Band Structure of Gallium Phosphide from Optical Experiments at High Pressure

Abstract

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 $p\ensuremath{-}n$ 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 ${{X}_{1}}^{c}$, ${{X}_{3}}^{c}$, ${{\ensuremath{\Gamma}}_{1}}^{c}$, ${{\ensuremath{\Gamma}}_{15}}^{c}$ are located at energies of 2.2, 2.5, 2.8, 3.7 eV, respectively, above the valence band maximum at ${{\ensuremath{\Gamma}}_{15}}^{v}$. 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: ${E}_{G}({{\ensuremath{\Gamma}}_{15}}^{v}\ensuremath{\rightarrow}{{X}_{1}}^{c})=2.22\ensuremath{-}1.1P$; ${E}_{0}({{\ensuremath{\Gamma}}_{15}}^{v}\ensuremath{\rightarrow}{{\ensuremath{\Gamma}}_{1}}^{c})=2.78+10.7P$; ${{E}_{0}}^{\ensuremath{'}}({{\ensuremath{\Gamma}}_{15}}^{v}\ensuremath{\rightarrow}{{\ensuremath{\Gamma}}_{15}}^{c})=3.71+5.8P$; $\ensuremath{\Delta}{E}_{2}({{X}_{1}}^{c}\ensuremath{\rightarrow}{{X}_{3}}^{c})=0.3+1P$. The coefficients of ${E}_{G}$ and ${{E}_{0}}^{\ensuremath{'}}$ 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\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ eV/\ifmmode^\circ\else\textdegree\fi{}K, compared to a value of about -5.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ eV/\ifmmode^\circ\else\textdegree\fi{}K for the 2.2 eV indirect gap.