Laser-writable high-k dielectric for van der Waals nanoelectronics

2D material compatible, writable, and high-k multifunctional oxide facilitates next-generation flexible van der Waals electronics.


INTRODUCTION
The high quality of the native oxide that can be grown on the surface of silicon has underpinned the wide success of modern micro-and nanoelectronics. In recent years, high-k dielectrics such as HfO 2 have been adopted to reduce the dimensions of nanoelectronic components and boost their performance (1). Recent work has shown similar native oxides in two-dimensional (2D) materials such as HfSe 2 , ZrSe 2 (2), TaS 2 (3) and TaSe 2 (4). However, use of these oxides embedded within van der Waals (vdW) heterostructures has not been shown.
In comparison to silicon, vdW heterostructure devices are likely to play an important role in future electronic device applications (5). With a rapidly growing family of layered 2D materials (6), the multitude of possible heterostructure combinations available will allow for device designs with unprecedented functionalities and improved performance (2). To date, many such vdW heterostructure devices have been shown, such as vertical tunneling transistors (7) with negative differential resistance (8), light-emitting quantum wells (9,10), photovoltaics (11)(12)(13)(14)(15)(16) and memory devices (17).
Contrary to the conventional molecular beam epitaxy growth of semiconductor devices, vdW heterostructures make it possible to produce atomically sharp interfaces between different materials (i.e., semiconductors, insulators, semimetals, etc.) without concerns for their intercompatibility during fabrication. The absence of dangling bonds on the surface of atomically thin materials allows for the creation of atomically sharp interfaces, eliminating the problem of interdiffusion known to impose severe limitations on the downscaling of devices fabricated by standard semiconductors. To date, the state-of-the-art vdW devices studied experimentally rely on the use of high-purity hexagonal boron nitride (hBN) as a gate dielectric, a tunnel barrier, or a high-quality substrate material (18). Such high-quality hBN crystals are not widespread, and scalable chemical vapor deposition versions typically contain impurities that lead to leakage current in transistor devices (19,20). Furthermore, the dielectric constant of hBN (k ≅ 4) is comparable to that of SiO 2 (k = 3.9), thus limiting the downscaling in vdW nanoelectronics (21). Common deposition techniques used for SiO 2 and HfO 2 are not directly compatible with 2D materials (22,23). In general, these methods tend to damage or modify the electronic properties of the underlying 2D crystal (24), especially when the 2D material is thinned to single-unit cell thickness. Other options include exploring atomically flat layered oxides such as mica or V 2 O 5 and assembling them layer by layer. However, these dielectrics also result in a significant level of charge transfer to neighboring 2D materials, large hysteresis in field-effect devices, and significant reduction of the mobility (25). Therefore, the search for alternative dielectrics or novel technologies, compatible with 2D materials, which give good interface quality and with high -k, is needed.
In this work, we demonstrate a route to embed ultrathin HfO x in vdW heterostructures using selective photo-oxidation of HfS 2 . HfS 2 is a layered semiconductor with an indirect bandgap of 2.85 eV in its bulk form (26)(27)(28) and has comparable surface roughness to other 2D crystals after exfoliation [see the Supplementary Materials for atomic force microscopy (AFM)]. We found that the photo-oxidation process can be enabled using laser light even when the HfS 2 is embedded within complex heterostructures and under metallic contacts. This fabrication technique eliminates the need for invasive sputtering or atomic layer deposition (ALD) methods (29). We demonstrate that the photo-induced HfO x has a dielectric constant k ≅ 15 and that this dielectric can be incorporated into four classes of devices enabling different applications: flexible field-effect transistors (FETs), resistive switching random access memories (ReRAMs), vertical light emitters, and photodetectors.

