Leejun Kim

Large nonlinear dielectric properties of artificial BaTiO3/SrTiO3 superlattices

BaTiO3(BTO)/SrTiO3(STO) artificial superlattices have been made on MgO (100) substrates. The periodicity of the BTO/STO layers in the superlattice was varied from one-unit cell to 125-unit cell thickness. The dielectric constant and its nonlinearity (or voltage tunability) showed similar behavior as the periodicity was varied. The voltage tunability of the superlattice increased with decreasing stacking periodicity of the BTO/STO within the critical thickness. Similarly, the lattice distortion, i.e., the ratio of the lattice parameter along surface normal to parallel, of the BTO and STO layers increased with decreasing the periodicity. Remarkable enhancement of the voltage tunability has been achieved. The superlattice exhibited large voltage tunability (94%, the highest value to date) at the periodicity of BTO2-unit cell/STO2-unit cell at which the maximum lattice distortion of each layer was obtained. This suggests that the nonlinear dielectric property of the superlattice is closely related with the la...

Strain manipulation in BaTiO3/SrTiO3 artificial lattice toward high dielectric constant and its nonlinearity

Enhancement of dielectric properties has been demonstrated in BaTiO3 (BTO)/SrTiO3 (STO) strained artificial lattice. Large variation of lattice distortion in the consisting BTO and STO layers was achieved by varying a stacking sequence of BTO and STO layers. From this strain manipulation, it is found that BTO and STO lattices have a dielectric constant sensitively influenced by the lattice distortion and, more importantly, maximum dielectric constant at a certain degree of lattice distortion. An appropriate degree of lattice distortions of the consisting layers is needed to obtain the large dielectric constant and its nonlinearity of the artificial lattice. The artificial lattice exhibited the large dielectric constant (1230) and extremely large nonlinearity (94%) at the periodicity of BTO2 unit cell/STO2 unit cell. These results suggest that the strain is a macroscopically important factor to influence the dielectric properties and can be manipulated via oxide artificial lattice to obtain large dielectri...

Effects of deposition parameters on the crystallinity of CeO2 thin films deposited on Si(100) substrates by r.f.-magnetron sputtering

Deposition temperature, r.f.-power and seed layer deposition time were important parameters effecting the crystallinity of CeO2 thin films deposited by r.f.-magnetron sputtering on Si(100) substrates. The CeO2 (200) peak was notable for a deposition temperature above 600°C. With decreased r.f.-power and thus lower deposition rate, the intensity of the CeO2(200) peak increased. When the seed layer deposition time was less than 20 s, the CeO2(200) peak dominated. Transmission electron microscopy (TEM) diffraction revealed that the deposited CeO2 thin film had a polycrystalline structure. Annealing at 950°C in O2 atmosphere for 30 min increased and sharpened the CeO2(200) peak.

Controllable capacitance–voltage hysteresis width in the aluminum–cerium-dioxide–silicon metal–insulator–semiconductor structure: Application to nonvolatile memory devices without ferroelectrics

The Al/CeO2/Si metal–insulator–semiconductor (MIS) structure showed a capacitance–voltage (C–V) hysteresis, which could be controlled by variation of the CeO2 thickness. For a sample with 3000 A CeO2, hysteresis width as high as ∼1.8 V was obtained. For nonvolatile field-effect transistors, the Al/CeO2/Si MIS structure with a reliable and controllable C–V hysteresis could be an alternative to metal–ferroelectric–semiconductor structures containing unstable, multicomponent ferroelectric materials.

Polarization of strained BaTiO3∕SrTiO3 artificial superlattice: First-principles study

We performed first-principles calculation to investigate the effect of epitaxial strain on lattice instabilities and polarization behavior of BaTiO3∕SrTiO3 artificial lattice with very short stacking period, i.e., BaTiO31unitcell∕SrTiO31unitcell(BTO∕STO). The structural analysis of BTO∕STO artificial superlattice under in-plane compressive state showed enhanced stability of the tetragonal phase. On the other hand, the stability of monoclinic phase was enhanced when the BTO∕STO was in the in-plane tensile state. The phase transition from tetragonal to the monoclinic phase occurs at the misfit strain of −0.25%. As the misfit strain of BTO∕STO superlattice increases from −0.25%to−1.5% (in-plane compressive state), the tetragonal superlattice exhibits an increasing polarization along the [001] direction. In the monoclinic phase, the polarization of the superlattice rotates progressively toward [110] direction with increasing the misfit strain, and the magnitude of the polarization simultaneously increases wit...

