Basal Plane Stacking Faults Research Articles

Optical investigation of semi-polar (11-22) AlxGa1-xN with high Al composition

Exciton localization generally disturbs uniform population inversion, leading to an increase in the threshold current for lasing. High Al content AlGaN is required for the fabrication of deep ultra-violet (DUV) laser diodes (LDs), and this also generates exciton localization. Temperature-dependent photoluminescence and room temperature cathodoluminescence measurements have been performed on high quality semi-polar (11-22) AlxGa1-xN alloys with high Al composition ranging from 0.37 to 0.56 in order to systematically study the optical properties (in particular, exciton localization) of both the near-band-edge emission and the basal-plane stacking faults related emission, demonstrating different behaviours. Further comparison with the exciton localization of their c-plane counterparts has been performed, exhibiting that the exciton localization in semi-polar (11-22) AlGaN is much smaller than that in c-plane AlGaN. As a consequence, semi-polar (11-22) AlGaN demonstrates a greater potential than its c-plane counterpart in terms of the growth of DUV LDs.

Indirect excitons in hydrogen-doped ZnO

We present a correlative experimental and theoretical study of bound excitons in hydrogen-doped ZnO, with a particular focus on the dynamics of their metastable state confined in the sub-surface region, using a combination of surface-sensitive characterisation techniques and density functional theory calculations. A metastable sub-surface emission at 3.31 eV found in H-doped ZnO is attributed to the radiative recombination of indirect excitons localised at basal plane stacking faults (BSFs) where the excitonic transition involves electrons bound to bond-centre hydrogen donors in the potential well of the BSF. Additionally, our work shows the electrical transport of ZnO Schottky junctions is dominated by electrons confined at BSFs in the near-surface region.

Formation of I2-type basal-plane stacking faults in In0.25Ga0.75N multiple quantum wells grown on a (101¯1) semipolar GaN template

In this work, I2-type basal-plane stacking faults (BSFs) were observed in In0.25Ga0.75N multiple quantum wells (MQWs) grown on a (101¯1) semipolar GaN template by high-resolution transmission electron microscopy. The structure and formation mechanisms of the I2-type BSFs at the GaN-InGaN interface were investigated in detail. The formation of the I2-type BSFs contributes to lattice mismatch accommodation within the InGaN QWs. Their density varies in different regions of the sample due to the inhomogeneous distribution of the In content in the InGaN layer. The relationship between the In content in the InxGa1-xN layer and the I2-type BSFs is 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.

Ohmic Contact Properties of Photo-Chemically Etched Nonpolar A-Plane GaN Films

Nonpolar a-plane GaN (a-GaN) was photo-chemically etched under ultraviolet illumination using alkali KOH solution, and the Ohmic contact properties on the etched a-GaN epitaxial layer were investigated. After photo-chemical etching, the striated surface morphology along the c-axis [0001] direction was obtained having triangular prisms consisting of m-plane facets. The properties of Ti/Al/Ni/Au Ohmic contact on surface-textured a-GaN films were studied with transmission line method patterns aligned along the specific crystal orientations. The minimum specific contact resistance of 9.97 × 10−5 Ω·cm2 was achieved along the c-axis on the etched GaN surface at the annealing temperature of 750°C. The etched a-GaN showed higher electrical conductivity along the c-axis than along m-axis. This anisotropic behavior is in contrast to that of the unetched a-GaN where carrier scattering by basal plane stacking faults in the direction perpendicular to the c-axis is a major factor in determining opposite anisotropic conductivity. The higher conductivity along the c-axis of the etched a-GaN film could be explained by more dominant effect of surface roughness scattering.

