Hosun Lee

Processing, Structure, Properties, and Applications of PZT Thin Films

There has been a resurgence of complex oxides of late owing to their ferroelectric and ferromagnetic properties. Although these properties had been recognized decades ago, the renewed interest stems from modern deposition techniques that can produce high quality materials and attractive proposed device concepts. In addition to their use on their own, the interest is building on the use of these materials in a stack also. Ferroelectrics are dielectric materials that have spontaneous polarization in certain temperature range and show nonlinear polarization–electric field dependence called a hysteresis loop. The outstanding properties of the ferroelectrics are due to non-centro-symmetric crystal structure resulting from slight distortion of the cubic perovskite structure. The ferroelectric materials are ferroelastic also in that a change in shape results in a change in the electric polarization (thus electric field) developed in the crystal and vice versa. Therefore they can be used to transform acoustic waves to electrical signal in sonar detectors and convert electric field into motion in actuators and mechanical scanners requiring fine control. In a broader sense the ferroelectric materials can be used for pyroelectric and piezoelectric sensors, voltage tunable capacitors, infrared detectors, surface acoustic wave (SAW) devices, microactuators, and nonvolatile random-access memories (NVRAMs), including the potential production of one transistor memory cells, and applications requiring nonlinear optic components. Another set of potential applications seeks to exploit the ferroelastic properties in stacked templates where they are juxtaposed to ferromagnetic materials. Doing so would allow the control of magnetic properties with electric field, which is novel. Such templates taking advantage two or more properties acquired a new name and now goes by multiferroics. After a brief historical development, this article discusses technological issues such as growth and processing, electrical and optical properties, piezo, pyro, and ferroelectric properties, degradation, measurements methods, and application of mainly lead-zirconate-titanate (PZT = PbZr1−xTixO3). The focus on PZT stems from its large electromechanical constant, large saturation polarization and large dielectric constant.

Microcrystalline silicon thin films studied using spectroscopic ellipsometry

We used spectroscopic ellipsometry to characterize four different microcrystalline silicon (μc-Si) films, which were fabricated by crystallizing a-Si:H films predeposited on glass substrates using solid phase crystallization (SPC), excimer laser annealing (ELA), Ni induced silicide-mediated crystallization (Ni-SMC), and field enhanced silicide-mediated crystallization (FESMC) method, respectively. A linear regression analysis, which took the effective dielectric function of μc-Si layer into account using effective medium approximation, showed that all these films were homogeneous throughout their thickness except the oxide overlayers, and completely crystallized regardless of the crystallization method. In our linear regression analysis, the complex dielectric function of silicon microcrystallites was represented by the Adachi model dielectric function (MDF) [T. Suzuki and S. Adachi, Jpn. J. Appl. Phys., Part1 32, 4900 (1993)], and the broadening parameters of the critical points (CPs) in MDF were allowed...

Effect of Ga∕In ratio on the optical and electrical properties of GaInZnO thin films grown on SiO2∕Si substrates

Amorphous GaInZnO and polycrystalline ZnO thin films are grown by rf magnetron sputtering. Their optical properties are investigated by spectroscopic ellipsometry. The optical gap of the GaInZnO film increases with the increase of Ga content and by annealing. These are attributed to the large band-gap energy of Ga2O3 and the structural relaxation after annealing, respectively. The changes in optical properties show a strong correlation to the device characteristics of GaInZnO thin film transistors: The turn-on voltage increases as the optical gap increases with increasing Ga∕In ratio. This study shows that the GaInZnO thin films are as excellent as transparent oxide semiconductors.

Optical properties of thermally annealed hafnium oxide and their correlation with structural change

We studied the optical properties of hafnium oxide as its structure changed. The shoulderlike feature that appears in crystalline HfO2 near the energy trough of the optical absorption edge after thermal anneal was found to have a direct correlation with the crystallization of the film. A luminescence feature near 4.1 eV could also be observed for some of the crystallized hafnium oxide films, but it did not have a direct correlation with crystallization or the shoulderlike feature. Our experimental results do not support the recently proposed self-trapped exciton (STE) model, where the shoulderlike absorption and the UV luminescence features are both attributed to STE. Simultaneous measurements of structural and optical properties of thermally annealed hafnium oxide films revealed that the shoulderlike feature is not related to defects, but rather is intrinsic to crystallized hafnium oxides. We discuss some possible origins of the shoulderlike feature based on our experimental results. Nevertheless, the UV...

