Publications

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This paper reports on trap-related shifts of the transfer curve and threshold voltage of power AlGaN/GaN HEMTs under switched bias operating life and reverse and forward DC bias stress. Opposite polarity threshold voltage shifts at room temperature under operating life and reverse bias stress conditions can be explained by means of drain current transient measurements under reverse bias stress conditions. A proposed model to explain the trapping/de-trapping behavior under different stress conditions is described and highlights the critical role of the electric field. Experimental evidence of the importance of the role of the electric field is seen in reduced parametric shift by improving the field plate design. © 2014 IEEE.

A method for extracting interface trap density (DIT) from subthreshold I-V characteristics is used to analyze data on a SiC MOSFET stressed for thirty minutes at 175°C with a gate bias of-20 V. Without knowing the channel doping, the change in DIT can be calculated when referenced to an energy level correlated with the threshold voltage. © 2014 IEEE.

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Emerging semiconductor switches based on the wide-bandgap semiconductor GaN have the potential to significantly improve the efficiency of portable power applications such as transportable energy storage. Such applications are likely to become more widespread as renewables such as wind and solar continue to come on-line. However, the long-term reliability of GaN-based power devices is relatively unexplored. In this paper, we describe joint work between Sandia National Laboratories and MIT on highvoltage AlGaN/GaN high electron mobility transistors. It is observed that the nature of current collapse is a strong function of bias conditions as well as device design, where factors such as Al composition in the barrier layer and surface passivation play a large role. Thermal and optical recovery experiments are performed to ascertain the nature of charge trapping in the device. Additionally, Kelvin-force microscopy measurements are used to evaluate the surface potential within the device. © The Electrochemical Society.

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Charge trapping and slow (10 s to > 1000 s) detrapping in AlGaN/GaN HEMTs designed for high breakdown voltage (> 1500 V) are studied to identify the impact of Al molefraction and passivation on trapping. Two different trapping components, TG1 (E a = 0.62 eV) and TG2 (with negligible temperature dependence) in AlGaN dominate under gale bias stress in the off-state. Al 0.15Ga 0.85N shows much more vulnerability to trapping under gate stress in the absence of passivation than does AlGaN with a higher Al mole fraction. Under large drain bias, trapping is dominated by a much deeper trap TD. Detrapping under illumination by monochromatic light shows TD to have E a ≈ 1.65 eV in Al 0.26Ga 0.74N and E a ≈ 1.85 eV in Al 0.15Ga 0.85N. This is consistent with a transition from a deep state (E c - 2.0 eV) in the AlGaN barrier to the 2DEG. © 2012 Materials Research Society.

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Charge trapping and slow (from 10 s to > 1000 s) detrapping in AlGaN/GaN high electron mobility transistors (HEMTs) designed for high breakdown voltages (> 1500 V) is studied through a combination of electrical, thermal, and optical methods to identify the impact of Al molefraction and passivation on trapping. Trapping due to 5-10 V drain bias stress in the on-state (V gs = 0) is found to have significantly slower recovery, compared with trapping in the off-state (V gs < V th, V ds = 0). Two different trapping components, i.e., TG1 (E a = 0.6 eV) and TG2 (with negligible temperature dependence), in AlGaN dominate under gate bias stress in the off-state. Al 0.15 Ga 0.85N shows much more vulnerability to trapping under gate stress in the absence of passivation than does AlGaN with a higher Al mole fraction. Under large drain bias, trapping is dominated by a much deeper trap TD. Detrapping under monochromatic light shows TD to have E a ≈ 1.65 eV. Carbon doping in the buffer is shown to introduce threshold voltage shifts, unlike any of the other traps. © 2012 IEEE.

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In order to elucidate how the degradation of individual components affects the state of the photovoltaic inverter as a whole, we have carried out SPICE simulations to investigate the voltage and current ripple on the DC bus. The bus capacitor is generally considered to be among the least reliable components of the system, so we have simulated how the degradation of bus capacitors affects the AC ripple at the terminals of the PV module. Degradation-induced ripple leads to an increased degradation rate in a positive feedback cycle. Additionally, laboratory experiments are being carried out to ascertain the reliability of metallized thin film capacitors. By understanding the degradation mechanisms and their effects on the inverter as a system, steps can be made to more effectively replace marginal components with more reliable ones, increasing the lifetime and efficiency of the inverter and decreasing its cost per watt towards the US Department of Energy goals.

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Carrier generation characteristics in n-type substrate SiC MOS capacitors induced by sub-bandgap energy light are reported. The generation rate is high enough to create an inversion layer in ∼20 minutes with monochromatic light (front side illumination) of energy 2.1 eV (intensity ∼5×10 16 cm-2s-1) in 4H-SiC for electric fields smaller than 1 MV/cm. Generation and recovery results strongly indicate involvement of a metastable defect whose efficiency as a generation center increases under hole-rich and decreases under electron-rich conditions. The generation dependence on bias history and light energy shows the defect to have properties consistent with the metastable silicon vacancy / carbon vacancy-antisite complex (VSi/Vc-CSi). © (2012) Trans Tech Publications.

