Optical Detection of Resonant Tunneling of Electrons in Quantum Wells

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The investigation of vertical transport in superlattices (SL's) and multiple quantum wells (MQW’s) has recently attracted much attention. Intense research was focused on basic quantum effects such as Bloch transport of electrons and holes in minibands,1 coherent and incoherent, resonant, sequential and Zener tunneling2 in double barriers,3,4 SL’s and MQW’s of GaAs/AlAs,5,6 GaAs/AlGaAs,7 and InGaAs/InP.2,8 In addition to the academic interest, the understanding of the mechanisms of escape from and travel through quantum wells is vital for the recently developed and constantly growing family of electro-optical devices using semiconductor quantum wells. Most of them are based on the quantum-confined Stark effect (QCSE), and they include bistable selfelectro-optic effect devices (SEED’s), tunable detectors, electro-absorption modulators and optical logic elements.9 The basic unit of all of these is an epitaxially grown p-i-n diode, with the quantum well layers in the intrinsic region. By reverse biasing the diode, an electrical field is applied on the quantum wells, controlling its optical absorption spectrum. Since operating wavelengths are close to the excitonic absorption peaks, photoexcited charge is created in the quantum wells and is transported to the electrodes. The mechanisms by which this transport occurs are ultimately responsible for the intrinsic maximum speed or operating intensity of these devices.