Superconductivity in overdoped semiconductors
Heavy doping of semiconductors beyond the equilibrium solid solubility (overdoping) can fundamentally modify their properties. For instance overdoped semiconductors can behave like metals or even superconductors. [wikipedia].
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Heavy doping without dopant precipitation requires special non-equilibrium preparation steps like ion implantation in combination with short-time flash lamp (~ ms) or laser annealing (ns..ms). Standard rapid thermal annealing (~ 10..100 s) can be used to synthesize superconducting, dopant-rich nanolayers at the interface between the semiconductor and the capping SiO2-layer (see figure: General preparation scheme).
General preparation scheme:
The nanolayer (~ 10 nm) at the SiO2/Si-interface with Ga-rich, amorphous precipitates becomes superconducting below 7 K.
Ga depth distribution in Si after implantation of 80 keV, 4x1016 Ga+ cm-2 (as-implanted) and after rapid thermal annealing at 650°C for 65 s and 70 s. A Ga-rich nanolayer has been formed at the SiO2/Si-interface after annealing.
Pseudocolor image of the silicon distribution in the cross section of a sample annealed at 600°C for 60 s obtained from energy filtered transmission electron microscopy (EFTEM). Ga accumulation and oxygen intermixing close to the interface lead to local Si depletion. The silicon concentration decreases from orange to blue. The multilayer structure is clearly mapped: glue, SiO2 layer, Ga rich nanolayer and heavily doped Si with few impurity rich precipitates (from left to right).
Temperature dependence of the sheet resistance for samples annealed at different temperatures for 60 s. Within a narrow annealing temperature window (600–700 °C), superconductivity is observed below 7 K. The inset demonstrates that superconductivity disappears when etching away the SiO2 cover including the Ga-rich nanolayer at the Si-SiO2 interface.
The Ga-heavily-doped Ge-layer (~ 50 nm) becomes supercondcuting below 0.5 K. There are no Ga-containing precipitates in the layer.
Sequence of XTEM micrographs of Ga-implanted (2x1016 cm-2, 100keV) and subsequently 3 ms flash-lamp-annealed [Link zu FLA] Ge samples. The insets give the Ga depth distribution as measured by SIMS. In the as implanted state (left) an amorphous layer of about 100 nm width has been formed. After annealing the layer consists of a single crystalline and a polycrystalline zone. With growing optical fluence (J/cm2) the annealing temperature increases. As a consequence broader single crystalline zones, interface roughening and grain coarsening in the polycrystalline top layer is observed. However, Ga loss and redistribution is negligible because of the short annealing time.
Temperature dependence of the electrical resistance of unimplanted Ge, as-implanted Ge:Ga and annealed Ge:Ga (53.6 J/cm2, 3 ms). Only annealed Ge:Ga exhibits superconductivity below 0.5 K, depending on the flashlamp fluence (see inset).
The global superconducting state in Ga-hyperdoped Si can disappear when precipitation of Ga is facilitated by annealing. In this case inhomogeneous layers with local superconducting regions are formed. These layers are insulators with interesting non-linear magnetotransport properties below the critical temperature. They behave like a network of Josephson-junctions with a superconductor –insulator transition at a critical resistance.
Sheet resistance of Si:Ga films as function of temperature. The film annealed at 600°C becomes superconducting below 7 K. An insulating film is formed after annealing at 700°C. The critical resistance for the superconductor-insulator transition is 13 kOhm. The background shows the inhomogeneous structure of the film.
Below the critical temperature of 7 K insulating Si:Ga layers have a large, strongly changing magnetoresistance.