Photo-oxidation of HfS 2 in vdW heterostructures
The procedure used to fabricate our devices is illustrated in Fig. 1A. Heterostructures are assembled using dry transfer of micromechanically exfoliated 2D crystals (25,30,31). A few-layer flake of HfS 2 crystal is placed in the stack where the dielectric is required (Fig. 1A, left). In more complex structures, additional layers are subsequently transferred (see section S1 for details). Once the device has been produced and the contacts have been defined by electron beam lithography, the desired region of oxide is selectively irradiated using visible laser light (Fig. 1A, center and right; see Materials and Methods). It has been shown that, upon laser irradiation, thin HfS 2 undergoes an oxidation reaction due to the charge transfer between the semiconductor and the water redox couple present on its surface (32) and that it converts into an oxide of hafnium.
By comparing laser irradiation effects of HfS 2 in vacuum and in atmosphere, we show that this process relies on the presence of atmospheric water and/or oxygen (Fig. 1E). Furthermore, the oxidation process is still found to occur even when the HfS 2 is sandwiched between neighboring 2D materials. Figure 1E shows a region of HfS 2 stacked within graphene and hBN after laser-assisted oxidation (red hatched region). The mechanism likely involves migration of interfacial water between the graphene-HfS 2 /HfO x interface to the reaction site that is being irradiated. Upon exfoliation, the surface layer of the HfS 2 will naturally oxidize in the 10 to 15 min before encapsulation, which could allow for diffusion of atmospheric water between the graphene and more hydrophilic HfO x surface. Similar diffusion effects have previously been observed for graphene on SiO 2 (33). Figure 1B shows a high-resolution scanning transmission electron microscopy (HR STEM) image (34,35) of the cross section of a graphene/HfO x / graphene device, where the few-layer top and bottom graphene electrodes are still clearly visible, while the long-range crystal order of HfS 2 is lost and the resultant material appears in an amorphous phase. Energy-dispersive x-ray spectroscopy (EDX) analysis confirms that the only species present in this phase are hafnium and oxygen, with only low levels of sulfur left after laser irradiation, as shown in Fig. 1B. We find that graphene encapsulated in laser-written HfO x (red curve) shows only a slight reduction in field-effect mobility (~1500 to 5000 cm 2 V −1 s −1 at a carrier concentration of ~10 11 cm −2 ) compared to graphene on our source of hBN (blue curve) and only a small level of n-type doping (Fig. 1F). The inset of Fig. 1F shows the Raman spectrum of graphene encapsulated by HfO x after photooxidation, in which a negligible D peak is seen. This indicates that graphene is not significantly structurally damaged by the laser irradiation process.
We characterized the insulating properties of the laser-written HfO x by fabricating a MoS 2 FET with an 8-nm HfS 2 flake separating a graphene gate electrode and the Cr/Au contacts, as shown in Fig. 1C. After laser exposure, the transparency of the HfS 2 film increases significantly, indicating an increase in the bandgap from 2.85 eV, consistent with the formation of an oxide [E g ~ 5.5 eV, expected for HfO x (36)]. Vertical electron transport through the oxide further supports the transformation to the oxide and indicates that oxidation occurs not only under the flakes of 2D materials but also under the thick Au contacts (d = 60 nm) facilitated by diffraction of the laser beam around the approximately micrometerwide contacts. Figure 1D shows the I sd -V sd characteristics for such a device. Before oxidation, the I sd -V sd shows the typical nonlinear behavior expected for electron transport through a series of semiconducting materials (37), with a low-bias vertical resistivity R ~ 20 × 10 6 ohm m 2 . After oxidation, the resistivity around V sd = 0 V increases to R ~10 11 ohm m 2 , consistent with an increased barrier height. The breakdown voltage of the laser-written oxide was measured using a graphite/HfO x /Cr (5 nm)/Au (60 nm) vertical electron tunneling device, schematically shown in the inset of Fig. 2A. Tunneling current can be measured when a source-drain bias is applied across the vertical junction, as shown in Fig. 2A. The tunnel current is found to increase exponentially until an electric field of E BD ~ 0.5 to 0.6 V/nm is applied, at which point the current discontinuously increases to the compliance level of the voltage source meter (Fig. 2B). This breakdown field is comparable to that of SiO 2 and hBN (0.6 to 2.5 V/nm and 1 V/nm, respectively) (38,39). The tunnel current is well fit by the Fowler-Nordheim tunneling model ( Fig. 2A, inset). We are able to estimate the barrier height value of  B ~ 1.15 eV (40). This value is smaller than expected for a graphene-HfO 2 barrier  B ~ 1.78 eV, likely related to the nonstoichiometry of the amorphous oxide and finite impurity content, leading to an impurity band forming below the conduction band edge. Scaling of the tunnel conductivity with oxide thickness was found to be unreliable with thin oxides d < 3 nm displaying significantly lower than expected resistivity (~10 6 to 10 7 ohm m 2 ), while oxides of thickness d > 10 nm show similar resistivity to 5-nm-thick oxides (~10 11 to 10 12 ohm m 2 ). This can be explained as follows: In thin flakes, there is a higher chance for electrical pinholes caused by impurities or defects that shunt the cur-rent away from high-resistance paths, whereas thicker flakes do not fully oxidize for the same irradiation energy (which was kept constant in this work), leading to higher than expected conductivity. Optimal thickness for uniform oxidation was found to be 4 to 8 nm. We expect that the oxide quality could easily be improved by optimizing for laser excitation energy, excitation power, and laser spot dwell time during the writing procedure. Current tunneling through thin HfO x dielectric was also measured using conductive AFM (CAFM; see fig. S5).
To better understand the dielectric properties of the laser-written HfO x , we fabricated dual-gated graphene FETs. An optical micrograph of a FET constructed on a Si/SiO 2 (285 nm) substrate from a stack of bilayer graphene/HfO x (7 nm) and Cr/Au contacts is shown in the inset of Fig. 3B. The metal contacts are placed directly on the bilayer graphene (contacts 1, 2, and 11) and on top of the HfO x (contacts 3 to 10). To form a contact between the top Cr/Au metal lead and the graphene underneath the HfO x , we rely on the formation of a stable conductive filament produced by the intentional breakdown of the dielectric. In this way, we can use, for example, contacts 7 and 8 in the inset of Fig. 3B as source and voltage probes and contacts 9, 10, and 11 as drain and voltage probes, while the other metal leads (3 to 6 and 8) are used as top gates. The I sd -V sd characteristics showing stable filament formation are shown in Fig. 3A, where the red curve shows the initial dielectric breakdown at a vertical electric field of ~0.5 V/nm. Further cycling of the source-drain bias with increasing current compliance leads to stable nonreversible filament formation that allows for direct contact of the underlying graphene channel. Typical contact resistances of ~5.5 kilohms are achieved after filamentation (as the area of conductive filament is unknown, we cannot estimate the resistivity in this case). Back gate (SiO 2 ) sweeps of the resistance show the bilayer graphene to be heavily p-typedoped with the charge neutrality point (CNP) lying at V CNP ~ 80 V. These p-type doping levels are attributed to the oxygen plasma cleaning of the Si-SiO 2 substrate, used to promote the adhesion of graphene before exfoliation. Similar hBN-graphene-HfO x stacks show negligible doping compared to graphene on hBN ( Fig. 1F and fig. S6). Figure 3C shows a four-terminal top gate-back gate contour plot of the four-point channel resistance between contacts 7 and 9 (contacted through filamentation), with contact 8 serving as the top gate electrode. From the slope of the neutrality point, dV tg /dV bg , and the thickness of the oxide (determined from the AFM data), we extract the dielectric constant of the HfO x material to be k ~ 15 ± 1. This  value is similar to literature values for amorphous HfO x (41,42). Therefore, having confirmed that the dielectric properties of our laserwritten HfO x are comparable to those of sputtered HfO x films, we turn our attention to its implementation in electronic devices.

