Unveiling defect-mediated carrier dynamics in monolayer semiconductors by spatiotemporal microwave imaging

Abstract

The optoelectronic properties of atomically thin transition-metal dichalcogenides are strongly correlated with the presence of defects in the materials, which are not necessarily detrimental for certain applications. For instance, defects can lead to an enhanced photoconduction, a complicated process involving charge generation and recombination in the time domain and carrier transport in the spatial domain. Here, we report the simultaneous spatial and temporal photoconductivity imaging in two types of WS₂ monolayers by laser-illuminated microwave impedance microscopy. The diffusion length and carrier lifetime were directly extracted from the spatial profile and temporal relaxation of microwave signals, respectively. Time-resolved experiments indicate that the critical process for photoexcited carriers is the escape of holes from trap states, which prolongs the apparent lifetime of mobile electrons in the conduction band. As a result, counterintuitively, the long-lived photoconductivity signal is higher in chemical-vapor deposited (CVD) samples than exfoliated monolayers due to the presence of traps that inhibits recombination. Our work reveals the intrinsic time and length scales of electrical response to photoexcitation in van der Waals materials, which is essential for their applications in optoelectronic devices.

Keywords: charge carriers; defects; laser-illuminated microwave impedance microscopy; spatiotemporal dynamics; transition-metal dichalcogenides.

Fig. 1.

Diffusion mapping of photoexcited charge carriers. (A) Schematic diagram of the iMIM setup with bottom illumination. Both the tip and the focused laser spot can scan with respect to the sample. The monolayer WS₂ flake grown on double-sided sapphire and coated by ALD Al₂O₃ is also shown in the schematic. (B) Illustration of the carrier diffusion from the illumination spot. (C) Optical reflection image of sample A with the laser spot in the middle. (D) iMIM images inside the dashed box in C at different laser power labeled by the intensity at the center P_c. The dashed lines show the contour of the flake. (E) Line profiles across the CCD (Upper) and iMIM images (Lower) for P_c = 1.6 × 10⁶ mW/cm². The solid line is a Gaussian fit to the laser profile with w = 1.5 μm. (F) Measured photoconductivity (orange stars) profiles at various laser intensity. The solid lines show the numerical fits with a diffusion length L = 2 μm. Dashed lines in the panel with P_c = 1.6 × 10⁶ mW/cm² correspond to L = 1 and 3 μm.

Fig. 2.

Time-resolved iMIM and carrier lifetime measurements. (A) Schematic of the tr-iMIM setup, in which the sample is illuminated by a diode laser with its intensity modulated by an EOM. The EOM is driven by a function generator, which also triggers the high-speed oscilloscope that averages the iMIM signals. (B) Time-resolved iMIM-Im signals (averaged over 8,192 cycles) at the center (green) and edge (magenta) of sample A. (Inset) AFM image of the flake, on which the two measured spots are labeled. (Scale bar, 4 μm.) The rise time of ∼10 ns, limited by the temporal resolution of the setup, is indicated in the plot. (C) Relaxation of tr-iMIM signals, averaged over 10 measurements in B, at the center and (D) at the edge of the flake. The solid lines are biexponential fits to the data with two relaxation time constants. (E) Same as C but at a higher laser intensity. A rapid drop of the signal with t_drop ∼ 10 ns, again limited by the temporal resolution, is indicated in the plot.

Fig. 3.

Time-resolved iMIM results on exfoliated WS₂. (A) Time-resolved data on sample B (optical image in the inset). Except for the lowest laser intensity of 4.7 × 10⁵ mW/cm², the tr-iMIM curves all show a sudden drop faster than our temporal resolution, followed by an exponential decay with time constants of 100 ∼150 ns. (B) Amplitudes of the fast (red) and slow (blue) processes in the tr-iMIM data as a function of laser power. The dashed lines are guides to the eyes. (C) Steady-state iMIM-Im signals at the center of the laser spot as a function of P_c, taken from Fig. 1D. The dashed line is a linear fit to the data.

Fig. 4.

STEM, DFT, and physical model of the spatiotemporal dynamics. (A) STEM images of the CVD-grown WS₂ sample, showing the presence of atomic defects such as S (orange), S-S (red), and W (black) vacancies. (B) DFT calculations of the energy band structures and density of states for three types of common atomic defects in WS₂ monolayers. (C) Schematic diagram of the band structure with midgap and band-tail states, as well as multiple processes for charge generation and recombination. τ_g, τ_th, τ_r, τ_n, τ_tr, and τ_esc represent the time constant for carrier generation, thermalization, radiative recombination, nonradiative recombination, trapping, and escaping from traps, respectively. Characteristic timescales of these processes are denoted in the plot. Note that the sub-ns exciton lifetime in the radiative recombination process is not relevant for the free-carrier dynamics probes by the iMIM. It is assumed that the electron traps are filled in n-type WS₂ samples, while the hole traps are dominant in the photoconductivity response.