Basal Plane Stacking Fault Density Research Articles

Structural and electrical anisotropies of Si-doped a-plane (11–20) GaN films with different SiNx interlayers

The effects of different SiNx interlayers on the structural and electrical properties of nonpolar Si-doped a-plane (11–20) GaN films grown on r-plane (1–102) sapphire were investigated. The surface roughness depends strongly on the SiNx coverage, deposition temperature and number of SiNx layers. The in-plane anisotropy of on-axis x-ray rocking curves (XRCs) (full width at half-maximum) was significantly decreased by the introduction of multiple SiNx-treated GaN interlayers, indicating coherently scattering domains of uniform size. Off-axis XRC measurements were also employed to investigate the effects on the mosaic twist corresponding to edge dislocation and the I1-type basal-plane stacking fault (BSF) density. Hall effect measurement showed that the electrical conductivity was the highest when multiple SiNx/GaN interlayers were employed. The measured sheet resistances (Rsh) along the c-axis were higher than those along the m-axis. These anisotropic conductivities could be explained by BSFs acting as carrier scattering centers. The ratios of Rsh along the two in-plane orientations also correlated well with the BSF densities.

Crystal phase engineered quantum wells in ZnO nanowires

We report the fabrication of quantum wells in ZnO nanowires (NWs) by a crystal phase engineering approach. Basal plane stacking faults (BSFs) in the wurtzite structure can be considered as a minimal segment of zinc blende. Due to the existing band offsets at the wurtzite (WZ)/zinc blende (ZB) material interface, incorporation of a high density of BSFs into ZnO NWs results in type II band alignment. Thus, the BSF structure acts as a quantum well for electrons and a potential barrier for holes in the valence band. We have studied the photoluminescence properties of ZnO NWs containing high concentrations of BSFs in comparison to high-quality ZnO NWs of pure wurtzite structure. It is revealed that BSFs form quantum wells in WZ ZnO nanowires, providing an additional luminescence peak at 3.329 eV at 4 K. The luminescence mechanism is explained as an indirect exciton transition due to the recombination of electrons in the QW conduction band with holes localized near the BSF. The binding energy of electrons is found to be around 100 meV, while the excitons are localized with the binding energy of holes of ∼5 meV, due to the coupling of BSFs, which form QW-like structures.

Analysis of the modified williamson-hall plot of non-polar a-plane GaN films

We report on the Williamson-Hall (W-H) plot analysis of non-polar a-plane GaN (a-GaN) films for a better understanding of the anisotropic peak broadening with X-ray in-beam direction. The full width at half maximum (FWHM) values of the symmetric reflections of the X-ray rocking curves at different angles phi for the a-GaN film clearly show the anisotropic behavior of its structural property. A modified W-H plot analysis of the symmetric and reflections shows that the mosaic blocks are highly elongated along the c-axis direction. The W-H plots of the peak widths from the n th order reflections in the asymmetric geometry with an angle chi of 30° (ϕ = 0°) can provide quite useful information on the microstructural state of a-GaN films. We found that a coherent scattering length of ∼285 nm could be obtained for an a-GaN film with high crystalline quality. We note that the coherent scattering lengths obtained from W-H analyses can significantly underestimate the density of basal-plane stacking faults for nonpolar a-GaN films.

Advantages and remaining issues of state-of-the-art m-plane freestanding GaN substrates grown by halide vapor phase epitaxy for m-plane InGaN epitaxial growth

Advantages and remaining issues of state-of-the-art m-plane freestanding GaN (FS-GaN) substrates grown by halide vapor phase epitaxy (HVPE) for m-plane InxGa1 − xN epitaxial film growth by metalorganic vapor phase epitaxy are described. Because of the low threading dislocation and basal-plane stacking fault densities, improved quantum efficiency and short radiative lifetime are achieved for the near-band-edge emission of 200–250 nm thick m-plane pseudomorphic InxGa1 − xN (x ⩽ 0.14) epilayers. Values of full-width at half-maximum for the x-ray ω-rocking curves remain unchanged as the substrate values being 80 and 60 arcsec for the (100) diffraction with 〈0001〉 and azimuths, respectively, and 80 arcsec for the (102) diffraction. As the surface flatness is greatly improved, the In-incorporation efficiency (ηIninc) is lower than the cases for conventional c-plane growth and m-plane growths on defective GaN bases. The former originates from nonidentical surface kinetics, and the latter is due to the reduction in the area of inclined and tilted planes. Sub-micrometer-wide zonary patterns parallel to the c-axis and 2 μm long axis figure-of-eight patterns parallel to the a-axis are clearly visualized in the monochromatic cathodoluminescence (CL) intensity images. Because the spatio-time-resolved CL measurement reveals very little spatial variation of low-temperature radiative lifetime, the slight peak energy variation is interpreted to originate from nonidentical ηIninc for the growing surfaces exhibiting various miscut angles. The figure-of-eight patterns are ascribed to originate from the anisotropic, severe m-plane tilt mosaic along the a-axis of the GaN substrate, and the zonary patterns may originate from the m-plane tilt mosaic along the c-axis. Further reduction in the tilt and twist mosaics is necessary for HVPE of FS-GaN substrates, in order to grow homogeneous InGaN epilayers.