Laser-written HfO x as high-k dielectric for 2D FETs
A drawback of graphene FETs is the absence of a bandgap, which prevents the use of this single layer of carbon atoms in practical FETs, where a suitable I on /I off ratio is required. In contrast, few-layer transition metal dichalcogenides (TMDCs) are semiconductors, and because they are atomically thin (43) they are ideally suited for transistor applications. We explored the fabrication of TMDC-FETs using the laser-defined ultrathin high-k HfO x on Si/SiO 2 and flexible polyethylene terephthalate (PET) substrates. We first studied the performance of these devices on a rigid Si/SiO 2 substrate, as schematically illustrated in the inset of Fig. 4A. Applying a voltage to the graphene electrode (V bg ) allows us to modulate the carrier injection into the MoS 2 channel. The two-terminal gate dependence of the sourcedrain channel current (I sd ) for a few-layer MoS 2 FET at different sourcedrain bias voltages (V sd ) is shown in Fig. 4 (A and B). We found that these devices have turn-on voltages V g ~ −0.4 V with I on /I off ~10 4 and subthreshold swings as low as 100 mV per decade. Negligible levels of hysteresis were observed in our device, as shown in Fig. 4B for a sweep rate of 0.3 V/min and V b = 10 mV. Higher levels of hysteresis are typically seen for TMDC FETs on SiO 2 substrates because of the presence of water and oxygen that act as electric field-dependent dopants (44)(45)(46). Field-effect mobilities in the linear region for our MoS 2 FETs were found to be  ~ 1 to 2 cm 2 V −1 s −1 , comparable to MoS 2 FETs on SiO 2 (47,48). The absence of significant hysteresis highlights the high-quality and low-impurity content of our dielectric. To further understand the level of charge traps in our HfO x , we systematically investigated the hysteretic behavior of graphene and MoS 2 devices in different dielectric environments (see figs. S6 and S7).
To test the suitability of our HfO x for flexible applications, we prepared a multilayer MoS 2 FET on a 0.5-mm-thick PET substrate and subjected it to uniaxial strains of up to 1.6% in a custom-made bending rig (Fig. 4D, inset). The I sd -V g sweeps are shown in Fig. 4C, where no significant change in the device performance is observed after applying increasing levels of strain. These devices operate over many bending cycles without degradation as shown in Fig. 4D, with a gate leakage current at V g = 1.5 V less than 40 pA and a small variation in the I sd at a bias voltage of V sd = 20 mV.

