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Pastor, David

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Pastor, David

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2015 - 201932020 - 20213

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Author's Publications: search.filters.isAuthorOfPublication.f6ca2cfc-103b-43a7-8b42-bc5d5041791c

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Now showing 1 - 6 of 6
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    Gold-Hyperdoped Germanium with Room-Temperature Sub-Band-Gap Optoelectronic Response
    (American Physical Society (APS), 2020-12-16) Gandhi, Hemi H.; Pastor, David; Tran, Tuan T.; Kalchmair, Stefan; Smilie, L.A.; Mailoa, Jonathan P.; Milazzo, Ruggero; Napolitani, Enrico; Loncar, Marco; Williams, James S.; Aziz, Michael; Mazur, Eric
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    Ion-beam synthesis and thermal stability of highly tin-concentrated germanium – tin alloys
    (Elsevier BV, 2017) Tran, Tuan T.; Gandhi, Hemi H.; Pastor, David; Aziz, Michael; Williams, J.S.
    A 9 at% Sn Ge-Sn alloy of good crystalline quality has been achieved by ion implantation followed by pulsed laser melting and resolidification. The concentration and crystallinity of the alloys are fully characterised by Rutherford backscattering spectrometry, X-ray diffraction, transmission electron microscopy and Raman spectroscopy. At high Sn concentrations, oxygen intermixing from a capping oxide layer, which is used to prevent ion-beam induced porosity, can interfere with the solidification process and compromise overall crystal quality. This indicates that the near surface layer containing oxygen after ion implantation must be removed before pulsed laser melting in order to obtain good crystal quality. The alloy's crystallinity is thermally stable under annealing up to View the MathML source for View the MathML source. This thermal budget is comparable to that of Ge-Sn produced by conventional MBE or CVD methods and suitable for subsequent device fabrication and post-processing.
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    Simultaneous high crystallinity and sub-bandgap optical absorptance in hyperdoped black silicon using nanosecond laser annealing
    (AIP Publishing, 2015) Franta, Benjamin; Pastor, David; Gandhi, Hemi H.; Rekemeyer, Paul H.; Gradečak, Silvija; Aziz, Michael; Mazur, Eric
    Hyperdoped black silicon fabricated with femtosecond laser irradiation has attracted interest for applications in infrared photodetectors and intermediate band photovoltaics due to its sub-bandgap optical absorptance and light-trapping surface. However, hyperdoped black silicon typically has an amorphous and polyphasic polycrystalline surface that can interfere with carrier transport, electrical rectification, and intermediate band formation. Past studies have used thermal annealing to obtain high crystallinity in hyperdoped black silicon, but thermal annealing causes a deactivation of the sub-bandgap optical absorptance. In this study, nanosecond laser annealing is used to obtain high crystallinity and remove pressure-induced phases in hyperdoped black silicon while maintaining high sub-bandgap optical absorptance and a light-trapping surface morphology. Furthermore, it is shown that nanosecond laser annealing reactivates the sub-bandgap optical absorptance of hyperdoped black silicon after deactivation by thermal annealing. Thermal annealing and nanosecond laser annealing can be combined in sequence to fabricate hyperdoped black silicon that simultaneously shows high crystallinity, high abovebandgap and sub-bandgap absorptance, and a rectifying electrical homojunction. Such nanosecond laser annealing could potentially be applied to non-equilibrium material systems beyond hyperdoped black silicon.
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    Carrier Dynamics and Absorption Properties of Gold-Hyperdoped Germanium: Insight Into Tailoring Defect Energetics
    (American Physical Society (APS), 2021-06-23) Dissanayake, Sashini Senali; Ferdous, Naheed; Gandhi, Hemi H.; Pastor, David; Tran, Tuan T.; Williams, Jim S.; Aziz, Michael; Mazur, Eric; Ertekin, Elif; Sher, Meng-Ju
    Hyperdoping germanium with gold is a potential method to produce room-temperature short-wavelength-infrared radiation (SWIR; 1.4–3.0 μm) photodetection. We investigate the charge carrier dynamics, light absorption, and structural properties of gold-hyperdoped germanium (Ge:Au) fabricated with varying ion implantation and nanosecond pulsed laser melting conditions. Time-resolved terahertz spectroscopy (TRTS) measurements show that Ge:Au carrier lifetime is significantly higher than that in previously studied hyperdoped silicon systems. Furthermore, we find that lattice composition, sub-band-gap optical absorption, and carrier dynamics depend greatly on hyperdoping conditions. We use density functional theory (DFT) to model dopant distribution, electronic band structure, and optical absorption. These simulations help explain experimentally observed differences in optical and optoelectronic behavior across different samples. DFT modeling reveals that substitutional dopant incorporation has the lowest formation energy and leads to deep energy levels. In contrast, interstitial or dopant-vacancy complex incorporation yields shallower energy levels that do not contribute to sub-band-gap light absorption and have a small effect on charge carrier lifetimes. These results suggest that it is promising to tailor dopant incorporation sites of Ge:Au for SWIR photodetection applications.
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    Chalcogen-hyperdoped germanium for short-wavelength infrared photodetection
    (AIP Publishing, 2020-07-01) Gandhi, Hemi H.; Pastor, David; Tran, Tuan T.; Kalchmair, Stefan; Smillie, Lachlan A.; Mailoa, Jonathan P.; Milazzo, Ruggero; Napolitani, Enrico; Loncar, Marko; Williams, James S.; Aziz, Michael; Mazur, Eric
    Obtaining short-wavelength-infrared (SWIR; 1.4 μm–3.0 μm) room-temperature photodetection in a low-cost, group IV semiconductor is desirable for numerous applications. We demonstrate a non-equilibrium method for hyperdoping germanium with selenium or tellurium for dopant-mediated SWIR photodetection. By ion-implanting Se or Te into Ge wafers and restoring crystallinity with pulsed laser melting induced rapid solidification, we obtain single crystalline materials with peak Se and Te concentrations of 1020 cm−3 (104 times the solubility limits). These hyperdoped materials exhibit sub-bandgap absorption of light up to wavelengths of at least 3.0 μm, with their sub-bandgap optical absorption coefficients comparable to those of commercial SWIR photodetection materials. Although previous studies of Ge-based photodetectors have reported a sub-bandgap optoelectronic response only at low temperature, we report room-temperature sub-bandgap SWIR photodetection at wavelengths as long as 3.0 μm from rudimentary hyperdoped Ge:Se and Ge:Te photodetectors.
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    Suppression of ion-implantation induced porosity in germanium by a silicon dioxide capping layer
    (AIP Publishing, 2016-08-22) Tran, Tuan; Alkhaldi, Huda; Gandhi, Hemi; Pastor, David; Huston, Larissa; Wong-Leung, Jennifer; Aziz, Michael; Williams, J.