Comparison of the Subband Transitions in Asymmetric and Symmetric GaAS/In x Ga 1-x As/Al y Ga 1-y AS Quantum Wells
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ABSTRACT Shallow symmetric and asymmetric GaAs/InGaAs/(Al)GaAs single quantum wells with an indium content of 5% and less, grown by MBE, show striking differences in their photoluminescence spectra. Unlike the symmetric case, the asymmetric quantum wells are found to have no bound ground state below a certain thickness, which depends on the indium content of the quantum well and the aluminium content of the barrier. For a set of asymmetric samples investigated here, the bound state vanishes at about 60A. Photoluminescence excitation measurements on shallow symmetric and asymmetric wells show a series of narrow lines with oscillator strengths many times exceeding those of the band to band transition. These lines can be identified with excited excitonic transitions including heavy and light holes.
INTRODUCTION
The optical investigation of symmetric single quantum wells, fabricated by various epitaxial growth techniques, has been a subject of interest for many years. The properties of the subband transitions in these structures are well understood [1-2]. As shown in text books on quantum mechanics [3] a rectangular quantum well always has at least one bound level if both potential barriers U 1, U2 are identical (symmetric quantum well: U1 = U2 ). In the case of an asymmetric quantum well, i.e. different barriers U 1 • U2 , the following condition has to be satisfied for the existence of a bound level of a particle with the mass m in a quantum well of the width Iz [3]: tz.X[•'• _
-'_
2
i1
arcsinFIVU2
This shows that for asymmetric quantum wells with different states are expected below a certain well thickness Iz0. For a barrier and U2 = 300meV for example, no bound conduction band state is well width of 5.5nm. lzo depends on the ratio UI/U 2 which is a asymmetry. 579
barriers non-bound ground configuration U 1 = 30meV expected for GaAs below a measure of the strength of
Asymmetric quantum wells have been investigated up to now predominately for HEMT applications [4]. In contrast there have been only few investigations on luminescence properties of these systems, regarding e.g. surface quantum wells [5-6] and asymmetric semiconductor structures [7]. In the present work, we investigate the optical transitions in asymmetric quantum wells with semiconductor-semiconductor interfaces on both sides. In order to obtain a significant ratio U11U 2 , we have fabricated asymmetric GaAs/InGaAs/AIGaAs quantum wells, with a very small indium content, which are compared to shallow symmetric GaAs/InGaAs/GaAs wells. The quantum wells were characterized by photoluminescence and photoluminescence excitation spectroscopy regarding the well widths dependence of the optical transitions.
MOLECULAR BEAM EPITAXY For a systematic investigation of the optical transitions in symmetric and asymmetric quantum wells we have grown a series of undoped structures with various single quantum well widths down to 15 A and an indium content between 3% and 5%. As described above for these shallow quantum wells a significant influence of the barrie
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