Ga 4s Research Articles

Growth of β-Ga2O3 nanostructures by thermal oxidation of GaN-on-sapphire for optoelectronic devices applications

Heterostructures of wide-bandgap semiconductors, like β-Ga2O3/ GaN, are being used in many high-power, high-frequency electronic and optoelectronic devices. This paper presents the growth of β-Ga2O3 thin film and nanostructures by thermally oxidizing (at 1000 °C) the planar and nano-structured GaN surfaces respectively. The nano-structures of GaN (having 300–500 nm long vertical nano-kinks) are fabricated by metal-assisted chemical etching of GaN surface for 1–2 hours. The thermally grown β-Ga2O3 samples are polycrystalline. The chemical analysis of the samples using X-ray photoelectron spectroscopy (XPS) showed enhancement of Ga-O peak intensity (at 20.21–20.3 eV) at the expense of the Ga-N peak intensity (at 19.2–19.7 eV) in the thermal oxidation process. The O1s XPS spectra of the oxidized samples exhibited prominent signature of O2- ions (530.7–530.9 eV) representing the formation of Ga2O3 with presence of adsorbed hydroxyl group at the surface (532.2 – 532.4 eV). The valance band XPS spectra disclosed the formation of an additional Ga(4 s)–O(2p) hybridization energy state, and the valence band maxima increased from 2.3 eV to 3.25 eV. The Raman spectra of the oxidized samples exhibited multiple Ag and Bg vibrational modes of β-Ga2O3 along with intense E2(high), A1(LO) modes of GaN. The time-resolved photoluminescence spectra of the samples demonstrated fast transition of charge carriers from excited state to ground state with 460–820 pico-seconds average career lifetime.

TiO2 supported-reduced graphene oxide co-doped with gallium and sulfur as an efficient heterogeneous catalyst for the selective photochemical oxidation of alcohols; DFT and mechanism insights

Achieving organic transformation reactions using green synthesis methods under mild conditions is always an aspiration of scientists. In this study, Ga, S co-doped TiO2/reduced graphene oxide (GaxSy@TRG) nanocomposite was synthesized by an innovative surfactant-free ultrasonic-assisted solvothermal method and used to catalyze the selective photocatalytic oxidation of benzyl alcohol under visible light irradiation. The prepared photocatalyst was analyzed via XRD, FT-IR, Raman, N2 adsorption–desorption, SEM, TEM, EDX, ICP-MS, UV–Vis absorption, and electrochemical impedance spectroscopy (EIS) measurement. The characterization outcomes indicated the prepared co-doped TiO2 nanoparticles with anatase crystal structure possessed a mesoporous texture and were uniformly distributed on the RGO surface. The conversion of GO to RGO was confirmed through FT-IR and Raman spectra. The prepared nanocomposites were examined for their ability in artificial photocatalysis and displayed remarkable catalytic activity for the selective oxidation of alcohols into benzaldehyde. The co-doped catalysts showed significantly enhanced photocatalytic ability, due to improved photogenerated electrons transfer, enhanced charge carriers separation and extended optical absorption to the visible light range. Moreover, the synergistic effect of dopants that could act as electron trapping centers and RGO as a powerful acceptor material efficiently reduced photogenerated electron-hole recombination and promoted charge migration. A high conversion rate of 85% was obtained over the Ga5.0S10.0@TRG photocatalyst after 110 min under visible light irradiation. The results from density functional theory (DFT) calculations revealed that the hybridization of Ga 4s and S 3p orbitals with valance band can remarkably affect the decreasing of band gap energy. The mechanism of photocatalytic oxidation of benzyl alcohol under visible light irradiation was proposed using the combination of experimental and computational methods.

Deep UV transparent conductive oxide thin films realized through degenerately doped wide-bandgap gallium oxide

Deep UV transparent thin films have recently attracted considerable attention owing to their potential in UV and organic-based optoelectronics. Here, we report the achievement of a deep UV transparent and highly conductive thin film based on Si-doped Ga2O3 (SGO) with high conductivity of 2500 S/cm. The SGO thin films exhibit high transparency over a wide spectrum ranging from visible light to deep UV wavelength and, meanwhile, have a very low work-function of approximately 3.2 eV. A combination of photoemission spectroscopy and theoretical studies reveals that the delocalized conduction band derived from Ga 4s orbitals is responsible for the Ga2O3 films’ high conductivity. Furthermore, Si is shown to act as an efficient shallow donor, yielding high mobility up to approximately 60 cm2/Vs. The superior optoelectronic properties of SGO films make it a promising material for use as electrodes in high-power electronics and deep UV and organic-based optoelectronic devices.

