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III-nitrides have revolutionized lighting technology and power electronics. Expanding the nitride semiconductor family to include heterovalent ternary nitrides opens up new and exciting opportunities for device design that may help overcome some of the limitations of the binary nitrides. However, the more complex cation sublattice also gives rise to new interactions with both native point defects and defect complexes that can introduce disorder on the cation sublattice. Here, depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy measurements of defect energy levels in ZnGeN2 combined with transmission electron microscopy and x-ray diffraction reveal optical signatures of mid-gap states that can be associated with cation sublattice disorder. The energies of these characteristic optical signatures in ZnGeN2 thin films grown by metal–organic chemical vapor deposition are in good agreement with multiple, closely spaced band-like defect levels predicted by density functional theory. We correlated spatially resolved optical and atomic composition measurements using spatially resolved x-ray photoelectron spectroscopy with systematically varied growth conditions on the same ZnGeN2 films. The resultant elemental maps vs defect spectral energies and intensities suggest that cation antisite complexes (ZnGe–GeZn) form preferentially vs isolated native point defects and introduce a mid-gap band of defect levels that dominate electron–hole pair recombination. Complexing of ZnGe and GeZn antisites manifests as disorder in the cation sub-lattice and leads to the formation of wurtzitic ZnGeN2 as indicated by transmission electron microscopy diffraction patterns and x-ray diffraction reciprocal space maps. These findings emphasize the importance of growth and processing conditions to control cation place exchange. [ABSTRACT FROM AUTHOR]

Intrinsic point defects are commonly present in and can strongly affect the electronic properties of complex oxides and their interfaces. The near- and subsurface characterization techniques, depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy, can measure the density distributions, energy levels, and optical transitions of intrinsic point defects in complex oxides on a near-nanometer scale. These measurements on SrTiO3, BaTiO3, and related materials reveal the sensitivity of intrinsic point defects to growth temperature, mechanical strain, crystal orientation, and chemical interactions. Spatial redistribution of these defects can vary significantly near surfaces and interfaces and can have strong electronic effects. The combination of these deep level spectroscopies along with other advanced characterization techniques provides an avenue to further expand the understanding and control of complex oxide defects in general. [ABSTRACT FROM AUTHOR]

The gallium vacancy is one of the dominant native point defects in β-Ga2O3, one that, together with its complexes, can have a major effect on free carrier densities and transport in this wide bandgap semiconductor. We used a combination of depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy to identify the optical and energy-level properties of these defects as well as how their defect densities and spatial distributions vary with neutron irradiation and temperature-dependent-forming gas anneals. These studies reveal optical signatures that align closely with theoretical energy-level predictions. Likewise, our optical techniques reveal variations in these defect densities that are consistent with hydrogen passivation of gallium vacancies as a function of temperature and depth from the free Ga2O3 surface. These techniques can help guide the understanding and control of dominant native point defects in Ga2O3. [ABSTRACT FROM AUTHOR]

We have studied the optical band and defect transitions of ScN, a group IIIB transition metal nitride semiconductor with electronic and optoelectronic applications. Recent works have focused on the degenerate nature of ScN by substitutional impurities ON and FN, which shift the direct (X–X) gap transition to higher energies via the Burstein–Moss effect. We used cathodoluminescence spectroscopy (CLS) to observe optical signatures of both the midgap VN precursor to ON doping as well as above the direct (X–X) bandgap corresponding to band-to-band transitions from four separate conduction bands near the Γ point with the valence band minimum, in agreement with the calculated band structure diagrams. Thin film ScN grown by reactive magnetron sputtering displays mild degenerate doping by substitutional oxygen as indicated by elevated (X–X) transition energies and the presence of Sc–O bonding determined via x-ray photoelectron spectroscopy (XPS), while ScN grown by physical vapor transport exhibited the intrinsic, non-degenerate (X–X) bandgap predicted by theory. CLS reveals a sharp, sub-bandgap emission at 1.26 eV for sputter grown ScN on GaN, which we attribute to nitrogen vacancies (VN) based on surface sensitive CLS and XPS chemical trends. This finding is in strong agreement with theoretical calculations for VN predicting the formation of a defect energy level within the gap. [ABSTRACT FROM AUTHOR]

