J. H. Baeck

Phase separation of a Ge2Sb2Te5 alloy in the transition from an amorphous structure to crystalline structures

Changes in the structural and electrical properties of a Ge2Sb2Te5 alloy thin film induced by phase transition were investigated using various analytical techniques. X-ray diffraction and scanning photoelectron microscopy showed that the phase separation occurred in a local area of the film during a phase transition when the amorphous structure was being transformed into crystalline structures. It was found that the heterogeneous distribution of Sb atoms that diffused during the phase transition accompanied the phase separation. Atomic force microscopy was used to examine the changes in surface morphology and roughness. The electrical conductance of the film was dramatically improved after the phase transition from an amorphous structure to crystalline structures as evidenced by the sheet resistance measurements. The sheet resistance changed from ∼109to∼102Ω∕sq. during the phase transition. Differential scanning calorimetry was used to determine the exact phase transition temperature (160–170°C) and the e...

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...

Electronic Structure of Te/Sb/Ge and Sb/Te/Ge Multi Layer Films Using Photoelectron Spectroscopy

Te/Sb/Ge and Sb/Te/Ge multilayer films with an atomically controlled interface were synthesized using effusion cell and e-beam techniques. The layers interacted during the deposition, resulting in films composed of Sb-Te+Sb-Sb/Ge and Sb/Sb-Te/Ge-Te/Ge respectively. Atomic diffusion and chemical reactions in films during the annealing process were investigated by photoemission spectroscopy. In the case of Te/Sb/Ge, Ge diffused into the Sb-Te region released Sb in Sb-Te bonds and interacted with residual Te, resulting in a change in valence band line shape, which was similar to that of a Ge(1)Sb(2)Te(4) crystalline phase. The Ge-Sb-Te alloy underwent a stoichiometric change during the process, resulting in a 1.2:2:4 ratio, consistent with the most stable stoichiometry value calculated by ab initio density-functional theory. The experimental results strongly suggest that the most stable structure is generated through a reaction process involving the minimization of total energy. In addition, Ge in the Sb/Te/Ge film diffused into Sb-Te region by thermal energy. However, Ge was not able to diffuse to the near surface because Sb atoms of the high concentration at the surface were easily segregated and hindered the diffusion of other elements.

Effect of ZrO2 incorporation into high dielectric Gd2O3 film grown on Si(111)

Gd2O3 films, in which ZrO2 was incorporated, were epitaxially grown on Si(111) using an electron-beam evaporation and effusion method. The crystalline structure and morphological characteristics were investigated by various measurements. A silicide layer was locally formed during the initial growth stage due to interactions between elemental Gd and Si in the Gd2O3 film, resulting in poor interfacial characteristics and extensive destruction of the crystalline structure. However, the incorporation of ZrO2 influenced the unit-cell structure of Gd2O3, which contains oxygen vacancies that is located diagonally, enhancing the structural stability owing to the effective suppression of the interfacial layer. The effect on the initial growth stage as the result of incorporation improves the crystalline quality of the epitaxial Gd2O3 film and structural coherence between the film and substrate.

Synthesis of self-ordered Sb2Te2 films with atomically aligned Te layers and the effect of phonon scattering modulation

In this work, we prepared multilayered films with repeated Sb and Te layers by a thermal evaporation method. The film structure was controlled by the layer thickness ratio of Sb to Te, resulting in the formation of self-ordered superlattice structured Sb2Te2/Te films and self-ordered Sb2Te3 films. The thermoelectrical properties and the thermoelectric figure of merit (ZT) closely correlated with the structural characteristics, i.e., the superlattice structure with semiconductor–semimetal layers contributed to both electron and phonon scattering resulting in an improvement in electrical conduction and an increase in phonon scattering. In particular, phonon scattering was significantly increased to further reduce thermal conductivity in the self-ordered superlattice structure of the {Sb2Te2/Te}n sample. As a result, a thermoelectric figure of merit of ZT = 1.43 was obtained at 400 K for {Sb(4)Te(6)}n, indicating that the self-ordered superlattice structure holds great promise as an alternative layered thin film thermoelectric material.

Electronic and structural characteristics of Zr-incorporated Gd2O3 films on strained SiGe substrates

Zr-incorporated Gd(2)O(3) films were grown on various substrates as a function of Zr content. The extent of interfacial reactions was found to be critically dependent on both the incorporated Zr content and the substrate type. Specifically, the silicide layer was suppressed and the Gd(2)O(3) phase was changed to ZrO(2) on a Si substrate with increasing Zr content. Crystalline Gd(2)Ge(2)O(7) was grown on a Ge substrate, as the result of interfacial reactions between Gd-oxide and the Ge substrate. However, interfacial reactions were not affected by the amount of Zr incorporated. On the SiGe/Si substrate, reactions between Gd-oxide and Si could be controlled effectively by the incorporation of Zr, while the extent of reactions with Ge was significantly enhanced as the Zr content increased. The formation of an interfacial layer between the film and the SiGe substrate resulted in a textured crystalline growth.