Properties of Charge Carriers in Ultrathin WSe2 · • A solid state top gate (initially graphene)...

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2015PPGUndergraduateResearchFellowshipProgram

Properties of Charge Carriers in Ultrathin WSe2Seamus O’Hara1, Jieun Lee2,3, Kin Fai Mak2,3, and Jie Shan2,3 (mentor)

1Department of Physics, Penn State University(PSU); 2Department of Physics, PSU; 3Center for Nanoscale Science, PSU

The PPG Undergraduate Research Fellowship Program is supported by the PPG Research Foundation through the Materials Research Institute

Experiment MotivationTungsten Diselenide (WSe2) posses two direct band gaps in momentum space, denoted as the K and K’ momentumvalleys. These valleys have spin degeneracy lifting as a result of Spin-Orbit coupling (SOC), leading to opticalselection properties which cause even greater spin splitting of energies in the valence and conduction bands. Suchproperties lead to spin-valley locking, where a given electron spin and momentum valley are paired together. Both theSpin Hall Effect (SHE) and Valley Hall Effect (VHE) push electrons of opposite spins and valleys, respectively, toopposite ends of a device. Optical observations such as Kerr Rotation (KR) measurements can be used to detect theSHE and VHE in a material, allowing us to observe both effects and test the strength of the SOC in a material, in thiscase WSe2.

Figure 1: The two physical phenomena of interest for this experiment. A) A simplification of the WSe2 momentum space band structure. The blue bandsindicate the valance band while the orange band marks the conduction band, with the up and down arrows corresponding to the electron spin. The redarrows indicate optical excitation, with the rotating blue arrow showing the optical selection properties of the given valley. B) A depiction of VHE, withelectrons 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

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Initial ObstaclesInitial transport tests:

• WSe2 was not conductive on Au electrodes.• WSe2 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 electrodeinstead.

• A solid state top gate (initially graphene) would beused.

Graphene top gate:• Achieved gating effect, but not to desired

conductivity.

Figure 2: An Energy diagram of Platinum and WSe2.From left to right, the energies depicted are the workfunction of Pt, the valence band energy, the channelenergy, and the conduction band energy of WSe2.

10K Transport Measurements

Charge carrier transport measurements were conducted at 10K. From these tests we were able to make a fewconclusions 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 toprovide 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 WSe2, then we should only observe signals for KRat the edges of the device, with opposite signals at opposite ends of the device. This was not the case, and we hypothesize a fewcauses for this discrepancy:

• The bias voltage was too high during KR measurements, creating a “source-gate coupling” effect.• The hBN was uneven with WSe2, causing a disturbance in charge carrier flow.• PDMS resides between the hBN layer and WSe2 layer, impairing the conductivity of the device.

Palladium (Pd) Top gate:• Used by other groups2 to achieve

maximum conductivity in WSe2.• Placed on top of hBN with e-beam

lithography and evaporation techniques.• Should allow for an Ohmic contact in the

p-doping regime2.

Figure 5: Device Structure. This diagram outlines the many layers of our WSe2device, along with the electrical contacts made at various layers.

Revised Structure

Figure 3: A depiction of the real space band structure for the Pt/WSe2contact region. The solid purple line indicates the Schottky barrier forelectrons in this structure.

ConclusionFrom the transport and KR measurements, we can see that our current device structure is an improvement, but not sufficient toobserve the VHE and SHE in WSe2. We believe a new fabrication method is necessary to insure optimal adhesion between the WSe2and hBN layers of the device. This will reduce straining of the WSe2 device and improve conductivity, creating an Ohmic contactwith the Pt electrode and WSe2. Greater conductivity will require lower amounts of bias voltage, which will reduce source-gatecoupling and give us a clearer edge signal in KR measurements.

Citations:1.Mak,K.F.andShan,J.PhotonicsandOptoelectronicsof2DsemiconductorTransitionMetalDichalcogenidesNaturePhotonics 201610,216-226

2Moova,H.C.P.etal High-MobilityHolesinDual-GatedWSe2 Field-EffectTransistors.ACSNano 2015,10,10401-10410.

1.Mak,K.F.andShan,J.PhotonicsandOptoelectronicsof2DsemiconductorTransitionMetalDichalcogenidesNaturePhotonics2016 10,216-226

Figure 4: A depiction of the real space band structure for the Au/WSe2contact region. The solid purple line indicates the Schottky barrier forholes in this structure.

Figure 6: The measured transport properties of bilayer WSe2 in 10K. A) The image of the device inside the test chamber. The bilayer sample of WSe2 isoutlined 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 ofCurrent vs. Back Gate Voltage. D) A plot of Current vs. Voltage bias.

a)

b)

c)

d)

a)

Currentvs.VoltageBias

Hole-doped Electron-doped

Currentvs.BackGateVoltage

Currentvs.TopGateVoltage

Vds =5VVbg =0V

Vds =5VVtg =10V

Vacuum Vacuum

Vacuum

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