Basal Plane Stacking Faults Research Articles

The Effect of Dislocation Density and Surface Morphology on the Optical Properties of InGaN/GaN Quantum Wells Grown on r-Plane Sapphire Substrates

The optical properties of non-polar InGaN/GaN multiple quantum wells grown on r-plane sapphire substrates are investigated as a function of threading dislocation density. The 6 K emission spectrum consists of a peak at 3.25 eV and a broad band centred around 2.64 eV. From microscopy and cathodoluminescence studies, the higher energy peak is assigned to recombination within quantum wells lying on the (1120) plane which are intersected by basal-plane stacking faults. The lower energy band is attributed to emission from sidewall quantum wells of varying width and composition which form on the various semi-polar facets of structural defects that develop during the quantum well growth.

Single phase [formula omitted] GaN on [formula omitted] sapphire grown by metal-organic vapor phase epitaxy

Heteroeptaxial growth of semipolar GaN on ( 1 0 1 ¯ 0 ) sapphire by metal-organic vapor phase epitaxy has been investigated using high temperature nucleation layers in order to provide large GaN templates for semipolar light emitters. The nitridation layer and nucleation conditions were optimized for phase pure { 1 1 2 ¯ 2 } layer orientation, minimum surface roughness and improved crystal quality. Single phase { 1 1 2 ¯ 2 } GaN layers were obtained by controlling the nitridation conditions and thus nitridation layer thickness. The crystal quality was improved by high temperature growth. Basal plane stacking faults were identified as the dominating defects by transmission electron microscopy. Photoluminescence measurements revealed that these defects are type I 1. A layer roughness of 4 nm was measured by atomic force microscopy.

Defect reduction in (1 1 2̄ 0) nonpolar a-plane GaN grown on r-plane sapphire using TiN interlayers

We report on the use of TiN interlayer to reduce the threading dislocation density in nonpolar a-plane GaN material grown by metal organic chemical vapor deposition (MOCVD), where the interlayer was formed by depositing the Ti metal on a GaN template followed by nitridize. By means of high resolution X-ray diffraction, transmission electron microscopy, and atomic force microscopy analyses, we found that the nonpolar a-plane GaN epitaxial grown on 10 nm-thick TiN interlayer, both on-axis and off-axis, exhibits a significant reduction in the full width at half maximum, the basal plane stacking faults (BSF), the threading dislocation density, and the root-mean-square roughness, respectively.

Biexciton emission and crystalline quality of ZnO nano-objects

The design of cost-effective standards for the quality of nano-objects is currently a keyissue toward their massive use for optoelectronic applications. The observation byphotoluminescence of narrow excitonic and biexcitonic emission lines in semiconductornanowires is usually accepted as evidence for high structural quality. Here, we performtime-resolved cathodoluminescence experiments on isolated ZnO nanobelts grown bychemical vapor deposition. We observe narrow emission lines at low temperature, togetherwith a clear biexciton line. Still, drastic alterations in both the CL intensity and lifetimeare observed locally along the nano-object. We attribute these to non-radiativerecombinations at edge dislocations, closing basal plane stacking faults, inhomogeneouslydistributed along the NB length. This leads us to the conclusion that the observation ofnarrow excitonic and biexcitonic emission lines is far from sufficient to grade the quality ofa nano-object.

The effect of indium concentration on the optical properties of a‐plane InGaN/GaN quantum wells grown on r‐plane sapphire substrates

AbstractWe report on the optical properties of a series of non‐polar a‐plane InGaN/GaN multiple quantum well (QW) structures of constant well width and varying indium concentration. The emission spectrum is dominated by recombination at regions of the QW intersected by basal plane stacking faults. The peak energy of this emission red‐shifts and the emission line width increases significantly with increasing average indium content in the QWs and indium–alloyed basal‐plane stacking faults. Furthermore, the spectral sensitivity of the luminescence decay time weakens as the indium content of the sample increases. These effects are attributed to the increase in compositional and hence electronic disorder of higher indium content samples resulting in stronger carrier localisation, which suppresses relaxation within the band of localised states.

