Ion-beam sputtered indium tin oxide for InP solar cells

The efficiency of indium tin oxide/indium phosphide (ITO/InP) solar cells depends on the rate of deposition of the ITO and the substrate temperature. In the case of rf sputter deposition, both of these parameters should be maintained as low as possible. However, it is still necessary to produce ITO window layers with as low a sheet resistance and high a transmittance as possible. This paper is concerned with the properties of such films fabricated by ion-beam sputtering under the above restrictive conditions. In particular, we have investigated the effects of varying the background partial pressure of oxygen, the film thickness, and the rate of deposition on the electro-optical properties of the films. (i) For very slow deposition rates (∼0.3 Å s−1) and low substrate temperatures (<40 °C), resistivities of <7×10−4 Ω cm have been obtained for films of thickness 250–1200 Å. This was for a partial pressure of oxygen, <10−6 Torr; for a partial pressure of 4×10−5 Torr, the resistivity of the films increased to 2×10−3 Ω cm. (ii) The refractive index increased from 1.95 to 2.00 over the same range of oxygen partial pressure. (iii) For very thin films, <200 Å in thickness, the refractive index decreased to ∼1.75. (iv) Carrier mobility shows a transition from lattice scattering ( μ∼25 cm2 V−1 s−1) to impurity scattering (25< μ<15 cm2 V−1 s−1) for a carrier density n≳3×1020 cm−3. (v) The optical constants vary significantly across the solar spectrum with oxygen partial pressure and film thickness. This should be taken into account in calculations of internal quantum efficiency of solar cells.