Dielectric properties of BaTiO3∕SrTiO3 ferroelectric thin film artificial lattice

Dielectric behavior on BaTiO3∕SrTiO3 artificial lattices has been investigated along with quantum mechanical simulation (first principles calculation). From the oxide artificial lattice approach, strain manipulation was performed to obtain a wide range of lattice deformation in the consisting BaTiO3 and SrTiO3 layers, which leads to two important consequences. First, we obtained enhanced dielectric constant and extremely large nonlinearity in the artificial lattices with very short stacking periods. Second, it is found that there exists a maximum dielectric constant in each BaTiO3 lattice and SrTiO3 lattice at a certain degree of lattice deformation. The first principles study successfully explains the dielectric behavior of strained BaTiO3 and SrTiO3 lattices, the existence of the maximum dielectric constant. The first principles study on BaTiO3∕SrTiO3 artificial lattices with very short stacking periods also reveals that the artificial lattice undergoes phase transition between the tetragonal and monocl...

Optical phonon softening in strained SrTiO3 thin film: First-principles study

The dielectric response and optical phonon of hydrostatically and anisotropically strained SrTiO3 lattices were studied using density functional theory. A structural analysis was performed on the strained SrTiO3 lattice. Following the structural analysis, the optical phonon frequency was calculated for a wide range of lattice distortion. The dielectric constant was calculated from the optical phonon frequency and the Born effective charges. It was found that significant optical phonon softening occurs at a certain degree of lattice distortion and a certain amount of volume change. In other words, the optical phonon frequency decreased and reached a minimum (50cm−1) at a specific lattice distortion (c∕a=0.985) as the SrTiO3 lattice was progressively distorted starting from the unstrained state. The phonon frequency increased as the lattice distortion was further increased. This phonon behavior, i.e., optical phonon softening, leads to a maximum dielectric constant at a certain degree of lattice distortion....

Thickness dependent dielectric property of BaTiO3/SrTiO3 artificial lattice

BaTiO3 (BTO)/SrTiO3 (STO) artificial lattices have been fabricated on (La,Sr)CoO3-coated MgO(100) single crystal substrate by pulsed laser deposition (PLD). Specifically, the artificial lattice with a stacking period of two-unit-cell-thick BTO and STO layers (BTO2/STO2) was investigated for the thickness-dependent dielectric property in the range of thickness from 100 nm to 10 nm. The structural analysis of the artificial lattice was carried out by synchrotron X-ray scattering. In the BTO2/STO2 artificial lattice with various thickness, the strained BTO and STO layers maintained the lattice coherence. The BTO layer in the artificial lattice was in a biaxial compressive stress, while the STO layer was in a biaxial tensile stress. As the thickness of the artificial lattice decreased, the BTO lattice became severely distorted while the lattice distortion of the STO lattice remained relatively unchanged. The corresponding dielectric constant of the artificial lattice decreased with decreasing thickness of the artificial lattice, although the dielectric constant of the artificial lattice was larger than other single layers such as STO, BTO, and BST layers with the same thickness. The finite-size effect was still observed in the artificial lattice with thickness down to 10 nm.

Dielectric Behavior of BaTiO3/SrTiO3 Oxide Artificial Lattice by Strain Manipulation

BaTiO3 (BTO)/SrTiO3 (STO) artificial lattices have been deposited on (La,Sr)CoO3/MgO (100) substrate by pulsed laser deposition (PLD). Strain manipulation in the oxide artificial lattice was carried out by varying stacking sequence and period of BTO and STO layers with layer-by-layer growth technique. A wide variation of the lattice distortion for both BTO and STO layers has been obtained from this strain manipulation. The dielectric constant of the BTO and STO lattices was sensitively influenced by the lattice distortion. Moreover, it is found that there exist a certain degree of lattice distortion of the strained BTO and STO lattices leading to a maximum value of their own dielectric constant of the BTO and STO lattices. Consequently, an appropriate combination of stacking period and sequence (resulting in a specific lattice distortion) led to the large dielectric constant (1230) and extremely large tunability (94%) in the BTO/STO oxide artificial lattice.

Growth of SrTiO3/(Sr1 - x,Lax)TiO3 Superlattices and Lattice Strain Development

The SrTiO3/(Sr1−x,Lax)TiO3 (STO/SLTO) superlattices have been grown on stepped-SrTiO3 substrate using laser-molecular beam epitaxy. The layer-by-layer growth was in-situ monitored by the intensity oscillation of specular spot in reflection high energy electron diffraction and also confirmed by atomic force microscopy image. A wide range of the lattice distortion of La modified SrTiO3 (SLTO) layer was obtained in the STO/SLTO superlattices by varying the stacking sequence of STO and SLTO layers. The lattice strain of the SLTO layer was in the range of 1.008 to 1.021. We have fabricated successfully STO/SLTO superlattices, in which La defect is distributed in a regular manner of one dimension and the La containing layer experiences the wide range of the lattice distortion.