Electronic structure modification in Fe-substituted β-Ga2O3 from resonant photoemission and soft x-ray absorption spectroscopies

Oriented thin films of β-(Ga1−x Fe x )2O3 were deposited by radio frequency magnetron sputtering on c-Al2O3 and GaN substrates. The itinerant character of the Fe 3d states forming the top of the valence band (VB) of the Fe-substituted β-Ga2O3 thin films has been determined from resonant photoelectron spectroscopy. Further, the admixture of the itinerant and localized characters of these Fe 3d states has been obtained for larger binding energies; i.e. deeper in the VB. The bottom of the conduction band (CB) for β-(Ga1−x Fe x )2O3 has been also found to have strongly hybridized states involving Fe 3d and O 2p states compared to that of Ga 4s in pristine β-Ga2O3. This suggests that β-Ga2O3 transforms from a band-like system to a charge-transfer system with Fe substitution. Furthermore, the bandgap red shifts with Fe composition, which has been found to be primarily related to the shift of the CB edge.

Study of New Infrared Non-Linear Optical Crystal BaGa4Se7 Based on First Principle

The electronic structure and optical properties of BaGa4Se7 crystals were studied by generalized gradient approximation (GGA) and Heyd–Scuseria–Ernzerhof (HSE). The electronic structure results showed that BaGa4Se7 is a nonlinear optical crystal with a direct wide band gap. The band gap was calculated to be 2.36 eV using the HSE06 method, which is basically equal to the experimental value. The band structure showed that the top of the valence band was largely contributed by Ga 4s, 4p and Se 4p states, while the bottom of the conduction band was mainly comprised of Ga 4s, 4p, Se 4p, and Ba 5d states. The optical properties showed strong absorption and reflection in the ultraviolet region and strong transmittance in the infrared region. The average static refractive index of BaGa4Se7 was 2.73, and the static birefractive index was 0.04. These values indicate that the material has a good phase-matching ability and a wide laser damage threshold. The above results indicate that BaGa4Se7 is a potential infrared nonlinear optical crystal material.

Electronic states of gallium oxide epitaxial thin films and related atomic arrangement

Amorphous, crystallized, and Mg-doped β-Ga2O3 thin films are investigated to understand the evolution of electronic states and the related atomic arrangement. In the process from the amorphous to crystallized phase of Ga2O3 films, disordered atoms transform into six-fold GaO6 octahedra. The denser structure creates more electron density. The crystallized phase widens the band gap, because of the conduction band changes attributed to the Ga 4s states hybridized with O 2s states. The Mg dopants create four-fold GaO4 tetrahedron deficiency. Moreover, the distances between Ga atoms become shorter for the doped film; however, the Ga–O distance is independent of the dopant. Mg dopants affect the oxygen states in valence band and Ga–O interaction at conduction band, which results in a slightly smaller band gap. The electronic evolution of the amorphous, crystallized and doped films is confirmed by the calculated density of states results. These results will be beneficial for the design of more effective semiconductive films.

Theoretical Study of Infrared Nonlinear Optical Crystal BaGa4Se7 Tetragonal System

The electronic structure and optical properties of BaGa4Se7 tetragonal crystals are systematically studied by first principles of density functional theory. The band gap (2.918 eV) of the tetragonal system of BaGa4Se7 is 1.12 times that of the orthorhombic system by using the Heyd-Scuseria-Ernzerh (HSE06) method. The band structure showed that the top of the valence band was largely contributed by Ga 4s, 4p and Se 4p states, while the bottom of the conduction band was mainly comprised of Ga 4s, 4p, Se 4p, and Ba 5d states. This wide band gap effectively avoid the two-photon absorption during laser pumping and increase the laser damage threshold of the material. The orbital coupling between Ga and Se atoms determines the optical properties of BaGa4Se7 tetragonal crystals, while Ba atoms make little contribution to the optical properties. The calculation of optical properties shows that the static birefringence (0.076) of the BaGa4Se7 tetragonal crystal is 1.2 times that of the orthorhombic crystal system. It shows that the BaGa4Se7 tetragonal crystal material has excellent phase matching performance in a wide range of wavelengths. BaGa4Se7 crystal shows strong absorption and reflection characteristics in the ultraviolet region, and strong transmittance in the infrared region. Theoretical results indicate that BaGa4Se7 is a very promising nonlinear optical crystal in the infrared region.

