Research

Defect physics and electron transport in wide bandgap semiconductors and other novel semiconductors

Ga2O3 is one of the few wide-band-gap semiconducting oxides that remains transparent well into the ultraviolet (UV), making it promising as a deep-UV TCO. In addition, in spite of its large bandgap (4.8 - 5.1 eV), Ga2O3 can be easily n-type doped by shallow donors (Si, Ge, Sn, H). These superior electronic properties, combined with the availability of high-quality but low-cost substrates, render Ga2O3 and its derivatives promising candidates for next generation high-power electronic and optoelectronic devices.

I explored a variety of fundamental properties of monoclinic Ga2O3 , other related polymorphs and their alloys. In particular, we look into enhanced thermal transport properties of Ga2O3 and its alloys [1], surface properties, kinetics of film growth [2-3], crack formation for epilayers of Al-alloyed Ga2O3 [4], conduction band offset between (AlxGa1-x)2O3 and Ga2O3 [5], carrier compensations [6], physical properties of shallow donors in Si [7], Ga2O3, phase stability of (AlxGa1-x)2O3 polymorphs [8], and carrier mobility in other novel semiconductors [9], etc.

  1. Sai Mu, H. Peelaers, C. G. Van de Walle, “Ab-initio study of enhanced thermal conductivity in ordered AlGaO3 alloy", Appl. Phys. Lett. 115, 242103 (2019). [DOI]

  2. Mengen Wang, S. Mu, Chris G. Van de Walle, “Role of Ga and In Adatoms in the epitaxial growth of the β-Ga2O3", Phys. Rev. B 102, 035303 (2020). [DOI]

  3. Mengen Wang, S. Mu, Chris G. Van de Walle, “Adsorption and diffusion of aluminum on β-Ga2O3 (010) surfaces", ACS Appl. Mater. Interfaces 13, 10650 (2021)[DOI]

  4. Sai Mu, M. Wang, H. Peelaers, Chris G. Van de Walle, “First-principles surface calculations for monoclinic Ga2O3 and Al2O3 and consequences for cracking of (AlxGa1−x)2O3 films" APL Materials, 8, 091105 (2020) (Featured article).[DOI]

  5. Sai Mu, H. Peelaers, Y. Zhang, M. Wang, and Chris G. Van de Walle, "Orientation-dependent band offsets between (AlxGa1−x)2O3 and Ga2O3", Appl. Phys. Lett. 117, 252104 (2020) (Editor's pick).[DOI]

  6. Sai Mu, M. Wang, J. Varley, J. Lyon, D. Wickramaratne, Chris G. Van de Walle, “Role of carbon and hydrogen in limiting n-type doping of monoclinic (AlxGa1−x)2O3" Phys. Rev. B, 105, 155201(2022) [DOI]

  7. Michael W. Swift, Hartwin Peelaers, Sai Mu, J. J. L. Morton, Chris G. Van de Walle, “Shallow donors in Silicon from first-principles: Hyperfine interaction, binding energy, and quadrupole coupling” npj Comput. Mater., 6, 181 (2020). [DOI]

  8. Sai Mu, Chris G. Van de Walle, "Phase stability of (AlxGa1−x)2O3 polymorphs: A first-principles study" Phys. Rev. Mater., 6, 104601 (2022). [DOI]

  9. Sai Mu, Andrew J. E. Rowberg, Joshua Leveillee, Feliciano Giustino and Chris G. Van de Walle “First-principles study of electron transport in ScN", Phys. Rev. B, 104, 075118 (2021). [DOI]

Effect of extreme disorder on the physical properties of concentrated solid solution alloys and high entropy alloys

Although, the alloys underpin modern life are the crowning achievement of an alloy design principle that dates back to the Bronze age: alloying one or two principle elements with limited amounts of secondary elements. Thanks to the recent discovery that alloys comprising equal proportions of many different elements can exhibit exceptional physical properties is resulting in the emergence of a new paradigm; one based on the exploitation of extreme chemical complexity. Unsurprisingly, these alloys present major challenges to both experiment and theory in relating the degree of disorder to their scientifically and technologically interesting properties.

We addressed the effect of extreme disorder on electronic structure and electron transport in concentrated solid solution binary, ternary, quaternary and high entropy alloys, mainly using multiple-scattering theory [1]. These studies on the electronic degree of freedom of alloys provide ingredients to manipulate the electronic transport and the energy dissipation of random alloys by choosing the number and the types of alloying elements. In particular, we delineated several different electron scattering mechanisms in multicomponent fcc disordered solid solution alloys at zero temperature [3] and at finite temperatures [2]. Optical conductivities of several fcc disordered solid solution alloys are explored, and a Mott-Ioffe-Regel limit is approached even at low temperature [5]. Besides, extreme Fermi surface smearing in NiFeCoCr alloy is demonstrated experimentally and contributions to resistivity from each Fermi surface sheet are sorted out [9]. The influence of disorder on the magnetism of some Mn-contained fcc solid solution alloys is investigated in Ref. [6]. In addition, we revealed a giant displacement scattering at finite temperatures in bcc refractory high entropy alloys [4].

We also studied lattice vibrations and phonon scattering in concentrated fcc alloys using the supercell phonon unfolding method and the itinerant CPA [8]. The calculated phonon dispersions and line widths match excellent with inelastic neutron scattering measurement. This study revealed the presence of a large, and heretofore unrecognized, impact of local chemical environments on the distribution of the species-pair-resolved force-constant disorder that can dominate phonon scattering.

The effect of charge transfer on the mechanical properties of concentrated fcc alloys was studied in Ref. [7].


