Aivars J. Lelis

Army Research Laboratory, Aberdeen Proving Ground, Maryland, United States

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Publications (170)107.7 Total impact

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    ABSTRACT: We use hybrid-functional density functional theory-based Charge Transition Levels (CTLs) to study the electrical activity of near-interfacial oxygen vacancies located in the oxide side of 4H-Silicon Carbide (4H-SiC) power Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). Based on the “amorphousness” of their local atomic environment, oxygen vacancies are shown to introduce their CTLs either within (permanently electrically active) or outside of (electrically inactive) the 4H-SiC bandgap. The “permanently electrically active” centers are likely to cause threshold voltage (Vth ) instability at room temperature. On the other hand, we show that the “electrically inactive” defects could be transformed into various “electrically active” configurations under simultaneous application of negative bias and high temperature stresses. Based on this observation, we present a model for plausible oxygen vacancy defects that could be responsible for the recently observed excessive worsening of Vth instability in 4H-SiC power MOSFETs under high temperature-and-gate bias stress. This model could also explain the recent electrically detected magnetic resonance observations in 4H-SiC MOSFETs.
    No preview · Article · Jul 2015 · Journal of Applied Physics
  • Daniel B. Habersat · Neil Goldsman · Aivars J. Lelis

    No preview · Article · Jun 2015 · Materials Science Forum
  • Ronald Green · Aivars Lelis · Mooro El · Daniel Habersat

    No preview · Article · Jun 2015 · Materials Science Forum
  • M.A. Anders · P.M. Lenahan · A.J. Lelis
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    ABSTRACT: The negative bias temperature instability (NBTI) has been investigated for quite some time in Si based MOSFETs. In these MOSFETs, the response has been interpreted in several ways, primarily in terms of the reaction diffusion model and newer model based on the occupation of a near interface oxide hole trap triggering the generation of silicon dielectric interface traps. SiC based MOSFETs have enormous promise for high power and high temperature applications. Consequently, device performance at elevated temperatures of these devices is a topic of great current interest. We have begun a magnetic resonance based study of NBTI in 4H-SiC devices and find, among other things, that elevated temperature and negative gate bias generates structural changes (associated with electrically active defects) within the SiC. These observations strongly suggest that SiC NBTI is significantly different and likely more complex than the NBTI processes taking place in silicon based devices. However, other observations suggest that one aspect of NBTI, the occupation of near-interfacial oxide hole traps called E' centers, takes place in both systems.
    No preview · Article · May 2015
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    ABSTRACT: In this paper, an exceptionally sensitive form of electron paramagnetic resonance called electrically detected magnetic resonance (EDMR) is utilized to investigate performance limiting imperfections at and very near the interface of 4H-silicon carbide MOSFETs. EDMR measurements are made over an extremely wide range of frequencies, 16 GHz-350 MHz. Multiple interface/near interface defects are identified and strong evidence for significant disorder at the interface region is presented.
    No preview · Article · Feb 2015 · IEEE Transactions on Electron Devices
  • Aivars J. Lelis · Ron Green · Daniel B. Habersat · Mooro El
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    ABSTRACT: A review of the basic mechanisms affecting the stability of the threshold voltage in response to a bias-temperature stress is presented in terms of the charging and activation of near-interfacial oxide traps. An activation energy of approximately 1.1 eV was calculated based on new experimental results. Implications of these factors, including the recovery of some bias-temperature stress-activated defects, for improved device reliability testing are discussed.
    No preview · Article · Feb 2015 · IEEE Transactions on Electron Devices
  • A.J. Lelis · R. Green · M. El
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    ABSTRACT: With the introduction of SiC power MOSFETs into the commercial market place, it is critically important to determine the reliability of these devices. A number of potential issues need to be addressed, including the stability of the device threshold voltage, and the reliability of both the body diode and the gate oxide. An ideal switch has minimal leakage current in the OFF state and very low resistance in the ON state. But an excess negative shift of the threshold voltage under high-temperature reverse-bias (HTRB) conditions can lead to a critical increase in OFF-state leakage current and potential device failure [1]. In a similar fashion, a large positive shift of the threshold voltage may occur under high-temperature gate-bias (HTGB) conditions, wherein a positive bias-temperature stress is applied. This can lead to a significant increase in the ON-state resistance. This work focuses on the stability of the device threshold voltage when subject to bias-temperature stressing, although the reliability of the body diode will also be discussed in the full paper. The primary defects that contribute to shifts in the threshold voltage are near-interfacial oxide traps [2, 3]. A certain number of defect states become activated during device processing. This can vary depending on the details of the processing steps, or may be a function of device design. Typical instabilities of the threshold voltage at room temperature are about 0.25 V. Much larger threshold-voltage instabilities are observed following bias-temperature gate-bias stressing. This is very likely due to the activation of additional oxide traps, which can then participate in the oxide-trap charging process [3]. Since this activation is a function of time at temperature (and bias), more stressful processing conditions can quicken the onset of significant shifts in the threshold voltage during bias-temperature stressing. (Similarly, stressing at higher temperatures also leads to an earlier onset of significant increases in threshold-voltage instability.) As a result, we have observed a marked difference in the onset of significant increases in threshold-voltage instability between different commercial vendors. This difference in bias-temperature stress time can vary in some cases by two orders of magnitude. Detailed results will be provided in the full paper. References 1. Lelis, et al., Mater. Sci. Forum, vols. 679-680, p. 599 (2011). 2. Lelis, et al., IEEE Trans. Elec. Dev., 55:8, 1835 (2008). 3. Lelis, et al., ECS Transactions, Vols. 41(8), p. 203 (2011).
    No preview · Conference Paper · Oct 2014

