Valence Band Density Of States Research Articles

Entropy-driven nonequilibrium phonon-stimulated electron-phonon coupling in tin dioxide nanorods.

Nonequilibrium (NEQ) phonon fluctuation in a nanosystem has been studied through the statistical assessment of the entropy-production and -consumption events in ultrasmall tin dioxide (SnO_{2}) nanorods. Size- and shape-dependent alteration in free energy leading to modulation of the probability distribution function of the phonon dynamics has been observed from the x-ray diffraction and Raman scattering characterizations. The Gallavotti-Cohen nonequilibrium fluctuation theorem has been utilized to qualitatively describe the aforementioned behaviors under the influence of a global flux. The observation of entropy consumption and thermodynamically favorable entropy-production events indicates the presence of NEQ fluctuations in the phonon modes. The effective energy scale of fluctuation in driven phonon modes, dissipating energy faster than relaxation time, is quantified on the order of nanojoules. From optical absorption and photoluminescence studies, the observation of the electron-phonon coupled state confirms the interaction of the NEQ phonons with electrons. The strength of the coupling has been estimated from the temperature-independent Barry center shift and found to be enhanced to 5.35. Valence band x-ray photoelectron spectroscopy and Fourier transformed infrared spectroscopy analyses reconcile NEQ phonon mediated alteration of the valence band density of states, activation of silent phonon modes, and superior excitonic transitions, suitable for the new generation of ultrafast quantum device applications.

Emerging 2D materials for tunneling field effect transistors

This work focuses on understanding the electronic properties of materials to enhance the performance of Tunnel Field Effect Transistor (TFET) through Density Functional Theory (DFT) simulations. Material selection prefers a p-type material with in-plane high density of state (DOS) (and low out-of-plane effective mass, m*, where defined for many layer systems), and high valence band maxima (VBM) energy stacked with an n-type material with low conduction band minimum (CBM) energy (large electron affinity (EA)) that creates a broken or nearly broken band alignment and has low lattice mismatch. SnSe2 is well-suited for an n-type 2D material due to high EA, while WSe2, Black phosphorous (BP) and SnSe are explored for p-type materials. Bilayers consisting of monolayers of WSe2 and SnSe2 show a staggered but nearly broken band alignment (gap of 24 meV) and a high valence band DOS for WSe2. BP-SnSe2 shows a broken band alignment and benefits from a low lattice mismatch. SnSe-SnSe2 shows the highest chemical stability, an optimal performance in terms of DOS of SnSe, tunability with an external field, and high VBM that also leads to a broken band alignment.

First Principle Calculation of Accurate Electronic and Related Properties of Zinc Blende Indium Arsenide (zb-InAs).

We carried out a density functional theory (DFT) study of the electronic and related properties of zinc blende indium arsenide (zb-InAs). These related properties include the total and partial densities of states and electron and hole effective masses. We utilized the local density approximation (LDA) potential of Ceperley and Alder. Instead of the conventional practice of performing self-consistent calculations with a single basis set, albeit judiciously selected, we do several self-consistent calculations with successively augmented basis sets to search for and reach the ground state of the material. As such, our calculations strictly adhere to the conditions of validity of DFT and the results are fully supported by the theory, which explains the agreement between our findings and corresponding, experimental results. Indeed, unlike some 21 previous ab initio DFT calculations that reported zb-InAs band gaps that are negative or zero, we found the room temperature measured value of 0.360 eV. It is a clear achievement to reproduce not only the locations of the peaks in the valence band density of states, but also the measured values of the electron and hole effective masses. This agreement with experimental results underscores not only the correct description of the band gap, but also of the overall structure of the bands, including their curvatures in the vicinities of the conduction band minimum (CBM) and of the valence band maximum (VBM).

