High-field transport and noise properties of sputter-deposited amorphous carbon silicon heterojunctions

Abstract

The electrical conductivity of heterojunctions of amorphous carbon (a-C) films (25 and 75 nm thick) grown on silicon by magnetron sputtering has been studied as a function of the applied electric field and temperature. At low electric fields and high temperatures, the conductivity exhibits thermally activated ohmic behaviour with activation energy 0.14 eV. At high electric fields, photoconductance measurements indicate that the conductivity is primarily due to a field-activated mobility with its activation energy decreasing as the electric field increases. At very high electric fields, band-to-band tunnelling is the dominant conduction mechanism. The mobility field-activated conduction model indicates an energy distribution of trapping states consisting of two exponential distributions. The exponential distributions correspond to tail states arising from clustering of sp 2 sites and to deep states caused by isolated sp 2 sites. Low-frequency noise measurements show that thicker a-C films contain a higher concentration of the trapping states. This result was explained by an increase of the sp 2 /sp 3 bonding ratio found from the analysis of Raman spectroscopic measurements.

Electrical transport properties of amorphous CNx/p‐Si heterostructures

“…2a, it is clear that in the voltage‐dependent current density J plots in the reverse voltage range measured at different temperatures that log J approaches a saturation state, as that reported by Hastas et al 9 even though in the present measurement range log J is not saturated. The current density J in the present heterostructure is almost in the same order of magnitude with those reported on the similar devices by other authors 9, 10–13. For analyzing the characteristic of the I – V curves in the low‐voltage range, Fig.…”

“…The log I –log V curves are not a straight line as temperature decreases from 120 to 90 K, suggesting that the electrical transport mechanism is different from that above 120 K. The slope of the log I –log V curves in the top‐right inset of Fig. 3b changes from 3.41 for the lower forward voltages to 4.89 at 305 K, and 5.03 to 13.15 at 130 K for the higher forward voltages, indicating the transition of the dominating current mechanisms from Ohmic to space charge limited current or tunneling emission mechanism as temperature decreases, which can be ascribed to the facts that the interface accommodates more degenerate defect energy levels from the defects and traps, presenting diffusions to limit the current 10–12. However, the Ohmic regime with the slope of 1 is never obtained, even for a small applied bias of 0.2 V, which can be explained by the facts that there are natural oxide SiO 2 layer between amorphous CN x and p‐Si substrates, and the electrons must tunnel through this layer at a small voltage.…”

“…Therefore, the slope of the log I –log V curves at small voltages is larger than 1. The linear log I –log V relations were also observed in the amorphous C and CN x /n‐Si heterostructures 10–14 and amorphous C/p‐Si structures 9. Meanwhile, in the bottom‐right inset of Fig.…”

Analytical derivation of the effective temperature for field-dependent hopping conductivity

“…In an attempt to elucidate the distribution in energy of the density of states (DOS) N(E ) and of the localization length À1 (E ) in amorphous semiconductors, high-electric-field transport has been extensively studied, particularly in films of amorphous germanium (Elliott et al 1974, Eray et al 1990), hydrogenated amorphous silicon (a-Si : H) (Stachowitz et al 1990, Nebel et al 1992a, b, Devlen et al 1993, Palsule et al 1994, Nagy et al 1995, Gu et al 1996 and hydrogenated amorphous carbon (Hastas et al 2002, Godet et al 2003. Strong nonlinearities in the field dependence of the dark conductivity, of the photoconductivity and of the carrier drift mobility have been observed in a temperature range where hopping transport unambiguously occurs.…”

Hopping current density in amorphous carbon/crystalline silicon heterojunctions

“…High electric field transport in metal/carbon/metal (MCM) or a-C/c-Si heterojunction devices [1][2][3][4][5][6][7] can be used as a characterization tool for the energy distribution of localized electronic p and p * states in amorphous carbon (a-C) films because the field enhanced conductivity results from increasing alignment of empty final bandtail states with filled initial states.…”