  • No preview · Conference Paper · Oct 2014
  • D.P. Ettisserry · N. Goldsman · A. Akturk · A. J. Lelis
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    ABSTRACT: In this work, we use density functional theory-based calculations to study the hole trapping properties of single carbon-related defects in silicon dioxide. We show that such interstitials are stable in the carboxyl configuration, where the interstitial carbon atom remains three-fold coordinated with chemical bonds to two Si atoms and an oxygen atom (Si-[C=O]-Si). Using formation energy calculations, we observed a +2 to neutral charge transition level for carboxyl defect within the 4H-SiC bandgap. This leads us to propose that carboxyl defects are likely to act as switching oxide border hole traps in the oxide and contribute to threshold voltage instabilities in a 4H-SiC MOSFET. Thus, we provide an additional candidate to the traditional oxygen vacancy hole traps in 4H-SiC MOS systems. The atomic structures of the defect in various charge states are presented. The stability-providing mechanism for the carboxyl defect in the doubly positive state is found to be the puckering of the Si atom, as in the case of positively charged oxygen vacancy hole traps.
    No preview · Conference Paper · Sep 2014
  • P. M. Lenahan · C.J. Cochrane · A.J. Lelis
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    ABSTRACT: Silicon carbide based transistors have great promise in high temperature and high power applications. In particular, the great promise of silicon carbide based metal oxide semiconductor field effect transistors (MOSFETs) has been somewhat limited by materials physics problems in the region near the silicon carbidesilicon dioxide interface. Materials physics problems in SiC bipolar junction transistors (BJTs) are also topics of current interest. Studies involving a combination of high, low, and zero field electrically detected magnetic resonance via spin dependent recombination provide a powerful approach for the understanding of SiC transistor materials physics problems. In this paper, we will review our group's work on multi-field spin dependent recombination of 4H SiC based MOSFETs and BJTs. We emphasize the physical principles involved in the measurements in part because they could potentially be quite useful if they were to be applied to other wide band gap semiconductor device systems.
    No preview · Article · Aug 2014 · ECS Transactions
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    ABSTRACT: In this report we present results comparing lateral MOSFET properties of devices fabricated on Si-face (0001) and A-face (11-20) 4H-SiC, with nitric oxide passivation anneals. We observe a field-effect mobility of 33 cm2/V.s on p-type 5×1015 doped Si-face. These devices have a peak field-effect mobility which increases with temperature, indicative of a channel mobility limited by coulomb scattering. On 1×1016 p-type A-face SiC, the peak channel mobility is observed to be 80 cm2/V.s, with a negative temperature dependence, indicating that phonon-scattering effects dominate, with a much lower density of shallow acceptor traps. This > 2x higher channel mobility would result in a substantial decrease in on-resistance, hence lower power losses, for 4H-SiC power MOSFETs with voltage ratings below 2 kV. However, MOS C-V and gate leakage measurements indicate very different oxide and interface quality on each SiC face. For example, the Fowler-Nordheim (FN) conduction-band (CB) barrier height for electron tunneling at the SiO2/SiC interface is 2.8 eV on Si-face SiC, while it is 2.5 eV or less on A-face SiC. For the valence-band side, the effective FN barrier height at the valence-band (VB) side of only 1.6 eV on A-face SiC, while the VB barrier height is about 3.1 eV on Si-face SiC. Moreover, C-V of the MOS gate on A-face indicates the presence of a high-density of deep hole traps. It is apparent that oxides on alternative crystal faces, very promising in terms of channel mobility, require further study for complete understanding and control of the interface properties.
