Observation of InGaAs / InAlAs Surface Quantum Wells by Photoreflectance and Photoluminescence Excitation Spectroscopies
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S. MONEGER *a, C. BRU *, T. BENYATTOU *, G. GUILLOT *, E. TOURNIE **b AND K. PLOOG **c * Laboratoire de Physique de la Matiere, URA CNRS 358, INSA-LYON, F69621 Villeurbanne cedex (France) ** Max-Planck-Institut fuir Festk6rperforschung, D-70569 Stuttgart (Germany)
ABSTRACT The samples under study consist of strained InAs/In0 .52AI 0 .48As single quantum wells, grown on InP substrates by molecular beam epitaxy. All samples were capped by a 15 nm-thick lattice matched InGaAs layer. By photoreflectance investigations, transitions which do not arise from the strained InAs quantum well have been observed. They have been interpreted as transitions from a surface quantum well formed by the InGaAs cap layer confined between the InAlAs barrier layer on one side and the sample surface on the other side. Photoluminescence excitation on the photoluminescence peak attributed to the first level of this well has been performed at 5 K. The obtained spectra are typical of a two-dimensional system. A modelisation of this structure is proposed and theoretical calculations of the expected quantum levels have been performed and fit well with experimental results. Finally, etching of the InGaAs cap layer has been done to confirm the origin of the observed transitions.
INTRODUCTION The initial aim of this work was to study the InAs/lnAlAs system which has a significant potential for optoelectronic devices [1]. A lot of recent publications have been devoted to the investigation by optical spectroscopy of InAs/InP quantum wells [2,3], but only few papers have been published about optical properties of InAs/In 0 .52A10. 48As system. The first photoluminescence of InAs/InAlAs quantum wells has been observed by Meynadier et al. in 1988 [4] and the first photoluminescence at room temperature by Toumid et al. in 1992 [5]. During the investigation by photoreflectance of such InAs/InAlAs quantum wells we have put in evidence the existence of surface quantum wells formed by the cap layer of these structures. Such surface or near surface quantum wells could be very interesting to study passivation effects as well as the photoinduced processes of electric field screening near the free semiconductor surface.
a) Present address Brooklyn College of CUNY, Brooklyn, NY 11210 (USA) b) Present address LPSES, CNRS, Sophia Antipolis, F-06560 Valbonne (France) c) Presentaddress : Paul-Drude-InstitutfiirFestk~rperelektronik,D-10117Berlin (Germany)
127 Mat. Res. Soc. Symp. Proc. Vol. 326. @1994 Materials Research Society
EXPERIMENT The InAs/InAlAs samples are grown by solid-source molecular beam epitaxy on highly S-doped InP substrates (ND-NA=5.1018 cm- 3 ), exactly (100) oriented. A 550 nm thick In 0 .52A10 .48 As buffer layer is deposited at 450 °C with a growth rate of 0.53 monolayers/sec and an As/group III ratio close to the stoechiometric minimum. The substrate temperature is then decreased to 380 0C for the deposition of the InAs quantum well. Sample well widths are listed in table I. Finally, a 165 nm thick In0 .52A10 .48As barrier and an In 0 .53
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