Uncoalesced GaN Research Articles

Radius-dependent homogeneous strain in uncoalesced GaN nanowires

We investigate the strain state of ensembles of thin and nearly coalescence-free self-assembled GaN nanowires prepared by plasma-assisted molecular beam epitaxy on Ti/Al2O3(0001) substrates. The shifts of Bragg peaks in high-resolution X-ray diffraction profiles reveal the presence of a homogeneous tensile strain in the out-of-plane direction. This strain is inversely proportional to the average nanowire radius and results from the surface stress acting on the nanowire sidewalls. The superposition of strain from nanowires with different radii in the same ensemble results in a broadening of the Bragg peaks that mimics an inhomogeneous strain on a macroscopic scale. The nanowire ensembles show a small blueshift of the bound-exciton transitions in photoluminescence spectra, reflecting the existence of a compensating in-plane compressive strain, as further supported by grazing incidence X-ray diffraction measurements carried out at a synchrotron. By combining X-ray diffraction and photoluminescence spectroscopy, the surface stress components fx and fz of the air-exposed GaN{11¯00} planes that constitute the nanowire sidewalls are determined experimentally to be 2.25 and −0.7 N/m, respectively.

Stress study of GaN grown on serpentine-channels masked Si(111) substrate by MOCVD

Abstract An advanced epitaxial lateral overgrowth (ELOG) structure named “serpentine-channels masked Si(111) substrate” has been introduced, which only has one high-defect region per period compared with the conventional ELOG method. We successfully obtained both coalesced and uncoalesced GaN layers on the substrate simultaneously by the metal-organic chemical vapor deposition (MOCVD) method. The stress states of these two kinds of GaN epilayers were investigated by room-temperature micro-Raman scattering technique. The stress level in the uncoalesced GaN layer was a little lower than that in the coalesced GaN layer. Raman spectra reveal the periodical variations of residual tensile stress in GaN by analyzing E2 (high) phonon mode. In addition, thermal stress distribution of GaN was simulated by elasticity theory using the finite-element method (FEM). The results of simulation are entirely consistent with the experimental results derived from micro-Raman measurements.

Self-assembled formation of long, thin, and uncoalesced GaN nanowires on crystalline TiN films

We investigate in detail the self-assembled nucleation and growth of vertically oriented GaN nanowires by molecular beam epitaxy on crystalline TiN films. We demonstrate that this type of substrate allows for the growth of long and thin GaN nanowires that do not suffer from coalescence, a problem common to the growth on Si and other substrates. Only beyond a certain nanowire length that depends on the nanowire density and exceeds here 1.5 μm, coalescence takes place by bundling, i.e. the same process as on Si. By analyzing the nearest neighbor distance distribution, we identify the diffusion-induced repulsion of neighboring nanowires as the main mechanism limiting nanowire density during nucleation on TiN. Since on Si the final density is determined by shadowing of the impinging molecular beams by existing nanowires, it is the difference in adatom surface diffusion that enables the formation of nanowire ensembles with reduced density on TiN. These nanowire ensembles combine properties that make them a promising basis for the growth of core–shell heterostructures.

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.

Characterization of growth defects in thin GaN layers with X‐ray microbeam

AbstractThe spatially resolved distribution of strain, misfit and threading dislocations, and crystallographic orientation in uncoalesced GaN layers grown on Si(111) by maskless cantilever epitaxy or by pendeo epitaxy on SiC was studied by white beam Laue X‐ray microdiffraction, scanning electron and orientation imaging microscopy. Tilt boundaries formed at the column/wing interface depending on the growth conditions. A depth dependent deviatoric strain gradient is found in the GaN. The density of misfit dislocations as well as their arrangement within different dislocation arrays was quantified. Two different kinds of tilt (parallel and perpendicular to the stripe direction) manifested themselves by mutually orthogonal displacements of the (0006) GaN Laue spot relative to the Si(444) Laue spot. The origin of the tilts is discussed with respect to the miscut of the Si(111) surface and misfit dislocations formed at the interface. Regular oscillations of the conventional wing tilt were observed. Irregular crystallographic tilts fluctuations were found in the direction parallel to stripes. The amplitude of fluctuations is an order of magnitude smaller for layers with a lower defects density. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The origin of wing tilt for uncoalesced GaN grown on maskless grooved sapphire fabricated by wet chemical etching

