Charge neutrality level in significantly cation-anion mismatched semiconductors

Abstract

The fundamental bulk and surface electronic properties of a novel class of semiconductors, characterised by a significant mismatch between the size and electro-negativity of the cation and anion (SCAMS), have been investigated. The characteristic examples of CdO, In2O3, and InN were studied using high-resolution x-ray photoemission spectroscopy, infrared reflectivity, optical absorption spectroscopy, and single-field Hall effect measurements. The behaviour of not only defects, dopants and impurities, which dominate the bulk electronic properties, but also surface states was shown to depend on the position of a single energy level, the charge neutrality level (CNL), unifying bulk and surface electronic properties of semiconductors. For the materials studied, the CNL was shown to be located within the conduction band (0.39 eV, »0:65 eV, and 1.19 eV above the conduction band minimum (CBM) in CdO, In2O3, and InN, respectively; see figure) in contrast to the vast majority of semiconductors where the CNL lies within the fundamental band gap (as, for example, in the classic case of GaAs). In CdO, this was shown to lead to native defects, hydrogen impurities and surface states all being donors, even in already n-type material. The donor surface states result in electron accumulation at the CdO surface. Such an electron accumulation is also present at InN surfaces, and this was shown to exhibit a remarkable independence on surface orientation, and to lead to inversion layers at the surface of p-type InN. The changes in surface space-charge regions were investigated across the In(Ga,Al)N composition range, for both undoped and Mg-doped alloys. The influence of the CNL position on interface properties and conductivity in InN was considered. Electron accumulation was observed in In2O3, in contrast to previous reports. Muonium, and by analogy hydrogen, was also shown to be a shallow donor in this material. The location of the CNL above the CBM in SCAMS was used to explain many of their striking bulk electronic properties, such as why materials like In2O3 are able to be conducting despite being optically transparent, two normally contradictory properties. The conclusions drawn from these studies are applicable to a wide variety of other materials, in particular other SCAMS such as ZnO or SnO2. Surface electron accumulation is treated here mainly within a one-electron semi-classical approximation. The final section of this work moves beyond this, using angle-resolved photoemission spectroscopy measurements and theoretical calculations to consider both the quantized nature of an electron accumulation layer, and the influence of many-body effects