Browsing by Author "Li, P. W."

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Designer germanium quantum dot phototransistor for near infrared optical detection and amplification
(2015-02-06)
Kuo, M. H.
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Lai, W. T.
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Hsu, T. M.
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Chen, Y. C.
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Chang, C. W.
We demonstrated a unique CMOS approach for the production of a high-performance germanium (Ge) quantum dot (QD) metal-oxide-semiconductor phototransistor. In the darkness, low off-state leakage (I-off similar to 0.27 pA mu m(-2)), a high on-off current ratio (I-on/I-off similar to 10(6)), and good switching behaviors (subthreshold swing of 175 mV/dec) were measured on our Ge-QD phototransistor at 300 K, indicating good hetero-interfacial quality of the Ge-on-Si. Illumination makes a significant enhancement in the drain current of Ge QD phototransistors when biased at both the on-and off-states, which is a great benefit from Ge QD-mediated photoconductive and photovoltaic effects. The measured photocurrent-to-dark-current ratio (I-photo/I-dark) and the photoresponsivities from the Ge QD phototransistor are as high as 4.1 x 10(6) and 1.7 AW(-1), respectively, under an incident power of 0.9mW at 850 nm illumination. A superior external quantum efficiency of 240% and a very fast temporal response time of 1.4 ns suggest that our Ge QD MOS phototransistor offers great promise as optical switches and transducers for Si-based optical interconnects.
"Embedded Emitters": Direct bandgap Ge nanodots within SiO2
(2016-12-21)
Kuo, M. H.
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Chou, S. K.
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Pan, Y. W.
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Lin, S. D.
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George, T.
Microdisk-arrays of vertically stacked 30-70nm Ge nanodots embedded within SiO2 were fabricated using thermal oxidation of Si0.75Ge0.25 abacus-shaped pillars and followed by post-annealing in oxygen-deficient conditions. The Ge nanodots are subjected to increasing quantum-confinement and tensile-strain by reducing dot size. We show that considerable quantum-confinement and tensile-strain can be generated within 30 nm Ge nanodots embedded in SiO2, as evidenced by large Raman red shifts for the Ge-Ge phonon lines in comparison to that for bulk Ge. These large quantum-confinement and tensile-strain facilitate direct-bandgap photoluminescence experimentally observed for the Ge nanodots, and are consistent with the strain-split photoluminescence transitions to the light-hole (LH) and heavy-hole (HH) valence bands at 0.83 eV and 0.88 eV, respectively. Time-resolved photoluminescence measurements conducted from 10-100K show temperature-insensitive carrier lifetimes of 2.7 ns and 5 ns for the HH and LH valence-band transitions, respectively, providing additional strong evidence of direct bandgap photoluminescence for tensile-strained Ge nanodots. Published by AIP Publishing.
Geometry-dependent phase, stress state and electrical properties in nickel-silicide nanowires
(2016-05-25)
Wang, C. C.
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Lai, W. T.
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Hsiao, Y. Y.
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Chen, I. H.
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George, T.
We report that the geometry of single-crystalline Si nanowires (NWs) prior to salicidation at 500 degrees C is the key factor controlling the phase, stress state, and electrical resistivity of the resulting NixSiy NWs of width less than 100 nm. This is a radical departure from previous observations of a single phase formation for nickel silicides generated from the silicidation of bulk Si substrates. The phase transition from NiSi for large NWs (W-Si NW = 250-450 nm) to Ni2Si for small NWs (W-Si NW = 70-100 nm) is well correlated with the observed volumetric expansion and electrical resistivity variation with the NW width. For the extremely small dimensions of NixSiy NWs, we propose that the preeminent, kinetics-based Zhang and d\'Heurle model for salicidation be modified to a more thermodynamically-governed, volume-expansion dependent NixSiy phase formation. A novel, plastic deformation mechanism is proposed to explain the observed, geometry-dependent NixSiy NW phase formation that also strongly influences the electrical performance of the NWs.
Self-organized gate stack of Ge nanosphere/SiO2/Si1-xGex enables Ge-based monolithically-integrated electronics and photonics on Si platform
(2018-01-01)
Liao, P. H.
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Kuo, M. H.
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Tien, C. W.
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Chang, Y. L.
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Hong, P. Y.
