Study of PtSi/Si(100) Interfaces by Ballistic-Electron-Emission Microscopy
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Introduction Ballistic-electron-emidssion microscopy (BEEM) and ballistic-electron-emission spectroscopy (BEES) are related STM-based techniques for the characterization of metal-semiconductor interfaces, first demonstrated by Kaiser and Bell1 . The STM. tip is used as a source of hot electrons or hot holes which are injected into a thin metal overlayer on a semiconductor substrate. For a sufficiently thin metal layer, the electrons or holes will travel to the semiconductor ballistically, without scattering. Normally, the metal layer is maintained at ground potential, and a collector current of the order of picoamps is measured at the semiconductor. In BEEM, the STM tip is maintained at a high constant bias, and is rastered across the sample surface as in a normal STM topographic scan. In normal STM practice, surface properties are determined by measuring the displacement of the tip in the z-direction. In BEEM, interface properties are determined by measuring the variations of the collector current. This technique promises to be able to observe inhomogeneities in interface properties with a spatial resolution on the order of tens of nanometers. It is also possible to operate in a spectroscopic mode, in which the STM tip is maintained in a constant position, while the tip bias is altered. We call this technique BEES. An ac variation of BEES uses a lock-in amplifier to add an ac: signal to the tip bias, and then select out the samefrequency signal at the semiconductor contact. The ac technique could also allow the metal layer to be dc biased, which in principle should allow noise to be reduced by reducing the leakage current. 217 Mat. Res. Soc. Symp. Proc. Vol. 320. @1994 Materials Research Society
In addition, the ac technique promises to provide a more reliable means of extracting Schottky barrier height from the BEES spectrum. Obtaining Schottky barrier heights from dc spectra is a model-dependent process, as the spectra have a quadratic 2 or 5/2 power3 threshold. The ac technique gives a linear threshold, and allows the detection of other features in the spectrum. We have applied the BEES technique to PtSi samples grown on n-type Si(l00) substrates, using both ac and dc modes of operation. The PtSi/Si system is of technological interest for several reasons, including the use of PtSi on p-type substrates to make infra-red detectors. Experimental Setup The Si wafers used for the preparation of the Au/PtSi/Si( 100) samples were prepared first with a standard RCA clean. They were then dipped into an HF solution immediately before being introduced into an evaporation system with a base pressure of 10-9 torr. E-beam evaporation was used to deposit Pt on the wafers, through a Ta shadow mask. Pressure during evaporation was less than 10-7 toff. Substrate temperature was 350 0C. The wafers were then annealed in situ for one hour, also at 350 0 C. After annealing, the wafers were removed from the evaporation system, dipped in HF and introduced into another evaporation chamber, where 50 A of Au was evaporated 4 onto them
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