Chul Hong Park

Bio: Chul Hong Park is an academic researcher from Pusan National University. The author has contributed to research in topics: Ferromagnetism & Doping. The author has an hindex of 26, co-authored 81 publications receiving 4627 citations. Previous affiliations of Chul Hong Park include National Renewable Energy Laboratory & Princeton University.

Origin of p -type doping difficulty in ZnO: The impurity perspective

Abstract: We investigate the p-type doping difficulty in ZnO by first-principles total-energy calculations. The dopants being considered are group-I elements Li, Na, and K and group-V elements N, P, and As. We find that substitutional group-I elements are shallow acceptors, while substitutional group-V elements such as P and As are deep acceptors. The AX centers that convert acceptors into deep donors are found to be unstable except for P and As. Without compensation by intrinsic defects, the most likely cause for doping difficulty is the formation of interstitials for group-I elements and antisites for group-V elements. Among all the dopants studied here, N is a relatively better candidate for p-type ZnO.

Study of diluted magnetic semiconductor: Co-doped ZnO

Abstract: We report on the high-temperature ferromagnetism in Co-doped ZnO films fabricated by the sol–gel method above 350 K. The lattice constant of c axis of wurtzite Zn1−xCoxO follows Vergard’s law for 0

Doping by large-size-mismatched impurities: the microscopic origin of arsenic- or antimony-doped p-type zinc oxide.

Abstract: Based on first-principles calculations, a model for large-size-mismatched group-V dopants in ZnO is proposed. The dopants do not occupy the O sites as is widely perceived, but rather the Zn sites: each forms a complex with two spontaneously induced Zn vacancies in a process that involves fivefold As coordination. Moreover, an As(Zn)-2V(Zn) complex may have lower formation energy than any of the parent defects. Our model agrees with the recent observations that both As and Sb have low acceptor-ionization energies and that to obtain p-type ZnO requires O-rich growth or annealing conditions.

Structural and electronic-properties of cubic, 2h, 4h, and 6h sic

Abstract: We study the structural and electronic properties of various polytypes of SiC through self-consistent ab initio pseudopotential calculations. For the wurtzite (2H), 4H, and 6H structures, the equilibrium lattice constants and bulk moduli are very similar to those for the cubic structure. The energies calculated for the polytypes considered here are very close to within 4.3 meV/atom, which may explain the polytypism of SiC. The 4H structure is found to be lowest in energy because of the attractive interactions between the alternating cubic and hexagonal stacking layers, while the wurtzite structure is most unstable among the polytypes. We find the asymmetric charge distribution for a Si-C bond to be on the boundary separating the zinc-blende and wurtzite phases, which should be related to the polytypism of SiC. In the hexagonal polytypes, the M conduction-band energy increases, while that of the K point decreases as the hexagonal close packing becomes more prominent. Thus, the conduction-band minimum state located at the X point for cubic SiC changes to the M point, and then to the K point for the 2H structure. For the cubic structure, the density of states near the conduction-band edge increases slowly with energy, while it shows very rapidly increasing behavior for the 6H polytype because its conduction-band edge states are flattened due to the band folding and the energy-increasing behavior of the M state when the hexagonal-close-packing nature is enhanced.

Microscopic study of oxygen-vacancy defects in ferroelectric perovskites

Abstract: We investigate the nature of atomic relaxations around oxygen-vacancy defects in the ferroelectric perovskite ${\mathrm{PbTiO}}_{3}$ through first-principles pseudopotential total energy calculations. A tail-to-tail polarization is one of the patterns that emerges from atomic relaxations around oxygen vacancies and its stability is found to be enhanced by charge trapping. Oxygen vacancies in Ti-O-Ti chains along the polarization axis are more favorable than those in Ti-O-Ti planes normal to the axis. The possible role of oxygen vacancies in fatigue and aging is discussed.

A comprehensive review of zno materials and devices

Abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

Fundamentals of zinc oxide as a semiconductor

Abstract: In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

Native point defects in ZnO

Abstract: We have performed a comprehensive first-principles investigation of native point defects in ZnO based on density functional theory within the local density approximation (LDA) as well as the $\mathrm{LDA}+U$ approach for overcoming the band-gap problem. Oxygen deficiency, manifested in the form of oxygen vacancies and zinc interstitials, has long been invoked as the source of the commonly observed unintentional $n$-type conductivity in ZnO. However, contrary to the conventional wisdom, we find that native point defects are very unlikely to be the cause of unintentional $n$-type conductivity. Oxygen vacancies, which have most often been cited as the cause of unintentional doping, are deep rather than shallow donors and have high formation energies in $n$-type ZnO (and are therefore unlikely to form). Zinc interstitials are shallow donors, but they also have high formation energies in $n$-type ZnO and are fast diffusers with migration barriers as low as $0.57\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$; they are therefore unlikely to be stable. Zinc antisites are also shallow donors but their high formation energies (even in Zn-rich conditions) render them unlikely to be stable under equilibrium conditions. We have, however, identified a different low-energy atomic configuration for zinc antisites that may play a role under nonequilibrium conditions such as irradiation. Zinc vacancies are deep acceptors and probably related to the frequently observed green luminescence; they act as compensating centers in $n$-type ZnO. Oxygen interstitials have high formation energies; they can occur as electrically neutral split interstitials in semi-insulating and $p$-type materials or as deep acceptors at octahedral interstitial sites in $n$-type ZnO. Oxygen antisites have very high formation energies and are unlikely to exist in measurable concentrations under equilibrium conditions. Based on our results for migration energy barriers, we calculate activation energies for self-diffusion and estimate defect-annealing temperatures. Our results provide a guide to more refined experimental studies of point defects in ZnO and their influence on the control of $p$-type doping.

First-principles calculations for defects and impurities: Applications to III-nitrides

Abstract: First-principles calculations have evolved from mere aids in explaining and supporting experiments to powerful tools for predicting new materials and their properties. In the first part of this review we describe the state-of-the-art computational methodology for calculating the structure and energetics of point defects and impurities in semiconductors. We will pay particular attention to computational aspects which are unique to defects or impurities, such as how to deal with charge states and how to describe and interpret transition levels. In the second part of the review we will illustrate these capabilities with examples for defects and impurities in nitride semiconductors. Point defects have traditionally been considered to play a major role in wide-band-gap semiconductors, and first-principles calculations have been particularly helpful in elucidating the issues. Specifically, calculations have shown that the unintentional n-type conductivity that has often been observed in as-grown GaN cannot be a...