16 research outputs found
Tunneling spectroscopy on semiconductors with a low surface state density
A detailed study of tunneling spectroscopy concerning semiconductors with a low surface state density is presented. For this purpose, I V curves under dark conditions and under illumination were measured on the (0001) van der Waals surface of a p-type WS2 single crystal, which is known to be free of intrinsic surface states. The measurements are interpreted by an analytical one-dimensional metal-insulator-semiconductor model, which shows that the presence of the finite tunneling current has to be considered in the calculation of the tip-induced bandbending. Rectification of the dark I V curves is explained by the absence of an inversion layer at the semiconductor surface. In contrast, the I V curves measured for different light intensities and tip-sample separations indicate the existence of an optically induced inversion layer. Since no surface recombination needs to be considered to model these spectra, we conclude that bulk recombination, diffusion and direct tunneling of photogenerated minority charge carriers are the dominant processes for semiconductors with a low density of surface states. In contrast to the standard interpretation of tunneling spectroscopy, which can be applied to semiconductors with a high surface state density, our results clearly show that in this case the normalized differential conductivity (dI/dU)/(I/U) cannot be used to determine the energetic distribution of the local surface state density
Investigation of acceptors in p type WS2 by standard and photo assisted scanning tunneling microscopy spectroscopy
Current imaging tunneling spectroscopy (CITS) and photo-assisted scanning tunneling spectroscopy (STS) is used to characterize dopants in p-type WS, single crystals. While the local charge distribution at ionized acceptor sites give rise to topographic depressions on a nm scale, CITS measurements reveal an additional bright ring. On the base of a one dimensional metal-insulator-semiconductor (MIS) model, the topographic contrast and the ring structure are explained by two competing current mechanisms involving tunneling into unoccupied states of the valence and the conduction band. Local surface photovoltage (LSPV) imaging allows to directly measure the local potential. We observe a nonoscillating reduction of the tip-induced bandbending in the vicinity of the acceptors and correlate this with the lateral extension of the topographic depressions and the ring structures observed in CITS measurements. In addition, photoinduced tunneling current (PITC) measurements do not show an enhanced minority charge carrier recombination at acceptor sites
Noncontact UHV AFM investigations of the growth of C59N films on layered materials
Submonolayer films of the heterofullerene C59N were grown by evaporation in ultra high vacuum onto the layered materials graphite HOPG , WSe2 and mica at different substrate temperatures. The growth of the C59N films was investigated by atomic force microscopy noncontact UHV BFM and compared with that of C 60. We found distinct differences in the shape of the islands formed by the two materials. Whereas C60 nucleates in well shaped trigonal and hexagonal islands, C59N forms extended dendritic agglomerates under similar conditions. Only at higher temperatures above 523 K are compact and oriented islands also formed for C59N. The formation of dendroids indicates that C59N sticks strongly to the edges of the islands. The mobility of C59N on HOPG and WSe2 is rather high and comparable to that of C60 on these materials. On mica, only small islands are observed for both C 60 and C59N, indicating a low mobility of both material
Kelvin probe force microscopy for the characterization of semiconductor surfaces in chalcopyrite solar cells
Kelvin probe force microscopy in ultrahigh vacuum is a powerful technique for the quantitative characterization of structural and electronic properties of semiconductor surfaces and interfaces on a nanometer scale. In chalcopyrite heterojunction solar cells the interfaces play a crucial role for the performance of the device. We studied chalcopyrite heterostructures based on epitaxial CuGaSe2 thin films prepared by MOVPE. Lateral variations of the contact potential difference and the surface photovoltage were investigated after different process steps, including the deposition of n CdS or n ZnSe buffer layers and the n ZnO window layer. Measurements on the CuGaSe2 absorber material show terraces with preferential orientation in the [110] direction in the topographic image. A negative surface photovoltage of 300 mV on the as grown CuGaSe2 absorber could be attributed to a highly doped p Cu2 xSe surface layer of a few nm thickness, which was removed by a KCN etch, resulting in a flat band condition. The deposition of the buffer layer alone does not lead to a significant band bending at the CuGaSe2 buffer interface and the deposition of the ZnO window layer seems to be crucial for the development of the band bending within the absorber
Investigations of photoinduced tunneling current and local surface photovoltage by STM
Photoassisted scanning tunneling microscopy was used to simultaneously image topoyraphy, photoinduced tunneling 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 photovoltage was observed