Optical and transport properties of p-i-n GaAs/AlAs resonant tunneling diode
Awan, Iram Taj
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In this thesis, we have investigated the optical and transport properties of a p-i-n GaAs-AlAs resonant tunneling diode (RTD). The possibility of controlling and significantly varying the density of carriers accumulated at different layers of this structure simply by applying an external bias makes it very useful to investigate various fundamental issues. Furthermore, the process of tunneling that critically depends on the alignment from confined energy levels and the injection of carriers that attain quasi - equilibrium distribution at distinct accumulation layers makes this structure very special for analyzing optical properties in general, and in special, spin-polarization effects when an external magnetic field is applied to the RTD. Particularly, two emission bands were observed for the quantum well (QW) of our structure and were associated to the recombination of electrons and holes that tunnel into the QW and may recombine either as an exciton involving the fundamental states of the QW or a transition involving an acceptor state in the QW. It was also observed that the relative intensity of these emission bands strongly depend on the applied bias voltage. The optical recombination involving acceptors states becomes relatively more efficient as compared to the excitonic recombination for higher densities of electrons in the QW. This effect was discussed considering how the electron carrier density depends on the applied voltage and other effects such the capture rate of holes by the acceptors and electron and hole differences concerning mobility, effective mass and tunneling processes. We have also investigated spin properties of the tunneling carriers in our device by measuring the polarization-resolved electroluminescence from the quantum well (QW) and the contact layers under low temperatures and high magnetic fields, up to 15 T. Under these conditions, we have observed that the QW emission presents a large negative polarization degree which depends on the external applied bias voltage. The VII QW spin polarization shows oscillations and abrupt changes at the electron resonant peak. The results are mainly attributed to the abrupt changes of intensity of the two QW emission lines. Furthermore, the contact-layer emission have also shown voltage dependent emission lines that were attributed to the two-dimensional electron gas formed at the accumulation layer under an applied bias. The contact-layer emission presents a large negative polarization degree which is also voltage dependent. The QW spin polarization degree was discussed considering different effects such as the presence of neutral acceptors in the QW, the voltage control of carrier densities in the device, hole and electron tunneling processes and the spin injection of spin polarized two-dimensional gases formed at the accumulation layers.