Moon Hyung Jang

Phase change behavior in oxygen-incorporated Ge2Sb2Te5 films

Oxygen-incorporated Ge2Sb2Te5 (GST) films were deposited using ion beam sputtering deposition. Sheet resistance in films with 16.7% oxygen content decreased at a higher annealing temperature than that of undoped GST films, while resistance in films with an oxygen content of over 21.7% decreased dramatically at lower temperatures. X-ray diffraction patterns showed crystallization to face-centered cubic phase was suppressed. However, phase separation to a hexagonal structure was observed in films with an oxygen content of over 21.7%. Extended x-ray absorption fine structure data of Ge K edge showed Ge was bonded to O as well as Te. Moreover, a stoichiometric GeO2 phase was not observed, while phase separation into Sb2O3 and Sb2Te3 occurred. The results indicate Ge–Te bonds with oxygen are related to structural stability.

Change in band alignment of HfO2 films with annealing treatments

Energy band alignment of a nitrided HfO2 film and dependence of the band gap (Eg) on annealing treatments with nitrogen plasma and ambient gases (N2 and O2) were studied by reflection electron energy loss spectra and x-ray photoelectron spectroscopy. We also investigated the nitrogen content in the film and its influence on the band alignment using medium energy ion scattering. The nitrogen incorporated into the HfO2 film by directed nitrogen plasma treatment significantly decreased the band gap and band offsets, i.e., the incorporated N in the film decreased both conduction and valance band offsets. The nitrogen content in depth direction was dependent on the postannealing conditions using O2 or N2.

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.

Effect of In incorporated into SbTe on phase change characteristics resulting from changes in electronic structure

The effects of the In content of InSbTe films with various stoichiometries (Sb2Te2.7, In0.5Sb2Te2.9, and In2.6Sb2Te2.9) on phase change characteristics were investigated. With increasing incorporation of In atoms into Sb2Te3, various crystalline phases, i.e., In2Te3, Sb, and In3SbTe2, were observed due to the bond energy between the constituent atoms, while only Sb2Te3 and the Sb2Te2 phases were observed in the case of Sb2Te2.7 and In0.5Sb2Te2.9 films. In addition, the shifts in binding energy of the Sb 3d and In 3d peaks in x-ray photoelectron spectra after the annealing treatment were directly related to the amount of incorporated In. The observed changes in electronic structure suggest that the changes in electrical conductivity and crystalline phase are directly related to the extent of In incorporation.

Characteristics of perylene-based organic thin-film transistor with octadecyltrichlorosilane monolayer

We fabricated perylene-based organic thin-film transistor (OTFT) with an octadecyltrichlorosilane (OTS) monolayer in an ultrahigh vacuum condition. By current–voltage characteristics, the saturation current and the field effect mobility of OTS–OTFT (gold∕perylene∕OTS∕SiO2∕p+-Si) increased over ∼100 times in comparison with normal-OTFT (gold∕perylene∕SiO2∕p+-Si), and on/off ratio increased over ∼100 times. The saturation current, field effect mobility, and on/off ratio were >1μA, 0.042cm2∕Vs, and >106, respectively. The uniformity of the OTS layer was confirmed by x-ray reflectivity. Perylene thin films on SiO2 and OTS∕SiO2 were compared by atomic force microscopy, scanning electron microscopy, x-ray diffraction, and x-ray photoscopy.

Ultrafast phase change and long durability of BN-incorporated GeSbTe

BN-incorporated amorphous Ge2Sb2Te5 (GST) films were deposited by an ion beam sputtering deposition (IBSD) method using GST and BN targets. Based on in situ sheet resistance measurements, we confirmed that as the amount of BN increased, the crystallization temperature (Tc) increased from 150 °C to 260 °C. It was demonstrated that the phase change speed of BN-incorporated GST is ten times faster than that of GST using a nanosecond laser. By evaluating Johnson–Mehl–Avrami (JMA) plots and scanning electron microscopy (SEM) images, it was confirmed that the one-dimensional grain growth is dominant during the fast phase change of BN-incorporated GST because BN impurities can act like nuclei during the initial stage of crystal growth. After the 100 iteration test under rigorous acceleration conditions of SET/RESET switching using the pulsed laser system, it was confirmed that the void formation and thickness variation are very limited in the BN-incorporated GST, as compared to GST. This result originates from the low phase change stress of the BN-incorporated GST films during one-dimensional growth. The electrical SET speed and cyclability of the BN-incorporated GST device also improved significantly compared to GST.

