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

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Abstract
The fundamental absorption-edge and the X1–X3absorption band (α1-band) were investigated at 300 K forn-GaP with various doping levels. The correction of the α1-band with the donor-electron-related absorptions can not be neglected for high donor concentrationsND. For increasingND, the optical gap shrinks asEg10=2.233-1.39×10-8N1/3D, the X1–X3threshold energy δ increases and decreases (increases) the effective mass ratiom2*/m1*(the effective massm1*), wherem1*is for the X1-band andm2*for the X3-band. Some discussions are made regarding the apparent increase inm1*. The increase in δ is attributed to a downward shift of the conduction band-edge (X1). Such shift of ∼5 meV evaluated experimentally is close to the shift of 8.8 meV calculated by Mahan's model with increasingNDfrom 2×1017to 2×1018cm-3.