Inversion Domain Boundaries Research Articles

Inversion domain boundaries in GaN studied by X‐ray microprobe

AbstractIn this study, we report on the application of synchrotron spectro‐microscopic techniques to the examination of inversion domain boundaries formed intentionally in a GaN‐based lateral polarity heterostructure. Using X‐ray sub‐microbeams, no evidence of field‐driven electrodiffusion effects has been observed on spatially separated inversion domain boundaries. In addition, XANES data around the Ga K‐edge strongly supported hexagonal Ga site configurations, suggesting high local order reconstruction. Based on inner‐shell excited luminescence on the micrometer scale, the uniform spectral distribution of the radiative centers was discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Displacement field measurement of metal sub-lattice in inversion domains of indium-doped zinc oxide

The displacement field of the metal sub-lattice in homologous compounds In 2O 3(ZnO) m is investigated by means of aberration-corrected high-resolution transmission electron microscopy. The elastic state in these compounds is characterised by plane strain where little column bending occurs due to surface relaxation. The compound contains inversion domain boundaries (IDBs) on basal and pyramidal planes of ZnO where the displacements are concentrated. The structure is imaged in <1 1¯ 0 0> of ZnO with negative spherical aberration and using bright atom contrast condition. Local atomic shifts are measured with precision of ca. 5 picometres in real space with a peak finding algorithm. The strain tensor and lattice rotations are calculated from displacements displaying dilatation, shear and rotation at pyramidal IDBs which reveal a mirror plane through the centre of each inversion domain.

Characterization of Crystalline Zinc Oxide in the Form of Hexagonal Bipods

We present a solution-phase preparation method for the synthesis of ZnO in an open reactor (with constant stirring) or in a closed reactor (autoclave). The method is based on a combination of the polyol method and the homogeneous precipitation of zinc nitrate with urea. ZnO particles with a rod-like shape and with a regular bipod structure were prepared in an open reactor, while needle-like shapes were prepared in the autoclave. A detailed characterization of the planar base-plane and prismatic defects obtained in the hexagonal ZnO bipod particles with electron microscopy revealed that in the middle of the bipod the planar defects contain a small amount of silicon. A rough estimation indicated that there was a single atomic layer of silicon at the defect. From convergent-beam electron-diffraction experiments, we found that these planar defects were inversion domain boundaries (IDBs) with a head-to-head orientation of the polar axis. In the needle-like particles, besides IDBs on the basal planes, prismatic IDBs with a small amount of silicon were also found. The formation mechanism for the prismatic IDB was thought to be the overgrowth of one part of the bipod over another. In this case, not just the head-to-head arrangement of the polar axis, but also the tail-to-tail arrangement, must be able to occur.

Surface transformation and inversion domain boundaries in gallium nitride nanorods

Phase transformation and subdomain structure in [0001]-oriented gallium nitride (GaN) nanorods of different sizes are studied using molecular dynamics simulations. The analysis concerns the structure of GaN nanorods at 300 K without external loading. Calculations show that a transformation from wurtzite to a tetragonal structure occurs along {011¯0} lateral surfaces, leading to the formation of a six-sided columnar inversion domain boundary (IDB) in the [0001] direction of the nanorods. This structural configuration is similar to the IDB structure observed experimentally in GaN epitaxial layers. The transformation is significantly dependent on the size of the nanorods.

Effects of defects on the morphologies of GaN nanorods grown on Si (111) substrates

Scanning electron microscopy and transmission electron microscopy images and selected area electron diffraction pattern showed that the one-dimensional GaN nanorods with [0001]-oriented single-crystalline wurzite structures were formed on Si (111) substrates by using hydride vapor-phase epitaxy without a catalyst. Although some stacking faults and inversion domain boundaries existed in the GaN nanorods, few other defects such as threading dislocations were observed. The formation of the facet plane in the N-polar region of the GaN nanorod containing an inversion domain boundary originated from the slow growth rate, followed by the lateral adatom diffusion from the Ga-polar region to reduce the length difference.

Microstructure of AlN with two-domain structure on (001) diamond substrate grown by metal-organic vapor phase epitaxy

We investigate the microstructures of domain boundaries in an AlN layer grown on a (001) diamond substrate by metal-organic vapor phase epitaxy. The AlN layer has a two-domain structure with crystal orientation along either <112̄0> AlN [named by AlN I domain] or <101̄0> AlN [named by AlN II domain] parallel to [110] direction of diamond. The AlN I and AlN II domains are not atomically bonded at two-domain boundary from initial to final step of growth, while an edge-type dislocation is generated at single-domain boundary (SDB). In addition, an inversion AlN I domain [named by AlN I⁎] is randomly-ordered at the initial stage of the coalescence between the AlN I domains. The AlN I⁎ is easily terminated with increasing the thickness of AlN I domain. The inversion domain boundary changes to the edge-type dislocation at the SDB with further growth, which reduces the defect density in the AlN I domains.