ReRAM devices
The formation of conducting filaments illustrated in Fig. 2 allows for switching between two resistance states, creating a device known as ReRAM element (49). ReRAM devices represent a promising emerging memory technology with several advantages over conventional technologies including increased speed, endurance, and device density. Of several groups of materials that show resistive switching, transition metal oxides including HfO x are promising candidates (50). More recently, these devices based on 2D materials are beginning to attract attention owing to their high mechanical flexibility, reduced power consumption, and potential for high-density memory devices based on stacks of vdW heterostructures (51). Figure 5 shows representative device characteristics for a typical resistive switching element based on photo-oxidized few-layer HfS 2 . Our devices consist of a Au top electrode with either titanium or chromium used as an adhesion layer deposited on top of the HfS 2 -graphite heterostructure (see Fig. 5A, inset). Following photo-oxidation of the HfS 2 , the device is subjected to repeated current-voltage sweeps, where the top metal electrode is voltage-biased with respect to the bottom graphite electrode. During the initial voltage sweeps, the current compliance and bias voltage are incrementally increased until stable and repeatable resistance cycling is achieved (increasing the current compliance and V b further will lead to nonreversible conductive filaments). Figure 5A shows a subsequent switching loop after the initial breakdown. At +1 V, an abrupt increase in current is observed as the device switches from a high-resistance state (HRS) to a low-resistance state (LRS), known as the SET process. The device maintains its LRS as the polarity is reversed and swept down to −1 V, at which a reduction in current for increasing negative voltage is observed, as the device switches back to the HRS, known as the RESET process. The use of thin flakes allows for low-voltage operation, with the SET/RESET voltages around |V sd | ~ 1 V. The memory window of devices measured here (R HRS /R LRS ) varies from ~5 up to 10 4 , with the larger values observed for Au/Ti top electrodes (see fig. S8). Figure 5B shows similar current-voltage behavior for the 1st and 100th cycle. The results of repeated cycling are shown in Fig. 5C in which R HRS /R LRS (with both resistance values extracted at V sd = 100 mV) shows little variation over 100 cycles. Last, we investigated the long-term stability of this ReRAM device (Fig. 5D) and found that the resistance levels, measured for V sd = 250 mV, were consistent and well defined for more than 10 4 s. We note that resistive switching in devices using graphene for both top and bottom electrodes was unreliable, and we postulate that electrode material asymmetry is crucial for reliable device performance. This bipolar switching is consistent with the formation and rupture of conducting filaments; however, further studies are required to optimize device performance and to better understand the role played by disorder, oxide thickness, and contact chemistry.

