Taking electrical control
Excitons—bound pairs of electrons and holes in a solid—can, in principle, be used as information carriers. However, their lifetime is limited because the electrons and holes tend to quickly recombine. One way to extend this lifetime is to physically separate electrons and holes—for example, by having them reside in different layers of a van der Waals heterostructure. Jauregui et al. used this strategy to form long-lived interlayer excitons in a heterostructure made out of monolayers of molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2). Through electrical control of the layers in the heterostructure, the researchers further increased exciton lifetime and formed and manipulated charged excitons.
Science, this issue p. 870
Abstract
A van der Waals heterostructure built from atomically thin semiconducting transition metal dichalcogenides (TMDs) enables the formation of excitons from electrons and holes in distinct layers, producing interlayer excitons with large binding energy and a long lifetime. By employing heterostructures of monolayer TMDs, we realize optical and electrical generation of long-lived neutral and charged interlayer excitons. We demonstrate that neutral interlayer excitons can propagate across the entire sample and that their propagation can be controlled by excitation power and gate electrodes. We also use devices with ohmic contacts to facilitate the drift motion of charged interlayer excitons. The electrical generation and control of excitons provide a route for achieving quantum manipulation of bosonic composite particles with complete electrical tunability.
Long-lived excitons can be potentially used for the realization of coherent quantum many-body systems (1–3) or as quantum information carriers (4, 5). In conventional semiconductors, the exciton lifetime can be increased by constructing double–quantum well (DQW) heterostructures, in which spatially separated electrons and holes form interlayer excitons (IEs) across the quantum wells (6–12). Strongly bound IEs can also be formed by stacking two single atomic unit cells of transition metal dichalcogenides (TMDs) into a van der Waals (vdW) heterostructure. TMD heterostructures, such as MoSe2/WSe2, MoS2/WS2, and MoS2/WSe2, have shown ultrafast charge transfer (13), the formation of IEs with a large binding energy of ~150 meV (14), and diffusion over long distances (15). Moreover, the tight binding and small exciton Bohr radius potentially allow for quantum degeneracy of these composite bosons, which may lead to exciton condensation at substantially elevated temperatures compared to those of, e.g., conventional Bose-Einstein condensates of cold atoms (2).
In this work, we fabricate individually electrically contacted optoelectronic devices using hexagonal boron nitride (h-BN)–encapsulated vdW heterostructures of MoSe2 and WSe2 (16, 17). Optically transparent electrical gates and ohmic electrical contacts realized for the individual atomic layers allow us to have complete control of the carrier densities in each TMD of the DQW while maintaining full optical access. The top and bottom insets of Fig. 1A show an optical image of a representative device with false-colored top gates and a schematic cross section, respectively [a detailed device scheme is illustrated in fig. S1 (18)]. The green and red false-colored gates depict the contact gates for doping the MoSe2 and WSe2 regions, respectively. These contact gates, together with the prefabricated Pt electrodes, provide ohmic contacts in the WSe2 p-channel (19).
(A) IE PL spectra versus electric field applied to the heterostructure
The presence of the top (optically transparent) and bottom electrical gates, in addition to the separately contacted TMD layers, allows us to control the carrier density in the individual TMD layers as well as the electric field across the TMD heterostructure, Ehs, using the voltage Vtg (or Vbg) applied to the top (or bottom) gate. For intrinsic TMD layers (i.e., no free carriers and the chemical potential located within the semiconducting gap), where the heterostructure can be approximated by a thin dielectric slab, Ehs is controlled by a gate operation scheme in which we apply opposite gate polarity [supplementary text section 1 in (18)]. Figure 1A shows the photoluminescence (PL) spectrum measured at temperature T = 4 K as a function of Ehs, keeping both TMD layers intrinsic. We observe a linear shift of PL peak energy with Ehs, suggesting a first-order Stark shift caused by the static electric dipole moment across the vdW heterostructure. By fitting the linear PL peak shift with the linear Stark shift formula
The IEs in the vdW heterostructure can live longer than intralayer excitons, owing to the spatial separation of electrons and holes in the heterostructure. Figure 1C shows the IE lifetime τ as a function of Ehs, measured by time-dependent PL after pulsed laser illumination. The lifetime τ increases as Ehs increases, reaching ≈600 ns for Ehs > 0.1 V/nm, an order of magnitude larger than in previous studies (14, 23). The observed dependence of τ on Ehs can be explained by changes in the overlap between the electron and hole wave functions [section 3 of (18)]. The recombination rate of an exciton is proportional to the probability that the electron and the hole occupy the same location. An electric field antiparallel to the IE dipole moment is therefore expected to reduce the recombination rate as it pulls the two carriers apart, consistent with our measurements. Considering the PL intensity modulation shown in Fig. 1A together with the measured τ, we also demonstrate that the emission efficiency (η) can be tuned with Ehs, reaching η ~ 80% when Ehs is aligned against the IE dipole moment, promoting the recombination process [section 4 of (18)].