Influence of stacking faults on the quality of GaN films grown on sapphire substrate using a sputtered AlN nucleation layer

This work describes the growth and characterization of GaN films on sapphire substrate by metal organic chemical vapor deposition (MOCVD) using a sputtered AlN nucleation layer. With this approach, the density of threading dislocations (TDs) in GaN is found to be dramatically reduced by 76% compared to the conventional growth method using MOCVD-grown AlN nucleation layer. Transmission electron microscope images reveal that basal-plane stacking faults are formed at different heights beyond the sputtered AlN nucleation layer, which block the extension of TDs along the growth direction, leading to a reduction in TDs density. The formation of basal-plane stacking faults can be attributed to the sputtered AlN nucleation layer and the preferred lateral growth condition of GaN. Furthermore, the surface morphology of GaN films also improved by adopting the sputtered AlN nucleation layer. This work demonstrates that a sputtered AlN nucleation layer helps to improve the quality of GaN epitaxial layer.

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.

Size effect on high temperature variable range hopping in Al+ implanted 4H-SiC

The hole transport properties of heavily doped 4H-SiC (Al) layers with Al implanted concentrations of 3 × 1020 and 5 × 1020 cm−3 and annealed in the temperature range 1950–2100 °C, have been analyzed to determine the main transport mechanisms. This study shows that the temperature dependence of the resistivity (conductivity) may be accounted for by a variable range hopping (VRH) transport into an impurity band. Depending on the concentration of the implanted impurities and the post-implantation annealing treatment, this VRH mechanism persists over different temperature ranges that may extend up to room temperature. In this framework, two different transport regimes are identified, having the characteristic of an isotropic 3D VRH and an anisotropic nearly 2D VRH. The latter conduction mechanism appears to take place in a rather thick layer (about 400 nm) that is too large to induce a confinement effect of the carrier hops. The possibility that an anisotropic transport may be induced by a structural modification of the implanted layer because of a high density of basal plane stacking faults (SF) in the implanted layers is considered. The interpretation of the conduction in the heaviest doped samples in terms of nearly 2D VRH is supported by the results of the transmission electron microscopy (TEM) investigation on one of the 5 × 1020 cm−3 Al implanted samples of this study. In this context, the average separation between basal plane SFs, measured along the c-axis, which is orthogonal to the carrier transport during electrical characterization, appears to be in keeping with the estimated value of the optimal hopping length of the VRH theory. Conversely, no SFs are detected by TEM in a sample with an Al concentration of 1 × 1019 cm−3 where a 3D nearest neighbor hopping (NNH) transport is observed.

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.

Optical emission related to basal-plane stacking faults inm-planeZn1−xMgxOepilayers for0≤x≤0.1

We investigate the optical properties of type-${\mathrm{I}}_{1}$ basal-plane stacking faults (BSFs) in ZnO and ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O}$ by cathodoluminescence spectroscopy supported by envelope function calculations. We report on a quantum-well-like band alignment of the ${\mathrm{I}}_{1}$ BSFs in ZnO taking into account the spontaneous polarization as well as an intrinsic self-screening effect on the polarization-related electric field. We present a systematic investigation of the luminescence properties associated with ${\mathrm{I}}_{1}$ BSFs in ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O}$ for varying Mg content $(0\ensuremath{\le}x\ensuremath{\le}0.1)$ using spatially and spectrally resolved cathodoluminescence spectroscopy. Both the near-band-edge emission and the luminescence line related to the ${\mathrm{I}}_{1}$ BSF exhibit the expected blueshift and line broadening with increasing Mg content. We propose a band diagram to describe the recombination mechanism of excitons in a ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O}$ film containing ${\mathrm{I}}_{1}$ BSFs. Based on a statistical analysis, we compile the experimentally obtained ${\mathrm{I}}_{1}$ BSF emission energies of ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O}$ samples and establish a linear dependence of the ${\mathrm{I}}_{1}$ BSF-related emission energy on the Mg content. This correlation provides an alternative way to identify the presence of ${\mathrm{I}}_{1}$ BSFs in ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O}$ without the necessity of sophisticated transmission electron microscopy investigations.

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.

