Showing papers by "Eric R. Heller published in 2019"

Multidimensional thermal analysis of an ultrawide bandgap AlGaN channel high electron mobility transistor

Abstract: Improvements in radio frequency and power electronics can potentially be realized with ultrawide bandgap materials such as aluminum gallium nitride (AlxGa1−xN). Multidimensional thermal characterization of an Al0.30Ga0.70N channel high electron mobility transistor (HEMT) was done using Raman spectroscopy and thermoreflectance thermal imaging to experimentally determine the lateral and vertical steady-state operating temperature profiles. An electrothermal model of the Al0.30Ga0.70N channel HEMT was developed to validate the experimental results and investigate potential device-level thermal management. While the low thermal conductivity of this III-N ternary alloy system results in more device self-heating at room temperature, the temperature insensitive thermal and electrical output characteristics of AlxGa1−xN may open the door for extreme temperature applications.

Instanton formulation of Fermi's golden rule in the Marcus inverted regime

Abstract: Fermi's golden rule defines the transition rate between weakly coupled states and can thus be used to describe a multitude of molecular processes including electron-transfer reactions and light-matter interaction. However, it can only be calculated if the wave functions of all internal states are known, which is typically not the case in molecular systems. Marcus theory provides a closed-form expression for the rate constant, which is a classical limit of the golden rule, and indicates the existence of a normal regime and an inverted regime. Semiclassical instanton theory presents a more accurate approximation to the golden-rule rate including nuclear quantum effects such as tunnelling, which has so far been applicable to complex anharmonic systems in the normal regime only. In this paper we extend the instanton method to the inverted regime and study the properties of the periodic orbit, which describes the tunnelling mechanism via two imaginary-time trajectories, one of which now travels in negative imaginary time. It is known that tunnelling is particularly prevalent in the inverted regime, even at room temperature, and thus this method is expected to be useful in studying a wide range of molecular transitions occurring in this regime.

First-Principles Assessment of the Structure and Stability of 15 Intrinsic Point Defects in Zinc-Blende Indium Arsenide

Abstract: Point defects are inevitable, at least due to thermodynamics, and essential for engineering semiconductors. Herein, we investigate the formation and electronic structures of fifteen different kinds of intrinsic point defects of zinc blende indium arsenide (zb-InAs ) using first-principles calculations. For As-rich environment, substitutional point defects are the primary intrinsic point defects in zb-InAs until the n-type doping region with Fermi level above 0.32 eV is reached, where the dominant intrinsic point defects are changed to In vacancies. For In-rich environment, In tetrahedral interstitial has the lowest formation energy till n-type doped region with Fermi level 0.24 eV where substitutional point defects In A s take over. The dumbbell interstitials prefer configurations. For tetrahedral interstitials, In atoms prefer 4-As tetrahedral site for both As-rich and In-rich environments until the Fermi level goes above 0.26 eV in n-type doped region, where In atoms acquire the same formation energy at both tetrahedral sites and the same charge state. This implies a fast diffusion along the t − T − t path among the tetrahedral sites for In atoms. The In vacancies V I n decrease quickly and monotonically with increasing Fermi level and has a q = − 3 e charge state at the same time. The most popular vacancy-type defect is V I n in an As-rich environment, but switches to V A s in an In-rich environment at light p-doped region when Fermi level below 0.2 eV. This study sheds light on the relative stabilities of these intrinsic point defects, their concentrations and possible diffusions, which is expected useful in defect-engineering zb-InAs based semiconductors, as well as the material design for radiation-tolerant electronics.

Simple ohmic contact formation in HEMT structures: application to AlGaN/GaN

Abstract: Low-resistance ohmic contacts on AlGaN/GaN HEMT devices presently require annealing at temperatures up to 850°C, which can adversely affect material properties. Here we investigate contacts with the metal directly contacting the 2DEG in the GaN, about 20 nm below the top surface. For convenience, we employed a 10-mm × 10-mm sample composed of 3-nm-GaN/16-nm-Al0.27Ga0.73N/1-nm-AlN/1.8-μm-GaN (Fe-doped). Four, 2-mm-long, 2-μm deep lines were scribed near the corners of the sample and filled with indium metal from a soldering iron. Hall measurements were then performed from 10 to 320 K at a current of 1 mA and with a magnetic field strength of 10 kG. At 10K (300K) the mobility was 1.96 × 104 (1.88 × 103) cm2·V-1·s-1; the sheet concentration, 9.39 × 1012 (9.35 × 1012) cm-2, and the sheet resistance, 33.9 (353) Ω/sq = rs. The contact resistance Rc was calculated from the average total resistance Rtot across each pair of contacts: Rtot = 2Rc + rs. At 10 K (300 K), Rc ≈ 1 (2) kΩ. Also, Rc has a much smaller temperature dependence than rs, implying tunneling, rather than thermionic current. From a Schrodinger-Poisson calculation, the peak volume carrier concentration in the 2DEG is n ≈ 3.7 × 1019 cm-3. The tunneling probability is P = exp[e(V – Vbi)/e00] and for e = 9.9evac and m* = 0.22m0, e00 = 0.077 eV = 894 K; thus, e00 < kT, further suggesting the dominance of tunneling current. This technique is immediately applicable to any HEMT-type structure, including AlScN/GaN.

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.

Combined Modeling and Experimental Approach to Improve Mechanical Impact Survivability of GaN Power FET

Abstract: An alternative approach was taken to improve the high-g shock tolerance of electronic devices. Rather than stiffening electronic devices with potting, the electronic device mass was reduced by an appropriate amount to match the compliance of the device to the circuit board. The devices studied were field effect transistors (FET) in bare die form factor and allowed a wafer thinning process to be utilized. A global-local finite element model was utilized to determine the ideal die thickness for matching the compliance. Test boards were populated with optimal thinned devices and stock devices for comparison on the same board. A three step thinning process was utilized in an effort to minimize the induced defects from the thinning process. The circuit boards with mounted FET’s were dropped from a shock drop tower to successively higher g-shocks up to 60,000-g. The electrical performance of each device was tested and verified after each level of mechanical shock. In general, most devices (both stock and thin) fail electrically before visual evidence of mechanical failure was present. The highest peak acceleration a device survived without failure is used as a figure of merit (e.g. the device failed on the next higher drop). The average of the “peak survived accelerations” for thinned devices is found to be about 25% higher for thin devices than for stock devices. However there was a wide variability in the results, which appears to be the greatest challenge to improving stock tolerance predictability and high confidence reliability of electronic devices.