Tuning the electronic structure of the trichloride honeycomb lattice by transition metal substitution

Transition metal trichlorides show peculiar and versatile magnetic properties. Whereas ${\mathrm{CrCl}}_{3}$ is a layered antiferromagnet with potential applications as an ultrathin two-dimensional magnet, $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ may host a spin-liquid state driven by Kitaev interactions. The interest to control their material properties by chemical modifications is immense, both from an application related and from a fundamental point of view. Here, by studying ${\mathrm{CrCl}}_{3}$, ${\mathrm{Cr}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_{3}$, and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ by photoemission and electron energy-loss spectroscopy, we find that transition metal substitution changes the optical properties of the host without compromising its underlying electronic structure. It does so by a Cr--Ru related charge transfer process across the Mott gap effectively opening up a new absorption channel below the principal gap edge of ${\mathrm{CrCl}}_{3}$. The Cr and Ru valencies as well as the respective valence band density of states remain stable for the mixed ${\mathrm{Cr}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_{3}$ compound. Our study underlines the potential of transition metal substitution as a means of material engineering of trichlorides.

Strategy to improve the efficiency of tin selenide based solar cell: A path from 1.02 to 27.72%

Solar energy is one of the most reliable, clean, and cheap renewable energy sources. One significant obstacle to harnessing this energy is cheap and efficient material to convert solar energy into electrical. Many materials are being searched like CIGS, CZTS, etc. Recently, there has been a focus on Sb2Se3 and SnSe types of material because they are made of few elements, have higher absorption coefficients, and are made from earth-abundant materials. Taking one experimental report as a base and using the SCAPS-1D simulation software, we have tried to explore the factors affecting the performance of tin selenide-based solar cells. The device structure was glass/Mo/SnSe/CdS/i-ZnO/ITO. An SnSe film with thickness 1.3 μm showed the efficiency of 1.02%. By adjusting the parameters the efficiency of the device was optimized. SnSe solar cell is sensitive to the shallow acceptor density, conduction and valence band density of states, and the radiative recombination coefficient (RRC). For a defect-free SnSe absorber (with RRC ≤ 1E−6 cm−3, VB-DOS 1.8E16 and CB-DOS 2.2E15) and back contact with WF = 5.2 eV (i.e. Ni) 27.72% efficiency can be achieved. Also, the thickness of the CdS layer is optimized to 20 nm concerning environmental issues. Results of this simulation can pave the strategy to improve the SnSe as an efficient solar cell absorber material.

PbBi3O4X3 (X = Cl, Br) with Single/Double Halogen Layers as a Photocatalyst for Visible-Light-Driven Water Splitting: Impact of a Halogen Layer on the Band Structure and Stability

Layered oxyhalides with a Sillén(−Aurivillius) structure have attracted considerable attention as promising photocatalysts for efficient visible-light-induced water splitting due to the elevation of the O-2p valence band maximum (VBM) derived from the strong s–p interaction between Pb/Bi-6s and O-2p electrons and the electrostatic destabilization of oxygen anions in the fluorite layers. However, the role of the halogen layer, consisting of single or double layers, was not known. In this study, we examined a Sillén-type layered oxyhalide PbBi3O4X3 (X = Cl, Br) with alternating stacks of single/double halogen layers and skeletal Bi-based oxide layers. Density functional theory (DFT) calculations of PbBi3O4X3 revealed that the two inequivalent halogen layers contribute differently to the valence band density of states; the halogen p orbitals in double layers are higher in energy and responsible for the VBM, which is understood in terms of Madelung site potentials, where the electrostatic repulsion between halide ions is at play. Another finding from the DFT calculation is the spatial separation of the conduction band minimum and the VBM based on the asymmetry of halogen layers, which may promote the charge separation. While both PbBi3O4Cl3 and PbBi3O4Br3 possess band levels appropriate for visible-light-induced water splitting, only the former showed photocatalytic O2 evolution activity. The stability of PbBi3O4Cl3 is due to the significant contribution of the O-2p orbitals to its VBM, in stark contrast to PbBi3O4Br3, where Br-4p predominately contributes to the VBM. PbBi3O4Cl3 functions as a stable O2-evolving photocatalyst in Z-scheme water splitting under visible light irradiation. These results show that the electrostatic repulsion of halogen layers is a key factor for tuning the valence band structure of oxyhalides while maintaining sufficient stability for water-splitting photocatalysis.

Monte Carlo Solution of High Electric Field Hole Transport Processes in Avalanche Amorphous Selenium.

