Research On The Nanofabrication Of Spin Hall Effect Devices Based On LaAlO₃/SrTiO₃ Heterostructure

The two-dimensional electron gas (2DEG) embedded at LaAlO3/SrTiO3 (LAO/STO) heterstructure has attracted enormous attention in the past decade due to its intriguing properties, including LAO/STO interface superconductivity, ferromagnetism, the coexistence of superconducting state and ferromagnetic state, and strong Rashba spin-orbit coupling (Rashba SOC). According to previous research on the Rashba SOC at the LAO/STO interface, the SOC strength can be effectively tuned by gate voltage, which means one could manipulate the spin procession and spin relaxation by simply controlling the gate voltage. The gate-tunable SOC makes LAO/STO interface a promising platform for spintronic devices, e.g., field effect spin transistor. Realizing spintronic devices requires firstly generating spin imbalance by spin injection or by other methods, then manipulating the spin procession and relaxation during its transport, and finally detecting the spin signal. Spin Hall effect (SHE) and inverse spin Hall effect (ISHE) are effect phenomena for spin imbalance generation and spin detection.Nevertheless, there is still a shortage of research on the spin imbalance generation and spin transport in LAO/STO interface, possibly due to the very short spin diffusion length, which is only of a few hundred nanometers according to the recent report. We hope to firstly realize imbalanced spin accumulation by SHE, then manipulate the spin by gate voltage, and finally detect the spin signal via ISHE. For that purpose, reducing the device size down to a few hundred nanometers which is comparable to the spin diffusion length is essential. Despite various nanofabrication methods have been proposed for patterning LAO/STO interface, challenges still remain in realizing reliable hundred-nanometer devices. This is mainly because the traditional dry-etching process will create oxygen vacancies in the STO substrate, which makes it conductive. The device will be unreliable with a conductive substrate. Furthermore, it is also required that the Rashba SOC should be robust against nanofabrication process, which means it still exists and can be tuned effectively in a hundred-nanometer scale device.We first investigate the reliability of the LAO/STO device with different size fabricated by low energy Ar+ ion beam irradiation, which is an effective method for patterning LAO/STO heterostructure. We find that the device remains reliable when the device size is of a few micrometers. However, the device becomes unreliable when further reducing the device size to hundreds of nanometers. Specifically, the area (hundreds nanometers wide) between two conducting channels becomes conductive. After further etching the spacing area with focused ion beam (FIB), the area becomes insulating. Successfully fabricating devices down to hundred-nanometer scale with high reliability makes realizing SHE device based on LAO/STO interface possible.We then investigate the tunability of the Rashba SOC in LAO/STO wires of different widths. By studying the magnetotransport properties of the LAO/STO wires, we find that the SOC strength can be effectively tuned by gate voltage when the wire width is of more than a few micrometers. However, the tunability of the SOC strength by gate voltage decreases and eventually vanishes if we decease the wire width down to hundreds nanometers. This is because the charge carriers are trapped by the trapping centers located at the wire boundaries, which deteriorates the effect of gate voltage. After we illuminate the device with laser and excite the trapped carriers back to the conducting band, we find that the SOC strength can be tuned again, this time, by the combined effect of gate voltage and laser irradiation. The recovery of tunable SOC implies that we can fabricate SHE devices and manipulate the spin.Our research lays foundation for fabricating reliable spintronics devices based on LAO/STO interface, e.g., SHE devices, of hundreds nanometers.