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Abstract
Tungsten diselenide (WSe2) thin films exhibit ultrafast carrier recombination lifetimes, which makes them promising candidates for high speed modulators. With pulsed optical excitation, they could be used to realize all-optical, frequency agile, terahertz devices. Looking into the potential of this material for such applications, time-resolved terahertz spectroscopy can provide significant insight into its free carrier and exciton dynamics such as recombination lifetimes, photo-induced conductivity and decay pathways. In this study, we measure transient terahertz conductivity and photo-generated carrier lifetimes in custom-grown large-area WSe2 thin-films. We discuss its dependence on grain size and number of layers. By analyzing the tradeoffs between carrier-lifetimes, photo-generated conductivity, grain size, and the number of layers, we show that the response of these films can be tailored by controlling the growth parameters. Customizing the film terahertz response can enable large modulation without the need for integration with bulk semiconductors, as widely reported in the literature, thereby achieve high terahertz photoconductivity and high-speed operation. Across samples, our measurements show carrier decay timescale on the order ~10 to 100 ps and a transient conductivity that shows non-Drude behavior. This deviation from a Drude response is dominant within the first few picoseconds (<10 ps) before changing into a Drude like free-carrier response at longer delays. Based on our grown films, we experimentally demonstrate a metamaterial terahertz modulator with WSe2 as the only active element, attaining ~40% modulation depth.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Corrections
Prashanth Gopalan, Ashish Chanana, Sriram Krishnamoorthy, Ajay Nahata, Michael A. Scarpulla, and Berardi Sensale-Rodriguez, "Ultrafast THz modulators with WSe2 thin films: erratum," Opt. Mater. Express 11, 2242-2243 (2021)https://opg.optica.org/ome/abstract.cfm?uri=ome-11-7-2242
1. Introduction
Semiconductors with fast carrier relaxation times are highly desirable for high-speed applications. Ultrafast response has been achieved by manipulating relaxation in GaAs/AlGaAs quantum wells [1], intersubband transitions [2], or more commonly through carrier recombination. Terahertz conductivity in materials, essentially stems from free carrier absorption. In semiconductors, this could be achieved by doping, electrostatic gating or more commonly, by photo-injection of carriers using pulsed excitation. For bulk semiconductors, the operation speed in devices based on such mechanism relies on carrier recombination lifetimes, which in turn are associated with intrinsic material properties. However, to impart ultrafast recombination beyond these fundamental limits, application specific defect engineering or hybrid structures have been employed [3]. In general, design of terahertz modulators, requires proper choice of semiconductor to provide high speed operation. In this regard, two-dimensional semiconducting transition metal dichalcogenides (TMDs) are an interesting addition to the materials library and offer an additional route to design ultrafast devices. Possessing a finite band-gap and parabolic density of states (DOS), these materials exhibit high (induced) charge carrier density through both electrical and optical injection. Although, the carrier mobility is quite small as compared to graphene [4–6], their finite bandgap allows for high ON-OFF ratio and thereby making them suitable candidates for low-power switches and logic-gates [7,8]. In these materials, the lifetimes are strongly influenced by the number of layers, as shown for instance by carrier lifetime studies carried out in bulk crystals and monolayers of MoS2 and WSe2. While mono- to few-layers exhibit lifetimes of a few picoseconds [9], bulk crystals show carrier lifetimes exceeding 200 - 300 ps [10,11]. However, based on these studies, it is observed that although monolayers can offer faster switching speeds, they absorb only a small portion of the incident optical excitation and consequently possess small conductivity modulation. In general, there is a tradeoff between number of layers, carrier lifetimes, and photogenerated conductivity. Even though, modulation efficiency has been augmented through their integration with bulk semiconductors such as Si and Ge [12–16], this imposes a huge trade-off between the carrier lifetimes and modulation depths. A loss in speed of operation by almost three orders of magnitude (picosecond to nanosecond) becomes inevitable. Instead, one could employ multilayer thin films of TMDs, wherein controlling its grain size as well as number of layers offers a window of choice between carrier lifetimes and photoconductivity.
