Quantization effects in the photoelectrochemistry of superlattice photoelectrodes

Abstract

Size quantization effects have created much recent interest. In solid state physics, for example, the novel effects exhibited by thin layered semiconductor structures in the form of multiple-quantum-wells (MQW) or superlattices (SL) are under intense study1–7. In photoelectrochemistry, quantum-size effects produced by very small (<100 A) semiconductor particles have been analysed8–11. We report here the first quantum size effects produced in a photo-electrochemical system using a superlattice photoelectrode. The electrode contained 40 alternating layers of undoped GaAs and GaAs0.5P0.5 each 250 A thick. This structure produces a strained-layer superlattice (SLS) with 20 GaAs quantum wells having a well width of 250 A; the GaAs0.5P0.5 barrier widths are also 250 A thick. The barrier heights for electrons in the conduction band and for holes in the valence band for this SLS are 0.28 eV and 0.30 eV, respectively6. The photocurrent–wavelength response shows remarkable structure at room temperature that arises from the presence of discrete energy levels in the GaAs quantum wells. This structure is readily resolvable into multiple photocurrent peaks that are in excellent agreement with theoretical predictions of the discrete energy levels in the quantum wells. These results have important implications for photoelectrochemical energy conversion and for the characterization and analysis of SL and MQW structures in general.