The comparison of the devices' J
) characteristics of SiOx
-Sinc and SiO2
-Sinc-based devices reveals a high rectifying behavior for the former while no rectifying behavior is observed for the latter (Figure ). J
) electrical characteristics for SiO2
-Sinc layer is typical of conduction in (semi) insulating material. Such a result suggests that the rectifying behavior of the SiOx
-Sinc layer could be explained by the presence of the incorporated nitrogen acting as N-type doping specie [15
]. In this case, at low forward bias (0 V ≤ V ≤ 0.8 V), current density taking account serial resistance can be described by:
is the electron charge, kB
the Boltzmann's constant, n
the ideality factor dealing with current dominated by carrier diffusions (n
= 1) and/or by carriers recombination processes at defects (n
= 2), R
the global serial resistance and J0
the saturation current density. Ideality factor n
and serial resistance R
are deduced by fitting our experimental results with the theoretical model (1). The resistance R
is likely to arise from minority carrier space charge, the bulk resistances, and finally contact resistance. In our case, n
was estimated to n
≈ 1.2 indicating that carrier injection is dominated by the carrier diffusion process. For such N-rich devices J
) plots have an excellent rectifying ratio (> 104
at V = ± 1 V) (Figure ) leading to a higher injected current level than reported in the literature [16
]. In addition, at high voltages (2 V > V > 0.8V), current deviates from the exponential behavior due to the low global resistance series (20 Ω <R
< 40 Ω).
Studies of bias and temperature dependence of the electrical properties are carried out to analyze the carrier transports for the two devices. Several models are currently used to understand the carrier injection. For SiOx
-Sinc-based devices biased in the reverse mode, the Poole-Frenkel (PF) model is the most convenient. In this approach, electrons are considered to be thermally emitted from the randomly distributed traps to the conduction band as a result of the lowering of the columbic potential barrier by an external electric field. This model is described by the following relation [19
where J is the current density, N the density of trapping sites, μ the effective carrier mobility, E(= V/d) the local electric field, Φ0 the zero field trapped energy barrier depth and βPF the PF coefficient.
This latter is extracted from the linear representation of ln(J/E) = f(E1/2) (Figure ) and is related to the permittivity of active layer, and thus to the material composition, following the relation:
Plots of the ln(J/E) vs E1/2 of Al/SiOxNy-Sinc/p-Si and Al/SiO2-Sinc/p-Si structures.
The permittivity obtained is also compared to the value deduced from quasi static C-V measurements. The coefficients deduced from the PF relation provides εr = 5.6 and εr = 4.4 for the 30-nm and the 65-nm-thick SiOxNy-Sinc layers, respectively while from C-V measurements, the corresponding permittivity obtained are εr = 5.1 and εr = 4.3. Such similar results are consistent with values obtained with the PF model to explain the reverse current behavior (V < -0.2 V). The difference of permittivity noticed could be explained either by a change of the density of Si nanoclusters (for the same Si content) or by a modification of the Si excess with the thickness. Considering that we observe an increase of the refractive index from 1.61 to 1.75 for 65 nm and the 30-nm layer thicknesses respectively, it suggests that during the first step of the deposition process, the starting growth process conditions could promote the incorporation of Si within a few nanometres thick due to the vicinity of the Si substrate.
Temperature measurements of the reverse current have been carried out in order to confirm the Poole-Frenkel mechanism for which the current is thermally activated. Thus, the corresponding activation energy Ea is defined by the following relation:
Arrhenius diagrams of ln(J
) vs 1/T
(Figure ) show a temperature dependence of the reverse current that confirms a thermal activation of the current and consequently that the carrier transport follows a PF mechanism. The carriers' emission from defects following such a mechanism is enhanced by a barrier lowering where the electrical field is the most important, that means at the PN junction interface. In this way, the Poole-Frenkel emission of carriers may likely occur from defects in the bulk of the SiOx
matrix located at Sinc/SiOx
] close to the p-type silicon substrate.
Arrhenius representations of the current density of the SiOxNy-Sinc layer made MIS structures.
For low temperatures, J
does not depend on the temperature indicating that it is more representative of a tunnel conduction way. This behavior is more pronounced at high reverse bias because of the linear decrease of the J
) plot (Figure ) according to the Fowler-Nordheim model given as [20
Fowler-Nordheim tunnel representation of Al/SiOxNy-Sinc/p-Si and Al/SiO2-Sinc/p-Si structures.
where C is a constant both depending on the elementary charge q, the barrier height ΦB, and the Planck's constant h, and where m* stands for the carriers' effective mass. This conduction mechanism could be effective between the silicon nanoclusters through the silicon oxynitride.
The same approach has been used for the N-free Si-rich layer. No current variation in the SiO2
-Sinc layer according to the electric field has been observed (Figure ) demonstrating that Poole-Frenkel mechanism is not the conduction mode in such an MIS structure. Fowler-Nordheim tunneling conduction has also been tested for this layer (see Figure ) and no variation is observed following equation 5. Therefore, the electrical behavior of this layer in both reverse and forward bias cannot be explained by Fowler-Nordheim tunneling conduction. However, the forward current characteristics plotted with a representation in ln(I
) = ln(V
) in Figure can be divided into three regions. In this case, the current follows a voltage power law (I Vn
). In the first region (V < 0.2 V) n
= 1 corresponding to an ohmic behavior where the deduced intrinsic resistivity of the material (ρ
= 2.7 × 1011
) is typical of (semi) insulator. For higher voltages (0.2 V < V < 0.4 V), conduction mechanism is due to space charge limited conduction dominated by a discrete trapping level in the second region (n
= 2) and by exponential distribution in the third region (n
> 2). From this characteristic, the density nt
of the trapped electrons can be extracted accordingly:
Ln I vs. ln V representation of MIS structure based on SiO2-Sinc layer.
is the permittivity of the material and VTFL
is the voltage at which the current significantly increases. The permittivity extracted from C-V measurements is εr
= 3.95. VTFL
is defined as the intersection between the linear parts of the second and the third region. The value of the trap density nt
, acting as quality factor of the SiO2
-Sinc layer, has been estimated to be 5.39 × 1016
. The presence of trap centers could be associated to the density of Si nanoclusters in the silicon oxide matrix as it has been reported in a previous work [21
]. In the case of SiOx
layer, as previously discussed, conduction mechanism appears to be different. In such a layer, the nitrogen is suspected to passivate the trap centers close to the silicon nanoclusters and thus promoting N-type doping effect responsible of pn junction creation between the active layer and the P-doped silicon substrate.