Microstructure, optical property, and electronic band structure of cuprous oxide thin films

Cuprous oxide (Cu2O) thin films were grown via radio frequency sputtering deposition at various temperatures. The dielectric functions and luminescence properties of the Cu2O thin films were measured using spectroscopic ellipsometry and photoluminescence, respectively. High-energy peaks were observed in the photoluminescence spectra. Several critical points (CPs) were found using second derivative spectra of the dielectric functions and the standard critical point model. The electronic band structure and the dielectric functions were calculated using density functional theory, and the CP energies were estimated to compare with the experimental data. We identified the high-energy photoluminescence peaks to quasi-direct transitions which arose from the granular structures of the Cu2O thin films.

Spectroscopic ellipsometry and Raman study of fluorinated nanocrystalline carbon thin films

Using spectroscopic ellipsometry and Raman spectroscopy, we measured the pseudodielectric function and the phonon frequencies of fluorinated nanocrystalline carbon (nc-C:F) thin films grown on silicon substrate at varying growth temperature and gas flux ratio of CH4 and CF4. Utilizing the Tauc–Lorentzian formula, we performed multilayer analysis to estimate the dielectric function of the fluorinated nanocrystalline carbon thin films. We also adopted Gaussian-like density-of-states model proposed by Demichelis et al. [Phys. Rev. B 45, 14364 (1992)] and estimated the amplitude A, the transition energy Eπ, and the broadening σπ of π→π* transitions. Based on this model, we explained the change of the optical gap and the refractive index in terms of the change of the amplitude A rather than the shift of transition energy Eπ of π→π* transitions. Raman and ellipsometric study suggested that the average size of nanocrystallites in the fluorinated carbon thin films was smaller than that of amorphous hydrogenated c...

Optical properties of (GeTe, Sb2Te3) pseudobinary thin films studied with spectroscopic ellipsometry

The authors measure the dielectric functions of (GeTe, Sb2Te3) pseudobinary thin films by using spectroscopic ellipsometry. By using standard critical point model, they obtained the optical transition (critical point) energies of the amorphous (crystalline) thin films. The optical (indirect band) gap energies of the amorphous (crystalline) phase are estimated from the linear extrapolation of the absorption coefficients. The band structure calculations show that GeTe, Ge2Sb2Te5, and Ge1Sb2Te4 have indirect gap whereas Ge1Sb4Te7 and Sb2Te3 have direct gap. The measured indirect band gap energies match well with electronic band structure calculations.

Atomistic origins of the phase transition mechanism in Ge2Sb2Te5

The fast and reversible phase transition mechanism between crystalline and amorphous phases of Ge2Sb2Te5 has been in debate for several years. Through employing first-principles density functional theory calculations, we identify a direct structural link between the metastable crystalline and amorphous phases. The phase transition is driven by the displacement of Ge atoms along the rocksalt [111] direction from stable octahedron to high energy unstable tetrahedron sites close to the intrinsic vacancy regions, which generates a high energy intermediate phase between metastable and amorphous phases. Due to the instability of Ge at the tetrahedron sites, the Ge atoms naturally shift away from those sites, giving rise to the formation of local-ordered fourfold motifs and the long-range structural disorder. Intrinsic vacancies, which originate from Sb2Te3, lower the energy barrier for Ge displacements, and hence, their distribution plays an important role in the phase transition. The high energy intermediate c...

Highly Transparent, Low Resistance, and Cost-Efficient Nb:TiO2/Ag/Nb:TiO2 Multilayer Electrode Prepared at Room Temperature Using Black Nb:TiO2 Target

Highly transparent, low resistance, and cost-efficient Nb-doped TiO 2 (NTO)/Ag/NTO multilayer electrodes were fabricated using tilted dual-target sputtering at room temperature without in situ or postannealing processing. Due to the effective antireflection and low resistivity properties of the Ag layer sandwiched between the NTO layers, an NTO/Ag (9 nm)/NTO electrode fabricated at room temperature showed a low sheet resistance of 5.26 Ω/□, a high optical transmittance of 86%, and a figure of merit value of 42.47 × 10- 3 Ω -1 even though the NTO layer had an amorphous structure. The NTO/Ag/NTO multilayer electrode is a promising indium-free and cost-efficient transparent conducting electrode substitute for conventional Sn-doped In 2 O 3 electrodes due to the low temperature process, low cost of required elements, and stability of the NTO layer.

Effect of density of localized states on the ovonic threshold switching characteristics of the amorphous GeSe films

We investigated the effect of nitrogen (N) doping on the threshold voltage of an ovonic threshold switching device using amorphous GeSe. Using the spectroscopic ellipsometry, we found that the addition of N brought about significant changes in electronic structure of GeSe, such as the density of localized states and the band gap energy. Besides, it was observed that the characteristics of OTS devices strongly depended on the doping of N, which could be attributed to those changes in electronic structure suggesting a method to modulate the threshold voltage of the device.