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ABSTRACT: The reaction pathways and phase evolution during synthesis of CuInSe2 (CIS) by a novel bilayer approach were investigated using in situ high-temperature X-ray diffraction. Two bilayer precursor structures, glass/Mo/g-In2Se3/b-CuSe + b-Cu2Se/Se and glass/ Mo/g-In2Se3/b-Cu2Se/Se, were examined in this study. Temperature ramp experiments revealed that the phase transformation sequence for each bilayer precursor qualitatively follows that predicted by the phase diagram and that the onset temperatures for decomposition of the sub-binary compounds depend on the Se partial pressure. Measurement of the isothermal rate of formation of CuInSe2 at six temperatures in the range 260 to 310�C for the gamma-In2Se3/beta-CuSe+beta-Cu2Se/Se bilayer suggests relatively slow nucleation followed by diffusion-limited reaction with estimated activation energy of 162(±7) and 225 (±16) kJ/mol from Avrami and parabolic models, respectively. Interestingly, the measured activation energy for the same precursor in a 4 mol % H2/He ambient (108 (±8) kJ/mol) was lower than that observed in pure N2 (158 (±16) kJ/mol). The results of isothermal measurements in the temperature range 250 to 300�C for the gamma-In2Se3/beta-Cu2Se/Se precursor film in an inert ambient are consistent with one-dimensional diffusion-limited growth with estimated activation energy from the Avrami and parabolic models of 194 (±10) and 203 (±12) kJ/mol, respectively.
Progress in Photovoltaics Research and Applications 10/2012; 50(5-10.1002/pip.2262):543. · 5.79 Impact Factor
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ABSTRACT: Thin films of W–B–N (10 nm) have been evaluated as diffusion barriers for Cu interconnects. The amorphous W–B–N thin films
were prepared at room temperature via reactive magnetron sputtering using a W2B target at various N2/(Ar + N2) flow ratios. Cu diffusion tests were performed after in-situ deposition of 200nm Cu. Thermal annealing of the barrier stacks
was carried out in vacuum at elevated temperatures for one hour. X-ray diffraction patterns, sheet resistance measurement,
cross-section transmission electron microscopy images, and energy-dispersive spectrometer scans on the samples annealed at
500°C revealed no Cu diffusion through the barrier. The results indicate that amorphous W–B–N is a promising low resistivity
diffusion barrier material for copper interconnects.
Applied Physics A 05/2012; 94(3):691-695. · 1.63 Impact Factor
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ABSTRACT: Recessed-gate high-electron-mobility transistors (HEMTs) were fabricated using a Cl2-based plasma etch to study device performance as a function of recess depth. Devices were fabricated with recess depths varying
from 0nm to 25nm on a standard HEMT structure using a controllable, low-power etch recipe. It is shown that the threshold
voltage approached zero as the recess approached the AlGaN/GaN heterojunction. At the same time, mobility decreased an order
of magnitude over the etch range studied, and sheet carrier density also decreased. In addition to direct-current (DC) I–V and Hall measurements, electroluminescence was also used to characterize plasma damage in these devices.
KeywordsGaN-HEMT-ICP etch-electroluminescence
Journal of Electronic Materials 04/2012; 39(5):478-481. · 1.47 Impact Factor
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Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE; 01/2011
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ABSTRACT: Molybdenum is widely used as a back contact for CIGS solar cells due to its low cost and proven electrical and morphological compatibility with CIGS. Furthermore, it is believed that MoSe<sub>2</sub> forms during the absorber synthesis step to yield ohmic contacts with good adhesion. In this study the reaction kinetics of MoSe<sub>2</sub> formation are investigated by reacting Mo films on glass with Se using in-situ high-temperature X-ray diffraction (HT-XRD). Isothermal experiments were performed and the kinetics parameters were obtained from analysis of the data using the Avrami and parabolic growth models. The activation energy evaluated from the Avrami and parabolic models yielded 101.2 and 101.7 kJ/mol, respectively.
Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
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ABSTRACT: High-quality CuInSe<sub>2</sub> nanoparticles have been prepared by the solution phase synthesis procedure utilizing copper acetate or cuprous chloride as the Cu-precursor, indium chloride as the In-precursor and selenium powder as Se precursor, respectively. The synthesis process was optimized to grow 20 to 150 nm sized nanostructures of CuInSe<sub>2</sub>, and the Cu to In ratio as well as the kind of Cu-precursor used was found to be most important in determining the structure and property of the nanostructures. The synthesized nanostructures were characterized by TEM (transmission electron microscopy), XRD (X-ray diffraction), PL (photoluminescence) and micro-Raman spectroscopy, and detailed microstructural analyses of synthesized samples were also carried out by selective area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM).
Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
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ABSTRACT: A novel device structure incorporating an ultrathin AlGaN barrier layer capped by an AlN layer in the source-drain access regions has been implemented to reliably control threshold voltage in AlGaN/GaN high-electron-mobility transistors. A recessed-gate structure has been used to decrease 2-D electron gas (2DEG) density under the gate, thus controlling threshold voltage while maintaining low on-resistance and high current density. The structure presented in this letter implements an ultrathin AlGaN structure grown by metal-organic chemical vapor deposition capped with AlN to maintain a high 2DEG density in the access regions. A selective wet etch using heated photoresist developer is used to selectively etch the AlN layer in the gate region to the AlGaN barrier. We have demonstrated a repeatable threshold voltage of +0.21 V with 4-nm AlGaN barrier layer thickness.
IEEE Electron Device Letters 01/2010; · 2.85 Impact Factor
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ABSTRACT: The properties of an Ir (5 nm)/TaN (5 nm) stacked layer as a copper diffusion barrier on Si have been investigated. Ir/TaN bilayer barriers were prepared at room temperature by magnetron sputtering followed by in situ Cu deposition for diffusion tests. Thermal annealing of the barrier stacks was carried out in vacuum at high temperatures for 1 h. X-ray diffraction patterns, cross sectional transmission electron microscopy images, and energy-dispersive spectrometer line scans on the samples annealed at 600 °C revealed no Cu diffusion through the barrier. The results indicate that the Ir/TaN bilayer is an effective diffusion barrier for copper metallization.
Applied Physics Letters 03/2008; 92(11):111917-111917-3. · 3.84 Impact Factor
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ABSTRACT: Equilibrium calculations were used to optimize conditions for the chemical vapor deposition (CVD) of zirconia. The results showed zirconia formation would occur at high oxygen to zirconium atomic ratios (>4), low hydrogen to carbon ratios (
Thin Solid Films 01/2008; 516(18):6133-6139. · 1.89 Impact Factor
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Hung-Ta Wang, T. J. Anderson,
B. S. Kang,
F. Ren,
Changzhi Li,
Zhen-Ning Low,
Jenshan Lin,
B. P. Gila,
S. J. Pearton,
A. Osinsky,
Amir Dabiran
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ABSTRACT: The use of TiB2-based Ohmic contacts on Pt-gate AlGaN/GaN heterostructure diode hydrogen sensors is shown to provide very stable operation for detection of 1% H2 in air under field conditions where temperature is allowed to vary. In contrast, the use of more conventional Ti/Al/Pt/Au Ohmic contacts led to higher background variations in current that affect the ultimate detection threshold of the sensors. Combined with a differential pair geometry that compares current from an active diode with Pt-gate contact and a passive diode with Ti/Au gate, the more stable TiB2-based Ohmic contacts reduce false alarms due to ambient temperature changes. The diodes exhibit a change in forward current of more than 1 mA at 1.5 V when 1% H2 is introduced into an air ambient.
Applied Physics Letters 06/2007; 90(25):252109-252109-3. · 3.84 Impact Factor
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ABSTRACT: The properties of Ge/HfNx have been investigated relative to its use as a diffusion barrier for Cu metallization. The Ge/HfNx bilayer was grown on p-Si (001) substrates by reactive sputtering, followed by in situ deposition of Cu. Individually annealed films at different temperatures (400–700 °C, 1 h) were characterized for evidence of Cu transport through the barrier bilayer to the Si substrate. The annealed structures were characterized by x-ray diffraction, energy-dispersive spectroscopy, and high-resolution transmission electron microscopy. The results indicate superior diffusion barrier properties of Ge/HfNx for Cu metallization on Si compared to that for HfNx (7 nm).
Applied Physics Letters 12/2006; 89(23):231914-231914-3. · 3.84 Impact Factor
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ABSTRACT: The effects of merit-based scholarships on engineering students are studied by relating the GPA, credit load, and retention rates of engineering student merit-based scholarship recipients and students who met the same SAT and high school GPA criteria but did not receive the scholarship because it was not offered at the time of their enrollment. Student record data for 10,167 engineering students at three institutions in the state of Florida from 1987 to 2002 are extracted from a nine-institution database with student records data from 1987 to 2002 compiled by the Southeastern University and College Coalition for Engineering Education (SUCCEED) and used to examine the differences between the two groups of students in terms of GPA, semester credit hours, and percent leaving engineering over time. The comparisons reveal evidence that merit-based scholarships led students to adopt three strategies for retaining their scholarships: scholarship students tended to have higher average GPAs, tended to take fewer credit hours, and were more likely to leave engineering than similar students matriculating prior to the implementation of the scholarship program
Frontiers in Education Conference, 36th Annual; 12/2006
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ABSTRACT: The diffusion barrier properties of Hf N <sub>x</sub> and Hf–Ge–N thin films for Cu metallization on Ge are examined. The diffusion barrier films were deposited by reactive sputtering on p -Ge (001) single crystal substrates with varying thicknesses. Cu thin films were then deposited in situ on the diffusion barrier. The multilayer film structure was subsequently annealed in an Ar atmosphere. X-ray diffraction was used to determine the film crystallinity and identify intermetallic phases due to reactions involving the film and substrate. The Hf N <sub>x</sub> and Hf–Ge–N diffusion barrier films remained amorphous for annealing temperatures up to 700 ° C . At thickness of 50 nm , the Hf N <sub>x</sub> films showed superior diffusion barrier properties as compared to Hf–Ge–N based on the appearance of secondary phases due to reactions and changes in the Cu morphology. These results suggest that Hf N <sub>x</sub> is an effective barrier material for Cu integration on Ge.
