Y.G. Kim

Changes in the electronic structures and optical band gap of Ge2Sb2Te5 and N-doped Ge2Sb2Te5 during phase transition

Changes in the electronic structures of Ge2Sb2Te5 (GST) and N-doped Ge2Sb2Te5 film during the phase transition from an amorphous to a crystalline phase were studied using synchrotron radiation high-resolution x-ray photoemission spectroscopy. The changes in tetrahedral and octahedral coordinated Ge 3d peaks are closely related to the changes in the chemical bonding state of GST films. The metallic Sb peak in the Sb 4d spectra of annealed GST films demonstrates that the metallic Sb atoms become segregated during thermal treatment resulting in phase separation. The incorporation of nitrogen into the GST film affects its structure and chemical bonding state, resulting in the suppression of crystallization. The incorporation of nitrogen also increases the optical band gap of the film due to the formation of a nitride.

PHENIX inner detectors

Abstract The timing, location and particle multiplicity of a PHENIX collision are determined by the Beam–Beam Counters (BBC), the Multiplicity/Vertex Detector (MVD) and the Zero-Degree Calorimeters (ZDC). The BBCs provide both the time of interaction and position of a collision from the flight time of prompt particles. The MVD provides a measure of event particle multiplicity, collision vertex position and fluctuations in charged particle distributions. The ZDCs provide information on the most grazing collisions. A Normalization Trigger Counter (NTC) is used to obtain absolute cross-section measurements for p–p collisions. The BBC, MVD and NTC are described below.

Change in electrical resistance and thermal stability of nitrogen incorporated Ge2Sb2Te5 films

Changes in the electrical resistance of nitrogen incorporated Ge2Sb2Te5 (NGST) films were investigated as a function of nitrogen content by four-point probe and ac two-point probe methods. Some nitrogen is initially located inside the cubic structure, resulting in a significant increase in crystalline temperature and electrical resistance. As the amount of incorporated nitrogen increases, excess nitrogen accumulates in the grain boundaries, which does not contribute substantially to the increase in electrical resistance and the crystallization temperature. The supersaturated nitrogen distorts the Ge2Sb2Te5 structure, resulting in a NGST film with a structure that is different from the metastable fcc structure. X-ray absorption spectroscopy revealed that Ge–N and N2 molecular states were present in the film and gradually increased in proportion to the amount of incorporated nitrogen. Moreover, the nitrogen states were very stably maintained even during the phase transition process.

Effects of N2+ ion implantation on phase transition in Ge2Sb2Te5 films

The phase transitions of Ge2Sb2Te5 (GST) films after bombardment with 40keV N2+ ions were investigated. Comparing the nitrogen incorporated GST films with a pure GST film, the suppression of a crystalline grain growth was more effective in the N2+ implanted GST film than in a nitrogen codeposited GST film, i.e., x-ray diffraction data showed that the intensities of the crystalline diffraction peaks were decreased and the full widths at half maximum were broader than that of a pure GST film. This suppression of crystallization owing to the incorporation of nitrogen drastically reduced the roughness of surface morphology and decreased the electrical conductivity of the crystalline film. A near edge x-ray absorption fine structure experiment and x-ray photoemission spectroscopy data demonstrated that the suppression of crystalline grain growth is due to the formation of Ge3N4 and interstitial N2 molecules. In N2+ implanted GST films, in particular, interstitial N2 molecules played a major role in the suppres...

Temperature-dependent photoluminescence of ZnSe/ZnS quantum dots fabricated under the Stranski–Krastanov mode

Self-assembled ZnSe/ZnS quantum dots (QDs) have been grown in the Stranski–Krastanov (S–K) mode using a metalorganic chemical vapor deposition technique under the atomic-layer epitaxy mode. Atomic-force-microscopy measurements on the uncapped ZnSe/ZnS QDs reveal that lens-shaped ZnSe QDs are formed after 1–2 monolayer ZnSe is deposited. The ZnSe QDs are estimated 1–2 nm in height and 25–35 nm in radius. The temperature-dependent behavior of confined carriers in the ZnSe QDs has been investigated through photoluminescence (PL) measurements. PL spectra show a substantial PL linewidth narrowing accompanied by a large redshift of the emission peak energy with increasing temperature. This unusual temperature-dependent behavior is interpreted as the dot-to-dot carrier transfer through the wetting layer, which is common to QDs grown in the S–K mode.

