The hot carrier diffusion coefficient of sub-10 nm virgin MoS₂: uncovered by non-contact optical probing

Abstract

We report a novel approach for non-contact simultaneous determination of the hot carrier diffusion coefficient (D) and interface thermal resistance (R) of sub-10 nm virgin mechanically exfoliated MoS nanosheets on c-Si. The effect of hot carrier diffusion in heat conduction by photon excitation, diffusion, and recombination is identified by varying the heating spot size from 0.294 μm to 1.14 μm (radius) and probing the local temperature rise using Raman spectroscopy. R is determined as 4.46-7.66 × 10 K m W, indicating excellent contact between MoS and c-Si. D is determined to be 1.18, 1.07, 1.20 and 1.62 cm s for 3.6 nm, 5.4 nm, 8.4 nm, and 9.0 nm thick MoS samples, showing little dependence on the thickness. The hot carrier diffusion length (L) can be determined without knowledge of the hot carrier's life-time. The four samples L is determined as 0.344 (3.6 nm), 0.327 (5.4 nm), 0.346 (8.4 nm), and 0.402 μm (9.0 nm). Unlike previous methods that are implemented by making electrical contact and applying an electric field for D measurement, our technique has the advantage of being truly non-contact and non-invasive, and is able to characterize the electron diffusion behavior of virgin 2D materials. Also it points out that hot carrier diffusion needs to be taken into serious consideration in Raman-based thermal property characterization of 2D materials, especially under very tightly focused laser heating whose spot size is comparable to the hot carrier diffusion length.

Very fast hot carrier diffusion in unconstrained MoS2on a glass substrate: discovered by picosecond ET-Raman

“…The thermal activation term (vn 0 /vT CW )DT CW /s gives the carrier generation due to the temperature increase; it is negligible at a relatively low temperature increase and for a low free-carrier density. 11,48,50 Term Fa represents the carrier photogeneration source. In eqn (2), hn (2.33 eV) is the photon energy of the laser beam.…”

“…[8][9][10] The interface thermal energy coupling of devices is a critical property because it determines their overall performance and lifetime. 11,12 However, accurate and straightforward measurement of this property is still very challenging due to the involvement of many different complicated physical phenomena. 13,14 Both theoretical and experiment-based methods have been developed and successfully applied to characterize interface thermal energy coupling.…”

“…Accordingly, optical-based measurements of the transport properties of hot carriers are preferred, such as transient absorption microscopy, 23 spatially and temporally resolved pump-probe techniques, 24 and Ramanbased techniques. 11 Currently, silicon wafer is a very popular substrate for standard integrated circuit processing; it is ideal for creating transistors due to its extreme uniformity and good chemical and mechanical strength. 25 Therefore, tremendous work has focused on the study of semiconductor materials supported on silicon substrates, such as graphene, 26 TMDs, 12 and black phosphorus.…”

Thermal conductance between water and nm-thick WS2: extremely localized probing using nanosecond energy transport state-resolved Raman

“…Hot carrier transfer inside TMD materials, such as WS2, is well-studied in our previous works. 42,48,52 Regarding the thermal transport in cross-plane direction of WS2 sample, it is assumed that temperature distribution in this direction is uniform. In the thickness direction, heat diffusion length ( L  ) under ns pulsed laser heating can be estimated as:…”

Energy and Charge Transport in 2D Atomic Layer Materials: Raman-Based Characterization

“…In addition to the modulation in time domain, this energy transport design can also be extended to spatial domain to control the energy transport states. Yuan et al reports a novel technique for non-contact simultaneous determination of interface thermal resistance ( R ) and hot carrier diffusion coefficient ( D ) of MoS 2 nanosheets on c-Si by varying the laser heating area [ 29 ]. In this work, the constructed two energy transport states are in spatial domain.…”