Magnesium adsorption and incorporation in InN (0 0 0 1) and [formula omitted] surfaces: A first-principles study

We present first-principles results obtained within the framework of density functional theory referring to Mg adsorption and its incorporation in two distinct InN surfaces ((0 0 0 1), ( 0 0 0 1 ¯ ) ) under various adsorption coverage conditions. In all deposition cases we deducted the bonding characteristics and the electronic character of the structures. It is found that in In rich conditions N-polarity facilitates the diffusion of In and Mg adatoms while In-polarity facilitated the Mg-induced weakening of adatom (In, Mg)–surface interactions for small doses of Mg. This implies that the Mg dopant decreases In and Mg diffusion barrier leading to smoother surfaces. Interestingly, we found significant differences of Mg incorporation in In- and N-polar surfaces: Mg incorporation is easier in bare N-polar than in the In-polar surface, except when an In contracted bilayer is formed on top of the InN. In the case of In-polarity, Mg impurities start to penetrate in the subsurface region as the Mg coverage increases. In the contracted In bilayer case, Mg incorporation in the InN is significantly enhanced. Furthermore, we found that the presence of Mg close to the ( 0 0 0 1 ¯ ) InN surface does not alter significantly the local structure, contrary to the In-polarity case in which a flattening of the bilayers is observed at the highest Mg coverage that may lead to the formation of basal inversion domain boundaries.

Structural and optical investigations of periodically polarity inverted ZnO heterostructures on (0001) Al2O3

We report the structural and optical properties of one-dimensional grating of ZnO consisting of periodically polarity inverted structures on (0001) Al2O3 substrates. The inversion domain boundaries (IDBs) between the Zn- and the O-polar ZnO regions were clearly observed by transmission electronic microscopy. The investigation of spatially resolved local photoluminescence (PL) revealed strong excitonic emission at the interfacial region including the IDBs. The possible mechanism of strong PL has been discussed by the consideration of atomic configuration and carrier collection including its lifetime and diffusion process in Zn- and O-polar regions. Therefore the authors conclude that the IDBs can be active for the strong emission not a nonradiative center.

Spontaneous transition in preferred orientation of GaN domains grown on r-plane sapphire substrate from [112¯] to [0001

GaN films were grown at 550 °C and subsequently at 1040 °C on sapphire (101¯2) (r-plane) substrates by using hydride vapor phase epitaxy with different layer thicknesses. As the thickness of a low-temperature-grown GaN layer was increased, a preferred orientation of GaN grown at 1040 °C changed from [112¯0] to [0001]. A detailed atomistic model reveals that this spontaneous transition in preferred orientation is due to the formation of inversion domain boundaries and stacking faults. This result has a significant implication that tailoring film characteristics in terms of controllability of preferred orientation may be possible independent of substrate orientation.

A TEM Study of Inversion Domain Boundaries Annihilation Mechanism in 3C-SiC during Growth

The aim of the present work is to study the evolution and the annihilation of inversion domain boundaries in 3C-SiC during growth. For this investigation conventional and high resolution transmission electron microscopy were employed. It is shown that the physical mechanism which results in the annihilation of inversion domain boundaries in 3C-SiC starting from the 3C-SiC/Si interface is the change of the crystallographic planes in which inversion domain boundaries propagate into the {111} ones. In all cases modeling and simulation analysis by EMS software [1] are in agreement with the experimental results.

Effect of AlN interlayers in the structure of GaN-on-Si grown by plasma-assisted MBE

The insertion of an AlN interlayer for tensile strain relief in GaN thin films grown on Si(1 1 1) on-axis and vicinal substrates by nitrogen rf plasma source molecular beam epitaxy has been investigated. The 15 nm AlN interlayer was inserted between a bottom 0.5 μm GaN layer and the top 1.0 μm GaN layer. The interlayer was effective in reducing the tensile stress to the level required for complete avoidance of microcracks, which were present in high densities in the case of GaN-on-Si heterostructures grown without an AlN interlayer. The strain in all the layers of the heterostructure was analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Reciprocal space mapping in XRD indicated that the 15 nm AlN interlayer was on average coherently strained with the GaN. However, TEM observations showed that the interlayer was partially relaxed in local regions. The AlN interlayer was also observed to interfere with the GaN growth process. In particular, above morphological features such as v-shaped surface depressions, GaN was overgrown with a high density of threading dislocations and inversion domain boundaries. A synergistic relaxation mechanism is proposed for the AlN interlayer leading to an elastically strained interlayer interconnected by plastically relaxed patches.

