Inversion Domain Boundaries Research Articles

Detection of an unintentional Si doping gradient in site-controlled GaN nanowires grown using a Si3N4 mask by spatially resolved cathodoluminescence and Raman spectroscopy

Highly uniform arrays of site-controlled GaN nanowires are synthesized by selective area growth using a Si3N4 mask and molecular beam epitaxy. Systematic modulation of the emission along the nanowire axis is observed in spectrally resolved cathodoluminescence linescans. We show that this intensity change is an indicator of unintentional Si incorporation during growth resulting from the interaction between the impinging Ga atoms and the mask material. The gradual reduction of the cathodoluminescence intensity along the nanowire highlights the important role of the growth geometry within the synthesis reactor, with shadowing from the elongating nanowires inhibiting the reaction with the mask. This gradient in Si doping is confirmed by the quenching of the longitudinal optical phonon line measured in Raman spectra along the nanowire axis. The corresponding carrier density is derived from the frequency of the coupled phonon–plasmon mode. The spectroscopic identification of inversion domain boundaries in the majority of the nanowires is also attributed to the Si incorporation. From temperature dependent cathodoluminescence experiments, we derive the activation energy for excitons bound to these defects.

Thermal Transport at the AlN-SiC Interface and Grain Boundary of AlN.

AlN is deposited on silicon carbide (SiC) for high-power electronics; in these devices, AlN acts as both a buffer layer for the growth of the active device and a thermal conductor. However, the mechanism of thermal transport through the AlN-SiC interfaces and through grain boundaries of AlN has not been clearly analyzed, even though AlN forms grain boundaries during the deposition process. The thermal properties of the AlN-SiC interface and the inversion domain boundaries (IDBs) of AlN were examined by a phonon transport model based on a nonequilibrium Green function formalism and first-principles calculations. The interface and grain boundary models were designed, and the thermal resistances (TRs) and origins of TR were examined. The TRs of the AlN-SiC interface and the IDB of AlN are much higher than the TRs of AlN and SiC of relevant thickness. Elemental intermixing and vacancy formation were modeled. The formation of charge-balanced defect of VAl + 3ON is thermodynamically favorable compared to other defects, indicating that ON induces formation of VAl. The charge-balanced defect combining VAl and ON increases the TRs of both AlN-SiC interfaces and AlN grain boundaries because vacancy defects induce larger changes in mass than all other defects, and TRs are proportional to changes in mass. In addition, VAl defects are increased by excess ON, resulting in a continuous increase in TR, and then, the calculated thermal boundary resistance (TBR) of the AlN-SiC interface with increased density of VAl by excess ON reaches the experimental TBR. Therefore, it is expected that the large increase in TR by the formation of VAl + ON would be suppressed by controlling the low O density during synthesis.

Atomic Structure and Electronic Properties of Pristine and O-Doped Basal-Plane Inversion Domain Boundaries in AlN

Atomic Structure and Electronic Properties of Pristine and O-Doped Basal-Plane Inversion Domain Boundaries in AlN

First-Principles Understanding on the Formation of Inversion Domain Boundaries of Wurtzite AlN, AlScN, and GaN

First-Principles Understanding on the Formation of Inversion Domain Boundaries of Wurtzite AlN, AlScN, and GaN

Unveiling interface structure and polarity of wurtzite ZnO film epitaxially grown on a-plane sapphire substrate

Atomic-scale structure properties of the epitaxial growth of the wurtzite ZnO film prepared on an a-plane sapphire (α-Al2O3) substrate have been investigated by using aberration-corrected transmission electron microscopy. The crystallographic orientation relationship of (0001)[1¯1¯20]ZnO//(112¯0)[0001]α-Al2O3 has been determined between the ZnO film and the α-Al2O3 substrate. Two types of oxygen-terminated a-plane α-Al2O3 substrate surfaces have been characterized, which leads to the formation of different heterointerface structures and ZnO domains with opposite lattice polarity. The coalescence of opposite polarity domains results in the appearance of inversion domain boundaries (IDBs) on prismatic planes, and kinks occur on basal planes during the propagation of IDBs within the film. Additionally, the structure of stacking mismatch boundaries in the film with threefold coordinated Zn and O atoms has been resolved. We believe that these findings can be helpful to advance the understanding of the complex propagation of planar defects (e.g., IDBs and stacking faults) in wurtzite films and the interface structure and polarity of wurtzite films on the a-plane sapphire substrate.

