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

Figure 1. Illustration of GPPC.

Figure 2. HRTEM characterization.

Figure 3. Vacancy determination.

Figure 4. Single and double vacancies.

Figure 5. GPPC and PL.

Figure 6. PL maps.

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. Time constants found in the literature before were between 100 s and 2 h at room temperature in case of irradiation with visible light [1] or up to 10 days when irradiated with UV light [2]. The persistent photoconductivity effect has been attributed to both, inhomogeneities in the substrate [1, 3] or inhomogeneous adsorbates [2,4] or inhomogeneities in the TMD surface. Nevertheless, photo-generated charge carriers could 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).

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) enabled 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 I_ds 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 V_g 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

Two transport regimes

By using such a model, we carry out a quantitative analysis of the transfer characteristics in two regimes. Die Feldeffektmobilität liegt bei 1,5 cm² V^-1 s^-1, which is a typical value for CVD-grown ML-MoS₂ [5,8].

At RT, the conductivity of the ML-MoS₂ is defined by thermally induced activation of localized electrons surpassing the percolation level (the mobility edge), EP. The EP is the characteristic energy above which electrons are delocalized, i.e., propagate along a conduction channel experiencing only weak scattering [18,19]. In this regime, the concentration of delocalized electrons depends strongly on the temperature as well as V_g and thus allows one to tune the conductivity. The conductivity in the thermal activation regime is obtained as σ_ph^RT=σ₀ exp(-(E_P-μ_e)/(k_B T)), (1) where µ_e is the quasi-Fermi energy level considering both equilibrium electrons (µ_e,D), i.e., the electrons which are present in the absence of irradiation, and nonequilibrium, photo-generated electrons (µ_e,l) (see Fig. 1d), and σ0 is a coefficient which depends only weakly on both V_g and the intensity of the UV radiation. In strongly disordered materials with many localized states, µ_e is determined by the concentration of electrons according to [18,20] n^eq (V_g )+n^non-eq (J)=n₀ ∫_-∞^μeg(E)dE (2) where neq is the concentration of equilibrium electrons given by neq(V_g)=ε_rε₀V_g/ed, where ε_r = 3.9 and d = 300 nm are the relative dielectrics constant and the thickness of the SiO₂ gate insulator, respectively, ε₀ is the vacuum permittivity, e is the electron charge, n^non-eq is the concentration of non-equilibrium electrons, J is the intensity of UV irradiation, n0 is the maximum possible concentration of electrons in the CB and g(E) is the density of states. The density of localized states in the bandgap is approximated by [19,21] g(E)=2/U₀ exp((2|E-E_CB⁰ |)/U₀ ),E≤E_CB⁰,|E-E_CB⁰ |≫U₀, (3) where E is the energy of localized states and E_CB⁰ is the bottom of the CB in the absence of spatial fluctuations. Note that for disordered two-dimensional (2D) materials the percolation level is given as E_P=E_CB⁰[19]. The calculated transfer characteristics for the pristine, i.e., non-irradiated sample, are in good agreement with the experimental data giving the values of U₀≅0.18 eV and ₀≅2.7×10¹⁸m^-2. Since the recombination time of photoexcited carriers is expressed as τ=τ₀ exp[E_a/(k_B T)], where the activation energy E_a≈2U₀, in the limit of U₀≫k_B T one can expect the GPPC effect.

At LT, a transition to Mott’s variable-range hopping regime is observed, and for 2D disordered semiconductor materials, the conductivity is expressed by [19] σ_ph^LT=σ₀ exp[-(T₀/T)^(1/3) ], (4) with T₀=13.8/(k_B g(μ_e)r_corr² ). Using (4) to fit the experimental transfer characteristics for the pristine sample, we extract the correlation radius of the random potential U(r), rcorr = 5 nm. Now, fitting our experimental transfer data at different UV irradiation doses I_ds^RT (V_g )=V_ds σ_ph^RT (V_g )and I_ds^LT (V_g )=V_ds σ_ph^LT (V_g) to the theoretical model we calculate the variation of the concentration photo-generated electrons, n^non-eq (J, t), with time at RT (inset Fig. 1c) and with UV intensity at RT and LT. In agreement with our experiment, the decay of the long-living photo-generated electrons with time shows a bi-exponential decay process (see Fig. 1c). We attribute the two different exponents to the variation of the spatial separation between photo-generated electrons and holes (i.e., a part of photo-generated electrons and holes are localized in close proximity and thus recombine faster), whereas another part is separated at longer distances further away from each other and therefore recombine slower [6]. Furthermore, the previously reported photogating effect [1,16] may also contribute to faster exponent. Summarizing this part, we conclude that our experimental transport data and the theoretical analysis suggest large spatial fluctuations of the band structures in MoS₂-FETs, with the minima of the CB and maxima of the VB serving as trap sites for the photo-generated carriers.

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

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