Insulator-to-Metal Transition in Selenium-Hyperdoped Silicon: Observation and Origin

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.Of all the experimentally measurable physical properties of materials, the electronic conductivity exhibits the largest variation, spanning a factor of 10 31 from the best metals to the strongest insulators 1 . Over the last century, the puzzle of why some materials are conductors and others insulators, and the mechanisms underlying the transformation from one to the other, have been carefully scrutinized; yet even after such a vast body of research over such a long period, the subject remains the object of controversy. In 1956, Mott introduced a model for the insulator-to-metal transition (IMT) in doped semiconductors, in which long-ranged electron correlations are the driving force 2 . Hyperdoping (doping beyond the solubility limit) creates a new materials playground to explore defect-mediated IMTs in semiconductors. In this letter, we identify a defect-induced IMT in silicon hyperdoped with selenium concentrations exceeding 10 20 cm −3(compared to the equilibrium solubility limit 3 of about 10 16 cm −3 ) and we explore the detailed nature of the transition with both experiment and computation. We find that the IMT is largely driven by electron correlation and most resembles a classic impurity-driven Mott transition. Additionally, we find that the high density of Se present at the IMT yields direct optical transitions and an absorption coefficient in excellent agreement with the measured sub-band gap optical properties 4 .Hyperdoping is currently being used to engineer new materials with unique and exotic properties. Silicon hyperdoped with chalcogens exhibits strong sub-band gap absorption down to photon energies as low as 0.5 eV 5-11 , sparking substantial recent interest in applications such as infrared detection and intermediate band photovoltaics [5][6][7][8][9][10][11] . The successful fabrication of rectifying junctions 10 and photodiodes 11-13 using S and Se hyperdoped silicon appears to justify such interest. While isolated S and Se dopants are well-established deep double donors in silicon 3,14 , the enhanced optical properties of hyperdoped silicon (in which these chalcogenic impurities are present at much higher concentrations) are not y...

Realizing the insulator-to-metal transition in Se-hyperdoped Si via non-equilibrium material processing

“…The Se peak concentration is chosen to cover the range of the insulator-to-metal transition predicted by density function theory (DFT) [14], Quantum Monte Carlo (QMC) calculations [14] and hybrid functional method [28]. The as-implanted samples were annealed by FLA with fixed pulse duration of 1.3 ms and an energy density of around 20 J/cm 2 in N 2 ambient.…”

“…Semi-insulating (100) Si wafers with a thickness of 525 μm and a resistivity as large as 10 4 √2πΔR P , where ΔR P is the longitudinal straggle [14,27]. The Se peak concentration is chosen to cover the range of the insulator-to-metal transition predicted by density function theory (DFT) [14], Quantum Monte Carlo (QMC) calculations [14] and hybrid functional method [28].…”

“…To electrically activate the dopants and to repair the lattice damage induced by energetic ions, a post-implantation annealing is necessary. Both pulsed laser annealing in the nanosecond range [8,14] and furnace annealing in the minute one [15,16] have been used to activate S and Se dopants in Si introduced by ion implantation. However, the long-time process required for furnace annealing or even few seconds for rapid thermal annealing will cause large possibilities for impurity diffusion, resulting in a low substitutional fraction [15] or a deactivation of dopants [17].…”

How to modify the van der Waals and Casimir forces without change of the dielectric permittivity

“…For a S-doped Si the critical concentration of free charge carriers falls in between n Si:S fc ≈ 1.8 × 10 20 cm −3 , when this semiconductor is an insulator, and n Si:S fc ≈ 4.3 × 10 20 cm −3 when it is of metallic-type [61]. For a Se-doped Si the critical concentration is confined between [62] n Si:Se fc ≈ 1.4 × 10 20 cm −3 (semiconductor of dielectric-type) and n Si:Se fc ≈ 4.9 × 10 20 cm −3 (semiconductor of metallic-type).…”

Electron-limiting defect complex in hyperdoped GaAs: TheDDXcenter

“…It is usually assumed that when the dopant concentration increases from the impurity limit to the hyperdoping limit, the general chemical trends of the stability of the defects are preserved, except that defect bands can form through the overlapping of the defect wavefunctions 12,13 . Because of this expectation, understanding obtained through studies of isolated defects in a semiconductor is usually extrapolated to interpret experimental results at high dopant concentrations.…”