In this paper, wing tilt was detected by high resolution X-ray rocking curve for uncoalesced GaN epilayer grown on maskless periodically grooved sapphire fabricated by wet chemical etching. Stress distribution was characterized by the frequency shift in micro-Raman spectroscopy measurement. It shows inhomogeneous stress distribution between the mesa and wing region, which is in agreement with the finite-element analysis simulations. Meanwhile, the wing tilt calculated by finite-element analysis agrees well with the rocking curve result. The results show that the different relaxation of stress causes inhomogeneous displacements between the mesa and wing region is the main reason of wing tilt formation.

Spatially resolved distribution of dislocations and crystallographic tilts in GaN layers grown on Si(111) substrates by maskless cantilever epitaxy

The spatial distribution of strain, misfit and threading dislocations, and crystallographic orientation in uncoalesced GaN layers grown on Si(111) substrates by maskless cantilever epitaxy was studied by polychromatic x-ray microdiffraction, high resolution monochromatic x-ray diffraction, and scanning electron microscopy. Tilt boundaries formed at the column/wing interface depending on the growth conditions. A higher lateral to vertical growth ratio suppressed sidewall deposition and was found to produce larger lattice tilts in the GaN films. Two kinds of crystallographic tilts are observed in the films. The measurements revealed that the free-hanging wings are tilted upward at room temperature in the direction perpendicular to the stripes. Finite element simulations of the thermally induced part of the wing tilt are presented. Moreover, a misorientation between the GaN(0001) and the Si(111) planes is observed in the parallel to the stripe direction. Its origin is discussed with respect to the strain of the epitaxial GaN on a miscut Si(111) surface and misfit dislocations formed at the interface.

Growth and characterization of pendeo-epitaxial GaN [formula omitted] on 4H–SiC [formula omitted] substrates

Growth on AlN/4H–SiC ( 1 1 2 ¯ 0 ) substrates of coalesced, non-polar GaN ( 1 1 2 ¯ 0 ) films having volumes of material with reduced densities of dislocations and stacking faults has been achieved from etched stripes via the statistical and experimental determination of the effect of temperature and V/III ratio on the lateral and vertical growth rates of the GaN{0 0 0 1} faces combined with pendeo-epitaxy. AFM of the uncoalesced GaN(0 0 0 1) and GaN ( 0 0 0 1 ¯ ) vertical faces revealed growth steps with some steps terminating at dislocations on the former and a pitted surface without growth steps, indicative of decomposition, on the latter. Coalescence was achieved via (a) a two-step route and the parameters of (1) T = 1100 ° C and V / III = 1323 for 40 min and (2) 1020 °C and V / III = 660 for 40 min and (b) a one-step route that employed T = 1020 ° C and a V/III ratio = 660 for 6 h. The densities of dislocations in the GaN grown vertically over and laterally from the ( 1 1 2 ¯ 0 ) stripes were ∼4×10 10 cm −2 and ∼2×10 8 cm −2, respectively; the densities of stacking fault in these volumes were ∼1×10 6 cm −1 and ∼2×10 4 cm −1, respectively. The defects in the wing material were observed primarily at the bottom of the film where lateral growth of the GaN occurred from the AlN and the SiC. Plan view AFM also revealed different microstructures and a reduction in the RMS roughness values from 1.2 to 0.95 nm in these respective regions.

