Chemical Interpretation of Charged Point Defects in Semiconductors: A Case Study of Mg2Si

Abstract

The number of excess charge carriers generated by a point defect, defined by the "charge state" of a defect, is oftentimes an important quantity used to engineer the electronic properties of semiconductors. Here, we develop a molecular orbital theory-based framework for interpreting the charge state(s) of a point defect, which is based on local chemical interactions between the defect and the atoms surrounding the defect site. We demonstrate how the framework can be applied to native defects in Mg2Si, such as interstitials, vacancies, and antisite defects, by utilizing symmetry principles and Density Functional Theory calculations. We anticipate that such an interpretive framework will guide efforts to engineer electronic and optical properties of semiconductors through manipulation of intrinsic and extrinsic defects.

Understanding the dopability of p-type Mg2(Si,Sn) by relating hybrid-density functional calculation results to experimental data

“…The Fermi level is determined by the charge-neutrality equation balancing charged defect densities and free charge carriers and determines the transport properties. It is not easily experimentally accessible, however, due to the exponential energy dependence of defect densities, the Fermi level is often close to the crossing point of the formation energies of the two lowest lying defects E cross [27,35,37,42,43]. For Mg 2 Si and Mg 2 Sn these are Li Mg and Li int , where Li Mg is an acceptor defect and the extrinsic defect behind highly doped p-type Mg 2 X: see figures S1 and S2 in the SI.…”

“…The vibrational entropy and temperature dependent lattice and band gap also would affect the charged defect formation energies and DOSs [37] which will shift the Fermi level and change the doping efficiency. In addition to that, the choice of exchange-correlation energy functional would affect the band gap correction and the defect total energy, hence resulting in the slight change in the defect formation energies [35]. Note also that, in this stage, our DFT-based carrier concentration prediction does not consider the experimentally observed reduction of the band gap with temperature [4,52].…”

“…Mg 2 Si and Mg 2 Sn crystallize in the antifluorite structure (Space Group No. 225, Fm 3 m) as displayed in [26,35]. Mg atoms occupy the 8c site while Si/Sn atoms occupy the 4a site.…”