Valley Zeeman splitting in semiconducting two-dimensional group-VI transition metal dichalcogenides
Date of Issue2018-12-31
School of Physical and Mathematical Sciences
Atomically thin semiconducting group-VI transition metal dichalcogenides (TMDs) have attracted enormous interest because of their as-born bandgaps and other unique properties giving great potential in next-generation electronic devices, valleytronics, photodetectors and flexible optoelectronics applications. Electrons at K and K^' valleys in 2D group-VI TMDs can be selectively excited by the circularly polarized light but energy degenerated due to the time-reversal symmetry, which is known as the valley degree of freedom. In the presence of an external out-of-plane magnetic field, the energy degeneracy is lifted thus there is an energy difference between the two emissions from the two valleys, known as the valley Zeeman splitting energy due to the breaking of time-reversal symmetry. Such unique features originating from the strong spin-orbital and spin-valley couplings make 2D group-VI TMDs highly competitive over the traditional semiconductors and even promising for the emerging valleytronics. In this thesis, circularly-polarized photoluminescence (PL) spectroscopy has been exploited to investigate valley Zeeman splitting behavior of emerging 2D group-VI TMDs under various circumstances. On the way towards the large-scale integration of potential valley Zeeman splitting based devices, one of the critical issues is whether the valley Zeeman splitting behavior changes with the strength of many body interactions induced by the different doping levels across the sample. Here, spatial variations of valley splitting evolution in exfoliated monolayer WSe2 are investigated through magneto-PL mapping measurements. It is found that for the neutral exciton emission, the valley Zeeman splitting behavior almost stays unchanged across the sample though the PL mapping measurements show the nonuniformity of the PL emission energy, which is caused by the unintentional doping during the sample preparation process. While for trion emission, the valley Zeeman splitting behavior changes a lot with the doping level from the sample center to the edge regions. In order to realize two stable binary states in potential valley Zeeman splitting based devices, a large valley Zeeman splitting energy is on demand even under a small magnetic field. Here, exfoliated monolayer WSe2 samples are transferred onto a ferromagnetic substrate of EuS. The net magnetization of EuS substrate results in a short-range magnetic exchange field (MEF) on the interface between the WSe2 and EuS. And this MEF further leads to enhanced valley Zeeman splitting energies for both trion and exciton emissions of WSe2 on the EuS substrate. The short-range MEF originating from proximity effect can be exploited to tune the valley Zeeman splitting behavior in future valleytronics. Hexagonal boron nitride (hBN) with a layered crystal structure has less lattice mismatch with the group-VI TMDs and is often used as a platform to improve the optical quality of the 2D group-VI TMDs by suppressing the unintentional doping from the oxide substrate. Here, a modified method is developed to directly grow WS2 and MoS2 monolayers on hBN with a high yield and high optical quality. Benefiting from the well-resolved and super sharp exciton and trion PL peaks, the intrinsic valley Zeeman splitting behavior in CVD-grown WS2 and MoS2 monolayers on hBN have been clearly revealed through in-situ magnetic-field-dependent PL imaging and spectroscopy at cryogenic temperature for the first time. This thesis manifests that, valley Zeeman splitting behavior in 2D group-VI TMDs can be tuned not only by the different substrates, but also by the doping levels in such 2D group-VI TMDs. These fundamental studies enable us to step further towards the future valleytronics.
DRNTU::Science::Physics::Optics and light