Effects of Si Incorporation and Electrical Activation on Intersubband Optical Absorption in MBE-grown GaAs/AlGaAs Multip
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EFFECTS OF Si INCORPORATION AND ELECTRICAL ACTIVATION ON INTERSUBBAND OPTICAL ABSORPTION IN MBE-GROWN GaAs/AlGaAs MULTIPLE QUANTUM WELL STRUCTURES J.D. Ralston, H. Ennen, M. Maier, M. Ramsteiner, B. Dischler, P. Koidl, and P. Hiesinger, Fraunhofer-Institut fUr Angewandte Festkbrperphysik, Eckerstrasse 4, 7800 Freiburg, West Germany.
ABSTRACT Using SIMS profiling, temperature-dependent Hall measurements, electronic Raman scattering, and infra-red absorption, a detailed study is presented examining the effects of Si dopant behaviour on intersubband absorption in GaAs/Al 32Ga cAs MQW's. The samples were grown by molecular beam epitaxy at substrate temfiperatures ranging from 520 to 680 0 C with Si doping in the GaAs quantum wells only. SIMS profiling reveals that, with increasing substrate temperature, substantial Si migration into the Al 32Ga As barrier layers occurs during epitaxial growth. Hall measurements indicate thtf bothoat elevated growth temperatures and under reduced incident As4 beam fluxes, the electron sheet concentration is reduced in the QW's. In both cases, loss of free carriers is attributed to enhanced Si compensation. Shifts in the absolute frequency of the infra-red absorption resonance, determined by electronic Raman scattering, as well as variations in the integrated absorption intensity, are both shown to directly reflect the growth-induced variations in the free-carrier concentration in the quantum wells.
INTRODUCTION In the past five years, several groups have successfully demonstrated efficient infrared (IR) detection at 8-12 gm based on intersubband absorption in GaAs/AIGaAs quantum wells (QW's) [1-3]. However, no studies have been presented detailing the important role played by MBE growth conditions in determining the IR optical properties of such structures. The GaAs 3 uantum wells are typically 40-80 Awide, doped with Si to a level on the order of 1018 cm-, and confined between undoped AI•Ga .,As barrier layers with an Al mole fraction x>0.3. In order to achieve the optical ana- electrical quality required for detector applications, substrate temperatures exceeding 600'C are generally necessary during MBE growth. A substantial body of literature exists which indicates that at the above doping levels and substrate temperatures, Si introduced in such a spatially localized fashion undergoes migration during MBE growth over distances on the order of tens to hundreds of Angstroms [4-7]. The resulting unintended doping of the AJGaAs barrier layers may lead to substantial reductions in the actual free-carrier population in the QW's. Such behaviour is expected to arise from enhanced autocompensation [6,8,9] and/or the formation of deep donor levels [6,10,11] in Si-doped AlGaAs. Furthermore, variations in substrate temperature and incident As flux during MBE growth can also alter the net free carrier concentration in GaAs for a fixed Si concentration [12]. The purpose of the present work was to study the effects of Si doping behaviour on the energy and intensity of the intersubband absorption
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