00 2015 PPG Undergraduate Research Fellowship Program Properties of Charge Carriers in Ultrathin WSe 2 Seamus O’Hara 1 , Jieun Lee 2,3 , Kin Fai Mak 2,3 , and Jie Shan 2,3 (mentor) 1 Department of Physics, Penn State University(PSU); 2 Department of Physics, PSU; 3 Center for Nanoscale Science, PSU The PPG Undergraduate Research Fellowship Program is supported by the PPG Research Foundation through the Materials Research Institute Experiment Motivation Tungsten Diselenide (WSe 2 ) posses two direct band gaps in momentum space, denoted as the K and K’ momentum valleys. These valleys have spin degeneracy lifting as a result of Spin-Orbit coupling (SOC), leading to optical selection properties which cause even greater spin splitting of energies in the valence and conduction bands. Such properties lead to spin-valley locking, where a given electron spin and momentum valley are paired together. Both the Spin Hall Effect (SHE) and Valley Hall Effect (VHE) push electrons of opposite spins and valleys, respectively, to opposite ends of a device. Optical observations such as Kerr Rotation (KR) measurements can be used to detect the SHE and VHE in a material, allowing us to observe both effects and test the strength of the SOC in a material, in this case WSe 2 . Figure 1: The two physical phenomena of interest for this experiment. A) A simplification of the WSe 2 momentum space band structure. The blue bands indicate the valance band while the orange band marks the conduction band, with the up and down arrows corresponding to the electron spin. The red arrows indicate optical excitation, with the rotating blue arrow showing the optical selection properties of the given valley. B) A depiction of VHE, with electrons in the K and K’Valleys being pushed to opposite sides of a device as the result of an in-plane electric field. a) b) k E Initial Obstacles Initial transport tests: • WSe 2 was not conductive on Au electrodes. • WSe 2 required doping from a top gate to achieve optimal conductivity. • Electrolyte top-gate was not practical at low temperatures. Solution: • Platinum (Pt) would be used as an electrode instead. • A solid state top gate (initially graphene) would be used. Graphene top gate: • Achieved gating effect, but not to desired conductivity. Figure 2: An Energy diagram of Platinum and WSe 2 . From left to right, the energies depicted are the work function of Pt, the valence band energy, the channel energy, and the conduction band energy of WSe 2 . 10K Transport Measurements Charge carrier transport measurements were conducted at 10K. From these tests we were able to make a few conclusions about our device: • The top gate voltage has a much greater effect on the conductivity than the back gate voltage. • The device demonstrates p-type conductivity. • A large amount of bias and gating is required to achieve noticeable charge carrier flow. • The contacts are Schottky in nature. Figure 7: Kerr Rotation Measurements. A) and B) are both two dimensional maps of measured Kerr Rotation measurements, with insets depicting Reflectance data to provide an optical image of the device. A) displays KR measurements conducted with a modulating 10V peak-to-peak bias voltage, 0V back gate, and 10V top gate. B) displays KR measurements with a modulating 10V peak-to-peak top gate bias, 0V back gate, and 0V bias voltage. a) b) Kerr Rotation KR measurements were conducted at 10K. If SOC and the VHE are prevalent in WSe 2 , then we should only observe signals for KR at the edges of the device, with opposite signals at opposite ends of the device. This was not the case, and we hypothesize a few causes for this discrepancy: • The bias voltage was too high during KR measurements, creating a “source-gate coupling” effect. • The hBN was uneven with WSe 2 , causing a disturbance in charge carrier flow. • PDMS resides between the hBN layer and WSe 2 layer, impairing the conductivity of the device. Palladium (Pd) Top gate: • Used by other groups 2 to achieve maximum conductivity in WSe 2 . • Placed on top of hBN with e-beam lithography and evaporation techniques. • Should allow for an Ohmic contact in the p-doping regime 2 . Figure 5: Device Structure. This diagram outlines the many layers of our WSe 2 device, along with the electrical contacts made at various layers. Revised Structure Figure 3: A depiction of the real space band structure for the Pt/WSe 2 contact region. The solid purple line indicates the Schottky barrier for electrons in this structure. Conclusion From the transport and KR measurements, we can see that our current device structure is an improvement, but not sufficient to observe the VHE and SHE in WSe 2 . We believe a new fabrication method is necessary to insure optimal adhesion between the WSe 2 and hBN layers of the device. This will reduce straining of the WSe 2 device and improve conductivity, creating an Ohmic contact with the Pt electrode and WSe 2 . Greater conductivity will require lower amounts of bias voltage, which will reduce source-gate coupling and give us a clearer edge signal in KR measurements. Citations: 1. Mak, K.F. and Shan, J. Photonics and Optoelectronics of 2D semiconductor Transition Metal Dichalcogenides Nature Photonics 2016 10, 216-226 2 Moova, H.C.P. et al High-Mobility Holes in Dual-Gated WSe 2 Field-Effect Transistors. ACS Nano 2015, 10, 10401-10410. 1. Mak, K.F. and Shan, J. Photonics and Optoelectronics of 2D semiconductor Transition Metal Dichalcogenides Nature Photonics 2016 10, 216-226 Figure 4: A depiction of the real space band structure for the Au/WSe 2 contact region. The solid purple line indicates the Schottky barrier for holes in this structure. Figure 6: The measured transport properties of bilayer WSe 2 in 10K. A) The image of the device inside the test chamber. The bilayer sample of WSe 2 is outlined in magenta. The light blue is the hBN sample, and the darker strip is the Pd top gate. B) A plot of the Current vs. Top Gate voltage. C) A plot of Current vs. Back Gate Voltage. D) A plot of Current vs. Voltage bias. a) b) c) d) a) Current vs. Voltage Bias Hole-doped Electron-doped Current vs. Back Gate Voltage Current vs. Top Gate Voltage V ds = 5V V bg = 0V V ds = 5V V tg = 10V Vacuum Vacuum Vacuum 6