Spatially Resolved Characterization of Microstructure, Defects and Tilts in GaN Layers Grown on Si(111) Substrates by Maskless Cantilever Epitaxy

ABSTRACTThe spatially resolved distribution of strain, misfit and threading dislocations, and crystallographic orientation in uncoalesced GaN layers grown on Si(111) substrates by maskless cantilever epitaxy was studied by white-beam Laue x-ray microdiffraction, scanning electron microscopy, and orientation imaging microscopy. Tilt boundaries formed at the column/wing interface with the misorientation strongly depending on the growth conditions. A depth-dependent deviatoric strain gradient is found in the GaN. Types and density of misfit dislocations as well as their arrangement within different dislocation arrays was quantified. The results are discussed with respect to the miscut of the Si(111) surface and misfit dislocations formed at the interface.

Characterization of crystallographic properties and defects via X‐ray microdiffraction in GaN (0001) layers

AbstractIntrinsic stresses due to lattice mismatch, high densities of threading dislocations, and extrinsic stresses resulting from the mismatch in the coefficients of thermal expansion, are present in almost all III‐Nitride heterostructures. Stress relaxation in the GaN layers occurs in conventional, cantilever (CE) and in pendeo‐epitaxial (PE) films via the formation of additional misfit dislocations, domain boundaries, elastic strain and wing tilt. Polychromatic X‐ray microdiffraction, high resolution monochromatic X‐ray diffraction and SEM analysis have been used to determine the crystallographic properties, misfit dislocations distribution and crystallographic tilts in uncoalesced GaN layers grown by PE and CE. The crystallographic tilt between the GaN(0001) and Si(111) planes was detected in the CE grown samples on Si(111). In contrast there was no tilt between GaN(0001) and SiC(0001) planes in PE grown samples. The wings are tilted upward for both the PE and CE grown uncoalesced GaN layers. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

White X‐ray microbeam analysis of strain and crystallographic tilt in GaN layers grown by maskless pendeoepitaxy

AbstractWe describe X‐ray and finite element analysis of dislocations, strain and crystallographic tilt in uncoalesced GaN layers grown by maskless pendeoepitaxy. The experimental results reveal that the GaN wings are tilted upward at room temperature. The distribution of strain and tilt depends on the width‐to‐height ratio of the GaN column. As the width‐to‐height ratio increases the wing tilt increases and changes more abruptly at the column/wing interface. Broadening of the GaN Laue reflections is more pronounced in the column region then in the wing region; this is in line with the inhomogeneity of both the strain and the defect density. A split in the Laue reflections from GaN films at the column/wing interface indicates the development of a low‐angle tilt boundary for samples with a large width‐to‐height ratio of the column. Spacing between the dislocations in the tilt boundary at the column/wing interface for GaN grown with maskless pendeoepitaxy is an order of magnitude larger than the dislocation spacing observed for masked lateral epitaxial growth. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of Stress Relaxation, Dislocations and Crystallographic Tilt Via X-ray Microdiffraction in GaN (0001) Layers Grown by Maskless Pendeo-Epitaxy

AbstractIntrinsic stresses due to lattice mismatch and high densities of threading dislocations and extrinsic stresses resulting from the mismatch in the coefficients of thermal expansion are present in almost all III-Nitride heterostructures. Stress relaxation in the GaN layers occurs in conventional and in pendeo-epitaxial films via the formation of additional misfit dislocations, domain boundaries, elastic strain and wing tilt. Polychromatic X-ray microdiffraction, high resolution monochromatic X-ray diffraction and finite element simulations have been used to determine the distribution of strain, dislocations, sub-boundaries and crystallographic wing tilt in uncoalesced and coalesced GaN layers grown by maskless pendeo-epitaxy. An important parameter was the width-to-height ratio of the etched columns of GaN from which the lateral growth of the wings occurred. The strain and tilt across the stripes increased with the width-to-height ratio. Tilt boundaries formed in the uncoalesced GaN layers at the column/wing interfaces for samples with a large ratio. Sharper tilt boundaries were observed at the interfaces formed by the coalescence of two laterally growing wings. The wings tilted upward during cooling to room temperature for both the uncoalesced and the coalesced GaN layers. It was determined that finite element simulations that account for extrinsic stress relaxation can explain the experimental results for uncoalesced GaN layers. Relaxation of both extrinsic and intrinsic stress components in the coalesced GaN layers contribute to the observed wing tilt and the formation of sub-boundaries.

