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.
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
Abstract: Hyperdoping has emerged as a promising method for designing semiconductors with unique optical and electronic properties, although such properties currently lack a clear microscopic explanation. Combining computational and experimental evidence, we probe the origin of sub-band-gap optical absorption and metallicity in Se-hyperdoped Si. We show that sub-band-gap absorption arises from direct defect-to-conduction-band transitions rather than free carrier absorption. Density functional theory predicts the Se-induced insulator-to-metal transition arises from merging of defect and conduction bands, at a concentration in excellent agreement with experiment. Quantum Monte Carlo calculations confirm the critical concentration, demonstrate that correlation is important to describing the transition accurately, and suggest that it is a classic impurity-driven Mott transition.
Pub.: 14 Feb '12, Pinned: 16 Aug '17
Abstract: Room-temperature infrared sub-band gap photoresponse in silicon is of interest for telecommunications, imaging and solid-state energy conversion. Attempts to induce infrared response in silicon largely centred on combining the modification of its electronic structure via controlled defect formation (for example, vacancies and dislocations) with waveguide coupling, or integration with foreign materials. Impurity-mediated sub-band gap photoresponse in silicon is an alternative to these methods but it has only been studied at low temperature. Here we demonstrate impurity-mediated room-temperature sub-band gap photoresponse in single-crystal silicon-based planar photodiodes. A rapid and repeatable laser-based hyperdoping method incorporates supersaturated gold dopant concentrations on the order of 10(20) cm(-3) into a single-crystal surface layer ~150 nm thin. We demonstrate room-temperature silicon spectral response extending to wavelengths as long as 2,200 nm, with response increasing monotonically with supersaturated gold dopant concentration. This hyperdoping approach offers a possible path to tunable, broadband infrared imaging using silicon at room temperature.
Pub.: 05 Jan '14, Pinned: 16 Aug '17
Abstract: Ion implantation followed by pulsed laser melting is an extensively-studied method for hyperdoping Si with impurity concentrations that exceed the equilibrium solubility limit by orders of magnitude. In the last decade, hyperdoped Si has attracted renewed interest for its potential as an intermediate band material. In this review, we first examine the important experimental results on both solid and liquid phase crystal regrowth from early laser annealing studies. The highly non-equilibrium regrowth kinetics following pulsed laser melting and its implications for dopant incorporation processes are discussed. We then review recent work in hyperdoped Si for enhanced sub-band gap photoresponse and give a brief discussion on photodetector device performance.
Pub.: 13 Nov '16, Pinned: 16 Aug '17