Scanning Photocurrent Microscopy Research Articles

Room-Temperature Geometrical Circular Photocurrent in Few-Layer MoS2.

Valleytronics, i.e., the manipulation of the valley degree of freedom, offers a promising path for energy-efficient electronics. One of the key milestones in this field is the room-temperature manipulation of the valley information in thick-layered material. Using scanning photocurrent microscopy, we achieve this milestone by observing a geometrically dependent circular photocurrent in a few-layer molybdenum disulfide (MoS2) under normal incidence. Such an observation shows that the system symmetry is lower than that of bulk MoS2 material, preserving the optical chirality-valley correspondence. Moreover, the circular photocurrent polarity can be reversed by applying electrical bias. We propose a model where the observed photocurrent results from the symmetry breaking and the built-in field at the electrode-sample interface. Our results show that the valley information is still retained even in thick-layered MoS2 at room temperature and opens up new opportunities for exploiting the valley index through interface engineering in multilayer valleytronics devices.

Engineering van der Waals Contacts by Interlayer Dipoles.

Tuning the interfacial Schottky barrier with van der Waals (vdW) contacts is an important solution for two-dimensional (2D) electronics. Here we report that the interlayer dipoles of 2D vdW superlattices (vdWSLs) can be used to engineer vdW contacts to 2D semiconductors. A bipolar WSe2 with Ba6Ta11S28 (BTS) vdW contact was employed to exhibit this strategy. Strong interlayer dipoles can be formed due to charge transfer between the Ba3TaS5 and TaS2 layers. Mechanical exfoliation breaks the superlattice and produces two distinguished surfaces with TaS2 and Ba3TaS5 terminations. The surfaces thus have opposite surface dipoles and consequently different work functions. Therefore, all the devices fall into two categories in accordance with the rectifying direction, which were verified by electrical measurements and scanning photocurrent microscopy. The growing vdWSL family along with the addition surface dipoles enables prospective vdW contact designs and have practical application in nanoelectronics and nano optoelectronics.

Validation of minority carrier recombination lifetimes in low-dimensional semiconductors found by analytical photoresponses

It is a formidable challenge to find the minority carrier recombination lifetime in low-dimensional devices as low-dimensionality increases the surface recombination rate and often reduces the recombination lifetime to a scale of picoseconds. In this work, we demonstrated a simple but powerful method to quantitatively probe the minority carrier recombination lifetime in silicon nanowires or microwires by fitting the experimental photoresponses with our recently established analytical photoresponse principle of photoconductors. The nanowires were passivated with small molecules and Al2O3 to suppress surface recombination, which will increase the minority recombination lifetimes. As expected, the minority carrier recombination lifetime found by this approach increases by orders of magnitude. These wires were also made into PIN diodes, the leakage of which was reduced at least 1 order of magnitude after surface passivation by Al2O3. The minority recombination lifetime found from the leakage current of these devices is largely consistent with what we found from our analytical photoresponse principle. As a further step, we performed scanning photocurrent microscopy to find the minority diffusion length from which we found that the minority recombination lifetime is close to what we found from the analytical photoresponses. In short, this work validated that our analytical response principle is a reliable method to find the minority recombination lifetime in low-dimensional semiconductors.

Probing the photocurrent in two-dimensional titanium disulfide

Generating photocurrent in a condensed matter system involves the excitation, relaxation, and transportation of charge carriers. As such, it is viewed a potent method for probing the dynamics of non-equilibrium carriers and the electronic band structure of solid state materials. In this research, we analyze the photoresponse of the mechanically exfoliated titanium disulfide (TiS2), a transition metal dichalcogenide whose classification as either a semimetal or a semiconductor has been the subject of debate for years. The scanning photocurrent microscopy and the temperature-dependent photoresponse characterization expose the appearance of a photovoltaic current primarily from the metal/TiS2 junction in an unbiased sample, while negative photoconductivity due to the bolometric effect is observed in the conductive TiS2 channel. The optoelectronic experimental results, combined with electrical transport characterization and angle-resolved photoemission spectroscopy measurements, indicate that the TiS2 employed in this study is likely a heavily-doped semiconductor. Our findings unveil the photocurrent generation mechanism of two dimensional TiS2, highlighting its prospective optoelectronic applications in the future.

Scanning Photocurrent Microscopy in Single Crystal Multidimensional Hybrid Lead Bromide Perovskites.

