Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (2024)

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Electronic structure and surface band bending of Sn-doped $β − {Ga}_{2} O_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations

Jiaye Zhang, Zhenni Yang, Siliang Kuang, Ziqi Zhang, Shenglong Wei, Joe Willis, Tien-Lin Lee, Piero Mazzolini, Oliver Bierwagen, Shanquan Chen, Zuhuang Chen, Duanyang Chen, Hongji Qi, David Scanlon, and Kelvin H. L. Zhang

Phys. Rev. B 110, 115120 – Published 11 September 2024

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$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (1)$

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Abstract

The bulk and surface electronic structures of Sn-doped $β − {Ga}_{2} O_{3}$ thin films have been studied by soft and hard x-ray photoemission spectroscopy (soft PES at 1486.6eV and HAXPES at 5920eV). The experimental spectra are compared with density functional theory calculated density of states in the valence band and conduction band. Excellent agreement was found between experimental spectra and calculated density of states by taking into account the photoionization cross section of different orbitals involved in the valence and conduction bands. The electronic states derived from Ga $4 s$ character are selectively enhanced by HAXPES. This allows us to infer that the states at the conduction band and bottom of the valence band contain pronounced Ga $4 s$ character. The occupation of the lower conduction band in degenerately Sn-doped ${Ga}_{2} O_{3}$ is clearly observed by HAXPES, which allows for direct measurement of Burstein-Moss shift and band-gap renormalization as a function of Sn doping. A comparison of the valence band spectra of Sn-doped ${Ga}_{2} O_{3}$ films with Si-doped samples suggests that Sn doping has different effects on the electronic structure than Si doping. An in-gap electronic state is observed for Sn-doped ${Ga}_{2} O_{3}$ , which is attributed to self-compensating $S n^{2 +}$ related defects. Furthermore, a larger band-gap renormalization is found in Sn-doped samples, because the Sn $5 s$ dopant orbital mixes strongly with the host Ga $4 s$ derived conduction band. Finally, a comparison of the valence band and core-level spectra excited with soft and hard x rays allows us to identify an upward band bending at the surface region of Sn-doped ${Ga}_{2} O_{3}$ films.

$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (3)$
$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (4)$
$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (5)$
$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (6)$
$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (7)$
$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (8)$
$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (9)$

Received 3 August 2023
Revised 7 May 2024
Accepted 14 June 2024

DOI:https://doi.org/10.1103/PhysRevB.110.115120

Physics Subject Headings (PhySH)

Research Areas

Doping effectsElectronic structureSurface states

Authors & Affiliations

Jiaye Zhang^1,*, Zhenni Yang^1,2,*, Siliang Kuang^1,2, Ziqi Zhang¹, Shenglong Wei¹, Joe Willis^3,4, Tien-Lin Lee⁴, Piero Mazzolini⁵, Oliver Bierwagen⁶, Shanquan Chen⁷, Zuhuang Chen⁷, Duanyang Chen⁸, Hongji Qi^2,8,†, David Scanlon^3,9,‡, and Kelvin H. L. Zhang^1,§

¹State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China
²Hangzhou Institute of Optics and Fine Mechanics, Hangzhou 311421, People's Republic of China
³Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom and Thomas Young Centre, University College London, Gower Street, London WC1E 6BT, United Kingdom
⁴Diamond Light Source Ltd., Harwell Science and Innovation Campus, Didcot OX11 0DE, United Kingdom
⁵Department of Mathematical, Physical and Computer Sciences, University of Parma, Parco Area delle Scienze 7/A, Parma 43124, Italy
⁶Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V., Hausvogteiplatz 5-7, 10117 Berlin, Germany
⁷School of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen 518055, People's Republic of China
⁸Key Laboratory of Materials for High Power Laser and Research Center of Laser Crystal, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, People's Republic of China
⁹School of Chemistry, The University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom

^*These authors contributed equally to this work.
^†Contact author: qhj@siom.ac.cn
^‡Contact author: d.scanlon@ucl.ac.uk
^§Contact author: Kelvinzhang@xmu.edu.cn

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Vol. 110, Iss. 11 — 15 September 2024

