Tunneling spectroscopy on semiconductors with a low surface state density (original) (raw)
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Applied Physics A: Materials Science & Processing, 1998
The van-der-Waals surfaces (0001) of the layered structure semiconductors WS 2 and WSe 2 are known to be free of intrinsic surface states. Therefore, they provide an ideal system for investigations of the influence of individual dopants on the local electronic properties, which can be measured by scanning tunneling microscopy (STM). Individual dopant sites were resolved as topographic depressions superimposed on the atomically resolved lattice. The apparent depth of these depressions showed a discrete statistical distribution and was attributed to the spatial depth of the dopant site. Using an STM-induced electrochemical process, we could locally expose the first and second sub-surface layer to correlate the previously recorded topographic contrast to the location of buried dopants. To our knowledge this is the first direct proof of the capability of STM to detect individual sub-surface dopants. An interpretation of the contrast mechanism is given in terms of tip-induced band-bending effects and current transport mechanisms involving minority charge carrier injection and majority charge carrier extraction.
Physica E: Low-dimensional Systems and Nanostructures, 2002
A novel technique is developed to follow the energetic position of the conduction and valence bands with respect to the Fermi level as a function of the lateral position on semiconductor surfaces. By combining high-resolution scanning-tunneling spectroscopy measurements with model calculations it is possible to relate the apparent change in conduction and valence band position to their real counterparts. This method allows one to determine the charge on surface artifacts like steps or vacancies. For a single step on p-type GaAs we ÿnd a charge of 0:9 ± 0:3q nm −1 . ?
Charge Transport in MoS2/WSe2 van der Waals Heterostructure with Tunable Inversion Layer
ACS Nano, 2017
Despite numerous studies on two-dimensional van der Waals heterostructures, a full understanding of the charge transport and photoinduced current mechanisms in these structures, in particular, associated with charge depletion/inversion layers at the interface remains elusive. Here, we investigate transport properties of a prototype multilayer MoS 2 /WSe 2 heterojunction via a tunable charge inversion/depletion layer. A charge inversion layer was constructed at the surface of WSe 2 due to its relatively low doping concentration compared to that of MoS 2 , which can be tuned by the back-gate bias. The depletion region was limited within a few nanometers in the MoS 2 side, while charges are fully depleted on the whole WSe 2 side, which are determined by Raman spectroscopy and transport measurements. Charge transport through the heterojunction was influenced by the presence of the inversion layer and involves two regimes of tunneling and recombination. Furthermore, photocurrent measurements clearly revealed recombination and space-charge-limited behaviors, similar to those of the heterostructures built from organic semiconductors. This contributes to research of various other types of heterostructures and can be further applied for electronic and optoelectronic devices.
Low-temperature scanning tunneling spectroscopy
Journal of Electron Spectroscopy and Related Phenomena, 2000
Low-temperature scanning tunneling spectroscopy measurements on semiconductor surface are described. We consider both surface which do not possess surface states within the bulk bandgap, such as GaAs , and surfaces which do have states within the gap, such as Ge and Ge(111)c(2×8). Band bending in the semiconductor due to the electric field in the vacuum penetrating the semiconductor is found to be a substantial effect in the former case. Transport limitations in the semiconductor give rise to additional voltage drops, which can be observed by making measurements over a wide range of tunnel current magnitudes.
Investigation of photoinduced tunneling current and local surface photovoltage by STM
Applied Surface Science, 1998
Photoassisted scanning tunneling microscopy was used to simultaneously image topoyraphy, photoinduced mnneling current and local surface photovoltage on an nm-scale. A novel interrupted z-feedback technique is presented which overcomes the limitations of previously reported techniques, that were restricted to semiconductor surfaces with a high density of surface states. As an example, measurements on the van der Waals surface of WS2 are shown. This semiconductor surface is known to be free of intrinsic surface states. In the vicinity of monolayer steps an enhanced minority charge carrier recombination and a reduced photovolta,oe was observed. G I998 Elsevier Science B.V.
