Person: Akey, Austin
Loading...
Email Address
AA Acceptance Date
Birth Date
Research Projects
Organizational Units
Job Title
Last Name
Akey
First Name
Austin
Name
Akey, Austin
6 results
Search Results
Now showing 1 - 6 of 6
Publication Au-rich filamentary behavior and associated subband gap optical absorption in hyperdoped Si(American Physical Society (APS), 2017) Yang, W.; Akey, Austin; Smillie, L. A.; Mailoa, J. P.; Johnson, B. C.; McCallum, J. C.; Macdonald, D.; Buonassisi, T.; Aziz, Michael; Williams, J. S.Au-hyperdoped Si, synthesized by ion implantation and pulsed laser melting, is known to exhibit a strong sub-band gap photoresponse that scales monotonically with the Au concentration. However, there is thought to be a limit to this behavior since ultrahigh Au concentrations (>1 × 1020 cm−3) are expected to induce cellular breakdown during the rapid resolidification of Si, a process that is associated with significant lateral impurity precipitation. This work shows that the cellular morphology observed in Au-hyperdoped Si differs from that in conventional, steady-state cellular breakdown. In particular, Rutherford backscattering spectrometry combined with channeling and transmission electron microscopy revealed an inhomogeneous Au distribution and a subsurface network of Au-rich filaments, within which the Au impurities largely reside on substitutional positions in the crystalline Si lattice, at concentrations as high as ∼3 at. %. The measured substitutional Au dose, regardless of the presence of Au-rich filaments, correlates strongly with the sub-band gap optical absorptance. Upon subsequent thermal treatment, the supersaturated Au forms precipitates, while the Au substitutionality and the sub-band gap optical absorption both decrease. These results offer insight into a metastable filamentary regime in Au-hyperdoped Si that has important implications for Si-based infrared optoelectronics.Publication Picosecond carrier recombination dynamics in chalcogen-hyperdoped silicon(AIP Publishing, 2014) Sher, Meng-Ju; Simmons, Christie B.; Krich, Jacob Jonathan; Akey, Austin; Winkler, Mark T.; Recht, Daniel; Buonassisi, Tonio; Aziz, Michael; Lindenberg, Aaron M.Intermediate-band materials have the potential to be highly efficient solar cells and can be fabricate by incorporating ultrahigh concentrations of deep-level dopants. Direct measurements of the ultrafast carrier recombination processes under supersaturated dopant concentrations have not been previously conducted. Here, we use optical-pump/terahertz-probe measurements to study carrier recombination dynamics of chalcogen-hyperdoped silicon with sub-picosecond resolution. The recombination dynamics is described by two exponential decay time scales: a fast decay time scale ranges between 1 and 200ps followed by a slow decay on the order of 1 ns. In contrast to the prior theoretical predictions, we find that the carrier lifetime decreases with increasing dopant concentration up to and above the insulator-to-metal transition. Evaluating the material’s figure of merit reveals an optimum doping concentration for maximizing performance.Publication Deactivation of metastable single-crystal silicon hyperdoped with sulfur(AIP Publishing, 2013) Simmons, C. B.; Akey, Austin; Krich, Jacob Jonathan; Sullivan, Joseph T.; Recht, Daniel; Aziz, Michael; Buonassisi, TonioSilicon supersaturated with sulfur by ion implantation and pulsed laser melting exhibits broadband optical absorption of photons with energies less than silicon's band gap. However, this metastable, hyperdoped material loses its ability to absorb sub-band gap light after subsequent thermal treatment. We explore this deactivation process through optical absorption and electronic transport measurements of sulfur-hyperdoped silicon subject to anneals at a range of durations and temperatures. The deactivation process is well described by the Johnson-Mehl-Avrami-Kolmogorov framework for the diffusion-mediated transformation of a metastable supersaturated solid solution, and we find that this transformation is characterized by an apparent activation energy of \(E_A=1.7 ± 0.1 eV\). Using this activation energy, the evolution of the optical and electronic properties for all anneal duration-temperature combinations collapse onto distinct curves as a function of the extent of reaction. We provide a mechanistic interpretation of this deactivation based on short-range thermally activated atomic movements of the dopants to form sulfur complexes.Publication Methodology for vetting heavily doped semiconductors for intermediate band photovoltaics: A case study in sulfur-hyperdoped silicon(AIP Publishing, 2013) Sullivan, J. T.; Simmons, C. B.; Krich, J. J.; Akey, Austin; Recht, Daniel; Aziz, Michael; Buonassisi, T.We present a methodology for estimating the efficiency potential for candidate impurity-band photovoltaic materials from empirical measurements. This methodology employs both Fourier transform infrared spectroscopy and low-temperature photoconductivity to calculate a “performance figure of merit” and to determine both the position and bandwidth of the impurity band. We evaluate a candidate impurity-band material, silicon hyperdoped with sulfur; we find that the figure of merit is more than one order of magnitude too low for photovoltaic devices that exceed the thermodynamic efficiency limit for single band gap materials.Publication Single-Phase Filamentary Cellular Breakdown Via Laser-Induced Solute Segregation(Wiley-Blackwell, 2015) Akey, Austin; Recht, Daniel; Williams, James S.; Aziz, Michael; Buonassisi, TonioNanosecond melting and quenching of materials offers a pathway to novel structures with unusual properties. Impurity-rich silicon processed using nanosecond-pulsed-laser-melting is known to produce nanoscale features in a process referred to as “cellular breakdown” due to destabilization of the planar liquid/solid interface. Here, we apply atom probe tomography combined with electron microscopy to show that the morphology of cellular breakdown in these materials is significantly more complex than previously documented. We observe breakdown into a complex, branching filamentary structure topped by a few nm of a cell-like layer. Singlephase diamond cubic silicon highly supersaturated with at least 10% atomic Co and no detectable silicides is reported within these filaments. In addition, the unprecedented spatio-chemical accuracy of the atom probe allows us to investigate nanosecond formation dynamics of this complex material. Previously-reported properties of these materials can now be reconsidered in light of their true composition, and this class of inhomogeneous metastable alloys in silicon can be explored with confidence.Publication Transition-Metal Single Atoms in a Graphene Shell as Active Centers for Highly Efficient Artificial Photosynthesis(Elsevier BV, 2017) Jiang, Kun; Siahrostami, Samira; Akey, Austin; Li, Yanbin; Lu, Zhiyi; Lattimer, Judith; Hu, Yongfeng; Stokes, Chris; Gangishetty, Mahesh; Chen, Guangxu; Zhou, Yawei; Hill, I.; Cai, Wen-Bin; Bell, David; Chan, Karen; Nørskov, Jens K.; Cui, Yi; Wang, HaotianUtilizing solar energy to fix carbon dioxide (CO2) with water into chemical fuels and oxygen, a mimic process of photosynthesis in nature, is becoming increasingly important but still challenged by the low selectivity and activity, especially in CO2 electrocatalytic reduction. Here we report transition metal atoms coordinated in graphene shell as active centers for aqueous CO2 reduction to carbon monoxide (CO), with high Faradaic efficiencies over 90 % under significant currents up to ~ 60 mA/mg (12 mA/cm2). Three-dimensional atom probe tomography was employed to directly identify the single Ni atomic sites in graphene vacancies. Theoretical simulations suggest that compared to metallic Ni, the Ni atomic sites present significantly different electronic structures which facilitate CO2 to CO conversion and suppress the competing hydrogen evolution reaction dramatically.