Browsing by Author "Jain, Nikhil"

Now showing 1 - 14 of 14
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    Band offset determination of mixed As/Sb type-II staggered gap heterostructure for n-channel tunnel field effect transistor application
    Zhu, Yizheng; Jain, Nikhil; Mohata, Dheeraj K.; Datta, Suman; Lubyshev, Dmitri; Fastenau, Joel M.; Liu, Amy K.; Hudait, Mantu K. (American Institute of Physics, 2013-01-14)
    The experimental study of the valence band offset (Delta E-v) of a mixed As/Sb type-II staggered gap GaAs0.35Sb0.65/In0.7Ga0.3As heterostructure used as source/channel junction of n-channel tunnel field effect transistor (TFET) grown by molecular beam epitaxy was investigated by x-ray photoelectron spectroscopy (XPS). Cross-sectional transmission electron micrograph shows high crystalline quality at the source/channel heterointerface. XPS results demonstrate a Delta E-v of 0.39 +/- 0.05 eV at the GaAs0.35Sb0.65/In0.7Ga0.3As heterointerface. The conduction band offset was calculated to be similar to 0.49 eV using the band gap values of source and channel materials and the measured valence band offset. An effective tunneling barrier height of 0.21 eV was extracted, suggesting a great promise for designing a metamorphic mixed As/Sb type-II staggered gap TFET device structure for low-power logic applications. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4775606]
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    Cell design in a cellular system using guard channels, call queueing and channel borrowing
    Jain, Nikhil (Virginia Tech, 1993-12-05)
    This dissertation develops an analytic framework to undertake cell design in a cellular system. The cell is modeled in a broader sense than ever done before. In our analytical model, we incorporated the use of guard channels, queueing of new calls, and hybrid channel allocation. A numerically stable and efficient solution to a queueing system with two arrival streams having reserved and borrowable servers has been developed. This queueing system is used to model the cell behavior. The model provides valuable insights into the behavior of the cell, and this in turn has been used to devise an efficient stochastic optimization algorithm for determining the minimum number of channels required by the cell. Our techniques are stable, easy to implement for practical systems and produce optimized solutions quickly. This is particularly useful because we expect that future designs of cellular systems may execute such algorithms on cell-site processors.
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    Defect assistant band alignment transition from staggered to broken gap in mixed As/Sb tunnel field effect transistor heterostructure
    Zhu, Yizheng; Jain, Nikhil; Vijayaraghavan, S.; Mohata, Dheeraj K.; Datta, Suman; Lubyshev, Dmitri; Fastenau, Joel M.; Liu, Amy K.; Monsegue, Niven; Hudait, Mantu K. (American Institute of Physics, 2012-11-01)
    The compositional dependence of effective tunneling barrier height (E-beff) and defect assisted band alignment transition from staggered gap to broken gap in GaAsSb/InGaAs n-channel tunnel field effect transistor (TFET) structures were demonstrated by x-ray photoelectron spectroscopy (XPS). High-resolution x-ray diffraction measurements revealed that the active layers are internally lattice matched. The evolution of defect properties was evaluated using cross-sectional transmission electron microscopy. The defect density at the source/channel heterointerface was controlled by changing the interface properties during growth. By increasing indium (In) and antimony (Sb) alloy compositions from 65% to 70% in InxGa1-xAs and 60% to 65% in GaAs1-ySby layers, the E-beff was reduced from 0.30 eV to 0.21 eV, respectively, with the low defect density at the source/channel heterointerface. The transfer characteristics of the fabricated TFET device with an E-beff of 0.21eV show 2x improvement in ON-state current compared to the device with E-beff of 0.30 eV. On contrary, the value of E-beff was decreased from 0.21 eV to -0.03 eV due to the presence of high defect density at the GaAs0.35Sb0.65/In0.7Ga0.3As heterointerface. As a result, the band alignment was converted from staggered gap to broken gap, which leads to 4 orders of magnitude increase in OFF-state leakage current. Therefore, a high quality source/channel interface with a properly selected E-beff and well maintained low defect density is necessary to obtain both high ON-state current and low OFF-state leakage in a mixed As/Sb TFET structure for high-performance and lower-power logic applications. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764880]
