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Browsing by Author "Allen, Noah P."

Now showing 1 - 5 of 5
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    Connection between Carbon Incorporation and Growth Rate for GaN Epitaxial Layers Prepared by OMVPE
    Ciarkowski, Timothy; Allen, Noah P.; Carlson, Eric; McCarthy, Robert; Youtsey, Chris; Wang, Jingshan; Fay, Patrick; Xie, Jinqiao; Guido, Louis J. (MDPI, 2019-08-01)
    Carbon, a compensator in GaN, is an inherent part of the organometallic vapor phase epitaxy (OMVPE) environment due to the use of organometallic sources. In this study, the impact of growth conditions are explored on the incorporation of carbon in GaN prepared via OMVPE on pseudo-bulk GaN wafers (in several cases, identical growths were performed on GaN-on-Al2O3 templates for comparison purposes). Growth conditions with different growth efficiencies but identical ammonia molar flows, when normalized for growth rate, resulted in identical carbon incorporation. It is concluded that only trimethylgallium which contributes to growth of the GaN layer contributes to carbon incorporation. Carbon incorporation was found to decrease proportionally with increasing ammonia molar flow, when normalized for growth rate. Ammonia molar flow divided by growth rate is proposed as a reactor independent predictor of carbon incorporation as opposed to the often-reported input V/III ratio. A low carbon concentration of 7.3 × 1014 atoms/cm3 (prepared at a growth rate of 0.57 µm/h) was obtained by optimizing growth conditions for GaN grown on pseudo-bulk GaN substrates.
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    Electrical Characterization of Gallium Nitride Drift Layers and Schottky Diodes
    Allen, Noah P. (Virginia Tech, 2019-10-09)
    Interest in wide bandgap semiconductors such as silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga 2 O 3 ) and diamond has increased due to their ability to deliver high power, high switching frequency and low loss electronic devices for power conversion applications. To meet these requirements, semiconductor material defects, introduced during growth and fabrication, must be minimized. Otherwise, theoretical limits of operation cannot be achieved. In this dissertation, the non-ideal current- voltage (IV) behavior of GaN-based Schottky diodes is discussed first. Here, a new model is developed to explain better the temperature dependent performance typically associated with a multi-Gaussian distribution of barrier heights at the metal-semiconductor interface [Section 3.1]. Application of this model gives researches a means of understanding not only the effective barrier distribution at the MS interface but also its voltage dependence. With this information, the consequence that material growth and device fabrication methods have on the electrical characteristics can be better understood. To show its applicability, the new model is applied to Ru/GaN Schottky diodes annealed at increasing temperature under normal laboratory air, revealing that the origin of excess reverse leakage current is attributed to the low-side inhomogeneous barrier distribution tail [Section 3.2]. Secondly, challenges encountered during MOCVD growth of low-doped GaN drift layers for high-voltage operation are discussed with focus given to ongoing research characterizing deep-level defect incorporation by deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) [Section 3.3 and 3.4]. It is shown that simply increasing TMGa so that high growth rates (>4 µm/hr) can be achieved will cause the free carrier concentration and the electron mobilities in grown drift layers to decrease. Upon examination of the deep-level defect concentrations, it is found that this is likely caused by an increase in 4 deep level defects states located at E C - 2.30, 2.70, 2.90 and 3.20 eV. Finally, samples where the ammonia molar flow rate is increased while ensuring growth rate is kept at 2 µm/hr, the concentrations of the deep levels located at 0.62, 2.60, and 2.82 eV below the conduction band can be effectively lowered. This accomplishment marks an exciting new means by which the intrinsic impurity concentration in MOCVD-grown GaN films can be reduced so that >20 kV capable devices could be achieved.