Tungsten di-selenide, a semiconducting member of the TMD family is a promising candidate for optoelectronics applications. Bulk WSe2 has a bandgap of ~1.2 eV which is comparable to commonly employed semiconductors. Possessing a smaller carrier effective mass than MoS2, it has the potential for improved performance in optoelectronic devices [17–19]. More importantly, it has a very high absorption coefficient of ~105 cm−1 at 800 nm [20,21] that is at least an order of magnitude higher than other semiconductors such as silicon (Si), gallium arsenide (GaAs) [22] and molybdenum disulfide (MoS2) [23]. Additionally, its absorption coefficient rises rapidly and becomes greater than 103 cm−1 within ~0.1 – 0.2 eV from its band edge, allowing us to utilize a conventional pulsed laser emission at 800 nm and not resort to higher energy excitations in order to attain substantial absorption. More interestingly, apart from free carriers, a strong excitonic response has been demonstrated in monolayer WSe2 [24], which directly impacts its frequency dependent photoconductivity.
In this work we study transient THz conductivity of WSe2 thin films on sapphire grown by a modified vapor transport method. We show that in these films we can control the thickness and grain size via controlling the thickness of a tungsten seed-layer and the growth temperature, respectively. We observe picosecond carrier recombination life times and study the relationship between grain sizes and induced photoconductivity. While influence of number of layers on carrier lifetimes is well known [9–11], the dependence of grain sizes on THz conductivity has not been studied in WSe2. With these measurements we aim to provide a qualitative framework for understanding the dependence between grain-size and photoconductivity that could prove useful in choosing appropriate thin film physical properties for the design of ultrafast THz modulators. Following these studies, we demonstrate a capacitively coupled terahertz metamaterial modulator manufactured via standard micro-fabrication processes.
2. Sample preparation and characterization
2.1. WSe2 growth and characterization
We grow large area WSe2 (~mm2) thin films on c-plane sapphire through chemical vapor transport (CVT). Tungsten thin films (~30 nm) are sputtered on clean sapphire substrates using DC sputtering. The substrates were then sealed in an evacuated ampoule (~10−3-10−5 torr) containing WSe2 powder (Alfa Aesar, 99.8%) that acts as a source of selenium at a growth temperature of 900 °C. This growth process was adapted from MoS2 growth demonstrated in [25]. The growth duration at set-point was ~4.5 hours followed by a controlled gradual cool-down at 0.5 °C/minute. Using powdered WSe2 source allowed for controlled supersaturation of selenium vapor pressure at high growth temperatures and thus allowing for the growth of large grain sizes (~1 µm). Although, this method precludes us from achieving monolayers, it provides a good degree of control over the final film thickness by adjusting the initial seed-metal thickness. This again plays into our requirement of multilayer films for achieving significant photoconductivity. The optimal growth temperature was decided based on thermogravimetric studies on WSe2 powders reported in [26] and a series of growths carried out at temperatures ranging from 750 °C – 900 °C.
X-ray diffraction measurements of the WSe2 thin film indicate high crystallinity, as seen in Fig. 1(a), and exhibiting only the family of (002) planes indicating the presence of a highly oriented (c-plane) thin film. Raman measurements shown in Fig. 1(b) further show an E12g mode at ~251.2 cm−1 consistent with the literature reports of multilayer films [27]. Optical transmission measurements were performed to ascertain absorbance trend and excitonic features as seen in Fig. 1(c) and SEM images reveal grain sizes ~1 μm or larger as shown in Fig. 1(d)). AFM measurements provide a height of ~50 - 60 nm (~2X starting metal film thickness). But more accurately, the final film thickness is a result of vapor transport of WSe2 from the source powder and the sticking coefficient of the WSe2 on the sapphire substrate.