Large bandgap tunability of GaN/ZnO pseudobinary alloys through combined engineering of anions and cations

Bandgap engineering of gallium zinc oxynitride (GaZnON) thin films has been performed by the GaN/ZnO pseudobinary alloying in a periodical superlattice order through the pulsed laser deposition technique. By tuning the growth temperature, the combined engineering of anions and cations in GaZnON quaternary alloys leads to a large tunability of the optical bandgap from 1.80 to 4.34 eV. In terms of the enthalpy of formation and kinetic dynamics of reactant species, nitrogen incorporation is effective to form Zn3N2-rich GaZnON quaternary alloys at low-temperature (<100 °C) conditions far from the equilibrium, while amorphous nitrogen deficient GaZnON is formed at high temperatures with ZnGa2O4 and β-Ga2O3 nanocrystalline structures embedded. The conduction band (CB) and valence band (VB) of GaZnON are determined by Zn 4s orbital electrons and the hybridization of N 2p and O 2p electrons, respectively, while the Ga 4s and O 2p are predominant to construct the CB and VB of O-rich GaON due to the low solubility of N at high temperature. The asymmetric band bowing effect of GaZnON quaternary alloy demonstrates a large bandgap tunability down to the visible spectral range, which provides significant potential applications in the harvest of solar energy technologies.

Electronic structure and optical properties of LiGa0.5In0.5Se2 single crystal, a nonlinear optical mid-IR material

Measurements of X-ray photoelectron core-level and valence-band spectra for pristine and irradiated with Ar+ ions surfaces of LiGa0.5In0.5Se2 single crystal, novel nonlinear optical mid-IR selenide grown by a modified vertical Bridgman-Stockbarger technique, are reported. Electronic structure of LiGa0.5In0.5Se2 is elucidated from theoretical and experimental points of view. Notably, total and partial densities of states (DOSs) of the LiGa0.5In0.5Se2 compound are calculated based on density functional theory (DFT) using the augmented plane wave + local orbitals (APW + lo) method. In accordance with the DFT calculations, the principal contributors to the valence band are the Se 4p states, making the main input at the top and in the upper part of the band, while its bottom is dominated by contributions of the valence s states associated with Ga and In atoms. The theoretical total DOS curve peculiarities are found to be in excellent agreement with the shape of the X-ray photoelectron valence-band spectrum of the LiGa0.5In0.5Se2 single crystal. The bottom of the conduction band of LiGa0.5In0.5Se2 is formed mainly by contributions of the unoccupied Ga 4s and In 5s states in almost equal proportion, with somewhat smaller contributions of the unoccupied Se 4p states as well. Our calculations indicate that the LiGa0.5In0.5Se2 compound is a direct gap semiconductor. The principal optical constants of LiGa0.5In0.5Se2 are calculated in the present work.

Electronic and magnetic properties of transition metal decorated monolayer GaS

Inducing controllable magnetism in two dimensional non-magnetic materials is very important for realizing dilute magnetic semiconductor. Using density functional theory, we have systematically investigated the effect of surface adsorption of various 3d transition metal (TM) atoms (Sc-Cu) on the electronic and magnetic properties of the monolayer GaS as representative of group-IIIA metal-monochalcogenide. We find that all adatoms favor the top site on the Ga atom. All the TM atoms, except for the Cr and Mn, can bond strongly to the GaS monolayer with sizable binding energies. Moreover, the TM decorated GaS monolayers exhibit interesting magnetic properties, which arise from the strong spin-dependent hybridization of the TM 3d orbitals with S 3p and Ga 4s orbitals. After examining the magnetic interaction between two same types of TM atoms, we find that most of them exhibit antiferromagnetic coupling, while Fe and Co atoms can form long-range ferromagnetism. Furthermore, we find that the electronic properties of metal decorated systems strongly rely on the type of TM adatom and the adsorption concentration. In particular, the spin-polarized semiconducting state can be realized in Fe doped system for a large range of doping concentrations. These findings indicate that the TM decorated GaS monolayers have potential device applications in next-generation electronics and spintronics.

