전기화학적 합성법을 이용한 칼코지나이드 반도체 합성과 그 응용에 관한 연구

Abstract

A chalcogenide material is a chemical compound consisting of at least one chalcogen ion that is an element in column VI of the periodic table, i.e. S, Se, and Te. Because of their wide spectrum of properties, such as electrical resistivity, energy band gap, and thermoelectric coefficient etc, the chalcogenide-based materials relate to a wide variety of existing and potential applications in electronic and optic devices, magnetics, photovoltatics, catalysis, passivation, chemical sensor, batteries, and fuel cells. In particular, high carrier mobility up to several hundred could be achieved with relatively low temperature processing compared to bulk Si-based microfabrications. This enables to be suitable for the material processing on flexible substrates. Solution-based material synthesis, such as spin coating, inkjet printing, spray coating, chemical bath deposition, and electrochemical synthesis, have attracted much attention on chalcogenide synthesis on the flexible device. Among the solution-based processes, the electrochemical synthesis is one of the promising materials processing according to following reasons; (1) precursor can be easily prepared with an aqueous type electrolyte, (2) thin-films with higher purity can be obtained from electrolyte not containing organic ligands, (3) lower consumption of materials, and (4) more safe process without using toxic chemicals, e.g. hydrazine. In this thesis, chalcogenide materials, such as ZnTe, ZnSe, CdSe, CdTe, CuIn(Se,S)2 were synthesized by the electrochemical deposition. This work started with designing electrolyte that contains metal ions, complex agent and supporting electrolyte. The specific experimental conditions such as applied potential, pH and bath temperature were also adjusted for obtaining chalcogenide materials with the stoichiometric composition. The objective of this work can be classified into two major categories. First, the experimental set-up and the mechanism of electrodeposition for the chalcogenides materials were focused to achieve high electronic qualities competitive to vacuum-based deposited thin films. The other one is to apply the chalcogenide materials electrochemically synthesized in this work for advanced electronic devices, such as thin-film transistors, chemical sensors and solar cells. First, the electrodeposition mechanism of ZnTe was investigated by cyclicvoltammetry test, resulting that Zn was underpotentially deposited on pre-deposited Te. This result can be attributed to the negative free energy of ZnTe formation. A potential range of −0.6 to −1 V vs. Ag/AgCl was determined for the formation of the stoichiometric ZnTe compound. The electrodeposited ZnTe thin film was polycrystalline phase strongly preferred to (111) direction observed by XRD and XPS. The electrical properties of the ZnTe thin film were investigated with doping of Cu. The amount of doped Cu proportionally increased with increasing concentration of the Cu precursor, and Cu excess was associated with Zn deficiency. A significant increase in carrier mobility (~1 cm2/Vs) and concentration (~1018) compared to undoped ZnTe thin films was achieved. Second, a novel electrochemical synthesis for the fabrication of a ZnTe thin film-based transistor was introduced, so-called lateral electrodeposition growth. This is an interesting phenomenon, in which, the lateral deposition rate of ZnTe along the surface of SiO2 starting from the metal substrate is much higher than that on the surface of the metal electrode. As result of this phenomenon, a channel of ZnTe thin film can be simply formed between two metal electrodes resulting in the structure of a thin-film transistor. A mechanism of the lateral electrodeposition growth is based on the adsorption of Te ions on the surface of SiO2 that is believed to introduce difference in reduction rate between on the metal surface and on the SiO2 surface: diffusion-controlled reaction on the metal surface and charge transfer reaction on the SiO2 surface verified by OCP, CV, and EIS. The ZnTe thin-film transistor in this study presented a field-effect mobility of 11.8 cm2/Vs, a threshold voltage of 20 V, and an on/off ratio of 1.27 x 104. Also, this device was applied for room-temperature operating NO2 sensor, showing 0.4 ppm detection level and fast response/recovery time. Third, the compact and uniform chalcogenide ZnSe thin films were electrochemically synthesized in an aqueous electrolyte containing ZnSO4, H2SeO3, and citrate. The pH of the electrolyte has important role to determine exiting form of Se related ions. Specifically, SeO32- ions that are dominant above pH 6.8 can drive the underpotential deposition of Zn, resulting in the formation of the stoichiometric ZnSe compound. The relatively low standard reduction potential of SeO32- is believed to introduce a small window for difference in the reduction potential between Se and Zn that enables to drive the UPD of Zn ions. While as-deposited ZnSe thin films had low crystallinity, the polycrystalline phase strongly preferred to (111) direction was obtained after annealing at 400 oC. Fourth, the fabrication and properties of CdSe/CdTe thin film photovoltaic devices with a dual back contact geometry were performed. Device fabrication involves CdSe electrodeposition on one of two interdigitated electrodes on a pre-patterned substrate followed by CdTe electrodeposition over the entire structure so that both electrodes are behind the active portion of the device. In contrast to traditional planar devices, illumination is on the electrode-free CdTe surface rather than through a window layer. Like previously detailed back-contact devices for other materials systems, all light that impinges on the device thus reaches the CdTe absorber, the surface being free of metallic electrodes, transparent conductors and window layers. Device efficiency of 2 % under simulated air mass 1.5 illumination for feature spacing of 2 μm is similar to that of three-dimensionally patterned CdS/CdTe devices detailed by other groups with similar feature height but much smaller 0.5 μm spacing. Fifth, the properties of thin film solar cells based on electrodeposited CuIn(Se,S)2 were investigated. The proposed solar cell fabrication method involves a single-step CuInSe2 thin film electrodeposition followed by sulfurization in a tube furnace to form a CuIn(Se,S)2 quaternary phase. A sulfurization temperature of 450 – 550 °C significantly affected the performance of the CuIn(Se,S)2 thin film solar cell in addition to its composition, grain size and bandgap. Sulfur(S) substituted for selenium(Se) at increasing rates with higher sulfurization temperature, which resulted in an increase in overall bandgap of the CuIn(Se,S)2 thin film. The highest conversion efficiency of 3.12 % under air mass (AM) 1.5 illumination was obtained from the 500 °C-sulfurized solar cell. The highest External Quantum Efficiency (EQE) was also observed at the sulfurization temperature of 500 °C.