Fig. 1 (a) XRD scans indicating highly crystalline WSe2 films showing only the (002) family of planes. (b) Raman mode at 251.2 cm−1, which agrees with reports in [24] for multilayer films. (c) UV-vis absorption measurements of WSe2 thin films indicating a strong excitonic absorption peak at ~740 nm. Based on this, optical excitation for OPTP and TRTS were chosen at 800 nm and 400 nm. (d) SEM image of WSe2 thin film shows grain size of ~1 µm.
3. Experimental results and discussion
3.1. Time-resolved and frequency dependent THz conductivity
Terahertz pulses were generated via optical rectification in a ZnTe crystal pumped with an 810 nm Ti-sapphire laser (1 kHz repetition rate). Optical delay lines control the time difference between the optical pump and terahertz probe at the sample. The transmitted THz signal is detected using a ZnTe crystal using the electro-optic sampling technique [28]. OPTP and time domain measurements were carried out to extract carrier recombination lifetimes and frequency dependent conductivity Δσ (ω), respectively. Figure 2(a) shows transient carrier dynamics from time resolved measurements at different optical fluences. In general, we observe increased photoconductivity with optical carrier injection and carrier recombination corresponding to the following rate equation:
From the above rate equation, the time dependent carrier density could be expressed as sum of exponentials. Here, G represents carrier generation. The decaying components constitute of mono- and bi-molecular mechanisms corresponding to rate constants and Given the optical pump excitation of 1.55 eV, the carriers would stem from contributions of indirect transitions (from Γ and K) and probable direct transitions (K-K). Following the generation, the recombination lifetimes extracted from the exponential fits show a slow recombination component, and a fast component, . Several observations can be made based on its behavior. First, the origin of the slow component, which is just observed at the lowest optical fluence, could possibly be attributed to: (i) phonon-assisted indirect e-h recombination, and (ii) possible band-band recombination. Second, increasing optical fluence results in an additional (fast) component of recombination. This could be attributed to, (i) enhanced carrier-carrier and carrier-lattice scattering due to the high population density, (ii) occurrence of many-body effects such as presence of excitons, and (iii) trap assisted recombination stemming from surface and grain boundary states. Table 1 summarizes the extracted time constants at different fluences at 800 nm.Fig. 2 (a) OPTP spectroscopy at 800 nm showing the recombination lifetimes of carriers in WSe2 thin films for different optical fluences – 35 μJ/cm2 (yellow), 85 μJ/cm2 (orange) and 125 μJ/cm2 (blue). We measure the change in the peak value of the incident THz electric field (ETHz). Increased optical fluence results in a change from single exponential decay to a bi-exponential decay. The bi-exponential decay has two components – fast (t2) lasting tens of picoseconds and a slow (t1) component extending for hundreds of picoseconds, (b) Excitation with 400 nm produces different recombination pathways with a short component of 42.05 picoseconds ( ± 1.18 ps) and a long component that extends to 1.86 nanoseconds ( ± 1.58 ns). The insets in both figures depict band structure of bulk WSe2 from ab initio calculation (extracted from [29]).
Table 1. Extracted carrier decay time constants (t1 and t2) under different optical fluence at 800 nm.
In general, the relative contributions of these mechanisms depend on the nature of the semiconductor, doping levels and more importantly their physical characteristics; as in our case dictated by the film microstructure. Given that our films are few-layer and polycrystalline with indirect bandgap, we believe some recombination mechanisms are more prominent at different fluence levels. Surface recombination could be an important mechanism to be considered in these measurements. While it is the dominant mechanism in monolayer samples, in thin films or bulk, they are limited by the diffusion of carriers from the bulk to the surface. Further limited by the polycrystallinity of the sample the out of plane diffusion drops exponentially with increasing thickness in a disordered film [30]. Additionally, since ultrafast exciton formation from induced e-h pairs has been reported in both monolayer (sub picosecond) and bulk (~1 ps) WSe2 [11,31], excitonic contributions (annihilation) could also be possibly taking place. While, the binding energy of excitons in monolayer samples are up to ~100 meV, in multilayer samples they could be close to ~25 meV. Therefore, their annihilation lifetimes typically fall in the timescales we measure. We believe the above-mentioned phenomena to be predominant in our case, however detailed studies are warranted to completely understand carrier dynamics in these films.