The electrostatic condition in our heterostructures is greatly modified if we change the gate operation scheme such that one of the TMD layers is doped electrostatically, introducing free charge carriers. Figure 1E shows the IE PL spectrum after the gating scheme depicted in Fig. 1D. In this scheme, as shown in the normalized reflection
Our measurements in the doped regime can be explained by the formation of charged IEs (CIEs) (24). As shown in the normalized reflection measured with the same gating scheme, we can identify (i) intrinsic/p, (ii) intrinsic/intrinsic, and (iii) n/intrinsic regions by the disappearance of the absorption dips for intralayer excitons in MoSe2 and WSe2 (25), which are well aligned with the sudden red shift observed in IE PL (vertical dashed lines in Fig. 1, D to G). Thus, this jump in energy can be related to the CIEs. Note that charged excitons can be referred to as trions (i.e., three-body bound states) (26–29) or, alternatively, attractive polarons (i.e., excitonic states dressed by a polarized fermionic sea, similar to those in monolayer TMDs) (30, 31). The value of the observed jump ≈10 meV (15 meV) for positive (negative) CIEs is in good agreement with the calculated binding energy of CIEs (24). The lifetime of CIEs is ≈100 ns near the band edge but decreases with increased doping, presumably owing to additional decay channels enabled by scattering with free carriers (Fig. 1G).
We create high densities of IEs by increasing laser power. In particular, for neutral IEs, we observe that the PL emission is shifted to higher energy with increasing power (Fig. 2A), consistent with a mean-field shift stemming from the repulsive dipole–dipole interaction between oriented IEs. Following the analysis based on a parallel plate-capacitance model used for GaAs DQW IEs (32), we obtain a lower bound for the IE density of ~5 × 1011 cm−2 [section 6 of (18)]. The high density of long-lived IEs and the large τ observed in our heterostructures can enable transport of IEs across the samples. Figure 2, B to D, shows the spatial map of the IE PL intensity at different laser powers. The PL signal can be detected far away from the diffraction-limited focused laser spot (<1 μm in diameter). At the highest power, PL can be observed many micrometers away from the excitation spot, strongly suggesting transport of IEs across the sample. From these maps, we obtain the normalized, radially averaged PL intensity,
(A) Power (P) dependence of the normalized PL spectra collected from the same spot as the excitation. The blue dashed line corresponds to the PL peak position versus power. (B to D) Spatial dependence of the intensity of the normalized PL for P = 10, 100, and 1000 μW, respectively. The white outlines depict the heterostructure area. The continuous wave laser excitation (λ = 660 nm) is fixed at the top left of the sample. Scale bar, 5 μm. All measurements were performed at 4 K. Experiments performed at higher temperatures provide smaller spatial extension of PL around the excitation [see section 12 of (18) for details]. (E) Power dependence of normalized radially averaged IPL (normalized IPL) versus r with the excitation fixed at the center of the sample. The red and black dashed lines represent
We also used a diffraction-limited focused pulsed laser to measure the temporal decay of the PL intensity. Figure 2F shows an estimate of the time-dependent exciton population (integrated PL signal along the heterostructure weighted by r2) after a laser pulse with peak power P. The time-dependent PL initially exhibits a faster decay process with characteristic time scale τ1 ~ 10 ns, followed by a slower decay process occurring on the time scale τ2 ~ 100 ns, suggesting that there are two different mechanisms for the PL intensity decay. The value of LD estimated above can be converted to a diffusion constant according to
We obtain further evidence for IE transport from time-dependent spatial PL maps with a pulsed laser illuminating the center of the sample. We measure the IE PL intensity
Unlike the neutral IEs discussed above, CIEs can be manipulated by an in-plane electric field. Figure 3A shows the spatial map of IPL overlaid with the device image when CIEs are optically excited at the center of the sample. Similar to neutral IEs, CIEs generated at the laser-illuminated spot can diffuse across the entire sample. Both the WSe2 and MoSe2 layers in our device have multiple electrical contacts away from the heterostructure edge (~10 μm away) that are used to control the lateral electric field while avoiding any local Schottky barrier effects. Figure 3B shows the spatial map of the PL intensity normalized as IPL(Vds)/IPL(Vds = 0) when applying an interlayer bias voltage of Vds = 3 V across the WSe2 layer. We observe that the grounded edge of the sample becomes brighter with increasing Vds (also see Fig. 3C for the normalized average emission intensity along the heterostructure channel). This increase in PL at the boundary between the heterostructure and the highly doped monolayer region can be explained by drift of CIEs under the applied bias voltage in the channel. The applied bias Vds creates an electric field to tilt the band structure in the direction of the WSe2 channel, driving positive (+) CIEs along the same direction as shown in the schematic diagram in Fig. 3D. At the boundary of the heterostructure, however, the +CIE cannot be transported to the WSe2 p-channel because current across the boundary must be preserved. Therefore, the transported +CIEs recombine to turn into a hole in the WSe2 p-channel. In figs. S13 and S14, we further confirm this picture of CIE transport by changing the doping polarity and Vds direction [see section 9 of (18) for details on gate configurations and alternative explanations]. We also note that, owing mainly to the limited resolution of our electrical setup, we do not observe any photocurrent >10 pA.
