Fabrication and subband gap optical properties of silicon supersaturated with chalcogens by ion implantation and pulsed laser melting

Topographically flat, single crystal silicon supersaturated with the chalcogens S, Se, and Te was prepared by ion implantation followed by pulsed laser melting and rapid solidification. The influences of the number of laser shots on the atomic and carrier concentration-depth profiles were measured with secondary ion mass spectrometry and spreading resistance profiling, respectively. We found good agreement between the atomic concentration-depth profiles obtained from experiments and a one-dimensional model for plane-front melting, solidification, liquid-phase diffusion, with kinetic solute trapping, and surface evaporation. Broadband subband gap absorption is exhibited by all dopants over a wavelength range from 1 to 2.5 microns. The absorption did not change appreciably with increasing number of laser shots, despite a measurable loss of chalcogen and of electronic carriers after each shot.One of the authors M.T. acknowledges the financial support of the Fulbright Program. This research was supported in part by the U.S. Army ARDEC under Contract No. W15QKN-07- P-0092

Photocarrier lifetime and transport in silicon supersaturated with sulfur

Doping of silicon-on-insulator layers with sulfur to concentrations far above equilibrium by ion implantation and pulsed laser melting can result in large concentration gradients. Photocarriers generated in and near the impurity gradient can separate into different coplanar transport layers, leading to enhanced photocarrier lifetimes in thin silicon-on-insulator films. The depth from which holes escape the heavily doped region places a lower limit on the minority carrier mobility-lifetime product of 10⁻⁸ cm²/V for heavily sulfur dopedsilicon. We conclude that the cross-section for recombination through S impurities at this concentration is significantly reduced relative to isolated impurities.Research at Rensselaer was supported by the Army Research Office under Contract No. W911NF0910470 and by the NSF REU program at Rensselaer. Research at Harvard was supported by US Army ARDEC under Contract No. W15QKN-07-P-0092. D.R. was supported in part by a National Defense Science and Engineering Graduate fellowship

Supersaturating silicon with transition metals by ion implantation and pulsed laser melting

We investigate the possibility of creating an intermediate band semiconductor by supersaturating Si with a range of transition metals (Au, Co, Cr, Cu, Fe, Pd, Pt, W, and Zn) using ion implantation followed by pulsed laser melting (PLM). Structural characterization shows evidence of either surface segregation or cellular breakdown in all transition metals investigated, preventing the formation of high supersaturations. However, concentration-depth profiling reveals that regions of Si supersaturated with Au and Zn are formed below the regions of cellular breakdown. Fits to the concentration-depth profile are used to estimate the diffusive speeds, v D, of Au and Zn, and put lower bounds on v D of the other metals ranging from 10² to 10⁴ m/s. Knowledge of v D is used to tailor the irradiation conditions and synthesize single-crystal Si supersaturated with 10¹⁹ Au/cm³ without cellular breakdown. Values of v D are compared to those for other elements in Si. Two independent thermophysical properties, the solute diffusivity at the melting temperature, D s(T m), and the equilibrium partition coefficient, k e, are shown to simultaneously affect v D. We demonstrate a correlation between v D and the ratio D s(T m)/k e ⁰·⁶⁷, which is exhibited for Group III, IV, and V solutes but not for the transition metals investigated. Nevertheless, comparison with experimental results suggests that D s(T m)/k e ⁰·⁶⁷ might serve as a metric for evaluating the potential to supersaturate Si with transition metals by PLM.Research at Harvard was supported by The U.S. Army Research Office under contracts W911NF-12-1-0196 and W911NF-09-1-0118. M.T.W. and T.B.’s work was supported by the U.S. Army Research Laboratory and the U.S. Army Research Office under Grant No. W911NF-10-1-0442, and the National Science Foundation (NSF) Faculty Early Career Development Program ECCS-1150878 (to T.B.). M.J.S., J.T.S., M.T.W., T.B., and S.G. acknowledge a generous gift from the Chesonis Family Foundation and support in part by the National Science Foundation (NSF) and the Department of Energy (DOE) under NSF CA No. EEC- 1041895. S.C. and J.S.W.’s work was supported by The Australian Research Council. J.M. was supported by a National Research Council Research Associateship