The Role of Growth-Pressure on the Determination of Anisotropy Properties in Nonpolar m-Plane GaN

This study reports the role of growth-pressure on the determination of anisotropy properties in nonpolar m-plane GaN. In the high-pressure-growth samples, unrelieved lattice misfit strain lead to larger anisotropic in-plane strains, a striated surface, lower densities of basal-plane stacking faults (BSF) and prismatic stacking faults (PSF), and a smaller anisotropic polarization. In contrast, the low-pressure-growth sample shows the opposite trend. It is suggested that the growth pressure determines the degree of relaxation of lattice misfit strain, which in turn affects the anisotropic in-plane strains, striation feature, densities of BSF and PSF, and degree of polarization. The research results can be used to optimize the growth of nonpolar m-plane GaN.

Comparative Study of the Characteristics of the Basal Plane Stacking Faults of Nonpolar a-Plane and Semipolar (112̄2) GaN

Nonpolar (112̄0) and semipolar (112̄2) GaN are grown on r-plane and m-plane sapphire by MOCVD to investigate the characteristics of basal plane stacking faults (BSFs). Transmission electron microscopy reveals that the density of BSFs for the semipolar (112̄2) and nonpolar a-plane GaN template is 3×105 cm−1 and 8×105 cm−1, respectively. The semipolar (112̄2) GaN shows an arrowhead-like structure, and the nonpolar a-plane GaN has a much smoother morphology with a streak along the c-axis. Both nonpolar (112̄0) and semipolar (112̄2) GaN have very strong BSF luminescence due to the optically active character of the BSFs.

Improvement of (1122) Semipolar GaN Crystal Quality with TiN Interlayer by Metal Organic Vapor Phase Epitaxy

The effect of a TiN interlayer on the crystalline quality of (1122) semipolar GaN grown on m-plane sapphire was studied by high-resolution X-ray diffraction (HRXRD). Before GaN growth, a TiN interlayer was formed by nitrided the Ti metal on the GaN seed layer. Symmetric and skew-symmetric HRXRD measurements demonstrate that both threading dislocation and basal plane stacking fault densities are reduced. Raman results also show an increased crystalline quality and a decreased threading dislocation density. (1122) semipolar GaN with an optimal Ti thickness of 2 nm has been obtained under the existing growth conditions.

Surface striation, anisotropic in-plane strain, and degree of polarization in nonpolar m-plane GaN grown on SiC

We report the roles of island coalescence rate and strain relaxation in the development of anisotropic in-plane strains, striation feature, and subsequent degree of polarization in NH3-flow-rate-dependent m-plane GaN. In the high-NH3-flow-rate samples, the results of cathodoluminescence, polarized Raman and in situ optical reflectance measurements reveal that a slower coalescence and unrelieved lattice misfit strain lead to larger anisotropic in-plane strains, striated surface and luminescence patterns, and a lower density of basal-plane stacking fault (BSF) and prismatic stacking fault (PSF). In contrast, a lower NH3 flow rate leads to more rapid island coalescence and fully relaxed lattice misfit strain such that relaxed in-plane strains, a reduced striation surface, and a higher density of BSF and PSF are observed. It is suggested that the anisotropic in-plane strains, striation feature, and BSF and PSF density are consequences of how rapidly coalescence occurs and the degree of relaxation of lattice misfit strain. In addition, the simulation results of the k · p perturbation approach confirm a larger anisotropic strain results in a smaller degree of polarization. The research results provide important information for optimized growth of nonpolar III-nitrides.

