Material-specific lattice defects linked to persistent photoconductivity in some MoS2 field-effect transistors

January 11, 2021 - Monolayer transition metal dichalcogenides (TMD) have numerous potential applications in ultrathin electronics and photonics. A team of scientists from the Universities of Jena and Ulm and the IPHT in Germany as well as the National University of Science and Technology in Russia and the Wayne State University in the USA has presented an experimental and theoretical investigation of persistent photoconductivity in CVD grown monolayer MoS2.

The persistent photoconductivity effect (PPC) was demonstrated for monolayer MoS2 (ML-MoS2) after irradiation with visible light. The time constants were between 100 s and 2 h at room temperature [1] or up to 10 days when irradiated with UV light [2]. It can be caused by inhomogeneities either in the substrate [1, 3] or in the adsorbates [2,4] or on the TMD surface. Nevertheless, photo-generated charge carriers can also originate from intrinsic, material-specific lattice defects, which has not yet been extensively studied in the literature previously for transition metal dichalcogenides (TMD).

The new study now showed that in field effect transistors (FETs) made of monocrystalline MoS2 produced by chemical vapor deposition (CVD) [5] extremely long-lived giant PPC (GPPC) can occur after exposure to UV light (λ = 365 nm) due to intrinsic defects.

The study illuminates the possible factors of the observed GPPC effect. Spectroscopic and microscopic examination of the ML-MoS2 at the atomic level showed atomic vacancies in the monolayer, which could be identified as the main factor for the effect.

The complex interactions that determine the GPPC effect were described by a model and are shown in Fig. 1. At room temperature (RT), photo-generated charge carriers lead to an increase in conductivity by a factor of up to ~ 107, which is shown in Fig. 1b. The state of high conductivity then persists for a long time with a time constant of up to 30 days (Fig. 1c).

Through the transport model, the scientists also enabled an insight into the cause of the experimental results. The presence of large spatial fluctuations in the potential energy of charge carriers (electrons and holes) in the ML-MoS2 leads to a separation of photo-generated charge carriers (see scheme in Fig. 1d), leading to a long recombination time [6]. 2 transport regimes became recognizable (i) the thermal activation regime at room temperature (RT) and (ii) the step regime with variable range hopping at low temperatures (LT).

“In addition, atomically resolved transmission electron microscopy (TEM) made it possible to determine the density of point defects in the samples,” said Ute Kaiser from Ulm University. “With our SALVE TEM we were able to measure the concentration of defects in the materials - which initially posed a particular challenge, especially in view of the high electron sensitivity of the material." In some regions of the material, large fluctuations in the concentration of highly localized states in the forbidden energy gap were also found with scanning tunneling spectroscopy (STS).

Observation of giant persistent photoconductivity in monolayer MoS2

The transport properties between 1 and 30 days are shown in Fig. 1b. The resulting decrease in Ids over time is shown in FIG. 1c. The data can be described by an exponential decay function. The 2 main time constants are τ1 ≈ 1 day and τ2 ≈ 34 days. At low temperatures (LT, 6 K), the absolute values of the measured currents were lower than at RT, indicating higher localization at low temperatures [7]. The different transport models are explained in Box 1.

The existence of spatial inhomogeneities in the band structure of ML-MoS2 could also be proven experimentally. To enable measurements with high sensitivity using Scanning Tunneling Spectroscopy (STS) [9], the CVD-grown ML-MoS2 was transferred to a Pt (111) single crystal, passivated by a monolayer of hexagonal boron nitride (h-BN). By evaluating the onset of the valence band (VB) and conduction band (CB) regions, the scientists also determined other characteristics of the fluctuations. The band gap fluctuations occur mainly due to trap states in the vicinity of the CB. Trapping states with dimensions of about 5-10 nm and an energy variation of 0.25 eV are formed. The variation in the band structure in the VB region is less pronounced. The spatial and energetic dimensions of the fluctuations exactly matched the parameters that one would expect based on the theoretical analysis of the transport data. This confirmed the model adopted for electronic transport.

Defects and strain correlate with the band structure fluctuations

The potential fluctuation can in principle be caused by various effects. Fluctuations in the band structure can be caused, for example, by defects in the atomic lattice. In addition, extrinsic causes, such as charges in the substrate, can also play a role. To further investigate the causes, the scientists measured the defect density with aberration-corrected high-resolution TEM (HRTEM) [10]. A representative HRTEM image of ML-MoS2 is shown in Fig. 2a. The Fourier filtering of the ML-MoS2 grating frequency allowed measuring of the exact defect concentration [11, 12].

The effect of the electron beam, which can cause imperfections in ML-MoS2, was precisely found by a time-dependent measurement. In this way, it was possible to distinguish between the newly generated and the initially present defects (see Fig. 3).

