Charge Density Wave-driven Carrier Transport in Layered Heterostructures
Metal-based electronics remain one of the longstanding goals of researchers to achieve ultra-fast and radiation-hard electronic circuits. Generally, metals are primarily used as passive conductors in modern electronics and do not play an active role. Nanoscale materials with distinctive size-dependent properties provide opportunities to achieve new device functionalities. Ta-based di-chalcogenides, particularly 1T-TaS2 and 2H-TaSe2, which form layered structures and exhibit charge density waves (CDW), are promising in this context. CDW is a macroscopic state shown by materials with reduced dimensions, for example, one-dimensional and layered two-dimensional crystals. It results from the modulation in the electronic charge arising due to a periodic modulation in the crystal lattice. 1T-TaS2 exhibits one of the strongest known CDW characteristics enabling temperature-dependent distinct resistivity phases. The nearly commensurate (NC) to the incommensurate (IC) CDW phase transition that usually occurs at 353 K and can be driven electrically at room temperature is of high practical interest. However, resistivity switching during this phase transition is weak (< 2) and cannot be modulated by an external gate voltage – limiting its widespread usage. Using a back-gated 1T-TaS2/2H-MoS2 heterojunction, we show resistivity switching up to 17.3, which is ~14.5-fold higher than standalone TaS2. We demonstrate a low barrier electrical contact between a TaS2 source and a MoS2 channel, promising “all-2D” flexible electronics. Additionally, we show that the usual resistivity switching in TaS2 due to different phase transitions is accompanied by a surprisingly strong modulation in the Schottky barrier height (SBH) at the TaS2/MoS2 interface – providing an additional knob to control the degree of the phase-transition-driven resistivity switching by an external gate voltage. In particular, the commensurate (C) to triclinic (T) CDW phase transition increases the SBH owing to a collapse of the Mott gap in TaS2. The change in SBH allows us to estimate an electrical Mott gap opening of ~71 ± 7 meV in the C phase of TaS2. The results show a promising pathway to externally control and amplify the CDW induced resistivity switching. Further, we achieve gate- and light-controlled negative differential resistance (NDR) characteristics in an asymmetric 1T-TaS2/2H-MoS2 T-junction by exploiting the electrically driven CDW phase transition of TaS2. The device operation is purely governed by majority charge carriers, making it distinct from typical tunneling-based NDR devices, thus avoiding the bottleneck of weak tunneling efficiency in van der Waals heterojunctions. Consequently, we achieve a peak current density over 10^5 nA μm^(-2), which is about two orders of magnitude higher than that obtained in typical layered material-based NDR implementations. An external gate voltage and photo-gating can effectively tune the peak current density. The device characteristics show a peak-to-valley current ratio (PVCR) of 1.06 at 290 K, increasing to 1.59 at 180 K. To exploit the low thermal conductivity of 1T-TaS2 and 2H-TaSe2 in a local heater structure, we insert 2H-TaSe2 in between TaS2 and MoS2 layers, thereby forming a triple-layered 1T-TaS2/2H-TaSe2/2H-MoS2 T-junction. TaSe2 acts as a buffer layer preventing the CDW-induced SBH modulation at TaS2/MoS2 interface. This will allow efficient thermionic switching of carriers resulting from sharp temperature rise in the junction due to electrically driven TaS2 phase transitions. Interestingly, the device can toggle between the current increment and NDR characteristics by simply changing the biasing conditions. At TaS2 biasing, the heterostructure device shows a current increment by a factor of 3 at 300 K, which gets enhanced up to ~10^3 at 77 K, beneficial for various switching circuits and sensing applications. However, under TaSe2 biasing, the device exhibits NDR characteristics with a PVCR of 1.04 and 1.10 at 300 K and 77 K, respectively. The external back-gate voltage can effectively tune the current enhancement factor and NDR. The devices mentioned above are robust against ambiance-induced degradation, and the characteristics repeat in multiple measurements over more than six months. Conventional metals, in general, do not exhibit strong photoluminescence. However, we found that 2H-TaSe2 exhibits a surprisingly strong optical absorption and photoluminescence resulting from inter-band transitions. We use this perfect combination of electrical and optical properties in several optoelectronic applications. We show a seven-fold enhancement in the photoluminescence intensity of otherwise weakly luminescent multi-layer MoS2 through non-radiative resonant energy transfer from TaSe2 transition dipoles. Using a combination of scanning photocurrent and time-resolved photoluminescence measurements, we also show that the hot electrons generated by light absorption in TaSe2 have a relatively long lifetime, unlike conventional metals, making TaSe2 an excellent hot-electron injector. Finally, we show a vertical TaSe2/MoS2/graphene photodetector demonstrating a responsivity greater than 10 AW^(-1) at 0.1 MHz - one of the fastest reported photodetectors using MoS2. The findings will boost device applications that exploit CDW phase transitions, such as ultra-broadband photodetection, negative differential conductance, thermal sensors, fast oscillator, and threshold switching in neuromorphic chips. These functionalities will enable the implementation of active metal-based circuits.