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    Demographic Trends in Paddle Lead Spinal Cord Stimulator Placement: Private Insurance and Medicare Beneficiaries
    Labaran, Lawal; Bell, Joshua; Puvanesarajah, Varun; Jain, Nikhil; Aryee, Jomar N. A.; Raad, Michael; Jain, Amit; Carmouche, Jonathan J.; Hassanzadeh, Hamid (2020-06)
    Objective: Although spinal cord stimulators (SCS) continue to gain acceptance as a viable nonpharmacologic option for the treatment of chronic back pain, recent trends are not well established. The aim of this study was to evaluate recent overall demographic and regional trends in paddle lead SCS placement and to determine if differences in trends exist between private-payer and Medicare beneficiaries. Methods: A retrospective review of Medicare and private-payer insurance records from 2007-2014 was performed to identify patients who underwent a primary paddle lead SCS placement via a laminectomy (CPT-63655). Each study cohort was queried to determine the annual rate of SCS placements and demographic characteristics. Yearly SCS implantation rates within the study cohorts were adjusted per 100,000 beneficiaries. A chi-square analysis was used to compare changes in annual rates. Results: A total of 31,352 Medicare and 2,935 private-payer patients were identified from 2007 to 2014. Paddle lead SCS placements ranged from 5.9 to 17.5 (p<0.001), 1.9 to 5.9 (p<0.001), and 5.2 to 14.5 (p<0.001) placements per 100,000 Medicare, private-payer, and overall beneficiaries respectively from 2007 to 2014. SCS placements peaked in 2013 with 19.6, 7.1, and 16.8 placements per 100,000 Medicare, private-payer, and overall patients. Conclusion: There was an overall increase in the annual rate of SCS placements from 2007 to 2014. Paddle lead SCS placements peaked in 2013 for Medicare, private-payer, and overall beneficiaries. The highest incidence of implantation was in the Southern region of the United States and among females. Yearly adjusted rates of SCSs were higher among Medicare patients at all time points.
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    Design of III-V Multijunction Solar Cells on Silicon Substrate
    Jain, Nikhil (Virginia Tech, 2011-05-06)
    With looming energy crisis across the globe, achieving high efficiency and low cost solar cells have long been the key objective for photovoltaic researchers. III-V compound semiconductor based multijunction solar cells have been the dominant choice for space power due to their superior performance compared to any other existing solar cell technologies. In spite of unmatched performance of III-V solar cells, Si cells have dominated the terrestrial market due to their lower cost. Most of the current III-V solar cells are grown on Ge or GaAs substrates, which are not only smaller in diameter, but are also more expensive than Si substrate. Direct integration of high efficiency III-V solar cells on larger diameter, cheaper and readily available Si substrate is highly desirable for increased density, low-cost and lightweight photovoltaics. However, the polar-on-nonpolar epitaxy, the thermal mismatch and the 4% lattice mismatch makes the direct growth of GaAs on Si challenging, rendering the metamorphic cell sensitive to dislocations. The focus of this work is to investigate and correlate the impact of threading dislocation density on the performance of lattice-mismatched single-junction (1J) GaAs and dual-junction (2J) InGaP/GaAs solar cells on Si substrate. Utilizing our calibrated dislocation-assisted modeling process, we present the design methodology to optimize the structure of 2J InGaP/GaAs solar cell on Si substrate. Our modeling results suggest an optimistic future for integrating III-V solar cell technology on Si substrate and will be useful for future design and prediction of metamorphic III-V solar cell performance on Si substrate.