Journal of Applied Physics 10/2006; · 2.17 Impact Factor
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ABSTRACT: The effect of annealing on the stability of
ohmic contacts on heavily Al-doped
n-ZnO is reported. Due to the high n-type doping levels in the ZnO, the as-deposited contacts are ohmic with an excellent
specific contact resistivity of
. Subsequent annealing produces a minimum value of
after processing at
. The absence of any temperature dependence to the contact resistance indicates that tunneling is the dominant current transport
mechanism. Annealing the contact structure at
produces Zn outdiffusion through the
, intermixing of Au and Ti, and roughening of the contact surface morphology. The ease of ohmic contact formation to the Al-doped
ZnO and extremely low contact resistance achievable even with low-temperature processing makes this metallization scheme attractive
in applications where high processing temperatures cannot be employed.
Journal of The Electrochemical Society. 04/2006; 153(5):G462-G464.
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ABSTRACT: Ti / Au Ohmic contacts on heavily Al-doped (n∼10<sup>19</sup> cm <sup>-3</sup>) n- ZnO produce low specific contact resistivity of 2.4×10<sup>-7</sup> Ω cm <sup>2</sup> in the as-deposited condition and extremely low minimum values of 6×10<sup>-8</sup> Ω cm <sup>2</sup> after annealing at 300 ° C .The contact resistance is independent of measurement temperature after low temperature anneals, suggesting that tunneling is the dominant transport mechanism in the contacts. The contact morphology roughens after annealing at 150 ° C and Auger electron spectroscopy depth profiling shows Zn outdiffusion through the metal and intermixing of Au and Ti. However, the morphology does not significantly worsen after anneals at 450 ° C . This metallization scheme looks very attractive for the n -electrode of ZnO -based light-emitting diode structures.
Applied Physics Letters 04/2006; · 3.84 Impact Factor
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ABSTRACT: Single crystal InN nanorods were successfully grown on c- Al <sub>2</sub> O <sub>3</sub> by hydride-metalorganic vapor phase epitaxy. The measured resistance of bare InN nanorods does not change upon exposure to hydrogen ambient. The addition of sputter-deposited clusters of Pt onto the surface of the InN nanorods, however, produced a significant change in the measured room temperature resistance. The measured resistance changed systematically by 0.5%–12% as the ambient hydrogen concentration in N <sub>2</sub> was varied between 10 and 250 ppm after 15 min exposure time. Importantly, a relatively low power consumption of ∼0.3 mW was measured under these conditions. There was no response at room temperature to O <sub>2</sub> , N <sub>2</sub> O , or NH <sub>3</sub> exposures.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 10/2005; · 1.34 Impact Factor
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ABSTRACT: The properties of W–Ge–N thin films are reported, focusing on issues relevant to their use as diffusion barriers for Cu metallization on silicon. The amorphous W–Ge–N thin films were deposited on thermally grown SiO2/Si using reactive sputter deposition. This was followed by in situ deposition of Cu films. Annealing studies for W–Ge–N were then carried out in a vacuum to investigate Cu diffusion and barrier film crystallization. X-ray diffraction was used to assess the crystallinity of the films upon annealing. The results show that W–Ge–N has a recrystallization temperature that is higher that that for WNx. Auger electron spectroscopy was used to measure the depth profile of Cu diffusion through the barrier layer. Little or no Cu diffusion was detected for a relatively high annealing temperature. The W–Ge–N films were conductive, although the resistivity is somewhat higher than that for WNx. The results suggest that W–Ge–N may be an attractive diffusion barrier material for Si or SiGe devices.
