In 2000, Zhores I. Alferov and Herbert Kroemer were awarded the Nobel Physics Prize for “the development of semiconductor heterostructures used in high-speed- and opto-electronics”. Their pioneering achievements have opened a new route for realizing functional electronic and optoelectronic devices with outstanding performances. In particular, these ideas enabled the invention of the high-electron-mobility transistors (HEMTs) based on the heterostructure of GaAs and AlxGa1-xAs. Such two-dimensional electron gas (2DEG) systems have advantages in high-speed electron transport because the impurity-induced carrier scattering is suppressed.
2D materials, covalently-bonded atomic layers weakly bonded by van der Waals forces, can be considered a naturally existing 2DEG, unlike the above-mentioned structures artificially created by epitaxial growth. Likewise, these materials need to be doped to implement various functions in the 2D-based electronic and optoelectronic devices. However, no matter what methods are used for doping, suppressing dopant-induced carrier scattering is intrinsically challenging because dopants are always in close proximity to the atomically thin material. The appropriate strategy to resolve such a critical issue has not been developed in these materials so far.
In this study, we first demonstrate a modulation doping in the WSe2/h-BN/MoS2 heterostructure that can diminish the doping-induced scattering. Electrons in the underlying MoS2 channel doped by remote charge transfer are spatially separated from molecular dopants on the WSe2 surface, as schematically shown in Fig. 1. Our low-temperature electrical measurements show that as decreasing the temperature, mobility of the modulation-doped device monotonically increases and begins to saturate below 100 K, whereas, for the direct-doped device, mobility peaks around 200 K and gradually decreases as further decreasing the temperature as shown in Fig. 2. Such a decreasing behavior of mobility at low temperature is not observed in the case of the undoped samples. In particular, the modulation-doped device exhibits over an order of magnitude enhancement in mobility at 10 K from 63 to 1,100 cm2V-1s-1 compared with the direct-doped counterpart. These results confirmed that the enhanced electron mobility in the doped WSe2/h-BN/MoS2 samples could be originated from the reduction of the extrinsic scattering sources, including ionized dopants owing to the spatial separation of electrons and their parent dopants. This finding paves the way for the development of high-speed 2D semiconductor electronics.
Figure 1. Schematic (a) and energy band diagram (b) of the modulation-doped van der Waals heterostructures.
Figure 2. a, Temperature dependent electron mobility in the modulation- and direct-doped MoS2 transistors. b, Phonon damping factor, γ, as a function of the temperature and gate voltages.
For more information, please refer to our recent publication in Nature Energy: "Remote modulation doping in van der Waals heterostructure transistors".