The Phase Change Effect of Oxygen-Incorporation in GeSbTe Films

Reflectivity changes in oxygen-incorporated Ge 2 Sb 2 Te 5 (GST) films were investigated via a laser-induced crystallization process. The crystallization process showed that the phase change speed and the laser power required for crystallization become faster and larger in GST films with a characteristic quantity of oxygen. We confirmed that a dominant grain growth mode during the laser crystallization is a major determinant for the speed of phase change in GST films with a characteristic quantity of oxygen. JMA results and changes in surface morphology indicate that the origin of the growth mode change is due to an increase in the number of initial nucleation sites produced in the oxygen-incorporated GST films. After the re-amorphization process, oxygen-incorporated GST films show more rapid and more stable phase change properties than that of GST films.

Phase selective synthesis of gadolinium silicide films on Si(111) using an interfacial SiO2 layer

We synthesized a single phase GdSi2 film on a Si(111) substrate with an interfacial SiO2 layer. In order to take account of the role of the interfacial SiO2 layer, systematic investigations on clean and oxidized Si substrates were done by using in situ reflection of high energy electron diffraction, x-ray diffraction, and atomic force microscopy of the silicides formed with post annealing. Our result showed that the interfacial SiO2 layer enhanced the structural transformation of the initial GdSi1.7 hexagonal phase into the GdSi2 orthorhombic phase above the decomposition temperature of SiO2(∼800 °C). We proposed a reaction mechanism for the GdSi2 film formation with the help of the interfacial SiO2 layer. The measured electrical resistivity of the Gd-silicide film strongly depends on the silicide phase.

Structural Stability and Phase-Change Characteristics of Ge2Sb2Te5 ∕ SiO2 Nano-Multilayered Films

The phase-change characteristics of Ge 2 Sb 2 Te 5 /SiO 2 multilayered films with various bilayer thicknesses were investigated using high-resolution transmission electron microscopy. The change in the electrical sheet resistance of the films shows that the metastable face-centered cubic (fcc) structure formation was significantly affected by the bilayer thickness of the multilayer film. That is, compared with the single-layered Ge 2 Sb 2 Te 5 film, the multilayered films transformed to hexagonal at lower temperatures. In particular, the transition temperature region of the fcc structure with semiconductor properties was significantly reduced. We observed that as the bilayer thickness decreased and annealing temperature increased, the structure of the multilayer film was transformed into Ge-deficient and hexagonal phases such as GeSb 2 Te 4 and Sb 2 Te 3 . This behavior was induced by the out-diffusion of Ge atoms from the outer surface layer, which can be caused by a difference in thermal strain at the interface between the Ge 2 Sb 2 Te 5 and SiO 2 films.

Effects of oxygen incorporation in GeSbTe films on electrical properties and thermal stability

Oxygen incorporated Ge2Sb2Te5 (GST) films were prepared by an ion beam sputtering deposition method. I-V curves of the oxygen incorporated GST active layer showed that the threshold voltage (Vth) varied, depending on the level of incorporated oxygen. In the case of a GST film with an elevated oxygen content of 30.8%, the GST layer melted at 9.02 V due to the instability conferred by the high oxygen content. The formation of Ge-deficient hexagonal phases such as GeSb2Te4 and Sb2Te3 appear to be responsible for the Vth variation. Impedance analyses indicated that the resistance in GST films with oxygen contents of 16.7% and 21.7% had different origins. Thermal desorption spectroscopy data indicate that moisture and hydrocarbons were more readily desorbed at higher oxygen content because the oxygen incorporated GST films are more hydrophilic than undoped GST films.