Yang-Koo Cho

Analytical and numerical approaches to calculating the escape function for the emission of medium-energy electrons from uniform specimens

A program for the simulation of electron transport by the Monte Carlo method has been developed. This program implies the model of single scattering and dielectric approach (to calculate the characteristics of an inelastic interaction). The escape functions for aluminum, germanium, and gold in the 0.012–20 keV energy range were calculated. The characteristic lengths determining the electron dynamics (the elastic and inelastic mean free paths, isotropization length, and integral path) were calculated using the differential cross sections for both elastic and inelastic interactions for these elements. The analysis of the relations between the characteristic lengths made it possible to determine the applicability range of the analytical expressions for the emission functions obtained in [1]. The comparison of the results obtained analytically and numerically confirmed the conclusion of [1] about the form of the analytical approximation of the emission function for electrons of various energies and showed the validity of the obtained analytical expressions for the escape lengths of electrons.

Annealing effects in ZnSe/GaAs heterostructure grown by molecular beam epitaxy

In the ZnSe/GaAs heterostructure, the problems of interdiffusion and thermal stability are very crucial. We have investigated the effects of annealing on ZnSe grown on GaAs by photoluminescence and double-crystal x-ray measurement. In order to investigate annealing effects of the ZnSe/GaAs heterostructure, we used 1.0-μm-thick and 0.2-μm-thick samples. Samples were annealed in the temperature range of 200–500 °C in an N2 ambient for 3 min using a face-to-face configuration with ZnSe epitaxial layer as a cap layer. In the annealing temperature above 450 °C, new emission peaks appeared in the range of 2.62–2.72 eV. The biaxial compressive strain due to lattice mismatch at the growth temperature is mostly relaxed at 400 °C, and the epilayer annealed at 500 °C has a biaxial tensile strain.

Variable Rowland radius laboratory vacuum surface-sensitive x-ray absorption fine structure spectrometer

A new laboratory x-ray spectrometer for surface-sensitive extended x-ray absorption fine structure [(S)EXAFS] and surface-sensitive x-ray absorption near-edge structure [(S)XANES] measurements is described. The spectrometer employs a 12 kV mA rotating anode generator. It has a monochromator equipped with a set of exchangeable curved crystals of Johann or Johansson type with different cell parameters, orientations, and Rowland radii. The computer controlled movement system based on nine stepping motors allows all the main elements of the spectrometer to be positioned freely relative to the x-ray source and gives an opportunity to use sophisticated scanning modes (for example, a mode with a focus spot position on a sample surface instead of an exit slit). The whole x-ray beam line is completely enclosed in a vacuum chamber that is directly connected to the x-ray generator, thereby preventing the absorption of x rays in the air. This layout allows a wide x-ray photon energy range from a few keV up to dozens of keV. A registration of x rays transmitted through the sample with proportional counter- and photoelectrons emitted from the sample with channeltron is used to carry out bulk- and surface-sensitive measurements, respectively. Using a 25 x 200 kV mA power regime of a rotating anode x-ray generator, a photon flux of 2.5 x 10(5) counts/s was registered at the Cu K edge, where the energy resolution was about 5 eV. High near surface sensitivity is demonstrated by the EXAFS spectra of Cu K and Hf LIII edges measured from 3 nm Cu and Hf oxide films.

Effective calculation of energy loss and scattering angle at electron-target material inelastic interaction

A Monte Carlo-based method for computing electron-target inelastic interaction is presented. It uses the double-differential inelastic scattering cross section. A resource-saving algorithm that simulates the interaction using the double-differential cross section was elaborated. The cross section was calculated from data on the optical permittivity of the target.

ZnSSe epilayers with extremely low defect density by the growth-temperature optimization

In order to realize ZnSSe/GaAs heteroepitaxial layers with lower defect density, we have introduced molecular beam epitaxy growth by a three-step temperature correction which takes account of the cell radiation and surface emissivity effects superposed on the Fabry - Perot interference oscillation in the pyrometric temperature. The optimum correction rates of growth temperature are determined by the analysis of double-crystal x-ray rocking curves, and an empirical profile for the control temperature as a function of growth time is proposed for the optimum ZnSSe growth. While the etch pit density is in normal molecular beam epitaxy growth, an etch pit density of , which is the lowest value known so far, is realized in ZnSSe/GaAs(100) epilayers grown by using the linearly corrected profile.

