Safeen, Mian Akif (2016) Transport Properties of 2DGEs at Oxide Interfaces under the Effect of Light and Electric Field. [Tesi di dottorato]

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Recently, the transport properties of the two-dimensional electron gas (2DEG) formed at LaAlO3/SrTiO3 interfaces has been intensively investigated. In particular, it was demonstrated that light can effectively enhance the carrier density, and that this effect is persistent after turning off the illumination. Another established technique to modify the carrier density relies on the fabrication of three terminal devices, where positive/negative gating voltages respectively determine the filling/depletion of the quantum well. In this thesis the electric transport properties of 2DEGs formed at the interface of LaAlO3/SrTiO3, Gamma-Al2O3/SrTiO3 and LaGaO3/SrTiO3 were probed under two external perturbations, i.e. light and electric field. The photoresponse of different samples under light illumination with wavelength in the range 365-625 nm and at variable intensities was found to increase exponentially as a function of photon energy for all samples. By studying the combined effect of electric field and light at room temperature, it was found that the response to the gate modulation can be enhanced by the light depending on photons energy. Interestingly, the samples showed no variation in resistance to the gate modulation in the presence of UV photons (3.4 eV). The low temperature field effect measurements showed that the virgin state of the samples is persistently changed after the first positive gate polarization. It was shown that, also according to the initial carrier density, the samples can be brought by a positive polarization pulse into a persistent insulating state. Such phenomenon is due to the formation of a “retention state” that is similar, under many aspects, to the analogous state obtained in floating-gate MOSFET devices. In low carrier-density samples, a non-volatile four-order-of-magnitude resistive switching to the insulating state was obtained. We argue that the obtained electrostatic configuration mimicks the charging of the floating gate in a MOSFET-type device. The recovery of the pristine metallic state can be either obtained by a long room-temperature field annealing, or, instantaneously, by a relatively modest dose of visible-range photons. Illumination caused in fact a sudden collapse of the electron system back to the metallic ground state, with a resistivity drop exceeding four orders of magnitude. Our experimental results provide some insight in the fundamental properties of the oxide interfaces that host a 2DEG, and in particular in the nature and role of defects in the optoelectronic response. Furthermore, they allow to envisage the application of oxide interfaces as non-volatile memories with electro-optical control.

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