The Growth of an Epitaxial ZnO Film on Si(111) with a Gd2O3(Ga2O3) Buffer Layer

High-quality (0001)-oriented ZnO epitaxial films have been grown by pulsed laser deposition on Si(111) buffered with a nanometer-thick Gd2O3(Ga2O3), GGO, layer. The structural characteristics of the ZnO epi-layers were studied by X-ray diffraction and transmission electron microscopy (TEM). The in-plane epitaxial relationship between the wurtzite ZnO, cubic GGO, and cubic Si follows {101̅0}ZnO∥{42̅2̅}Gd2O3∥{4̅22}Si and the ZnO/GGO interface can be described by the domain matching epitaxy. TEM contrast analysis reveals that the major defect structures in the ZnO films are edge-type threading dislocations and intrinsic basal plane stacking faults. All the ZnO epi-films studied are under a tensile biaxial strain, but no cracks were found. Temperature-dependent photoluminescence results show the excellent optical properties of the obtained ZnO layers, and the origins of the spectral features are discussed.

Microstructure and mechanical properties of Mg–Gd–Dy–Zn alloy with long period stacking ordered structure or stacking faults

The Mg–6.5Gd–2.5Dy–1.8Zn (wt.%) alloy with high strength and ductility was prepared by conventional casting method. At room temperature, the as-cast alloy with 14H long period stacking ordered (LPSO) structure exhibits an ultimate tensile strength of 276 MPa and elongation to failure of 10.8%, while they are 392 MPa and 6.1% for the peak-aged alloy with basal plane stacking faults (SF). The results indicate that the kinking of LPSO structure is beneficial for both work hardening and plasticity, and 14H LPSO structure contributes more to the improvement of ductility while SF is more effective in increasing strength.

Exciton Localization Behaviors of Basal Stacking Faults in a-Plane AlGaN Alloys

We study the basal plane stacking faults (BSFs) related optical properties in a-plane AlGaN alloys with different Al composition ranging from 0 to 0.28. The low-temperature photoluminescence (PL) spectra for AlGaN show two dominant peaks attributed to the emission of near band edge and BSFs-bound excitons, respectively. The PL integrated intensity ratio of the BSFs to NBE is found to correlate to the density of BSFs observed by the transmission electron microscopy. Finally, the exciton localization behaviors of BSFs in a-plane AlGaN alloys is observed and discussed in this study.

Impacts of anisotropic tilt mosaics of state-of-the-art m-plane freestanding GaN substrates on the structural and luminescent properties of m-plane AlxGa1−xN epilayers

Impacts of grown-in anisotropic tilt mosaics of state-of-the-art m-plane freestanding GaN (FS-GaN) substrates on the structural and luminescent properties of m-plane AlxGa1−xN epilayers are described. The results of cross-sectional transmission-electron microscopy and transmission-electron diffraction analyses reveal the following plane defects formation, even in mostly pseudomorphic epilayers: (i) The formation of basal-plane stacking faults (BSFs) is hard to avoid in the case of lattice-mismatched AlxGa1−xN thick film growth and (ii) the anisotropic greater m-plane tilt mosaic along the a-axis (twist in c-plane) of FS-GaN gives rise to pronounced formation of surface striations along the c-axis and planar-defect network (PDN) located on prismatic {101¯0} planes and pyramidal {11¯0n} planes, which are often terminated by BSF, especially in the case of the epilayers grown by NH3-source molecular-beam epitaxy. These PDNs are assigned by spatially resolved cathodoluminescence (CL) and time-resolved photoluminescence measurements to associate with a characteristic emission peak approximately 200–300 meV lower than the near-band-edge (NBE) emission peak. Based on our database made with the aid of the positron-annihilation technique, three deep-state CL bands are correlated with cation vacancies (VIII) and cation-vacancy complexes. The intensity of the emission band originating from VIII-oxygen complexes was weaker by more than two orders of magnitude than the NBE peak.

Interfacial sources of extended defects in nonpolar and semipolar III-Nitride semiconductors

The principal characteristic of nonpolar and semipolar heteroepitaxial interfaces is their anisotropy that leads to distinct ways of misfit accommodation along two in-plane directions. One direction can require misfit dislocations with out-of-plane Burgers vectors that induce epilayer tilts and low-angle grain boundaries, and the other necessitates the introduction of multiple basal-plane stacking faults. The reduced bicrystalline symmetry also favours the coexistence of distinct epilayer orientation relationships. This may result in nanocrystalline interfacial zones comprising low-energy grain boundaries that act as defect sources for the matrix epilayer, generating threading dislocations and stacking faults.

I2basal plane stacking fault in GaN: Origin of the 3.32 eV luminescence band

We investigate the $3.32 \mathrm{eV}$ defect-related emission band in GaN correlating transmission electron microscopy and spatially and spectrally resolved cathodoluminescence at low temperature. The band is unambiguously associated with basal plane stacking faults of type ${I}_{2}$, which are a common defect type in semi- and nonpolar GaN grown on foreign substrates. We ascribe the luminescence to free-to-bound transitions. The suggested intrinsic acceptors involved have an ionization energy of $\ensuremath{\approx}$$0.17 \mathrm{eV}$, and are located at the ${I}_{2}$-type basal plane stacking faults.

