Paper TF-ThP23
Effect of Vacuum Annealing on Charge Transport and Trapping in a-SiC:H/c-Si Heterostructures

Thursday, November 12, 2009, 6:00 pm, Room Hall 3

a-Si_1-XC_X:H alloys are of interest in detectors and white light emitting devices, but their properties under thermal anneal have not been reported so far. This paper considers the processes of charge transfer and trapping in a-Si_1-XC_X:H films deposited on crystalline p-type Si wafers and annealed in vacuum (10^-6 Torr) over temperature range of 300 to 850^oC. The a-Si_1-XC_X:H films were deposited by reactive magnetron sputtering using of the Ar /CH₄ as working gases. An Au/Ti multilayer was used as a contact to the a-Si_1-XC_X:H film and Al as back contact to the Si wafer. Raman scattering spectroscopy, photoluminescence (PL) spectroscopy and electron paramagnetic resonance (EPR) measurements were carried out at room temperature. Current-voltage (I-V) and capacitance-voltage characteristics were measured in the temperature range 100 - 350K.

Evaluation of the I-V characteristics of the initial structure and the structures annealed at 450 ^oC and 650^oC demonstrates that maximum coefficient of rectification is observed for the heterostructure annealed at 450^oC and equals 4x10² for ±5V. For this material the maximum optical band gap and minimum paramagnetic defect concentration are observed. The dielectric constant is found to be 6.5. After 650^oC vacuum annealing the forward and reverse currents are higher than those of both the initial and 450^oC- annealed structures, and are associated with amorphous carbon cluster formation observed by Raman scattering after such thermal annealing. Temperature dependence of forward current of the initial structure demonstrates that variable-range hopping (VRH) conductivity at the Fermi level is dominant up to 1V. The density of states at the Fermi level is estimated at ~8x10¹⁹ cm^-3eV^-1, that is on the order of the concentration of Si and C dangling bonds, determined by EPR technique. Increase of the forward voltage from 0.1V to 1.0V results in increase of average hopping distance from 4.2 nm to 7 nm. Annealing at 450^oC results in change of current transport mechanism: now the forward current can be described by Pool-Frenkel emission from levels with energy ~ 0.11 eV. Annealing at 650^oC considerably reduces the temperature dependence of current, testifying to the emergence of tunneling processes for charge movement. The process of VRH conductivity through a large density of state at Fermi level again appears, with an estimated density of the states around 5x10²⁰ cm^-3eV^-1. Increase of applied voltage beyond |1V| results in a decrease in current with increase of temperature. It is surmised that the observed phenomenon is associated with charge trapping in local regions separated from main matrix by high potential barriers.