Surface relaxation and rumpling of Sn-doped β−Ga2O3(010) (original) (raw)

Surface morphology and electronic structure of bulk single crystal β-Ga[sub 2]O sub 3

Applied Physics Letters, 2009

Experimental studies of the surface morphology and electronic structure of bulk single crystals of the transparent and wide gap semiconductor gallium oxide ͑␤-Ga 2 O 3 ͒ have been conducted using scanning tunneling microscopy ͑STM͒, low-energy electron diffraction ͑LEED͒, and angle-resolved photoemission spectroscopy ͑ARPES͒. Atomically resolved STM and LEED results for the ␤-Ga 2 O 3 ͑100͒ surface clarify that the predominant surface termination contains both gallium and oxygen, and this surface does not exhibit a reconstruction. The valence band structure was obtained with ARPES and shows good agreement with existing theoretical works at the zone center and along the a ‫ء‬ and c ‫ء‬ directions, except that the calculated bandwidth is ϳ7% too small. There is poorer agreement along the b ‫ء‬ direction, where the experimental bands disperse more strongly than the calculations.

The electronic structure of "-Ga 2 O 3

The electronic structure of "-Ga 2 O 3 thin films has been investigated by ab initio calculations and photoemission spectroscopy with UV, soft, and hard X-rays to probe the surface and bulk properties. The latter measurements reveal a peculiar satellite structure in the Ga 2p core level spectrum, absent at the surface, and a core-level broadening that can be attributed to photoelectron recoil. The photoemission experiments indicate that the energy separation between the valence band and the Fermi level is about 4.4 eV, a valence band maximum at the point and an effective mass of the highest lying bands of -4.2 free electron masses. The value of the bandgap compares well with that obtained by optical experiments and with that obtained by calculations performed using a hybrid density-functional, which also reproduce well the dispersion and density of states.

Band bending and surface defects in β-Ga2O3

Applied Physics Letters, 2012

Surface band bending and surface defects on the UV-transparent conducting oxide b-Ga 2 O 3 (100) are studied with hard x-ray photoemission spectroscopy and scanning tunneling microscopy. Highly doped b-Ga 2 O 3 shows flat bands near the surface, while the bands on nominally undoped (but still n-type), air-cleaved b-Ga 2 O 3 are bent upwards by & 0:5 eV. Negatively charged surface defects are observed on vacuum annealed b-Ga 2 O 3 , which also shows upward band bending. Density functional calculations show oxygen vacancies are not likely to be ionized in the bulk, but could be activated by surface band bending. The large band bending may also hinder formation of ohmic contacts. V

Structure and gap of low-x (Ga 1 x In x ) 2 O 3 alloys

We study the electronic and local structural properties of pure and In-substituted -Ga2O3 using density functional theory. Our main result is that the structural energetics of In in Ga2O3 causes most sites to be essentially inaccessible to In substitution, thus limiting the maximum In content to somewhere between 12 and 25 % in this phase. We also find that the band gap variation with doping is essentially due to "chemical pressure", i.e. volume variations with doping.

Surface morphology and electronic structure of bulk single crystal

2000

First-principles study of the structural, electronic, and optical properties of Ga2O3 in its monoclinic and hexagonal phases

Physical Review B, 2006

We report the results of a comprehensive study on the structural, electronic, and optical properties of Ga 2 O 3 in its ambient, monoclinic ͑␤͒ and high-pressure, hexagonal ͑␣͒ phases in the framework of all-electron density functional theory. In both phases, the conduction band minimum is at the zone center while the valance band maximum is rather flat in the k space. The calculated electron effective mass m e * / m 0 comes out to be 0.342 and 0.276 for ␤-Ga 2 O 3 and ␣-Ga 2 O 3 , respectively. The dynamic dielectric function, reflectance, and energy-loss function for both phases are reported for a wide energy range of 0 -50 eV. The subtle differences in electronic and optical properties can be attributed to the higher symmetry, coordination number of Ga atoms, and packing density in ␣-Ga 2 O 3 relative to that in ␤-Ga 2 O 3 .

Atomic-scale characterization of structural and electronic properties of Hf doped β-Ga2O3