Indium-contacted van der Waals gap tunneling spectroscopy for van der Waals layered materials
Scientific Reports, 2021
The electrical phase transition in van der Waals (vdW) layered materials such as transition-metal dichalcogenides and Bi2Sr2CaCu2O8+x (Bi-2212) high-temperature superconductor has been explored using various techniques, including scanning tunneling and photoemission spectroscopies, and measurements of electrical resistance as a function of temperature. In this study, we develop one useful method to elucidate the electrical phases in vdW layered materials: indium (In)-contacted vdW tunneling spectroscopy for 1T-TaS2, Bi-2212 and 2H-MoS2. We utilized the vdW gap formed at an In/vdW material interface as a tunnel barrier for tunneling spectroscopy. For strongly correlated electron systems such as 1T-TaS2 and Bi-2212, pronounced gap features corresponding to the Mott and superconducting gaps were respectively observed at T = 4 K. We observed a gate dependence of the amplitude of the superconducting gap, which has potential applications in a gate-tunable superconducting device with a SiO...
Nano letters, 2016
Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), we examine the electronic structure of transition metal dichalcogenide heterostructures (TMDCHs) composed of monolayers of MoS2 and WS2. STS data are obtained for heterostructures of varying stacking configuration as well as the individual monolayers. Analysis of the tunneling spectra includes the influence of finite sample temperature, yield information about the quasi-particle bandgaps, and the band alignment of MoS2 and WS2. We report the band gaps of MoS2 (2.17 ± 0.04 eV) and WS2 (2.39 ± 0.06 eV) in the individual materials and type II band alignment for the heterostructure with an interfacial band gap of 1.45 ± 0.06 eV.
Influence of surface states on tunneling spectra of n-type GaAs (110) surfaces
2009
We show that surface states within the conduction band of n-type GaAs͑110͒ surfaces play an important role in reducing the tunneling current out of an accumulation layer that forms due to an applied potential from a nearby probe tip. Numerical computation of the tunneling current combined with an electrostatic potential computation of the tip-induced band bending ͑TIBB͒ reveals that occupation of the surface states limits the TIBB, thus leading to the limitation of the accumulation. As a result, the tunneling current out of the accumulation layer is strongly suppressed, which is in quantitative agreement with the experiment.
Effect of point defects on the optical and transport properties of MoS2 and WS2
Physical Review B, 2014
Imperfections in the crystal structure, such as point defects, can strongly modify the optical and transport properties of materials. Here, we study the effect of point defects on the optical and DC conductivities of single layers of semiconducting transition metal dichalcogenides with the form M S2, where M =Mo or W. The electronic structure is considered within a six bands tight-binding model, which accounts for the relevant combination of d orbitals of the metal M and p orbitals of the chalcogen S. We use the Kubo formula for the calculation of the conductivity in samples with different distributions of disorder. We find that M and/or S defects create mid-gap states that localize charge carriers around the defects and which modify the optical and transport properties of the material, in agreement with recent experiments. Furthermore, our results indicate a much higher mobility for p-doped WS2 in comparison to MoS2.
Tunneling Photocurrent Assisted by Interlayer Excitons in Staggered van der Waals Hetero-Bilayers
Advanced materials (Deerfield Beach, Fla.), 2017
Vertically stacked van der Waals (vdW) heterostructures have been suggested as a robust platform for studying interfacial phenomena and related electric/optoelectronic devices. While the interlayer Coulomb interaction mediated by the vdW coupling has been extensively studied for carrier recombination processes in a diode transport, its correlation with the interlayer tunneling transport has not been elucidated. Here, a contrast is reported between tunneling and drift photocurrents tailored by the interlayer coupling strength in MoSe2 /MoS2 hetero-bilayers (HBs). The interfacial coupling modulated by thermal annealing is identified by the interlayer phonon coupling in Raman spectra and the emerging interlayer exciton peak in photoluminescence spectra. In strongly coupled HBs, positive photocurrents are observed owing to the inelastic band-to-band tunneling assisted by interlayer excitons that prevail over exciton recombinations. By contrast, weakly coupled HBs exhibit a negative phot...