Aluminium oxide passivation films by liquid phase deposition for TiO2 ultraviolet solid–liquid heterojunction photodetectors

https://doi.org/10.1016/j.surfcoat.2020.125684Get rights and content

Highlights

  • Aluminium oxide passivation layer was used to improve the properties of TiO2 ultraviolet photodetector.

  • The aluminium oxide film was prepared by liquid phase deposition.

  • Aluminium oxide passivation layer can reduce the oxygen vacancy of TiO2 film.

  • The responsivity can be improved to from 133 mA/W to 208 mA/W after the passivation layer treatment.

Abstract

In this study, an aluminum oxide passivation layer generated through liquid-phase deposition (LPD) was used to improve the properties of a TiO2 ultraviolet solid–liquid heterojunction photodetector. An aluminum oxide deposition solution was used to reduce the oxygen vacancies of TiO2. Moreover, the surface roughness and porosity of the TiO2 film improved after aluminum oxide deposition. An aluminum oxide passivation layer was deposited on a TiO2/indium-tin-oxide (ITO)/glass photodetector with the precursors of aluminum sulfate and sodium bicarbonate. The responsivity and turn-off time of the TiO2/ITO photodetector were 133 mA/W and 34.6 s, respectively. A dense and uniform film surface could be obtained after the aluminum oxide passivation layer was deposited on TiO2 to form an Al2O3–TiO2 composite film. After deposition, the responsivity and turn-off time improved to 208 mA/W and 27.3 s, respectively. Thus, the LPD passivation film is highly favorable for improving the properties of photodetectors.

Introduction

Ultraviolet photodetectors have been widely used in recent years in daily life activities, industry, and military. The commonly used ultraviolet photodetectors are fabricated using semiconductor materials such as silicon carbide (SiC) [1,2] and gallium nitride [3,4]. However, these compounds have complex manufacturing processes and contain rare metals. Because most of the working electrodes in the photodetector device are fabricated using atomic layer deposition (ALD) [5,6] or chemical vapor deposition (CVD) [7,8], expensive vacuum equipment and multiple processes are required to fabricate the p–n junction structure. Thus, the manufacturing cost is greatly increased. Therefore, in this study, a solid–liquid heterojunction (SLHJ) was utilized to create an ultraviolet photodetector with excellent spectral selectivity, high photoelectric conversion responsivity, high environmental adaptability, and low cost. The assembly process of the proposed device is simple and does not require expensive vacuum equipment. The key feature of the device is that it utilizes pure water as the electrolyte. Most of the existing photodetectors cannot measure in a high-humidity environment. However, the use of water as an electrolyte in the proposed device enables the device to detect in a high-humidity environment. TiO2, a porous metal oxide semiconductor [9], is used as the primary material for working electrodes. TiO2 has a wide bandgap (Eg = 3.0–3.2 eV) and responds to ultraviolet light with a wavelength less than 387 nm. Moreover, TiO2 has stable chemical properties in the anatase phase and is extremely suitable as an ultraviolet photodetector material. However, many studies have applied chemical passivation to improve device efficiency and quality because metal oxides have an oxygen vacancy defect [10].

Passivation layers have been used in many studies to enhance the performance of optoelectronic devices, such as field effect transistors, organic light emitting diodes, and solar cells. Aluminum oxide (Al2O3) [[11], [12], [13]] is the most common material used for creating passivation films. A passivation layer can repair the surface defects that occur at the interface between a film and semiconductor, thus effectively improving device performance. Such a layer is widely used in silicon solar cells to prevent carrier recombination, obtain high effective minority carrier lifetime (τeff), and improve photoelectric conversion efficiency. Al2O3 films are usually fabricated using vacuum process techniques, such as plasma-enhanced CVD [14], ALD [15,16], and sputtering [17]. Films fabricated using these methods have uniform thicknesses and stable electrical properties. However, the vacuum deposition technology used for film fabrication is expensive and is unsuitable for batch-type mass production, especially for devices used in large-area coating. Moreover, the use of vacuum process technology to uniformly coat porous TiO2 materials is difficult and the optimal passivation effect cannot be achieved. Therefore, an appropriate liquid-phase passivation coating technology should be developed for improving the performance of SLHJ ultraviolet photodetectors.

The liquid-phase deposition (LPD) [18,19] method was employed in this study to produce an Al2O3 passivation film. This method features low cost, high uniformity, and high adhesion capabilities, and it can be suitably used for mass production and large-area film deposition. Such a film can be uniformly coated on porous TiO2 working electrodes for greatly improving the performance of SLHJ ultraviolet photodetectors. The passivation technology of SLHJ ultraviolet photodetectors has been rarely discussed by experts. Thus, this study used Al2O3 films, which were fabricated using the LPD method, as the passivation layer for the porous TiO2 working electrode to produce a SLHJ ultraviolet photodetector. The passivation mechanism and device performance of the passivation layer are discussed in this paper.

Section snippets

Aluminum oxide preparation

The LPD system comprises a thermostatically controlled water sink that provides a precise deposition temperature of ±0.1 °C and a Teflon container containing the deposition solution. The Al2O3 film was deposited using the LPD method. For obtaining the deposition solution, 50 g aluminum sulfate (Al2(SO4)3·18H2O) and 14 g sodium bicarbonate (NaHCO3) were mixed with 50 mL deionized (DI) water. This mixture was filtered through 6- and 1-μm filter papers. After filtration, 20 mL Al(OH)3 (pH = 3.3)

Analysis of TiO2 and Al2O3–TiO2 composite films

A previous study [20] reported that the deposition rate of the LPD-Al2O3 film fabricated using LPD is linear within the pH range of 3.1–3.4. Moreover, the deposition rate increases with an increase in the pH of the growth solution. However, when the pH of the growth solution is 3.4, a large amount of Al2O3 powder is deposited on the film surface. Thus, the surface roughness of the film increases, which influences the subsequent deposition of the film. Therefore, the optimum growth solution has

Conclusions

In this study, a low-cost LPD method was used to deposit an Al2O3 film on a TiO2 working electrode of a SLHJ ultraviolet photodetector. The reaction solution had sufficient oxygen atoms during the deposition of the LPD-Al2O3 film, reducing the Ti3+ and Ti2+ defects in the TiO2 film. The Al2O3–TiO2 composite film developed a Tisingle bondOsingle bondAl bond in the intermediate oxidation state after high-temperature annealing was conducted in the N2 atmosphere, which improved the oxygen vacancy defect of the TiO2

CRediT authorship contribution statement

Jung-Jie Huang: Conceptualization, Methodology, Writing - review & editing, Project administration. Ching-Huang Lin: Supervision, Validation. Ying-Rong Ho: Investigation, Formal analysis, Writing - original draft. Yu-Han Chang: Validation, Data curation.

Declaration of competing interest

The authors declare that they have no competing interests.

Acknowledgements

The authors thank the Ministry of Science and Technology of the Republic of China, Taiwan, for financially supporting this research under Contract No. 108-3116-F-212-001-CC2 and No. 108-2221-E-212-005.

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