We investigated solution-grown single crystals of multidimensional 2D-3D hybrid lead bromide perovskites using spatially resolved photocurrent and photoluminescence. Scanning photocurrent microscopy (SPCM) measurements where the electrodes consisted of a dip probe contact and a back contact. The crystals revealed significant differences between 3D and multidimensional 2D-3D perovskites under biased detection, not only in terms of photocarrier decay length values but also in the spatial dynamics across the crystal. In general, the photocurrent maps indicate that the closer the border proximity, the shorter the effective decay length, thus suggesting a determinant role of the border recombination centers in monocrystalline samples. In this case, multidimensional 2D-3D perovskites exhibited a simple fitting model consisting of a single exponential, while 3D perovskites demonstrated two distinct charge carrier migration dynamics within the crystal: fast and slow. Although the first one matches that of the 2D-3D perovskite, the long decay of the 3D sample exhibits a value two orders of magnitude larger. This difference could be attributed to the presence of interlayer screening and a larger exciton binding energy of the multidimensional 2D-3D perovskites with respect to their 3D counterparts.

Anisotropic photocurrent response at MnBi2Te4-metal interface

The magnetic topological insulator MnBi2Te4 has attracted a lot of research interests for its exotic properties due to the interplay between nontrivial topology and magnetism. Here, we report the photocurrent (PC) response of MnBi2Te4 flakes under the excitation wavelengths between 633 nm and 4000 nm measured by scanning PC microscopy. We observe a significant polarization dependent PC response at the interface between metal electrode and MnBi2Te4, while the PC response remains polarization-independent at MnBi2Te4 layer steps. The polarization dependent PC at the MnBi2Te4-metal electrode interface, which is attributed to the polarization dependent light absorption at the interface, preserves in the whole tested wavelength range. The responsivity of the device is 80 μA W−1. This responsivity as well as PC polarity is consistent with the results calculated based on a photo-thermoelectric generation mechanism, thus we infer that photo-thermoelectric effect dominates in the PC generation at MnBi2Te4-metal interface. Our results reveal the photoelectric response mechanism of the emerging material MnBi2Te4 for its potential optoelectronic applications.

Nanoscale Characterization of Photocurrent and Photovoltage in Polycrystalline Solar Cells.

We investigate the role of grain structures in nanoscale carrier dynamics of polycrystalline solar cells. By using Kelvin probe force microscopy (KPFM) and near-field scanning photocurrent microscopy (NSPM) techniques, we characterize nanoscopic photovoltage and photocurrent patterns of inorganic CdTe and organic-inorganic hybrid perovskite solar cells. For CdTe solar cells, we analyze the nanoscale electric power patterns that are created by correlating nanoscale photovoltage and photocurrent maps on the same location. Distinct relations between the sample preparation conditions and the nanoscale photovoltaic properties of microscopic CdTe grain structures are observed. The same techniques are applied for characterization of a perovskite solar cell. It is found that a moderate amount of PbI2 near grain boundaries leads to the enhanced photogenerated carrier collections at grain boundaries. Finally, the capabilities and the limitations of the nanoscale techniques are discussed.

Single-material MoS2 thermoelectric junction enabled by substrate engineering

To realize a thermoelectric power generator, typically, a junction between two materials with different Seebeck coefficients needs to be fabricated. Such differences in Seebeck coefficients can be induced by doping, which renders it difficult when working with two-dimensional (2d) materials. However, doping is not the only way to modulate the Seebeck coefficient of a 2d material. Substrate-altered electron–phonon scattering mechanisms can also be used to this end. Here, we employ the substrate effects to form a thermoelectric junction in ultrathin, few-layer MoS2 films. We investigated the junctions with a combination of scanning photocurrent microscopy and scanning thermal microscopy. This allows us to reveal that thermoelectric junctions form across the substrate-engineered parts. We attribute this to a gating effect induced by interfacial charges in combination with alterations in the electron–phonon scattering mechanisms. This work demonstrates that substrate engineering is a promising strategy for developing future compact thin-film thermoelectric power generators.