Local strain, defects, and crystallographic tilt in GaN(0001) layers grown by maskless pendeo-epitaxy from x-ray microdiffraction

Polychromatic x-ray microdiffraction, high-resolution monochromatic x-ray diffraction, and finite element simulations have been used to determine the distribution of strain, defects, and crystallographic tilt in uncoalesced GaN layers grown by maskless pendeo-epitaxy. An important materials parameter was the width-to-height ratio of the etched columns of GaN from which occurred the lateral growth of the wings. Tilt boundaries formed at the column/wing interface for samples with a large ratio. Formation of the tilt boundary can be avoided by using smaller ratios. The strain and tilt across the stripe increased with the width-to-height ratio. The wings were tilted upward at room temperature.

Microscopic mapping of strain relaxation in uncoalesced pendeoepitaxial GaN on SiC

Mapping of the strain distribution in uncoalesced pendeoepitaxial GaN on SiC by spatially resolved micro-Raman and microphotoluminescence, and lattice constants measured by high-resolution x-ray diffraction provide a consistent picture of relaxation of inhomogeneous and anisotropic strain. The pendeoepitaxial wings show a nearly complete strain relaxation and high optical quality with extremely narrow donor bound exciton peaks. The narrow exciton linewidths result in the determination of c-axis strain to an accuracy of $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}.$

Effect of the Nucleation Layer on Stress during Cantilever Epitaxy of GaN on Si (111)

We have investigated the effect of the nucleation layer on subsequent film stress of GaN grown on Si (111) patterned with periodic mesas, using the cantilever epitaxy technique. GaN was nucleated on the unetched substrate mesa and proceeded to grow laterally over the trench region with low dislocation density material resulting in the wing region - the area over the trench. We have shown that by reducing the thickness of the AlN nucleation layer to 500 A, the uncoalesced GaN stripes are essentially crack free. We have also studied the effect of inserting a linear grade from the AlN nucleation layer to GaN. The grade increased the crack density from 4.65 × 10 -4 μm -1 with no grade to values ranging from 2.25 × 10 -2 to 1.29 × 10 -3 μm -1 . As the thickness of the grade increased the crack density approaches that obtained with no grade. X-ray diffraction was also used to characterize the nucleation layers effect on both, the structural quality and crystallographic wing tilt.

Surface instability and associated roughness during conventional and pendeo-epitaxial growth of GaN(0 0 0 1) films via MOVPE

Investigations of the origins of surface roughening in GaN(0 0 0 1) have resulted in the development of a growth process route having an optimum temperature of 1020°C and a film thickness beyond 2.5 μm which results in films with the smoothest surface morphology. Atomic force microscopy (AFM) revealed uncoalesced GaN islands and hillocks for non-optimum temperatures below and above 1020°C, respectively. Uncoalesced islands were a result of insufficient lateral growth. Hillocks were a result of the rotation of heterogeneous steps formed at pure screw or mixed dislocations which terminated on the (0 0 0 1) surface. Growth of the latter features was controlled kinetically by temperature through adatom diffusion. The 10 6 cm −2 density of hillocks was reduced through growth on thick GaN templates and regions of pendeo-epitaxy (PE) overgrowth with lower pure screw or mixed dislocations. Smooth PE surfaces were obtained at temperatures that reduced the lateral to vertical growth rate but also retarded hillock growth that originated in the stripe regions. The (1 1 2 ̄ 0) PE sidewall surface was atomically smooth, with a root mean square roughness value of 0.17 nm, which was the noise limited resolution of the AFM measurements.

Strain and crystallographic tilt in uncoalesced GaN layers grown by maskless pendeoepitaxy

The strain in thin GaN layers grown by maskless pendeoepitaxy has been investigated using high-resolution x-ray diffraction and finite-element simulations. The crystallographic tilt of the free-hanging wings was determined to result from the strain relaxation of the seed stripes along [0001]. The impact of the dimensions of the pendeostructure and of the formation of crystal defects on the expected wing tilt is discussed.