Crystals of S-doped material, compensated by in-diffused Cu, exhibited gap modes at 272.5 and 266.2/cm which arose from the isolated ionized single donors, 32SP+ and 34SP+, respectively. New gap modes at 311.5 and 304.7/cm were attributed to 2 isotopes of S which were present as SP+-CuGa2- nearest-neighbour pairs that were single acceptors. The 32SP+-CuGa2- pair also gave rise to a localized vibrational mode at 408.1/cm. There was evidence that pairing ceased when electrical compensation was achieved: that is, when [SP+] = [SP+-CuGa2-]. A comparison of experimental frequencies, with the values simulated by using Green’s functions methods (based upon ab initio perfect lattice modes), showed that the stretching-force constant of the S-Ga bond increased when pairing occurred. The strong force constant of the S-Cu nearest-neighbour pair explained the presence of the localized vibrational mode. The SAs+-CuGa2- pairs in GaAs also gave rise to a localized vibrational mode, but isolated SAs+ donors did not.

Vibrational Modes of Sulphur-Copper Donor-Acceptor Pairs in GaP: Effects of Increasing Local Force Constants by Impurity Pairing. E.G.Grosche, M.J.Ashwin, R.C.Newman, D.A.Robbie, M.J.L.Sangster: Journal of Physics - Condensed Matter, 2001, 13[10], 2117-25