In-plane mixed-dimensional 2D/2D/1D MoS2/MoTe2/Mo6Te6 heterostructures for low contact resistance optoelectronics

Mixed-dimensional heterostructures formed by combining 2D materials and other dimensional (0D, 1D, and 3D) materials provide new opportunities for various applications owing to their novel properties. However, in-plane mixed-dimensional heterostructures have been rarely investigated. Here, we report a novel flux-controlled chemical vapor deposition method for synthesizing in-plane mixed-dimensional heterostructures composed of monolayer MoS2 and low-dimensional Mo/Te compounds. By adjusting the Te flux and growth time, we controlled the composition, dimension, and phase of the Mo/Te compounds interfaced with the MoS2. While in-plane 2D/1D MoS2/Mo6Te6 and 2D/2D/1D MoS2/2H MoTe2/Mo6Te6 heterostructures were obtained with a low Te flux, in-plane 2D/2D MoS2/mixed 2H-1T’ MoTe2 and 2D/2D MoS2/2H MoTe2 heterostructures were synthesized with a high Te flux. We investigated the transport properties of the devices fabricated with in-plane mixed-dimensional 2D/2D/1D MoS2/2H MoTe2/Mo6Te6 heterostructures and imaged localized electronic band structures using scanning photocurrent microscopy. Under the low bias condition, the device exhibited an Ohmic-like behavior, which has not been achieved in conventional devices with stacked van der Waals junctions. Under the high bias condition, the device showed a rectifying behavior because of band-bending formed at the heterojunctions; this is consistent with the electronic band-alignment expected from the bandgap and electron affinity of the materials.

Microsphere‐Aided Super‐Resolution Scanning Spectral and Photocurrent Microscopy for Optoelectronic Devices

AbstractDielectric microspheres naturally possess unique optical properties by which the light's focus and confinement can be manipulated on a microscale. Combining microspheres and optical or Raman microscopy, super‐resolution imaging beyond the diffraction limit and enhancement of Raman signals are demonstrated to provide abundant spectroscopic information on materials. However, microsphere‐aided super‐resolution scanning photocurrent imaging remains challenging to date. Here, based on the photonic nanojet mechanism, a super‐resolution photocurrent and spectral microscopy equipped with a scanning tip with silica dielectric microspheres are presented. With such microsphere‐aided microscopy, order of magnitude enhancements for single‐point Raman and photoluminescence signals can be achieved, and the spatial resolutions for photocurrent and spectral mapping surpass the best resolution of the original confocal system restricted by the diffraction limit. This versatile system enables correlative super‐resolution spectral and photocurrent imaging, serving as a reliable tool for comprehensively understanding and uncovering the optoelectronic properties of materials.

Visualization of bulk and edge photocurrent flow in anisotropic Weyl semimetals

Materials that rectify light into current in their bulk are desired for optoelectronic applications. In inversion-breaking Weyl semimetals, bulk photocurrents may arise due to nonlinear optical processes that are enhanced near the Weyl nodes. However, the photoresponse of these materials is commonly studied by scanning photocurrent microscopy (SPCM), which convolves the effects of photocurrent generation and collection. Here, we directly image the photocurrent flow inside the type-II Weyl semimetals WTe2 and TaIrTe4 using high-sensitivity quantum magnetometry with nitrogen-vacancy center spins. We elucidate an unknown mechanism for bulk photocurrent generation termed the anisotropic photothermoelectric effect (APTE), where unequal thermopowers along different crystal axes drive intricate circulations of photocurrent around the photoexcitation. Using simultaneous SPCM and magnetic imaging at the sample's interior and edges, we visualize how the APTE stimulates the long-range photocurrent collected in our Weyl semimetal devices through the Shockley-Ramo theorem. Our results highlight an overlooked, but widely relevant source of current flow and inspire novel photodetectors using homogeneous materials with anisotropy.

Laser trimming for lithography-free fabrications of MoS2 devices

Single-layer MoS2 produced by mechanical exfoliation is usually connected to thicker and multilayer regions. We show a facile laser trimming method to insulate single-layer MoS2 regions from thicker ones. We demonstrate, through electrical characterization, that the laser trimming method can be used to pattern single-layer MoS2 channels with regular geometry and electrically disconnected from the thicker areas. Scanning photocurrent microscope further confirms that in the as-deposited flake (connected to a multilayer area) most of the photocurrent is being generated in the thicker flake region. After laser trimming, scanning photocurrent microscopy shows how only the single-layer MoS2 region contributes to the photocurrent generation. The presented method is a direct-write and lithography-free (no need of resist or wet chemicals) alternative to reactive ion etching process to pattern the flakes that can be easily adopted by many research groups fabricating devices with MoS2 and similar two-dimensional materials.

