Short-period GaAs/AIAs superlattices grown along the [110]direction have orthorhombic symmetry and thus exhibit two different dielectric functions e(u) along the main axes perpendicular to the growth direction ([001] and [110] direction). Spectroscopic ellipsometry as well as ab initio calculations have been used to determine the optical properties of these superlattices. The E1 transitions split into two components with diR'erent strengths for the two principal polarizations. This effect can be qualitatively interpreted on the basis of a k p model for the E1 transitions and confinement arguments. The Ez transition for [001] polarization is lower in energy than the one for [110]polarization. In addition, we observe a transition slightly above Ez, which occurs predominantly in [001] polarization. Two other structures specific to the superlattices are identified, in analogy to the case GaAs/AIAs superlattices grown along [001]. Confinement effects on optical transitions for various superlattice periods are also discussed.The linear optical properties of short-period GaAs/A1As superlattices (SL's) grown along the [001] direction have been studied in detail with both theoretical as well as experimental methods. The eH'ects of the additional periodicity have been shown to be important for the dielectric response, producing two structures that are specific to the superlattice and which are thus not found in Al Gai As alloys. 7 Lately, the growth of device-quality GaAs on (110) oriented surfaces and GaAs/A1As (110) superlattices by molecular beam epitaxy (MBE) and Raman studies of these superlattices have been reported. Recent photoluminescence experiments on (110)-oriented quantum wells show an in-plane polarization anisotropy of the lowest optical transitions. ii The GaAs/A1As (110) superlattices are of importance for many applications, such as avalanche devices and optical modulators for integrated optics (see Ref. 8 and references therein). In addition, the nonpolar (110) GaAs surface is known to produce no intrinsic surface states in the fundamental band gap and thus makes it a preferred orientation for the growth of polar-nonpolar interface (e.g. , Si on GaAs) systems. i2 Theoretical work on GaAs/AIAs (110) SL's has focused on the electronic band structure and the crossover from a direct to an indirect gap and on the stability of these SL's. Here, we present both a theoretical and an experimental study of the dielectric functions of (GaAs)"/(AIAs) (110) SL's with emphasis on the orthorhombic anisotropy, which does not occur in GaAs/AlAs SL's grown along the [001] direction. The latter have tetragonal symmetry (optically uniaxial) and thus only two diA'erent tensor components of the dielectric function e(io). The SL's grown along the [110] direction, however, have orthorhombic symmetry, and of the three components of the dielectric function the two perpendicular to the growth direction along the main axes ([001] and [110]directions) are experimentally accessible, whereas for light polarized along the growth d...
We present experimental as well as theoretical data for the linear optical response of symmetrically strained Ge4Si6 [001] superlattices. Ab initio calculations show that they have a direct gap. The complex dielectric function has been measured ellipsometrically. Agreement of the experimental secondderivative spectrum d 2 €i/d(D 2 with the theory is obtained, both for the shape and position of bulklike transitions and for new superlatticelike transitions, after including lifetime effects in the calculated curves through Lorentzian convolution.PACS numbers: 78.65. Gb, 73.20.Dx, 73.60.Gx High-quality Ge/Si strained-layer superlattices (SLS's) have been grown up to a thickness of 200 nm on thin homogeneous Ge^Sii-* buffers, 1 causing the strain to be equally distributed in the Ge and the Si layers ("strain symmetrized"). A stimulus for the investigation and growth of Ge/Si SLS's is the hope to obtain new materials with a direct or quasidirect gap that can be integrated on well-established silicon technology. This was suggested long ago, 2 and recently confirmed with the local-density, pseudopotential method 3 for an overall period of 10 monolayers (ML) and a minimum amount of tensile strain in the Si layers, and demonstrated experimentally by Pearsall et al. A and Zachai et a/., 5 although the details of their interpretation are still under debate. 6 For device applications, the optical response over a wide energy range is of great importance. We present such experimental data here, as well as theoretical calculations that allow a detailed analysis of the observed structures in terms of interband transitions. For the freestanding Ge4Si6 [001] SLS studied here we find various strain-split peaks that can be ascribed to bulklike transitions, not only in the calculations but also in the spectrum measured with the spectroellipsometric method. From the shape and the magnitude of the various peaks, we can estimate the effects of lifetime broadening on these transitions. In addition, we observe several new peaks that can be ascribed to "superlattice" transitions, such as zone-folded transitions that originate from the k z direction (zll[00lD.The calculations are performed within the localdensity approximation (LDA) by means of the linearmuffin-tin-orbitals (LMTO) method. 7 Corrections for the LDA "band-gap problem" were made by adding external potentials on the atomic sites. 8,9 These sharply peaked potentials are included self-consistently in the calculation and yield the correct conduction bands and deformation potentials of the bulk materials Si and Ge. 9 When these correction parameters are transferred to the SLS's, our calculated direct transition energies agree within «0.1 eV with electroreflectance 4,10 and photoreflectance 11 experiments. The SLS structure has been relaxed with a valence force-field scheme. 12,13 The Ge4Si6 SLS has orthorhombic symmetry 14 [space group />2 2 if(/mwfl)], although the unit cell is biaxial. As the difference of the eigenvalues along the two inequivalent [100] and [010] direction...
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