Volume 39A, number 4 PHYSICS LETTERS 22 May 1972 INFRARED LATTICE VIBRATIONS OF InP-InAs SOLID SOLUTIONS N.P. KEKELIDZE, G.P. KEKELIDZE and Z.D. MAKHARADZE Tbilisi State University, Tbilisi, USSR Received 11 April 1972 Reflection spectra of InP, InAs crystal and their solid solutions of AsxPi_xln have been measured. Generalized model of Chen-Shocldey-Pearson has been used for result interpretation. On the basis of.comparison of the theory with experiment variations of oscillator forces and frequencies characterizing the vibrations of phosphorous and arsenium atoms depending upon x were determined. The local parts of the effective charges of InP and InAs have been calculatud. In recent time the considerable attention is associa- ted with the problem of studying reflection spectra of the disordered systems of A~Bj_~C. Earlier two types of behaviour of mixed polar crystals have been noted. It was reported that crystals of some alloys display the only one reststrahlen band for all values of x. The characteristic frequencies of this band are in- termediate ones for the characteristic frequencies of the last components [1]. The systems of mixed crys- tals displaying such type of behaviour have been called the “one mode” systems. The crystals in which two sharply expressed maxima for the compounds of any composition are observed in the long wave range are called the “two modes” system [2]. In the mixed crystals a type of behaviour being intermediate one between the above discussed “one mode” and “two mode” systems has also been noted [3]. The present paper deals with measurements of re- flection spectra of relatively pure crystals of InP, InAs and their solid solutions of As~Pi_~ In in the range of (22-47)p. Some characteristic reflection spectra of the above crystals are shown in fig. 1. In InP-InAs solid solution reflection spectra two sharply expressed maxima for all values of x are re- vealed. It is noteworthy that earlier the low frequency maximum had not been detected in As~Pi_~In re- flection spectra. In Oswald’s paper [4] only low fre- quency peak has been detected. In order to understand the features of vibrational spectra of As~Pi_~In one should choose the corre- sponding model of interatomic interactions of ionic crystals, providing microscopic interpretation of the obtained results. For this case modified model of Chen-Shockly-Pearson was used [5, 6]. Fig. 1. Experimental reflection spectra of As~Pi_~In solid solutions. By comparing of theory with experiment the fol- lowing was determined: frequencies of longitudinal WL and transverse .oT optical vibrations of the lattice, oscillator force 4irp the spectral dependence of dielec- trical constants e(w) = e~ + ie~ and the spectral de- pendence of e 1(w) and e2(w) for all crystals under investigation as well. Dependence of longitudinal and transverse optical phonon frequencies as a function of As~Pi_~In com- position is given in fig. 2; fig. 3 shows the dependence of oscillator forces upon composition of solid solu- tions. The local part of the effective charges of InAs, e1 = 4.OX 10~-~ CGS units and InP, e1= 5.OX 10—10 CGS units have been calculated. It is interesting to compare the transverse optical phonon frequencies with the data obtained from the theory of local vibrations [7]. In the case of InAs 80 60 40 20 22 26 30 ~ ~ 42 ~ 273