High-Performance van der Waals Photodetectors Based on 2D Ruddlesden–Popper Perovskite/MoS2 Heterojunctions

Perovskites have prevailed in the field of photovoltaics over the past several years thanks to their outstanding optoelectronic properties. Compared to three-dimensional (3D) perovskites, the emerging two-dimensional (2D) Ruddlesden–Popper (RP) phase perovskites have demonstrated superior stability due to their hydrophobic organic capping ligands, which can protect the inside inorganic octahedrons against moisture. Meanwhile, RP perovskites broaden the family of 2D materials, enriching the building blocks for van der Waals heterostructures. Here, we first demonstrated a vertical heterojunction incorporating RP perovskite (C6H5C2H4NH3)2PbBr4 and MoS2. The heterojunction photodetector exhibits an extremely low dark current down to 0.1 pA, which can be attributed to the combining effect of the low carrier density in RP perovskites and the band structure of the heterojunction. The spatially resolved scanning photocurrent microscopy (SPCM) was carried out to confirm the charge transport mechanism and band alignment of the heterojunction. Moreover, the photodetector benefits from the heterojunction structure, leading to a high photoresponsivity (7.98 mA W–1). In addition, the device can be operated under a self-driven mode with a responsivity of 0.168 mA W–1. Our work proves the compatibility of RP perovskites in van der Waals devices, promising a scalable pathway to further applications.

Hexagonal Network of Photocurrent Enhancement in Few-Layer Graphene/InGaN Quantum Dot Junctions

Strain in two-dimensional (2D) materials has attracted particular attention because of the remarkable modification of electronic and optical properties. However, emergent electromechanical phenomena and hidden mechanisms, such as strain-superlattice-induced topological states or flexoelectricity under strain gradient, remain under debate. Here, using scanning photocurrent microscopy, we observe significant photocurrent enhancement in hybrid vertical junction devices made of strained few-layer graphene and InGaN quantum dots. Optoelectronic response and photoluminescence measurements demonstrate a possible mechanism closely tied to the flexoelectric effect in few-layer graphene, where the strain can induce a lateral built-in electric field and assist the separation of electron-hole pairs. Photocurrent mapping reveals an unprecedentedly ordered hexagonal network, suggesting the potential to create a superlattice by strain engineering. Our work provides insights into optoelectronic phenomena in the presence of strain and paves the way for practical applications associated with strained 2D materials.

Distinct junction transformations of pixel-arrayed long-wavelength infrared detectors by photocurrent mapping

Pixel-arrayed long-wavelength HgCdTe is of great significance in the development of infrared detection. However, the PN junction formation process for long-wavelength HgCdTe is complex. The damage induced by boron ion implantation and subsequent low-temperature annealing may results in changes in the effective doping concentration and geometry of the actual n-type implantation region. Therefore, an in-situ, non-destructive scanning-photocurrent-microscopy (SPCM) method is needed to test the performance and electroactive defects of HgCdTe devices. Here, the pixel-arrayed long-wavelength infrared detectors with n-on-p structure formed by B+ ion implantation in Hg vacancy-doped Hg0.78Cd0.22Te crystals were fabricated. The SPCM method is used to characterize the PN junction performance of long-wavelength Hg0.78Cd0.22Te array detectors. It is found that the long-wavelength device transformed from n-on-p structure to n-on-n− structure at about 175 K, and further turns into an n−-on-n-on-n− structure with the temperature increasing. In addition, when the temperature increases, the minority carrier diffusion length in the B+ ion implantation region gradually decreases with the activation of the recombination center. Our work provides an effective theoretical and experimental basis for the HgCdTe junction formation process.

Band Bending and Trap Distribution along the Channel of Organic Field-Effect Transistors from Frequency-Resolved Scanning Photocurrent Microscopy

The scanning photocurrent microscopy (SPCM) method is applied to pentacene field-effect transistors (FETs). In this technique, a modulated laser beam is focused and scanned along the channel of the transistors. The resulting spatial photocurrent profile is attributed to extra free holes generated from the dissociation of light-created excitons after their interaction with trapped holes. The trapped holes result from the local upward band bending in the accumulation layer depending on the applied voltages. Thus, the photocurrent profile along the conducting channel of the transistors reflects the pattern of the trapped holes and upward band bending under the various operating conditions of the transistor. Moreover, it is found here that the frequency-resolved SPCM (FR-SPCM) is related to the interaction of free holes via trapping and thermal release from active probed traps of the first pentacene monolayers in the accumulation layer. The active probed traps are selected by the modulation frequency of the laser beam so that the FR-SPCM can be applied as a spectroscopic technique to determine the energy distribution of the traps along the transistor channel. In addition, a crossover is found in the FR-SPCM spectra that signifies the transition from empty to partially empty probed trapping states near the corresponding trap quasi-Fermi level. From the frequency of this crossover, the energy gap from the quasi-Fermi Etp level to the corresponding local valence band edge Ev, which is bent up by the gate voltage, can be estimated. This allows us to spatially determine the magnitude of the band bending under different operation conditions along the channel of the organic transistors.

