K. Jeong

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.

Structural and electrical characteristics of Y2O3 films grown on oxidized Si(100) surface

Heteroepitaxial Y2O3 films were grown on oxidized and clean Si (100) surfaces by ion assisted evaporation under an ultrahigh vacuum. The crystalline structure, crystallinity, morphology, and electrical properties were investigated using various techniques. The crystallinity assessed by x-ray diffraction and Rutherford backscattering spectroscopy shows that the films grown on the oxidized Si substrates have better crystallinity and smoother morphology compared to those on the clean Si. As the annealing temperature increases, the crystallinity and morphology are stable for the films grown on the oxidized Si, while those of the films grown on clean Si substrates degrade. The difference between the two films is attributed to the formation of hillocks and a chemical reaction at the interface between the film and SiO2. The low crystallinity, strain change, and the reaction of excess Y in the films grown on the clean Si contribute to the crystalline structure and the formation of hillock. These changes of crysta...

Annealing effects of aluminum silicate films grown on Si(100)

The annealing effects of the thin aluminum silicate films grown on Si(100) by sputtering method were investigated using various physical and electrical measurements. All the films grown at the temperature of 300 °C using sputtering Al2O3 target show an amorphous structure as examined by x-ray diffraction and transmission electron microscopy. The amorphous structure is maintained up to 700 °C and then transformed to crystalline Al1.7SiO0.15O2.85 or mullite phase above the annealing temperature of 800 °C. The conduction process, charge trapping and detrapping characteristics, and trap charge density in metal–oxide–semiconductor structure are influenced by the annealing temperature. The depth profiling data using x-ray photoelectron spectroscopy show that the properties are closely related to the change of the interfacial layer and chemical state under the high temperature annealing. The breakdown characteristics are degraded after the annealing temperature of 900 °C due to the rapid change of the interfacia...

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...

Evolution of tungsten-oxide whiskers synthesized by a rapid thermal-annealing treatment

Tungsten oxide whiskers were prepared on a tungsten thin film by oxidation with H2O and a subsequent annealing treatment at a temperature of over 900 °C in a vacuum. The tungsten oxide formed by oxidation was transformed into smooth, straight whiskers with a monoclinic-crystalline structure after the vacuum annealing treatment. The whiskers showed an oxygen-deficient stoichiometry and a crystalline structure consistent with W18O49, which was dependent on the annealing temperature and vacuum used. The competition between the whisker growth and the dissociation of W oxide has a significant effect on the crystal shape, as well as the size of the whiskers. A change in the binding state during whisker formation indicates that some of the dissociated W oxide contributes to whisker formation and that crystalline whiskers are grown at nucleation sites through this process.

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.

Effects of Surface Chemical Structure on the Mechanical Properties of Si1–xGex Nanowires

The Youngs modulus and fracture strength of Si(1-x)Ge(x) nanowires (NWs) as a function of Ge concentration were measured from tensile stress measurements. The Youngs modulus of the NWs decreased linearly with increasing Ge content. No evidence was found for a linear relationship between the fracture strength of the NWs and Ge content, which is closely related to the quantity of interstitial Ge atoms contained in the wire. However, by removing some of the interstitial Ge atoms through rapid thermal annealing, a linear relationship could be produced. The discrepancy in the reported strength of Si and Ge NWs between calculated and experimented results could be related to SiO(2-x)/Si interfacial defects that are found in Si(1-x)Ge(x) NWs. It was also possible to significantly decrease the number of interfacial defects in the NWs by incorporating a surface passivated Al2O3 layer, which resulted in a substantial increase in fracture strength.

Chemical structure of ultrathin SiO2 film with nitrogen incorporated by remote nitrogen plasma

The nitrided ultrathin SiO2 films using a remote nitrogen plasma source were investigated by high-resolution x-ray photoelectron spectroscopy. At the low nitridation temperature of 500 °C, the nitrogen is effectively incorporated in 15 A SiO2 film. The chemical shifts of the N 1s peaks show that the quantity of the second-nearest neighbors of oxygen atoms and N–O bonds influences the difference of the peak shift depending on the nitridation temperature and post-annealing in ultrahigh vacuum. The peak intensity changes of the N 1s peak at the different take-off angles indicate that the nitridation dominantly occurs at the interfacial region as the nitridation temperature increases, which suggests that the highly incorporated nitrogen at the surface region can be accomplished even with a low temperature nitridation process using a remote nitrogen plasma source. The defect formation due to the nitrogen incorporation resulted in a negative shift of the capacitance–voltage curve, and the difference is increase...

Induction of the surface plasmon resonance from C-incorporated Au catalyst in Si1−xCx nanowires

Si1−xCx nanowires (NWs) were synthesized by varying the ratio of SiH4 and CH3SiH3 gases using a vapor–liquid–solid (VLS) procedure using Au as a catalyst. The growth rate of the Si1−xCx NWs and the change in the wire shape from straight to helical near the Au tip were found to be closely related to the ratio of the CH3SiH3 content. The large concentration of C in the Si1−xCx NWs was proportional to the CH3SiH3 content, overcoming the extremely low solubility of C in Si, resulting in an interstitial incorporation of C atoms in the wire. This incorporation can be attributed to the cleavage of Si–C bonds in the CH3SiH3 compound through the Au catalyst (an Au–Si liquid-state cluster of about 70–100 nm) during wire growth by the VLS method. Simultaneously supplying CH3SiH3 and SiH4 gases enhanced the diffusion of Au atoms from the tip to the sidewall of the wire, while also deforming the shape of the Au tip. When the CH3SiH3 gas was increased to 1.5 sccm, the number of Au nanoparticles (2–3 nm in size) at the lateral surface induced a surface plasmon resonance (SPR) and improved the optical conductivity (σ) of the Si1−xCx NWs. For 2 sccm of CH3SiH3, a remarkable increase in the number of C atoms incorporated in the Au nanoparticles along the sidewall red shifted the SPR peak, suggesting that the SPR can be modulated by the Au–C interactions in the nanoparticles.

Characteristics of ultrathin SiO2 films using dry rapid thermal oxidation and Pt catalyzed wet oxidation

The SiO2/Si interfaces of the 2 nm thick dry oxide using rapid thermal oxidation and wet oxide using H2O formed by catalysis of Pt were investigated in order to understand the difference between the two oxides relevant to the device reliability using various methods. The interstitial defects at the interfacial region were detected using medium energy of ion scattering spectroscopy in the sample of dry oxidation, but not in the wet oxidation. The interfacial roughness and the interstitial defects strongly depended on the oxidation process, although the interfacial strain differs very little between the dry and wet oxidation processes. The results of high resolution x-ray photoelectron spectroscopy using a synchrotron radiation source clearly provided the difference of the defects between two oxides. The scanning tunneling microscope image of the SiO2 layers subjected to hot electron injection from the tip showed that wet oxide had superior quasi-breakdown characteristics to dry oxide. Time-dependent dielec...