Abstract
The band alignment at an Al2O3/SrTiO3 heterointerface forming a two-dimensional electron gas (2DEG) was investigated using scanning photocurrent microscopy (SPCM) in an electrolyte-gated environment. We used a focused UV laser source for above-the-bandgap illumination on the SrTiO3 layer, creating electron-hole pairs that contributed to the photocurrent through migration towards the metal electrodes. The polarity of the SPCM signals of a bare SrTiO3 device shows typical p-type behavior at zero gate bias, in which the photogenerated electrons are collected by the electrodes. In contrast, the SPCM polarity of 2DEG device indicates that the hole carriers were collected by the metal electrodes. Careful transport measurements revealed that the gate-dependent conductance of the 2DEG devices exhibits n-type switching behavior. More importantly, the SPCM signals in 2DEG devices demonstrated very unique gate-responses that cannot be found in conventional semiconducting devices, based on which we were able to perform detailed investigation into the electronic band alignment of the 2DEG devices and obtain the valence band offset at the heterointerface.
Highlights
The extraordinarily high carrier density at the heterostructures can be interpreted through the polar catastrophe mechanism[15,16,17,18], where the metallic channel is created above the critical thickness of the LAO film
Using an scanning photocurrent microscopy (SPCM) coupled with a UV laser source, we studied the electronic band alignment at an Al2O3/STO heterointerface forming a 2DEG layer
The energy of the UV laser source was greater than the bandgap of the STO layer, enabling carrier generation that contributes to the photocurrent through migration towards the metal electrode
Summary
The extraordinarily high carrier density at the heterostructures can be interpreted through the polar catastrophe mechanism[15,16,17,18], where the metallic channel is created above the critical thickness of the LAO film. The formation of the 2DEG when using the amorphous oxides has been attributed to the creation of oxygen vacancies (VO) on the STO surface, inducing the creation of high-density free electrons[20,21,22,23,24] These amorphous LAO and Al2O3 layers have been grown using atomic layer deposition (ALD)[20, 23], which is widely used to grow functional films of high quality, providing mass production compatibility[25]. Generated in the STO layer and are collected by the nearby metal electrodes This enabled us to study the band alignment of 2DEG systems in conjunction with their gate-dependent photocurrent response
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