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    Electrical characterization of RuOx/n-GaN Schottky diodes formed by oxidizing ruthenium thin-films in normal laboratory air
    Allen, Noah P.; Ciarkowski, Timothy; Carlson, Eric; Chakraborty, Amrita; Guido, Louis J. (2020-01)
    Schottky diodes were formed by oxidizing Ru thin films deposited on n-type GaN at 400, 500, and 600 degrees C in normal laboratory air, and their electrical behavior was compared to that of a Ru/n-GaN reference device. The GaN epitaxial layers were grown via metalorganic chemical vapor deposition. The ruthenium films were deposited by electron beam evaporation. The Schottky barriers were characterized via current vs voltage (IV) and deep-level transient spectroscopy (DLTS) measurements between 70 and 400 K. The temperature dependent forward bias IV characteristics were fit, and the extracted temperature dependence of the effective barrier height for each device was shown to be caused by inhomogeneity at the metal/semiconductor interface. It was found that barrier inhomogeneity could be well described by a modified log-normal distribution. In reverse bias, it was shown that the low-energy tail of the barrier distribution is an important factor in determining leakage current. Favorable results occur for diodes oxidized at 400 and 500 degrees C, but raising the oxidation temperature to 600 degrees C results in a drastic increase in leakage current. DLTS measurements reveal one electron trap at E-C - 0.57 eV in each of the samples. It was found that the concentration of this 0.57 eV trap increases substantially at 600 degrees C and that trap-assisted tunneling likely contributes an additional pathway for reverse leakage current. (c) 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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    Electrical Characterization of Ruthenium Dioxide Schottky Contacts on GaN
    Allen, Noah P. (Virginia Tech, 2014-12-09)
    A film which is optically transparent and electrically conductive is difficult to come by but can be realized in ways such as doping an oxidized film or by oxidizing a metallic film resulting in what is known as a transparent conducting oxide (TCO). TCO's have many important uses in electronics, especially as the top contact in to solar cells where efficient transmission of light and low electrical resistivity allow for higher efficiency solar cells and as the gate contact in AlGaN/GaN HFET's allowing for optical characterization of the subsurface transistor properties. Because these devices rely heavily on the characteristics of its material interfaces, a detailed analysis should be done to investigate the electrical effects of implementing a TCO. In this work, the electrical characterization of ruthenium dioxide (RuO₂) Schottky contacts to gallium nitride (GaN) formed by evaporating ruthenium with a subsequent open-air annealing is presented. The results gathered from the current-voltage-temperature and the capacitance-voltage relationships were compared to ruthenium (Ru) on GaN and platinum (Pt) on GaN. Additionally, the measurement and analysis procedure was qualified on a similar structure of nickel on GaAs due to its well-behave nature and presence in the literature. The results indicate that an inhomogeneous Gaussian distribution of barrier heights exists at the RuO₂/GaN interface with an increase of 83meV in the mean barrier height when compared to Ru/GaN.
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    Heteroepitaxial Ge MOS Devices on Si Using Composite AlAs/GaAs Buffer
    Nguyen, Peter D.; Clavel, Michael B.; Goley, Patrick S.; Liu, Jheng-Sin; Allen, Noah P.; Guido, Louis J.; Hudait, Mantu K. (IEEE, 2015-07-01)
    Structural and electrical characteristics of epitaxial germanium (Ge) heterogeneously integrated on silicon (Si) via a composite, large bandgap AlAs/GaAs buffer are investigated. Electrical characteristics of N-type metal-oxide-semiconductor (MOS) capacitors, fabricated from the aforementioned material stack are then presented. Simulated and experimental X-ray rocking curves show distinct Ge, AlAs, and GaAs epilayer peaks. Moreover, secondary ion mass spectrometry, energy dispersive X-ray spectroscopy (EDS) profile, and EDS line profile suggest limited interdiffusion of the underlying buffer into the Ge layer, which is further indicative of the successful growth of device-quality epitaxial Ge layer. The Ge MOS capacitor devices demonstrated low frequency dispersion of 1.80% per decade, low frequency-dependent flat-band voltage, VFB , shift of 153 mV, efficient Fermi level movement, and limited C-V stretch out. Low interface state density (Dit) from 8.55 × 1011 to 1.09 × 1012 cm-2 eV-1 is indicative of a high-quality oxide/Ge heterointerface, an effective electrical passivation of the Ge surface, and a Ge epitaxy with minimal defects. These superior electrical and material characteristics suggest the feasibility of utilizing large bandgap III-V buffers in the heterointegration of high-mobility channel materials on Si for future high-speed complementary metal-oxide semiconductor logic applications.