Yiou Zhang

Pseudo-Hydrogen Passivation: A Novel Way to Calculate Absolute Surface Energy of Zinc Blende (111)/(͞1 ͞1 ͞1) Surface.

Determining accurate absolute surface energies for polar surfaces of semiconductors has been a great challenge in decades. Here, we propose pseudo-hydrogen passivation to calculate them, using density functional theory approaches. By calculating the energy contribution from pseudo-hydrogen using either a pseudo molecule method or a tetrahedral cluster method, we obtained (111)/ surfaces energies of Si, GaP, GaAs, and ZnS with high self-consistency. This method quantitatively confirms that surface energy is determined by the number and the energy of dangling bonds of surface atoms. Our findings may greatly enhance the basic understandings of different surfaces and lead to novel strategies in the crystal growth.

New approaches for calculating absolute surface energies of wurtzite (0001)/(000 1¯): A study of ZnO and GaN

The accurate absolute surface energies of (0001)/(000-1) surfaces of wurtzite structures are crucial in determining the thin film growth mode of important energy materials. However, the surface energies still remain to be solved due to the intrinsic difficulty of calculating dangling bond energy of asymmetrically bonded surface atoms. In this study, we used a pseudo-hydrogen passivation method to estimate the dangling bond energy and calculate the polar surfaces of ZnO and GaN. The calculations were based on the pseudo chemical potentials obtained from a set of tetrahedral clusters or simple pseudo-molecules, using density functional theory approaches. And the surface energies of (0001)/(000-1) surfaces of wurtzite ZnO and GaN we obtained showed relatively high self-consistencies. A wedge structure calculation with a new bottom surface passivation scheme of group I and group VII elements was also proposed and performed to show converged absolute surface energy of wurtzite ZnO polar surfaces, and the result were also compared with the above method. These calculations and comparisons may provide important insights to crystal growths of the above materials, thereby leading to significant performance enhancements of semiconductor devices.The accurate absolute surface energies of (0001)/(000 1¯) surfaces of wurtzite structures are crucial in determining the thin film growth mode of important energy materials. However, the surface energies still remain to be solved due to the intrinsic difficulty of calculating the dangling bond energy of asymmetrically bonded surface atoms. In this study, we used a pseudo-hydrogen passivation method to estimate the dangling bond energy and calculate the polar surfaces of ZnO and GaN. The calculations were based on the pseudo chemical potentials obtained from a set of tetrahedral clusters or simple pseudo-molecules, using density functional theory approaches. The surface energies of (0001)/(000 1¯) surfaces of wurtzite ZnO and GaN that we obtained showed relatively high self-consistencies. A wedge structure calculation with a new bottom surface passivation scheme of group-I and group-VII elements was also proposed and performed to show converged absolute surface energy of wurtzite ZnO polar surfaces, and th...

A novel strategy to control defects and secondary phases of CZTS by surfactant Potassium

Cu2ZnSnS4 (CZTS) is a promising photovoltaic absorber material with earth abundant and nontoxic elements. However, the detrimental native defects and secondary phases of CSTS will largely reduce the energy conversion efficiencies. To understand the origin of these problems during the growth of CZTS, we investigated the kinetic processes on CZTS (-1-1-2) surface, using first principles calculations. A surface Zn atom was found to occupy the subsurface Cu site easily due to a low reaction barrier, which may lead to a high ZnCu concentration and a secondary phase of ZnS. These n-type defects may create deep electron traps near the interface and become detrimental to device performance. To reduce the population of ZnCu and the secondary phase, we propose to use K as a surfactant to alter surface kinetic processes. Improvements on crystal quality and device performance based on this surfactant are consistent with early experimental observations.

Stepping Stone Mechanism: Carrier-Free Long-Range Magnetism Mediated by Magnetized Cation States in Quintuple Layer*

The long-range magnetism observed in group-V tellurides quintuple layers is the only working example of carrier-free dilute magnetic semiconductors (DMS), whereas the physical mechanism is unclear, except the speculation on the band topology enhanced van Vleck paramagnetism. Based on DFT calculations, we find a stable long-range ferromagnetic order in a single quintuple layer of Cr-doped Bi2Te3 or Sb2Te3, with the dopant separation more than 9 A. This configuration is the global energy minimum among all configurations. Different from the conventional super exchange theory, the magnetism is facilitated by the lone pair derived anti-bonding states near the cations. Such anti-bonding states work as stepping stones merged in the electron sea and conduct magnetism. Further, spin orbit coupling induced band inversion is found to be insignificant in the magnetism. Therefore, our findings directly dismiss the common misbelief that band topology is the only factor that enhances the magnetism. We further demonstrate that removal of the lone pair derived states destroys the long-range magnetism. This novel mechanism sheds light on the fundamental understanding of long-range magnetism and may lead to discoveries of new classes of DMS.

Defect properties of Na and K in Cu2ZnSnS4 from hybrid functional calculation

In-growth or post-deposition treatment of

Realization of stable ferromagnetic order in a topological insulator: Codoping-enhanced magnetism in 4 f transition metal doped B i 2 S e 3

Cu_{2}ZnSnS_{4}

Violation of the symmetry rule for the [ 2 + 2 ] addition in the chemisorption of C 2 H 4 on Si(100)

(CZTS) absorber layer had led to improved photovoltaic efficiency, however, the underlying physical mechanism of such improvements are less studied. In this study, the thermodynamics of Na and K related defects in CZTS are investigated from first principle approach using hybrid functional, with chemical potential of Na and K established from various phases of their polysulphides. Both Na and K predominantly substitute on Cu sites similar to their behavior in

A brief review of co-doping

Cu(In,Ga)Se_{2}

Induced effects by the substitution of Zn in Cu 2ZnSnX4 (X = S and Se)

, in contrast to previous results using the generalized gradient approximation (GGA). All substitutional and interstitial defects are shown to be either shallow levels or highly energetically unfavorable. Defect complexing between Na and abundant intrinsic defects did not show possibility of significant incorporation enhancement or introducing deep n-type levels. The possible benefit of Na incorporation on enhancing photovoltaic efficiency is discussed. The negligible defect solubility of K in CZTS also suggests possible surfactant candidate.

Hydrogen-surfactant-assisted coherent growth of GaN on ZnO substrate

The quantum anomalous Hall effect (QAHE) originates from a combination of the spin-orbital coupling and the breaking of time-reversal symmetry due to intrinsic ferromagnetic ordering and was recently observed in Cr and V doped magnetic topological insulators (TIs). However, it was only observed at extremely low temperatures due to the low ferromagnetic Curie temperature and the tiny magnetically induced gap. To fully understand the mechanism of the ferromagnetic ordering, thereby improving the ferromagnetism, we investigated