Basal Plane Stacking Faults Research Articles

Crystal-phase quantum dots in GaN quantum wires

We study the nature of excitons bound to I1 basal plane stacking faults in ensembles of ultrathin GaN nanowires by continuous-wave and time-resolved photoluminescence spectroscopy. These ultrathin nanowires, obtained by the thermal decomposition of spontaneously formed GaN nanowire ensembles, are tapered and have tip diameters down to 6 nm. With decreasing nanowire diameter, we observe a strong blue shift of the transition originating from the radiative decay of stacking fault-bound excitons. Moreover, the radiative lifetime of this transition in the ultrathin nanowires is independent of temperature up to 60 K and significantly longer than that of the corresponding transition in as-grown nanowires. These findings reveal a zero-dimensional character of the confined exciton state and thus demonstrate that I1 stacking faults in ultrathin nanowires act as genuine quantum dots.

High-quality, 2-inch-diameter m-plane GaN substrates grown by hydride vapor phase epitaxy on acidic ammonothermal seeds

In this paper, we discusse the origin of basal-plane stacking faults (BSFs) generated in the homoepitaxial hydride vapor phase epitaxy (HVPE) growth of m-plane gallium nitride (GaN). We investigated the effects of seed quality, especially dislocation density, on BSF generation during homoepitaxy. The results clearly identify basal-plane dislocation in the seed as a cause of BSF generation. We realized high-quality m-plane GaN substrates with a 2-in. diameter using HVPE on low-dislocation-density m-plane seeds.

Atomic-scale investigation of structural defects in GaN layer on c-plane sapphire substrate during initial growth stage

Structural defects in the initial growth stages of GaN on sapphire, including stacking faults (SFs), threading dislocations (TDs), and mosaic structure containing grain boundaries, are investigated at the atomic scale. Individual grains in the as-deposited low temperature-GaN buffer layer are found to have twists correlated with those of the adjacent grains. These grains have little similarity on the stacking sequences, and the atomic arrangement on each side of the grain boundaries may be rearranged by annealing to achieve higher similarity in the stacking sequence. The TD identified as a-type at the top of the SFs-rich interfacial region is thought to originate from Frank partial dislocations. The Frank partial dislocation produces a distorted wurtzite-type structure. At the intermediate region of the basal-plane stacking fault between Frank and Shockley partial dislocations, the TD relieves the distortion in the wurtzite-type structure. In the TD, the wurtzite structure slips relative to the surrounding wurtzite.

Optical properties of one- and two-dimensional excitons in m-plane ZnO/MgZnO multiple quantum wells

Five pairs of ZnO/Zn0.9Mg0.1O multiple quantum wells (MQWs) with fixed barrier thickness of 55 nm and three well widths of 4, 8 and 16 nm have been grown on m-plane sapphires by pulsed laser deposition. Due to 2D quantum confinement with decreasing well width, the emission of excitons bound to the basal-plane stacking faults, which are one-dimensionally confined in MQWs, encounters larger blue shift than that of the near-band edge excitons. Furthermore, remarkably reducing coupling of free excitons with A1 longitudinal optical phonons is closely correlated with increasing exciton binding energy but enhanced coupling of E2-low phonons is a result of increasing interaction with the interface phonons with decreasing well width.

Influence of trench period and depth on MOVPE grown [formula omitted] GaN on patterned r-plane sapphire substrates.

In this article, the influence of the trench period and depth of pre-structured r-plane sapphire substrates on the metalorganic vapor phase epitaxy (MOVPE) growth of (112¯2) GaN is investigated. We found that a larger trench period is beneficial for a small basal plane stacking fault (BSF) and threading dislocation (TD) density on the wafer surface, because it facilitates a better formation of a coalescence gap, which effectively blocks defects from penetrating to the surface. Further, the amount of BSFs emerging from the −c-wings of the uncoalesced GaN stripes is directly related to the trench period. With the help of in situ deposited marker layers we studied the development of the individual GaN stripes and observed that the trench depth and hence the ratio of the sapphire c-plane area relative to the total surface area heavily influences the coalescence process and defect development. Moreover, it is observed that the parasitic donor concentration increases for samples with smoother wafer surface.