  1. Sai Mu, Zongrui Pei, Xianglin Liu, G. M. Stocks, “Electronic transport and phonon properties of maximally disordered alloys: from binaries to high entropy alloys", J. Mater. Res. 33, 2857 (2018).[DOI]

  2. G. D. Samolyuk, Sai Mu, A. F. May, B. C. Sales, S. Wimmer, S. Mankovsky, H. Ebert, G. M. Stocks, “Temperature dependent electronic transport in concentrated solid solutions of the 3d-transition metals Ni, Fe, Co and Cr from first principles", Phys. Rev. B 98,165141, (2018). [DOI]

  3. Sai Mu, G. D. Samolyuk, S. Wimmer, M. C. Troparevsky, S. Khan, S. Mankovsky, H. Ebert and G. M. Stocks, “Uncovering electron scattering mechanisms in alloys possessing extreme disorder", npj Comput. Mater. 5,1, (2019). [DOI]

  4. Sai Mu, S. Wimmer, S. Mankovsky, H. Ebert, G. M. Stocks “Influence of local lattice distortions on electrical transport of refractory high entropy alloys", Scr. Mater. 170, 189 (2019). [DOI]

  5. G. D. Samolyuk, C. C. Homes, A. F. May, Sai Mu, K. Jin, H. Bei, G. M. Stocks and B. C. Sales “Prediction of the optical conductivity of metal alloys with residual resistivities near or above the Mott-Ioffe-Regel limit”, Phys. Rev. B 100, 075128 (2019) [DOI]

  6. Sai Mu, J. Yin, G. D. Samolyuk, S. Wimmer, Z. Pei, M. Eisenbach, S. Mankovsky, H. Ebert and G. M. Stocks, “Hidden Mn magnetic-moment disorder and its influence on the physical properties of medium-entropy NiCoMn alloy ", Phys. Rev. Mater. 3, 014411, (2019)[DOI]

  7. H. S. Oh, S. J. Kim, K. Odbadrakh, W. H. Ryu, K. N. Yoon, S. Mu, F. Körmann, Y. Ikeda, C. C. Tasan, D. Raabe, T. Egami, E. S. Park, “Engineering atomic-level complexity in high-entropy and complex concentrated alloys", Nat. Commun. 10, 2090 (2019).[DOI]

  8. Sai Mu*, R. J. Olsen*, B. Dutta*, L. Lindsay, G. D. Samolyuk, T. Berlijn, E. D. Specht, K. Jin, H. Bei, T. Hickel, B. C. Larson and G. M. Stocks, “Unfolding the complexity of phonon quasi-particle physics in disordered materials" (* Equal Contribution). npj comput. mater., 6, 4 (2020) [DOI]

  9. H. C. Robarts, T. E. Millichamp, D. A. Lagos, J. Laverock, D. Billington, J. A. Duffy, D. O’Neill, S. R. Giblin, J. W. Taylor, G. Kontrym-Sznajd, M. Samsel-Czekala, H. Bei, S. Mu, G. D. Samolyuk, G. M. Stocks, and S. B. Dugdale, “Extreme Fermi surface smearing in a maximally disordered alloys", Phys. Rev. Lett., 124, 046402 (2020) [DOI]

Magnetic properties and magnetoelectric effects in transition metal antiferromagnets

Antiferromagnets exhibiting a linear magnetoelectric effect are of great interest for applications aiming to achieve electric control of magnetism. In such materials, there is a term −EαH in the free energy density, where α is the magnetoelectric tensor. Due to this term, the electric field induces a magnetization and the magnetic field induces a dielectric polarization, both in linear order. Benefit from this effect, Cr2O3 can be used as a pinning layer in voltage-controlled exchange bias devices which are attractive for magnetic memory and logic applications due to their nonvolatility and low power consumption.

We are interested in several important topics towards practical device applications. Despite of several superior properties of Cr2O3, its practical applications is hindered by its Neel temperature (307K). To facilitate room-temperature operations of voltage-controlled exchange bias devices, we proposed several avenues (alloying and strains) to increase the Neel temperature of magnetoelectric antiferromangetic Cr2O3 (direct exchange) [1] and Fe2TeO6 (superexchange) [2]. And our proposed B doping towards high Neel temperature has been demonstrated experimentally later.

Besides the transition temperature, magnetic anisotropy energy (MAE) is another pivotal parameter for device applications. It affects the thermal stability of the stored information and the coercivity of the antiferromagnet, which, in turn, controls the exchange bias, switching voltage, and switching speed. It also controls the domain wall width, which is important for domain-wall-mediated memory cells. We explored the MAE of Cr2O3 using first-principles calculations and investigated its response to epitaxial strain, chemical substitution, and applied electric field [4].

We also formulated a microscopic model for finite temperature exchange-driven magnetoelectric response and successfully applied this model for Cr2O3. We sorted out the electronic and lattice-mediated contributions and resolved the latter by normal displacement modes. A substantial electronic contribution was found, which is opposite to the ionic part [3].


  1. Sai Mu, A. L. Wysocki, K. D. Belashchenko, “Effect of substitutional doping on the Néel temperature of Cr2O3", Phys. Rev. B 87, 054435 (2013)[DOI].

  2. Sai Mu, K. D. Belashchenko, “Strategies for increasing the Néel temperature of magnetoelectric Fe2TeO6", J. Phys.: Condens. Matter. 27, 022203 (2015) [fast track communication]. [DOI]

  3. Sai Mu, A. L. Wysocki, K. D. Belashchenko, “First-principles microscopic model of exchange-driven magnetoelectric response with application to Cr2O3", Phys. Rev. B 89, 174413 (2014). [DOI]

  4. Sai Mu, K. D. Belashchenko, “Influence of strain and chemical substitution on the magnetic anisotropy of antiferromagnetic Cr2O3: an ab-initio study", Phys. Rev. Mater. 3, 034405 (2019). [DOI]