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    Eigenanalysis solution for quasi birth and death process
    Jain, Nikhil (Virginia Tech, 1991-04-05)
    The behavior of many systems of practical interest in communications and other areas is well modeled by a single server exponential queueing system in which the arrival and service rates are dependent upon the state of a Markov chain, the dynamics of which are independent of the queue length. Formal solution to such models based on Neuts's matrix geometric approach have appeared frequently in the literature. A major problem in using the matrix geometric approach is the computation of the rate matrix, which requires the solution of a matrix polynomial. In particular, computational times appear to be unpredictable and excessive for many problems of practical interest. Alternative techniques which employ eigenanalysis have been developed. These techniques are polynomially bounded and yield results very quickly compared to iterative routines. On the other hand, the class of systems to which the eigenanalysis based techniques apply have been somewhat restricted. In this thesis, we modify the eigenanalysis approach initially presented in order to remove some of these restrictions.
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    Heterogeneous Integration of Epitaxial Ge on Si using AlAs/GaAs Buffer Architecture: Suitability for Low-power Fin Field-Effect Transistors
    Hudait, Mantu K.; Clavel, Michael B.; Goley, Patrick S.; Jain, Nikhil; Zhu, Yan (Nature Publishing Group, 2014-11-07)
    Germanium-based materials and device architectures have recently appeared as exciting material systems for future low-power nanoscale transistors and photonic devices. Heterogeneous integration of germanium (Ge)-based materials on silicon (Si) using large bandgap buffer architectures could enable the monolithic integration of electronics and photonics. In this paper, we report on the heterogeneous integration of device-quality epitaxial Ge on Si using composite AlAs/GaAs large bandgap buffer, grown by molecular beam epitaxy that is suitable for fabricating low-power fin field-effect transistors required for continuing transistor miniaturization. The superior structural quality of the integrated Ge on Si using AlAs/GaAs was demonstrated using high-resolution x-ray diffraction analysis. High-resolution transmission electron microscopy confirmed relaxed Ge with high crystalline quality and a sharp Ge/AlAs heterointerface. X-ray photoelectron spectroscopy demonstrated a large valence band offset at the Ge/AlAs interface, as compared to Ge/GaAs heterostructure, which is a prerequisite for superior carrier confinement. The temperature-dependent electrical transport properties of the n-type Ge layer demonstrated a Hall mobility of 370 cm2/Vs at 290 K and 457 cm2/Vs at 90 K, which suggests epitaxial Ge grown on Si using an AlAs/ GaAs buffer architecture would be a promising candidate for next-generation high-performance and energy-efficient fin field-effect transistor applications.
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    Heterogeneous Integration of III-V Multijunction Solar Cells on Si Substrate: Cell Design and Modeling, Epitaxial Growth and Fabrication
    Jain, Nikhil (Virginia Tech, 2015-05-07)
    Achieving high efficiency solar cells and concurrently driving down the cell cost has been among the key objectives for photovoltaic researchers to attain a lower levelized cost of energy (LCOE). While the performance of silicon (Si) based solar cells have almost saturated at an efficiency of ~25%, III-V compound semiconductor based solar cells have steadily shown performance improvement at approximately 1% (absolute) increase per year, with a recent record efficiency of 46%. However, the expensive cost has made it challenging for the high efficiency III-V solar cells to compete with the mainstream Si technology. Novel approaches to lower down the cost per watt for III-V solar cells will position them to be among the key contenders in the renewable energy sector. Integration of such high-efficiency III-V multijunction solar cells on significantly cheaper and large area Si substrate has the potential to address the future LCOE roadmaps by unifying the high-efficiency merits of III-V materials with low-cost and abundance of Si. However, the 4% lattice mismatch, thermal mismatch polar-on-nonpolar epitaxy makes the direct growth of GaAs on Si challenging, rendering the metamorphic cell sensitive to dislocations. The focus of this dissertation is to systematically investigate heterogeneously integrated III-V multijunction solar cells on Si substrate. Utilizing a combination of comprehensive solar cell modeling and experimental techniques, we seek to better understand the material properties and correlate them to improve the device performance, with simulation providing a very valuable feedback loop. Key technical design considerations and optimal performance projections are discussed for integrating metamorphic III-V multijunction solar cells on Si substrates for 1-sun and concentrated photovoltaics. Key factors limiting the “GaAs-on-Si” cell performance are identified, and novel approaches focused on minimizing threading dislocation density are discussed. Finally, we discuss a novel epitaxial growth path utilizing high-quality and thin epitaxial Ge layers directly grown on Si substrate to create virtual “Ge-on-Si” substrate for III-V-on-Si multijunction photovoltaics. With the plummeting price of Si solar cells accompanied with the tremendous headroom available for improving the III-V solar cell efficiencies, the future prospects for successful integration of III-V solar cell technology with Si substrate looks very promising to unlock an era of next generation of high-efficiency and low-cost photovoltaics.
