Of the available materials for thermophotovoltaic cells, quaternary compounds of the GaSb family have the advantage of optimal band gap and excellent minority carrier transport in InGaAsSb/GaSb TPV diodes. To take full advantage of these devices high quality transparent ohmic contacts are required. At present the front contact metallization relies on the use of photolitographically patterned thin film metallic structures that introduce a shadowing loss. The use of transparent conducting oxides (TCOs) could potentially eliminate this loss. However, TCOs may introduce other losses due to electrical resistance.
The most successful oxide to date for such application has been indium tin oxide, which has seen widespread use in thin photovoltaic cells. However, no results have been reported on its use over wider spectral range. Moreover, recent studies of other oxides of the same family, such as GaInO₃ and CdO indicate that other material systems may offer better transparency in mid-infrared range.
In this work comparative studies of transparent ohmic contacts to GaSb-based materials have been performed. Thin conducting films of ITO, SnO₂, CdO, and ZnO have been deposited by reactive magnetron sputtering. (100) GaSb wafers and LPE-grown InGaAsSb and AlGaAsSb epilayers were used as substrates. The influence of parameters of deposition on electrical and optical properties of TCO films was assessed by measurements of resistivity and photospectrometry. Electrical properties of TCO/semiconductor contacts were extracted from I-V and contact resistivity measurements. Contact microstructure was analysed using XRD, SIMS, SEM, and AFM.
We show that highly transparent (T>90% in the spectral range up to 2.4 μm) TCO films have been produced enabling fabrication of ohmic contacts with the specific resistance about 10^-4 Ωcm² to GaSb-based semiconducting materials.
Work supported by the grant from the State Committee for Scientific Research 3 T11B 009 026.