Silicon


Preparation Technique

Silicon, when exposed to air, forms a few nm thick oxide layer. In order to perform our measurements this layer has to be removed. This is accomplished by direct current heating in UHV [1]. A current is passed through the sample which heats it to 950 °C. The sample is held at this temperature for several minutes until the pressure within the UHV chamber has settled. The the temperature is raised to 1200 °C for 30 s. The pressure should not exceed 1·10-9 mbar. This might take several tries.

A special sample holder and heating stage was designed for this process. The heating stage features an additional contact, which can be lowered onto the insulated side of the silicon sample in the sample holder. The other side of the sample is connected to the sample holder and thereby to the heating stage.


Preliminary Results

One of the goals of our project is to characterize single donors in silicon. In preliminary studies, performed inside a commercial Omicron system, we cleaned a Si (100) sample by direct current heating. The topography shows rows of Si dimers (2x1 reconstruction). Using scanning tunneling spectroscopy one can identify individual donors close to the silicon surface by studying shifts in the band gap.

Clean Si (100) at room temperature.
(-2 V, 400 pA, scale bar: 10 nm)
Topography and spectroscopy of clean Si at room temperature
(-1.5 V, 250 pA, scale bar: 100 nm)

First AFM images of Si prepared inside the UHV chamber of the 4K system. Unfortunately, the pressure during preparation exceeded 10-9 mbar, leading to the formation of silicon carbide on the surface. Silicon carbide forms as pyramidal mounds on the sample surface.

 
(5x5 μm2) (3x3 μm2)

First LT-STM images of p doped Si (100). We used silicon sample with a burried layer of highly doped silicon 50 nm below the surface (delta doped). The samples proved conductive enough to optain atomic resolution.

[1] B. S. Swartzenhuber, Y.-W. Mo, M.W. Webb, and M. G. Lagally, “Scanning tunneling microscopy studies of structural disorder and steps on Si surfaces,” J. Vac. Sci. Technol. A, vol. 7, pp. 2901–2905, 1989.