Rules of Structure Formation for the HomologousInMO3(ZnO)nCompounds

The formation mechanisms that lead to the layered M-modulated InMO3(ZnO){n} structures (M=In, Ga, and Al; n=integer) are revealed and confirmed by first-principles calculations based on density functional theory. We show that all ground state structures of InMO3(ZnO){n} satisfy the octahedron rule for the InO2 layers; they contain an inversion domain boundary located at the M and Zn fivefold trigonal bipyramid sites and maximize the hexagonality in the (MZn{n})O{n+1} layers. They also obey the electronic octet rule. This understanding provides a solid basis for studying and understanding the physical properties of this group of homologous materials.

Direct Observation of Inversion Domain Boundaries of GaN on c‐Sapphire at Sub‐ångstrom Resolution

Inversion domain boundaries (IDBs) of GaN are studied by a high-resolution technique. The IDB separates adjacent domains of opposite polarity. The image shows a GaN IDB in the [bar;2110] projection. The theoretical IDB structure fits the experimentally obtained structure well. The inset is an image acquired from a very thin region on the right side of the IDB. It can indicate the polarity of GaN directly.

Polarity Analysis of Self-Seeded Aluminum Nitride Crystals Grown by Sublimation

Self-seeded aluminum nitride (AlN) crystals were successfully grown in a tungsten crucible by the sublimation method. The polarities along the growth direction of these AlN samples were characterized by chemical etching, combined with high-resolution transmission electronic microscopy (HRTEM) and Raman spectroscopy. It has been proven by our experimental results that etching in KOH solution and in molten KOH-NaOH can be a reliable method to determine the surfaces with different polarities. Chemically active N-polar (0001) AIN surfaces were removed appreciably, while only etch pits related to threading dislocations were observed on Al-polar surfaces after chemical etching. HRTEM images revealed that there are noticeable inversion domain boundaries in the samples. Raman spectrum measurements showed the vibration mode change before and after etching in the same geometrical arrangement.

Interatomic potential calculations of III(Al, In)–N planar defects with a III‐species environment approach

AbstractIII–N compound semiconductors are nowadays widely used in electronic device technology. Due to the complexity of their structures planar and linear defects may have various atomic configurations. Since in the wurtzite structure of AlN and InN the second‐neighbor distance is very close to the stable “metallic” Al–Al and In–In distances respectively, a III‐species environment approach based on a Tersoff empirical bond order interatomic potential is developed in which the cut‐off distance for Al–Al and In–In interactions is tuned. In particular, the work is focused on two issues: the development of an approach for the calculation of defected structures in III‐nitrides and the application of this method on a series of planar defects in wurtzite structure. Various structural and energy‐related conclusions are drawn that are attributed to the complexity of the III–III metal type and N–N interactions in connection with the difference of the lattice parameters and the elastic constants. Molecular dynamic simulations are led to the conclusion that structural transformations may also occur. The Austerman–Gehman and Holt models for the inversion domain boundary (IDB) on the (10$ \bar 1 $0) plane are higher in energy than the IDB* model of Northrup, Neugebauer, and Romano. The model of Blank et al. for the translation domain boundary (TDB) on the {1$ \bar 2 $10} plane is unstable with respect to Drum's model. The Austerman model for the IDB on the {1$ \bar 2 $10} plane is unstable with respect to the IDB* model appropriate for this plane. The Austerman {10$ \bar 1 $0} IDB model is recognized as a strong candidate, among the IDB atomic configurations. Moreover, models for IDBs on {10$ \bar 1 $0} planes in which the boundary plane intersects two bonds (type‐2 models) are less stable than models in which the boundary plane intersects one bond (type‐1 models), in all cases considered. It is confirmed that the III‐species environment approach describes the “wrong”‐bonded defect local configuration structures more realistically with respect to the standard approach. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Characterization of GaN‐based lateral polarity heterostructures

AbstractGaN‐based lateral polarity heterostructures (LPH) were grown on sapphire substrates using molecular beam epitaxy. The LPH samples consist of periodical N‐face (000‐1) (grown directly on Al2O3) and Ga‐face (0001) (grown on a AlN bufferlayer) GaN stripes. The GaN‐based LPH have been characterized using surface sensitive techniques. The GaN‐based LPH consist of smooth Ga‐face (0.4 nm RMS) and N‐face regions of higher roughness (7 nm RMS) with small (&lt;70 nm as measured by atomic force microscopy) and well‐defined inversion domain boundary regions (IDBR). (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structural characterization of non-polar (1 1 2 0) and semi-polar (1 1 2 6) GaN films grown on r-plane sapphire

