PCSI2018 Session PCSI-SuA: 2D Surfaces I

Atomically thin crystals, such as monolayer transition metal dichalcogenides (TMDs), provide a new platform to investigate the electrons in low dimensional solid state systems. In these materials, two inequivalent energy band extrema occur at the edges of the Brillouin zone, known as valleys, which serve as a binary degree of freedom of electrons similar like spins [1]. The unique control of valley pseudospins by optical and electrical means are not only fundamentally interesting, but may also find applications in valley-based electronics and optoelectronics [2,3].

In this talk, we discuss methods to manipulate the valley polarization in 2D TMD materials. First, we describe the observation of the valley Hall effect in monolayer molybdenum disulfide (MoS₂), in which the valley current is generated transverse to the charge current. We use optical techniques to directly image the valley polarization accumulated at the edges of MoS₂ channel with spatial resolution [4]. We will also discuss the possibility to tune the valley Hall conductivity by controlling the crystal’s inversion symmetry in bilayer. Second, we apply strain to monolayer MoS₂ to break the crystal’s 3-fold rotational symmetry which leads to the generation of valley magnetoelectricity [5]. The observed valley magnetization switches the sign when the in-plane bias direction flips and the magnitude scales with the amount of channel current. We discuss the dependence of the observed magnetization on bias, gate, strain direction, and external magnetic fields.

[1] D. Xiao, W. Yao and Q. Niu, Phys. Rev. Lett. 99, 236809 (2007).

[2] J.R. Schaibley, H. Yu and G. Clark, et al., Nat. Rev. Mater. 1, 16055 (2016).

[3] K.F. Mak and J. Shan, Nat. Photon. 10, 216 (2016).

[4] J. Lee, K.F. Mak and J. Shan, Nat. Nanotech. 11, 421 (2016).

[5] J. Lee, Z. Wang, H. Xie, K.F. Mak and J. Shan, Nat. Mater. 16, 887 (2017).

MoS₂ is attractive for a variety of nanoelectronic devices due to its ability to maintain desirable semiconducting properties at the single layer limit [1]. Understanding the behavior of metal/MoS₂ interfaces is important for developing low-resistance contacts for scaled transistors and other emerging applications of MoS₂. Our recently published work on Ag/MoS₂ contacts shows that after annealing in Ar at 250 and 300 °C, the contact resistance R_C is reduced from 0.8–3.5 kΩ·μm to 0.2–0.7 kΩ·μm, likely due to the incorporation of Ag donors between layers of MoS₂ [2]. This result is very good relative to the state-of-the-art. More recently, we have verified using transmission electron microscopy and electron energy loss spectroscopy that Ag diffuses into MoS₂ at low levels.

Now we have discovered that Ni also diffuses into MoS₂ — without altering its structure — after annealing in Ar at a temperature as low as 200 °C. Therefore, we fabricated Ni-based contacts to MoS₂ and characterized them before and after annealing. However, annealing caused an increase in R_C in every Ni-contacted device. As deposited, R_C varied from 2.5–8.0 kΩ·μm, but it increased by 50% after annealing at 200 °C, and increased by 650% after annealing at 300 °C. While Ag acts as a donor when intercalated in MoS₂ [3], Ni might not. Our further efforts towards understanding the effects of diffusion of Ag, Ni, and possibly other transition metals into MoS₂ may ultimately guide us in achieving even lower contact resistances.

[1] K. F. Mak et al., Phys. Rev. Lett. 105, 136805(2010).

[2] M. Abraham and S. E. Mohney, J. Appl. Phys. 122, 115306 (2017).

[3] D. M. Guzman et al., J. Appl. Phys. 121, 055703(2017).

