STM-REBIC study of nanocrystalline and crystalline silicon.
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G7.6.1
STM-REBIC study of nanocrystalline and crystalline silicon. E. Nogales, B. Méndez, J. Piqueras, R. Plugaru*. Departamento. Física de Materiales, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Madrid, Spain. * Permanent address: National Institute for Research and Development in Nanotechnology-IMT Bucharest Romania
ABSTRACT Electrically active regions of nanocrystalline silicon (nc-Si) films as well as of a p-type crystalline silicon (c-Si) wafer have been investigated by using a scanning electron microscope/scanning tunneling microscope (SEM/STM) combined instrument. The nc-Si films were obtained by boron implantation of amorphous silicon layers with an average nanocrystal size of about 10 nm. STM current constant images reveal a cell structure in the nc-Si films which was also revealed in the STM remote electron beam induced current (REBIC) images with a resolution of up to 20 nm. The contrast in the STM-REBIC images indicate the existence of space charge regions at the boundaries. The influence of the thermal treatment on the cell structure was studied. For comparison, SEM-REBIC and STM-REBIC images from c-Si wafer were obtained.
INTRODUCTION In the past years the physical properties of nanocrystalline silicon (nc-Si) have attracted much interest due to its possible applications in the optoelectronics technology [1]. Electronic characterization of the structural features of nc-Si is of interest due to the important role of the defects that appear in the surface of the nanocrystals, or in the interface between the nanocrystals and an oxide or amorphous phase, on the luminescence properties of this material [2]. In addition, electrical characterization of optoelectronic materials is important in order to optimize their efficiency. The REBIC mode in the SEM has been successfully applied in the past to investigate electrically active defects in high resistive semiconductors [3] while STM-EBIC has been only occasionally used to image defects in materials such as CuInSe2 [4], diamond [5] or sintered zinc oxide [6]. The signal generation in SEM - REBIC is mainly due to the presence of space charge regions and it provides direct evidence for electrically active barrier structures [7]. In the case of STM, the excitation conditions are different compared with a SEM, but the results show the capability of the STM-REBIC technique to characterize electrically active defects with a very high spatial resolution. In this work, REBIC mode of operation in a STM and in a conventional SEM were employed to image electrically active barriers either in Si wafers or nc-Si films with a resolution of about 10 nm. In the case of nc-Si films the STM-REBIC contrast has been found to depend on the implantation dose and the thermal treatment. To complement the STM study, CITS measurements have been also performed.
G7.6.2
EXPERIMENTAL DETAILS Amorphous silicon films, with a thickness of about 2 µm, were obtained by LPCVD on ptype silicon wafers at a temperature of 570ºC and a pressure of 0.4 torr. In order to obtai
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