Applied Physics Letters 09/2005; 87(11):111902-111902-3. · 3.84 Impact Factor
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ABSTRACT: Brain lesions and degeneration can lead to a decreased ability to perform the physical and cognitive functions necessary for safe driving. A battery of computerized sensory-motor and cognitive tests (SMCTeststrade) has been developed to quantify sensory-motor and cognitive dysfunction, with particular application to the assessment of driving abilities in patients with neurological disorders. SMCTests and an on-road driving assessment were applied to 50 subjects with brain lesions referred to the Driving and Vehicle Assessment Service at Burwood Hospital, Christchurch (36 males, 14 females; age range 43-85 years, mean age 71.3 years; 35 stroke, 4 traumatic brain injury, 4 Alzheimer's disease, 7 with other diagnoses). Two techniques were used to build model equations for prediction of on-road driving ability based on SMCTests performance - binary logistic regression and nonlinear causal resource analysis (NCRA). Binary logistic regression correctly classified 94% of referrals as on-road pass or fail, while NCRA correctly classified 90% of referrals. Leave-one-out cross-validation analysis estimated that binary logistic regression would correctly predict the classification of 86% of an independent referral group as on-road pass or fail, while NCRA would correctly predict 76%. Results indicate that the predictive model based on binary logistic regression would be slightly more accurate than NCRA at predicting on-road pass or fail in an independent subject group. Conversely, the NCRA model also provides valuable information on the extent to which a subject would pass or fail an on-road driving assessment and identifies the deficit which most limits their driving ability
Engineering in Medicine and Biology Society, 2005. IEEE-EMBS 2005. 27th Annual International Conference of the; 02/2005
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ABSTRACT: From a cardiovascular standpoint, the safety of cyclooxygenase-2 (COX-2) blockers has been a topic of increasing concern. This concern stemmed from observations indicating that the COX-2 isoform is the major source of endothelium-derived prostacyclin and, hence, that selective blockade of this enzyme may impair endothelial health. To investigate this matter, we examined the effects of 7 days of treatment with rofecoxib versus naproxen on endothelial function in healthy volunteers.
Thirty-five healthy volunteers were randomized to receive 7-day treatment with either rofecoxib (25 mg/d, n=18) or naproxen (750 mg/d, n=17). Vascular response measurements were conducted using forearm strain-gauge plethysmography. Changes in forearm blood flow in response to the endothelium-dependent vasodilator acetylcholine (3, 10, and 30 microg/min) and the endothelium-independent vasodilator sodium nitroprusside (1 and 10 microg/min) were assessed before and after treatment. Acetylcholine evoked a dose-dependent increase in forearm blood flow in all groups. Importantly, treatment resulted in no change in acetylcholine-mediated increases in forearm blood flow in either group (naproxen, P=0.27; rofecoxib, P=0.58). Similarly, there was no change in forearm blood flow in response to sodium nitroprusside (naproxen, P=0.55; rofecoxib, P=0.63).
We herein describe, for the first time, the effects of COX-2-selective inhibition on endothelium-dependent vasodilatation in healthy adults. COX-2 blockade, when used at the doses employed therapeutically (which are known to inhibit vascular prostacyclin production) did not result in significant changes in endothelial vasodilator responses in healthy volunteers. The effects of COX-2 inhibitors on vasodilator responses in patients with coronary artery disease remain to be determined.
Circulation 01/2002; 104(24):2879-82. · 14.74 Impact Factor
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ABSTRACT: This study was designed to assess the effect of metformin on impaired endothelial function in type 2 diabetes mellitus.
Abnormalities in vascular endothelial function are well recognized among patients with type 2 (insulin-resistant) diabetes mellitus. Insulin resistance itself may be central to the pathogenesis of endothelial dysfunction. The effects of metformin, an antidiabetic agent that improves insulin sensitivity, on endothelial function have not been reported.
Subjects with diet-treated type 2 diabetes but without the confounding collection of cardiovascular risk factors seen in the metabolic syndrome were treated with metformin 500 mg twice daily (n = 29) or placebo (n = 15) for 12 weeks. Before and after treatment, blood flow responses to intraarterial administration of endothelium-dependent (acetylcholine), endothelium-independent (sodium nitroprusside) and nitrate-independent (verapamil) vasodilators were measured using forearm plethysmography. Whole-body insulin resistance was assessed on both occasions using the homeostasis model (HOMA-IR).
Subjects who received metformin demonstrated statistically significant improvement in acetylcholine-stimulated flows compared with those treated with placebo (p = 0.0027 by 2-way analysis of variance), whereas no significant effect was seen on nitroprusside-stimulated (p = 0.27) or verapamil-stimulated (p = 0.40) flows. There was a significant improvement in insulin resistance with metformin (32.5% reduction in HOMA-IR, p = 0.01), and by stepwise multivariate analysis insulin resistance was the sole predictor of endothelium-dependent blood flow following treatment (r = -0.659, p = 0.0012).
Metformin treatment improved both insulin resistance and endothelial function, with a strong statistical link between these variables. This supports the concept of the central role of insulin resistance in the pathogenesis of endothelial dysfunction in type 2 diabetes mellitus. This has important implications for the investigation and treatment of vascular disease in patients with type 2 diabetes mellitus.
Journal of the American College of Cardiology 05/2001; 37(5):1344-50. · 14.16 Impact Factor