    No preview · Conference Paper · Apr 2014
  • D. P. Ettisserry · N. Goldsman · A. Lelis
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    ABSTRACT: In this paper, we present a methodology for the identification and quantification of defects responsible for low channel mobility in 4H-Silicon Carbide (SiC) power metal-oxide-semiconductor field-effect transistors (MOSFETs). To achieve this, we use an algorithm based on 2D-device simulations of a power MOSFET, density functional simulations, and measurement data. Using physical modeling of carrier mobility and interface traps, we reproduce the experimental I-V characteristics of a 4H-SiC doubly implanted MOSFET through drift-diffusion simulation. We extract the position of Fermi level and the occupied trap density as a function of applied bias and temperature. Using these inputs, our algorithm estimates the number of possible trap types, their energy levels, and concentrations at 4H-SiC/SiO2 interface. Subsequently, we use density functional theory (DFT)-based ab initio simulations to identify the atomic make-up of defects causing these trap levels. We study silicon vacancy and carbon di-interstitial defects in the SiC side of the interface. Our algorithm indicates that the Dit spectrum near the conduction band edge (3.25 eV) is composed of three trap types located at 2.8-2.85 eV, 3.05 eV, and 3.1-3.2 eV, and also calculates their densities. Based on DFT simulations, this work attributes the trap levels very close to the conduction band edge to the C di-interstitial defect.
    No preview · Article · Feb 2014 · Journal of Applied Physics
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    ABSTRACT: Commercial SiC MOSFETs were exposed to ionizing radiation to characterize the radiation response and to compare the observed threshold voltage (VT) instability post-radiation exposure, with the VT instability following bias temperature stress (BTS) testing. As expected, a large number of positively charged oxide traps were present in these devices following irradiation, resulting in a significant negative VT shift. However, the observed VT instability following irradiation was much smaller than that for similarly processed devices exposed to a BTS. Irradiated devices subjected to unbiased thermal treatments experienced a significant annealing of trapped holes above 100 °C. However, isochronal annealing treatments did not significantly alter the number of switching oxide traps, suggesting that a large portion of the traps activated by irradiation may lie deeper within the SiO2, beyond the tunneling distance from the SiC.
    No preview · Article · Feb 2014 · Materials Science Forum
  • A. Lelis · D. Habersat · R. Green · E. Mooro
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    ABSTRACT: A brief review of the key results and issues regarding the threshold-voltage instability effect in SiC MOSFETs is presented. These include the basic effect, the strong dependence on measurement conditions, the effect of high-temperature bias stressing, and the implications for reliability testing.
    No preview · Conference Paper · Oct 2013
  • P.M. Lenahan · C.J. Cochrane · A. Lelis