With the advent of atomically thin van der Waals materials, it is now possible to combine p and n doped 2D semiconductors to realize p-n junctions at their ultimate thickness. [1-3] Due to the lack of a depletion width in atomically thin van der Waals materials, however they are conceptually different compared to conventional diode. [4]Here, we demonstrated strong rectification behaviour for a p-n junction made of bilayer n-type MoS₂ and ultrathin (10 nm) p-type GaSe with a rectification ratio of 10⁴. The threshold voltage is determined to be 0.57 V. The I – V characteristics under illumination using below band gap excitation reveals a marked photovoltaic effect suggesting efficient exciton dissociation due to the presence of an electric field at the interface. The below band gap excitation also suggests the presence of accessible states in the forbidden gap of MoS₂. To validate our experimental observations we also performed DFT calculations on such heterostructure. Our theoretical findings indicate that the electronic band structure of bilayer MoS₂ is modified by the interaction with GaSe. This interaction creates accessible states in the forbidden gap of MoS₂ and may explain the below band gap excitation and the rectification behaviour of the p-n junction.

1. Lee, C.-H.; Lee, G.-H.; van der Zande, A. M.; Chen, W.; Li, Y.; Han, M.; Cui, X.; Arefe, G.; Nuckolls, C.; Heinz, T. F.; Guo, J.; Hone, J.; Kim, P. Nat Nano 2014,9 (9), 676-681.

2. Huang, C.; Wu, S.; Sanchez, A. M.; Peters, J. J. P.; Beanland, R.; Ross, J. S.; Rivera, P.; Yao, W.; Cobden, D. H.; Xu, X.; Nat Mater 2014,13 (12), 1096-1101.

3. Ross, J. S.; Klement, P.; Jones, A. M.; Ghimire, N. J.; Yan, J.; Mandrus, D. G.; Taniguchi, T.; Watanabe, K.; Kitamura, K.; Yao, W.; Cobden, D. H.; Xu, X.; Nat Nano 2014,9 (4), 268-272.

4. Li, H.-M.; Lee, D.; Qu, D.; Liu, X.; Ryu, J.; Seabaugh, A.; Yoo, W. J.; Nature Communications 2015,6, 6564.

The sublimation of Si atoms from a SiC surface by annealing in UHV is a familiar method for the epitaxial growth of graphene. However, a problem is the pitted morphology of the graphene/SiC(0001) structure after simple annealing above 1100°C in UHV. It is well known that, prior to graphene growth above 1100°C, a buffer layer is formed at around 1000°C. The pitted morphology is probably caused by the insufficient amount of liberated carbon atoms on the SiC surface to form a uniform buffer layer because the rapid sublimation of Si atoms occurs on an area not covered by a buffer layer, causing the formation of pits. Thus, the control of the carbon concentration at the monolayer level on an initial SiC surface is important.

We have found a new chemical route to achieve this, which is the plasma oxidation of a SiC surface near room temperature followed by HF etching. This is referred to as the plasma-assisted process hereafter. We took the change in O1s XPS spectra caused by the plasma-assisted process. It indicated that the initial untreated SiC surface is terminated by OH species, which are the origin of the hydrophilic property of the surface. In contrast, after the plasma-assisted process, a shoulder peak corresponding to C-O bonds appeared in the O1s spectrum. Taken together with the C1s spectrum, we consider that a mixture of C-C and C-O bonds exists, which agrees with the slightly hydrophobic property of the treated SiC surface. We speculate that carbon species composed of C-C and C-O bonds accumulated at the SiO₂/SiC interface during plasma oxidation near room temperature. When the SiO₂ layer was stripped off by subsequent HF etching, additional carbon species with a thickness of 1-2 monolayers appeared on the SiC surface. When this surface was annealed at 1100°C for 30 min in UHV, a graphene/SiC structure with a low pit density was obtained, which is completely different from the structure on the untreated SiC surface. This is probably because the additional carbon species produced by the plasma-assisted process contribute to the formation of a uniform buffer layer, which suppresses random Si sublimation at elevated temperatures of above 1000°C.

[1] N. Saito, K. Arima et al., Carbon 80, 440 (2014).

[2] K. Arima et al., Meeting Abstracts of 230th ECS Meeting, 2062 (2016).