Mechanisms of Intersubband Transition in n-Type III-V Quantum Well Superlattice and Improvement on Absorption for TE Pol
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ABSTRACT In this paper, the theoretical study of intersubband transitions in quantum well infrared photodetectors (QWIPs) applying the eight bands k.p model incorporated with envelope function approximation is described. The focus of the work is on the intersubband transition in n-type IIIV QWIP based on AIGaAs/GaAs and AlGaAs/InGaAs material system, with particular emphasis placed on the physics of TE excited transition and the improvement of resulted absorption. Various theoretical absorption spectra of the two material systems are compared, the distinct mechanisms of the intersubband transition for the two material systems are proposed. Possible ways of improving on the absorption for such excitation are also investigated and discussed. INTRODUCTION The response of QWIP to TE polarized field would determine the efficiency of QWIP in responding to normal incident infrared photons, thus simplifying the fabrication of QWIP array. However, the possibility of large TE mode infrared response remains controversial, as many theoretical [1-6] and experimental [7-10] results obtained in recent years have been contradictory. The proper understanding of the mechanism of TE field absorption is important, as it not only provide insight into the physics, but also facilitates the engineering of QWIP. An eight bands k.p model incorporated with envelope function approximation had been accomplished [11-14], with the effects of strain included. In this work, the valence band envelope functions were expressed analytically in terms of envelope functions associated to the zone-center conduction band edges, and the Schroedinger-like equations were derived for the envelope wavefunctions. Analytical forms of the energy dependent effective mass and the effective potential for conduction band electron can be easily identified from these equations. The model had been used to study the intersubband transition due to TM and TE polarized fields in n-type QWIP made of AIGaAs/GaAs and AIGaAs/InGaAs material system. The unique differences and mechanisms in the absorption spectrum for the two systems were identified. The material systems were found to influence the absorption spectrum through the bulk momentum interband transition matrix element, and the band gaps. Well and barrier regions were also found to play very different roles in the absorption of TE polarized field for the two systems. This understanding will help to shed some light on the controversial question about the TE absorption in n-type QWIP and can also lead to several possible ways of improving TE absorption via material and the structure that alters the symmetry of the wavefunctions. THEORY The band structure of III-V compound semiconductor exhibits the symmetry of Td group. The electron in III-V semiconductor compound superlattice forms the sp 3 hybrid bond [16] as a result of the mixing of s-type conduction band and three p-type valence bands. The inclusion of 223 Mat. Res. Soc. Symp. Proc. Vol. 607 ©2000 Materials Research Society
spins results in eight bands, i.e., double
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