Supersaturating silicon with transition metals by ion implantation and pulsed laser melting

We investigate the possibility of creating an intermediate band semiconductor by supersaturating Si with a range of transition metals (Au, Co, Cr, Cu, Fe, Pd, Pt, W, and Zn) using ion implantation followed by pulsed laser melting (PLM). Structural characterization shows evidence of either surface segregation or cellular breakdown in all transition metals investigated, preventing the formation of high supersaturations. However, concentration-depth profiling reveals that regions of Si supersaturated with Au and Zn are formed below the regions of cellular breakdown. Fits to the concentration-depth profile are used to estimate the diffusive speeds, v [subscript D], of Au and Zn, and put lower bounds on v [subscript D] of the other metals ranging from 10[superscript 2] to 10[superscript 4] m/s. Knowledge of v [subscript D] is used to tailor the irradiation conditions and synthesize single-crystal Si supersaturated with 10[superscript 19] Au/cm[superscript 3] without cellular breakdown. Values of v [subscript D] are compared to those for other elements in Si. Two independent thermophysical properties, the solute diffusivity at the melting temperature, D [subscript s](T [subscript m]), and the equilibrium partition coefficient, k [subscript e], are shown to simultaneously affect v [subscript D]. We demonstrate a correlation between v [subscript D] and the ratio D [subscript s](T [subscript m])/k [subscript e] [superscript 0.67], which is exhibited for Group III, IV, and V solutes but not for the transition metals investigated. Nevertheless, comparison with experimental results suggests that D [subscript s](T [subscript m])/k [subscript e] [superscript 0.67] might serve as a metric for evaluating the potential to supersaturate Si with transition metals by PLM.National Science Foundation (U.S.) (Faculty Early Career Development Program ECCS-1150878)Chesonis Family FoundationUnited States. Army Research Laboratory (United States. Army Research Office Grant W911NF-10-1-0442)National Science Foundation (U.S.) (United States. Dept. of Energy NSF CA EEC-1041895

Emergence of Very Broad Infrared Absorption Band by Hyperdoping of Silicon with Chalcogens

We report the near through mid-infrared (MIR) optical absorption spectra, over the range 0.05–1.3 eV, of monocrystalline silicon layers hyperdoped with chalcogen atoms synthesized by ion implantation followed by pulsed laser melting. A broad mid-infrared optical absorption band emerges, peaking near 0.5 eV for sulfur and selenium and 0.3 eV for tellurium hyperdoped samples. Its strength and width increase with impurity concentration. Its strength decreases markedly with subsequent thermal annealing. The emergence of a broad MIR absorption band is consistent with the formation of an impurity band from isolated deep donor levels as the concentration of chalcogen atoms in metastable local configurations increases.Engineering and Applied Science