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    Interfacial band alignment and structural properties of nanoscale TiO2 thin films for integration with epitaxial crystallographic oriented germanium
    Jain, Nikhil; Zhu, Yizheng; Maurya, Deepam; Varghese, Ronnie; Priya, Shashank; Hudait, Mantu K. (American Institute of Physics, 2014-01-14)
    We have investigated the structural and band alignment properties of nanoscale titanium dioxide (TiO2) thin films deposited on epitaxial crystallographic oriented Ge layers grown on (100), (110), and (111) A GaAs substrates by molecular beam epitaxy. The TiO2 thin films deposited at low temperature by physical vapor deposition were found to be amorphous in nature, and high-resolution transmission electron microscopy confirmed a sharp heterointerface between the TiO2 thin film and the epitaxially grown Ge with no traceable interfacial layer. A comprehensive assessment on the effect of substrate orientation on the band alignment at the TiO2/Ge heterointerface is presented by utilizing x-ray photoelectron spectroscopy and spectroscopic ellipsometry. A band-gap of 3.33 +/- 0.02 eV was determined for the amorphous TiO2 thin film from the Tauc plot. Irrespective of the crystallographic orientation of the epitaxial Ge layer, a sufficient valence band-offset of greater than 2 eV was obtained at the TiO2/Ge heterointerface while the corresponding conduction band-offsets for the aforementioned TiO2/Ge system were found to be smaller than 1 eV. A comparative assessment on the effect of Ge substrate orientation revealed a valence band-offset relation of Delta E-V(100)> Delta E-V(111)> Delta E-V(110) and a conduction band-offset relation of Delta E-C(110) > Delta E-C(111)> Delta E-C(100). These band-offset parameters are of critical importance and will provide key insight for the design and performance analysis of TiO2 for potential high-k dielectric integration and for future metal-insulator-semiconductor contact applications with next generation of Ge based metal-oxide field-effect transistors. (C) 2014 AIP Publishing LLC.