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.

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 .

Structural and Optical Properties of Nonpolar AlN(1120) Films Grown on ZnO(1120) Substrates with a Room-Temperature GaN Buffer Layer

We have grown a-plane AlN films on ZnO substrates using room temperature epitaxial GaN buffer layers by pulsed laser deposition (PLD). The basal plane stacking fault density of these AlN(1120) films, as estimated from X-ray rocking curve measurements, is as low as 1.1×105 cm-1, which is attributed to the use of ZnO substrates with a small lattice mismatch. Cathode luminescence spectra measured at 300 K exhibited a clear near band-edge peak of AlN at around 5.9 eV. The use of a PLD low-temperature growth technique and ZnO substrates would be quite attractive for the fabrication of ultraviolet light-emitting devices.

Effect of Controlled Growth Dynamics on the Microstructure of Nonpolar a-Plane GaN Revealed by X-ray Diffraction

This paper reports the effect of controlled growth dynamics, as monitored by in situ optical reflectance, on the microstructure of nonpolar a-plane GaN films grown on r-plane sapphire. The mosaic microstructure of a-plane GaN and its anisotropy are evaluated by X-ray rocking curve (XRC) measurements. By inserting a pronounced islanding stage followed by an enhanced lateral growth, pit-free a-plane GaN has been achieved showing an XRC linewidth of ∼0.18 and ∼0.3° for on- and off-axes planes, respectively, with only minor anisotropy. The density of basal-plane stacking faults is reduced by ∼70% as determined by a modified Williamson–Hall X-ray analysis.

Effect of NH 3 flow rate on m-plane GaN growth on m-plane SiC by metalorganic chemical vapor deposition

This paper reports a study of the effect of NH 3 flow rate on m-plane GaN growth on m-plane SiC with an AlN buffer layer. It is found that a reduced NH 3 flow rate during m-plane GaN growth can greatly improve the recovery of in situ optical reflectance and the surface morphology, and narrow down the on-axis (1 0 1¯ 0) X-ray rocking curve (XRC) measured along the in-plane a-axis. The surface striation along the in-plane a-axis, a result of GaN island coalescence along the in-plane c-axis, strongly depends on the NH 3 flow rate, an observation consistent with our recent study of kinetic Wulff plots. The pronounced broadening of the (1 0 1¯ 0) XRC measured along the c-axis is attributed to the limited lateral coherence length of GaN domains along the c-axis, due to the presence of a high density of basal-plane stacking faults, most of which are formed at the GaN/AlN interface, according to transmission electron microscopy.

Defect reduction in non‐polar (11$ \bar 2 $0) GaN grown on (1$ \bar 1 $02) sapphire

AbstractThis work assesses the relative effectiveness of different techniques to reduce defect density in heteroepitaxial, non‐polar, a ‐plane GaN films grown on r ‐plane sapphire by MOVPE. Plan view TEM was used to obtain the defect density of films grown by different methods. The as‐grown material was found to have a high dislocation and basal plane stacking fault (BSF) density (1.9 (± 0.2) × 1011 cm–2 and 1.1 (± 0.9) × 106 cm–1 respectively). The four defect reduction techniques tested were: 3D–2D growth, SiNx interlayers, ScN interlayers and epitaxial lateral overgrowth (ELOG). Both dislocation and BSF density were reduced by all methods compared to the as‐grown material. The lowest defect density was achieved in the (0001) wing of the ELOG sample and was <1 × 106 dislocations cm–2 and 2.0 (± 0.7) × 104 BSFs cm–1. On the wafer scale, ScN interlayers were most effective: A single 5 nm thick ScN interlayer reduced the BSF density to 5.9 (± 0.8) × 105 cm–1 and the dislocation density was reduced by two orders of magnitude to 1.8 (± 0.2) × 109 cm–2 compared to the as‐grown material. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Defect reduction in (112¯2) semipolar GaN grown on m-plane sapphire using ScN interlayers

The effect of ScN interlayer thickness on the defect density of (112¯2) semipolar GaN grown on m-plane sapphire was studied by transmission electron microscopy. The interlayers comprised Sc metal deposited on a GaN seedlayer that was nitrided before GaN overgrowth by metal-organic vapor-phase epitaxy. Both interlayer thicknesses reduced the dislocation density by a factor of 100 to low-108 cm−2. The 8.5 nm interlayer produced regions that were free from basal plane stacking faults (BSF) and dislocations. The overall BSF density here was reduced by a factor of 5, to (6.49±0.07)×104 cm−1, without the need for an ex situ mask patterning step.