Amorphous selenium lacks the structural long-range order present in crystalline solids. However, the stark similarity in the short-range order that exists across its allotropic forms, augmented with a shift to non-activated extended-state transport at high electric fields beyond the onset of impact ionization, allowed us to perform this theoretical study, which describes the high-field extended-state hole transport processes in amorphous selenium by modeling the band-transport lattice theory of its crystalline counterpart trigonal selenium. An in-house bulk Monte Carlo algorithm is employed to solve the semiclassical Boltzmann transport equation, providing microscopic insight to carrier trajectories and relaxation dynamics of these non-equilibrium “hot” holes in extended states. The extended-state hole–phonon interaction and the lack of long-range order in the amorphous phase is modeled as individual scattering processes, namely acoustic, polar and non-polar optical phonons, disorder and dipole scattering, and impact ionization gain, which is modeled using a power law Keldysh fit. We have used a non-parabolic approximation to the density functional theory calculated valence band density of states. To validate our transport model, we calculate and compare our time of flight mobility, impact ionization gain, ensemble energy and velocity, and high field hole energy distributions with experimental findings. We reached the conclusion that hot holes drift around in the direction perpendicular to the applied electric field and are subject to frequent acceleration/deceleration caused by the presence of high phonon, disorder, and impurity scattering. This leads to a certain determinism in the otherwise stochastic impact ionization phenomenon, as usually seen in elemental crystalline solids.

Quantitative material analysis using secondary electron energy spectromicroscopy

This paper demonstrates how secondary electron energy spectroscopy (SEES) performed inside a scanning electron microscope (SEM) can be used to map sample atomic number and acquire bulk valence band density of states (DOS) information at low primary beam voltages. The technique uses an electron energy analyser attachment to detect small changes in the shape of the scattered secondary electron (SE) spectrum and extract out fine structure features from it. Close agreement between experimental and theoretical bulk valance band DOS distributions was obtained for six different test samples, where the normalised root mean square deviation ranged from 2.7 to 6.7%. High accuracy levels of this kind do not appear to have been reported before. The results presented in this paper point towards SEES becoming a quantitative material analysis companion tool for low voltage scanning electron microscopy (LVSEM) and providing new applications for Scanning Auger Microscopy (SAM) instruments.

(Invited) Tensile Strain Engineering and Defects Management in GeSn Laser Cavities

We present GeSn material engineering to address both defect managements and band structure via tensile strain in laser cavities. We show that GeSnOI technologies allow to address both issues, with drastic reduction of power density thresholds in lasing devices at 2-3 µm wavelengths range.First set of analysis were performed on GeSn micro-disks with removed defects from their active region. The GeSn layers, grown on Ge strain-relaxed-buffers (SRBs) on silicon, were etched into micro-disks cavities. A specific chemistry were used to remove the dense misfit dislocation array from the suspended area of the GeSn disk, which constitutes the laser gain media. We obtained reduced lasing thresholds from hundreds of kW/cm2, as commonly obtained in the literature at low temperature, to around 10 kW/cm2 in pulsed regime with Sn content as low as 7 % to 10.5 %.In a second set of analysis we demonstrate a new approach relied on GeSn layers transfer on insulator, GeSnOI, with the aim to introduce SiNx stressor layers in an all-around geometry. [1,2] Tensile strain transfer from the stressor layers to GeSn micro-disks of 1.6% were reached, homogenously distributed in the disks volume. The defective interface were removed from the GeSn layer once it has been transferred on the host Si-substrate. This approach was applied to a GeSn layer with only 5.4% of Sn-content, which is nominally an indirect band gap semiconductor that cannot support lasing. The applied tensile strain transformed the Ge0.95Sn0.05 alloy into a direct-band gap material which support lasing with dramatically reduced thresholds, to 0.8 kW/cm2 at low temperature [3]. Tensile strain indeed yields to reduction of the valence band density of state by lifting the HH-LH degeneracy at the Γ-point, and thus contributes to reduce the lasing thresholds. The tensile strained GeSn lasers operate under continuous-wave-pumping up to 70 K thanks to further thermal management of the GeSnOI devices [3]. These results show the potential of new GeSn material engineering for future integration of laser sources on silicon platform using fully CMOS-compatible technology.[1] M. El Kurdi, M. Prost, A. Ghrib, et.al. ACS Photonics 3, 443 (2016).[2] A. Ghrib, M. El Kurdi, M. Prost, et. al. Adv. Opt. Mater. 3, 353 (2015).[3] A. Elbaz et. al. Nature Photonics, March (2020) doi.org/10.1038/s41566-020-0601-5