Furthermore, we studied these dynamics at 400 nm (~3.10 eV) optical excitation. In this case we observe a fast component of 42 ps and a long component of 1.86 ns. The different slow component of lifetime observed is consistent with the fact that this energy leads to carrier excitation into conduction band side-valleys (aside from Г-point), which can exhibit different recombination pathways as observed in Fig. 2(b) and its inset depicting the possible transitions (pink arrow).
To further elucidate the nature of charge species in this material we measured the frequency dependent conductivity at different pump-probe delay intervals: <1ps, 20ps, 100ps and 200ps. We extract the transient conductivity (Δσ (ω)) using the formula,
where ‘ns’ is the substrate refractive index, ‘Zo’ is the free space impedance and ‘Efilm(ω)’ and ‘Esub(ω)’ are the complex THz electric field passing through the WSe2 thin film and the substrate respectively. This formalism can be derived by assuming a transmission line geometry and where the thin film thickness is much lesser than the skin depth (applicable to our case). From the extracted behavior, shown in Fig. 3(b), we see that the real part of conductivity could not be explained by a simple Drude model. Deviation is more prominent close to excitation (less than 10 ps), while it reverts to a Drude-like behavior at longer delays. Deviation from Drude-like behavior has been reported in monolayer WSe2 and few-layer WS2 [9,32]. Different contributing factors such as internal resonant state of trions and collective oscillations of charged particles have been proposed, respectively. This non-Drude behavior coupled with the fact that strong excitonic resonances have been observed in monolayer WSe2 at room temperatures [33] could point towards strong carrier-carrier interactions in our thin film samples with possible quasiparticle formation. Although, the positive slope in transient conductivity could also be attributed to scattering events at grain boundary and possibly a Drude-Smith behavior, the time dependent change in this slope indicates that the contributing phenomena is transient and thus closely related to the injected carrier population. Given that the energy of the THz probe is not enough to probe the internal excited states of an exciton, its contribution to the conductivity can conclusively be ascertained only from measurements over a larger frequency range than what is currently possible in our setup. In a similar fashion, 400 nm excitation, being far above the exciton binding energy, depicted in Fig. 4(d), also exhibits a similar trend in Δσ (ω) with non-Drude behavior being dominant at smaller delays. The trend is more evident in this case as the excitation energy is much larger than the exciton energy and we believe that a relatively larger fraction of excited carriers (as compared to 800 nm) bind to form electron-hole pairs.Fig. 3 (a) Change in peak value of the THz electric field at different time delays between optical pump (800 nm) and THz probe. (b) Corresponding extracted real part of transient conductivity. (c & d) THz electric field versus time delay and corresponding real part of conductivity for a 400 nm excitation. We can observe from (b) and (d) that the slope of the extracted conductivity changes over time and doesn’t follow a classic Drude like fee-carrier response for all time delays.
Fig. 4 (a) OPTP measurements of different grain sized samples (I, II and III/ ~10 nm, ~100 nm and ~1 µm) show significant differences in recombination lifetimes. This indicates that the size of the grains (relative to the probe length) plays a significant role in the induced THz photoconductivity. Smaller grains have a faster response but a smaller value of induced photoconductivity at the same optical fluence. The schematic depicted in the inset illustrates the role of grain boundary as recombination centers. The scale bar in all the SEM images is 2 µm. (b) Schematic illustration showing the observed tradeoff between photoconductivity and carrier lifetime with grain-size. An optimum window exists between the two, which can enable high-speed THz modulators with simultaneous large modulation depth.
