Presence Of Basal Stacking Faults Research Articles

Narrow Excitonic Lines in Core–Shell Nanorods With InGaN/GaN Quantum Wells Intersected by Basal Stacking Faults

Individual core–shell GaN nanorods (NRs) with InGaN quantum wells (QWs), lying in non‐polar, semi‐polar, and polar planes, are investigated by micro‐photoluminescence (μ‐PL) spectroscopy. Complementary transmission electron microscopy (TEM) studies reveal the presence of basal stacking faults (BSFs), which intersect the QWs. Narrow peaks of excitonic emission were detected at the NR tips, where the BSFs can both cross the semi‐polar QWs and penetrate through the thick polar well. In the NRs without the polar wells, the peaks appear in the blue spectral range only, while in the NRs with such wells, the peaks also appear in the green spectral range. Polarization resolved μ‐PL measurements reveal the scattering of linear polarization degree among the narrow peaks, as well as among individual NRs, although the dominant polarization of the narrow lines was normal to the NR axis. In general, these findings are consistent with the origination of the narrow excitonic lines from the intersections of the BSFs with QWs of different types.

Fabrication of InxGa1-xN Nanowires on Tantalum Substrates by Vapor-Liquid-Solid Chemical Vapor Deposition.

InxGa1−xN nanowires (NWs) have drawn great attentions for their applications in optoelectronic and energy conversion devices. Compared to conventional substrates, metal substrates can offer InxGa1−xN NW devices with better thermal conductivity, electric conductivity, and mechanic flexibility. In this article, InxGa1−xN NWs were successfully grown on the surface of a tantalum (Ta) substrate via vapor-liquid-solid chemical vapor deposition (VLS-CVD), as characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), scanning and transmission electron microscope (STEM), and photoluminescence spectroscopy (PL). It was found that the surface pretreatment of Ta and the composition of metallic catalysts played important roles in the formation of NWs. A dimpled nitrided Ta surface combined with a catalyst of nickle is suitable for VLS-CVD growth of the NWs. The obtained InxGa1−xN NWs grew along the [1100] direction with the presence of basal stacking faults and an enriched indium composition of ~3 at.%. The successful VLS-CVD preparation of InxGa1−xN nanowires on Ta substrates could pave the way for the large-scale manufacture of optoelectronic devices in a more cost-effective way.

Dislocation transmutation by tension twinning in magnesium alloy AZ31

The interaction between dislocations and {101¯2}〈1¯011〉 twins appears to play an important role in the strain hardening behavior of Mg. Detailed transmission electron microscopy study was performed to investigate the concept of dislocation “transmutation” across twin boundaries. A previously proposed dislocation transmutation reaction is confirmed. For (101¯2) twins, the transmutation reactions involve [a1] or [a3] matrix dislocations resulting in 〈c±a2〉 dislocations, which populate the vicinity of the twin boundary. No other 〈c+a〉 slip systems are observed within the twins, despite the fact that the 〈c±a2〉 dislocations have similar or lower resolved shear stress, as compared to other slip systems. This suggests the 〈c+a〉 slip mode is source limited, since the observed slip systems are the only ones that can result from transmutation. The formation of a unit 〈c+a〉 dislocation in the twin is proposed to involve two consecutive reactions, necessitating dislocation pile-up in the matrix, and the associated dislocation configurations are evaluated in terms of elastic strain energy considerations. Dislocation reactions are proposed which could explain the presence of basal stacking faults of either I1 or I2 type inside twin. At the low stress levels typical of {101¯2}〈1¯011〉 twinning dominated flow of textured polycrystals, the observed 〈c±a2〉 dislocations are most likely sessile. However, this could serve as a source mechanism for later deformation, and as a forest hardening mechanism against other slip systems. The interfacial reaction products could result in a dragging effect on twin boundary advancement, and establish a basis for subsequent rapid hardening.

