High efficiency of solar energy conversion is a main challenge of many fields in novel nanotechnologies. Various nanostructures have been proposed early (Pillai et al., 2007; Hun et al., 2007; Johnson et al., 2007; Slaoui & Collins, 2007). However, every active element cannot function without electrodes. Thus, the problem of performing effective contacts is of particular interest.
The unique room-temperature electrical characteristics of the porous metallic nanocluster – based structures deposited by the wet chemical technology on conventional silicon-based solar cells were described in (Laptev & Khlyap, 2008). We have analyzed the current-voltage characteristics of Cu-Ag-metallic nanocluster contact stripes and we have registered for the first time dark currents in metallic structures. Morphological investigations (Laptev & Khlyap, Kozar et al., 2010) demonstrated that copper particles are smaller than 0.1 pm and smaller than the pore diameter in silver.
Electrical measurements were carried out for the nanoclustered Ag/Co-contact stripe (Fig.18, inset) and a metal-insulator-semiconductor (MIS) structure formed by the silicon substrate, SiNx cove layer, and the nanocluster stripe. Fig. 18 plots experimental room – temperature current-voltage characteristics (IVC) for both cases.
As is seen, the nanocluster metallic contact stripe (function 3 in Fig. 18) demonstrates a current-voltage dependence typical for metals. The MIS-structure (functions 1 and 2 in Fig. 18) shows the IVC with a weak asymmetry at a very low applied voltage; as the external electric field increases, the observed current-voltage dependence transforms in a typical "metallic" IVC. More detailed numerical analysis was carried out under re-building the experimental IVCs in a double-log scale.
Fig. 18. Room-temperature current-voltage characteristics of the investigated structures <8see text above): functions 1 and 2 are "forward" and "reverse" currents of the MIS – structure (contacts 1-2), and the function 3 is a IVC for the contacts 1-3.
Fig. 19 illustrates a double-log IVCs for the investigated structure. The numerical analysis has shown that both "forward" and ‘reverse" currents can be described by the function
I = f(Va)m,
where I is the experimental current (registered under the forward or reverse direction of the applied electric field), and Va is an applied voltage. The exponential factor m changes from
1. 7 for the "forward" current at Va up to 50 mV and then decreases down to ~1.0 as the applied bias increases up to 400 mV; for the "reverse" current the factor m is almost constant (~1.0) in the all range of the external electric field.
Thus, these experimental current-voltage characteristics (we have to remember that the investigated structure is a metallic cluster-based quasi-nanowire!) can be described according to the theory (Sze & Ng, 2007) as follows: the first section of forward current
I = TtunAel(48/9L2)(2e/m*)1/2(Va)3/2 (ballistic mode) and the second one as
I = TtunAel(2svs/L2)Va,
and the reverse current is
I = TtunAel(2svs/L2)Va
(velocity saturation mode). Here Ttun is a tunneling transparency coefficient of the potential barrier formed by the ultrathin native oxide films, Ael and L are the electrical area and the length of the investigated structure, respectively, є is the electrical permittivity of the structure, m* is the effective mass of the charge carriers in the metallic Cu-Ag-nanoclucter structure, and vs is the carrier velocity (Kozar et al., 2010). These experimental data lead to the conclusion that the charge carriers can be ejected from the pores of the Cu-Ag-nanocluster wire in the potential barrier and drift under applied electric field (Sze & Ng, 2007; Peleshchak & Yatsyshyn, 1996; Datta, 2006; Ferry & Goodnick, 2005; Rhoderick, 1978).
Fig. 19. Experimental room-temperature current-voltage characteristic of the examined
structure in double-logarithmic scale.