Electronic structure and optical properties of defect chalcopyrite HgGa2Se4

We report on studies from an experimental and theoretical viewpoint of the electronic structure of mercury digallium selenide, HgGa2Se4, a very promising optoelectronic material. In particular, the method of X-ray photoelectron spectroscopy (XPS) was used to evaluate binding energies of the constituent element core electrons and the shape of the valence band for pristine and Ar+-ion bombarded surfaces of HgGa2Se4 single crystal. First principles band-structure calculations were performed in the present work using the augmented plane wave + local orbitals (APW+lo). These calculations indicate that the Se 4p states are the main contributors at the top and in the upper portion of the valence band with slightly smaller contributions of the Ga 4p states in the upper portion of the band as well. Further, the central portion of the valence band is determined mainly by contributions of the Ga 4s states, and the Hg 5d states are the principal contributors to the bottom of the valence band. These theoretical data are in fair agreement when matching on a common energy scale of the X-ray emission bands giving information on the energy distribution of the Se 4p and Ga 4p states and the XPS valence-band spectrum of the HgGa2Se4 crystal. The principal optical constants are elucidated from the DFT calculations.

Enhancing s, p–d exchange interactions at room temperature by carrier doping in single crystalline Co0.4Zn0.6O epitaxial films

Magnetic doping of semiconductors has been actively pursued because of their potential applications in the spintronic devices. Central to these efforts is a drive to control the mutual interactions between their magnetic properties (supported by d electrons of the magnetic ions) and their semiconductor properties (supported by s and/or p electrons) at room temperature (RT). Despite the long, intensive efforts, the experimental evidence of thermally robust s, p–d coupling in a semiconductor remains scarce and controversial. Here, we report the enhancement of RT ferromagnetic s, p–d exchange interaction by means of carrier doping in single crystalline Co0.4Zn0.6O epitaxial films with a high Co concentration. Magneto-transport measurements reveal that spin-polarized conducting carriers are produced at RT and are increased with the carrier density through Ga3+ doping, owing to the s, p–d coupling between Ga (4s), O (2p), and Co (3d) orbitals. With the ability to individually control carrier density and magnetic doping, single crystalline Ga(Co, Zn)O films can lay a solid foundation for the development of practical semiconductor spintronic devices operable at RT.

Electronic structure and optical properties of noncentrosymmetric LiGaSe2: Experimental measurements and DFT band structure calculations

We report on measurements of X-ray photoelectron (XP) spectra for pristine and Ar+ ion-irradiated surfaces of LiGaSe2 single crystal grown by Bridgman-Stockbarger method. Electronic structure of the LiGaSe2 compound is studied from a theoretical and experimental viewpoint. In particular, total and partial densities of states of LiGaSe2 are investigated by density functional theory (DFT) calculations employing the augmented plane wave + local orbitals (APW + lo) method and they are verified by data of X-ray spectroscopy measurements. The DFT calculations indicate that the main contributors to the valence band of LiGaSe2 are the Se 4p states, which contribute mainly at the top and in the upper portion of the valence band, with also essential contributions of these states in the lower portion of the band. Other substantial contributions to the valence band of LiGaSe2 emerge from the Ga 4s and Ga 4p states contributing mainly at the lower ant upper portions of the valence band, respectively. With respect to the conduction band, the calculations indicate that its bottom is composed mainly from contributions of the unoccupied Ga s and Se p states. The present calculations are confirmed experimentally when comparing the XP valence-band spectrum of the LiGaS2 single crystal on a common energy scale with the X-ray emission bands representing the energy distribution of the Ga 4p and Se 4p states. Measurements of the fundamental absorption edges at room temperature reveal that bandgap value, Eg, of LiGaSe2 is equal to 3.47 eV and the Eg value increases up to 3.66 eV when decreasing temperature to 80 K. The main optical characteristics of the LiGaSe2 compound are clarified by the DFT calculations.

First-Principles Study of CuGaO2 Polymorphs: Delafossite α-CuGaO2 and Wurtzite β-CuGaO2.

