Spin splitting in high-density GaN two-dimensional electron gas

Abstract

Two-dimensional electron gas (2DEG) in gallium nitride (GaN) has been widely investigated because of its interesting properties for electronics. It has been proposed also for devices which exploit the spin degree of freedom (“spintronics”); to this aim, the spin splitting at zero magnetic field due to spin-orbit interaction (SOI) must be investigated. In GaN, however, theoretical studies have highlighted the interplay of different mechanisms in SOI: experimental results are conflicting, and the SOI strength is still debated. The most common and suitable experimental technique for investigation of SOI is magnetoresistance measurement at cryogenic temperature, exploiting two effects: ShubnikovdeHaas effect (SdH) and weak antilocalisation (WAL). In this thesis, these measurements were performed on two high carrier density GaN 2DEG samples. SdH appears at high magnetic field, as periodic resistance oscillations; spin splitting causes beatings in the oscillations. However, beatings can be hidden by oscillation damping, and other effects can also give beatings patterns, so that standard analyses can be not very effective. A novel analysis was thus developed in this thesis, studying amplitude and phase modulations of SdH: the two spin split states were univocally identified and characterised. WAL is a small correction to resistance due to quantum interference in presence of SOI; it has a characteristic pattern observable at very low field. Different theories have been developed to extract the spin splitting from WAL. In this thesis, the experimental data were compared to four of them, representative of different theoretical approaches. Three comprehensive theories reproduce the data, and gave comparable results for the spin splitting. Thanks to the careful analyses, in both samples a spin-splitting energy of about 1 meV is reliably measured. In the literature a large variance is found for spin splitting in high density 2DEGs, and values measured by SdH are systematically larger than those measured by WAL. Using the analysis developed in this thesis, SdH are found in agreement with the WAL measurements, which thus are validated as good zero-field spin splitting estimation.