Current transport mechanism in Al/Si_3N_4/p-Si (MIS) Schottky barrier diodes at low temperatures

摘要

The current-voltage (Ⅰ-Ⅴ) and capacitance-voltage (C-Ⅴ) characteristics of metal-insulator-semiconductor (Al/Si_3N_4/p-Si) Schottky barrier diodes (SBDs) were measured in the temperature range of 80-300 K. By using the thermionic emission (TE) theory, the zero-bias barrier height Φ_(B0) calculated from Ⅰ-Ⅴ characteristics was found to increase with increasing temperature. Such temperature dependence is an obvious disagreement with the negative temperature coefficient of the barrier height calculated from C-Ⅴ characteristics. Also, the ideality factor decreases with increasing temperature, and especially the activation energy plot is nonlinear at low temperatures. Such behaviour is attributed to Schottky barrier inhomogeneties by assuming a Gaussian distribution of barrier heights (BHs) at interface. We attempted to draw a Φ_(B0) versus q/2kT plot to obtain evidence of a Gaussian distribution of the BHs, and the values of Φ_(B0) = 0.826 eV and α_o = 0.091 Ⅴ for the mean barrier height and standard deviation at zero-bias, respectively, have been obtained from this plot. Thus, a modified ln (Ⅰ_o/T~2) - q~2σ_o~2/2(kT)~2 versus q/kT plot gives Φ_(B0) and Richardson constant A~* as 0.820 eV and 30.273 A/cm~2 K~2, respectively, without using the temperature coefficient of the barrier height. This value of the Richardson constant 30.273 A/cm~2 K~2 is very close to the theoretical value of 32 A/cm~2 K~2 for p-type Si. Hence, it has been concluded that the temperature dependence of the forward Ⅰ- Ⅴ characteristics of the Al/Si_3N_4/p-Si Schottky barrier diodes can be successfully explained on the basis of TE mechanism with a Gaussian distribution of the barrier heights. In addition, the temperature dependence of energy distribution of interface state density (N_(SS)) profiles was determined from the forward Ⅰ-Ⅴ measurements by taking into account the bias dependence of the effective barrier height and ideality factor.