Depth-resolved cathodoluminescence spectroscopy (DRCLS) reveals the evolution of surface and near surface defects at polar surfaces with remote oxygen plasma (ROP) treatment. Furthermore, this evolution exhibits significant differences that depend on surface polarity. ROP decreased the predominant 2.5 eV defect emission related to oxygen vacancies on the O face, while creating a new 2.1 eV defect emission on the Zn face that increases with ROP time. The surface-located 2.1 eV emission correlates with carrier profiles from capacitance-voltage measurements and a shift of the E3 trap to higher binding energy from deep level transient spectroscopy (DLTS). This result suggests that ROP generates Zn vacancies on the Zn face which act as compensating acceptors at the surface and in the near surface region. Secondary ion mass spectrometry (SIMS) shows no polarity dependence due to impurities. We conclude that the near-surface deep level optical emissions and free carrier densities of ZnO depend strongly on the ROP modulation of native defects related to Zn or O vacancies. [ABSTRACT FROM AUTHOR]

We used depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy to measure the effects of near-surface plasma processing and neutron irradiation on native point defects in β-Ga2O3. The near-surface sensitivity and depth resolution of these optical techniques enabled us to identify spectral changes associated with removing or creating these defects, leading to identification of one oxygen vacancy-related and two gallium vacancy-related energy levels in the β-Ga2O3 bandgap. The combined near-surface detection and processing of Ga2O3 suggests an avenue for identifying the physical nature and reducing the density of native point defects in this and other semiconductors. [ABSTRACT FROM AUTHOR]

Abstract: Ohmic and rectifying metal contacts to semiconductor nanowires are integral to electronic device structures and typically require different metals and different process techniques to form. Here we show how a noble metal ion beam of Pt commonly used to pattern conducting contacts in electron microscopes can form both ohmic and Schottky/blocking contacts on ZnO nanowires by controlling native point defects at the intimate metal‐semiconductor interface. Spatially‐resolved cathodoluminescence spectroscopy on a nanoscale both laterally and in depth gauges the nature, density, and spatial distribution of specific native point defects inside the nanowires and at their metal interfaces. Combinations of electron and ion beam deposition, annealing, and sculpting of the same nanowire provide either low contact resistivity ohmic contacts or a high Schottky/blocking barrier with a single metal source. These results highlight the importance of native point defects distributed inside nanowires and their variation near interfaces with sculpting and annealing. [ABSTRACT FROM AUTHOR]

Depth-resolved cathodoluminescence spectroscopy (DRCLS) studies of LNO/LSMO/STO interfaces display an ability to detect optical transitions between orbital-derived energy levels with filled states near the Fermi level of ultrathin complex oxides and to detect changes in the electronic structure at their interfaces on a near-nanometer scale. A differential form of DRCLS (DDRCLS) provides a unique capability to measure electronic features at buried interfaces of ultrathin complex oxide films. DDRCLS measurements demonstrate the abruptness of LNO/LSMO interfaces but atomic layer distortions and altered optical emissions at the LSMO/STO heterojunction. The capability to probe electronic structure at buried complex oxide interfaces with enhanced depth resolution can reveal changes in energy levels within nanometers of interfaces, band alignments across interfaces, and the possible effect of local defects on these energy levels. [ABSTRACT FROM AUTHOR]

Semi-insulating Gallium nitride was irradiated by fast and thermal neutrons with fluences from 1014to 1016 n/cm2. Depth-resolved cathodoluminescence spectroscopy was used to determine defects changes before and after irradiation. The results revealed two kinds of defects affected near-band emission recombination from two opposite directions. One was attributed to irradiation-induced N vacancies that contribute to near-band emission recombination. Another was attributed to irradiation-induced deep level defects that contribute to sub-band gap recombination and thus decrease the near-band emission recombination. [ABSTRACT FROM AUTHOR]

The effect of dimensional confinement on the optical band gap of SrTiO3 is investigated by periodically introducing one extra SrO monolayer every n SrTiO3 layers. The result is the n = 1-5 and 10 members of the Srn+1TinO3n+1 Ruddlesden-Popper homologous series. Spectroscopic ellipsometry, optical transmission, and cathodoluminescence measurements reveal these Srn+1TinO3n+1 phases to have indirect optical band gaps at room temperature with values that decrease monotonically with increasing n. First-principles calculations suggest that as n increases and the TiO6 octahedra become connected for increasing distances along the c-axis, the band edge electronic states become less confined. This is responsible for the decrease in band gaps with increasing n (for finite n) among Srn+1TinO3n+1 phases. [ABSTRACT FROM AUTHOR]