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$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (14)$
Figure 1
(a) Crystal structure of monoclinic $β$ -phase ${Ga}_{2} O_{3}$ ; ${Ga}_{1}$ and ${Ga}_{2}$ are located at $T_{d}$ and $O_{h}$ coordination, respectively. (b) Out of plane $θ − 2 θ$ XRD pattern at the vicinity of ${Ga}_{2} O_{3}$ (020) reflection. (c) Room-temperature Hall mobility as a function of carrier concentration for Sn- and Si-doped ${Ga}_{2} O_{3}$ thin films. (d) Temperature dependence of resistivity for ${({Sn}_{x} {Ga}_{1 – x})}_{2} O_{3}$ with different x.
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$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (15)$
Figure 2
(a) Soft PES and HAXPES measured valence band spectra of 0.01% Sn-doped ${Ga}_{2} O_{3}$ sample. (b) DFT-calculated total and partial DOS for ${Ga}_{2} O_{3}$ around the valence and conduction band, with instrumental (0.6eV FWHM Gaussian) and lifetime (0.2eV FWHM Lorentzian) broadening. (c) One-electron photoionization cross sections (PICSs) of Ga $4 s$ , Ga $4 p$ , Ga $3 d$ , and O $2 s$ , O $2 p$ as a function of photon energy taken from Yeh and Lindau [33] and Scofield [32]. (d) Shirley background subtracted (top) soft PES and (bottom) HAXPES valence band photoemission spectrum of 0.01% Sn-doped ${Ga}_{2} O_{3}$ sample and the corresponding DFT-calculated total and partial DOS weighted by PICS for ${Ga}_{2} O_{3}$ .The measured valence band photoemission is rigidly shifted to lower energies by 4.95 for soft PES and 5.05eV for HAXPES to align the VBM at 0eV binding energy as for the calculation.
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$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (16)$
Figure 3
(a) The HAXPES measured VB structure and expanded view (80×) of the CB state for ${({Sn}_{x} {Ga}_{1 – x})}_{2} O_{3}$ with different x. (b) Expanded view of the HAXPES measured VB edges. The HAXPES measured (c) Ga $2 p$ and (d) O $1 s$ core level for ${({Sn}_{x} {Ga}_{1 – x})}_{2} O_{3}$ with different x. (e) The HAXPES measured binding energy (BE) shift of VBM (ΔVBM) and core level (ΔCL) are with resepct to the lowest Sn-doped sample, $x = 0.01 %$ . The Burstein-Moss shift (ΔBM) derived from the free-electron model using the $n_{e}$ determined from Hall measurement. The ΔCL is the average of the shift values for Ga $2 p_{3 / 2}$ , O $1 s$ , Ga $3 s$ , and Ga $3 p$ . (f) The schematic diagram for the band structure of the undoped and degenerately doped samples. Doping produces an increase of optical band gap $(E_{opt})$ , consisting of the contributions from the Burstein-Moss shift (ΔBM) and compensating band-gap renormalization (ΔRN); i.e., $Δ E_{opt} = Δ BM - Δ RN$ .
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$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (17)$
Figure 4
(a) The HAXPES measured VB and CB structure for ${({Sn}_{x} {Ga}_{1 – x})}_{2} O_{3}$ and ${({Si}_{x} {Ga}_{1 – x})}_{2} O_{3}$ with different x. Inset: expanded view of gap states and CB states close to the Fermi energy $(E_{F})$ . (b) The diagram of the proposed band edge evolution for undoped ${Ga}_{2} O_{3}$ , and Si-doped ${Ga}_{2} O_{3}$ and Sn-doped ${Ga}_{2} O_{3}$ . Calculated DOS for (c) Sn- and (d) Si-doped ${Ga}_{2} O_{3}$ . The Sn $5 s$ , Sn $5 p$ , Si $3 s$ , and Si $3 p$ partial DOS in VB and CB are multiplied by 10 and 100, respectively; the CB total DOS is multiplied by 10.
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$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (18)$
Figure 5
(a) The schematic diagram of the probing depths for photoelectrons with kinetic energies of 360 and 4800eV, corresponding to Ga $2 p_{3 / 2}$ excited by soft PES and HAXPES. (b) The soft PES and (c) HAXPES measured Ga $2 p_{3 / 2}$ spectra of ${({Sn}_{x} {Ga}_{1 – x})}_{2} O_{3}$ with different Sn concentration. (d) Soft PES and HAXPES measured binding energies Ga $2 p_{3 / 2}$ with different Sn concentration. (e) Soft PES and (f) HAXPES measured VB spectra. Inset: expanded view of CB states $(20 \times)$ close to the Fermi energy.
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$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (19)$
Figure 6
(a) The band bending widths $(W_{t})$ as a function of band bending potentials $(V_{bi})$ up to 1.0eV and comparison with escape lengths of Ga $2 p_{3 / 2}$ spectra from soft PES and HAXPES; (b) a schematic diagram of the band bending width of the 0.01% Sn and (c) 3% Sn samples at the band bending potential of 0.1, 0.2 and 0.4eV compared with the escape length of the Ga $2 p_{3 / 2}$ spectrum of soft PES and HAXPES. The percentage in the figure is the proportion of soft PES or HAXPES signal intensity provided by the band bending region.
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$Electronic structure and surface band bending of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films studied by x-ray photoemission spectroscopy and ab initio calculations (20)$
Figure 7
(a) Fitting diagram of surface energy band bending of the 3% Sn-doped ${Ga}_{2} O_{3}$ . (b) The schematic diagram for surface band bending $(V_{bi})$ of the 0.01%, 1%, and 3% Sn-doped ${Ga}_{2} O_{3}$ . (c) Band line-up with respect to the charge neutrality level (CNL) for ${Ga}_{2} O_{3}$ and other oxide semiconductors including CdO, ${In}_{2} O_{3}$ , ${SnO}_{2}$ , and ZnO. The CNLs of all materials are aligned to 0eV.
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