arXiv subjects

John T. Heron

Publications and source records attributed to John T. Heron.

25 records · Page 2Linked to original sources

arXiv2019

Rutile GeO2: an ultrawide-band-gap semiconductor with ambipolar doping

Ultra-wide-band-gap (UWBG) semiconductors have tremendous potential to advance electronic devices as device performance improves superlinearly with increasing gap. Ambipolar doping, however, has been a major challenge for UWBG materials as dopant ionization energy and charge compensation generally increase with increasing band gap and significantly limit the semiconductor devices that can currently be realized. Using hybrid density functional theory, we demonstrate rutile germanium oxide (r-GeO2) to be an alternative UWBG (4.68 eV) material that can be ambipolarly doped. We identify SbGe, AsGe, and FO as possible donors with low ionization energies and propose growth conditions to avoid charge compensation by deep acceptors such as VGe and NO. On the other hand, acceptors such as AlGe have relatively large ionization energies (0.45 eV) due to the formation of localized hole polarons and are likely to be passivated by VO, Gei, and self-interstitials. Yet, we find that the co-incorporation of AlGe with interstitial H can increase the solubility limit of Al and enable hole conduction in the impurity band. Our results show that r-GeO2 is a promising UWBG semiconductor that can overcome current doping challenges and enable the next generation of power electronics devices.

cond-mat.mtrl-sci↗

arXiv2019

Spin Seebeck imaging of spin-torque switching in antiferromagnetic Pt/NiO heterostructures

As electrical control of Néel order opens the door to reliable antiferromagnetic spintronic devices, understanding the microscopic mechanisms of antiferromagnetic switching is crucial. Spatially-resolved studies are necessary to distinguish multiple nonuniform switching mechanisms; however, progress has been hindered by the lack of tabletop techniques to image the Néel order. We demonstrate spin Seebeck microscopy as a sensitive, table-top method for imaging antiferromagnetic order in thin films, and apply this technique to study spin-torque switching in NiO/Pt and Pt/NiO/Pt heterostructures. We establish the interfacial antiferromagnetic spin Seebeck effect in NiO as a probe of surface Néel order, resolving antiferromagnetic spin domains within crystalline twin domains. By imaging before and after applying current-induced spin torque, we resolve spin domain rotation and domain wall motion, acting simultaneously. We correlate the changes in spin Seebeck images with electrical measurements of the average Néel orientation through the spin Hall magnetoresistance, confirming that we image antiferromagnetic order.

cond-mat.mtrl-sci↗

arXiv2018

Point defects and dopants of boron arsenide from first-principles calculations: donor compensation and doping asymmetry

We apply hybrid density functional theory calculations to identify the formation energies and thermodynamic charge transition levels of native point defects, common impurities, and shallow dopants in BAs. We find that boron-related defects such as V_B, B_As, B_i-V_B complexes, and antisite pairs are the dominant intrinsic defects. Native BAs is expected to exhibit p-type conduction due to the acceptor-type characteristics of V_B and B_As. Among the common impurities we explored, we found that C substitutional defects and H interstitials have relatively low formation energies and are likely to contribute free holes. Interstitial hydrogen is surprisingly also found to be stable in the neutral charge state. Be_B, Si_As and Ge_As are predicted to be excellent shallow acceptors with low ionization energy (< 0.03 eV) and negligible compensation by other point defects considered here. On the other hand, donors such as Se_As, Te_As, Si_B, and Ge_B have a relatively large ionization energy (~0.15 eV) and are likely to be passivated by native defects such as B_As and V_B, as well as C_As, H_i, and H_B. The hole and electron doping asymmetry originates from the heavy effective mass of the conduction band due to its boron orbital character, as well as from boron-related intrinsic defects that compensate donors.

cond-mat.mtrl-sci↗

arXiv2018

Unexpected Termination Switching and Polarity Compensation in LaAlO3/SrTiO3 heterostructures