We report the first-of-its-kind, self-organized gate stack of Ge nanosphere (NP) gate/SiO2/Si1-xGex channel fabricated in a single oxidation step. Process-controlled tunability of the Ge NP size (5-90nm), SiO2 thickness (2-4nm), and Ge content (x = 0.65-0.85) and strain engineering (epsilon(comp) = 1-3%) of the Si1-xGex are achieved. We demonstrated Ge junctionless (JL) n-FETs and photoMOSFETs (PTs) as amplifier and photodetector, respectively, for Ge receivers. L-G of 75nm JL n-FETs feature I-ON/I-OFF > 5x10(8), I-ON > 500 mu A/mu m at V-DS = 1V, T= 80K. Ge-PTs exhibit superior photoresponsivity > 1,000A/W and current gain linearity ranging from nW-mW for 850nm illumination. Size-tunable photo-luminescence (PL) of 300-1600nm (NUV-NIR) are observed on 5-100nm Ge NPs. Our gate stack of Ge NP/SiO2/Si1-xGex enables a practically achievable building block for monolithically-integrated Ge electronic and photonic ICs (EPICs) on Si.
Single-fabrication-step Ge nanosphere/SiO2/SiGe heterostructures: a key enabler for realizing Ge MOS devices
(2018-05-18)
Liao, P. H.
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Peng, K. P.
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Lin, H. C.
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George, T.
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Li, P. W.
We report channel and strain engineering of self-organized, gate-stacking heterostructures comprising Ge-nanosphere gate/SiO2/SiGe-channels. An exquisitely-controlled dynamic balance between the concentrations of oxygen, Si, and Ge interstitials was effectively exploited to simultaneously create these heterostructures in a single oxidation step. Process-controlled tunability of the channel length (5-95 nm diameters for the Ge-nanospheres), gate oxide thickness (2.5-4.8 nm), as well as crystal orientation, chemical composition and strain engineering of the SiGe-channel was achieved. Single-crystalline (100) Si1-xGex shells with Ge content as high as x = 0.85 and with a compressive strain of 3%, as well as (110) Si1-xGex shells with Ge content of x = 0.35 and corresponding compressive strain of 1.5% were achieved. For each crystal orientation, our high Ge-content, highly-stressed SiGe shells feature a high degree of crystallinity and thus, provide a core 'building block' required for the fabrication of Ge-based MOS devices.
'Symbiotic' semiconductors: unusual and counter-intuitive Ge/Si/O interactions
(2017-03-15)
George, T.
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Li, P. W.
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Chen, K. H.
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Peng, K. P.
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Lai, W. T.
Since the inception of the first transistors in the 1940s, the immense body of work on the Group IV semiconductors, Si and Ge, has spearheaded spectacular advances in modern integrated-circuit (IC) technology that has enabled a vast landscape of device applications in logic, memory, and computing. Although initially Si supplanted Ge as the material of choice for metal-oxide-semiconductor field-effect transistors, Ge-based devices are now breaking new ground. Widespread and innovative Ge-based applications exist in optoelectronics, communications, microelectro-mechanical systems, and energy harvesting/savings. On the fundamental, materials science front, while it is well known that Ge and Si are fully miscible in each other, the nature and extent of their attraction for each other has largely been unexplored. In this paper, we report a rather curious interplay between Ge and Si that occurs at high temperature (similar to 900 degrees C) and that can be best described as 'symbiotic'. Each element appears to facilitate reactions in the other which would otherwise not be possible. Oxygen intersititials also appear to play a major role in these reactions. Our experimental work has allowed us to classify four distinct regimes where these reactions occur. We describe these conditions and provide the necessary theoretical explanations for these results.
The pivotal role of oxygen interstitials in the dynamics of growth and movement of germanium nanocrystallites
(2015-01-01)
Chen, K. H.
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Wang, C. C.
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Lai, W. T.
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George, T.
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Li, P. W.
We report an unusual "symbiotic" behavior of oxygen interstitials acting in concert with Ge and Si interstitials at high temperature inducing morphology changes and autonomous migration of Ge nanocrystallites within SiO2/Si3N4 layers. The Ge nanocrystallites were originally generated by the selective oxidation of SiGe nano-pillars grown and lithographically patterned over buffer Si3N4 layers on Si substrates. The coalescence and movement of these Ge nanocrystallites appear to be very sensitive to the presence and flux of oxygen interstitials especially at the Ge nanocrystallite/buffer Si3N4 interface. A range of different morphologies are observed for Ge nanocrystallites that are directly attributable to the influence of oxygen interstitial concentration and consequently the interstitial Si and Ge concentrations. In combination with Si and Ge interstitials, oxygen interstitials activate the coalescence of sparsely-distributed Ge nanocrystallites and concurrently their migration towards the source of Si interstitials, i.e. the buffer Si3N4 layers, through catalytically-enhanced local decomposition and subsequent oxidation of both the SiO2 and Si3N4 buffer layers. We also show that these symbiotic effects are "tunable" by increasing the Ge content of the SiGe nano-pillars. Dense distributions of Ge nanocrystallites generated from high Ge content SiGe nano-pillars remain static and they show no changes in their morphology possibly because oxygen interstitials are simply unable to penetrate these clusters and consequently incapable of inducing symbiotic Si and Ge interstitial generation.