Observation of thermally activated glassiness and memory dip in a-NbSi insulating thin films

Abstract

We present electrical conductance measurements on amorphous NbSi insulating thin films. These films display out-of equilibrium electronic features that are markedly different from what has been reported so far in disordered insulators. Like in the most studied systems (indium oxide and granular Al films), a slow relaxation of the conductance is observed after a quench to liquid helium temperature which gives rise to the growth of a memory dip in MOSFET devices. But unlike in these systems, this memory dip and the related conductance relaxations are still visible up to room temperature, with clear signatures of a temperature dependent dynamics. [5,6] and thallium oxide films [7]. Until very recently, the glassy features like the memory dip and its activationless logarithmic slow relaxation were believed to be of universal character. However a new behaviour was evidenced two years ago in discontinuous metal films [6]: the conductance relaxations were found to be strongly suppressed below a well-defined temperature T*, whereas nothing similar was seen in indium oxide [8,9] and granular Al films [10], the two most extensively studied systems so far.These glassy features remain for a large part unexplained but they might be the experimental signature of an electron glass [2,11,12]. This hypothesis is supported by the fact that all systems in which out-of-equilibrium effects are observed have a large charge carrier density compared to standard doped semiconductors close to the metal-insulator transition [7,13]. In indium oxide films, the charge carrier density was indeed found to influence the gate voltage width of the memory dip and the conductance dynamics itself [13], even if aspects of this last result were questioned recently [14].In order to make some progress towards the understanding of these phenomena, it is of crucial importance to identify among the observed properties what is universal and what is specific to each system. In this respect, the exploration of new systems is an incomparable source of information. We present here the first investigation of out of equilibrium phenomena in amorphous (a-) insulating NbSi thin films. We show that this system also displays slow conductance relaxations after a quench from room to liquid helium temperature, as well as gate voltage (V g ) history memory. However the characteristics of the memory dip as well as the effects of temperature are different from all known systems and strongly indicate a thermal activation of the dynamics [8][9][10].Our NbSi films were obtained by co-deposition of Nb and Si at room temperature and under ultrahigh vacuum (typically a few 10 −8 mbar) [15]. Samples for conductance relaxation measurements (see below) were deposited on sapphire substrates coated with a 25 nm thick SiO underlayer designed to smooth the substrate and were protected from oxidation by a 25 nm thick SiO overlayer. Samples for electrical field effect measurements were deposited on Si++ wafers (the gate) coated with 100 nm of thermally grown SiO 2 (the gate insulator). ...

Charging of highly resistive granular metal films

“…Indeed in some of them, in the appropriate resistance range, these occur up to room temperature. [9][10][11] The technique can be very useful to analyze memory effects, and combined with conductance measurements, may shine light on the mechanisms involved.…”

Glassy Dynamics in Disordered Electronic Systems Reveal Striking Thermal Memory Effects

“…These phenomena were predicted theoretically several decades ago [4][5][6][7][8] and were termed the electron glass [7] because the glassy properties are attributed to the conduction electrons. Experimentally, a growing number of systems have been reported to show such glassy behavior including discontinuous Au [9,10], amorphous and polycrystalline indium oxide films [11][12][13][14][15], ultrathin Pb film [16], granular aluminum [17,18], thin beryllium films [19], NbSi [20], Tl 2 O 3−x [21], GeSbTe [22], and discontinuous films of Ag, Al, and Ni [10,23]. The conductance in these systems was shown to decay logarithmically with time after an abrupt cooldown or an electrical excitation out of equilibrium,…”

Electron glass effects in amorphous NbSi films

“…Most films were protected from oxidation, in-situ, by a 12.5 nm-thick SiO overlayer. The SiO overlayer was found to play no significant role in the glassy behavior described below [32].…”

“…This is what is done in the present study. In a preliminary work on a-NbSi films, we found a strong slow down of the dynamics and the freezing of prominent broad MDs upon cooling from room temperature [32], evidencing activated dynamics in a continuous amorphous system. Here we report on a comprehensive study of electron glassiness in a series of NbSi films of different Nb contents and thicknesses which confirms the general character of the activated dynamics and shed some light on the glassy phenomenology in this system.…”

“…Let us now focus on the MD centred on V geq that is visible for all the films in Figure 2. It actually results from the growth of a narrow 4.2 K contribution (hereafter called the 4.2 K MD) on top of a broad and time independent contribution inherited from the cooling history of the sample [32]. If we plot the differences between the G(V g ) curve measured at the longest time and the ones measured at different times after cooling, they can be superposed for each sample on a single curve simply by using an ad hoc vertical scaling (see Figure 5).…”