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Carrier generation characteristics in n-type substrate silicon carbide (SiC) metal oxide semiconductor capacitors induced by sub-bandgap energy light are reported. The generation rate is high enough to create an inversion layer in approximately 20 min with monochromatic front side illumination of energy 2.1 eV in 4H-SiC for electric fields less than 1 MV/cm. Generation and recovery results strongly indicate involvement of a metastable defect whose efficiency as a generation center increases under hole-rich and decreases under electron-rich conditions. The generation dependence on bias history and light energy shows the defect to have properties consistent with the metastable silicon vacancy/carbon vacancy-antisite complex (VSi/Vc-C Si). © 2011 American Institute of Physics.

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Flexible AC Transmission Systems (FACTS) use advanced power electronics to minimize reactive power loss on the grid. Power devices used in FACTS systems must be capable of switching several thousand amps at voltages of 1-10 kV. Traditionally, these systems have relied on silicon thyristors, but recently have also began to incorporate insulated gate bipolar transistors. FACTS systems present an opportunity for emerging SiC and GaN power transistors, which offer major efficiency gains. However, for these advanced materials to be considered for use in high consequence grid level systems like FACTS controllers, excellent reliability must be demonstrated. ©The Electrochemical Society.

The project's goals are: (1) use experiments and modeling to investigate and characterize stress-related failure modes of post-silicon power electronic (PE) devices such as silicon carbide (SiC) and gallium nitride (GaN) switches; and (2) seek opportunities for condition monitoring (CM) and prognostics and health management (PHM) to further enhance the reliability of power electronics devices and equipment. CM - detect anomalies and diagnose problems that require maintenance. PHM - track damage growth, predict time to failure, and manage subsequent maintenance and operations in such a way to optimize overall system utility against cost. The benefits of CM/PHM are: (1) operate power conversion systems in ways that will preclude predicted failures; (2) reduce unscheduled downtime and thereby reduce costs; and (3) pioneering reliability in SiC and GaN.

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Based on the wurtzite crystal structure, large (MV/cm) polarization-induced electric fields are known to exist in InGaN single quantum wells (SQWs) grown perpendicular to the GaN c-axis, and these fields may impact optical device performance due to the quantum-confined Stark effect (QCSE). In general, the QCSE has experimentally been found to be smaller than the theoretical value expected for a coherently strained InGaN QW, and subsequently the InGaN/GaN QW polarization field is often under-estimated as well. In this study, we measure the QCSE in modulation-doped, InGaN/GaN SQW LEDs. The well-behaved capacitance-voltage (majority-carrier) characteristics of these devices allow us to unambiguously determine the applied field with bias. With this analysis, we de-couple the QCSE from the QW polarization field and show that although the applied field approaches the opposing QW polarization field theoretical value (i.e., flatband), the QCSE remains too small. We propose a localized-hole picture of the InGaN QW which explains our optical and electrical measurements. © 2006 Materials Research Society.

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Materials studies of high Al-content (> 30%) AlGaN epilayers and the performance of AlGaN-based LEDs with emission wavelengths shorter than 300 nm are reported. N-type AlGaN films with Al compositions greater than 30% reveal a reduction in conductivity with increasing Al composition. The reduction of threading dislocation density from the 1-5 x10{sup 10} cm{sup -2} range to the 6-9 x 10{sup 9}cm{sup -2} range results in an improvement of electrical conductivity and Al{sub 0.90}Ga{sub 0.10}N films with n= 1.6e17 cm-3 and f{acute Y}=20 cm2/Vs have been achieved. The design, fabrication and packaging of flip-chip bonded deep UV LEDs is described. Large area (1 mm x 1 mm) LED structures with interdigitated contacts demonstrate output powers of 2.25 mW at 297 nm and 1.3 mW at 276 nm when operated under DC current. 300 f{acute Y}m x 300 f{acute Y}m LEDs emitting at 295 nm and operated at 20 mA DC have demonstrated less than 50% drop in output power after more than 2400 hours of operation. Optimization of the electron block layer in 274 nm LED structures has enabled a significant reduction in deep level emission bands, and a peak quantum well to deep level ratio of 700:1 has been achieved for 300 f{acute Y}m x 300 f{acute Y}m LEDs operated at 100 mA DC. Shorter wavelength LED designs are described, and LEDs emitting at 260 nm, 254nm and 237 nm are reported.