In Fourier-filtered Cc/Cs-corrected HRTEM images, the exact position of the vacancies can be clearly found. Fig. 4 shows a Cc / Cs-corrected HRTEM image of MoS2 with single (S1) and double (S2) sulfur vacancies. To differentiate between two types of vacancies. The contrast for individual S1 vacancies is noticeably reduced compared to an untouched S column.

In ML-MoS2, which was produced by CVD, a biaxial strain can be produced, since the expansion coefficients of the 2D material and the SiO2 substrate underneath do not match [13]. Strain can also lead to fluctuations in the band structure, which can contribute to the observed photoconductivity in the MoS2 FETs. To precisely find the strain relative to exfoliated ML-MoS2, the research team used the Grüneisen parameters (𝛾) and electron doping factors (𝑘𝑛) from literature [14] and a method for Raman-derived quantitative analysis / derivation of the exciton transition energy. The strain in ML-MoS2 grown using the CVD method was estimated to be 0.34 ± 0.08% compared to exfoliated MoS2 flakes. For 2D materials exfoliated from the bulk crystal, it is assumed that they are not strained [13]. Lattice deformation leads to smaller fluctuations in the band structure and, in addition to deep-lying states such as atomic vacancies. Both, strain and defects can cause PPC effects and lead to characteristic relaxation times.

Disentangling substrate effects and intrinsic effects

In order to further investigate the substrate effect on GPPC in the FETs, the scientists first carried out a comparative study with WS2-FETs and MoS2-FETs made from CVD-grown monolayers. By evaluating the HRTEM data (Fig. 3c, d) they found that the total intrinsic concentration of sulfur vacancies in ML-WS2 is lower by a factor of 1.6 than in ML-MoS2. In addition, the concentration of S2 vacancies was about a factor of 3 lower than in ML-WS2. The lower concentration of defects thus correlates with the lower relaxation time.

In addition, the scientists carried out further electrical measurements with short gate pulses. [1] If the surrounding material of the MoS2 layers, especially the substrate, played an important role for the PPC effect, the application of a short gate pulse could lead to a discharge or reorientation of the photo-induced changes and thus to a decrease of the observed photocurrent. [1] On the other hand, in the case of intrinsic origins such as fluctuations in the band structure, no significant decrease of the photocurrent should be expected after applying a gate pulse.

After applying several short gate pulses from -60 V to 0 V, the photocurrent almost returned to the initial value in the case of WS2, but remained almost unchanged in the case of MoS2. As a result, external factors are to be assumed for the WS2-FETs [1,15,16], while internal factors should contribute to the observed PPC effect in the MoS2-FETs.

The hysteresis, i.e. the difference in the respective transfer curves between the forward and backward Vg runs of UV-irradiated devices, can also provide information about the origin of the PPC effect. If there was a high density of charge traps at the interface between the MoS2 channel and the SiO2 surface, one might expect a hysteresis in the transmission properties of the FETs. [15,16,17]. A large hysteresis would therefore show that external factors play a role in the transfer characteristic. The hysteresis was much larger for WS2 FETs than for MoS2 FETs. This is in good agreement with these results. In the case of MoS2-FETs, the substrate seems to play a subordinate role, and the defect-related spatial fluctuations in the band structure are the outstanding mechanism of GPPC.

Modification of the optical properties of ML-MoS2

In addition to the modification of the electronic properties, the researchers also expected a modification of the optical properties through UV irradiation. After the irradiation, a large number of electrons is localized, which should lead to a quenching of the photoluminescence (PL). As in Fig. 5a and b. As can be seen, the PL emission is indeed reduced after the UV irradiation.

Interestingly, the effect was also found with suspended ML-MoS2 on TEM grids, see Fig. 6. Such a grid is a thin carbon film with an array of holes 2 µm in diameter spaced 2 µm apart. However, an effect of the UV irradiation could still be clearly observed on the partially suspended samples. The GPPC effect is thus also observed with free-standing layers. Their results showed that the investigation of atomic defects in materials can play a crucial role in their optoelectronic properties. It is therefore extremely important to understand the defect-related properties to further develop the field of TMD-based electronics and optoelectronics.

Highlighted Topics

Two transport regimes

Resource: George, A.; Fistul, M. V.; Gruenewald, M.; Kaiser, D.; Lehnert, T.; Mupparapu, R.; Neumann, C.; Hübner, U.; Schaal, M.; Masurkar, N.; Arava, L. M. R.; Staude, I.; Kaiser, U.; Fritz, T.; Turchanin, A. Giant persistent photoconductivity in monolayer MoS2 field-effect transistors. npj 2D Materials and Applications 5, 1, 15. (2021), doi: 10.1038/s41699-020-00182-0.

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