Optoelectronic devices
As discussed above, vertical electron transport in HfO x formed from thin one-to three-layer parent HfS 2 crystals allowed for much higher tunneling currents. While these high leakage currents are detrimental in transistor applications, other device types require higher elec-tron transparency and higher injection rates. As such, we made use of this property of thinner oxides to realize light-emitting and lightdetecting tunneling transistors.
Such vertically stacked heterostructures of 2D materials provide a framework for the creation of large-area, yet atomically thin and flexible optoelectronic devices with photodetectors (11,13,52) and light-emitting diodes (9,10,53). To date, only hBN tunnel barriers have been demonstrated; however, other wide-gap material oxides have not been explored when combined with vdW heterostructures. Here, we demonstrate the use of ultrathin HfO x tunnel barriers in vertical light-emitting tunneling transistor device geometries. Figure 6A shows a current-voltage curve of a HfO x single-quantum well (SQW) device formed by the encapsulation of monolayer MoS 2 in 1 to 2 nm of HfO x . Applying a bias voltage between the top and bottom graphene electrodes (G t and G b ) allows a current to tunnel  through the thin HfO x layers and into the MoS 2 . There is a negligible temperature dependence of the measured source-drain current, indicating a tunneling mechanism rather than transport through lowenergy impurity states (see fig. S9). As we increase the bias voltage from zero, the current increases nonlinearly. Outside of a low-bias regime (|V sd | > 1 V), we observe an increase in the current due to tunneling into the conduction band of MoS 2 . In addition, an asymmetry between the current at positive and negative bias voltage is observed, which is likely due to both a variation in doping between G t and G b and a different thickness of the top and bottom HfO x . This behavior is similar to previous work using hBN tunnel barriers (9).
To determine the active area of the heterostructure, we used scanning photocurrent microscopy, whereby a laser beam is rastered across the device while photocurrent is acquired simultaneously (see Materials and Methods). Figure 6B shows that, under a moderate bias (V sd = −1 V), the photocurrent is predominately localized to regions of overlap between the top and bottom graphene flakes, each outlined in light green. Photoexcited carriers in MoS 2 (red outline) are separated by the graphene electrodes because of the applied vertical electric field. Away from this region, the photocurrent (I pc ) drops from >65 to <10 nA. In Fig. 6C, we measure a reduction in the magnitude of the photocurrent as we increase the light modulation frequency. By normalizing this to the value of the photocurrent at low frequencies I pc 0 , we can ascertain the −3-dB bandwidth of the device, which we found to be f −3dB = 40 kHz. From this, we can estimate the rise time using t r = 0.35/f −3dB ~ 8.8 s, which is in agreement with our analysis of the temporal response of the photocurrent (see the Supplementary Materials). The inset of Fig. 6C shows multiple iterations of the photocurrent obtained at 1.8 kHz. The measured response time is 10 3 to 10 6 times faster than that of typical planar MoS 2 photodetectors (54), a result arising from the use of a vertical, as opposed to lateral, contact geometry. The small electrode separation of ~6 nm and large electric fields of ~0.1 to 0.2 V/nm minimize the transit time of the photoexcited carriers. Hence, these vertical heterostructures of MoS 2 encapsulated in HfO x are a promising high-speed light-detection architecture.
As the bias voltage is further increased, the quasi-Fermi levels of the graphene electrodes allow for simultaneous injection of electrons into the conduction band of MoS 2 and holes into the valence band. The carrier confinement set by the HfO x tunnel barriers allows for exciton formation in the MoS 2 . The subsequent decay of those excitons leads to light emission at the excitonic gap of MoS 2 . Figure 6D shows the electroluminescence (EL) intensity map as a function of photon energy and bias voltage, where the main EL band appears at 1.78 eV. Line plots of the EL spectra at 0.1-V increments are shown in the Supplementary Materials.
Briefly, EL is not observed when V sd > −2 V. Only upon reducing the bias voltage below −2 V can EL be detected with a more intense signal recorded by increasing |V sd |. The emergence of EL at −2 V corresponds well with the single-particle bandgap of monolayer MoS 2 (55,56) while the negative threshold voltage can be attributed to the asymmetric device structure. Figure 6E shows a false-color charge-coupled device image of the EL overlaid on a monochrome image of the device at an applied bias voltage of −2.5 V. The EL is localized to the active area of the device previously identified in Fig. 6B through photocurrent mapping. To further understand the emission, normalized EL and photoluminescence (PL) spectra are shown in Fig. 6F. The main PL emission peak is assigned to the A exciton seen at an energy of 1.8 eV. The energy of the main EL band red-shifts from that of PL by 53 meV. Typically, the exfoliated monolayer MoS 2 is n-doped, which favors the formation of negatively charged excitons (57), which have a lower emission energy than that of the neutral exciton by ~30 meV. Therefore, we attribute the main feature in EL spectra at 1.78 eV to the radiative recombination of the charged exciton. Moreover, the dissociation energy (i.e., energy shift referring to that of neutral exciton) of the charged exciton is proportional to the doping concentration (57). Therefore, it is likely that the large energy difference between EL and PL is an indication of high doping in monolayer MoS 2 , which is due to doping of the as-exfoliated natural MoS 2 flakes.