(A) Spatial dependence of IPL with the laser excitation fixed at the center of the heterostructure (blue arrow). An optical image of the device with false-colored top gates that cover the WSe2 and MoSe2 contacts is overlaid. An in-plane electric field is applied by a voltage (Vds) in one of the WSe2 contacts while the other contact remains grounded. GND corresponds to the grounded electrode. Scale bar, 5 μm. (B) Spatial dependence of IPL normalized according to
Finally, we demonstrate the electrical generation of IEs by free-carrier injection using ohmic contacts in individual TMD layers. Because our heterostructure forms type II–aligned p- and n-layers, the charge transport across WSe2 to MoSe2 is expected to show diode-like rectifying behaviors (34, 35). Figure 4A shows interlayer current (Ids) versus interlayer bias (Vds) curves, whose characteristics can be modulated by Vtg and Vbg. Changing Vtg and Vbg adjusts the band offset in the type II heterojunction and the doping in each layer. The inset to Fig. 4A shows a map of the forward bias current at a fixed bias voltage. One can identify the region in which both WSe2 and MoSe2 layers remain intrinsic, consistent with the absorption spectrum discussed in Fig. 1. Notably, we find that this p-n device generates detectable electroluminescence (EL) at sufficiently high bias. A particularly interesting EL condition occurs when both TMD layers are intrinsic, thus allowing electrons and holes to recombine through the formation of IEs. Figure 4, B and C, shows the EL maps of the heterostructure region. The local EL intensity in the heterostructure depends on the local recombination current density, which can be controlled by Vtg and Vbg [section 10 of (18)]. We find that the EL spectrum resembles the PL spectrum in the same (Vtg, Vbg) configuration (34). Figure 4D shows EL versus Ehs. Similar to the PL shown in Fig. 1A, the EL spectrum shifts linearly with Ehs, which can be attributed to the IE Stark effect. More direct evidence that the EL process in our heterostructure is mediated through the IE formation by carrier injection is provided by the EL lifetime. Figure 4E shows the EL intensity as a function of time when we pulse Vds at a fixed Vtg and Vbg. We measure the EL at the falling edge of the pulse. Long and short lifetimes of ≈150 and ≈25 ns are obtained for the gate voltages of Vtg = 10 and −10 V with Vbg = 16.3 V, corresponding to the neutral IE and charged IE formation regime, respectively (Fig. 4A, inset).
(A) I-V curves at various top (Vtg) and bottom (Vbg) gate configurations, with corresponding indicators in the inset. (Inset) Ids versus Vtg and Vbg (with Vds = −3 V on MoSe2 and grounded WSe2). The white dashed line represents the compensated electric field where
The electrical generation of long-lived interlayer excitons provides an electrically driven near-infrared light source with an energy tunability that ranges over several hundreds of milli–electron volts and spatial control of the emission. Achieving high-density IEs without optical excitation could pave a way to realize quantum condensates in solid-state devices. Large valley polarization (23, 36) strongly coupled to the spin may also lead to optoelectronic devices based on electrically driven CIEs. The spin degree of freedom in such devices could be potentially used for both classical and quantum information processing.
Supplementary Materials
science.sciencemag.org/content/366/6467/870/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S18
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