Abstract
The (111)-2 × 1 surface of in situ cleaved heavily P- or B-doped Si is investigated by scanning tunnelling microscopy and spectroscopy at room temperature and at low temperature. P atoms have been identified on different sites of the Si(111)-2 × 1 surface by their characteristic voltage-dependent contrast for positive as well as negative buckling of the π-bonded chains. The distributions of dopants per surface area and of nearest-neighbour distances are found to be in agreement with a random arrangement of dopants in Si up to doping levels well above the metal–insulator transition. In addition, P atoms have been identified by their depth-dependent contrast down to the third layer beneath the surface with a volume density in agreement with the bulk doping density. The random electronic disorder supports the view of an Anderson transition driven by disorder close to the critical concentration or critical uniaxial stress.
Highlights
The cleaving procedure without further thermal treatment ensures that the distribution of dopants at the Si surface represents the original distribution in the bulk
We note that the distribution of nearest-neighbour distances of P atoms in subsurface layers agrees well with a Poisson distribution which further supports a random arrangement of P dopants in the bulk
At the cleaved Si(111)-2×1 surface, individual P and B dopants were identified by their voltage-dependent contrast
Summary
Clean (111) surfaces of P- and B-doped Si were obtained by cleaving Czochralski grown single crystals at room temperature (RT) under UHV at a base pressure below 10−10 mbar. The assignment of P donors to the four different sites has been investigated for negatively buckled π -bonded chains as well, see figures 5(f)–(k). These contrast patterns can be assigned to substitutional P atoms at sites 2–4. Apart from the different voltages at which the contrast patterns occur, the signatures for P atoms at site 3 or 4 are identical for positive or negative buckling, see figures 5(c), (d), (h) and (i), respectively. These values are in good agreement with the low-distance cut-off observed for Si:P and Si:B which seem to be the characteristic minimum P–P or B–B distance governed by interactions between the dopants
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