CONCLUSION
In conclusion, we show that ultrathin few-layer HfS 2 can be incorporated into a variety of vdW heterostructures and selectively transformed into an amorphous high-k oxide using laser irradiation. In contrast to sputtering or ALD, the use of photo-oxidized HfS 2 allows for clean interfaces without damaging the underlying 2D materials. We demonstrate that the laser-written HfO x has a dielectric constant k ~ 15 and a breakdown field of ~0.5 to 0.6 V/nm. These properties allow us to demonstrate several promising high-quality vdW heterostructure devices using this oxide: (i) ReRAM elements that operate in the voltage limit of ~1 V; (ii) flexible TMDC-FETs with I on /I off > 10 4 , subthreshold swings of 100 mV per decade, and good resilience to bending cycles; and (iii) optoelectronic devices based on quantum well architectures, which can emit and detect light in the same device, with EL intensities and drive voltages comparable to devices with hBN barriers and photodetection response times up to 10 6 times faster than equivalent planar MoS 2 devices. Moreover, the high-k dielectric constant, the compatibility with 2D materials, and the ease of laser-writing techniques (58) will allow for significant scaling improvements and greater device functionality, which we predict to be an important feature for future flexible semi-transparent vdW nanoelectronics.

Device fabrication
Devices were fabricated using standard mechanical exfoliation of bulk crystals and dry transfer methods used to form the heterostructures (see the Supplementary Materials for details). Following heterostructure production, the contacts were structured using either optical or electron beam lithography, followed by thermal evaporation of Cr/Au (5/60 nm) electrodes.
After vdW assembly, photo-oxidation of the HfS 2 layer was performed by rastering either ultraviolet (UV) ( in = 375 nm) or visible ( in = 473 nm) laser light focused to a diffraction-limited spot in a custom-built setup (59). A typical energy density of 53 mJ/m 2 was used for exposures lasting 1 to 2 s per point of the HfS 2 layer. The focused spot size was d s = 264 nm for the UV laser and d s = 445 nm for the visible wavelength.

Materials characterization STEM imaging
A cross-sectional specimen for HR STEM was prepared in an FEI Dual Beam Nova 600i instrument incorporating a focused ion beam and a scanning electron microscope in the same chamber. Using 30-kV ion milling, platinum deposition, and lift-out with a micromanipulator, a thin cross section of material was secured on an Omniprobe TEM grid and thinned down to electron transparency with low-energy ions. HR STEM images were acquired using a probe side aberrationcorrected FEI Titan G2 80 to 200 kV with an X-FEG electron source. Bright-field (BF) imaging and high-angle annular dark-field (HAADF) imaging were performed at 200 kV using a probe convergence angle of 21 mrad, an HAADF inner angle of 48 mrad, and a probe current of ∼80 pA. The lamellae were aligned with the basal planes parallel to the incident electron probe. Correct identification of each atomic layer within BF and HAADF images was achieved by elemental analysis with EDX spectrum imaging. Atomic force microscopy AFM was performed using a Bruker Innova system operating in the tapping mode to ensure minimal damage to the sample's surface. The tips used were Nanosensors PPP-NCHR, which have a radius of curvature smaller than 10 nm and operate in a nominal frequency of 330 kHz.

Electrical measurements
I sd -V sd values were collected using a Keithley 2400 voltage/current source meter. Electrical characterization of graphene and TMDC FETs was performed using standard low-noise AC lock-in techniques using a Signal Recovery 7225 lock-in amplifier and a Keithley 2400 source meter providing the gate voltage.
All electrical transport measurements were performed in either a vacuum of 10 −3 mbar or a dry helium atmosphere at room temperature, unless otherwise stated. The flexible MoS 2 FET produced on PET was measured under ambient conditions.

Optoelectronic characterization
Optoelectronic measurements were performed using a custom-built setup (59). Photocurrent measurements were performed using a continuous-wave laser ( in = 514 nm, P = 15 W/cm 2 ) rastered on the devices to produce spatial maps of the photoresponse. The electrical signal was acquired by a DL Instruments Model 1211 current amplifier connected to a Signal Recovery model 7124 digital signal processing lock-in amplifier. The frequency modulation of the lasers was 73.87 Hz. EL and PL measurements were performed in the same setup using a Princeton Instruments SP2500i spectrometer and a PIXIS400 camera. All measurements were performed at room temperature in vacuum (P = 10 −5 mbar).