Eric R. Heller

Bio: Eric R. Heller is an academic researcher from Wright-Patterson Air Force Base. The author has contributed to research in topics: High-electron-mobility transistor & Gallium nitride. The author has an hindex of 24, co-authored 88 publications receiving 2523 citations. Previous affiliations of Eric R. Heller include University of Alabama in Huntsville & Wright State University.

Avalanche ballistic electron emission microscopy with single hot-electron sensitivity

Abstract: We discuss an implementation of ballistic electron emission microscopy (BEEM), in which the metallic or metal–insulator “stack” of interest is formed directly over an avalanche p–n diode. This allows nanometer-resolution studies of hot-electron transport through technologically important device stacks with up to single electron sensitivity and >10 kHz measurement bandwidth when the avalanche diode is cooled to <200 K.

Local trap spectroscopy on cross-sectioned AlGaN/GaN devices with in situ biasing

Abstract: Scanning probe deep-level transient spectroscopy (SP-DLTS) is applied to cross-sectioned, fully processed, commercially sourced AlGaN/GaN Schottky barrier diodes (SBDs) and high electron mobility transistors (HEMTs) biased in situ. The SBD and HEMT structures had been specially designed to allow two- and three-terminal biasing after cross-sectioning. The cross-sectioning procedure exposes electrically active regions throughout the length and depth of the devices while also preserving electrical functionality. Spatially resolved SP-DLTS surface potential transients (SPTs) measured on the appropriately cross-sectioned faces of the devices reveal the presence of two traps in the GaN buffer layer which are shown to be consistent with traps detected in macroscopic deep-level transient spectroscopy measurements performed on an intact AlGaN/GaN SBD made at the same time as the HEMT device. This indicates that, for an appropriate cross-sectioning process, the cross-sectioned surface does not screen or mask defects in the bulk GaN from the probe tip. SP-DLTS maps collected over the cross-sectioned faces in active device regions also reveal the spatial variation in trapping-induced SPTs. These measurements demonstrate an avenue for exploring the energies, concentrations, and spatial distributions of traps located throughout GaN-based devices with potential applications to other material and device systems.

Thermoreflectance Imaging of (Ultra)wide Band-Gap Devices with MoS2 Enhancement Coatings.

Abstract: Measuring the maximum operating temperature within the channel of ultrawide band-gap transistors is critically important since the temperature dependence of the device reliability sets operational limits such as maximum operational power. Thermoreflectance imaging (TTI) is an optimal choice to measure the junction temperature due to its submicrometer spatial resolution and submicrosecond temporal resolution. Since TTI is an imaging technique, data acquisition is orders of magnitude faster than point measurement techniques such as Raman thermometry. Unfortunately, commercially available LED light sources used in thermoreflectance systems are limited to energies less than ∼3.9 eV, which is below the band gap of many ultrawide band-gap semiconductors (>4.0 eV). Therefore, the semiconductors are transparent to the probing light sources, prohibiting the application of TTI. To address this thermal imaging challenge, we utilize an MoS2 coating as a thermoreflectance enhancement coating that allows for the measurement of the surface temperature of (ultra)wide band-gap materials. The coating consists of a network of MoS2 nanoflakes with the c axis aligned normal to the surface and is easily removable via sonication. The method is validated using electrical and thermal characterization of GaN and AlGaN devices. We demonstrate that this coating does not measurably influence the electrical performance or the measured operating temperature. A maximum temperature rise of 49 K at 0.59 W was measured within the channel of the AlGaN device, which is over double the maximum temperature rise obtained by measuring the thermoreflectance of the gate metal. The importance of accurately measuring the peak operational temperature is discussed in the context of accelerated stress testing.

Degradation of AlGaN/GaN High Electron Mobility Transistors from X-Band Operation

Abstract: AlGaN/GaN HEMTs with a gate length of 0.125 µm were stressed at Class AB condition, 10 GHz under 3 dB compression for up to 350 hours. Devices exhibited excellent RF stability up to a drain bias of 20 V. Rapid degradation was observed for a drain bias of 25 V. Substantial Schottky contact degradation was observed for all three bias conditions. Electroluminescence indicates localized points of failure along the channel length, and micro- photoluminescence indicates an increase in non- radiative trap formation in regions of failure.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

A review of Ga2O3 materials, processing, and devices

Abstract: Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

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