The electronic structures of delafossite α-CuGaO2 and wurtzite β-CuGaO2 were calculated based on density functional theory using the local density approximation functional including the Hubbard correction (LDA+U). The differences in the electronic structure and physical properties between the two polymorphs were investigated in terms of their crystal structures. Three major structural features were found to influence the electronic structure. The first feature is the atomic arrangements of cations. In the conduction band of α-CuGaO2 with a layered structure of Cu2O and Ga2O3, Cu and Ga states do not mix well; the lower part of the conduction band mainly consists of Cu 4s and 4p states, and the upper part consists of Ga 4s and 4p states. By contrast, in β-CuGaO2, which is composed of CuO4 and GaO4 tetrahedra, Cu and Ga states are well-mixed. The second feature is the coordination environment of Cu atoms; the breaking of degeneracy of Cu 3d orbitals is determined by the crystal field. Dispersion of the Cu 3d valence band of β-CuGaO2, in which Cu atoms are tetrahedrally coordinated to oxygen atoms, is smaller than those in α-CuGaO2, in which Cu atoms are linearly coordinated to oxygen atoms; this results in a larger absorption coefficient and larger hole effective mass in β-CuGaO2 than in α-CuGaO2. The interatomic distance between Cu atoms-the third feature-also influences the dispersion of the Cu 3d valence band (i.e., the effective hole mass); the effective hole mass decreases with decreasing interatomic distance between Cu atoms in each structure. The results obtained are valuable for understanding the physical properties of oxide semiconductors containing monovalent copper and silver.

Electronic structure and optical properties of F-doped β-Ga2O3 from first principles calculations**Project supported by the Innovation Project of Shandong Graduate Education, China (No. SDYY13093) and the National Natural Science Foundation of China (No. 10974077).

The effects of F-doping concentration on geometric structure, electronic structure and optical property of β-Ga2O3 were investigated. All F-doped β-Ga2O3 with different concentrations are easy to be formed under Ga-rich conditions, the stability and lattice parameters increase with the F-doping concentration. F-doped β-Ga2O3 materials display characteristics of the n-type semiconductor, occupied states contributed from Ga 4s, Ga 4p and O 2p states in the conduction band increase with an increase in F-doping concentration. The increase of F concentration leads to the narrowing of the band gap and the broadening of the occupied states. F-doped β-Ga2O3 exhibits the sharp band edge absorption and a broad absorption band. Absorption edges are blue-shifted, and the intensity of broad band absorption has been enhanced with respect to the fluorine content. The broad band absorption is ascribed to the intra-band transitions from occupied states to empty states in the conduction band.

First principles calculations of ternary wurtzite β-CuGaO2

The electronic structure of β-CuGaO2 was studied by first principles calculations and X-ray photoelectron spectroscopy (XPS), and the expected electrical and optical properties of this material were discussed. Density functional theory calculations using the local density approximation with corrections for on-site Coulomb interactions (LDA + U) with U = 5–7 eV reproduced well the experimentally obtained crystal structure and valence-band XPS spectrum. The calculated electronic structure indicates that β-CuGaO2 is a direct band gap semiconductor and its conduction band minimum and valence band maximum consist mainly of highly delocalized Ga 4s and Cu 4s states and relatively localized Cu 3d and O 2p states, respectively. The effective electron mass obtained under parabolic approximation is small (me*/m0 = 0.21), similar to common n-type oxide semiconductors, and the effective hole mass is relatively large (mh*/m0 = 1.7–5.1) although p-type conduction is experimentally observed. The direct and allowed band gap and large density of states near the valence band maximum result in a high absorption coefficient of 1 × 105 cm−1 near the absorption edge.

