Sang-Ho Yun

Growth of HgBa2Ca2Cu3O8+δ thin films on LaAlO3 substrates using fast temperature ramping Hg‐vapor annealing

The fast temperature ramping Hg‐vapor annealing (FTRA) process has been used for growth of superconducting Hg‐based cuprate thin films on (100) LaAlO3 substrates. The film/substrate interface chemical reactions and the formation of a CaHgO2 impurity phase have been effectively reduced with adoption of the FTRA process. A zero‐resistance superconducting transition temperature of 128 K and critical current densities of up to 1.4×106 A/cm2 at 77 K and 2.5×105 A/cm2 at 110 K and zero field have been obtained.

Epitaxial growth of HgBa2CaCu2O6+δ thin films on SrTiO3 substrates

Abstract High quality c -axis oriented HgBa 2 CaCu 2 O 6+δ thin films have been grown epitaxially on SrTiO 3 substrates with the a -axis of the film aligned with the (100)-axis of the substrates. The film-substrate interface quality can be greatly improved by using a fast temperature ramping Hg-vapor annealing process (FTRA). Zero-resistance T c values above 120 K are routinely obtained for FTRA processed HgBa 2 CaCu 2 O 6+δ thin films.

Fabrication and characterization of boron-related nanowires

Abstract A thermal vapor transport process has been employed for fabrication of boron-related (boron, boron–silicon alloys, and MgB2) nanowire films on Au-coated Si and MgO substrates. Tangled polycrystalline as well as aligned single-crystalline boron nanowires (BNWs) have been obtained and their growth mechanism investigated at different growth temperatures, cooling rates, and vapor sources. The growth temperature was found critical to the nucleation of the BNWs and different growth modes were observed in different temperature ranges. The temperature ramping rate in the cooling process after the high-temperature growth of the BNWs was found crucial for the formation of crystalline structures and for controlling the alignment of BNWs. We have observed that slow cooling at 1–5 °C/min resulted in non-textured BNWs, while fast cooling at ∼90 °C/min induced crystallization of the non-textured BNWs. We have also found that the alignment of the BNWs depended on the cooling rate. At a slow cooling rate the BNWs were heavily tangled while at a higher cooling rate they aligned well with the normal of the substrate. By manipulating the growth parameters, we have obtained two types of nanowire junctions. One was via fusing two nanowires together and the other, via joining them with another material.

Microstructures of mercury-based cuprate thin films

Abstract The microstructures of Hg-based cuprate thin films have been studied using a scanning electron microscope and an energy dispersive X-ray spectrometer. Besides a rough surface, several types of defects including the HgCaO 2 phase are identified on those films made using a slow temperature ramping Hg-vapor annealing process. By ramping the sample temperature rapidly to the annealing temperature, the surface morphology of the film is significantly improved and the impurity phases, especially the HgCaO 2 phase, are effectively reduced.

Magnetic flux pinning enhancement in HgBa2CaCu2O6+δ films on vicinal substrates

High-temperature superconducting HgBa2CaCu2O6+δ films were fabricated on the vicinal surfaces of 4°-miscut SrTiO3 single crystal substrates in a cation-exchange process, with the purpose to induce additional growth defects via a step-flow growth mode on the miscut substrates. The critical current densities (Jc) of these films were measured using a standard four-probe method with magnetic fields applied perpendicular to the film surface. The vicinal Hg-1212 films indeed showed higher Jc’s in magnetic fields and irreversibility fields (Hirr) than that on 0°-cutSrTiO3. Hirr at 77K is 2.1T for the film grown 0°-cut SrTiO3, and is enhanced to 2.7T for the one grown on 4°-miscut SrTiO3. If optimized so that the shape and the density of the defects can be controlled experimentally, this may be a promising way of improve magnetic flux pinning for many practical applications related to coated conductors.

