ICMCTF 2023 Session C1-2-WeA: Optical Materials and Thin Films II

Recently, ternary perovskite oxides have attracted great attention as alternative transparent conducting oxides (TCOs) because their structures are compatible with many other perovskite oxides that allow devices to be fabricated comprised entirely of perovskite oxides. Among these perovskite oxides, BaSnO₃ has gained considerable attention as a promising TCO because of its high mobility at room temperature (~320 cm²V^-1s^-1 in bulk single crystals and ~100 cm²V^-1s^-1 in epitaxial thin films) and high temperature stability in oxygen atmospheres compared to other TCOs, such as Sn-doped In₂O₃, Al-doped ZnO, and F-doped SnO₂. We have grown epitaxial La-doped BaSnO₃ (LBSO) thin films on (001) SrTiO₃ and (001) MgO substrates by pulsed laser deposition and investigated their structural, electrical, and optical properties as a function of the oxygen pressure and substrate temperature during deposition. By adjusting the oxygen pressure and substrate temperature during deposition, we were able to control the film crystallinity and strain, which modified the electrical and optical properties. The LBSO films grown at the optimum conditions (780 °C and 100 mTorr of oxygen) show the highest conductivity (3.6 x 10³ S cm^-1) with a carrier concentration of 3.5 x 10²⁰ cm^-3 and a carrier mobility of 65 cm²V^-1s^-1. This observed high conductivity corresponds to the film with the best crystallinity and the lowest strain state. The permittivity of the LBSO films can also be modified as a function of the oxygen pressure and temperature during deposition allowing tuning of their epsilon-near-zero (ENZ) wavelength from 2 µm to 6 µm. We will present details of the deposition conditions on the properties of LBSO films and the ability to tune the permittivity in infrared range.

This work was supported by the Office of Naval Research (ONR) through the Naval Research Laboratory basic research program.

Zinc oxide (ZnO) with a wide bandgap energy of ~3.37 eV is expected to be applied to ultraviolet (UV) detectors. In general, ZnO exhibits n-type conduction because unintentionally doped native defects and/or residual hydrogen (H) atoms act as donors. On the other hand, it is difficult to obtain p-type conduction with good reproducibility by intentional impurity doping. Therefore, we have fabricated the UV detectors composed of the heterojunctions between the ZnO nanorods (NRs) and poly(3,4-ethylenedioxythiophne) poly(styrenesulfonate) (PEDOT:PSS) instead of those of ZnO pn homojunctions. In this paper, generation mechanisms of both hysteresises on the voltage-current (V-I) curves and optical responses of the PEDOT:PSS/ZnO NRs/ZnO:Ga (GZO) heterojunctions will be discussed.

The GZO seed layers were deposited on alkali-free glass substrates by ion-plating (IP) with a DC arc discharge using a sintered ZnO pellet containing Ga₂O₃ powder of 4.0 wt.%. Preparations of ZnO NRs layers were done by chemical bath deposition (CBD) using the mixed aqueous solution of zinc nitrate hexahydrate and hexamethylenetetramine. The PEDOT:PSS layer was spin-coated on the surface of the ZnO NRs layer at 3000 rpm for 30 s, followed by thermal annealing in air at 80 °C.

The V-I curves of the PEDOT:PSS/ZnO NRs/GZO heterojunctions exhibited a rectification behavior with hysteresis loops both in forward and reverse voltage regions. Under the irradiation of the ultraviolet (UV) light of 360 nm, the hysteresis loop area in the forward voltage region decreased, but that in the reverse voltage region increased. Both the ln V vs. ln I plots and Fowler-Nordheim (F-N) plots, 1/V vs. ln (I/V²) plots, for the voltage increase in the forward voltage region in a dark state can be clearly divided into three characteristic regions. The V-I curve showed an ohmic characteristic in the low voltage region, whereas the current was approximately proportional to the fourth power of the forward voltage in the high voltage region. Therefore, the possible transport mechanisms in the low and high voltage regions are direct tunneling and F-N tunneling, respectively. In the medium forward voltages, the current was approximately proportional to the square of the forward voltage, which is characteristic of space-charge-limited conduction. The increase in repetition number of V-I measurement under the forward voltage (0→3→0 V) in a dark state led to the increase in maximum forward current.

This work was supported by JSPS KAKENHI Grant Number JP22K04220.

XPS depth profiling is a widely employed analytical technique to determine the chemical composition of thin films, coatings and multi-layered structures, due to its ease of quantification, good sensitivity and chemical state information. Since the introduction of XPS as a surface analytical technique more than 50 years ago, depth profiles have been performed using ion beam sputtering. However, many organic and inorganic materials suffer from ion beam damage, resulting in incorrect chemical compositions to be recorded during the depth profile. This problem has been resolved for most polymers through the use of argon gas cluster ion beams (GCIBs), but the use of GCIBs does not solve the issue for inorganics. A prototype XPS depth profiling instrument has been constructed which employs a femtosecond laser rather than an ion beam for XPS depth profiling purposes. This novel technique has shown the capability of eradicating chemical damage during XPS depth profiling for all initial inorganic, compound semiconductor and organic materials examined. The technique is also capable of profiling to much greater depths (10s - 100s microns) and is much faster than sputter XPS sputter depth profiling. FESLA XPS results will be shown for selected bulk, thin film and multi-layered materials employed in optical and electrical applications.