Domain boundaries and growth manner of a-plane ZnO film on symmetric (0 0 1)LaAlO3 substrate

Non-polar a-plane ZnO film grown on (001)LaAlO3 (LAO) substrate has been systematically characterized by employing transmission electron microscope. On (001)LAO substrate, a-plane ZnO film comprises two main orientation types of domains perpendicular to each other. In one orientation type of a-plane ZnO domains, stacking mismatch boundary (SMB) and inversion domain boundary (IDB) form in the coalescence of two domain islands parallel to each other. SMB is composed of m- and r-planes. IDB extends along m-plane. The in-plane growth of a-plane ZnO domains extends along the direction with ∼45°-rotation to [0001]ZnO. The boundaries of two perpendicular a-plane ZnO domains consist of 45°-boundaries and r-plane twin boundaries. The profile of a-plane ZnO domain appears as L-shape.

In-Grain Ferroelectric Switching in Sub-5 nm Thin Al0.74 Sc0.26 NFilms at 1 V.

Analog switching in ferroelectric devices promises neuromorphic computing with the highest energy efficiency if limited device scalability can be overcome. To contribute to a solution, one reports on the ferroelectric switching characteristics of sub-5 nm thin Al0.74 Sc0.26 Nfilms grown on Pt/Ti/SiO2 /Siand epitaxial Pt/GaN/sapphiretemplates by sputter-deposition. In this context, the study focuses on the following major achievements compared to previously available wurtzite-type ferroelectrics: 1) Record low switching voltages down to 1V are achieved, which is in a range that can be supplied by standard on-chip voltage sources. 2) Compared to the previously investigated deposition of ultrathin Al1-x Scx Nfilms on epitaxial templates, a significantly larger coercive field (Ec ) to breakdown field ratio is observed for Al0.74 Sc0.26 Nfilms grown on silicon substrates, the technologically most relevant substrate-type. 3) The formation of true ferroelectric domains in wurtzite-type materials is for the first time demonstrated on the atomic scale by scanning transmission electron microscopy (STEM) investigations of a sub-5 nm thin partially switched film. The direct observation of inversion domain boundaries (IDB) within single nm-sized grains supports the theory of a gradual domain-wall driven switching process in wurtzite-type ferroelectrics. Ultimately, this should enable the analog switching necessary for mimicking neuromorphic concepts also in highly scaleddevices.

Moiré patterns and inversion boundaries in graphene/hexagonal boron nitride bilayers

In this paper a systematic examination of graphene/hexagonal boron nitride (g/hBN) bilayers is presented, through a recently developed two-dimensional phase field crystal model that incorporates out-of-plane deformations. The system parameters are determined by closely matching the stacking energies and heights of g/hBN bilayers to those obtained from existing quantum-mechanical density functional theory calculations. Out-of-plane deformations are shown to reduce the energies of inversion domain boundaries in hBN, and the coupling between graphene and hBN layers leads to a bilayer defect configuration consisting of an inversion boundary in hBN and a domain wall in graphene. Simulations of twisted bilayers reveal the structure, energy, and elastic properties of the corresponding moir\'e patterns and show a crossover as the misorientation angle between the layers increases from a well-defined hexagonal network of domain boundaries and junctions to smeared-out patterns. The transition occurs when the thickness of domain walls approaches the size of the moir\'e patterns and coincides with the peaks in the average von Mises and volumetric stresses of the bilayer.

In Situ Imaging of an Anisotropic Layer-by-Layer Phase Transition in Few-Layer MoTe2

Understanding the phase transition mechanisms in two-dimensional (2D) materials is a key to precisely tailor their properties at the nanoscale. Molybdenum ditelluride (MoTe2) exhibits multiple phases at room temperature, making it a promising candidate for phase-change applications. Here, we fabricate lateral 2H-Td interfaces with laser irradiation and probe their phase transitions from micro- to atomic scales with in situ heating in the transmission electron microscope (TEM). By encapsulating the MoTe2 with graphene protection layers, we create an in situ reaction cell compatible with atomic resolution imaging. We find that the Td-to-2H phase transition initiates at phase boundaries at low temperatures (200-225 °C) and propagates anisotropically along the b-axis in a layer-by-layer fashion. We also demonstrate a fully reversible 2H-Td-2H phase transition cycle, which generates a coherent 2H lattice containing inversion domain boundaries. Our results provide insights on fabricating 2D heterophase devices with atomically sharp and coherent interfaces.

