Shibghatullah Muhammady

Polarity enhancement in high oriented ZnO films on Si (100) substrate

Zinc oxide films with hexagonal crystal structures have been grown on Si substrate (100) using the DC-unbalanced magnetron sputtering at temperature of 300°C with growth time variation. The films have been characterized using X-Ray Diffraction and Fourier Transform Infrared measurement to show the crystal parameter and atomic bonding. The results show that the films have a dominant orientation in the hkl plane (002), while peak positions are shift to lower diffraction angle with addition of growth time. In addition, the bonding between Zn and O atoms (Zn-O) tend to be weaker as indicated by transmittance peak shifting to lower energy. The weakening of Zn-O bonding is due to the contribution of addition of bond length. These conditions make the films tend to have a high polarity. Further analysis of these studies will bring us to have a good undestanding to explain the ferroelectric properties of the ZnO films.

Effect of Ta concentration on the refractive index of TiO2:Ta studied by spectroscopic ellipsometry

We have investigated optical properties of Ta-doped TiO2 thin film on LaAlO3 (LAO) substrate using Spectroscopic Ellipsometry (SE) at room temperature. Amplitude ratio Ψ and phase difference L1 between p- and s- polarized light waves are obtained by multiple incident angles measurement (60°, 70°, and 80°) at energy range of 0.5 – 6.5 eV. In order to obtain optical properties for every Ta concentrations (0.01, 0.4, and 5 at. %), multilayer modelling was performed simultaneously by using Drude-Lorentz model. Refractive index and optical dispersion parameters were determined by Wemple-DiDomenico relation. In general, refractive index at zero photon energy n(0) increases by increasing Ta concentration. Furthermore, optical band gap shows a significant increasing due to presence of Ta dopant. In addition, other optical constants are discussed as well.

Electrical properties of ZnO-based photodetector prepared by room temperature DC unbalanced magnetron sputtering

We study electrical properties of ZnO thin films on p-Si (100) substrates as ultraviolet (UV) photodetector. ZnO films with thickness of ~400nm were deposited by room temperature (RT) DC unbalanced magnetron sputtering (DC-UBMS), and we also used ZnO film deposited at 260°C for the comparison. Metal-semiconductor-metal (MSM) planar structure was fabricated by using silver (Ag) contact on ZnO thin films as photodetector. X-ray diffraction (XRD) spectroscopy shows the amorphous structure of RT growth of ZnO thin films in contrast to relatively good crystallinity of ZnO film grown at 260°C. I-V characteristics of reverse-forward bias of UV photodetector were taken in dark and light conditions. As the results, amorphous phase of RT ZnO shows the high value of dark current and photocurrent. High stability of photo detection is also observable over the specific time as investigated by switching condition. In addition, we found that the mechanism of photo detection is strongly related to the oxygen adsorption on the ZnO surface. This study provides an alternative way to have a simple and high sensitive UV photodetector.

Contribution of on-site Coulomb repulsion energy to structural, electronic and magnetic properties of SrCoO3 for different space groups: first-principles study

Abstract We report structural, electronic, and magnetic properties of SrCoO3 in Pm3̅m and P4/mbm space groups, which are calculated by using generalized gradient approximation corrected with on-site Coulomb repulsion U and exchange energies J. The cubic lattice parameter a and local magnetic moments of Co (μCo) are optimized by varying U at Co 3d site. Employing ultrasoft pseudopotential, the values of U = 8 eV and J = 0.75 eV are the best choice for Pm3̅m space group. We found the value of μCo = 2.56 μB, which is consistent with the previous results. It was also found that Co 3d, hybridized with O 2p, is the main contributor to ferromagnetic metallic properties. Besides, norm-conserving pseudopotential promotes a, which is in good agreement with experimental result. However, it is not suitable for P4/mbm space group. By using ultrasoft pseudopotential, the value of U = 3 eV (J = 0.75) is the most suitable for P4/mbm group. Ferromagnetic metallic properties, Jahn-Teller distortion, and reasonable lattice parameters have been obtained. This study shows that U has significant contribution to the calculated properties and also points out that P4/mbm space group with US-PP is suitable to describe experimental results.

Electronic and Thermoelectric Properties of Layered Oxychalcogenides (BiO)CuCh (Ch = S, Se, Te)

We study the new details of electronic and thermoelectric properties of polycrystalline layered oxychalcogenide systems of (BiO)Cu Ch ( Ch = Se, Te) prepared by using a solid-state reaction. The systems were characterized by using photoemission (PE) spectroscopy and four-probe temperature-dependent electrical resistivity ρ( T). PE spectra are explained by calculating the electronic properties using the generalized-gradient approximation method. PE spectra and ρ( T) show that (BiO)CuSe system is a semiconductor, while (BiO)CuTe system exhibits the metallic behavior that induces the high thermoelectric performance. The calculation of electronic properties of (BiO)Cu Ch ( Ch = S, Se, Te) confirms that the metallic behavior of (BiO)CuTe system is mainly induced by Te 5p states at Fermi energy level, while the indirect bandgaps of 0.68 and 0.40 eV are obtained for (BiO)CuS and (BiO)CuSe systems, respectively. It is also shown that the local symmetry distortion at Cu site strongly stimulates Cu 3d-t2g to be partially hybridized with Ch p orbitals. This study presents the essential properties of the inorganic systems for novel functional device applications.

The enhancement of ferromagnetism in Ta-doped anatase TiO2 system by iron co-doping

The ferromagnetic properties of Ta-doped and (Ta,Fe)-doped anatase TiO2 as diluted ferromagnetic materials has been studied within spin-polarized generalized gradient approximation (GGA) and GGA + U method. We observed a ferromagnetic properties in Ta- doped anatase TiO2, with a total magnetic moment of 1.00 μ B /supercell, which mainly arised due to Ti 3d and O 2p states upon Ef. Furthermore, the enhancement of ferromagnetism, mainly arising due to Fe 3d states, in (Ta,Fe)-doped anatase TiO2 was observed up to 5.00 μB /supercell, 5 times larger than that in Ta-doped TiO2. Our results confirmed the important role of Fe doping for the ferromagnetism enhancement in Ta-doped anatase TiO2.