Insun Hwang

Photoreflectance study of etching and annealing effect on AlGaAsGaAs heterostructure

Abstract We investigated the PR spectra of as-grown, annealed and etched modulation-doped AlGaAs GaAs heterostructure samples. As annealing time increases, the built-in electric fields are strengthened as a result of the diffusion of SiSi pairs. As the etching time increases the broad signal near GaAs band-gap energy and the single peak shape (SPS) signal near AlGaAs band-gap energy change to the step-like signal and the double peak shape (DPS) signal, respectively.

Photoluminescence due to hole capturing of DX− centers in In0.32Ga0.68P : S

The photoluminescence (PL) characteristics of S-doped In0.32Ga0.68P epilayer grown by liquid phase epitaxy have been studied in the temperature range of 20–300 K. In addition to the two peaks of the near band-edge emission and the donor-to-acceptor transition, a third peak of 2.199 eV at 74 K is observed in a narrow sample temperature range. It is shown that the appearance of this peak is related to the DX centers. The transition is attributed to the recombination of the neutral donor with a hole bound to the DX− center.

The incorporation behaviour of Co impurities in a ZnSe crystal

This work is a study on the incorporation phenomena of cobalt magnetic ions in ZnSe. Zn1-xCoxSe (x<0.02) polycrystals were prepared by the melt growth technique. The influences of Co impurities on the ZnSe crystal were investigated using X-ray diffractometry and Raman spectroscopy. The solid solubility of melt grown crystals is about x=0.02(+or-0.0015). It was found that the lattice constant of the crystal increases linearly up to x=0.0086 and then saturates as the amount of Co impurities increases. It is also observed that the TO2 and LO1 Raman modes in Zn1-xCoxSe soften for x<0.0086 by the addition of Co atoms. The oscillator strength, which is calculated using the energies of the TO2 and LO1 phonon modes, decreases as the cobalt content increases. There is a linear relationship between the ionicity and the oscillator strength with a slope of -7.02*1025 s-2. From these observations, it is clear that the instability of the Zn1-xCoxSe crystal is mainly due to the ionicity difference between Zn and Co atoms.

Origin of short period oscillation near GaAs band-gap energy in photoreflectance

Abstract We investigated the origin of the short-period oscillation (SPO) in photoreflectance (PR) spectra of selectively doped GaAs samples. In the PR spectra of the samples the SPO are separated into two distinct signals. The intensity of the lower-energy signal decreases rapidly as the temperature is lowered. Therefore, we conclude that the short-period oscillation is due to the hole-ionized acceptor ( h - A − ) pair modulation and the free exciton modulation in the cap layer.

Possible Site of Co2+ Ions in Zn1-xCdxGa2Se4 Crystals

Zn1-xCdxGa2Se4 crystals and Co2+-doped ones with values between x=0.0 and x=1.0 at intervals of 0.2 were investigated by XRD and Raman spectroscopy in order to determine the possible site of Co2+ ions in these crystals. It seems from the changes of the lattice parameters and Raman shifts of these samples that Co2+ ions possibly locate at the site of Ga atoms. Since Co2+ ions will act as acceptors in this case, we can also discuss the valence band offset in Zn1-xCdxGa2Se4/ZnGa2Se4 heterojunctions.

Co Impurity Effect in Zn1-xCoxSe Crystals Grown under Equilibrium Condition

The influences of Co impurity in the Zn1-xCoxSe crystal were investigated by X-ray diffractometry (XRD) and Raman spectroscopy using melt grown Zn1-xCoxSe polycrystals. The solid solubility of Co atom in melt grown ZnSe crystal is about x=0.02±0.0015. The solid solubility of magmetic ions may depend on the electronegativity and covalent radius. It was found that the lattice constant of the crystal increases linearly up to x=0.0086 and then saturates as the amount of Co impurity increases. It is also observed that the TO2 and LO1 Raman modes in Zn1-xCoxSe grown under equilibrium condition soften in the value of x<0.0086 by the addition of Co atoms. There is a linear relationship between the ionicity and the oscillator strength which is calculated using the energies of TO2 and LO1 phonon modes with the slope of -7.02×1025 (s-2). From these observations it is clear that the instability of Zn1-xCoxSe crystal is mainly due to the ionicity difference between Zn and Co atoms since both covalent radii are almost the same.