Band-Edge Shift and X₁–X₃Absorption Depending on Donor Concentration in GaP

The fundamental absorption-edge and the X 1 –X 3 absorption band (α 1 -band) were investigated at 300 K for n -GaP with various doping levels. The correction of the α 1 -band with the donor-electron-related absorptions can not be neglected for high donor concentrations N D . For increasing N D , the optical gap shrinks as E g10 =2.233-1.39×10 -8 N 1/3 D , the X 1 –X 3 threshold energy δ increases and decreases (increases) the effective mass ratio m 2 * / m 1 * (the effective mass m 1 * ), where m 1 * is for the X 1 -band and m 2 * for the X 3 -band. Some discussions are made regarding the apparent increase in m 1 * . The increase in δ is attributed to a downward shift of the conduction band-edge (X 1 ). Such shift of ∼5 meV evaluated experimentally is close to the shift of 8.8 meV calculated by Mahan's model with increasing N D from 2×10 17 to 2×10 18 cm -3 .