Hyejin Choi

Dielectric characteristics of Al2O3–HfO2 nanolaminates on Si(100)

The structural characteristics and the chemical state of a HfO2–Al2O3 nanolaminate structure, depending on the postannealing temperature, were examined by x-ray diffraction and x-ray photoelectron spectroscopy. The structural stability is significantly enhanced up to 870 °C and so is able to sustain its amorphous and laminate structure. However, the laminate structure is drastically broken at the annealing temperature of 920 °C and the crystallization is locally generated. In particular, the formation of the interfacial layer during the postannealing treatment is effectively suppressed in the nanolaminated structure. The dielectric constant of the nanolaminate structure calculated from the accumulation capacitance increases from ∼10 to ∼17 as the annealing temperature increases. This change is closely related to the degree of the mixture composed by Al2O3 and HfO2.

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...

Reversible Fermi Level Tuning of a Sb2Te3 Topological Insulator by Structural Deformation

For three-dimensional (3D) topological insulators that have a layered structure, strain was used to control critical physical properties. Here, we show that tensile strain decreases bulk carrier density while accentuating transport of topological surface state using temperature-dependent resistance and magneto-resistance measurements, terahertz-time domain spectroscopy and density functional theory calculations. The induced strain was confirmed by transmittance X-ray scattering measurements. The results show the possibility of reversible topological surface state device control using structural deformation.

Evolution of the surface state in Bi2Se2Te thin films during phase transition

Topological insulators, a new quantum state of matter, have created exciting opportunities for studies in topological quantum physics and for exploring spintronics applications due to their gapless helical metallic surface states. In this study, thin films composed of alternate layers of Bi and Se (Te) ({Bi(3 Å)Te(9 Å)}n/{Bi(3 Å)Se(9 Å)}n) were fabricated by controlling the layer thickness within the atomic scale using thermal evaporation techniques. The high-purity growth of uniform Bi2Se2Te1 thin films has not yet been achieved using a thermal evaporation method. However, as a result of a self-ordering process during annealing, an as-grown amorphous film with p-type polarity could transform into single crystalline Bi2Se2Te1 with n-type polarity. Using THz-time domain spectroscopy (THz-TDS) and ultraviolet photoemission spectroscopy (UPS), we concluded that the conductivity is dominated by the Drude contribution, suggesting the presence of a quantum well state and surface states. Moreover we demonstrated that the emission of terahertz waves from the (001) surface of the single crystalline Bi2Se2Te1 thin film would be possible under the excitation of 790 nm femtosecond optical pulses, indicating the presence of a Dirac-fermion, a photo-Dember effect at the surface state and the transient current within the surface depletion region. The results reported herein provide useful information regarding a valuable deposition method that can be useful in studies of the evolution of surface state electrons in topological insulators.

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.

Tuning the Fermi level with topological phase transition by internal strain in a topological insulator Bi2Se3 thin film

In a three-dimensional topological insulator Bi2Se3, a stress control for band gap manipulation was predicted but no systematic investigation has been performed yet due to the requirement of large external stress. We report herein on the strain-dependent results for Bi2Se3 films of various thicknesses that are grown via a self-organized ordering process. Using small angle X-ray scattering and Raman spectroscopy, the changes of d-spacings in the crystal structure and phonon vibration shifts resulted from stress are clearly observed when the film thickness is below ten quintuple layers. From the UV photoemission/inverse photoemission spectroscopy (UPS/IPES) results and ab initio calculations, significant changes of the Fermi level and band gap were observed. The deformed band structure also exhibits a Van Hove singularity at specific energies in the UV absorption experiment and ab initio calculations. Our results, including the synthesis of a strained ultrathin topological insulator, suggest a new direction for electronic and spintronic applications for the future.

Enhancement of carrier lifetime by spin–orbit coupling in a topological insulator of an Sb2Te3 thin film

Electrons and phonons in chalcogenide-based materials are important factors in the performance of optical data-storage media and thermoelectric devices. However, the fundamental kinetics of carriers in chalcogenide materials remains controversial, and active debate continues over the mechanism responsible for carrier relaxation. In this study, we used optical-pump terahertz-probe spectroscopy, which permits the relationship between structural phase transition and optical property transitions to be examined, to investigate the ultrafast carrier dynamics in a multilayered [Sb(3 Å)/Te(9 Å)]n thin film during the transition from the disordered to crystalline phase. Using terahertz time-domain spectroscopy and a contact-free optical technique, we demonstrated that the optical conductance and carrier concentration vary as functions of annealing temperature. Moreover, we observed that the topological surface state (TSS) affects the enhancement of the carrier lifetime, which is closely related to the degree of spin-orbit coupling (SOC). The combination of the optical technique and proposed carrier relaxation mechanism provides a powerful tool for monitoring TSS and SOC. Consequently, it was determined that the response of the disordered phase is dominated by an electron-phonon coupling effect, while that of the crystalline structure is controlled by a Dirac surface state and SOC effects. These results are important for understanding the fundamental physics of phase change materials and for optimizing and designing materials with better performance in optoelectronic devices.