Investigating stacking faults in nonpolar gallium nitride films using X-ray diffraction

Nonpolar (11–20) GaN films with different basal-plane stacking fault (BSF) densities (determined using transmission electron microscopy) were investigated using X-ray diffraction. Diffuse streaking from I 1 and I 2 BSFs was observed in reciprocal space maps of the 10–10 and 20–20 reflections. X-ray calibration curves for BSF density determination can be plotted using the diffusely scattered intensity of open detector 10–10 or 20–20 ω -scans measured at a fixed, large separation from the peak maximum. However, ab initio determination of stacking fault densities is not possible due to additional broadening from other defects. Similarly, ω -scan peak widths are poor indicators of BSF densities.

Assessment of defect reduction methods for nonpolar a-plane GaN grown on r-plane sapphire

This work assesses the relative effectiveness of four techniques to reduce the defect density in heteroepitaxial nonpolar a-plane GaN films grown on r-plane sapphire by metalorganic vapour phase epitaxy (MOVPE). The defect reduction techniques studied were: 3D–2D growth, SiNx interlayers, ScN interlayers and epitaxial lateral overgrowth (ELOG). Plan-view transmission electron microscopy (TEM) showed that the GaN layer grown in a 2D fashion had a dislocation and basal-plane stacking fault (BSF) density of (1.9±0.2)×1011cm−2 and (1.1±0.9)×106cm−1, respectively. The dislocation and BSF densities were reduced by all methods compared to this 2D-grown layer (used as a seed layer for the interlayer and ELOG methods). The greatest reduction was achieved in the (0001) wing of the ELOG sample, where the dislocation density was <1×106cm−2 and BSF density was (2.0±0.7)×104cm−1. Of the in-situ techniques, SiNx interlayers were most effective: the interlayer with the highest surface coverage that was studied reduced the BSF density to (4.0±0.2)×105cm−1 and the dislocation density was lowered by over two orders of magnitude to (3.5±0.2)×108cm−2.

Defect reduction in nonpolar and semipolar GaN using scandium nitride interlayers

Nonpolar (112¯0) and semipolar (112¯2) GaN films were grown on sapphire by metalorganic vapour phase epitaxy using ScN interlayers of varying thicknesses. A 5nm interlayer reduced basal plane stacking fault (BSF) densities in nonpolar films by a factor of 2 and threading dislocation (TD) densities by a factor of 100 to (1.8±0.2)×109cm−2. An 8.5nm interlayer reduced BSF densities in semipolar films by a factor of 5 and reduced TD densities by a factor of 200 to (1.5±0.3)×108cm−2. Nonpolar film surface roughnesses were reduced by a factor of 20.

Reduction of stacking fault density in m-plane GaN grown on SiC

We report the reduction in basal-plane stacking faults (BSFs) in m-plane GaN grown on m-plane SiC. The origin of BSFs is linked to heteronucleation of m-plane GaN and the presence of N-face basal-plane sidewalls of three-dimensional islands. Graded AlGaN layers help to alleviate mismatched nucleation and the generation of BSFs. Transmission electron microscopy shows that the density of BSFs is decreased to the low 105cm−1. Anisotropy in on-axis x-ray rocking curves, a salient feature in m-plane GaN heteroepitaxial layers, is greatly reduced. A possible mechanism of BSF generation, and the demonstration of improved InGaN∕GaN quantum well emission are presented.

Basal Plane Stacking-Fault Related Anisotropy in X-ray Rocking Curve Widths of m-Plane GaN

A model of basal plane stacking faults as boundaries between incoherently scattering domains in m-plane GaN films is reviewed. m-Plane GaN films are analyzed with a modified version of the Williamson–Hall analysis in order to determine the length-scale of coherent scattering and tilt-mosaic contribution to X-ray rocking curve widths for the primary in-plane directions. This analysis shows that basal plane stacking faults are the predominant source of rocking-curve width anisotropy in the m-plane films, and indicate that the modified Williamson–Hall analysis can be used as a non-destructive technique for measuring basal plane stacking fault densities in m-GaN films.