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    Opioids and Spinal Cord Stimulators: Pre- and Postoperative Opioid Use Patterns and Predictors of Prolonged Postoperative Opioid Use
    Labaran, Lawal; Aryee, Jomar N. A.; Bell, Joshua; Jain, Nikhil; Puvanesarajah, Varun; Raad, Michael; Jain, Amit; Carmouche, Jonathan J.; Hassanzadeh, Hamid (2020-03)
    Objective: The aim of the study was to compare trends and differences in preoperative and prolonged postoperative opioid use following spinal cord stimulator (SCS) implantation and to determine factors associated with prolonged postoperative opioid use. Methods: A database of private-payer insurance records was queried to identify patients who underwent a primary paddle lead SCS placement via a laminectomy (CPT-C3655) from 2008-2015. Our resulting cohort was stratified into those with prolonged postoperative opioid use, opioid use between 3- and 6-month postoperation, and those without. Multivariate logistic regression was used to determine the effect preoperative opioid use and other factors of interest had on prolonged postoperative opioid use. Subgroup analysis was performed on preoperative opioid users to further quantify the effect of differing magnitudes of preoperative opioid use. Results: A total of 2,374 patients who underwent SCS placement were identified. Of all patients, 1,890 patients (79.6%) were identified as having prolonged narcotic use. Annual rates of preoperative (p = 0.023) and prolonged postoperative narcotic use (p < 0.001) decreased over the study period. Significant independent predictors of prolonged postoperative opioid use were age < 65 years (odds ratio [OR], 1.52; p = 0.004), male sex (OR, 1.33; p = 0.037), preoperative anxiolytic (OR, 1.55; p = 0.004) and muscle relaxant (OR, 1.42; p = 0.033), and narcotic use (OR, 15.04; p < 0.001). Increased number of preoperative narcotic prescriptions correlated with increased odds of prolonged postoperative use. Conclusion: Patients with greater number of preoperative opioid prescriptions may not attain the same benefit from SCSs as patients with less opioid use. The most significant predictor of prolonged narcotic use was preoperative opioid use.
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    Quasi-zero lattice mismatch and band alignment of BaTiO3 on epitaxial (110)Ge
    Hudait, Mantu K.; Zhu, Yizheng; Jain, Nikhil; Maurya, Deepam; Zhou, Y.; Priya, Shashank (American Institute of Physics, 2013-07-14)
    Growth, structural, and band alignment properties of pulsed laser deposited amorphous BaTiO3 on epitaxial molecular beam epitaxy grown (110) Ge layer, as well as their utilization in low power transistor are reported. High-resolution x-ray diffraction demonstrated quasi-zero lattice mismatch of BaTiO3 on (110) Ge. Cross-sectional transmission electron microscopy micrograph confirms the amorphous nature of BaTiO3 layer as well as shows a sharp heterointerface between BaTiO3 and Ge with no traceable interfacial layer. The valence band offset, Delta E-v, of 1.99 +/- 0.05 eV at the BaTiO3/(110) Ge heterointerface is measured using x-ray photoelectron spectroscopy. The conduction band offset, Delta E-c, of 1.14 +/- 0.1 eV is calculated using the bandgap energies of BaTiO3 of 3.8 eV and Ge of 0.67 eV. These band offset parameters for carrier confinement and the interface chemical properties of the BaTiO3/(110) Ge system are significant advancement towards designing Ge-based p-and n-channel metal-oxide semiconductor field-effect transistors for low-power application. (C) 2013 AIP Publishing LLC.
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    Reduced erbium-doped ceria nanoparticles: one nano-host applicable for simultaneous optical down- and up-conversions
    Shehata, Nader; Meehan, Kathleen; Hassounah, Ibrahim; Hudait, Mantu K.; Jain, Nikhil; Clavel, Michael B.; Elhelw, Sarah; Madi, Nabil (Springer, 2014-05-13)
    This paper introduces a new synthesis procedure to form erbium-doped ceria nanoparticles (EDC NPs) that can act as an optical medium for both up-conversion and down-conversion in the same time. This synthesis process results qualitatively in a high concentration of Ce3+ ions required to obtain high fluorescence efficiency in the down-conversion process. Simultaneously, the synthesized nanoparticles contain the molecular energy levels of erbium that are required for up-conversion. Therefore, the synthesized EDC NPs can emit visible light when excited with either UV or IR photons. This opens new opportunities for applications where emission of light via both up- and down-conversions from a single nanomaterial is desired such as solar cells and bio-imaging.
