Switching and control of electric field configurations along multi-quantum well structures

Abstract

The phenomenon of high-field domain formation and expansion along a superlattice/multi-quantum well system has been known since the first pioneering works on these structures. Experimentally it has usually been observed by monitoring negative differential resistance oscillations in the electric characteristics (I-V curves) of the samples. It is due to the fact that a uniform field distribution becomes unstable with respect to the formation of high and low electric field domains. When increasing the applied field, one can observe the progressive inducement of additional quantum well periods into the high-field domain, at voltages correlated with the alignment energies and with the periods of the oscillations in the I-V characteristics. Recently, optical tools were used to probe, in a more direct way, the field distributions along such systems. These experiments can be performed using interband or intersubband spectroscopies. The behaviour of electric field configurations along superlattices/multi-quantum well structures is strongly linked to the availability of free-space charge. Heavy doping or strong illumination providing this charge will induce stable field configurations. A supply of charge which is less than needed may result in a spatial oscillation of space charge among different sites. The regime of lowest charge supply can produce transient field domains resulting in spontaneous current oscillations reminiscent of transferred electron devices and Gunn diodes; however, the negative differential resistance driving the effect originates from low-dimensional transport properties rather than from bulk inter-valley transfer. In addition to its importance for transport studies, the presence of space-charge accumulation at definite sites along a superlattice/multi-quantum well system, together with the presence of different field domains, raise the possibility of spatial control, localization and/or modulation of optical nonlinear effects along such structures. We review previous studies, which used intersubband spectroscopy to study field domain switching, and present new results on how to use photoluminescence quenching of interband transitions, under the application of an applied field, to further investigate domain formation and expansion in multi-stack samples.