Raman study of Zn1−xBexSe/GaAs systems with low Be content (x⩽0.20)

Abstract

We present an experimental and theoretical investigation of the Raman line shape of long-wavelength phonons with longitudinal optical (LO) symmetry from both sides of Zn1−xBexSe/GaAs (001) heterojunctions with special emphasis on samples with low Be content (x⩽0.20). First the built-in p-type LO phonon–plasmon (LO–P) coupled mode at the near-interfacial substrate is used as a sensitive probe to investigate the interfacial quality. The corresponding hole gas is reinforced when the ZnBeSe layers are nominally p doped by nitrogen. This provides clear evidence for effective hole transfer across the junction, and thereby indicates a minimized density of interfacial defects. In the nitrogen-doped layers hole densities as high as 1017 cm−3, in accordance with capacitance–voltage measurements, are directly inferred from clear weakening of the ZnSe-like LO mode due to LO–P coupling. Concerning the intrinsic properties of the alloys, we demonstrate that the asymmetric broadening of the ZnSe-like LO mode is determined by topological disorder only, and not by other possible mechanisms such as structural disorder, nonhomogeneity in the alloy composition, a distribution of tensile strain, or a Fano-type interference. This reveals a high structural quality that parallels the high interfacial quality. We also show that a spatial correlation model with Gaussian distribution applies to Zn1−xBexSe. Most of this study is supported by a quantitative treatment. We extend the phenomenological approach of Hon and Faust to equations of motion and polarization derived from the modified-random-element-isodisplacement model in order to achieve line shape analysis of the alloy-related LO and LO–P modes.

Long-wave phonons inZnSe−BeSemixed crystals: Raman scattering and percolation model

“…Recently we have proposed 14 an extension of the Hon and Faust formalism 23 to the equations of motion and polarization given by the MREI model, 2 which is the standard one-bond→ one-mode description for the long-wave A-C and B-C phonons in A 1−x B x C zinc blende alloys. Here we use a simplified form obtained by neglecting the second-nearest neighbors in the per-atom force estimate, i.e., by taking the so-called K ij terms in the MREI notation equal to zero.…”

“…Extensive detail is given elsewhere. 14 The Zn-Se domain is located at lower frequency, i.e., ϳ225 cm −1 . Disorder-activated longitudinal acoustical (DALA) bands at the X and L zone edges, referred as DALA X and DALA L , bring parasitical Raman contributions in this region.…”

“…4,5,14,15 Most of the attention was awarded to the long-wave transverse-optical (TO) modes, because these consist of quasi-independent oscillators, and thereby carry clear information. Complexity arises in the longitudinal-optical (LO) symmetry, due to coupling via the macroscopic polarization field.…”

Optical and vibrational properties of Be-Zn chalcogenide alloys and superlattices

“…In certain alloy systems where the sizes of the anions and cations are radically different one might suspect that preserving the cation-anion bond lengths become increasingly difficult which, in turn, would considerably affect the phonon behavior of the ternary alloys. In addition to the small size, the light mass of Be is expected to cause atypical phonon properties of Be chalcogenide alloys 7,[49][50][51][52][53][54][55][56][57] and SLs compared to the Zn-based II-VI compound semiconductors of similar crystal structure. Consequently, one would anticipate new phonon features to occur in ͑BeX͒ m / ͑ZnY͒ n SLs ͑Refs.…”

“…7,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] Of particular interest are the TO modes as they consist of quasi-independent oscillators exhibiting 1-bond→ 2-mode behavior in the percolation scheme. 7,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] In Be x Zn 1−x Se͑Te͒ alloys this atypical phonon mode behavior is explained recently by Pagès et al 49 by using a double-branch percolation model which discriminates between vibrations within the randomly formed hard Be-rich host region and the soft Zn-rich region. Here we have used a traditional formulation of the multilayer 66 optics with appropriate alloy dielectric functions and calculated the IR reflectance spectra of thin Be x Zn 1−x Se/ GaAs͑001͒ epilayer at an oblique incidence ͑cf.…”