The Low Temperature Magnetoconductance of Delta-Doped Silicon

Abstract

The low temperature electronic transport properties of delta-doped silicon are established here, in an effort to determine the technological limitations facing the construction of semiconductor wafers with monolayer doping profiles. Both phosphorous (Si:P) and boron (Si:B) delta-doped silicon wafers prepared by molecular beam epitaxy have been investigated. Devices fabricated from samples of these wafers were cooled to approximately 130 mK in a dilution refrigerator, where four wire resistance measurements were performed in the presence of magnetic fields both perpendicular and parallel to the plane of the devices. The data obtained was interpreted using the theory of weak localization. For Si:P samples, the effective thickness of the delta-doped region can be inferred by comparing the magnetoconductance signal in parallel magnetic fields to that of perpendicular fields. Using this technique on several different samples which were annealed at 850 C, an estimate of the diffusivity of phosphorus in silicon at this temperature is established. In addition, it is shown that the primary mechanism for the loss of quantum mechanical phase coherence in these samples at low temperatures is the electron-electron interaction, and provide evidence for lattice defects enhancing this dephasing. For Si:B samples, the spin-orbit interaction is shown to dominate the dephasing, and the temperature dependence of this interaction is found to be substantially different from that observed in Si:P devices. Based on these results, I conclude that specific epitaxial growth techniques must be followed if monolayer doping profiles are ever to be achieved in silicon. In addition, because this weak localization technique for measuring the thickness of delta-doped regions is highly sensitive and non-destructive, it may ultimately prove to be the most effective method yet established for probing ultra-thin doping profiles. Finally, several theoretical desiderata are established, which would enable a more accurate interpretation of experimental data.