Van de Walle
Computational Materials Group
vandewalle@mrl.ucsb.edu | (805) 893-7144

Materials Department, University of California, Santa Barbara, CA 93106-5050

 


Oxides

Controlling the conductivity oxides is a formidable task, but is a fundamental step towards enabling a new range of devices and achieving ambipolar doping.  Many as-grown oxides are unintentionally conductive.  E.g., ZnO and SnO2 often exhibit unintentional n-type conductivity. Because of its correlation with oxygen partial pressure, this unintentional n-type conductivity has long been attributed to oxygen vacancies. However, our first-principles calculations have shown that oxygen vacancies in these materials are high-energy defects, and do not act as shallow donors.  We propose that unintentionally incorporated impurities, such as hydrogen, are responsible for the observed conductivity.

Zinc Oxide

Tin Oxide

Gallium Oxide, Aluminum Oxide

Zinc Oxide

ZnO is a material that has received renewed interest for optoelectronic applications.  Contrary to the conventional wisdom, we have shown that oxygen vacancies are deep donors with high formation energies in n-type conditions and thus not responsible for n-type conductivity. Additionally, we have found that both interstitial and substitutional hydrogen acts as a shallow donor.  Hydrogen favorably substitutes on the oxygen site, forming a multicenter bond with the neighboring Zn atoms and contributing to the observed conductivity. Our results for the calculated configuration coordinate diagrams elegantly explain recent optically detected paramagnetic-resonance measurements of oxygen vacancies in ZnO. We are also currently investigating other unintentional dopants with the aim of achieving greater control over the conductivity of ZnO.

Silicon has recently been observed to be a common impurity in as-grown ZnO crystals. We have investigated the various ways in which Si can incorporate into ZnO, and find that its most stable configuration is to sit on the zinc site, where it acts as a shallow double donor. Thus, Si impurities likely contribute to the background n-type conductivity. We calculate both the Si and Ge donors to have moderate formation energies in ZnO, making them potential candidates for intentional n-type doping of ZnO.

p-type doping has also proven to be a significant challenge in ZnO. While there are many reports of p-type material, all-ZnO devices remain elusive. Nitrogen is often considered as the most suitable p-type dopant, because it has been used to p-type dope similar systems (ZnSe), and it is very similar in size to O. Our calculations show that, contrary to the accepted view, nitrogen is in fact a very deep acceptor in ZnO and cannot lead to p-type conductivity.

 

J. L. Lyons, A. Janotti, and C. G. Van de Walle, Appl. Phys. Lett. 95, 252105 (2009)

J. L. Lyons, A. Janotti, and C. G. Van de Walle, Phys. Rev. B 80, 205113 (2009).

A. Janotti and C. G. Van de Walle, Nature Materials 6, 44 (2007).

A. Janotti and C. G. Van de Walle, Appl. Phys. Lett. 87, 122102 (2005).

 

Hydrogen multicenter bonds in ZnO and MgO
Hydrogen multicenter bonds in ZnO and MgO.
Links:
Oxides as Semiconductors: An Interdisciplinary Research Group in the Materials Research Laboratory
UCSB Engineering News
Chemistry World

Tin oxide

SnO2 is another wide-band-gap oxide that has been used commercially for decades.  Applications include transparent conductors as well as sensors, based on its surface sensitivity to adsorbates.  Despite the widespread use, few efforts have been made to understand the properties of high quality samples of these materials and the role that defects play in their special properties.  We are investigating native point defects as well as a number of potential dopants, while searching for routes to achieve additional control over the conductivity.

A. Schleife, J.B. Varley, F. Fuchs, C. Rödl, F. Bechstedt, P. Rinke, A. Janotti, C.G. Van de Walle, Phys. Rev. B 83, 035116 (2011).

W.M.H. Oo, S. Tabatabaei, M.D. McCluskey, J.B. Varley, A. Janotti, and C.G. Van de Walle, Phys. Rev. B 82, 193201 (2010).

J.B. Varley, A. Janotti, C.G. Van de Walle, Phys. Rev. B 81, 245216 (2010).

J.B. Varley, A. Janotti, A.K. Singh, C.G. Van de Walle, Phys. Rev. B 79, 245206 (2009).

A. K. Singh, A. Janotti, M. Scheffler, and C. G. Van de Walle, Phys. Rev. Lett. 101, 055502 (2008)

Valence charge density of the neutral AlSn-Hi complex in SnO2
Valence charge density of the neutral AlSn-Hi complex in SnO2. These complexes have lower formation energies than the isolated ionized acceptors and interstitial hydrogen.


Gallium Oxide, Aluminum Oxide

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. It is also one of the less understood TCOs in terms of its bulk and defect-induced properties. We are actively studying a variety of native and foreign impurities to further understand this material

Al2O3 is being studied as a possible high-k dielectric for novel CMOS devices. We are looking at defects in these materials with the goal of understanding Fermi-level pinning at the dielectric/semiconductor interface. In addition, we are modeling the interfacial structure.

 

M. Mohamed, C. Janowitz, R. Manzke, Z. Galazka, R. Uecker, R. Fornari, J.R. Weber, J.B. Varley, and C.G. Van de Walle, Appl. Phys. Lett. 97, 211903 (2010).

J.B. Varley, J.R. Weber, A. Janotti, and C.G. Van de Walle, Appl. Phys. Lett. 97, 142106 (2010).

J.R. Weber, A. Janotti, C.G. Van de Walle, J. Appl. Phys. in press (2010).

B. Shin, J.R. Weber, R.D. Long, P.K. Hurley, C.G. Van de Walle, P.C. McIntyre, Appl. Phys. Lett. 96, 152908 (2010).

A dielectric/semiconductor interface between Ga2O3 and GaAs.
A dielectric/semiconductor interface between Ga2O3 and GaAs

Vacancy migration in Ga2O3

 

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