Carrier Transport in Thin Silicon Films

Films of p-type and n-type silicon 1–2 μ thick have been epitaxially grown on single-crystal sapphire. The Hall mobilities of films with carrier concentrations varying between 1013/cc to mid-1020/cc have been measured. For carrier concentrations above 1017/cc the mobility decreased with increasing carrier concentration similar to the drop in mobility observed in bulk silicon, indicating the dominance of impurity scattering at higher carrier concentrations. At lower carrier concentrations the scattering from crystalline defects limited the mobility to values approaching 30% of the mobility of bulk silicon. The presence of crystalline defects acting as electron traps was demonstrated by measuring the electron concentration in films of constant donor density but varying dislocation density and noting that the electron concentration dropped as the defect density rose. This has lead to an estimate of the electronically active defect density in thin silicon films grown epitaxially on sapphire of about 1015/cc-1017/cc. Optical absorption data have indicated that there is considerable absorption at energies below 1.1 eV.