High Resolution Transmission Electron Microscopy Observations on Sintering Processes in KNbO3 Ceramics

Piezoelectric devices such as the ultrasonic motors, transformers, and actuators have received considerable attention due to their wide range of applications (Saito et al., 2004). Lead-zirconate-titanate (Pb(Zr,Ti)O3 or PZT)based ceramics have been used for these devices because of their outstanding piezoelectric properties (Jaffe et al, 1971). However, PZT-based ceramics contain more than 60 wt% of PbO, which causes serious environmental problems. There has been an increasing number of investigations on leadfree piezoelectric ceramics such as BaTiO3, (Bi1/2Na1/2)TiO3, and (Na0.5K0.5)NbO3 (NKN) (Rodel et al., 2009). Among these materials, NKN-based ceramics have attracted much attentions because of their high piezoelectric properties and high Curie temperature (Tc) (Jarupoom et al., 2008). However, NKN ceramics decompose easily in water, and Na2O (or K2O) evaporates during sintering at high temperatures (>1,000C). Therefore, it is very difficult to obtain dense NKN ceramics with homogeneous composition and reliable piezoelectric properties. To overcome this problem and to improve the piezoelectric properties, many investigations have been conducted on NKN-based solid solutions (Park et al., 2010). In particular, (1-x)NKN-xLi(Nb, Ta, Sb) solid solutions have been studied extensively because of their promising piezoelectric properties (Guo et al., 2004). The lowtemperature sintering of NKN-based ceramics has been also studied to prevent the evaporation of Na2O (or K2O) during sintering. The KNbO3 (KN) ceramic could also be a candidate for leadfree piezoelectric ceramics since KN single crystal exhibited good piezoelectric properties and high Tc (Nakamura et al., 2002). However, densification of the KN ceramics using the conventional solid-state method was very difficult because of the evaporation of K2O and formation of unwanted secondary phases (Birol et al., 2005). Therefore, special

Microstructural Characteristics of (Na0.5Ko.5)NbO3 Ceramics with Additives: Transmission Electron Microscopy Study

Na0.5K0.5NbO3 (NKN) has attracted much attention as an alternative to Pb(Zr1-xTix)O3 (PZT) ceramics and lead-free piezoelectric materials because of its high piezoelectric properties and a high Curie temperature (Tc) [1, 2]. However, the high sintering temperature for NKN is an obstacle for realizing multilayer devices because they require low driving force, miniaturization, and hybridization. Specifically, the NKN ceramics have to be sintered at around 900 C because the melting point of silver (Ag), commonly used as an electrode in multilayer devices, is 961 C and the Na2O evaporates during the sintering process at temperatures above 1000 C [3, 4].

Calculation of the electron escape function and medium-energy photoelectron yield from layer-on-substrate composites

Analytical expressions for the electron escape function and X-ray-induced photoemission yield from layer-on-substrate composites are derived by approximately solving the kinetic equation for medium-energy (several keV) electrons. Relationships obtained by Monte Carlo calculation qualitatively agree with the analytical dependences and make it possible to numerically find parameters entering into them.

Molecular beam epitaxial growth and properties of high-quality ZnSxSe1-x on GaAs(0 0 1) substrate

Abstract We have studied molecular beam epitaxial growth of high-quality ZnSSe on GaAs substrates. From the results obtained by pyrometric oscillation behavior and X-ray rocking curve simulation, a new technique, called a three-step temperature correction method, is introduced. In this method, the temperature correction region is divided into T1 for compensating the heating effect by radiation, T2 for connecting the inverse gradient of temperature in T1 and T2, and T3 for compensating the temperature drop during ZnSxSe1 − x growth. Applying this method to ZnSxSe1 − x epitaxial layer growth on GaAs substrates, layers with better crystallinity are reproducibly grown as evaluated by the full-width at half-maximum of the double-crystal X-ray rocking curves, etch pit density, and photoluminescence. An etch pit density of ⩽ 5 × 103 cm−2 and photoluminescence broadening of about 1.8 meV at 4.2 K could be obtained for ZnSxSe1 − x with x = 0.06.