Conduction Band-Tail Estimated from the Temperature Dependence of X1–X3 Threshold Energy in Heavily-Doped n-GaP

Abstract

Temperature dependences of the threshold energy, δ, between the X1-band with effective mass m1* and the X3-band with m2*, and the ratio m2*/m1* were obtained for the donor(Te)-doped GaP with a concentration of ND=1.8∼60×1017 cm-3, from the X1–X3 absorption bands measured at various temperatures T and the theoretical curve fittings. δ increases smoothly with decreasing T for 135<T<300K for the lightly-doped sample, whereas δ shows anomalous increase below about 190K for the heavily-doped samples. This anomaly is explained by the electrons falling deep into the band-tail. It can be confirmed by the calculations of electron energy distributions in the parabolic part and band-tail of the conduction band using a density-of-states model. The calculations give the average depth of the band-tail as 11 meV for ND=20.9×1017 cm-3.