Effect of in-plane and out-of-plane misorientation on the ferroelectric properties of thin film ferroelectric PZT infrared sensors on Si substrates

Abstract Pb(Zr,Ti)O 3 (PZT) , a promising material for uncooled infrared detection, is an anisotropic perovskite with the best pyroelectric effect observed along the c-axis. Although c-axis orientated PZT films can be easily obtained on single crystal substrates with minimal lattice mismatch, it remains a challenge in practical cases when they must be grown on non-textured polymer based sacrificial coatings over Si substrates. To address this issue, we have been focused on development of thin textured MgO templates on non-textured substrates, such as amorphous SiO 2 /Si and polymer coated SiO 2 /Si, using an ion-beam-assisted deposition (IBAD) technique. C-axis-oriented multi-layered LaNiO 3 /Pb(Zr,Ti)O 3 /LaNiO 3 have been achieved and the ferroelectric properties, that impact the figure of merit for IR sensors, have been characterized. Keywords: ferroelectric, ion beam assisted deposition, texturing template, and infrared detectors. 1. Introduction Infrared (IR) detection has been extensively investigated ever since the discovery of IR radiation about two hundred years ago. It has wide usage in both commercial and military applications. The IR spectrum can be divided into short-wave IR (SWIR) (1 to 3 µm), medium-wave IR (MWIR) (3 to 5 µm), long-wave IR (LWIR) (8 to 12 µm), and very-long-wave IR (VLWIR) (> 12 µm). Besides many civilian related applications, large format IR focal plane arrays (IRFPAs) at 12-18 µm are very useful in detecting cold objects such as ballistic missiles in midcourse. VLWIR sensors are very important in strategic missile defenses and space applications. Such needs have been the driving force for research and development of IRFPAs in the LWIR and VLWIR spectra. There are two classes of IR detectors: cooled and uncooled based on different mechanisms of IR detection. Most cooled IR detectors are photodetectors so the materials used for these detectors have a semiconductor band gap that falls into the infrared range. The uncooled IR detectors, on the other hand, employ quite different mechanisms from their cooled counterparts and most uncooled IR detectors studied so far are either bolometers or ferroelectric/pyroelectric. A ferroelectric infrared (IR) detector, such as the thin film ferroelectric (TFFE) device, has a multilayer structure including layers for top and bottom electrodes, ferroelectric, mechanical supporting, and sacrificial-that will be removed later to form a suspended structure [1]. The quality of the ferroelectric film, such as phase purity, is critical to the overall performance of the TFFE IR sensors. Among several others, Pb(Zr,Ti)O

Fast temperature ramping Hg-vapor annealing process for fabrication of Hg-based superconducting thin films

Growth mechanism of superconducting Hg-based cuprate thin films has been studied by changing the sample heating rate in Hg-vapor annealing process. A fast temperature ramping Hg-vapor annealing (FTRA) process has been found to be beneficial to the growth of high quality c-axis oriented epitaxial HgBa/sub 2/CaCu/sub 2/O/sub 6+/spl delta// films with zero-resistance T/sub c/ up to 124 K and HgBa/sub 2/Ca/sub 2/Cu/sub 3/ O/sub 8+/spl delta// films with zero-resistance T/sub c/ up to 130 K on SrTiO/sub 3/ and LaAlO/sub 3/ substrates. The high J/sub c/s carried by these films makes them promising for many applications.

Systematic investigation of the growth of LaNiO3/PZT/LaNiO3/Si and LaNiO3/PZT/LaNiO3/polymer/Si for IR detector applications

We have been focused on growth of multi-layered LaNiO3/Pb(Zr,Ti)O3/LaNiO3 on bare Si and polymer-coated Si substrates for infrared detector applications. A unique ion-beam assisted pulsed laser deposition (IBAD-PLD) has been employed to address two critical issues related to these thin film ferroelectric (TFFE) devices: to reduce the thermal budget and to enhance the texture of the devices. IBAD has been a well-known technique for deposition of thin films due to the ability to control morphology, adhesion, texture, and stoichiometry of the film by providing extra kinetic energies to, and to generate desired textures in films by preferential sputtering of the growing surface of the film. We have studied the role of several processing parameters of IBAD-PLD process, including ion-beam energy, current density, IBAD time, and substrate temperature in order to identify the best processing window for LaNiO3/Pb(Zr,Ti)O3/LaNiO3.