Stacking fault‐related luminescence features in semi‐polar GaN

AbstractWe report about defect‐related emission bands in GaN. We establish a direct correlation between results from spatially and spectrally resolved cathodoluminescence bands at 3.32, 3.23, 3.20, 3.16, and 3.07 eV with findings obtained by transmission electron microscopy. The band around 3.32 eV was unambiguously assigned to I2 basal plane stacking faults (BSFs). The bands at 3.23, 3.20, 3.16, and 3.07 eV were detected in sample regions, where BSFs exist, but were not assigned to specific defect types, except for the 3.07 eV line which is a phonon replica of the transition at 3.16 eV.

One-dimensional exciton luminescence induced by extended defects in nonpolar GaN/(Al,Ga)N quantum wells

In this study, we present the optical properties of nonpolar GaN/(Al,Ga)N single quantum wells (QWs) grown on either a- or m-plane GaN templates for Al contents set below 15%. In order to reduce the density of extended defects, the templates have been processed using the epitaxial lateral overgrowth technique. As expected for polarization-free heterostructures, the larger the QW width for a given Al content, the narrower the QW emission line. In structures with an Al content set to 5 or 10%, we also observe emission from excitons bound to the intersection of I1-type basal plane stacking faults (BSFs) with the QW. Similarly to what is seen in bulk material, the temperature dependence of BSF-bound QW exciton luminescence reveals intra-BSF localization. A qualitative model evidences the large spatial extension of the wavefunction of these BSF-bound QW excitons, making them extremely sensitive to potential fluctuations located in and away from BSF. Finally, polarization-dependent measurements show a strong emission anisotropy for BSF-bound QW excitons, which is related to their one-dimensional character and that confirms that the intersection between a BSF and a GaN/(Al,Ga)N QW can be described as a quantum wire.

Heavy Si doping: The key in heteroepitaxial growth of a‐plane GaN without basal plane stacking faults?

AbstractIn this paper, we compare the effectiveness of two different methods regarding the reduction of defect densities in heteroepitaxially grown a‐plane GaN by heavy Si doping. The insertion of well‐established in situ SixNy nanomasks leads to locally heavy Si δ‐doped GaN. By increasing the SixNy deposition time in the range from 0 to 300 s the full width at half maxima (FWHM) of the X‐ray diffraction ω‐scans at in‐plane GaN(1${\bar {1}}$00) and GaN(0002) Bragg reflections decreases from 0.55° to 0.24° and from 0.45° to 0.16°, respectively. When growing without any SixNy interlayer but instead with continuously heavy Si‐doping, these values are further decreased to 0.13° and 0.15°, respectively. By measuring several higher order reflections and detailed evaluation of the ω‐scan broadening in Williamson–Hall‐plots (WHPs) a considerable reduction in defect densities and no hint of basal plane stacking faults (BSFs) were found for the heavy Si doped a‐plane GaN sample. To verify this result the micro‐structural properties of this sample were additionally investigated by transmission electron microscopy and cathodoluminescence (CL).

Epitaxial lateral overgrowth of non-polar GaN(1 1̄ 0 0) on Si(1 1 2) patterned substrates by MOCVD

m-Plane GaN was grown selectively by metal–organic chemical vapor deposition (MOCVD) on patterned Si(1 1 2) substrates, where grooves aligned parallel to the Si〈1 1 0〉 direction were formed by anisotropic wet etching to expose the vertical Si{1 1 1} facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, and ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied with the aim of achieving coalesced m-plane GaN films. The epitaxial relationship was found to be GaN(1 1̄ 0 0) || Si(1 1 2), GaN[0 0 0 1] || Si[1 1 –1], GaN[1̄ 1̄ 2 0] || Si[1 1̄ 0]. Among all growth parameters, the ammonia flow rate was revealed to be the critical factor determining the growth habits of GaN. The distribution of extended defects, such as stacking faults and dislocations, in the selectively grown GaN were studied by transmission electron microscopy in combination with spatially resolved cathodoluminescence and scanning electron microscopy. Basal-plane stacking faults were found in the nitrogen-wing regions of the laterally overgrown GaN, while gallium-wings were almost free of extended defects, except for the regions near the GaN/Si{1 1 1} vertical sidewall interface, where high dislocation density was observed.