Tracing fcc iron in oxide dispersion strengthened steel by photoelectron emission, Mössbauer spectroscopy, and X-ray diffraction

The crystal structure, the magnetic properties, and the valence band density of states of an oxide dispersion strengthened (ODS) steel with nominal composition of Fe–18%Cr–13%Ni-2.5%Mo-3.2%Si-0.1%C +1 wt% Y2O3 was investigated applying volumetric and surface sensitive methods. The sample volume contained majority fcc and minority bcc phases according to XRD and volumetric magnetic measurements, while the top layer of the grains contained only the paramagnetic fcc phase, revealed by conversion electron Mössbauer spectrometry (CEMS) and by photoelectron measurements. The valence band photoelectron emission spectra reflected well the average distribution of the theoretically expected DOS, however, there were no signs of abrupt peaks in the region of 0–4 eV below the Fermi level that could have been attributed to 3d Fe states (bcc phase). It was demonstrated that the photoelectron emission can be utilized to distinguish the Fe atoms located in different crystallographic structures. The valence band data of the ODS steel was compared to that of originating from a bcc iron based magnetic alloy.

Electronic Structure and Optoelectronic Properties of Bismuth Oxyiodide Robust against Percent‐Level Iodine‐, Oxygen‐, and Bismuth‐Related Surface Defects

AbstractIn the search for nontoxic alternatives to lead‐halide perovskites, bismuth oxyiodide (BiOI) has emerged as a promising contender. BiOI is air‐stable for over three months, demonstrates promising early‐stage photovoltaic performance and, importantly, is predicted from calculations to tolerate vacancy and antisite defects. Here, whether BiOI tolerates point defects is experimentally investigated. BiOI thin films are annealed at a low temperature of 100 °C under vacuum (25 Pa absolute pressure). There is a relative reduction in the surface atomic fraction of iodine by over 40%, reduction in the surface bismuth fraction by over 5%, and an increase in the surface oxygen fraction by over 45%. Unexpectedly, the Bi 4f7/2 core level position, Fermi level position, and valence band density of states of BiOI are not significantly changed. Further, the charge‐carrier lifetime, photoluminescence intensity, and the performance of the vacuum‐annealed BiOI films in solar cells remain unchanged. The results show BiOI to be electronically and optoelectronically robust to percent‐level changes in surface composition. However, from photoinduced current transient spectroscopy measurements, it is found that the as‐grown BiOI films have deep traps located ≈0.3 and 0.6 eV from the band edge. These traps limit the charge‐carrier lifetimes of BiOI, and future improvements in the performance of BiOI photovoltaics will need to focus on identifying their origin. Nevertheless, these deep traps are three to four orders of magnitude less concentrated than the surface point defects induced through vacuum annealing. The charge‐carrier lifetimes of the BiOI films are also orders of magnitude longer than if these surface defects were recombination active. This work therefore shows BiOI to be robust against processing conditions that lead to percent‐level iodine‐, bismuth‐, and oxygen‐related surface defects. This will simplify and reduce the cost of fabricating BiOI‐based electronic devices, and stands in contrast to the defect‐sensitivity of traditional covalent semiconductors.

