A complete knowledge of absolute surface energies with any arbitrary crystal orientation is important for the improvements of semiconductor devices because it determines the equilibrium and nonequilibrium crystal shapes of thin films and nanostructur
es. It is also crucial in the control of thin film crystal growth and surface effect studies in broad research fields. However, obtaining accurate absolute formation energies is still a huge challenge for the semi-polar surfaces of compound semiconductors. It mainly results from the asymmetry nature of crystal structures and the complicated step morphologies and related reconstructions of these surface configurations. Here we propose a general approach to calculate the absolute formation energies of wurtzite semi-polar surfaces by first-principles calculations, taking GaN as an example. We mainly focused on two commonly seen sets of semi-polar surfaces: a-family (11-2X) and m-family (10-1X). For all the semi-polar surfaces that we have calculated in this paper, the self-consistent accuracy is within 1.5 meV/{AA}^2. Our work fills the last technical gap to fully investigate and understand the shape and morphology of compound semiconductors.
In-growth or post-deposition treatment of $Cu_{2}ZnSnS_{4}$ (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 a
nd 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 $Cu(In,Ga)Se_{2}$, 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.
Based on the density functional theory with hybrid functional approach, we have studied the structural and thermodynamic stabilities of Cu2MSnX4 (M = Zn, Mg, and Ca; X = S and Se) alloy, and have further investigated the electronic and optical proper
ties of stable Cu2MgSnS4 and Cu2MgSnSe4 phases. Thermal stability analysis indicates that Cu2MgSnS4 and Cu2MgSnSe4 are thermodynamically stable, while Cu2CaSnS4 and Cu2CaSnSe4 are unstable. The ground state configuration of the compound changes from kesterite into stannite structure when Zn atoms are substitued by larger Mg or Ca atoms. An energy separation between stannite and kesterite phase similar to that of CZTS is observed. Calculated electronic structures and optical properties suggest that Cu2MgSnS4 and Cu2MgSnSe4 can be efficient photovoltaic materials.
InGaN is an ideal alloy system for optoelectronic devices due its tunable band gap. Yet high-quality InGaN requires high In concentration, which is a challenging issue that limits its use in green-light LEDs and other devices. In this paper, we inves
tigated the surfactant effect of Sb on the In incorporation on InGaN (000-1) surface via first-principles approaches. Surface phase diagram was also constructed to determine surface structures under different growth conditions. By analyzing surface stress under different structures, we found that Sb adatom can induce tensile sites in the cation layer, enhancing the In incorporation. These fi ndings may provide fundamental understandings and guidelines for the growth of InGaN with high In concentration.
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 o
rigin 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.
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, 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.