Optical properties of ZnFe2O4 nanoparticles and Fe-decorated inversion domain boundaries in ZnO.

The maximum efficiency of solar cells utilizing a single layer for photovoltaic conversion is given by the single junction Shockley-Queisser limit. In tandem solar cells, a stack of materials with different band gaps contribute to the conversion, enabling tandem cells to exceed the single junction Shockley-Queisser limit. An intriguing variant of this approach is to embed semiconducting nanoparticles in a transparent conducting oxide (TCO) solar cell front contact. This alternative route would enhance the functionality of the TCO layer, allowing it to participate directly in photovoltaic conversion via photon absorption and charge carrier generation in the nanoparticles. Here, we demonstrate the functionalization of ZnO through incorporation of either ZnFe2O4 spinel nanoparticles (NPs) or inversion domain boundaries (IDBs) decorated by Fe. Diffuse reflectance spectroscopy and electron energy loss spectroscopy show that samples containing spinel particles and samples containing IDBs decorated by Fe both display enhanced absorption in the visible range at around 2.0 and 2.6 eV. This striking functional similarity was attributed to the local structural similarity around Fe-ions in spinel ZnFe2O4 and at Fe-decorated basal IDBs. Hence, functional properties of the ZnFe2O4 arise already for the two-dimensional basal IDBs, from which these planar defects behave like two-dimensional spinel-like inclusions in ZnO. Cathodoluminescence spectra reveal an increased luminescence around the band edge of spinel ZnFe2O4 when measuring on the spinel ZnFe2O4 NPs embedded in ZnO, whereas spectra from Fe-decorated IDBs could be deconvoluted into luminescence contributions from bulk ZnO and bulk ZnFe2O4.

Comparative studies of interatomic potentials for modeling point defects in wurtzite GaN

In this paper, a new version of the Stillinger–Weber (SW) potential for wurtzite GaN is presented, by which we systematically explore the structural and thermodynamical properties of native point defects and their complexes. In parallel, the semi-empirical Modified Embedded-Atom Method (MEAM) potential is selected for comparison. The SW and MEAM potentials are assessed by the reproduction of the fundamental properties of wurtzite GaN and by the ability to describe the inversion domain boundaries and the wurtzite–rocksalt phase transition. Then the structural search of native point defects and their complexes in GaN is implemented using both SW and MEAM potentials with the benchmark of Density Functional Theory (DFT) calculations. Besides vacancies and antisites, four N and five Ga interstitials are confirmed by refining the DFT calculations, among which two N split interstitials N+−N⟨21̄1̄0⟩ and N+−Ga⟨011̄0⟩, and two Ga split interstitials, Ga+−Ga⟨011̄0⟩−g and Ga+−N⟨011̄0⟩, are observed for the first time. The SW potential correctly predicts the octahedral occupation GaOct to be the most stable Ga interstitial, while the MEAM potential predicts the ground state of the N+−N⟨011̄0⟩ split interstitial (N+−N⟨011̄0⟩−g) as the most stable N interstitial. However, neither of the two potentials could simultaneously generate the most stable configurations of N and Ga interstitials. The investigations of point defect complexes reveal that N octahedral Frenkel [FrenkelOct(N)] and paired antisite (NGaGaN) defects are unstable and get converted into VN⊕N+−N⟨011̄0⟩−g configurations with different separations between VN and N+−N⟨011̄0⟩−g point defects based on the DFT calculations. The formation energies calculated by the DFT and SW potential demonstrate that Schottky, Ga octahedral Frenkel [FrenkelOct(Ga)], and VN⊕N+−N⟨011̄0⟩−g point defect complexes are energetically feasible and that they should not dissociate into two isolated point defects. In contrast, the MEAM potential predicts the dissociation to be exothermic for Schottky and VN⊕N+−N⟨011̄0⟩−g. Overall, the structural features concerned with N–N or Ga–Ga bonds relaxed by the SW potential are more consistent with DFT calculations than the MEAM counterpart.

