Optical Studies of Free and Acceptor-Bound Excitons in Gaas/Algaas Symmetric Coupled Double Quantum Well Structures
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with and (b) without an electric field across the double wells. Energy levels and allowed optical
transitions are also indicated. The exciton states are formed from either symmetric or antisymmetric hole and electron one-particle wavefunctions, and are consequently denoted "symmetric" and "antisymmetric" excitons. Both types of excitons have even parity. The splitting of the energy levels due to the coupling between the two wells is very sensitive to the barrier width, but it is also dependent on the well widths. The exciton effects and band mixing in CDQW's have often been neglected in the past, however, recently their importance was realised [1,2,3]. In this report we present an experimental studies of the free excitons and the acceptor bound excitons in CDQW's. EXPERIMENTAL The samples were grown in a Varian Gen II modular MBE machine. Four samples have been used in this study, each sample contains three different CDQW structures with the same thickness of the A10. 3Ga0.7 As barrier layer. Sample A contains three double wells with nominal widths of Lz = 124.5, 79.2 and 39.6 A, respectively, while sample B contains three double wells with nominal widths of Lz = 150.0, 99.1 and 59.4 A, respectively. All the double wells in both samples have nominal A10. 3Ga0.7As barrier widths of Lb =14.2 A. The sample C contains three double wells with nominal width of Lz = 79.2, 59.4 and 39.6 A, and a nominal AI0.3Gao. 7As barrier width of Lb = 19.8 A. For the sample (sample AS 133) used to study the acceptor bound exciton (BE), the Be impurities are always doped in the center of the wells, the doping concentration is 1017 cm- 3 , and the doping volume is 20% of the well width. 383 Mat. Res. Soc. Symp. Proc. Vol. 326. ©1994 Materials Research Society
Sample AS 133 contains three CDQW's with nominal well widths of Lz = 150.0 A, 99.1 A, 59.4 A, respectively, and a nominal A10.3Gao.7As barrier thickness of Lb =14.2 A. The detailed growth conditions of the samples were reported in Refs. 3 and 4. I E
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Forbidden transitions
(a)
(b)
at zero field
Fig. 1. Schematic illustration of the energy levels and radiative transitions in a CDQW: (a) under flat band conditions, and (b) with an electric field applied perpendicular to the layers. The photoluminescence (PL), PL excitation (PLE) and polarized PLE (PPLE) measurements were done in an exchange-gas-type He cryostat, where the temperature could be varied from 1.8 K up to room temperature. The excitation source was either an Ar+ laser (5145 A) or a tunable Sapphire: Ti solid state laser pumped with the Ar+ laser, which covers the wavelength range from 700 to 1000 nm. A double-grating monochromator and a GaAs photomultiplier were used to disperse and detect the PL signals. For the PPLE experiments we have used a photo-elastic modulator (PEM), whereby the intensity difference between a + and 5 - polarization can be measured in PPLE experiments. For the magnetic field
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