Resonance Raman scattering of light in GaAs

Abstract

The Raman scattering of light in GaAs has been investigated using a back scattering technique by many authors. The majority of the papers are concerned with the investigation of the spectra from the surfaces of chips of perfect unalloyed crystals [1-4], of the effects of various different treatments on the surface [5-7], and of external pressure [8] on the contours and positions of the allowed single phonon lines. At the same time the effect of alloying impurities and defects on the resonance Raman spectrum (RRS) has barely been studied at all. In [9, i0] the resonance Raman scattering of light in GaAs alloyed with AI, P, and Si was investigated and it was shown that, upon strong doping with these impurities, additional peaks arise which are attributable to the local modes of these impurities. In the present work we have investigated the effect of disorder in the surface layer and the degree of doping by Te, Si, and Cr impurities on the resonance Raman scattering of light in GaAs. The resonance Raman spectra were investigated using a back scattering method at room temperature. The radiation from a He--Ne laser with k = 632.8 nm (50 mW) which had been focussed into a spot with a diameter of 200-300 ~m was incident along a normal to the surface of the sample. A DFS-24 spectrometer was employed to analyze the scattered light. The effective depth to which the light penetrated into the crystal was of the order of i ~m. The polarization was not specially studied. Since the reflected light, apart from the resonance Raman scattering, also contained lines due to plasma emission from the He--Ne laser and grating "ghosts", the need arose to identify these lines. For this purpose we recorded the plasma radiation lines during reflection from the sample in the absence of the laser llne at 632.8 nm and also the scattering from a glass disk and a metallic mirror with the laser line at 632.8 nm included. The spectrum of specially undoped samples of p-GaAs with a free electron concentration n ~ 2"1016 cm -3 is shown in Fig. i. In order to interpret the observed spectra we recall that GaAs has the zinc blende structure. Its symmetry is characterized by the Td 2 space group. This structure contains two different atoms in the unit cell. Its vibrational spetrum for a selected direction in K-space consists of two branches, an acoustic branch and an optic branch. Threefold degenerate vibrational modes are split into longitudinal modes and twofold degenerate transverse modes. According to the selection rules in the dipole approximation [ii], only the lines of the TO- and LO-phonons with a wave vector close to zero, i.e. To(r) and LO(r), are allowed in the first order in the case of perfect crystals. An interpretation of the observed lines, which was carried out taking account of the data on the coherent scattering of slow neutrons [12], is presented in Table i. It can be seen that 26 lines are found in the Stokes region from 60-500 cm -I in the spectrum of undoped GaAs which is a significantly greater number than has hitherto been observed [1-4]. Together with second-order lines, we also observed single mode resonances which are forbidden according to the selection rules. The first-order lines were also recorded in the anti-Stokes region (Fig. 2). Activation of the first-order lines may be due to thefollowing causes. Since the samples which were investigated had been polished using fine abrasives (d < 0.5~m) there were dislocation irregularities [13] with inhomogeneous lattice deformations and stresses on the surface which was mirrorlike with respect to its external form. Inhomogeneous deformations, on the one hand, reduce the local symmetry of the crystals which leads to a breakdown of the selection Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 41, No. 2, pp. 272-277, August,