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.

In situ asymmetric island sidewall growth of high-quality semipolar (112̄2) GaN on m-plane sapphire

In situ asymmetric island sidewall growth (AISG) was developed to enhance Ga-face facet growth and improve the crystalline quality of (112) GaN epilayers on m-plane sapphire substrates. In the early growth stage island shaping and sidewall faceting were distinct and controlled by growth design. Using in situ AISG, {0002} instead of {103} sidewall facets were formed on the Ga-rich island surface, which eliminated formation of a {103} phase during subsequent layer growth of semipolar GaN. Enhanced Ga-face sidewall facet growth led to +c regions overlapping −c regions, which reduced defect density. Pure semipolar (112) epilayers with a reduced surface striation density and a basal-plane stacking fault density of 8 × 103 cm−1 were obtained. The observation of a narrow EH2 peak and an intense E1(LO) peak in Raman spectra indicates that almost strain-free high-quality semipolar (112) GaN films were achieved. The photoluminescence emission intensity from the (112) GaN film prepared by in situ AISG was dominated by band-edge emission and enhanced ∼4 times more than that from conventional (112) GaN.

Toward defect‐free semi‐polar GaN templates on pre‐structured sapphire

The microstructure of semi‐polar (11–22) GaN templates grown on pre‐structured r‐plane sapphire by MOVPE has been characterized by TEM. Cross‐sectional observations indicate that defects are generated in three regions of the layers: threading dislocations at the inclined GaN/sapphire interface, basal plane stacking faults (BSFs) at the c−‐wing, BSFs and threading dislocations at the coalescence between neighboring GaN stripes. An in situ SiN interlayer deposited at an early stage of the growth is shown to be effective in blocking the propagation of dislocations, which is mainly attributed to SiN formed on the c‐plane rather than on the (11–22) plane. Si‐doped marker layers have been used to study the evolution of the growth front before coalescence as a function of temperature. A high growth temperature is associated with the formation of highly faceted GaN stripes. Dislocations originally running along the c‐direction are bent to the [11–20] direction driven by a progressing (11–22) facet. An efficient defect reduction is realized as a result of terminating these dislocations at voids partially defined by the (11–20) facet.

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.

Semipolar (202̅3) nitrides grown on 3C–SiC/(001) Si substrates

Heteroepitaxial growth of GaN buffer layers on 3C–SiC/(001) Si templates (4°-offcut towards [110]) by metalorganic vapour phase epitaxy has been investigated. High-temperature grown Al0.5Ga0.5N/AlN interlayers were employed to produce a single (202̅3) GaN surface orientation. Specular crack-free GaN layers showed undulations along [11̅0] with a root mean square roughness of about 13.5 nm (50 × 50 μm2). The orientation relationship determined by x-ray diffraction (XRD) was found to be [1̅21̅0]GaN ∥[11̅0] and [3̅034]GaN ∥[110]3C − SiC/Si. Low-temperature photoluminescence (PL) and XRD measurements showed the presence of basal-plane stacking faults in the layers. PL measurements of (202̅3) multiple-quantum-well and light-emitting diode structures showed uniform luminescence at about 500 nm emission wavelength. A small peak shift of about 3 nm was observed in the electroluminescence when the current was increased from 5 to 50 mA (25–250 A cm−2).

Optical properties of defects in nitride semiconductors

Abstract

Nanoscale cathodoluminescence of stacking faults and partial dislocations in a‐plane GaN

In GaN, the basal plane stacking fault type I1 is a two‐dimensional defect characterized by a cubic inclusion within the wurtzite structure. Excitons are bound at the BSF I1 similar to the localization in a quantum well heterostructure leading to an efficient radiative recombination. In this study, we present the optical and structural properties of basal plane stacking faults occurrent in silicon doped a‐plane GaN layer by means of highly spatially resolved cathodoluminescence spectroscopy performed in a scanning transmission electron microscope. Drastically reduced panchromatic intensity in the vicinity of partial dislocations terminating the stacking faults, points to their non‐radiative character. Originating from intersection of two‐dimensional defects, the emission at 379.6 nm could be attributed to optically active stair‐rod dislocations.

