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PhD candidate, The Australian National University


Using non-equilibrium techniques to enhance the optical properties of the most common semiconductor

Our current telecommunications industry relies on electronic signals to transmit data. With recent advances in optical telecommunications, optical links (optical fibres), which are both smaller and faster than their electronic counterparts (cables), are expected to revolutionise the telecommunications industry. It is therefore necessary to produce hybrid, optoelectronic chips that will enable the integration of optical interconnects with standard electronic chips.

Silicon, the semiconductor material ubiquitous in the microelectronics industry, is unfortunately a poor candidate for photonic applications in its typical form. In particular, the band-gap of silicon is too wide for absorbing the near infra-red light that is used in the telecommunications industry. However, as a material with unprecedented technological maturity and scientific advances, the enormous practical advantages of silicon makes it worthwhile to enhance its otherwise non-ideal optical properties.

With that as a premise, my research explores the formation of an intermediate band within the band gap of silicon as a way to enhance its sub-band gap absorption. In particular, ion implantation and pulsed laser melting, both of which are non-equilibrium techniques, are used to incorporate certain deep-level impurities into silicon at concentrations that exceed the equilibrium solubility by orders of magnitude. This unnaturally high concentration of impurities changes the band structure of silicon, so that the resultant ‘hyperdoped’ silicon can then be carefully engineered to facilitate sub-band gap absorption. By studying the microscopic structure and properties of hyperdoped silicon, my project focuses on both optimising the sub-band gap absorption and, more fundamentally, understanding the intriguing compositional regime of hyperdoped silicon.


Understanding of sub-band gap absorption of femtosecond-laser sulfur hyperdoped silicon using synchrotron-based techniques.

Abstract: The correlation between sub-band gap absorption and the chemical states and electronic and atomic structures of S-hyperdoped Si have been extensively studied, using synchrotron-based x-ray photoelectron spectroscopy (XPS), x-ray absorption near-edge spectroscopy (XANES), extended x-ray absorption fine structure (EXAFS), valence-band photoemission spectroscopy (VB-PES) and first-principles calculation. S 2p XPS spectra reveal that the S-hyperdoped Si with the greatest (~87%) sub-band gap absorption contains the highest concentration of S(2-) (monosulfide) species. Annealing S-hyperdoped Si reduces the sub-band gap absorptance and the concentration of S(2-) species, but significantly increases the concentration of larger S clusters [polysulfides (Sn(2-), n > 2)]. The Si K-edge XANES spectra show that S hyperdoping in Si increases (decreased) the occupied (unoccupied) electronic density of states at/above the conduction-band-minimum. VB-PES spectra evidently reveal that the S-dopants not only form an impurity band deep within the band gap, giving rise to the sub-band gap absorption, but also cause the insulator-to-metal transition in S-hyperdoped Si samples. Based on the experimental results and the calculations by density functional theory, the chemical state of the S species and the formation of the S-dopant states in the band gap of Si are critical in determining the sub-band gap absorptance of hyperdoped Si samples.

Pub.: 23 Jun '15, Pinned: 16 Aug '17