Band Alignment at Oxide Semiconductor Interfaces
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B9.10.1
Band alignment at oxide semiconductor interfaces F. Säuberlich, A. Klein Institute of Materials Science, Darmstadt University of Technology Petersenstrasse 23, 64287 Darmstadt, Germany
ABSTRACT Transparent conductive oxides (TCOs) are important contact materials in thin film solar cells. It is thus important to understand their basic interface properties as the band alignment. We present results on the determination of interfacial properties of TCOs using photoelectron spectroscopy. Large interface dipole potentials are generally observed at interfaces between conducting oxides ZnO, SnO2, In2O3 and TiO2 and chalcogenide semiconductors CdS, CdTe and Cu2S, leading to small conduction band discontinuities in the case of ZnO, SnO2 and In2O3 and to large conduction band discontinuities for TiO2. In addition to the band alignment the Fermi level position at the interface determines the contact properties of TCOs in thin film solar cells.
INTRODUCTION Transparent conductive oxides are essential parts of all thin film solar cells (Fig. 1 (a)) and other optoelectronic devices. The solar cell conversion efficiencies are strongly affected by the properties of the TCOs. While the dependencies of the optical and electrical properties of TCOs are fairly well understood, their surface and interface properties are hardly studied. At semiconductor heterointerfaces the energy bands of the two materials in contact align to each other in a certain way to accommodate the difference in electron affinity (distance between the vacuum energy Evac and the conduction band minimum (ECB)) and band gap (Eg). The discontinuities which develop at the valence band maximum and the conduction band minimum at the interface (∆EVB and ∆ECB, respectively) form barriers for carrier transport across the interface. It is thus important to know the absolute values of these barriers. The simplest way to estimate these is by using the electron affinity rule (EAR), which states that the conduction band offset is given by the difference in electron affinity of the two materials: ∆ECB=χB-χA (see Fig. 1(b)) [1]. This is, however, generally not observed. Interface dipoles exist at most semiconductor interfaces due to charge transfer into electronic states localized at the interface as a result of the heteronuclear chemical interface bonds. This results in a dipole potential δ at the interface, which modifies the band alignment from its value given by the EAR (for review of band alignment at semiconductor interfaces see Ref. [2]).
EXPERIMENTAL Photoelectron spectroscopy is a versatile tool to determine the band alignment at semiconductor heterointerfaces [2]. The experimental procedure is sketched in Fig. 2. Starting from a clean substrate surface, the contact material is stepwise deposited. After each deposition step the sample is analyzed using XPS and UPS.
B9.10.2
(a)
a-Si:H
Ag
CIGS
CdTe
ZnO
metal
CdS
CdTe
dye
organic
(b)
EAR
general δ
Evac
Pt
metal
χA
χB
χB χA
a-Si:H
CIGS
CdS
electrolyte
organic
ZnO/SnO2
Mo
ITO/SnO2
TiO
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