Thick GaN films, with (1 1 2 0) or (1 1 2 6) planes parallel to the r-plane of sapphire, were grown by molecular beam epitaxy using AlN or GaN buffer layers. Characterization by transmission electron microscopy revealed a high density of basal-plane stacking faults (BSFs) in the (1 1 2 0) non-polar GaN ( a-GaN) films. {1 1 2 0} and {1 0 1 0} prismatic-plane and {1 1 0 2} pyramidal-plane stacking faults (SFs) were observed to terminate some BSFs. The SFs on {1 0 1 0} and {1 1 0 2} planes generally formed closed domains. For (1 1 2 6) semi-polar GaN ( s-GaN) films, most threading dislocations were located at small-angle grain boundaries. Many BSFs were observed close to the AlN/GaN interface but, in comparison with a-GaN, the s-GaN films had much lower BSF density at the top surface. Inversion domain boundaries (IDBs) on {1 0 1 0} planes were observed to form closed domains. GaN/AlGaN multiple quantum wells (MQWs) grown on s-GaN or a-GaN followed the morphology of the GaN surface. Some IDBs in the s-GaN propagated through the GaN/AlGaN MQWs to the top surface.

Effects of Stress-relieving AlN Interlayers in GaN-on-Si Grown by Plasma-assisted Molecular Beam Epitaxy

ABSTRACTThe insertion of an AlN interlayer for tensile strain relief in GaN thin films grown on Si (111) on-axis and vicinal substrates by nitrogen rf plasma source molecular beam epitaxy has been investigated. The 15 nm AlN interlayer was inserted between the bottom 0.5 micron GaN layer and the top 1.0 micron GaN layer. The interlayer was very effective to reduce the tensile stress in the overall 1.5 micron GaN/Si film to the level required for complete avoidance of microcracks, which were present in high densities in GaN/Si heterostructures grown without an AlN interlayer. The strain of the AlN interlayer, as well as the strain in all the layers of the entire GaN/Si heterostructure was analyzed by x-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Reciprocal space map in XRD indicated that the 15 nm AlN interlayer was coherently strained with the GaN films. However TEM observations revealed that the AlN interlayer was partially relaxed in local regions. The AlN interlayer was also observed to interfere with the GaN growth process. In particular, above morphological features such as V-defects, GaN was overgrown with a large density of threading dislocations and inversion domain boundaries.

ZnO with Additions of Fe2O3: Microstructure, Defects, and Fe Solubility

Zinc oxide (ZnO) crystals with additions of iron (III) oxide exhibit a characteristic inversion domain microstructure with domain boundaries on two different habit planes: parallel to {0001} basal planes and parallel to {2115} pyramidal planes of ZnO. The structural inversion of the domains is proved by electron diffraction experiments. In the present transmission electron microscopy study, emphasis is placed on the early stages of domain formation in sintered polycrystalline material and in diffusion couples with single crystals of ZnO. For solute iron content &gt;0.5 at.% of the cations, defects nucleate at the surface and in the interior of ZnO grains at &gt;900°C. These primary defects propagate along the basal planes of ZnO and gradually widen in the positive c‐axis direction of the ZnO host crystal. The widening along c is promoted by a second defect on {211l} planes that moves away from the basal plane defect. The c‐axis orientation in the ZnO region swept by the second defect is inverted, finally resulting in the inversion domain microstructure. A low iron content ≈0.1 at.% was measured in the inverted domains. Energy‐filtered imaging and quantitative electron energy‐loss spectroscopy show that the inversion domain boundaries (IDBs) parallel to the basal planes contain a full close‐packed monolayer of iron whereas the pyramidal IDBs are occupied by iron with ≈2/3 of the content of the basal IDB. Based on experimental observations and arguments of structural chemistry, a mechanism is proposed explaining the nucleation and oriented growth of the inversion domains that are finally induced and driven by the trivalent iron ions at octahedral sites in the primary defects.

Defects in interfacial layers and their role in the growth of ZnO nanorods by metallorganic chemical vapor deposition

We report the evolution of ZnO nanorods by metalorganic chemical vapor deposition on sapphire substrates and an investigation of their microstructure. Well-aligned ZnO nanorods with a high aspect ratio were grown on an interfacial layer with several types of defects at a lower reactor pressure. Planar defects such as stacking mismatch boundaries and inversion domain boundaries were formed in the interfacial layer during the coalescence of the islands, and finally constituted the side facets of the nanorods. Based on the microstructural changes and origin of the defects in the interfacial layers, we propose a model to explain the growth evolution of ZnO nanorods on sapphire substrates.