Optically excited multi-band conduction in LaAlO3/SrTiO3 heterostructures

Abstract

The low-temperature resistance of a conducting LaAlO 3 /SrTiO 3 interface with a 10 nm thick LaAlO 3 film decreases by more than 50% after illumination with light of energy higher than the SrTiO 3 band-gap. We explain our observations by optical excitation of an additional high mobility electron channel, which is spatially separated from the photo-excited holes. After illumination, we measure a strongly non-linear Hall resistance which is governed by the concentration and mobility of the photo-excited carriers. This can be explained within a two-carrier model where illumination creates a high-mobility electron channel in addition to a lowmobility electron channel which exists before illumination. 9-11 Several mechanisms are suggested to describe the origin of conductivity at the LAO/STO interface.12-17 However, the relative contribution of each mechanism strongly depends on the LAO film thickness and on the LAO growth conditions such as substrate temperature, oxygen partial pressure and the post annealing treatment.18 In particular, growing 5-10 layers of LAO on STO yields a metallic interface with relatively high mobility and low electron concentration, 19,20 whereas growing 26 LAO layers with the same conditions results in a low mobility, high concentration electron system with interesting magnetic properties.3 Tuning the transport properties at such a complex oxide interface by modulating the carrier density with light can both contribute to the understanding of its physics and open new pathways towards oxide-based optoelectronic device applications. It has been shown previously that interface conductivity in oxide heterostructures can be tuned by light or by an electric field. 21-23In this Letter we report our investigation of the interface of a LAO/STO sample with 26 monolayers of LAO, using low-temperature (4.2 K) magnetotransport experiments under selective illumination. Illuminating the sample with UV light of energy greater than the STO band gap results in a sharp and persistent decrease of electrical resistance. Using Hall effect measurements, we show that before illumination there is a single, low mobility electron conduction band, and that the resistance drop on illumination can be explained by the creation of a parallel conducting channel containing optically excited a) Electronic mail: u.zeitler@science.ru.nl high mobility electrons.Our sample was grown by pulsed laser deposition and has a 10 nm thick (26 unit cells) LAO film on a TiO 2 -terminated single crystal STO [001] substrate (treatment described in Ref. 24). The LAO film was deposited at a substrate temperature of 850• C and an oxygen pressure of 2 × 10 −3 mbar, using a single-crystal LaAlO 3 target. The growth of the LAO film was monitored using in situ reflection high-energy electron diffraction. After the growth, samples were cooled to room temperature in the deposition pressure.The sample was mounted on a ceramic chip carrier and electrical contacts were made with an ultrasonic wirebonder, using aluminium wires. The magnetoresistance an...

Note: Fiber optic transport probe for Hall measurements under light and magnetic field at low temperatures: Case study of a two dimensional electron gas

“…The observed increase in R PI (at t = 0) with increasing light energy (1.55-3.06 eV) is also in agreement with earlier reports, 13 where finite absorption by in-gap states in STO band gap was responsible for such a behavior. As seen further from Figs.…”

“…We need to mention that in Ref. 13, the photo-induced change in the resistance for light energy greater than the STO band-gap has been attributed to a transfer of photoexcited carrier to a subband where the carrier mobility is high; this two-band picture enables Guduru et al 13 to explain the non-linear Hall data. A fiber optic transport probe capable of electrical transport measurements in a range of T = 1.8-300 K, B = 0-7 T (or higher), and light wavelengths between near UV to near IR has been designed, developed, and tested.…”

“…[6][7][8] A photo-induced response study of these heterointerfaces has been important to explore the underlying mechanism and optoelectronic effects in 2DEG. [9][10][11][12][13] Naturally, it is important to supplement these studies with measurements of photo-induced carrier concentration changes.…”

Stimulating Oxide Heterostructures: A Review on Controlling SrTiO3‐Based Heterointerfaces with External Stimuli

“…Similar illumination experiments were carried out by Guduru et al at the LAO/STO interface with a LAO layer thickness of 10 nm, as shown in Figure a. Here, they illuminated the interface with different photon energies ranging from 1.44 to 3.65 eV . Upon illumination, the resistance decreased gradually with increasing photon energy, however, with a relatively small decrease observed when the photon energy was smaller than the band gap of STO at approximately 3.4 eV.…”

Thermally excited multiband conduction in LaAlO3/SrTiO3heterostructures exhibiting magnetic scattering

“…[1][2][3] Interest is largely focused on the conducting interface between two band-insulating perovskite oxides SrTiO 3 (STO) and LaAlO 3 (LAO), 4 which exhibits properties such as superconductivity, 5 magnetism, [6][7][8][9][10][11] and tunable switching of high mobility interface conductivity. [12][13][14] Although several mechanisms [15][16][17][18] are proposed to be responsible for the interface conductivity, the exact origin and nature of the charge carriers at the interface is still under debate. The major difficulty in achieving consensus about the intrinsic electronic nature of the interfaces is due to the fact that their properties strongly depend on external factors such as the growth conditions of the LAO layer, 6 LAO layer thickness, 19,20 and the configuration of the heterostructures.…”