Electrical and optical bandgaps of Gex Si1-x strained layers

Theoretical and experimental evidence is presented to show that the effective mass of holes is reduced due to strain in the Ge_x Si_1-x layers grown on Si(100) substrate. It is shown theoretically that due to this change in the hole effective mass, the reduction of bandgap of a heavily doped (i.e., more than ~1×10¹⁸ cm^-3) Ge_xSi_1-x strained layer base determined by measuring the collector current of the heterostructure bipolar transistor (HBT) is smaller than the bandgap reduction obtained from optical measurements. When uncertainties in the value of the mass and in the experimental results are taken into account, agreement between the theoretical and experimental bandgap reduction values is satisfactory. A significant result obtained is that a high Ge and doping concentrations in the base suppress the collector current and make it smaller than the value that would be obtained if heavy doping effects are neglected