Resonance of one-phonon and two-phonon states in mixed semiconductors

Abstract

RESONANCE OF ONE-PHONON AND TWO-PHONON STATES IN MIXED SEMICONDUCTORS V. V. Artamonov, M. Ya. Valakh, A. P. Litvinehuk, V. I. Sidorenko, and A. M. Yaremko UDC 535.343.2 Studies in recent years have revealed multifaceted resonance phenomena in the interaction of exeitons, biexcitons, plasmons, electronic excitations, and phonons in solid materials [1-5]. Interesting anomalies in the resonant anharmonic interactions have been observed in a study of polar[ton Raman scattering in crystals which lack a center of inversion symmetry [6] and in "soft" phonon modes in ferroelectric crystals [7]. In the case of polariton resonance the energy mismatch of the resonating excitations may be varied by changing the scattering angle, which in turn changes the polariton frequency. The resonance conditions in ferroelee- trics are varied by changing the crystal temperature. In this work we present the results of experimental and theoretical studies of the anharmonic resonance of phonon excitations in mixed II-VI semiconductors. In semiconducting crystals the phonon branches typically exhibit significant dispersion and, consequently, there is a broad band with a complex frequency distribution function. Therefore the spectral manifestation of the resonance of discrete and continuous excitations in such materials is rather complex and in many cases may exhibit a Fano contour [8 ] with its characteristically asymmetric bandshapes and a sharp valley in the intensity between them (the so-called "Fano antiresonance"). Since the resonant interaction is determined to a significant degree by the energy mismatch and by the relationship of the intensities of the resonating states [ 1, 8, 9], mixed sere[conducting crystals are conve- nient systems for study. In crystals with a one-mode character the rearrangement of the phonon spectrum as the composition is changed includes a significant change in the frequency of the long wavelength optic phonons. Therefore, it is most likely that in some interval of composition the one-phonon state will cross a band of two- phonon excitations of the appropriate symmetry. In the case of crystals with a two-mode character, the rear- rangement of the spectrum consists mainly of a redistribution of the intensities of the phonon lines due to scattering by the individual components, which also may influence the resonance phenomena. EXPERIMENTAL RESULTS Experimental studies of Raman scattering were carried out in a right angle geometry on an apparatus based on a DFS-24 spectrometer. The light source utilized was a He-Ne laser (~ = 632.8 nm). An FEU-79 photomultiplier was used as a detector, with a photon-counting system. In Fig. 1 we show the tlaman spectra of crystals of Zn1_xCdxSe for various compositions. In the compo- sition range 0 _< x _< 0.4 they are characterized by the structure of sphalerite. ]Preliminary experimental data on the resonance phenomena in this system have been reported previously [10]. In Zn0.89Cd0.11Se lines are ob- served at ~ = 140, 186, 204, and 250 cm -1 (curve 1, Fig. la). The latter two are due to scattering by the long lines (symmetry A1) are due to scattering processes involving simultaneous excitation of two TA phonons at the special points X and W in the Brillouin zone [11]. In the polarization studied (yy), corresponding to the fully symmetric component A1, the two-phonon peak at ~ = 140 cm -1 has a significant intensity. The appear- ance of the bands of F2 symmetry in the spectra is due to incomplete polarization of the exciting and scatter- ing light and also to some lifting of the selection rules caused by the disorder of the mixed crystals. As the CD content of the mixed crystals is increased the frequency of the long wavelength LO phonons decreases monotonically to ~ = 232 cm -t for x = 0.60 (curves 1-3, Fig. la). No significant change in the in- tensiW, halfwidth, or shape of this band is observed as this takes place. At the same time the changes in the vicinity of the transverse optical vibrations are significant. The TO band, which approaches the sharp two- phonon peak at 186 cm -~, causes a significant increase in the intensity of the latter (curve 2, Fig. la). In other words, there is a transfer of intens[ W between the one-phonon and two-phonon peaks. Such a process is known Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 38, No. 2, pp. 272-279, February, 1983. Original article submitted January 4, 1982. 0021-9037/83/3802-0223507.50 9 1983 Plenum Publishing Corporation 223