Optical Properties of the Group IV Elements Carbon and Silicon in Gallium Phosphide

Two special open-tube furnace systems, respectively containing all fused silica and all alumina furnace-area components, and in which P is supplied through the reaction of wet H2 with AlP to form PH3, have been constructed for the growth of GaP crystals from Ga solution. Careful attention has been paid to the purification of the PH3 and H2 transport gas. Unwanted contamination from the flow tubes and furnace area has been minimized. The crystals are then suitable for the controlled study of the optical properties of the impurities C and Si, persistent residual impurities in GaP crystals grown under less stringent conditions. The residual concentrations of S and N have also been dramatically reduced in this system. Clearly defined chemical evidence indicates that carbon rather than silicon is the shallow acceptor (ionization energy EA ∼48 meV) in the residual green donor-acceptor pair luminescence spectrum in GaP. Analysis of sharp line donor-acceptor pair transitions observed at the high-energy tail of a broad red luminescence band (peak energy ∼1.96 eV) characteristic of silicon-doped crystals indicates that silicon is a deep acceptor (EA∼204 meV) on P lattice sites and a shallow donor (ED∼80 meV) on Ga lattice sites in GaP. Sharp line green spectra observed for recombinations at C–Si and Zn–Si pairs confirm this value of (ED)Si. The shallow green pair transitions involving Si donors are strongly coupled to ∼17.5 meV and ∼29 meV low-energy phonons, unlike pair recombinations involving P site donors. This marked difference in phonon coupling invalidates judgment of the relative values of ED for S, Te, and Si donors from the relative positions of the peak intensities of the green or red pair spectra involving these donors. Small shifts in the no-phonon discrete pair lines between crystals doped with Si28 and Si30 confirm the suggested role of Si in these spectra.