Microstructural, optical, and electrical characterization of semipolar (112¯2) gallium nitride grown by epitaxial lateral overgrowth

Semipolar (112¯2) gallium nitride (GaN) films have been grown on m-plane (11¯00) sapphire by epitaxial lateral overgrowth. Transmission electron microscopy (TEM) studies show that the inclination of the [0001] axis at 32° from the film surface combined with the high [0001] growth rate under the reactor conditions used, allowed a low defect density (LDD) wing growing along [0001] to partially overgrow the highly defective window region and the other wing, resulting in a coalescence boundary, at which stacking faults and dislocations appear to terminate. Low temperature cathodoluminescence (CL) was performed to correlate the optical properties with the different stages of the growth process. It is found that emission from the LDD wing is dominated by near band edge recombination, whereas an emission band at 3.42 eV related to basal plane stacking faults and a broad band from 3.15–3.38 eV possibly related to emission from prismatic stacking faults and partial dislocations were observed in the window region. Scanning capacitance microscopy (SCM) studies showed that almost the entire film is unintentionally n-doped. By correlating the spatial variation in the unintentional doping to the overgrowth process, different impurity incorporation rates between the (112¯2) and (0001) facets have been observed. Dislocations of a or a+c type were found to bend over toward the growth direction in the LDD wing by TEM appear to be nonradiative by cross-sectional CL and SCM studies have revealed that they also impact the local electrical properties.

An X‐ray diffraction technique for analyzing basal‐plane stacking faults in GaN

AbstractAn X‐ray diffraction (XRD) technique for analyzing basal‐plane stacking faults (BSFs) is introduced and tested on GaN. The analysis considers the coexistence of multiple X‐ray broadening terms including tilt, twist, limited coherence length, and inhomogeneous strain. By measuring and fitting a series of symmetric and asymmetric reflections planes, we deduce the lattice tilt, twist, lateral coherence length, and inhomogeneous strain contributions, which allow us to determine the existence of any additional broadening in m‐plane GaN. We found that in fact certain ω‐scans do have excess broadening and that their lattice plane dependence is similar to the functional dependence of BSFs derived for powder XRD (θ/2θ‐scan). Applying such functional dependence allows us to estimate the BSF densities (I1‐ and I2‐type). The typical stacking fault density was estimated to be 1 × 106/cm in m‐plane GaN grown on m‐plane sapphire by MOCVD, while the relative densities of I1‐ and I2‐type are sample dependent. In contrast to the m‐plane GaN, stacking faults in GaN on c‐plane sapphire do not contribute significantly to Bragg peak broadening and are below the detection limit.

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.

Influence of buffer layers on the microstructure of MOVPE grown a-plane InN

We report a comparative study of the microstructure of a-plane ( 1 1 2 ¯ 0 ) InN epilayers grown on different buffer layers via metalorganic vapour phase epitaxy. Under optimized growth conditions, the crystalline quality of the InN epilayer is found to be best on an AlN buffer layer compared to that on GaN buffer layers or InN nucleation layers. All a-plane InN epilayers show an anisotropy in the in-plane mosaicity with the in-plane tilt value being different along the c- and m-directions. We also observe a Nagai-like tilt in all the epilayers. The magnitude and nature of this anisotropy and the Nagai-like tilt is also strongly influenced by the buffer layer. Our investigations show that InN grown on an AlN buffer has smaller in-plane tilt, basal plane stacking fault density, and background carrier concentration resulting in a higher mobility and a low band gap energy of ∼ 0.70 eV .

Growth kinetics of AlN and GaN films grown by molecular beam epitaxy on R-plane sapphire substrates

This paper reports the growth by molecular beam epitaxy of AlN and GaN thin films on R-plane sapphire substrates. Contrary to previous findings that GaN grows with its (112¯0) A-plane parallel to the (11¯02) R-plane of sapphire, our results indicate that the crystallographic orientation of the III-nitride films is strongly dependent on the kinetic conditions of growth for the GaN or AlN buffer layers. Thus, group III-rich conditions for growth of either GaN or AlN buffers result in nitride films having (112¯0) planes parallel to the sapphire surface, and basal-plane stacking faults parallel to the growth direction. The growth of these buffers under N-rich conditions instead leads to nitride films with (112¯6) planes parallel to the sapphire surface, with inclined c-plane stacking faults that often terminate threading dislocations. Moreover, electron microscope observations indicate that slight miscut (∼0.5°) of the R-plane sapphire substrate almost completely suppresses the formation of twinning defects in the (112¯6) GaN films.