Joohwi Lee

Thermodynamic stability of various phases of zinc tin oxides from ab initio calculations

Thermodynamic stabilities of various phases in ZnO–SnO2 systems were investigated based on the Gibbs energy obtained from density functional theory (DFT) calculations. The pressure–temperature (p–T) phase diagram was determined; the coexistence of ZnO and SnO2 was the most stable phase in the low temperature region at zero external pressure, while Zn2SnO4 with the inverse spinel structure and ZnSnO3 with the lithium niobate structure were stable at the high temperature and high pressure region. Various octahedral configurations of the inverse spinel structures of Zn2SnO4 were considered. The calculated results showed feasible agreement with experimental data on the phase stability and explained well the experimental observation of the mixed state of Zn2SnO4, ZnO and SnO2 at mid-range temperatures and pressures. Considering the atomic structures, bulk moduli and thermodynamic stabilities, the local density approximation calculations were found to describe experimental observations more accurately than the generalized gradient approximation calculations. The phase transitions in the ZnO–SnO2 system were found to be dominated by the changes in both the Zn–O bond length and the coordination number of Zn, rather than changes in the bond length of Sn–O and the coordination number of Sn.

Effects of Annealing Environment on Interfacial Reactions and Electrical Properties of Ultrathin SrTiO3 on Si

SrTiO 3 films were grown by radio-frequency magnetron sputtering and postdeposition annealing (PDA) in N 2 , O 2 , or NH 3 atmospheres. A Sr silicate layer formed at the interface between the SrTiO 3 film and Si substrate due to Si diffusion into the films during deposition. This resulted in an inhomogeneous composition of SrTiO 3 films along the vertical direction, which was enhanced by PDA. While the thick SrTiO 3 film was crystallized after PDA and the permittivity increased (>220), the thin SrTiO 3 films remained amorphous even after PDA due to the diffused Si. TiO x in the amorphous SrTiO 3 layer was easily nitrided after PDA in an NH 3 atmosphere but TiO x in the crystalline SrTiO 3 layer was barely nitrided. The electrical properties of the SrTiO 3 films were improved by PDA.

Study on the defects in metal–organic chemical vapor deposited zinc tin oxide thin films using negative bias illumination stability analysis

This study experimentally examined the physical and electrical characteristics of ZnxSnyOz (ZTO) thin films grown by a metal–organic chemical vapor deposition (MOCVD) method with various Zn/Sn atomic compositions. The corresponding defect structures of the deposited films were investigated in detail using negative bias illumination stability (NBIS) analysis in the thin film transistor (TFT) structure. The ZTO thin films were deposited at a substrate temperature of 400 °C and post-deposition annealed at 600 °C, which still resulted in the amorphous structure except for the Sn-rich film where a nano-crystalline SnO2 phase is detected. Among the films with different Zn/Sn atomic compositions, the film with a Zn/Sn atomic composition of ∼50/50 showed the best electrical performance. After applying NBIS stress for 1000 s, the transfer curves of the Zn-rich ZTO TFT showed a hump, but the transfer curves of the Sn-rich ZTO TFT exhibited a parallel shift to the negative bias direction. These phenomena were attributed to the difference in the oxygen vacancy energy states generated in the ZTO band gap by light illumination. The Zn- and Sn-related oxygen vacancies generated deep donor like trap states at ∼0.3 eV and shallow states at ∼0.1 eV from the conduction band (or mobility) edge, respectively, which were identified by the quantitative simulations of the transfer curves of the TFTs.

Effect of oxygen vacancy on the structural and electronic characteristics of crystalline Zn2SnO4

The effects of oxygen vacancies on the atomic and electronic structures in crystalline Zn2SnO4 were examined by ab initio calculations using a screened hybrid density functional. The formation energy and the electronic structure indicated that the neutral oxygen vacancy was stable in the n-type region with the formation of a deep level in the bandgap. The ionization energy of the oxygen vacancy was calculated to be around 2.0 eV, which can be excited by visible photon energy. The atomic movements and the Bader charge analyses around the neutral oxygen vacancy showed that Sn played a dominant role in changing the electronic properties by forming the Sn2+–VO0 pair, which was accompanied by a reduction of the charge state of Sn due to its multiple oxidation states. In contrast, the electronic state of Zn hardly changed, despite its more predominant atomic shifts than Sn around the oxygen vacancy.

Systematic study on bias temperature instability of various high-k gate dielectrics ; HfO 2 , HfZr x O y and ZrO 2

HfO<inf>2</inf>, HfZr<inf>x</inf>O<inf>y</inf> and ZrO<inf>2</inf> gate dielectrics are systematically compared in terms of the nMOS PBTI and pMOS NBTI. Compared to HfO<inf>2</inf>, ZrO<inf>2</inf> exhibits higher capacitance and superior nMOS mobility characteristics. In addition, as the ZrO<inf>2</inf> content increases, V<inf>th</inf> shift under the nMOS PBTI stress is dramatically reduced. This is mainly contributed to the lower density of pre-existing bulk traps related to the oxygen vacancies.

Reduction of Charge Trapping in

The dielectric performance and charge trapping properties of <formula formulatype=inline><tex Notation=TeX>

\hbox{HfO}_{2}

hbox{HfO}_{2}

Film on Ge Substrates by Atomic Layer Deposition of Various Passivating Interfacial Layers

</tex></formula> on a Ge substrate with various passivating interfacial layers (PILs), such as <formula formulatype=inline><tex Notation=TeX>

Optimized Electrical Properties and Chemical Structures of SrTiO3 Thin Films on Si Using Various Interfacial Barrier Layers

hbox{SiO}_{x}hbox{N}_{y}

Binge eating disorder and depressive symptoms among females of child-bearing age: the Korea Nurses' Health Study

</tex></formula>, <formula formulatype=inline><tex Notation=TeX>