A combination of depth-resolved electronic and structural techniques reveals that native point defects can play a major role in ZnO Schottky barrier formation and charged carrier doping. Previous work ignored these lattice defects at metal--ZnO interfaces due to relatively low point defect densities in the bulk. At higher densities, however, they may account for the wide range of Schottky barrier results in the literature. Similarly, efforts to control doping type and density usually treat native defects as passive, compensating donors or acceptors. Recent advances provide a deeper understanding of the interplay between native point defects and electronic properties at ZnO surfaces, interfaces, and epitaxial films. Key to ZnO Schottky barrier formation is a massive redistribution of native point defects near its surfaces and interfaces. It is now possible to measure the energies, densities, and in many cases the type of point defects below the semiconductor-free surface and its metal interface with nanoscale precision. Depth-resolved cathodoluminescence spectroscopy of deep level emissions calibrated with electrical techniques show that native point defects can (1) increase by orders of magnitude in densities within tens of nanometers of the semiconductor surface, (2) alter free carrier concentrations and band profiles within the surface space charge region, (3) dominate Schottky barrier formation for metal contacts to ZnO, and (4) play an active role in semiconductor doping. The authors address these issues by clearly identifying transition energies of leading native point defects and defect complexes in ZnO and the effects of different annealing methods on their spatial distributions on a nanoscale. These results reveal the interplay between ZnO electronic defects, dopants, polarity, and surface nanostructure, C highlighting new ways to control ZnO Schottky barriers and doping. [ABSTRACT FROM AUTHOR]

Abstract: Defects and intermediate chemical phases at nanoscale heterointerfaces of GaN, AlGaN, and SiC can dominate their macroscopic electronic properties. We have used low energy electron-excited nanoscale luminescence spectroscopy in combination with secondary ion mass spectrometry and internal photoemission spectroscopy to correlate interface physical and electronic properties for a variety of Schottky barrier and heterointerfaces involving these semiconductors. These results demonstrate the key role of initial surface processing and subsequent chemical interaction on the heterointerface electronic states, barriers, and carrier concentrations. [Copyright &y& Elsevier]

Depth-resolved cathodoluminescence spectroscopy (DRCLS), time-resolved surface photovoltage spectroscopy, X-ray photoemission spectroscopy (XPS), and current-voltage measurements together show that fast versus thermal neutrons differ strongly in their electronic and morphological effects on metal-GaN Schottky diodes. Fast and thermal neutrons introduce GaN displacement damage and native point defects, while thermal neutrons also drive metallurgical reactions at metal/GaN interfaces. Defect densities exhibit a threshold neutron fluence below which thermal neutrons preferentially heal versus create new native point defects. Scanning XPS and DRCLS reveal strong fluence- and metal-dependent electronic and chemical changes near the free surface and metal interfaces that impact diode properties. [ABSTRACT FROM AUTHOR]

We used depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy to measure the effects of near-surface plasma processing and neutron irradiation on native point defects in β-Ga2O3. The near-surface sensitivity and depth resolution of these optical techniques enabled us to identify spectral changes associated with removing or creating these defects, leading to identification of one oxygen vacancy-related and two gallium vacancy-related energy levels in the β-Ga2O3 bandgap. The combined near-surface detection and processing of Ga2O3 suggests an avenue for identifying the physical nature and reducing the density of native point defects in this and other semiconductors. [ABSTRACT FROM AUTHOR]

The tantalum oxynitride perovskitesATaO2N (A = Ca,Sr, and Ba) and PrTaON2are promising candidates for thephotocatalytic splitting of water under illumination with visiblelight. A combination of X-ray photoemission spectroscopy (XPS), Kelvinprobe force microscopy (KPFM), UV–vis spectroscopy, and depth-resolvedcathodoluminescence spectroscopy (DRCLS) has been used to determinethe absolute conduction and valence band energy levels of these fourcompounds. All have conduction band edges that lie above the reductionpotential for water and therefore are suitable for the photocatalyticproduction of hydrogen, whereas the valence band edges lie near theoxidation potential of water. The position of the conduction bandedge is closely linked to the Ta–O/N–Ta bond anglesand hence tilting of the octahedra, whereas the position of the valenceband edge is more sensitive to the oxygen-to-nitrogen ratio. [ABSTRACT FROM AUTHOR]