Gap tuning and effective electron correlation energy in amorphous silicon: A first principles density functional theory-based molecular dynamics study

Abstract

Abstract First principles density functional theory (DFT)-based molecular dynamics (MD) is used to study some physical and electronic properties of amorphous silicon (a-Si) samples, as-quenched and annealed containing dangling and floating bonds (pertinent to the threefold- and fivefold-coordinated defects, respectively) as well as distorted tetrahedral bonds. Surprisingly, except for the work of Pantelides (1986) who gave a rough estimate for the effective electron correlation energy, U eff of a floating bond on the fivefold-coordinated Si, to date, there are no theoretical studies in the literature for the calculation of U eff pertinent to this type of defect. In this work, U eff for each type of defect, namely, threefold- and fivefold-coordinated atoms which are present in our generated annealed a-Si sample at 300 K is calculated by the current ab initio framework. We found that, U eff for the fivefold-coordinated Si varies from + 0.32 to + 0.41 eV , whereas for the threefold-coordinated Si it ranges between - 0.33 to + 0.04 eV . The electronic, optoelectronic, and transport properties of a-Si semiconductors are directly influenced by gap tuning which in turn is controlled by the applied strains. The effects of temperature and strain on the mobility gap and the electronic density of states (DOS) for the a-Si samples are of particular interest. For the unstrained as-quenched and annealed samples at T = 0 K , the mobility gap is calculated to be equal to 1.42 and 1.47 eV , respectively; whereas, at T = 300 K these values change to 1.17 and 1.24 eV , respectively. At T = 0 K , for both samples under the uniaxial tensile strains below 0.070 , the calculated mobility gap is about 1.4 eV which sharply decreases by applying strain beyond 0.070 . As it will be seen, the gap regions for both the unstrained sample and the strained sample with ∊ 33 = 0.070 contain midgap states, but for the strained samples with the higher strains of ∊ 33 = 0.140 and 0.210 the midgap states disappear.