Enhanced high-temperature volume resistivity of AlN by doping

The addition of various oxide dopants for AlN were investigated aiming to increase the AlN volume resistivity at high temperature. It was confirmed that the volume resistivity of 1 mol% MgO doped AlN shows four orders of magnitude higher than that of non-doped AlN. To further clarify the effect of MgO doping, we sintered various kinds of AlN with different MgO concentration and grain sizes, and the microstructures were investigated by transmission electron microscopy. The results suggest the inversion domain boundary appeared in the MgO doped AlN might be the possible reason for the increase of volume resistivity.

Direct Determination of Band Gap of Defects in a Wide Band Gap Semiconductor.

Crystal defects play an important role in the degradation and failure of semiconductor materials and devices. Direct determination of band gap of defects is a critical step for clarifying how the defects affect the physical properties of semiconductors. Here, high-quality aluminum nitride (AlN) thin films were grown epitaxially on single-crystal Al2O3 substrates via pulsed laser deposition. The atomic structure and band gap of three types of inversion domain boundaries (IDBs) in AlN were determined using aberration-corrected transmission electron microscopy and atomic-resolution valence electron energy-loss spectroscopy. It was found that the band gap of all of the IDBs reduces evidently compared to that of the bulk AlN. The maximum band gap reduction of the IDBs is 1.0 eV. First-principles calculations revealed that the band gap reduction of the IDBs is mainly due to the rise of pz orbital at the valence band maximum, which originates from the elongated Al-N bonds along the [0001] direction at the IDBs. The successful band gap determination of defects paves an avenue for quantitatively evaluating the effect of defects on the performance of semiconductor materials and devices.

Impact of Inversion Domain Boundaries on the Electronic Properties of 3C‐SiC

Inversion domain boundaries are extended defects affecting cubic silicon carbide (3C‐SiC) layers grown on the on‐axis (001) Si wafers. Their impact on the device performance is still debated and a clear connection to their electronic properties at the atomistic scale is still missing. By comparing the first‐principles atomistic models and scanning transmission electron microscopy imaging, the most relevant structures of inversion domain boundaries laying both in the {110} and {111} planes are identified. Their calculated electronic structures shed light on the relevance that these extended defects may have in the degradation of the performances of the 3C‐SiC devices.

An ab initio-based approach for the formation of pyramidal inversion domain boundaries in highly Mg-doped GaN

The formation of pyramidal inversion domain (PID) boundaries in highly Mg-doped GaN is investigated on the basis of ab initio calculations. Results for the absolute interface energies of PID boundaries reveal that their upper (0001) and side {113} facets hosting Mg atoms are stable owing to the electron counting rule. Furthermore, with these absolute interface energies, the formation energies for various PID boundaries of varying sizes and concentrations were estimated. Energy comparisons with those of substitutional Mg demonstrate that the formation of large PID boundaries is preferred when numerous Mg atoms are incorporated in the GaN matrix. The results suggest that the formation of PID boundaries is crucial for Mg doping and the resulting hole concentrations.

Quantitative microstructural and spectroscopic investigation of inversion domain boundaries in sintered zinc oxide ceramics doped with iron oxide

Abstract It is known that sintering of powders of zinc oxide (ZnO) with small additions of iron oxide results in a ceramic with grains exhibiting a characteristic inversion domain micro-structure with planar inversion domain boundaries (IDBs) on two different habit planes. This study concentrates on a quantitative analysis, by a combination of different transmission electron microscopy methods, of those IDBs that are parallel to {0001} basal planes of the wurtzite structure of ZnO. Electron diffraction and dark-field imaging prove the nature of the inversion. High-resolution annular dark field scanning transmission electron microscopy allows measurement of the rigid body displacements across these IDBs and of the local lattice contraction related to the octahedral interstices that form the boundaries. Energy-selected imaging, electron energy-loss spectroscopy and energy-dispersive X-ray spectroscopy have been combined to determine the chemical composition of the IDBs quantitatively. It is thus shown unambiguously that every such fault consists of precisely one basal plane of octahedral interstices that are completely occupied by Fe3+ ions and that these FeO6 octahedra are themselves contracted along the <0001> direction. A local charge balance model explains the observations.

