Yong Youn

Enhanced amorphous stability of carbon-doped Ge2Sb2Te5: Ab Initio investigation

The effects of carbon doping on structural and electronic properties of amorphous Ge2Sb2Te5 are studied by using ab initio molecular dynamics simulations. In comparison with Si, N, and O dopants, C dopants are found to fundamentally alter the local order of amorphous network by increasing the population of tetrahedral Ge atoms significantly. In addition, the density of ABAB-type squared rings is much smaller than for the undoped case. The present results indicate that carbon dopants are very effective in extending covalent nature in amorphous Ge2Sb2Te5 and enhancing amorphous stability.

Identification of ground-state spin ordering in antiferromagnetic transition metal oxides using the Ising model and a genetic algorithm

Abstract We identify ground-state collinear spin ordering in various antiferromagnetic transition metal oxides by constructing the Ising model from first-principles results and applying a genetic algorithm to find its minimum energy state. The present method can correctly reproduce the ground state of well-known antiferromagnetic oxides such as NiO, Fe2O3, Cr2O3 and MnO2. Furthermore, we identify the ground-state spin ordering in more complicated materials such as Mn3O4 and CoCr2O4.

Effect of annealing temperature on the phase transition, band gap and thermoelectric properties of Cu2SnSe3

The effect of annealing temperature on the phase transition of Cu2SnSe3 was investigated in order to study the thermoelectric (TE) properties of the various Cu2SnSe3 phases. The stoichiometric composition of Cu2SnSe3 was synthesized by melt solidification and annealing at various temperatures followed by water quenching. Rietveld refinement was used to calculate the amount of monoclinic and cubic phases for each sample. XRD analyses reveal that the samples annealed at 720 and 820 K have mostly a monoclinic phase along with a small amount of cubic phase. The Cu2SnSe3 annealed at 960 K was mostly cubic. TE properties of the cubic phase Cu2SnSe3 were studied for the first time, and it was found that it has much higher ZT (∼0.09) than the monoclinic phase at 600 K. Better TE performance of the cubic phase can be attributed to the smaller band gap (∼0.92 eV) compared to that of monoclinic Cu2SnSe3 (∼1.0 eV) at room temperature. First principles calculations further confirmed the conductive metallic nature of the cubic phase Cu2SnSe3. The power factor (S2σ) of the cubic phase, 0.24 mW m−1 K−2, was higher than that of the monoclinic phase, 0.096 mW m−1 K−2, at 600 K, but the difference between the thermal conductivities of the two phases was very small. Small polymorphic modification with increasing annealing temperature results in compositionally similar but different crystallographic phases, which is one of the possible reasons for the very similar thermal conductivities of the two phases. The electrical conductivity of the cubic phase, which is larger than that of the monoclinic phase, and the similar thermal conductivities of the two phases lead to the higher ZT of the cubic Cu2SnSe3.

Computational discovery of p-type transparent oxide semiconductors using hydrogen descriptor

The ultimate transparent electronic devices require complementary and symmetrical pairs of n-type and p-type transparent semiconductors. While several n-type transparent oxide semiconductors like InGaZnO and ZnO are available and being used in consumer electronics, there are practically no p-type oxides that are comparable to the n-type counterpart in spite of tremendous efforts to discover them. Recently, high-throughput screening with the density functional theory calculations attempted to identify candidate p-type transparent oxides, but none of suggested materials was verified experimentally, implying need for a better theoretical predictor. Here, we propose a highly reliable and computationally efficient descriptor for p-type dopability—the hydrogen impurity energy. We show that the hydrogen descriptor can distinguish well-known p-type and n-type oxides. Using the hydrogen descriptor, we screen most binary oxides and a selected pool of ternary compounds that covers Sn2+-bearing and Cu1+-bearing oxides as well as oxychalcogenides. As a result, we suggest La2O2Te and CuLiO as promising p-type oxides.Transparent oxides: p-type huntingComputational studies screen transparent oxide semiconductors based on the formation energy of a particular defect, and reveal promising hole-doped candidates. Although p-doped semiconductors are useful for electronic devices, their performance so far is not comparable to their n-type counterparts: most p-type oxides have stability issues or suffer from poor transparency. Now a team from Seoul National University and the Electronics and Telecommunications Research Institute in Daejeon, South Korea, are computationally looking for p-doped oxides with high conductivity and good transparency, simultaneously. Their screening relies on defect chemistry and assesses the dopability of the materials by using the formation energy of hydrogen interstitial defect, as well as the evaluation of the hole mass. Authors put forth a few promising p-type binary oxides and ternary compounds, which can be very useful for implementing electronic devices, once their properties have been verified experimentally.

High-throughput ab initio calculations on dielectric constant and band gap of non-oxide dielectrics

High-k dielectrics, materials having a large band gap (Eg) and high dielectric constant (k) simultaneously, constitute critical components in microelectronic devices. Because of the inverse relationship between Eg and k, materials with large values in both properties are rare. Therefore, massive databases on Eg and k will be useful in identifying optimal high-k materials. While experimental and theoretical data on Eg and k of oxides are accumulating, corresponding information is scarce for non-oxide dielectrics with anions such as C, N, F, P, S, and Cl. To identify promising high-k dielectrics among these material groups, we screen 869 compounds of binary carbides, nitrides, sulfides, phosphides, chlorides, and fluorides, through automated ab initio calculations. Among these compounds, fluorides exhibit an Eg-k relation that is comparable to that of oxides. By further screening over ternary fluorides, we identify fluorides such as BiF3, LaF3, and BaBeF4 that could serve as useful high-k dielectrics.

All-atom simulation of molecular orientation in vapor-deposited organic light-emitting diodes

Molecular orientation in vapor-deposited organic semiconductor films is known to improve the optical and electrical efficiencies of organic light-emitting diodes, but atomistic understanding is still incomplete. In this study, using all-atom simulation of vapor deposition, we theoretically investigate how the molecular orientation depends on various factors such as the substrate temperature, molecular shape, and material composition. The simulation results are in good agreement with experiment, indicating that the all-atom simulation can predict the molecular orientation reliably. From the detailed analysis of the dynamics of molecules, we suggest that the kinetics of molecules near the surface mainly determines the orientation of the deposited film. In addition, the oriented films have higher density and thermal stability than randomly oriented films. We also show that higher mobility of laterally oriented films can be explained in terms of the site-energy correlation.