Ellipsometric characterization of random and random-dimerGaAs−AlxGa1−x

Abstract

We studied the optical properties of ordered and intentionally disordered GaAs-Al x Ga 1Ϫx As superlattices, with and without dimer-type correlations in the disorder, by means of spectroscopic ellipsometry. The electronic structure of the superlattices has been calculated and compared with the experiments. The optical transitions in ordered, correlated, and uncorrelated disordered superlattices show specific features that we relate to their different electronic structures. DOI: 10.1103/PhysRevB.66.193310 PACS number͑s͒: 73.20.Jc, 73.21.Ϫb, 78.66.Ϫw Latest advances in nanotechnology make it possible to grow artificial semiconductor superlattices ͑SL's͒ with tailored physical properties. This feature opened the possibility to verify experimentally former theoretical predictions, such as Stark-Wannier ladders, 1 Bloch oscillations, 2 Anderson localization by uncorrelated disorder, 3 and electron delocalization by correlations in the disorder. 4 Concerning this last finding, photoluminescence ͑PL͒ and electron-transport experiments in intentionally disordered SL's showed that spatial correlations of the disorder lead to electron delocalization of states in low-dimensional systems, 4,5 as previous theoretical calculations suggested, 6 in contrast to the earlier belief that all eigenstates might be localized. This is a validation of basic physical phenomena, i.e., the inhibition of Anderson localization in quasi-one-dimensional systems with correlated disorder, predicted theoretically at the beginning of the 1990's. [7][8][9] Spectroscopic ellipsometry ͑EL͒ is an optical technique that measures the complex ratio ˜ϭ r / i between the polarization states of the reflected and incident waves r and i , respectively. In the case of ambient-bulk material interfaces described by the two-phase model, this ratio is directly related to the dielectric function of the material. In GaAs-Al x Ga 1Ϫx As SL's, being multilayered structures, the two-phase model no longer applies. Nevertheless, it is useful and very common to represent the complex reflectance ratio as a pseudodielectric function in a two-phase model since this allows a direct comparison with the data for bulk GaAs and AlAs. This procedure has been successfully used by other authors to study dielectric and optical properties of ordered GaAs-AlAs SL's. [10][11][12] In the present work, we report on EL measurements performed in ordered and intentionally-uncorrelated and correlated-disordered GaAs-Al 0.35 Ga 0.65 As SL's. We present a physical interpretation of the EL spectra in the energy region close to the near-band-edge optical transitions on the basis of their different electronic states. In particular, the EL experiments provide further support of previous theoretical claims that correlated and uncorrelated disordered SL's exhibit a remarkably different electronic structure. 6 We characterized three undoped SL's grown by molecular beam epitaxy. All the SL's have 200 periods and Al 0.35 Ga 0.65 As barriers 3.2 nm thick. In the ordered SL ͑OSL͒ all the 200 wells are...

Spin-valley polarization and tunneling magnetoresistance in monomer, dimer, and trimer magnetic silicene superlattices

“…However, that one seed is repeated in dimer and trimer fashion multiplies (fragments) the transmission minibands and minigaps in a specific energy region at the same pace. Random dimer and trimer semiconductor superlattices have also been investigated experimentally [27][28][29][30][31] and theoretically [32][33][34][35][36][37][38][39]. For instance, the correlated disorder in random trimer InAs/InGaAs superlattices causes the delocalization of electron states owing to trimer tunneling states [40].…”