Joseph T. Sullivan

Extended infrared photoresponse and gain in chalcogen-supersaturated silicon photodiodes

Highly supersaturated solid solutions of selenium or sulfur in silicon were formed by ion implantation followed by nanosecond pulsed laser melting. n+p photodiodes fabricated from these materials exhibit gain (external quantum efficiency >3000%) at 12 V of reverse bias and substantial optoelectronic response to light of wavelengths as long as 1250 nm. The amount of gain and the strength of the extended response both decrease with decreasing magnitude of bias voltage, but >100% external quantum efficiency is observed even at 2 V of reverse bias. The behavior is inconsistent with our expectations for avalanche gain or photoconductive gain.

Room-temperature sub-band gap optoelectronic response of hyperdoped silicon

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.

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

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.

Soft x-ray emission spectroscopy studies of the electronic structure of silicon supersaturated with sulfur

We apply soft x-ray emission spectroscopy (XES) to measure the electronic structure of crystalline silicon supersaturated with sulfur (up to 0.7 at. %), a candidate intermediate-band solar cell material. Si L2,3 emission features are observed above the conventional Si valence band maximum, with intensity scaling linearly with S concentration. The lineshape of the S-induced features change across the insulator-to-metal transition, indicating a significant modification of the local electronic structure concurrent with the change in macroscopic electronic behavior. The relationship between the Si L2,3XESspectral features and the anomalously high sub-band gap infrared absorption is discussed.

Extended X-ray absorption fine structure spectroscopy of selenium-hyperdoped silicon

Silicon doped with an atomic percent of chalcogens exhibits strong, uniform sub-bandgap optical absorptance and is of interest for photovoltaic and infrared detector applications. This sub-bandgap absorptance is reduced with subsequent thermal annealing indicative of a diffusion mediated chemical change. However, the precise atomistic origin of absorptance and its deactivation is unclear. Herein, we apply Se K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy to probe the chemical states of selenium dopants in selenium-hyperdoped silicon annealed to varying degrees. We observe a smooth and continuous selenium chemical state change with increased annealing temperature, highly correlated to the decrease in sub-bandgap optical absorptance. In samples exhibiting strong sub-bandgap absorptance, EXAFS analysis reveals that the atoms nearest to the Se atom are Si at distances consistent with length scales in energetically favorable Se substitutional-type point defect complexes as calculated by d...

Controlling dopant profiles in hyperdoped silicon by modifying dopant evaporation rates during pulsed laser melting

We describe a method to control the sub-surface dopant profile in “hyperdoped” silicon fabricated by ion implantation and pulsed laser melting. Dipping silicon ion implanted with sulfur into hydrofluoric acid prior to nanosecond pulsed laser melting leads to a tenfold increase in the rate of sulfur evaporation from the surface of the melt. This results in an 80% reduction of the near-surface dopant concentration, effectively embedding the hyperdoped region in a layer up to 180 nm beneath the surface. This method should facilitate the development of blocked impurity band devices.

Targeted Search for Effective Intermediate Band Solar Cell Materials

Recent years have seen a number of candidate materials for intermediate band (IB) solar cells, but none has demonstrated a high-efficiency device. We explain this deficit by means of a figure of merit, which predicts the potential effectiveness of candidate IB materials in advance of device fabrication. This figure of merit captures in a single parameter the inherent tradeoff between enhanced absorption and enhanced recombination within an IB material, and it suggests a path toward efficient IB materials. We illustrate a screening approach based on this figure of merit for a specific class of IB material systems: a dopant-induced impurity band in silicon. We show, in this case, that the optical and nonradiative electrical trapping cross sections of impurities, widely studied properties that can be measured in bulk materials, determine the potential performance of IB solar cell devices. We conclude with a list of appealing and unappealing candidate material systems.

Effect of layer thickness on device response of silicon heavily supersaturated with sulfur

We report on a simple experiment in which the thickness of a hyperdoped silicon layer, supersaturated with sulfur by ion implantation followed by pulsed laser melting and rapid solidification, is systematically varied at constant average sulfur concentration, by varying the implantation energy, dose, and laser fluence. Contacts are deposited and the external quantum efficiency (EQE) is measured for visible wavelengths. We posit that the sulfur layer primarily absorbs light but contributes negligible photocurrent, and we seek to support this by analyzing the EQE data for the different layer thicknesses in two interlocking ways. In the first, we use the measured concentration depth profiles to obtain the approximate layer thicknesses, and, for each wavelength, fit the EQE vs. layer thickness curve to obtain the absorption coefficient of hyperdoped silicon for that wavelength. Comparison to literature values for the hyperdoped silicon absorption coefficients [S.H. Pan et al. Applied Physics Letters 98, 12191...

In situ PL imaging toward real-time plating process control

Light induced plating (LIP) of front grid contacts is an industry-scalable potential alternative to silver paste, but LIP requires an additional patterning step to create openings in the silicon nitride (SiNx) antireflection coating (ARC) layer for metallization. One approach for patterning SiNx is masking and wet chemical etching. However, nitride etch rates can vary from cell to cell depending on the SiNx PECVD deposition parameters, previous processing steps, and etching solution usage and maintenance. Under-etching results in poor contact adhesion and over-etching results in undercutting and possible emitter damage. We demonstrate in situ real-time photoluminescence imaging (PLI) as a method to determine the point when SiNx has been fully removed. This method has the potential to be integrated into a commercial processing line to improve process control, uniformity, and repeatability.

Hyperdoped silicon sub-band gap photoresponse for an intermediate band solar cell in silicon

Hyperdoping silicon with impurities is considered an attractive method to develop an intermediate band solar cell in silicon with the potential to increase the photovoltaic cell efficiency beyond that of the Shockley-Queisser limit by utilizing sub-band gap photons for energy generation. Unfortunately, to date sub-band gap photoresponse has not been observed in singlecrystal hyperdoped silicon at room temperature, which is crucial for the development of intermediate band solar cells. In this contribution, we report and analyze room-temperature sub-band gap photoresponse of single-crystal silicon hyperdoped with gold. We further discuss the potential of using gold-hyperdoped silicon for IBSC in silicon.