developed method has been applied to semipolar GaN structures grown on patterned sapphire substrates (PSSs). The fitting of the diffuse scattering intensity profiles along the stacking fault streaks with simulations based on a Monte Carlo approach has delivered an accurate determination of the basal plane stacking fault density. Three-dimensional reciprocal-space mapping is shown to be a method sensitive to the influence of crystallographic surface orientation on basal stacking fault densities during investigation of semipolar (11{\overline 2}2) GaN grown on an r-plane (1{\overline 1}02) PSS and semipolar (10{\overline 1}1) GaN grown on an n-plane (11{\overline 2}3) PSS. Moreover, the influence of HVPE overgrowth at reduced temperature on the quality of semipolar (11{\overline 2}2) GaN has been studied.

Effect of Basal-plane Stacking Faults on X-ray Diffraction of Non-polar (1120) a-plane GaN Films Grown on (1102) r-plane Sapphire Substrates

We report the effect of basal-plane stacking faults (BSFs) on X-ray diffraction (XRD) of non-polar (1120) a-plane GaN films with different SiNx interlayers. Complete SiNx coverage and increased three-dimensional (3D) to two-dimensional (2D) transition stages substantially reduce BSF density. It was revealed that the Si-doping profile in the Sidoped GaN layer was unaffected by the introduction of a SiNx interlayer. The smallest in-plane anisotropy of the (1120) XRD ω-scan widths was found in the sample with multiple SiNx layers, and this finding can be attributed to the relatively isotropic GaN mosaic resulting from the increase in the 3D-2D growth step. Williamson-Hall (WH) analysis of the (h0h0) series of diffractions was employed to determine the c-axis lateral coherence length (LCL) and to estimate the mosaic tilt. The c-axis LCLs obtained from WH analyses of the present study’s representative a-plane GaN samples were well correlated with the BSFrelated results from both the off-axis XRD ω-scan and transmission electron microscopy (TEM). Based on WH and TEM analyses, the trends in BSF densities were very similar, even though the BSF densities extracted from LCLs indicated that the values were reduced by a factor of about twenty.

Growth and doping of semipolar GaN grown on patterned sapphire substrates

In order to achieve large area semipolar GaN layers with high crystal quality, we have etched trenches into n-plane and r-plane sapphire wafers exposing c-plane-like side-walls, from which GaN stripes can be grown by metalorganic vapor phase epitaxy mainly in c-direction, forming semipolar {101¯1} or {112¯2} surfaces after coalescence. Here, we describe how to improve such layers by optimizing the side-facet orientation and by including a SiN nanomask interlayer in situ into the growth process, eventually resulting in a basal plane stacking fault density below 5×103cm−1. Moreover, doping experiments have revealed a substantially lower Mg incorporation efficiency on the {112¯2} surface as compared to the c-plane, whereas Si does not show such differences.

Three-dimensional reciprocal space mapping of diffuse scattering for the study of stacking faults in semipolar (\bf 11{\overline 2}2) GaN layers grown from the sidewall of anr-patterned sapphire substrate

Three-dimensional reciprocal space mapping of semipolar (11{\overline 2}2) GaN grown on stripe-patternedr-plane (1{\overline 1}02) sapphire substrates is found to be a powerful and crucial method for the analysis of diffuse scattering originating from stacking faults that are diffracting in a noncoplanar geometry. Additionally, by measuring three-dimensional reciprocal space maps (3D-RSMs) of several reflections, the transmission electron microscopy visibility criteria could be confirmed. Furthermore, similar to cathodoluminescence, the 3D-RSM method could be used in future as a reliable tool to distinguish clearly between the diffuse scattering signals coming from prismatic and from basal plane stacking faults and from partial dislocations in semipolar (11{\overline 2}2) GaN. The fitting of the diffuse scattering intensity profile along the stacking fault streaks with a simulation based on the Monte Carlo approach has delivered an accurate determination of the basal plane stacking fault density. A reduction of the stacking fault density due to the intercalation of an SiN interlayer in the GaN layer deposited on the sidewall of the pre-patterned sapphire substrate has led to an improvement of the optoelectronic properties, influenced by the crystal quality, as has been demonstrated by a locally resolved cathodoluminescence investigation.