Investigation of phase transition of Ge2Sb2Te5 and N-incorporated Ge2Sb2Te5 films using x-ray absorption spectroscopy

For this study, the phase-change materials Ge2Sb2Te5 and N-doped Ge2Sb2Te5 films were investigated using x-ray absorption near-edge structure and extended x-ray absorption fine structure. During the phase transition, change in electronic structure is observed by the shift of the absorption edge energy, i.e., structural coordination of Ge–Te changes from tetrahedral to octahedral coordination, of which the interatomic distances are 3.12 and 2.83A, respectively. In addition, nitrogen incorporation into the film led to a p-p orbital hybridization and a different crystallization behavior. The hybridization caused by the formation of a Ge–N bond was related to suppression of the phase transition.

Strain-sensitive size modulations in ZnSe∕ZnS quantum dots grownon GaAs substrates

Strain effects on sizes and emission energies of ZnSe∕ZnS quantum dots (QDs) have been investigated. The initial strain in the ZnSe QD layer was altered by adjusting the thickness of the ZnS buffer. Consistent blueshifts of the ground-state emission from 4 monolayer ZnSe QDs were observed with increasing the thickness of the ZnS buffer from 20 to 40nm. Atomic-force microscopy revealed that the blueshifts are due to a continuous QD size reduction. We estimated the emission energy as a function of the initial strain in the ZnSe QD layer, which shows that the band-gap engineering is possible through the strain modification.

Impact of the crystallization of the high-k dielectric gate oxide on the positive bias temperature instability of the n-channel metal-oxide-semiconductor field emission transistor

The positive bias temperature instability (PBTI) characteristics of the n-channel metal-oxide-semiconductor field emission transistors which had different kinds of high-k dielectric gate oxides were studied with the different stress-relaxation times. The degradation in the threshold voltage followed a power-law on the stress times. In particular, we found that their PBTI behaviors were closely related to the structural phase of the high-k dielectric gate oxide. In an amorphous gate oxide, the negative charges were trapped into the stress-induced defects of which energy level was so deep that the trapped charges were de-trapped slowly. Meanwhile, in a crystalline gate oxide, the negative charges were trapped mostly in the pre-existing defects in the crystallized films during early stage of the stress time and de-trapped quickly due to the shallow energy level of the defects.

Defect-mediated spontaneous emission enhancement of plasmon-coupled CuInS 2 and CuInS 2 /ZnS

The studies of plasmon-coupled excitons at the surface-/interface-, shallow-, and deep-trapped states of copper-indium-disulfide (CIS) with/without zinc-sulfide (ZnS) shell revealed the defect-mediated spontaneous emission enhancement. The PL enhancement with spectral blue-shift of plasmon-coupled excitons in CIS quantum dots (QDs) indicates the large reduction of nonradiative decay at the surface- and shallow-trapped states with strong spectral overlapping. The PL enhancement with spectral red-shift of plasmon-coupled excitons in CIS/ZnS QDs is accredited to the defect-mediated PL enhancement by the higher fractional amplitude at the interface-trapped state around the longer spectral region. The spontaneous emission enhancement of plasmon-coupled CIS QDs were ~2.1, ~2.2, and ~2.8-folds compared to the decay rates of CIS, and those of plasmon-coupled CIS/ZnS QDs were ~24.1, ~32.8, and ~24.9-folds compared to the decay rates of CIS/ZnS at shorter, intermediate, and longer spectral regions due to relatively stable charge carriers and close to the surface plasmon resonance. The PL enhancements of plasmon-coupled CIS at room temperature and 6 K were two-fold and three-fold compared to the integrated CIS PLs, and the PL enhancements of plasmon-coupled CIS/ZnS at room temperature and 6 K were five-fold and eight-fold compared to the integrated CIS/ZnS PLs. The large PL enhancement is attributable to the plasmon-exciton coupling through Coulomb interaction and the local field enhancement. The larger PL enhancement of plasmon-coupled CIS/ZnS compared to that of plasmon-coupled CIS is accredited to the larger spontaneous emission enhancement.

The PHENIX Multiplicity and Vertex Detector

Abstract We describe the design and expected performance of the PHENIX Multiplicity and Vertex Detector (MVD) sub-system of the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC).