Quantum-Confined Stark Effect in II-VI Semiconductor Coupled Quantum Wells
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QUANTUM-CONFINED
STARK EFFECT IN II-VI COUPLED QUANTUM WELLS
Department F. Schetzina, and J. Z. Yang, Carolina State University, Raleigh, NC 27695
SEMICONDUCTOR of
Physics,
North
ABSTRACT II-VI semiconductor The Quantum-Confined Stark Effect in coupled quantum wells is studied theoretically. It is found that because of the difference in localization of the wavefunctions of the heavy hole and the electron subbands involved, large band gap shifts can be induced by an external electric field for quantum wells with zero-field band gaps in the spectrum region from 0.4 Rm Several potential device applications based on this to 12 gm. effect are proposed. INTRODUCTION The Quantum-Confined Stark Effect (QCSE) in III-V semiconductor quantum wells (QWs) has been studied extensively in recent years, and various device applications based on the effect have been [1-3]. The effect in Hg-based narrow gap II-VI demonstrated semiconductor QWs was recently studied theoretically, and large band gap shifts were predicted in coupled QW structures [4,5]. In this paper we extend our previous work [5) to II-VI semiconductor QWs with zero-field band gaps ranging from 0.4 pim to 12 Vm. Novel device applications are also proposed. QUALITATIVE
ANALYSIS
A typical coupled QW structure chosen for study is shown in In the structure two Hgl-xCdxTe well layers are Fig. l(a). separated by a thin Hgl-yCdyTe (y > x) wider-band-gap barrier layer, and the three layers are sandwiched between two thick CdTe barrier layers. The structure can be repeated many times to form thick samples. The two wells separated by the thin low barrier within each structure are coupled while, because of the thick CdTe barriers, each structure is isolated from one another. The QCSE in the coupled wells can be qualitatively analyzed in the following way: With proper x and y values and layer thicknesses, the electronic subbands in the structure are such two heavy hole subbands, Hi and H2, are above the that the first their valence band edge of the low barrier and therefore wavefunctions are mostly localized in the left-side well and in conduction subband El thb right-side well respectively. The first lies above the low barrier conduction band edge with its wavefunction thus extending over both the wells and the low barrier region. The light hole subband Li is below the valence wavefunction also spreads band edge of the low barrier and its over the entire structure. Its contribution to the QCSE is thus is applied field When an external electric very limited. perpendicular to the well layers in the direction shown in Fig. l(b) (defined as positive bias), an additional potential V(z) exists, where z is the coordinate along the field direction. If the center point of the structure is chosen as z = 0 and V(0) = 0, Mat. Res. Soc. Symp. Proc. Vol. 161. @1990 Materials Research Society
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V(z) first
is then positive when z < 0 and negative when z > 0. The order perturbation to the energy of a particular subband is AE = f IP(z)12 V(z) dz,
where P(z) is the envelope wavefunc
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