Defect-mediated nanostructures and luminescence centres in silicon

Research over the past decade has indicated that ion implantation provides an attractive way of forming nanocrystals in solids that exhibit some exciting new material properties. However, the size and size distribution of these nanoparticles are both very difficult parameters to control, especially when a subsequent thermal anneal is needed to form well-defined precipitates. This usually leads to somewhat uncontrolled growth by the Ostwald ripening effect. In addition, ion implantation is known to create structural damage to crystals (especially semiconductors such as silicon), which evolves into many types of defects depending on the annealing regime and degree of damage. Some of these defects in silicon are optically active and can be detected by photoluminescence (PL), while others are inactive and may act as non-radiative recombination centres. The first part of this thesis concentrates on the encapsulation of Au nanoparticles in silicon and SiO2 using a combination of ion implantation, high temperature annealing, and wet oxidation. Structural analysis was undertaken by Rutherford backscattering spectrometry and channeling (RBS-C) in conjunction with transmission electron microscopy (TEM). The final structure of Au embedded at a precise depth in SiO{u2082} was achieved by wet oxidising the top silicon layer of a silicon-on-insulator (SOl) wafer containing Au precipitates. Several interesting phenomena including the segregation of Au precipitates at the oxidising interface, Au-enhanced oxidation, and preferential reprecipitation of Au on the Si-SiO{u2082} interface after dissolution are observed. The role of excess silicon-interstitials, which are injected into the underlying silicon during the oxidation process and mechanisms have been proposed to explain the results. In the second part, Si implantation to a range of fluences and subsequent annealing under various conditions were used to form different types of interstitial-based defects in crystalline silicon. Results show that low dose implantation (10{u00B9}{u2070} to 10{u00B9}{u00B3} cm{u207B}{u00B2}) and a low thermal budget annealing process (up to 525{u00B0}C for 2 minutes) are favourable for the observation of luminescence from small interstitial-related defect clusters (principally as the W-line at 1218 nm and the X-line at 1193 nm). Higher fluences create higher damage levels that lead to a formation of extended {311} defects and dislocations by annealing at higher temperatures, as can be identified by the R-line luminescence (at 1375 nm), as well as D-bands (around 1428 nm and 1530 nm), respectively. The W-line, in particular, is so sharp and bright that it was exploited to realise a sub-bandgap silicon light emitting diode (LED). The quantum efficiency of the fabricated LEDs is, however, poor especially when additional boron is introduced to create a p{u207A} layer that serves the purpose of making a good electrical contact at low temperature. An extended study on the effect of boron is consequently carried out in the last experimental chapter, where the competing formation between optically active silicon-interstitial defects and boron-interstitial clusters (BICs) is argued for this luminescence intensity loss. The results presented in this thesis have provided a better understanding of defect-defect and defect-impurity interactions and may inspire future research on silicon-based applications

Self-assembled Au nanoparticles in SiO 2 by ion implantation and wet oxidation

Implantation, annealing, and oxidation processes have been used to form Au nanoparticles with a narrow size and depth distribution in a SiO2 layer. Different approaches have been attempted: in particular, the gettering of Au to fill preformed nanocavities (obtained by H-implantation and annealing) and thus overcome the broad particle size distribution that is normally associated with nanoparticles formed by implantation and annealing. The results suggest that nanocavities cannot be directly formed in SiO2 by H-implantation and a subsequent annealing due partly to the high mobility of H atoms in SiO2. However, cavities formed in Si are useful in obtaining a narrow size and depth distribution of Au precipitates: the Si substrate can then be oxidized to form Au nanoparticles in SiO2. Sequential wet oxidations of Si samples containing Au nanoparticles have revealed several interesting phenomena, namely, segregation of Au particles at a growing oxide interface, Au-enhanced oxidation, dissolution and reprecipitation of Au precipitates during oxidation, and preferential wetting of Au on the oxide layer. In particular, the Au dissolution and reprecipitation processes are Si interstitial mediated. By completely oxidizing the top Si layer, an array of Au precipitates can be confined at a precise depth within a SiO2 layer corresponding to the front interface of a buried oxide layer. The size distribution of the resulting Au precipitates in SiO2 is smallest when Au is first gettered to cavities and vacancies are subsequently introduced into the Si layer prior to oxidation

Enhanced visible and near-infrared optical absorption in silicon supersaturated with chalcogens

We show that single-crystal silicon supersaturated with sulfur (S), selenium (Se), or tellurium (Te) displays a substantially enhanced absorption coefficient for light with wavelengths of 400 to 1600 nm. Alloys were prepared in silicon on insulator wafers by ion implantation followed by nanosecond pulsed laser melting. Measurements of the absorption coefficient were made by direct transmission through freestanding thin films and by spectroscopic ellipsometry