Band Structure of Gallium Phosphide from Optical Experiments at High Pressure

The effect of hydrostatic pressure on the following optical properties of GaP has been measured at room temperature: the fundamental absorption edge region from 2.2 to 2.7 eV, an infrared absorption band appearing in $n$-type material at 0.3-0.5 eV, peaks in the reflectivity spectrum at 2.8 and 3.7 eV, and recombination radiation in forward-biased $p-n$ junctions at 1.7-2.3 eV. The results have been interpreted by means of a proposed energy band structure in which the conduction band states ${X_1}^c$, ${X_3}^c$, ${{\Gamma }_1}^c$, ${{\Gamma }_{15}}^c$ are located at energies of 2.2, 2.5, 2.8, 3.7 eV, respectively, above the valence band maximum at ${{\Gamma }_{15}}^v$. The following pressure coefficients have been measured (the transition involved is given in parenthesis), where energy is expressed in eV and pressure in ${10}^6$ bars: $E_G({{\Gamma }_{15}}^v\to {X_1}^c)=2.22-1.1P$; $E_0({{\Gamma }_{15}}^v\to {{\Gamma }_1}^c)=2.78+10.7P$; ${E_0}^{\prime }({{\Gamma }_{15}}^v\to {{\Gamma }_{15}}^c)=3.71+5.8P$; $\Delta E_2({X_1}^c\to {X_3}^c)=0.3+1P$. The coefficients of $E_G$ and ${E_0}^{\prime }$ are close to those for the corresponding transitions in Si; that of $E_0$ is close to the corresponding coefficient in Ge. The weak reflectivity peak at 2.8 eV, the direct gap, shifts with temperature at a rate of about -4.6×${10}^{-4\mathrm{}}$ eV/°K, compared to a value of about -5.2×${10}^{-4\mathrm{}}$ eV/°K for the 2.2 eV indirect gap.