Assessing model-dielectric-dependent hybrid functionals on the antiferromagnetic transition-metal monoxides MnO, FeO, CoO, and NiO

Recently, two nonempirical hybrid functionals, dielectric-dependent range-separated hybrid functional based on the Coulomb-attenuating method (DD-RSH-CAM) and doubly screened hybrid functional (DSH), have been suggested by Chen et al (2018 Phys. Rev. Mater. 2 073803) and Cui et al (2018 J. Phys. Chem. Lett. 9 2338), respectively. These two hybrid functionals are both based on a common model dielectric function approach, but differ in the way how to non-empirically obtain the range-separation parameter. By retaining the full short-range Fock exchange and a fraction of the long-range Fock exchange that equals the inverse of the dielectric constant, both DD-RSH-CAM and DSH turn out to perform very well in predicting the band gaps for a large variety of semiconductors and insulators. Here, we assess how these two hybrid functionals perform on challenging antiferromagnetic transition-metal monoxides MnO, FeO, CoO, and NiO by comparing them to other conventional hybrid functionals and the GW method. We find that single-shot DD0-RSH-CAM and DSH0 improve the band gaps towards experiments as compared to conventional hybrid functionals. The magnetic moments are slightly increased, but the predicted dielectric constants are decreased. The valence band density of states (DOS) predicted by DD0-RSH-CAM and DSH0 are as satisfactory as HSE03 in comparison to experimental spectra, however, the conduction band DOS are shifted to higher energies by about 2 eV compared to HSE03. Self-consistent DD-RSH-CAM and DSH deteriorate the results with a significant overestimation of band gaps.

A facile method to tune the crystal lattice/morphology/electronic state/photocatalytic performance of BiOCl

A series of BiOCl samples were synthesized in the vapor environment of isopropanol and deionized water with different volume ratios. It was observed that the crystal lattice, nucleation and growth, morphology, specific surface area, elemental chemical states, valence band density of states, optical properties and photocatalytic performance of BiOCl can be easily tuned by the component and volume of the vapor. The degradation rate of RhB upon BiOCl changes in the broad range of 5.7 × 10−2 min−1–18.2 × 10−2 min−1. The photoluminescence intensity is approximately proportional to the total defect concentration of Bi+3−x, Bi+3+x, –OH and Ov, indicating that these defects act as the recombination centers, detrimental to the photocatalytic performance. Therefore, the decrease in the defect concentration enables to improve the photocatalytic performance. Furthermore, the degradation rate of RhB by BiOCl is almost proportional to the ratio of exposed (001) face. This work reports a simple, effective and low-cost method to synthesize the catalyst with high performance.

Majority Carrier Properties of Single Crystal Cu2−xZnSnSe4 with Varying Copper Composition

The dependence of carrier concentration on copper content in Cu2−xZnSnSe4 single crystals, grown without fluxing agents, is presented over the composition range Cu/(Zn + Sn) of 0.77–0.93 and Zn/Sn from 1.3 (low Cu content) to 1.3 (high Cu content). Single phase mono‐crystals with facets up to 5 mm are synthesized from high‐purity elements and validated using Laue diffraction. A direct optical band gap of 0.98 eV is observed for a crystal with Cu/(Zn + Sn) = 0.85. Van der Pauw–Hall resistance analysis shows decreasing hole concentration, from 1019 cm−3 to 2 × 1015 cm−3, with decreasing copper content over the composition range. Hole mobility varies from 50 cm2 V−1 s−1 to 160 cm2 V−1 s−1, with no dependence on copper concentration. A linear temperature dependence of the Seebeck coefficient is found for all crystals. The Seebeck coefficient–carrier concentration dependence is used to determine the valence band density of states, NV, and corresponding effective mass, μh*. Crystals with Cu/(Zn + Sn) < 0.9 have NV = 4 × 1018 cm−3 and μh* = 0.4 m0 while crystals with elevated copper concentration, Cu/(Zn + Sn) > 0.9, exhibit higher density of states, NV = 3 × 1019 cm−3 and hole effective mass μh* = 0.9 m0. The increased valence band density of states at elevated copper composition is consistent with more Cu–Zn exchange sites contributing to bulk defects and band edge fluctuations. Temperature‐dependent conductivity measurements indicate a transition from metallic‐type to semiconductor‐type conduction at Cu/(Zn + Sn) < 0.9. At lower copper content, for a crystal with Cu/(Zn + Sn) = 0.83 and p = 1017 cm−3 the activation energy for intrinsic conduction is 60 meV. This behavior is attributed to the copper vacancy defect, VCu, and is comparable to reported values obtained in thin films and by first‐principles predictions.

Band Alignments, Band Gap, Core Levels, and Valence Band States in Cu3BiS3 for Photovoltaics.