Applied Physics Letters

In this letter we investigate the atomic and electronic structure of Hf-doped β-Ga2O3 single crystal using high resolution scanning transmission electron microscopy imaging and electron energy loss spectroscopy. UV-vis-NIR absorption measurements and density functional theory calculations are performed to further connect the nanoscale observation to the macroscale properties arising from the atomic structure. The Hf-doped sample was grown from the melt with a nominal Hf concentration of 0.5%at. We show that the Hf dopants prefer to occupy octahedral over tetrahedral sites by 0.68 eV and have some resistance to form precipitates due to a repulsive interaction of 0.17 eV between Hf atoms on neighboring sites. Also, the presence of Hf atoms on either tetrahedral or octahedral sites do not significantly affect the crystal structure of β-Ga2O3. Finally, the bandgap values of the Hf doped β-Ga2O3 obtained by EELS and UV-Vis-spectroscopy were Eg = 4.83 ± 0.1 eV and 4.75 ± 0.02 eV respectively, similar to the values reported for unintentionally doped β-Ga2O3 crystals. All these results make Hf an excellent dopant candidate for β-Ga2O3. The most thermally stable polymorph of Ga2O3, beta-gallium oxide (-Ga2O3), is an exciting semiconductor that combines an ultrawide bandgap (Eg ~ 4.8 eV) with a reasonable mobility (~ 100 cm 2 /V) and a high breakdown field with a predicted value of 8 MV/cm: properties which makes it a great candidate in high-power electronics, optical devices, and gas sensing detectors [1, 2]. In addition, the flexibility and tunability of its electronic properties, that can be achieved through doping, makes it an extremely promising candidate for future electronic device design [3]. Several dopants for -Ga2O3 have been studied, including Si, Sn, Ge, Ta and Nb, resulting in free-electron densities ranging from 1 x 10 17 to 2 x 10 19 cm-3 and mobilities from 25 to 130 cm 2 /V•s [4, 5, 6, 7, 1, 8]. These dopants thus have been shown to allow tunable ntype conductivity in -Ga2O3 [4, 5, 6, 7, 1], enabling applications like charge-transfer devices and optoelectronic devices. Saleh et al. have recently studied Zr as an alternative dopant and demonstrated tunable n-type conductivity in Zr-doped bulk -Ga2O3 grown from the melt, with

Compensation of Shallow Donors by Gallium Vacancies in Monoclinic β - Ga2O3

Physical Review Applied

Knowledge of the origin of deep levels and their impact on electrical properties is critical for device applications of β-Ga 2 O 3. By annealing under an oxygen (O 2) atmosphere, the resistivity in shallowdonor (zirconium) doped β-Ga 2 O 3 :Zr single crystals is found to increase by more than 10 orders of magnitude to (7 ± 4) × 10 10 cm, which is comparable to the resistivity achieved by iron (Fe) acceptor doping of (5 ± 3) × 10 11 cm. We combine thermoelectric effect spectroscopy and positron annihilation spectroscopy (PAS), which are sensitive to deep levels and concentration of open-volume defects, with modeling of the electrical properties, to study these strongly compensated crystals. We find the compensating level in the O 2-annealed β-Ga 2 O 3 :Zr sample to be located at (0.727 ± 0.021) eV (E2*) below the conduction band, which correlates with a vacancy signal from PAS data. The defect is most likely the relaxed split Ga vacancy V i Ga , rather than a simple gallium vacancy, considering theoretical predictions of a small energy barrier to relax. We observe that, due to the unique nature of these vacancies and anisotropy in the monoclinic lattice, the Doppler-broadening parameter is rather small compared with other widegap compounds, and in such a case the positron diffusion length is a suitable parameter to estimate the open-volume defect concentration.

Energetics and migration of point defects in Ga2O3

Physical Review B, 2005

The results of a theoretical study on the point defects of monoclinic ␤-Ga 2 O 3 are reported here. The point defects considered here are vacancies, interstitials together with dopant ions such as Be, Mg, In, Cr, Si, Ge, Sn, and Zr. Since the low symmetry of the monoclinic lattice does not provide an unambiguous location of interstitial sites and migration paths, we propose a unique way for their identification in terms of the electron density topology. Special attention has also been given to the preference among the lattice and interstitial sites for the impurity defects, and its explanation in terms of structural, electrostatic, and electron density arguments. The calculated results find the most prominent features in the lattice to be the existence of ͑i͒ empty channels along the b direction, and ͑ii͒ atomic layers perpendicular to them. Their interplay governs the stability and mobility of the point defects in ␤-Ga 2 O 3 . The anionic Frenkel pair consisting of the oxygen vacancy and oxygen interstitial is predicted to dominate the defect structure in the lattice. The dopants considered here are likely to be stabilized at the octahedral gallium sites, except for Be +2 , which prefers a tetrahedral gallium site in the lattice. Some of the possible migration paths have been determined, and the pseudoactivation energies for the intrinsic, oxygen-rich, and oxygen-deficient conditions are computed as a function of temperature. It is suggested that tuning the concentration of oxygen can lead to a change in the anisotropy of the ionic conductivity in ␤-Ga 2 O 3 .

First-Principles Studies for Electronic Structure and Optical Properties of Strontium Doped β-Ga2O3

Micromachines, 2021

The crystal structure, electron charge density, band structure, density of states, and optical properties of pure and strontium (Sr)-doped β-Ga2O3 were studied using the first-principles calculation based on the density functional theory (DFT) within the generalized-gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE). The reason for choosing strontium as a dopant is due to its p-type doping behavior, which is expected to boost the material’s electrical and optical properties and maximize the devices’ efficiency. The structural parameter for pure β-Ga2O3 crystal structure is in the monoclinic space group (C2/m), which shows good agreement with the previous studies from experimental work. Bandgap energy from both pure and Sr-doped β-Ga2O3 is lower than the experimental bandgap value due to the limitation of DFT, which will ignore the calculation of exchange-correlation potential. To counterbalance the current incompatibilities, the better way to complete the theoretical...

Surface relaxation and rumpling of Sn-doped β−Ga2O3(010) (original) (raw)

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