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

• First principles density functional theory-based molecular dynamics.
• Effects of temperature and strain on the mobility gap and the electronic DOS.
• Correlation of the midgap states with the fivefold-coordinated defects.
• The effective electron correlation energy corresponding to dangling and floating bonds.
• Omission of the midgap states via the strain level.

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, UeffUeff of a floating bond on the fivefold-coordinated Si, to date, there are no theoretical studies in the literature for the calculation of UeffUeff pertinent to this type of defect. In this work, UeffUeff for each type of defect, namely, threefold- and fivefold-coordinated atoms which are present in our generated annealed a-Si sample at 300K is calculated by the current ab initio   framework. We found that, UeffUeff for the fivefold-coordinated Si varies from +0.32+0.32 to +0.41eV, whereas for the threefold-coordinated Si it ranges between -0.33-0.33 to +0.04eV. 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=0K, the mobility gap is calculated to be equal to 1.421.42 and 1.47eV, respectively; whereas, at T=300K these values change to 1.171.17 and 1.24eV, respectively. At T=0K, for both samples under the uniaxial tensile strains below 0.0700.070, the calculated mobility gap is about 1.4eV which sharply decreases by applying strain beyond 0.0700.070. As it will be seen, the gap regions for both the unstrained sample and the strained sample with ∊33=0.070∊33=0.070 contain midgap states, but for the strained samples with the higher strains of ∊33=0.140∊33=0.140 and 0.2100.210 the midgap states disappear.

The electronic DOS of the strained a-Si under ∊33 = 0.070, 0.140, and 0.210. The details of the mobility gap region for the cases of ∊33 = 0.070, 0.140, and 0.210 are also depicted. For ∊33 = 0.070 the midgap states are observed.Figure optionsDownload as PowerPoint slide