New Publication! S. Chae, , J. Lee, K. A. Mengle, J. T. Heron, and E. Kioupakis Appl. Phys. Lett. 114, 102104 (2019)

Abstract: Ultra-wide-band-gap (UWBG) semiconductors have tremendous potential to advance electronic devices as device performance improves superlinearly with the increasing gap. Ambipolar doping, however, has been a major challenge for UWBG materials as dopant ionization energy and charge compensation generally increase with the increasing bandgap 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 Sb_Ge, As_Ge, and F_O as possible donors with low ionization energies and propose growth conditions to avoid charge compensation by deep acceptors such as V_Ge and N_O. On the other hand, acceptors such as Al_Ge have relatively large ionization energies (0.45 eV) due to the formation of localized hole polarons and are likely to be passivated by V_O, Ge_i, and self-interstitials. Yet, we find that the co-incorporation of Al_Ge 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.

Full text available from Applied Physics Letters

New Publication! S. Chae, K. Mengle, J. T. Heron & E. Kioupakis Appl. Phys. Lett. 113, 212101 (2018)

Abstract: 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 AsB antisites, boron-related defects such as VB, BAs, and Bi-VB 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 VB and BAs. 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. BeB, SiAs, and GeAs 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 SeAs, TeAs SiB, and GeB have a relatively large ionization energy (∼0.15 eV) and are likely to be passivated by native defects such as BAs and VB, as well as CAs, Hi, and HB. 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.

Full text available from Applied Physics Letters

“The ability to use an electric or magnetic field to manipulate the orientation of electric dipoles or magnetic moments associated with atoms, ions or molecules in a material provides a vast array of functions. In rare materials called magnetoelectric multiferroics, the dipoles are intimately coupled to the moments, and a single field can control both¹. After the field is applied, however, the dipoles and moments typically all have the same orientation, and the original pattern that they formed is lost. In a paper Nature, Leo et al.² show that, in two particular materials, a magnetic field can flip each of the dipoles or moments while preserving the structure of the original pattern. The work illustrates how the complex coupling in these materials could be used to uncover other, previously unobserved electric and magnetic effects.”

Full text available from Nature

New Publication!- P. B. Meisenheimer, S. Novakov, N. M. Vu, J. T. Heron  Journal of Applied Physics 123, 240901 (2018).

Abstract: Since the resurgence of multiferroics research, significant advancement has been made in the theoretical and experimental investigation of the electric field control of magnetization, magnetic anisotropy, magnetic phase, magnetic domains, and Curie temperature in multiferroic heterostructures. As a result of these advances, multiferroic heterostructures are on a trajectory to impact spintronics applications through the significantly reduced energy consumption per unit area for magnetization switching (1–500 μJ cm⁻²) when compared to that of current-driven magnetization switching (0.2–10 mJ cm⁻²). Considering this potential impact, it becomes necessary to understand magnetoelectric switching dynamics and characteristic switching times. The body of experimental work investigating magnetoelectric switching dynamics is rather limited, with the majority of room temperature converse magnetoelectric switching measurements reported having employed relatively long voltage pulses. Recently, however, the field has started to consider the kinetics of the switching path in multiferroic (and ferroelectric) switching. Excitingly, the results are challenging our understanding of switching processes while offering new opportunities to engineer the magnetoelectric effect. Considering the prospects of multiferroics for beyond-CMOS applications and the possible influence on operational speed, much remains to be understood regarding magnetoelectric switching kinetics and dynamics, particularly at reduced dimensions and under the influence of boundary effects resulting from strain, electrostatics, and orientation. In this article, we review magnetoelectric switching in multiferroic heterostructures for the electric field control of magnetism. We then offer perspectives moving toward the goal of low energy-delay spintronics for computational applications.

Full text available from Journal of Applied Physics

New Publication!- S. Sivakumar*, E. Zwier*, P. B. Meisenheimer*, J. T. Heron J. Vis. Exp. (135), e57746, (2018).

Abstract: Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (Cu variant) entropy-stabilized oxides. Phase pure and chemically homogeneous (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (x = 0.11, 0.27) ceramic pellets are synthesized and used in the deposition of ultra-high quality, phase pure, single crystalline thin films of the target stoichiometry. A detailed methodology for the deposition of smooth, chemically homogeneous, entropy-stabilized oxide thin films by pulsed laser deposition on (001)-oriented MgO substrates is described. The phase and crystallinity of bulk and thin film materials are confirmed using X-ray diffraction. Composition and chemical homogeneity are confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The surface topography of thin films is measured with scanning probe microscopy. The synthesis of high quality, single crystalline, entropy-stabilized oxide thin films enables the study of interface, size, strain, and disorder effects on the properties in this new class of highly disordered oxide materials.

Full text available from Journal of Visualized Experiments