New horizon for high performance Mg-based biomaterial with uniform degradation behavior: Formation of stacking faults.

Designing the new microstructure is an effective way to accelerate the biomedical application of magnesium (Mg) alloys. In this study, a novel Mg–8Er–1Zn alloy with profuse nano-spaced basal plane stacking faults (SFs) was prepared by combined processes of direct-chill semi-continuous casting, heat-treatment and hot-extrusion. The formation of SFs made the alloy possess outstanding comprehensive performance as the biodegradable implant material. The ultimate tensile strength (UTS: 318 MPa), tensile yield strength (TYS: 207 MPa) and elongation (21%) of the alloy with SFs were superior to those of most reported degradable Mg-based alloys. This new alloy showed acceptable biotoxicity and degradation rate (0.34 mm/year), and the latter could be further slowed down through optimizing the microstructure. Most amazing of all, the uniquely uniform in vitro/vivo corrosion behavior was obtained due to the formation of SFs. Accordingly we proposed an original corrosion mechanism for the novel Mg alloy with SFs. The present study opens a new horizon for developing new Mg-based biomaterials with highly desirable performances.

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.

In situ stress measurements during direct MOCVD growth of GaN on SiC

Abstract

Microstructural dependency of optical properties of m-plane InGaN multiple quantum wells grown on 2° misoriented bulk GaN substrates

A non-polar m-plane structure consisting of five InGaN/GaN quantum wells (QWs) was grown on ammonothermal bulk GaN by metal-organic vapor phase epitaxy. Surface step bunches propagating through the QW stack were found to accommodate the 2° substrate miscut towards the -c direction. Both large steps with heights of a few tens of nanometres and small steps between one and a few atomic layers in height are observed, the former of which exhibit cathodoluminescence at longer wavelengths than the adjacent m-plane terraces. This is attributed to the formation of semi-polar facets at the steps on which the QWs are shown to be thicker and have higher Indium contents than those in the adjacent m-plane regions. Discrete basal-plane stacking faults (BSFs) were occasionally initiated from the QWs on the main m-plane terraces, but groups of BSFs were frequently observed to initiate from those on the large steps, probably related to the increased strain associated with the locally higher indium content and thickness.

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.

Stacking fault related luminescence in GaN nanorods

Optical and structural properties are presented for GaN nanorods (NRs) grown in the [0001] direction on Si(111) substrates by direct-current reactive magnetron sputter epitaxy. Transmission electron microscopy (TEM) reveals clusters of dense stacking faults (SFs) regularly distributed along the c-axis. A strong emission line at ∼3.42 eV associated with the basal-plane SFs has been observed in luminescence spectra. The optical signature of SFs is stable up to room temperatures with the activation energy of ∼20 meV. Temperature-dependent time-resolved photoluminescence properties suggest that the recombination mechanism of the 3.42 eV emission can be understood in terms of multiple quantum wells self-organized along the growth axis of NRs.

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.

Structural characterization of selective area growth GaN nanowires by non-destructive optical and electrical techniques

The growth selectivity and structural quality of GaN nanowires obtained by plasma-assisted molecular beam epitaxy on pre-patterned GaN(0001) templates are investigated by means of non-destructive techniques. Optimum control over the nanowire arrangement and size requires a pitch between the mask apertures below twice the diffusion length of Ga atoms. Lower pitches, however, seem to slightly diminish the structural quality of the material, as revealed by the increase of the Raman peak linewidths. The photoluminescence spectra of the nanowires show a considerable presence of basal plane stacking faults, whose density increases for decreasing nanowire diameter. The capabilities of Kelvin probe force microscopy for imaging these kind of defects are also demonstrated.