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    Role of InAs and GaAs terminated heterointerfaces at source/channel on the mixed As-Sb staggered gap tunnel field effect transistor structures grown by molecular beam epitaxy
    Zhu, Yizheng; Jain, Nikhil; Vijayaraghavan, S.; Mohata, Dheeraj K.; Datta, Suman; Lubyshev, Dmitri; Fastenau, Joel M.; Liu, W. K.; Monsegue, Niven; Hudait, Mantu K. (American Institute of Physics, 2012-07-15)
    The structural, morphological, defect properties, and OFF state leakage current mechanism of mixed As-Sb type-II staggered gap GaAs-like and InAs-like interface heterostructure tunnel field effect transistors (TFETs) grown on InP substrates using linearly graded InxAl1-xAs buffer by molecular beam epitaxy are investigated and compared. Symmetric relaxation of >90% and >75% in the two orthogonal < 110 > directions with minimal lattice tilt was observed for the terminal GaAs0.35Sb0.65 and In0.7Ga0.3As active layers of GaAs-like and InAs-like interface TFET structures, respectively, indicating that nearly equal numbers of alpha and beta dislocations were formed during the relaxation process. Atomic force microscopy reveals extremely ordered crosshatch morphology and low root mean square roughness of similar to 3.17 nm for the InAs-like interface TFET structure compared to the GaAs-like interface TFET structure of similar to 4.46 nm at the same degree of lattice mismatch with respect to the InP substrates. The GaAs-like interface exhibited higher dislocation density, as observed by cross-sectional transmission electron microscopy, resulting in the elongation of reciprocal lattice point of In0.7Ga0.3As channel and drain layers in the reciprocal space maps, while the InAs-like interface creates a defect-free interface for the pseudomorphic growth of the In0.7Ga0.3As channel and drain layers with minimal elongation along the Delta omega direction. The impact of the structural differences between the two interface types on metamorphic TFET devices was demonstrated by comparing p(+)-i-n(+) leakage current of identical TFET devices that were fabricated using GaAs-like and InAs-like interface TFET structures. Higher OFF state leakage current dominated by band-to-band tunneling process due to higher degree of defects and dislocations was observed in GaAs-like interface compared to InAs-like interface where type-II staggered band alignment was well maintained. Significantly lower OFF state leakage current dominated by the field enhanced Shockley-Read-Hall generation-recombination process at different temperatures was observed in InAs-like TFET structure. The fixed positive charge at the source/channel heterointerface influences the band lineup substantially with charge density greater than 1 x 10(12)/cm(2) and the band alignment is converted from staggered gap to broken gap at similar to 6 x 10(12)/cm(2). Clearly, InAs-like interface TFET structure exhibited 4x lower OFF state leakage current, which is attributed primarily to the impact of the layer roughness, defect properties on the carrier recombination rate, suggesting great promise for metamorphic TFET devices for high-performance, and ultra-low power applications. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737462]
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    Study of Optical and Structural Characteristics of Ceria Nanoparticles Doped with Negative and Positive Association Lanthanide Elements
    Shehata, Nader; Meehan, Kathleen; Hudait, Mantu K.; Jain, Nikhil; Gaballah, Soha (Hindawi, 2014-11-30)
    This paper studies the effect of adding lanthanides with negative association energy, such as holmium and erbium, to ceria nanoparticles doped with positive association energy lanthanides, such as neodymium and samarium. That is what we called mixed doped ceria nanoparticles (MDC NPs). In MDC NPs of grain size range around 6 nm, it is proved qualitatively that the conversion rate from Ce4+ to Ce3+ is reduced, compared to ceria doped only with positive association energy lanthanides. There are many pieces of evidence which confirm the obtained conclusion. These indications are an increase in the allowed direct band gap which is calculated from the absorbance dispersion measurements, a decrease in the emitted fluorescence intensity, and an increase in the size of nanoparticles, which is measured using both techniques: transmission electron microscope (TEM) and X-ray diffractometer (XRD). That gives a novel conclusion that there are some trivalent dopants, such as holmium and erbium, which can suppress Ce3+ ionization states in ceria and consequently act as scavengers for active O-vacancies in MDC. This promising concept can develop applications which depend on the defects in ceria such as biomedicine, electronic devices, and gas sensors.