Impact of charged basal stacking faults on the mobility of two-dimensional electron gas in nonpolar a-plane AlGaN/GaN heterostructures

The development of nonpolar nitride enhancement-mode high electron mobility transistors (E-HEMTs) has been severely hindered by the low and strongly anisotropic mobility of two-dimensional electron gas (2DEG) in nonpolar nitride heterostructures. The poor 2DEG transport properties could be correlated with the presence of basal stacking faults (BSFs) in nonpolar nitride heteroepitaxial films. A theoretical model of 2DEG transport in a-plane AlGaN/GaN heterostructures containing a large number of charged BSFs is developed. The electric current along an applied field is taken to be formed by a process in which two-dimensional electrons tunnel through a BSF and diffusively transport between consecutive BSFs. The room temperature 2DEG mobility of an a-plane Al0.3Ga0.7N/GaN heterostructure influenced by charged BSFs is calculated, and the result is consistent in magnitude and anisotropy with the experimental data in the literature. Analysis shows that charged BSFs lower the 2DEG mobility to about one tenth or less in the 2DEG density range from 1.0 × 1012 cm−2 to 1.5 × 1013 cm−2, and the effect is dependent on the direction of the external field. The anisotropy of the 2DEG mobility is dominated by the anisotropic electron tunneling process, but the 2DEG mobility nearly parallel to BSFs can be decreased obviously by the enhanced interface roughness scattering at relatively higher 2DEG density.

Charged basal stacking fault scattering in nitride semiconductors

A theory of charge transport in semiconductors showing built-in polarization (polar) is developed in the presence of basal stacking faults. The theory is based on quantum tunneling in conjunction with the semiclassical description of diffusive charge transport. It is shown that the presence of basal stacking faults leads to anisotropy in carrier transport. The theory is applied to carrier transport in gallium nitride films consisting of a large number basal stacking faults, and the result is compared with experimental data.

Effects of Basal Stacking Faults on Electrical Anisotropy of Nonpolar a-Plane ($11\bar{2}0$) GaN Light-Emitting Diodes on Sapphire Substrate

We report on the effects of basal stacking faults (BSFs) on the electrical anisotropy and the device characteristics of nonpolar a-plane GaN (1120) light-emitting diodes (LEDs) on r-plane (1102 ) sapphire substrates. The sheet resistance in the direction parallel to the c-axis [0001] is 18%-70% higher than the one in the direction parallel to the m-axis [1100 ]. The anisotropic conductivity of faulted a-plane GaN films can be explained by carrier scatterings from BSFs. It is also shown that the output power of nonpolar a-plane GaN LEDs are significantly influenced by the presence of BSFs, which laterally hampers the carrier transport in the n-GaN layer, especially in the direction parallel to the c-axis in faulted nonpolar nitride films.

Strain relaxation in short-period polar GaN/AlN superlattices

We have investigated the strain relaxation mechanisms in short-period polar GaN/AlN superlattices deposited by plasma-assisted molecular-beam epitaxy, and designed to display intersubband transitions at 1.55 μm. In a first stage, we have identified the growth conditions to minimize strain relaxation, using a Ga excess to reduce the (0001) surface free energy of both GaN and AlN. Under these growth conditions, crack propagation is not observed, even for the tensile-strained superlattices grown on GaN templates. The initial misfit relaxation in the vicinity of the buffer occurs by the formation of a-type dislocations. The final strain state of the superlattice, reached after 10–20 periods, is independent of the substrate (either GaN or AlN templates). Once the steady-state conditions are reached, we observe a periodic partial relaxation of quantum wells and barriers. High-resolution transmission electron microscopy indicates that the periodic relaxation can be related to the presence of basal and prismatic stacking faults creating clusters with an in-plane length of tens of nanometers. The effect of these defects on the optical performance of the superlattices is discussed by simulation of electronic structure using an 8×8 k⋅p Schrödinger–Poisson solver. In the presence of basal stacking faults at the quantum well interfaces, the deviation of the e1-e2 intersubband transition with respect to the nominal value is expected to be smaller than the measured absorption line width.

Microstructural Characterization of Semipolar GaN Templates and Epitaxial-Lateral-Overgrown Films Deposited on M-Plane Sapphire by Metalorganic Vapor Phase Epitaxy

The microstructure of GaN templates and epitaxial lateral overgrown (ELO) films deposited on M-plane sapphire is investigated by transmission electron microscopy. The epitaxial relationship is [1120]sapphire∥[1100]GaN in-plane with a growth plane close to (1122)GaN. The microstructure of the templates is dominated by the presence of basal stacking faults with a density of around 3×105 cm-1. Basal stacking faults are either terminated by partial dislocations or connected by prismatic stacking faults. Perfect dislocations are also observed. ELO films with stripes parallel to [1100]GaN allow us to reduce the density of basal stacking faults in the overgrown regions down to 5×103 cm-1. In these overgrown materials, the density of perfect dislocations is 108 cm-2 while neither partial dislocation nor prismatic stacking faults are observed.