Polar crystals composed of charged ionic planes cannot exist in nature without acquiring surface changes to balance an ever-growing dipole. The necessary changes can manifest structurally or electronically. An electronic asymetry has long been observed in the LaAlO3/SrTiO3 system. Electron accumulation is observed near the LaAlO3/TiO2-SrTiO3 interface, while the LaAlO3/SrO-SrTiO3 stack is insulating. Here, we observe evidence for an asymmetry in the surface chemical termination for nominally stoichiometric LaAlO3 films in contact with the two different surface layers of SrTiO3 crystals, TiO2 and SrO. Using several element specific probes, we find that the surface termination of LaAlO3 remains AlO2 irrespective of the starting termination of SrTiO3 substrate surface. We use a combination of cross-plane tunneling measurements and first principles calcula- tions to understand the effects of this unexpected termination on band alignments and polarity compensation of LaAlO3/SrTiO3 heterostructures. An asymmetry in LaAlO3 polarity compensation and resulting electronic properties will fundamentally limit atomic level control of oxide heterostructures.

cond-mat.mtrl-sci↗

arXiv2017

Clocked Magnetostriction-Assisted Spintronic Device Design and Simulation

We propose a heterostructure device comprised of magnets and piezoelectrics that significantly improves the delay and the energy dissipation of an all-spin logic (ASL) device. This paper studies and models the physics of the device, illustrates its operation, and benchmarks its performance using SPICE simulations. We show that the proposed device maintains low voltage operation, non-reciprocity, non-volatility, cascadability, and thermal reliability of the original ASL device. Moreover, by utilizing the deterministic switching of a magnet from the saddle point of the energy profile, the device is more efficient in terms of energy and delay and is robust to thermal fluctuations. The results of simulations show that compared to ASL devices, the proposed device achieves 21x shorter delay and 27x lower energy dissipation per bit for a 32-bit arithmetic-logic unit (ALU).

physics.app-ph↗

arXiv2016

Spin-Hall Torques Generated by Rare-Earth (Lanthanide) Thin Films

We report an initial experimental survey of spin-Hall torques generated by the rare-earth metals Gd, Dy, Ho, and Lu, along with comparisons to first-principles calculations of their spin Hall conductivities. Using spin torque ferromagnetic resonance (ST-FMR) measurements and DC-biased ST-FMR, we estimate lower bounds for the spin-Hall torque ratio, $ξ_{SH}$, of $\approx$ 0.04 for Gd, $\approx$ 0.05 for Dy, $\approx$ 0.14 for Ho, and $\approx$ 0.014 for Lu. The variations among these elements are qualitatively consistent with results from first principles (density functional theory, DFT, in the local density approximation with a Hubbard-U correction). The DFT calculations indicate that the spin Hall conductivity is enhanced by the presence of the partially-filled $f$ orbitals in Dy and Ho, which suggests a strategy to further strengthen the contribution of the $f$ orbitals to the spin Hall effect by shifting the electron chemical potential.

cond-mat.mes-hall↗

arXiv2015

High Dynamic Range Pixel Array Detector for Scanning Transmission Electron Microscopy

We describe a hybrid pixel array detector (EMPAD - electron microscope pixel array detector) adapted for use in electron microscope applications, especially as a universal detector for scanning transmission electron microscopy. The 128 x 128 pixel detector consists of a 500 um thick silicon diode array bump-bonded pixel-by-pixel to an application-specific integrated circuit (ASIC). The in-pixel circuitry provides a 1,000,000:1 dynamic range within a single frame, allowing the direct electron beam to be imaged while still maintaining single electron sensitivity. A 1.1 kHz framing rate enables rapid data collection and minimizes sample drift distortions while scanning. By capturing the entire unsaturated diffraction pattern in scanning mode, one can simultaneously capture bright field, dark field, and phase contrast information, as well as being able to analyze the full scattering distribution, allowing true center of mass imaging. The scattering is recorded on an absolute scale, so that information such as local sample thickness can be directly determined. This paper describes the detector architecture, data acquisition (DAQ) system, and preliminary results from experiments with 80 to 200 keV electron beams.

physics.ins-det↗