Bright green emission and temperature dependent localized bound exciton transitions from undoped ZnO films

Bright green luminescence is achieved from undoped ZnO films prepared by the reaction of oxygen and zinc powder. The green emission band is considered to be mainly due to the singly ionized oxygen vacancies. Temperature dependence of exciton transitions in the undoped ZnO films has been investigated in the range from 10 to 270K. The PL spectrum at 10K is dominated by neutral donor-bound exciton (D°X) emissions. The dominant emission centered at about 3.303eV at above 45K can be attributed to the localized bound exciton (LBX) transitions related to basal plane stacking faults. LBX transitions can survive up to near room temperature due to the LBX binding energy of 68meV.

Semipolar AlN and GaN on Si(100): HVPE technology and layer properties

Hydride vapor phase epitaxy (HVPE) growth of semipolar AlN and GaN layers on planar Si(100) substrates with SiC nanolayer is investigated. It is shown experimentally that the solid-phase epitaxial formation of a specially oriented SiC nucleation layer followed by epitaxy of AlN layer by HVPE at low rates enables growth of aluminum and gallium nitrides in the semipolar direction. For the best GaN(20-23) layers obtained, the full width at half maximum (FWHM) value for the x-ray diffraction rocking curve is 24arcmin. The photoluminescence spectrum of the semipolar GaN measured at 4K exhibits bands related to basal-plane and prismatic stacking faults (BSF and PSF).

High Temperature Variable Range Hopping in Heavy Al Implanted 4H-SiC

In this work, we confirm and extend the results of a previous study where a variable range hopping transport through localized impurity states has been found to dominate the electrical transport properties of 3×1020 cm-3 and 5×1020 cm-3 Al+ implanted 4H-SiC layers after 1950-2000 °C post implantation annealing. In this study, samples with longer annealing times have been taken into account. The temperature dependence of these sample conductivity follows a variable range hopping law, consistent with a nearly two-dimensional hopping transport of non-interacting carriers that in the highest doped samples, persists up to around room temperature. This result indicates that the hole transport becomes strongly anisotropic on increasing the doping level. At the origin of this unusual electrical behavior, may be the presence of basal plane stacking faults, actually observed by transmission electron microscopy in one of the 5×1020 cm-3 samples

Temperature Dependent Stability of Stacking Fault in Highly Nitrogen-Doped 4H-SiC Crystals

The formation of basal plane stacking faults in highly nitrogen-doped 4H-SiC crystals was theoretically investigated. A novel theoretical model based on the so-called quantum well action (QWA) mechanism was proposed; the model considers several factors, which were overlooked in a previously proposed model, and explains well the annealing-induced formation of double layer Shockley-type stacking faults in highly nitrogen-doped 4H-SiC crystals. We further revised the model to consider the carrier distribution in the depletion regions adjacent to the stacking fault and were successful in explaining the shrinkage of stacking faults during annealing at even higher temperatures.

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.

Stacking fault emission in GaN: Influence of n-type doping

We present spatially and spectrally resolved cathodoluminescence investigations on the cross section of semipolar (112¯2) gallium nitride epitaxial layers with high background doping level. The locally varying high carrier concentration leads in emission to a free electron recombination band (FERB) governed on the high energy side by conduction band filling. For the basal plane stacking fault (BSF) of type I1, typically emitting at ≈3.41 eV in low doped GaN, we find a blue shift in emission correlated to the FERB high energy tail. This shift can be perfectly modeled and understood in a quantum well model for the BSF, taking also into account the varying doping level in the barrier region. Thus, the carrier concentration can be finally calculated either from the actual position of the I1 BSF or alternatively from the FERB-related near band edge emission.

