This award supports computational research on point defects and doping in a new class of wide band gap semiconductors which may have optoelectronic applications. It focuses on II-IV-N2 semiconductors in which the group III element, e.g. Ga in GaN, is replaced by group II and group IV elements, hence the name heterovalent ternary. In preliminary work, the PI found that these materials have promising optoelectronic properties similar to those of the group-III nitrides. The PI will explore whether the point defect physics of these II-IV-N2 semiconductors is more conducive to doping than the III-nitrides. The flexibility arising from having two cation sublattices with different valences enriches the point defect and doping physics. The PI will participate in a close collaboration with an experimentalist who works on the same family of heterovalent ternary compounds.
An integral part of the proposal is to improve current methodologies for point defect calculations in two areas. First, in the context of defect physics, the PI will focus on the problem of underestimating the band gaps, as is the tendency of density functional theory in the local density approximation. Using the framework of the recently developed quasiparticle self-consistent GW approach as implemented in the full-potential linearized muffin-tin orbital method, the PI will exploit the possibility of representing the self-energy in a real-space basis set, hence reducing the large computational demand. Second, the PI will focus on a challenging problem in point defect physics related to the accuracy of the calculations for shallow donors and acceptors. The PI will revive the effective mass approximation method with central cell corrections extracted from first-principles calculations and generalize the approach to the lower symmetry Hamiltonian required for the particular class of semiconductors under study. To further address the supercell finite size effects, the PI will redevelop the Green's function approach as a more accurate means to obtain one-electron defect levels with respect to the band edges and explore how long-range Coulomb tail effects can be included in this method.
The award also supports the training of two graduate students in electronic structure methods and defect physics. The PI will also mentor and involve undergraduate students in research through senior projects in an existing Research Experience for Undergraduates program. The PI's planned collaboration with a colleague from Mexico on parts of this project will allow student exchanges between the two groups and be useful in attracting underrepresented minorities to science and technology.
NONTECHNICAL SUMMARY
This award supports computational research on method development related to modeling of isolated imperfections and impurities that are intentionally introduced into crystalline materials as well as the application of the developed methods and techniques to a new class of semiconductors that can be potentially useful for manipulating optical properties of electronic devices.
Isolated imperfections or impurities, generally called "point defects", are ubiquitous in crystalline solids. Intentionally introduced impurities, called dopants, and native defects, such as missing atoms or atoms in the wrong place in the crystal, play very crucial roles in determining various properties of semiconducting materials. Some defects are essential for, while some are detrimental for the useful operation of a semiconductor device. Accordingly, obtaining a fundamental understanding of which defects give rise to what properties in semiconductor materials is a problem of both fundamental and technological relevance. While theoretical and computational studies, aided by the vast increase in computational power and improved algorithmic developments, have played a major role toward achieving this goal during the last three decades, even the most widely used parameter-free computational methods still suffer from certain deficiencies when it comes to making accurate and reliable predictions about various properties of native defects and impurities in solids. The PI will address such problems that arise in the modeling of structural, electronic, and optical properties of defects by revisiting existing methodologies, combining them with state-of-the-art capabilities, and extending them to produce more sophisticated tools and methods that can increase the accuracy and reliability of parameter-free defect computations.
The PI will then apply these new methods to a new class of nitride semiconductors, composed of three elements, that can potentially help to overcome certain technological difficulties associated with existing group-III nitrides, which are composed of two elements, such as gallium nitride, for optical applications. This new class of semiconductors is important for photovoltaic energy conversion in solar cells as well as the development of solid-state lighting by light emitting diodes. If successful, the project will provide a rationale for further experimental development of these materials.
The award also supports the training of two graduate students in electronic structure methods and defect physics. The PI will also mentor and involve undergraduate students in research through senior projects in an existing Research Experience for Undergraduates program. The PI's planned collaboration with a colleague from Mexico on parts of this project will allow student exchanges between the two groups and be useful in attracting underrepresented minorities to science and technology.