3.2. Influence of grain sizes on carrier lifetimes and THz conductivity
As mentioned earlier, the grain sizes tend to have a direct influence on carrier recombination lifetimes and induced photoconductivity. Growths were carried out to attain different grain sizes (~10 - 30 nm, ~100 nm and ~1 μm). Their corresponding photoconductivity and carrier lifetimes are shown in Fig. 4 (I, II and III) respectively. The OPTP measurements on these different grain-sized samples were carried out at an optical fluence of 125 µJ/cm2. As shown in Table 1, I possess two components of decay lifetimes. II also exhibits a two-component lifetime, fast component (t2) of 3.53 ps ( ± 0.11 ps) and a slow component (t1) of 49.26 ps ( ± 2.79 ps). III exhibits only a single fast component of 2.22 ps ( ± 0.24ps). These measurements indicate that photoconductivity and carrier lifetimes are very sensitive to grain size with the smallest (spherical) grains having the lowest photoconductivity (very close to the noise floor) and the shortest carrier recombination lifetime. Transient conductivity (Δσ (t)) can be expressed as Δσ (t) = n(t)eµ(t), with both the induced carrier concentration ‘n’ and mobility ‘μ’ being a function of time and extremely sensitive to grain boundaries [34]. Small grains tend to impact this in two major ways – (i) they produce fewer electron-hole pairs per cross-sectional area i.e. reduced carrier concentration and, (ii) carrier transport in small grains are more susceptible to scattering at the grain boundary resulting in reduced mobility. The probe length of THz pulses, assuming a diffusive transport model was calculated to be ~30 to 120 nm [35,36]. The combination of these two factors result in an overall reduction of photoconductivity as seen in Fig. 4(a).
Also, from the table, we observe that both the components of lifetimes become smaller upon decreasing grain sizes i.e. accelerated carrier recombination. This leads us to believe that, firstly, the grain boundaries possibly introduce a series (or band) of near mid-gap states that act as efficient carrier recombination centers. This phenomenon and mechanism have been observed and proposed in other members of the TMD family [37,38]. Secondly, electron-phonon interactions also strongly influence the excited state lifetimes of carriers in materials. Time domain atomistic modelling performed on methyl-ammonium perovskites have shown that grain boundaries typically enhance electron-phonon coupling in the material by contributing to electron and hole wavefunctions and allow higher order modes to exist thereby providing additional energy transfer pathways [39]. This formalism could be adapted to our material system as well and would indicate that electron-phonon scattering becomes more dominant due to increased concentration of grain boundaries. These comparative measurements indicate that an optimum window of grain size and thickness exists, which provides an important degree of control for designing ultrafast thin-film based devices.
3.3. Ultrafast THz modulators with hybrid metamaterials
Metamaterials can provide unprecedented control over the amplitude, phase, frequency and polarization of electromagnetic waves. The meta-atoms, i.e. unit-cells, composing the metamaterial typically made from metal precludes tunable operation, which necessitates integration with a tunable element and wherein the stimulus could be electronic, optical, thermal or others. Several hybrid device concepts for terahertz modulation involving traditional semiconductors (GaAs, Si and Ge) [40–45], methylammonium perovskites [46–49], superconductors [50–52] and phase change materials [53,54] have been demonstrated. In this regard, a tunable semiconducting element with pulsed ultrafast excitation helps in achieving a more efficient temporal control of device performance. In the TMDs family, a variety of terahertz modulators have also been reported in the literature typically using MoS2-Si hybrid [12,13] and WS2-Si hybrid structures [14] to maximize its modulation efficiency. However, these are limited in their speed of operation. Recently, ultrafast operation using (standalone) MoS2-metamaterial hybrid has been realized via spin-on technique allowing for easy fabrication [55]. Given WSe2’s promising attributes we demonstrate that it could be employed in a similar fashion providing both a fast speed and large modulation depth.