Engel-Vosko GGA Approach Within DFT Investigations of the Optoelectronic Structure of the Metal Chalcogenide Semiconductor CsAgGa2Se4

Metal chalcogenide semiconductors have a significant role in the development of materials for energy and nanotechnology applications. First principle calculations were applied on CsAgGa2Se4 to investigate its optoelectronic structure and bonding characteristics, using the full-potential linear augmented plane wave method within the framework of generalized gradient approximations (GGA) and Engel-Vosko GGA functionals (EV-GGA). The band structure from EV-GGA shows that the valence band maximum and conduction band minimum are situated at Γ with a band gap value of 2.15 eV. A mixture of orbitals from Ag 4p 6/4d 10, Se 3d 10, Ga 4p 1, Se 4p 4 , and Ga 4s 2 states have a primary role to lead to a semiconducting character of the present chalcogenide. The charge density iso-surface shows a strong covalent bonding between Ag-Se and Ga-Se atoms. The imaginary part of dielectric constant reveals that the threshold (first optical critical point) energy of dielectric function occurs 2.15 eV. It is obvious that with a direct large band gap and large absorption coefficient, CsAgGa2Se4 might be considered a potential material for photovoltaic applications.

Electronic structure analysis of GaN films grown on r- and a-plane sapphire

The electronic structure and surface properties of epitaxial GaN films grown on r- and a-plane sapphire substrates were probed via spectroscopic and microscopic measurements. X-ray photoemission spectroscopic (XPS) measurements were performed to analyse the surface chemistry, band bending and valence band hybridization states. It was observed that GaN/a-sapphire display a downward band bending of 0.5 eV and possess higher amount of surface oxide compared to GaN/r-sapphire. The valence band (VB) investigation revealed that the hybridization corresponds to the interactions of Ga 4s and Ga 4p orbitals with N 2p orbital, and result in N2p–Ga4p, N2p–Ga4s∗, mixed and N2p–Ga4s states. The energy band structure and electronic properties were measured via ultraviolet photoemission spectroscopic (UPS) experiments. The band structure analysis and electronic properties calculations divulged that the electron affinity and ionization energy of GaN/a-sapphire were 0.3 eV higher than GaN/r-sapphire film. Atomic Force Microscopic (AFM) measurements revealed faceted morphology of GaN/r-sapphire while a smooth pitted surface was observed for GaN/a-sapphire film, which is closely related to surface oxide coverage.

Ab initio study of gallium stabilized δ-plutonium alloys and hydrogen–vacancy complexes

All-electron density functional theory was used to investigate δ-plutonium (δ-Pu) alloyed with gallium (Ga) impurities at 3.125, 6.25, 9.375 atomic (at)% Ga concentrations. The results indicated that the lowest energy structure is anti-ferromagnetic, independent of the Ga concentration. At higher Ga concentrations (>3.125 at%), the position of the Ga atoms are separated by four nearest neighbor Pu–Pu shells. The results also showed that the lattice constant contracts with increasing Ga concentration, which is in agreement with experimental data. Furthermore with increasing Ga concentration, the face-centered-cubic structure becomes more stably coupled with increasing short-range disorder. The formation energies show that the alloying process is exothermic, with an energy range of −0.028 to −0.099 eV/atom. The analyses of the partial density of states indicated that the Pu–Ga interactions are dominated by Pu 6d and Ga 4p hybridizations, as well as Ga 4s–4p hybridizations. Finally, the computed formation energies for vacancy and hydrogen–vacancy complexes within the 3.125 at% Ga cell were 1.12 eV (endothermic) and −3.88 eV (exothermic), respectively. In addition, the hydrogen atom prefers to interact much more strongly to the Pu atom than the Ga atom in the hydrogen–vacancy complex.

Effect on the electronic structures and optical bandgaps of Ga-doped wurtzite TM0.125Zn0.875O(TM=Be, Mg)

The optimized structure parameters, electron density of states, energy band structures and optical bandgaps of the TM0.125Zn0.875O (TM=Be, Mg) alloys and Ga-doped TM0.125Zn0.875O are calculated and analyzed by using the ultra-soft pseudopotential approach of the plane-wave based upon density functional theory. The theoretical results show the Ga-doped TM0.125Zn0.875O materials are easily obtained and their structures are more stable. The Ga-doped TM0.125Zn0.875O are good n-type materials and their energy bandgaps are determined by Ga 4s states of the conduction band minimum and O 2p states of the valence band maximum. Compared with the TM0.125Zn0.875O alloys, the optical bandgaps of Ga-doped TM0.125Zn0.875O become wider due to the Burstein-Moss shift and many-body effects, which is consistent with previous experimental data. The Ga-doped TM0.125Zn0.875O materials are suitable as TCO films for the UV and deep UV optoelectronic device.