Electron glass effects in amorphous NbSi films

Abstract

We report on non equilibrium field effect in insulating amorphous NbSi thin films having different Nb contents and thicknesses. The hallmark of an electron glass, namely the logarithmic growth of a memory dip in conductance versus gate voltage curves, is observed in all the films after a cooling from room temperature to 4.2 K. A very rich phenomenology is demonstrated. While the memory dip width is found to strongly vary with the film parameters, as was also observed in amorphous indium oxide films, screening lengths and temperature dependence of the dynamics are closer to what is observed in granular Al films. Our results demonstrate that the differentiation between continuous and discontinuous systems is not relevant to understand the discrepancies reported between various systems in the electron glass features. We suggest instead that they are not of fundamental nature and stem from differences in the protocols used and in the electrical inhomogeneity length scales within each material.

Electron glass signatures up to room temperature in disordered insulators

“…among disordered insulating systems. Indeed, they were also observed in discontinuous and ultrathin continuous films of metals [4][5][6], in granular Al [7,8], in oxidized Be [9], in microcrystalline Tl 2 O 3−x [10], in amorphous and microcrystalline metal-semiconductor alloys (NbSi [11,12], GeSbTe [13], GeTe [14], GeBiTe [15]), with the notable exception of standard doped semi-conductors. It was soon suggested that these glassy features are the signature of the electron glass [1].…”

“…The amorphous NbSi film is 2.5 nm thick and has a Nb content of 13%. This sample, already measured at 4.2 K in previous works [11,12] was made by the co-deposition of Nb and Si at room T under high vacuum (typically a few 10 −8 mbar) [24]. Its sheet resistance at room T is 25 kΩ (100 MΩ at 4.2 K).…”

Electron glass signatures up to room temperature in disordered insulators

“…This MD behaviour and other related features, first investigated in detail in amorphous and microcrystalline indium oxide (InOx) films [2,3], have since then proven to be rather common among disordered insulating systems. Indeed, they were also observed in discontinuous and ultrathin continuous films of metals [4,5,6], in granular Al [7,8], in oxidized Be [9], in microcrystalline Tl 2 O 3−x [10], in amorphous and microcrystalline metal-semiconductor alloys (NbSi [11,12], GeSbTe [13], GeTe [14], GeBiTe [15]), with the notable exception of standard doped semi-conductors. It was soon suggested that these glassy features are the signature of the electron glass [1].…”

“…In the amorphous NbSi film measured in Refs. [11,12] (R s300K = 25 kΩ), a symmetrical conductance minimum centred on the equilibrium gate voltage value is visible at room T after the subtraction of a linear field effect contribution (left panel of Fig. 8).…”

“…Taking the free electron formula for N (E F ) and a relative dielectric constant of 10 [2], we finally obtain U CG ≈ 0.1 eV. In NbSi films, the charge carrier density is not known precisely but it may be larger than 10 22 e/cm 3 [12], which will correspond to a Coulomb gap charging energy above 0.2 eV (again assuming a relative dielectric constant of 10). In granular Al films, U CG should be of the order of the charging energy of the Al grains.…”