The earth-abundant semiconductor Cu3BiS3 (CBS) exhibits promising photovoltaic properties and is often considered analogous to the solar absorbers copper indium gallium diselenide (CIGS) and copper zinc tin sulfide (CZTS) despite few device reports. The extent to which this is justifiable is explored via a thorough X-ray photoemission spectroscopy (XPS) analysis: spanning core levels, ionization potential, work function, surface contamination, cleaning, band alignment, and valence-band density of states. The XPS analysis overcomes and addresses the shortcomings of prior XPS studies of this material. Temperature-dependent absorption spectra determine a 1.2 eV direct band gap at room temperature; the widely reported 1.4-1.5 eV band gap is attributed to weak transitions from the low density of states of the topmost valence band previously being undetected. Density functional theory HSE06 + SOC calculations determine the band structure, optical transitions, and well-fitted absorption and Raman spectra. Valence band XPS spectra and model calculations find the CBS bonding to be superficially similar to CIGS and CZTS, but the Bi3+ cations (and formally occupied Bi 6s orbital) have fundamental impacts: giving a low ionization potential (4.98 eV), suggesting that the CdS window layer favored for CIGS and CZTS gives detrimental band alignment and should be rejected in favor of a better aligned material in order for CBS devices to progress.

ZnO nanorods decorated with nanocrystalline (nc) Au Particles:Electronic structure and magnetic behaviours

Electronic structures and magnetic behaviours of zinc oxide nanorods (ZnO-NRs) decorated with nanocrystalline “Au”-particles (nc-Au) have been studied using different spectroscopies, superconducting quantum interference device-type magnetometer and plane wave pseudopotential density functional theory calculations. The presence of intrinsic defects viz. zinc interstitials (Zni) and/or oxygen vacancies (VO) are established using different spectroscopies measurements. Ultraviolet photoelectron spectroscopy and valence band photoemission spectroscopy suggests that the valence band density of states of O 2p - Zn 4sp hybridized states of nc-Au/ZnO-NRs is higher than pure ZnO-NRs that could be correlated with its higher ferromagnetic behaviour. An enhanced room temperature ferromagnetism has been observed when nanocrystalline (nc) gold (Au) particles are decorated on the surface of ZnO-nanorods (ZnO-NRs). This enhanced magnetic moment in nc-Au/ZnO-NRs comes from the metallic electrons of nc-Au that induces large orbital magnetic moment at the nc-Au/ZnO-NR interface. Theoretical calculations shows the enhancement of the density of states at the valence band region of ZnO with a decoration of optimum content nc-Au particles along with the increase of oxygen vacancies in nc-Au/ZnO-NRs; that further confirms the enhancement of magnetization.

Electronic structure and magnetic behaviors of exfoliated MoS2 nanosheets

The correlation of electronic structure and magnetic behaviors of layered molybdenum disulfide (MoS2) nanosheets, mechanically exfoliated from pristine hexagonal crystal (2H-MoS2) have been studied. Raman spectra show the energy difference (ΔE) between two Raman peaks A1g and was about 20.2 cm−1, indicating the formation of mono-/bi-layered MoS2 nanosheets as obtained after mechanical exfoliation from pristine 2H-MoS2. The absence of the reflection peak (0 0 2) in x-ray diffraction patterns confirms the formation of few-layered and mono-/bi-layered MoS2 nanosheets with reduced thickness. Mo 3d and S 2p XPS core level peaks shifted to higher energy with the reduction of the number of layers in exfoliated MoS2. As the number of layers decreased, valence band maximum position increased from 1.11 eV (pristine MoS2) to 1.57 eV (mono-/bi-layered MoS2 nanosheets), whereas the surface work function (Ф) reduced from 4.85 eV (pristine MoS2) to 4.47 eV (mono-/bi-layered MoS2 nanosheets), as observed from UPS (He-I) measurements. UPS (He-II) spectra, as well as VB-PES spectra of mono-/bi-layered MoS2 nanosheets, exhibits an enhanced valence band density of states (DOS) of S 3p —derived states near Fermi level (Ef). Mo LII-edge and S K-edge x-ray absorption near edge structure spectra of mono-/bi-layered MoS2 nanosheets show the splitting of different peaks that cause a noticeable change in their band structure. Magnetic M–H hysteresis loops measurement clearly demonstrates the increase of room temperature ferromagnetism from pristine to mono-/bi-layer MoS2, due to the existence of defects (‘S’-vacancies or defects at the grain boundaries region) and the increase of DOS.

