Electrostatic control of charge density in multiple quantum wells

We have fabricated a semitransparent metal gate at the surface of the sample having a thick barrier undoped region and ohmic contacts to the (modulation-doped) multiple-quantum-well (MQW) region to control the charge density in MQWs with minimal electric field in the quantum wells. The charge density was controlled by applying a bias between the gate and the ohmic contact region. The advantage of this technique is that the potential profile of the quantum wells is not significantly affected by the applied bias voltage, i.e., by electrically connecting all the quantum wells we minimized the electric field in the quantum-well region. Applying electric field by sandwiching MQWs between two parallel capacitance contacts has been used in MQW structures, however, in this case, the electric field modifies not only the charge density but also the potential profile of quantum wells. Depletion of the charge in the MQWs up to 90 percent was directly observed from cyclotron resonance measurements and corresponding feature changes were observed in PL. The peak position of the PL signal did not show any significant shift with bias which demonstrates that there is neglegible electric field in the quantum wells. * This work was supported by the ONR-MFEL program.