Forward-to-reverse bias step-recovery measurements were performed on In.07Ga.93N/GaN and Al.36Ga.64N/Al.46Ga.54N quantum-well (QW) light-emitting diodes grown on sapphire. With the QW sampling the minority-carrier hole density at a single position, distinctive two-phase optical decay curves were observed. Using diffusion equation solutions to self-consistently model both the electrical and optical responses, hole transport parameters tp = 758 {+-} 44 ns, Lp = 588 {+-} 45 nm, and up = 0.18 {+-} 0.02 cm2/Vs were obtained for GaN. The mobility was thermally activated with an activation energy of 52 meV, suggesting trap-modulated transport. Optical measurements of sub-bandgap peaks exhibited slow responses approaching the bulk lifetime. For Al.46Ga.54N, a longer lifetime of tp = 3.0 us was observed, and the diffusion length was shorter, Lp = 280 nm. Mobility was an order of magnitude smaller than in GaN, up = 10-2 cm2/Vs, and was insensitive to temperature, suggesting hole transport through a network of defects.

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This SAND report is the final report on Sandia's Grand Challenge LDRD Project 27328, 'A Revolution in Lighting -- Building the Science and Technology Base for Ultra-Efficient Solid-state Lighting.' This project, which for brevity we refer to as the SSL GCLDRD, is considered one of Sandia's most successful GCLDRDs. As a result, this report reviews not only technical highlights, but also the genesis of the idea for Solid-state Lighting (SSL), the initiation of the SSL GCLDRD, and the goals, scope, success metrics, and evolution of the SSL GCLDRD over the course of its life. One way in which the SSL GCLDRD was different from other GCLDRDs was that it coincided with a larger effort by the SSL community - primarily industrial companies investing in SSL, but also universities, trade organizations, and other Department of Energy (DOE) national laboratories - to support a national initiative in SSL R&D. Sandia was a major player in publicizing the tremendous energy savings potential of SSL, and in helping to develop, unify and support community consensus for such an initiative. Hence, our activities in this area, discussed in Chapter 6, were substantial: white papers; SSL technology workshops and roadmaps; support for the Optoelectronics Industry Development Association (OIDA), DOE and Senator Bingaman's office; extensive public relations and media activities; and a worldwide SSL community website. Many science and technology advances and breakthroughs were also enabled under this GCLDRD, resulting in: 55 publications; 124 presentations; 10 book chapters and reports; 5 U.S. patent applications including 1 already issued; and 14 patent disclosures not yet applied for. Twenty-six invited talks were given, at prestigious venues such as the American Physical Society Meeting, the Materials Research Society Meeting, the AVS International Symposium, and the Electrochemical Society Meeting. This report contains a summary of these science and technology advances and breakthroughs, with Chapters 1-5 devoted to the five technical task areas: 1 Fundamental Materials Physics; 2 111-Nitride Growth Chemistry and Substrate Physics; 3 111-Nitride MOCVD Reactor Design and In-Situ Monitoring; 4 Advanced Light-Emitting Devices; and 5 Phosphors and Encapsulants. Chapter 7 (Appendix A) contains a listing of publications, presentations, and patents. Finally, the SSL GCLDRD resulted in numerous actual and pending follow-on programs for Sandia, including multiple grants from DOE and the Defense Advanced Research Projects Agency (DARPA), and Cooperative Research and Development Agreements (CRADAs) with SSL companies. Many of these follow-on programs arose out of contacts developed through our External Advisory Committee (EAC). In h s and other ways, the EAC played a very important role. Chapter 8 (Appendix B) contains the full (unedited) text of the EAC reviews that were held periodically during the course of the project.

To observe the effects of polarization fields and screening, we have performed contacted electroreflectance (CER) measurements on In{sub 0.07}Ga{sub 0.93}N/GaN single quantum well light emitting diodes for different reverse bias voltages. Room-temperature CER spectra exhibited three features which are at lower energy than the GaN band gap and are associated with the quantum well. The position of the lowest-energy experimental peak, attributed to the ground-state quantum well transition, exhibited a limited Stark shift except at large reverse bias when a redshift in the peak energy was observed. Realistic band models of the quantum well samples were constructed using self-consistent Schroedinger-Poisson solutions, taking polarization and screening effects in the quantum well fully into account. The model predicts an initial blueshift in transition energy as reverse bias voltage is increased, due to the cancellation of the polarization electric field by the depletion region field and the associated shift due to the quantum-confined Stark effect. A redshift is predicted to occur as the applied field is further increased past the flatband voltage. While the data and the model are in reasonable agreement for voltages past the flatband voltage, they disagree for smaller values of reverse bias, when charge is stored in the quantum well, and no blueshift is observed experimentally. To eliminate the blueshift and screen the electric field, we speculate that electrons in the quantum well are trapped in localized states.