Metal-insulator transition and superconductivity in boron-doped diamond

Abstract

We report on a detailed analysis of the transport properties and superconducting critical temperatures of boron-doped diamond films grown along the ͕100͖ direction. The system presents a metal-insulator transition ͑MIT͒ for a boron concentration ͑n B ͒ on the order of n c ϳ 4.5ϫ 10 20 cm −3 , in excellent agreement with numerical calculations. The temperature dependence of the conductivity and Hall effect can be well described by variable range hopping for n B Ͻ n c with a characteristic hopping temperature T 0 strongly reduced due to the proximity of the MIT. All metallic samples ͑i.e., for n B Ͼ n c ͒ present a superconducting transition at low temperature. The zero-temperature conductivity 0 deduced from fits to the data above the critical temperature ͑T c ͒ using a classical quantum interference formula scales as 0 ϰ ͑n B / n c −1͒ with ϳ 1. Large T c values ͑ജ0.4 K͒ have been obtained for boron concentration down to n B / n c ϳ 1.1 and T c surprisingly mimics a ͑n B / n c −1͒ 1/2 law. Those high T c values can be explained by a slow decrease of the electron-phonon coupling parameter and a corresponding drop of the Coulomb pseudopotential * as n B → n c .

Metal-to-insulator transition and superconductivity in boron-doped diamond

“…Typical values of 10 6 K and crossovers from the VRH to the ES regime and then to a mZ1 variation at even lower temperature have been reported for type IIb single crystals with n B Z2!10 19 cm K3 (Sato et al 2000). Closer to the transition, x loc is expected to increase, leading to lower T 0 values, so that the VRH regime extends to lower temperatures, as is the case in figure 2 where T 0 is of the order of a few 10 3 K for n B Z2.4!10 20 cm K3 and several 10 2 K for n B Z4!10 20 cm K3 with a VRH regime extending down to 10 K (Klein et al 2007). Above the critical concentration, the normal-state conductivity can be extrapolated to a finite value s 0 as T/0 K, as deduced from the resistivity variations shown in figure 2.…”

“…Above the critical concentration, the normal-state conductivity can be extrapolated to a finite value s 0 as T/0 K, as deduced from the resistivity variations shown in figure 2. As a matter of fact, as shown in the insert of figure 3a, for n B greater than 6!10 20 cm K3 (Klein et al 2007), and below a certain temperature at which the inelastic mean free path becomes of the order of the elastic one, the resistivity increases slowly when the temperature is reduced. In this regime, where weak localization effects arising from electron-phonon scattering as well as other electronic correlations are expected, the experimental temperature dependence of the conductivity was found (Klein et al 2007) to follow between 3 and 30 K an expression of the type: sZs 0 CAT 1/2 CB e-ph T. Pronounced 'weak localization' effects have also been detected in heavily boron-doped polycrystalline (Winzer et al 2005) and nanocrystalline (Mareš et al 2006) diamond, while a sZs 0 CAT 1/3 variation has been observed in ion-implanted samples (Tshepe et al 1999).…”

“…Given the 10% uncertainty of secondary ion mass spectrometry (SIMS) absolute calibrations, these experimental results are in excellent agreement with the zero-temperature model calculations of the vanishing ionization energy or of the chemical potential for both the insulating and the metallic phase (Kačmarčik et al 2005), which yield a critical boron concentration n c of approximately 5.2!10 20 cm K3 . A generalized Drude approach, taking into account the influence of temperature but neither the hopping transport nor weak localization effects, has also been applied to this system (Klein et al 2007) and yielded a similar critical concentration value lying between 4 and 5!10 20 cm K3 . Following Mott, we may assume that this transition results from boron-related hydrogenic states with a Bohr radius a H Z3a 0 /m à , which overlap when n B reaches n c with a H n 1=3 c Z 0:26 as observed in numerous materials.…”

Local boron doping quantification in homoepitaxial diamond structures

“…The sample has been grown by MPCVD in a vertical silica tube reactor as described elsewhere [17] on a (100)-oriented HPHT type Ib diamond substrate. After a 2 hour-long pure hydrogen plasma at 880°C, a thin non intentionally doped "buffer" layer was grown at 830°C first from 0.25% CH 4 in purified H 2 , and then the methane concentration was raised to 4% and B 2 H 6 (B/C = 1800 ppm) was added to the gas mixture without turning off the plasma, and a thicker (almost 4 µm) heavily boron-doped layer was grown.…”

Influence of the substrate type on CVD grown homoepitaxial diamond layer quality by cross sectional TEM and CL analysis

“…Three samples were grown by microwave plasma chemical vapour deposition (MPCVD) in a vertical silica tube reactor as described elsewhere [10] on different diamond substrates. They were grown the same week with exactly identical growing conditions that correspond to optimum parameters for low doping.…”