Infrared reflectivity studies of gax in1-x asy p1-y quaternary compounds

Reflectivity spectra have been measured on Ga_xIn_1-xAS_yP_1-y quaternary alloys, lattice-matched to InP substrates, covering the complete composition range from y = 0 (InP) to y = 1 (Ga_0.47 In_0.53 As). The spectra have been analysed to obtain parameters of both the free carriers and lattice vibrations. The reflectivity method has been used to measure the carrier density and mobility, the relatively low mobilities obtained confirming the electrical measurements explained previously by the presence of strong alloy scattering in this system. Four phonon modes, corresponding to InAs-, GaAs-, InP- and GaP-like vibrations, were observed over most of the composition range, except for y ≲ 0.25 where the GaAs mode was not observed and the GaP mode was a weak mode within the reststrahlen band of the InP mode. Thus the behaviour of the system may be termed "two"-four mode. The oscillator strengths of the four modes are consistent with this behaviour and their variation may be explainable by assuming that Ga_xIn_1-xAS_yP_1-y is a random alloy.