    No preview · Conference Paper · Oct 2013
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    ABSTRACT: We observe an unusual instability in the SiC DMOSFET transistor characteristics. From a series of bias conditions at elevated temperatures, we conclude that a high density of hole traps in the oxide near the SiO2/SiC interface are responsible.
    No preview · Conference Paper · Oct 2013
  • C. J. Cochrane · P. M. Lenahan · A. J. Lelis
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    ABSTRACT: We use three electrically detected magnetic resonance (EDMR) approaches to explore nitric oxide (NO) annealing in 4H SiC metal-oxide-semiconductor field-effect transistors (MOSFETs). One approach is sensitive to defects at the interface and those extending into the SiC. Two of these approaches are particularly sensitive to SiC/SiO2 interface defects. They show that NO anneals decrease the EDMR response. Since this and earlier studies indicate the ubiquitous presence of silicon vacancy centers in SiC MOSFETs, our results provide strong circumstantial evidence that these defects play an important role in limiting device performance and that NO anneals are effective in reducing their populations.
    No preview · Article · May 2013 · Applied Physics Letters
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    ABSTRACT: In this paper, we report our recently developed 2nd Generation, large-area (56 mm2 with an active conducting area of 40 mm2) 4H-SiC DMOSFET, which can reliably block 1600 V with very low leakage current under a gate-bias (VG) of 0 V at temperatures up to 200°C. The device also exhibits a low on-resistance (RON) of 12.4 mΩ at 150 A and VG of 20 V. DC and dynamic switching characteristics of the SiC DMOSFET have also been compared with a commercially available 1200 V/ 200 A rated Si trench gate IGBT. The switching energy of the SiC DMOSFET at 600 V input voltage bus is > 4X lower than that of the Si IGBT at room-temperature and > 7X lower at 150°C. A comprehensive study on intrinsic reliability of this 2nd generation SiC MOSFET has been performed to build consumer confidence and to achieve broad market adoption of this disruptive power switch technology.
    No preview · Conference Paper · May 2013
  • Source
    A. Lelis · R. Green · M. El · D. Habersat
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    ABSTRACT: A review of the threshold-voltage instability effect in SiC MOSFETs and the issues regarding the effect of stress and measurement conditions in determining the reliability of these devices is presented. The complex response of near-interfacial oxide traps, as well as other interfacial charge, to bias and temperature with time is discussed, and how it affects the accuracy of reliability measurements. The strong dependence on measurement speed is also addressed. All these issues point to the need for a separate, appropriate reliability test standard for SiC MOSFETs, rather than using the existing standard based on Si technology.
    Full-text · Article · Mar 2013 · ECS Transactions
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    ABSTRACT: The 4H-SiC/SiO2 interface in MOSFET devices contains a high density of electrically active traps. Recent work has revealed an inverse relationship between the SiC-SiO2 transition layer width and FET channel mobility. Interfacial N and P, introduced by nitric oxide (NO) anneals, nitrogen plasma (N2P), or phosphosilicate glass (PSG) passivations improve carrier mobility, but a relationship to transition layer width is lacking. We present a characterization of the SiC/SiO2 transition layer as a function of NO anneal time using high resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning TEM (HAADF-STEM), and electron energy-loss spectroscopy (EELS). The transition layer was measured with HRTEM and HAADF-STEM and characterized by the evolution of the C/Si and O/Si composition ratios and the Si-L2,3 edge in the EEL spectra across the interface. We show an inverse relationship of NO anneal time and transition layer width, which correlates with improved channel mobility, increased N interfacial density, and reduced interface trap density. No excess C was noted at the interface. NO annealed samples are compared to N2P and PSG passivations.
    No preview · Article · Mar 2013

Publication Stats

2k Citations
107.70 Total Impact Points


  • 1993-2015
    • Army Research Laboratory
      Aberdeen Proving Ground, Maryland, United States
  • 2006
    • Tel Aviv University
      • School of Electrical Engineering
      Tell Afif, Tel Aviv, Israel
  • 2005
    • University of Maryland, College Park
      • Department of Electrical & Computer Engineering
      CGS, Maryland, United States
  • 2001
    • Loyola University Maryland
      Baltimore, Maryland, United States
  • 1996
    • Pennsylvania State University
      • Department of Engineering Science and Mechanics
      University Park, MD, United States