On the polarity determination and polarity inversion in nitrogen-polar group III-nitride layers grown on SiC

We investigate the interfaces and polarity domains at the atomic scale in epitaxial AlN and GaN/AlN grown by hot-wall metal organic chemical vapor epitaxy on the carbon face of SiC. X-ray diffraction, potassium hydroxide (KOH) wet chemical etching, and scanning transmission electron microscopy combined provide an in-depth understanding of polarity evolution with the film thickness, which is crucial to optimize growth. The AlN grown in a 3D mode is found to exhibit N-polar pyramid-type structures at the AlN–SiC interface. However, a mixed N-polar and Al-polar region with Al-polarity domination along with inverted pyramid-type structures evolve with increasing film thickness. We identify inclined inversion domain boundaries and propose that incorporation of oxygen on the ⟨40–41⟩ facets of the N-polar pyramids causes the polarity inversion. We find that mixed-polar AlN is common and easily etched and remains undetected by solely relying on KOH etching. Atomic scale electron microscopy is, therefore, needed to accurately determine the polarity. The polarity of GaN grown on mixed-polar AlN is further shown to undergo complex evolution with the film thickness, which is discussed in the light of growth mechanisms and polarity determination methods.

Repeatable mechanical energy absorption of ZnO nanopillars

We show that repeatable energy absorption can be obtained via the reversible wurtzite-to-hexagonal phase transformation of ZnO nanopillars at room temperature. The effect is demonstrated using molecular dynamics simulations and available experimental data. With uniaxial compressive strains up to 22.1% along the [0001] orientation, a ZnO nanopillar with a lateral dimension of 5.5 nm can produce average specific energy absorption on the order of 26.7 J g−1 under quasistatic cyclic loading and 11.1 J g−1 under rapid loading. The theoretical maximum of the specific energy absorption is 41.0 J g−1 which can be approached at nanopillars with lateral sizes above 55 nm. These values are comparable to that of widely used aluminum foams. The effects of inversion domain boundaries and sample size on the repeatable energy absorbing capacity are discussed. The findings open an avenue for ZnO nanostructures in mechanical energy absorption and dissipation applications.

Low-defect-density ZnO homoepitaxial films grown by low-temperature ALD

We report atomic layer deposition (ALD) of ZnO thin films on O-polar surface crystalline ZnO substrates at the relatively low temperatures of 120, 150, and 200 °C. The as-grown ZnO films are studied with aberration-corrected transmission electron microscopy and diffraction contrast, photoluminescence (PL), and surface photovoltage (SPV) spectroscopy. We find that the homoepitaxial films have a monocrystalline structure with the density of basal stacking faults comparable to that of the substrate (∼1011 cm−2) and that the stacking faults can induce high lattice strain due to their interaction with the inversion domain boundaries. The narrow excitonic PL linewidth (2 meV at 8 K) and the sharp SPV bandgap transition confirm the high quality of the ZnO films. Despite similarities in the film properties, the growth temperature has an effect on the density and spatial distribution of intrinsic defects. Our results demonstrate a considerable potential of ALD ZnO homoepitaxy for fabricating high-quality ZnO nanostructures and attaining viable p-type ZnO.

Mobility of the {0110} inversion domain boundary in ZnO nanopillars

The apparent activation energy of a nanostructure is difficult to directly measure experimentally. In this letter, we present a computational method for estimating the apparent activation energy of a range of semiconductor nanostructures. This method allows the activation energy to be obtained from experimentally measured average activation time or propagation speed at various temperatures of the phase boundary associated with the transformation. The approach entails analyzing the mobility of the transformation in question using a model based on the Arrhenius relation. The specific analysis carried out uses the {0110} inversion domain boundary in wurtzite ZnO nanopillars as example. Molecular dynamics simulations are conducted over the temperature range of 300–900 K of the corresponding available experimental data. The approach and analysis offer a means for experimentally establishing the apparent activation energy of the {0110} inversion domain boundary in a variety of wurtzite-structured II-VI, III-V and IV-IV binary compounds.