Electrical and Optical Properties of High-Resistivity Gallium Phosphide

Static and dynamic photoconductive properties of single-crystal high-resistivity (compensated) GaP have been studied in the intrinsic and near infrared spectral region at 300°, 77°, and 27°K. Room-temperature resistivities in excess of ${10}^{14}$ Ω-cm have been produced by copper diffusions into either $n$- or $p$-type GaP. Large photoconductivity gains have been measured for intrinsic radiation: Gains in some cases exceeded ${10}^4$ for fields of ${10}^3$ V/cm. The photoconductivity is strongly influenced by traps. In $p$-type material infrared radiation produces stimulation of the dark conductivity. But in $n$-type material, at 300°K, infrared radiation stimulates only the dark conductivity and quenches the intrinsic photoconductivity at the same photon energies. Thermal-probe measurements indicate that the stimulations are due to increased hole current. At 27°K, only stimulation is observed regardless of the level of intrinsic photoconductivity, but this latter response is due to increased electron current. At the intermediate temperature of 77°K, stimulation (electron and hole) and quenching are present, and the over-all behavior is more complex. Also discussed are room-temperature measurements of space-charge-limited currents which indicate the presence of deep electron traps. An energy-level diagram is presented which can explain these observations. Principal features include electron traps about 0.6 eV below the conduction band, recombination levels near the center of the band gap, and "sensitizing" hole traps about 0.7 eV above the valence band. In addition, measurements of photovoltaic currents and photocapacitance of surface-barrier junctions are presented and shown to represent independent verification of the presence and energy of the "sensitizing" hole-trap levels and the polarity of the charge carriers released by the infrared radiation: Extrinsic photovoltaic response is present and is stimulated by intrinsic radiation in $n$-type material only; strong infrared quenching is seen of intrinsic photovoltaic currents in $p$-type material and of intrinsic photocapacitance in $n$-type material.