Valence-band density of states and surface electron accumulation in epitaxialSnO2films

S K Vasheghani Farahani,G W Watson,C F Mcconville,T D Veal,M E White,J S Speck,D O Scanlon,J J Mudd,O Bierwagen

doi:10.1103/physrevb.90.155413

Abstract

The surface band bending and electronic properties of SnO2(101) films grown on r-sapphire by plasma-assisted molecular beam epitaxy have been studied by Fourier-transform infrared spectroscopy (FTIR), x-ray photoemission spectroscopy (XPS), Hall effect, and electrochemical capacitance-voltage measurements. The XPS results were correlated with density functional theory calculation of the partial density of states in the valence-band and semicore levels. Good agreement was found between theory and experiment with a small offset of the Sn 4d levels. Homogeneous Sb-doped SnO2 films allowed for the calculation of the bulk Fermi level with respect to the conduction-band minimum within the k⋅p carrier statistics model. The band bending and carrier concentration as a function of depth were obtained from the capacitance-voltage characteristics and model space charge calculations of the Mott-Schottky plots at the surface of Sb-doped SnO2 films. It was quantitatively demonstrated that SnO2 films have downward band bending and surface electron accumulation. The surface band bending, unoccupied donor surface-state density, and width of the accumulation region all decrease with increasing Sb concentration.

Highlights

In materials with crystal structures, the perfect periodicity is broken at the surface, which introduces surface states with complex Bloch wave vectors
The reflectance was measured at an incident light angle of 56◦ with respect to the surface normal and simulated assuming a three-layer and a fourlayer stratified medium for the unintentionally doped (UID) and Sb-doped SnO2 films, respectively
We present a detailed experimental and theoretical study of the surface electronic properties of high-quality SnO2(101) films grown on r-sapphire by plasma-assisted molecular beam epitaxy (PAMBE)

Summary

INTRODUCTION

In materials with crystal structures, the perfect periodicity is broken at the surface, which introduces surface states with complex Bloch wave vectors. Binary TCOs that have attracted intensive research into their optical and electronic properties, namely, In2O3 [8,9], CdO [10,11,12,13,14], ZnO [11,15,16], and SnO2 [17,18,19], have all been shown to exhibit surface electron accumulation This follows from the fact that the charge neutrality level (CNL), defined as the boundary energy at which defect states change from donor type to acceptor type with reference to the Fermi level, lies above the conduction-band minimum (CBM) in these materials. Variation of the unoccupied donor surface-state density is obtained as a function of Sb-doping level

EXPERIMENTAL AND THEORETICAL METHODS

Infrared reflectance and Hall effect

Electronic density of states of the valence-band and semicore levels

Surface band bending and the bulk Fermi level

ECV measurements and space charge calculations

SUMMARY AND CONCLUSIONS