Van der Waals heterostructures based on 2D layered materials: Fabrication, characterization, and application in photodetection

Construction of heterostructures has provided a tremendous degree of freedom to integrate, exert, and extend the features of various semiconductors, thereby opening up distinctive opportunities for the upcoming modern optoelectronics. The abundant physical properties and dangling-bond-free interface have enabled 2D layered materials serving as magical “Lego blocks” for building van der Waals heterostructures, which bring about superior contact quality (atomically sharp and distortionless) and the combination of functional units with various merits. Therefore, these heterostructures have been the focus of intensive research in the past decade. This Tutorial begins with a variety of strategies for fabricating van der Waals heterojunctions, categorized into the transfer-stacking method and in situ growth assembly method. Then, the techniques commonly exploited for characterizing the structure, morphology, band alignment, interlayer coupling, and dynamics of photocarriers of van der Waals heterojunctions are summarized, including Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy, conductive atomic force microscopy, Kelvin probe force microscope, ultraviolet photoelectron spectroscopy, transfer characteristic analysis, scanning photocurrent microscopy, etc. Following that, the application of various van der Waals heterojunctions for diverse photoelectric detection is comprehensively overviewed. On the whole, this Tutorial has epitomized the fabrication, characterization, and photodetection application of van der Waals heterostructures, which aims to provide instructive guidance for the abecedarians in this emerging field and offer impetus of advancing this rapidly evolving domain.

Direct mapping and characterization of the surface local field in InGaAs/InP avalanche photodetectors

In this paper, InGaAs/InP avalanche photodetectors with a variety of guard ring structures are investigated. The electric distribution in the device is characterized by experiment and simulation methods to better understand the dark current mechanisms. It is found that deeper guard rings are more effective in preventing edge breakdown for the depressed lateral electric field. Scanning photocurrent microscopy measurement is used to characterize the effect of guard ring on InP layer. Our results show that deep guard ring design can reduce the surface electric field. In addition, the 1550 nm response distribution of optimized photodetectors under different operating modes is measured and compared. It shows guard ring can inhibit the edge breakdown. This work demonstrates the significance of guard ring designs of InGaAs/InP avalanche photodetectors, and the characterization method will provide an effective way to understand the source of the current generation mechanism.

New insight into carrier transport in 2D layered perovskites

2D layered halide perovskites exhibit anisotropic carrier transport. Using scanning photocurrent microscopy, in this issue of <i>Chem</i>, Shrestha et al. report novel findings in 2D layered Ruddlesden-Popper perovskites, including a long carrier in-plane diffusion length and exciton behavior and dissociation, unveiling new insight not previously obtained through spectroscopic techniques.

Two-Dimensional Cs2AgBiBr6/WS2 Heterostructure-Based Photodetector with Boosted Detectivity via Interfacial Engineering.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers have been widely used for optoelectronic devices because of their ultrasensitivity to light detection acquired from their direct gap properties. However, the small cross-section of photon absorption in the atomically thin layer thickness significantly limits the generation of photocarriers, restricting their performance. Here, we integrate monolayer WS2 with 2D perovskites Cs2AgBiBr6, which serve as the light absorption layer, to greatly enhance the photosensitivity of WS2. The efficient charge transfer at the Cs2AgBiBr6/WS2 heterojunction is evidenced by the shortened photoluminescence (PL) decay time of Cs2AgBiBr6. Scanning photocurrent microscopy of Cs2AgBiBr6/WS2/graphene reveals that improved charge extraction from graphene leads to an enhanced photoresponse. The 2D Cs2AgBiBr6/WS2/graphene vertical heterostructure photodetector exhibits a high detectivity (D*) of 1.5 × 1013 Jones with a fast response time of 52.3 μs/53.6 μs and an on/off ratio of 1.02 × 104. It is worth noting that this 2D heterostructure photodetector can realize self-powered light detection behavior with an open-circuit voltage of ∼0.75 V. The results suggest that the 2D perovskites can effectively improve the TMDC layer-based photodetectors for low-power consumption photoelectrical applications.