Transition from electron accumulation to depletion at β-Ga2O3 surfaces: The role of hydrogen and the charge neutrality level

The surface electronic properties of bulk-grown β-Ga2O3 (2¯01) single crystals are investigated. The band gap is found using optical transmission to be 4.68 eV. High-resolution x-ray photoemission coupled with hybrid density functional theory calculation of the valence band density of states provides insights into the surface band bending. Importantly, the standard linear extrapolation method for determining the surface valence band maximum (VBM) binding energy is found to underestimate the separation from the Fermi level by ∼0.5 eV. According to our interpretation, most reports of surface electron depletion and upward band bending based on photoemission spectroscopy actually provide evidence of surface electron accumulation. For uncleaned surfaces, the surface VBM to Fermi level separation is found to be 4.95 ± 0.10 eV, corresponding to downward band bending of ∼0.24 eV and an electron accumulation layer with a sheet density of ∼5 × 1012 cm−2. Uncleaned surfaces possess hydrogen termination which acts as surface donors, creating electron accumulation and downward band bending at the surface. In situ cleaning by thermal annealing removes H from the surface, resulting in a ∼0.5 eV shift of the surface VBM and formation of a surface electron depletion layer with upward band bending of ∼0.26 eV due to native acceptor surface states. These results are discussed in the context of the charge neutrality level, calculated bulk interstitial hydrogen transition levels, and related previous experimental findings.

Electronic structures and impurity cluster features in Mg-Zn-Y alloys with a synchronized long-period stacking ordered phase

Photoemission and inverse-photoemission spectroscopy (PES and IPES) measurements were carried out on polycrystalline Mg97Zn1Y2, Mg85Zn6Y9, and Mg75Zn10Y15 alloys with a synchronized long-period stacking ordered phase together with non-doped Mg to investigate the valence- and conduction-band electronic structures, respectively. It was found from the valence-band PES spectra that the non-doped Mg includes at most 25% of an intrinsic oxidation portion. With increasing impurity concentration, the valence-band density of states (DOS) very slightly changes, while the conduction-band DOS drastically varies. The contributions of the impurity elements are clarified using a density functional theory calculation for the Mg85Zn6Y9 alloy. The core-level PES measurements were also performed at the Mg 2s, 2p, Zn 3p, 3d, and Y 3d levels to study the chemical natures of the corresponding elements. The Y 3d core spectrum exhibits mostly three doublets in Mg97Zn1Y2, while mostly only one in the heavily doped alloys, reflecting a coexistence of three different sites of the Y atoms; ones in the Zn6Y8 L12 clusters, ones in their fragments, and isolated ones in Mg97Zn1Y2.

Heavy and light hole minority carrier transport properties in low-doped n-InGaAs lattice matched to InP

Minority carrier diffusion lengths in low-doped n-InGaAs using InP/InGaAs double-heterostructures are reported using a simple electrical technique. The contributions from heavy and light holes are also extracted using this methodology, including minority carrier mobilities and lifetimes. Heavy holes are shown to initially dominate the transport due to their higher valence band density of states, but at large diffusion distances, the light holes begin to dominate due to their larger diffusion length. It is found that heavy holes have a diffusion length of 54.5 ± 0.6 μm for an n-InGaAs doping of 8.4 × 1015 cm–3 at room temperature, whereas light holes have a diffusion length in excess of 140 μm. Heavy holes demonstrate a mobility of 692 ± 63 cm2/Vs and a lifetime of 1.7 ± 0.2 μs, whereas light holes demonstrate a mobility of 6200 ± 960 cm–2/Vs and a slightly longer lifetime of 2.6 ± 1.0 μs. The presented method, which is limited to low injection conditions, is capable of accurately resolving minority carrier transport properties.