Our choice of design is primarily based on the ease of integration since post-growth patterning and lithography techniques could lead to surface defects and introduce impurities that could alter carrier recombination and photoconductivity. In order to preserve the film’s attributes, we employ a capacitively coupled metasurface as shown in Fig. 5 (a) that provides for amplitude modulation at resonance. This design was adapted from the work reported in [56] and the frequency of operation is at ~1.3 THz since at high frequencies our films exhibit the larger conductivity modulation. In general, the operation frequency, i.e. frequency at which the structure exhibits largest modulation of transmission, can be tuned by choosing appropriate aperture and unit cell dimensions. For device fabrication, a Parylene C film of approximately ~1.2 µm was deposited on the WSe2 thin film and a frequency selective surface (FSS), as shown in Fig. 5(a), was patterned on top of it employing Cr/Au (20 nm/100nm) using standard lithography techniques. As seen in Fig. 5(c), with this design, we achieve ~40% modulation depth in transmitted terahertz power (~125 μJ/cm2 optical fluence) with a ~50 nm WSe2 film. Our estimations show that the modulation efficiency can be increased to almost 100% by increasing WSe2 film thickness to ~100 nm while still retaining its high-speed operation and requiring no integration with bulk semiconductors. Given WSe2’s higher absorption coefficient, as compared to MoS2, it is able to absorb a large fraction of the incident radiation with a smaller overall thickness. Full-wave electromagnetic simulations were carried out so to simulate the behavior of the hybrid metasurface using ANSYS HFSS. The experimentally extracted value of WSe2’s complex conductivity at different time delays was utilized when defining the constitutive parameters of WSe2 in the simulation. Figure 5(b) depicts the corresponding simulated device response. Interestingly, although the thin film exhibits carrier lifetimes of several hundred picoseconds (long lived decay), in a metasurface hybrid structure the contrast of WSe2 conductivity as compared to the metal becomes relevant. As a result, the hybrid device reverts back to its OFF-state operation at much shorter timescales (~100 ps) contrary to its thin film behavior. The small deviation between simulated and experimental transmission levels seen in Fig. 5(b-c) could be attributed to differences in device dimensions after fabrication. By choosing a more robust device design, e.g. with larger gaps as in [57], a less sensitive response to such variations might result.
Fig. 5 (a) Schematic representation of capacitively coupled WSe2 / metal hybrid metasurface. (b) Transmission spectra obtained through full-wave simulations. (c) Experimental measurements of the fabricated metasurface exhibiting a similar response to the simulations in (b). The inset in (c) shows an optical image of the fabricated WSe2 – hybrid metasurface. The scale bar in the inset corresponds to 100 µm.
4. Conclusion
In conclusion, we demonstrated modified-CVT grown WSe2 thin films, which as a result of an ultrafast response and large photo-generated conductivity can be well suited for application in ultrafast THz devices. Assessing its carrier recombination (lifetimes and potential mechanisms) through time-resolved photoconductivity studies reveals interesting pathways and decay mechanisms that could be attributed to both free-carriers and excitons. More importantly, we provide crucial insights into the dependence of THz conductivity on grain size and demonstrate the existence of an optimum window of choice providing for a good tradeoff between photoconductivity and recombination lifetime thus modulation depth and speed. Based on these observations, we demonstrate the integration of our WSe2 films with a metallic frequency selective surface in a metamaterial structure that exhibits ~40% modulation depth in terms of transmitted THz power. Overall, our observations pave a road for new investigations on the role of grains in the ultrafast terahertz properties of WSe2 as well as introduce the idea of harnessing grain size and film thickness through controlling two growth parameters, i.e. temperature and tungsten seed layer thickness, as a way to customize the ultrafast properties of these films and obtain a tailored device response.
Funding
National Science Foundation (NSF) (Award # 1407959, 1351389 and DMR 1121252).
Acknowledgements
This work was performed in part at the Utah Nanofab sponsored by the College of Engineering, Office of the Vice President for Research, and the Utah Science Technology and Research (USTAR) initiative of the State of Utah and made use of University of Utah shared facilities of the Micron Technology Foundation Inc. Microscopy Suite sponsored by the College of Engineering, Health Sciences Center, Office of the Vice President for Research, and the Utah Science Technology and Research (USTAR) initiative of the State of Utah. The authors appreciate the support of the staff and facilities that made this work possible.
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