Positional dependence of defect distribution in semipolar hydride vapor phase epitaxy-GaN films grown on patterned sapphire substrates

We have investigated the position dependence of crystalline quality and defect distribution in a semipolar hydride vapor phase epitaxy (HVPE)-GaN film grown on a patterned sapphire substrate (PSS). Position-dependent X-ray microdiffraction (XRMD) measurement clearly revealed the periodic fluctuation of the lattice plane tilting in HVPE-GaN films. This correlated with the periodic distribution of (a + c)-type dislocations owing to the patterning pitch of the PSS as confirmed by transmission electron microscopy (TEM). In the three-dimensional reciprocal lattice space map, the diffuse streak exactly along the c-axis can be clearly detected, indicating the presence of basal plane stacking faults in HVPE-GaN films. Furthermore, we have quantitatively estimated the defect densities from the results of XRMD and TEM measurements. From the obtained results of XRMD and TEM measurements, the fluctuation of the lattice plane tilting and the defect distribution in HVPE-GaN films grown on two types of metalorganic vapor phase epitaxy-GaN templates will be discussed in detail.

Theoretical investigation of the formation of basal plane stacking faults in heavily nitrogen-doped 4H-SiC crystals

The formation of basal plane stacking faults in heavily nitrogen-doped 4H-SiC crystals was theoretically investigated. A novel theoretical model based on the so-called quantum well action mechanism was proposed; the model considers several factors, which were overlooked in a previously proposed model, and provides a detailed explanation of the annealing-induced formation of double layer Shockley-type stacking faults in heavily nitrogen-doped 4H-SiC crystals. We further revised the model to consider the carrier distribution in the depletion regions adjacent to the stacking fault and successfully explained the shrinkage of stacking faults during annealing at even higher temperatures. The model also succeeded in accounting for the aluminum co-doping effect in heavily nitrogen-doped 4H-SiC crystals, in that the stacking fault formation is suppressed when aluminum acceptors are co-doped in the crystals.

Optical and Electric Properties of Cu-Doped ZnO Films Grown on MgO (100) Substrate

This work studied the luminescent and electric properties of Cu-doped ZnO films grown on MgO (100) substrate by plasma-assisted molecular beam epitaxy. The doping quantity was controlled by regulating Cu Kundsen cell temperature. The crystallinity and surface morphology of the Cu-doped ZnO films were characterized by high resolution X-ray diffraction, atomic force microscopy, and scanning electron microscopy. The chemical state and elemental compositions was analyzed by X-ray photoelectron spectroscopy (XPS). The near-band-edge emission (NBE) and defect-related emission (DLE) were analyzed by photoluminescence spectroscopy. The electric properties were determined by Hall measurement and four-point probe measurement. The experimental results revealed that ZnO films grown epitaxially on MgO substrate with (10-10)ZnO//(100)MgO. X-ray rocking curve measurements showed that the (10-10)ZnO reflection exhibits a full width at half maximum value of about 0.7o-1.0o. The XPS analysis showed that the doped copper atoms have a monovalent state in the ZnO films grown at Cu Kundsen cell temperatures lower than 900oC. The amount of Cu incorporation was estimated to be in the range of 1x1019 to 1x1021 atoms/cm3. The NBE peak of the Cu-doped ZnO films can be deconvoluted into three sub-peaks: a free exciton emission at 3.29 eV, a basal-plane stacking fault emission at 3.19 eV, and a third peak at 3.25 eV. The third peak might be related to an acceptor-bound excitons due to the doping of copper, since it disappeared after annealing. Hall measurement indicated that both pure and Cu-doped ZnO films are n-type semiconductor. The electron concentration of undoped ZnO film is 1x1018 cm-3, whereas that of the Cu-doped ZnO films are in the range of 5x1015~5x1016 cm-3. Obviously, the electron concentration of ZnO is compensated by the substitution of Cu+ to Zn2+ which generates acceptor levels. However, the electron concentration of the Cu-doped ZnO films rose back to the order of magnitudes 1018 cm-3 after annealed in oxygen or hygrogen, indicating the acceptor states were eliminated. Moreover, in this study, the incorporation of copper in ZnO does not result in an increase of the yellow-green luminescence.