Ultimate thin vertical p–n junction composed of two-dimensional layered molybdenum disulfide

Semiconducting two-dimensional crystals are currently receiving significant attention because of their great potential to be an ultrathin body for efficient electrostatic modulation, which enables to overcome the limitations of silicon technology. Here we report that, as a key building block for two-dimensional semiconductor devices, vertical p–n junctions are fabricated in ultrathin MoS2 by introducing AuCl3 and benzyl viologen dopants. Unlike usual unipolar MoS2, the MoS2 p–n junctions show ambipolar carrier transport, current rectification via modulation of potential barrier in films thicker than 8 nm and reversed current rectification via tunnelling in films thinner than 8 nm. The ultimate thinness of the vertical p–n homogeneous junctions in MoS2 is experimentally found to be 3 nm, and the chemical doping depth is found to be 1.5 nm. The ultrathin MoS2 p–n junctions present a significant potential of the two-dimensional crystals for flexible, transparent, high-efficiency electronic and optoelectronic applications.


Figure 4
Carrier transport in vertical monolayer MoS 2 p-n junction. was not tested as the solar cells in a solar simulator.)

Supplementary Note 1 Comparative study of chemically doped n-MoS 2 and p-MoS 2 FETs
The effects of the chemical doping on carrier transport and device performance

Supplementary Note 3 Doping profile in MoS 2 flakes
We experimentally measured the thickness limit for a vertical MoS 2 p-n junction to be 3 nm (4 layers). The chemical doping depth along the direction perpendicular to the layers was estimated to be 1.5 nm (2 layers) for both p-and n-type doping. In order to confirm the doping depth, a direct observation of doping profile in MoS 2 flakes by using secondary ion mass spectroscopy (SIMS) was made, as shown in Fig. 3. The doping depth was found to be 2 nm for p-type doping (Au atoms in AuCl 3 ), and to be 1.5 nm for n-type doping (C and H atoms in BV).

Supplementary Note 4 Carrier transport in vertical monolayer MoS 2 p-n junction
Another vertical monolayer MoS 2 p-n junction was fabricated with the same process as described in the Methods of manuscript. It had double top electrodes (T1 and T2) and double bottom electrodes (B1 and B2) in order to confirm the carrier transport type on the top and bottom surfaces, respectively, as shown in Fig. 4(a). The bottom electrodes were made of Cr/Pd/Cr (5 nm / 50 nm / 5 nm), and the top electrodes were made of Cr/Pd (5 nm / 50 nm), in order to provide symmetric metal contact to MoS 2 flake.
The layer number of monolayer MoS 2 was confirmed by Raman spectroscopy, as shown in Fig. 4(b). The difference of Raman shift between E 2g 1 and A 1g peaks was measured as ~19 cm -1 , suggesting a monolayer structure of MoS 2 flake 10 .
Output characteristics of vertical monolayer MoS 2 p-n junction was measured by employing T1 and B1 electrodes under vacuum conditions (10 mTorr) at room temperature. It showed "reversed" current rectification in which a tunneling-dominated large current was observed at the reversed bias, as shown in Fig. 4(c). This was consistent with the electrical behavior of monolayer MoS 2 p-n junction as shown in the manuscript (see Fig. 5(e) in the main manuscript), suggesting the good reproducibility and reliability of the vertical MoS 2 p-n junction in this work.
In order to reduce the external thermal interference, transfer characteristics of vertical monolayer MoS 2 p-n junction were measured at the low-temperature (120 K) by employing T1 and B1 electrodes, as shown in Fig. 4(d). The monolayer MoS 2 p-n junction showed unipolar electron transport over a wide V G range. This was consistent with the electrical behavior of another device shown in the manuscript (see Fig. 6 surfaces showed electron-dominated carrier transport, suggesting the compromise of ptype doping and the overwhelming of n-type doping (see Fig. 4(d)). In other words, the pn junction cannot be properly formed by the chemically doping in monolayer MoS 2 . Here we also note that even the overall monolayer MoS 2 showed n-type carrier transport, the dominance of electron transport in each individual transfer curves was not as clear as that shown in the few-layer MoS 2 (see Fig. 1(d)). The degradation of n-type doping in the monolayer MoS 2 suggested that the n-type doping was also compromised partially by the p-type doping, and therefore its doping effect was suppressed. This also agreed with our theory.
To quantitatively analyze the metal-semiconductor contact condition, the metal- where A is the area of the contact junction, A * is the effective Richardson constant, q is the electronic charge, k B is Boltzmann constant, and T is the temperature. Under a high V D , the contact at the drain end was reversely biased [exp(-qV D /k B T)<<1], and I D became proportional to T 2 exp(-ϕ MS /k B T). A linear relation between ln(I D /T 2 ) and q/k B T can be plotted for various V G levels, and the gate-dependent ϕ MS for a given V D was estimated from the slope of each curve, as shown in Fig. 4