Showing papers by "Eric R. Heller published in 2018"

Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

Abstract: J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

Thermal characterization of gallium oxide Schottky barrier diodes.

Abstract: The higher critical electric field of β-gallium oxide (Ga2O3) gives promise to the development of next generation power electronic devices with improved size, weight, power, and efficiency over current state-of-the-art wide bandgap devices based on 4H-silicon carbide (SiC) and gallium nitride (GaN). However, it is expected that Ga2O3 devices will encounter serious thermal issues due to the poor thermal conductivity of the material. In this work, self-heating in Ga2O3 Schottky barrier diodes under different regimes of the diode operation was investigated using diverse optical thermography techniques including thermoreflectance thermal imaging, micro-Raman thermography, and infrared thermal microscopy. 3D coupled electro-thermal modeling was used to validate experimental results and to understand the mechanism of heat generation for the diode structures. Measured top-side and cross-sectional temperature fields suggest that device and circuit engineers should account for the concentrated heat generation that occurs near the anode/Ga2O3 interface and/or the lightly doped drift layer under both forward and high voltage reverse bias conditions. Results of this study suggest that electro-thermal co-design techniques and top-side thermal management solutions are necessary to exploit the full potential of the Ga2O3 material system.

Transient Thermal Characterization of AlGaN/GaN HEMTs Under Pulsed Biasing

Abstract: The development of steady-state thermal characterization techniques for AlGaN/GaN high-electron mobility transistors (HEMTs) has been used to measure the device’s peak temperature under DC conditions. Despite these methods enabling the accurate quantification of the device’s effective thermal resistance and power density dependence, transient thermometry techniques are necessary to understand the nanoscale thermal transport within the active GaN layer where the highly localized joule heating occurs. One technique that has shown the ability to achieve this is transient thermoreflectance imaging (TTI). The accuracy of TTI is based on using the correct thermoreflectance coefficient. In the past, alternative techniques have been used to adjust the thermoreflectance coefficient to match the correct temperature rise in the device. This paper provides a new method to accurately determine the thermoreflectance coefficient of a given surface and is validated via an electrical method: gate resistance thermometry (GRT). Close agreement is shown between the temperature rise of the passivated gate metal measured by TTI and the averaged gate temperature monitored by GRT. Overall, TTI can now be used to thermally map GaN HEMTs under pulsed conditions providing simultaneously a submicrosecond temporal resolution and a submicrometer spatial resolution.

Atomic-Scale Simulation for Pseudometallic Defect-Generation Kinetics and Effective NIEL in GaN

Abstract: We have employed large-scale molecular dynamics simulations to study defect production, clustering, and its evolution in GaN for energies of a primary knock-on atom ranging from 500 eV to 40 keV. In the presence of proton radiation, a large number of atoms will be displaced during the collisional phase with a compacted cascade volume, but a great number of displaced atoms recombine significantly with vacancies at the same time, i.e., a pseudometallic behavior (PMB). This leads to the result that the majority of surviving defects are just single interstitials or vacancies for all recoil energies considered with only a small number of defects forming clusters. The total number of defects simulated in GaN can be very well predicted by the simplified Norgett, Robison, and Torrens (NRT) formula due to the PMB, in contrast to GaAs where the defect number becomes much larger than the NRT value. Moreover, the damage density within a cascade core is evaluated and applied to construct a model to calculate an energy-partition function for studying the nonionizing energy loss (NIEL) in GaN. The calculated NIEL in GaN is often found to be smaller than that predicted by a model based on the simple Kinchin–Pease formula. The comparisons of defect creation, density, and effective NIEL in GaN to those of GaAs suggest that GaN may be much more resistant to displacement damage than GaAs at low temperatures.

In situ transmission electron microscopy of transistor operation and failure

Abstract: Microscopy is typically used as a post-mortem analytical tool in performance and reliability studies on nanoscale materials and devices. In this study, we demonstrate real time microscopy of the operation and failure of AlGaN/GaN high electron mobility transistors inside the transmission electron microscope. Loading until failure was performed on the electron transparent transistors to visualize the failure mechanisms caused by self-heating. At lower drain voltages, thermo-mechanical stresses induce irreversible microstructural deformation, mostly along the AlGaN/GaN interface, to initiate the damage process. At higher biasing, the self-heating deteriorates the gate and catastrophic failure takes place through metal/semiconductor inter-diffusion and/or buffer layer breakdown. This study indicates that the current trend of recreating the events, from damage nucleation to catastrophic failure, can be replaced by in situ microscopy for a quick and accurate account of the failure mechanisms.

A Comparison of Electroluminescence Spectra From Plan View and Cross-Sectioned AlGaN/GaN Devices

Abstract: We report on a comparison of electroluminescence (EL) spectra measured in plan view versus on a cross-sectioned face of the same AlGaN/GaN device for the first time. Because EL emission can be more intense under optically opaque metal structures, a difference was expected. We observed and quantified the difference in EL intensity and the extracted electron temperature. We discuss the effect on the rate of proposed channel hot-carrier degradation. Device simulations were used to gain insight on the expected spatial distribution of hot electrons within the channel. Last, spectra acquired on the cross-sectioned face were found to be free of interference artifacts common in plan view that could complicate the interpretation of the spectral data.

Proton radiation as a tool for selective degradation and physics based device model test and calibration

Abstract: A method of evaluating localized degradation of a III-V compound semiconductor. The method includes preparing first and second III-V compound semiconductors. The second III-V compound semiconductor that is similar to the first III-V compound semiconductor and further comprises a shield layer that is configured to alter exposed portions of channels of the second III-V compound semiconductor. The first and second III-V compound semiconductors and irradiated and then electrically tested. Results of the electrical testing of the first and second III-V compound semiconductors are compared.

Atomic-level based non-ionizing energy loss: an application to GaAs and GaN semiconductor materials

Abstract: Large-scale molecular dynamics (MD) simulations, along with bond-order interatomic potentials, have been employed to study defect production, clustering and their evolution within high energy displacement cascades in semiconductors. Based on the MD results, the damage density within a cascade core is evaluated, and used to describe a new energy partition function. In addition, we have further developed a model to determine the non-ionizing energy loss (NIEL) for semiconductors, which can be used to predict the displacement damage degradation induced by space radiation on electronic components. The atomic-level based NIEL model has been applied to GaAs and GaN. At low energies, the most surviving defects are single interstitials and vacancies, and only 20% of the interstitial population is contained in clusters in GaAs, but a direct-impact amorphization in GaAs occurs with a high degree of probability during the cascade lifetime for Ga PKAs (primary knock-on atoms) with energies higher than 2 keV. However, a large number of atoms will be displaced during the collisional phase with a compacted cascade volume in GaN, and consequently, a great number of displaced atoms recombine significantly with vacancies at the same time, i.e., a pseudo-metallic behavior (PMB). This leads to the result that the majority of surviving defects are just single interstitials or vacancies for all recoil energies considered with only a small number of defects forming clusters. The total number of defects simulated in GaN can be very well predicted by the simplied Norgett, Robison and Torrens (NRT) formula due to the PMB, in contrast to GaAs where the defect number becomes much larger than the NRT value. The calculated NIEL in GaN is often found smaller than that predicted by a model based on the simple Kinchin-Pease formula. The comparisons of defect creation, density and effective NIEL in GaN to those of GaAs suggest that GaN may be much more resistant to displacement damage than GaAs, and therefore, very suitable for use in high-power space-energy systems and space-probe applications.