Spatially correlated microstructure and superconductivity in polycrystalline boron-doped diamond

Abstract

Scanning tunneling spectroscopies are performed below 100 mK on polycrystalline boron-doped diamond films characterized by transmission electron microscopy and transport measurements. We demonstrate a strong correlation between the local superconductivity strength and the granular structure of the films. The study of the spectral shape, amplitude, and temperature dependence of the superconductivity gap enables us to differentiate intrinsically superconducting grains that follow the BCS model, from grains showing a different behavior involving the superconducting proximity effect. DOI: 10.1103/PhysRevB.82.033306 PACS number͑s͒: 73.22.Ϫf, 73.61.Cw, 74.45.ϩc, 74.81.Bd Over the last few years, superconductivity has been discovered in heavily doped group IV covalent semiconductors, 1 in particular, diamond 2 and silicon. 3 In the case of diamond, low-temperature superconductivity appears at the same doping level than the metallic state created by heavy boron doping. 4 Evidence for a pairing mechanism mediated by phonons in the weak-coupling limit has been provided among others by very low-temperature scanning tunneling spectroscopy of single-crystal epilayers. 5Polycrystalline diamond films can be a new model system for the general issue of the nature of superconductivity in strongly disordered metals. 6 In such systems, disorder sits either at the atomic scale, in which case electronic excitations can become localized so that superconductivity vanishes 7 or at a larger scale, for instance, that of a granular structure, in which case the two competing mechanisms are the Coulomb blockade and the superconducting proximity effect. 8,9 Nevertheless, recent studies of polycrystalline diamond films 10,11 did not provide a clear picture on the coexistence between superconductivity and disorder in these films.In this Brief Report, we report a study of the local superconducting and structural properties of high-quality polycrystalline boron-doped diamond by very low-temperature scanning tunneling microscopy ͑STM͒. The granular structure was consistently characterized by STM and transmission electron microscopy ͑TEM͒. In contrast with epitaxial films, a strong correlation is observed between the granular microstructure and the superconductivity local strength. The spatial evolution and temperature dependence of the local electronic density of states are consistent with the picture of an assembly of grains, which either follow the BCS model or present another superconducting behavior involving the superconducting proximity effect.Boron-doped polycrystalline diamond thin films of different thicknesses were grown as described elsewhere 12,13 by microwave plasma-enhanced chemical-vapor deposition from hydrogen-rich methane-trimethylborane-hydrogen gaseous mixtures on ultrasonically seeded quartz ͑sample A͒ and oxidized silicon ͑sample B͒ substrates. As shown by Figs. 1͑a͒ and 1͑b͒ displaying, respectively, a very lowtemperature STM ͑Refs. 14 and 15͒ topography of sample A and a TEM cross section in bright-field condition of sa...

Superconducting Density of States in B-Doped Diamond

“…2, the superconducting gap in the density of states is gradually suppressed with increase of temperature and the estimated superconducting critical temperature T c = 7 ± 0.5 K. Such measurements were performed at numerous locations across the sample surface, yielding identical results within experimental error. We did not observe any spatial variation of SDOS as reported in previous studies of polycrystalline boron doped diamond [11][12][13][14]. Notably, the value of the differential conductance inside the superconducting gap is equal to zero (see Fig.…”

“…3). This is in steep contrast with previous studies on polycrystalline samples [11][12][13][14]. To our knowledge, such homogeneous hard gap was observed only in the case of a single crystal [15].…”

The impact of heavy Ga doping on superconductivity in germanium

“…The former indicated that diamond valence-band holes establish the Fermi surface of BDD, thus rendering a possible impurity-band scenario unlikely. STM data revealed that Si:B epilayers exhibit BCS-like superconductivity [13] while in nanocrystalline BDD the proximity effect has been found [14,15]. A distinct carbon but less clear boron isotope effect has further unraveled the phonon-mediated nature of superconductivity in BDD [16,17].…”

“…That is why further insight in microstructure and local analysis is highly desirable. Most important recent work include angle-resolved photo-emission spectroscopy [12] and low-temperature scanning tunnelling microscopy (STM) [13][14][15]. The former indicated that diamond valence-band holes establish the Fermi surface of BDD, thus rendering a possible impurity-band scenario unlikely.…”

P-type and N-type Conductivity in Nanodiamond Films

“…2 An unusual property of metallically doped NCD films is its superconductivity at low temperatures, as shown in Figure 14.7. 5,[15][16][17][18][19][20] The resistivity of NCD increases with decreasing temperature above the critical temperature (T c ), in contrast with metallically doped single crystal diamond. The T c is around 2 K in this case; however, higher values have been reported since this work.…”