Ultraviolet photodetectors based on ZnO nanoparticles

Abstract

Ultraviolet (UV) photodetectors based on ZnO nanoparticles (NPs) were fabricated and their optoelectronic properties were examined. The dominant photoluminescence (PL) peak of the ZnO NPs was located at a wavelength of 380 nm under the illumination of 325-nm wavelength light. The direct bandgap transition of the charge carriers at λ = 380 nm contributed to the photocurrent. The ratio of the photocurrent to the dark current (on/off ratio) was as high as 10⁶, which is favorable for photodetectors. The decay time constant in the photoresponse was relatively small, while the rise time constant was relatively large. The reasons for the high on/off ratio and photoresponse characteristics are discussed in this paper.

Introduction

ZnO is one of the n-type semiconducting oxide materials which is widely utilized in the fields of transparent conducting electrodes, varistors, gas sensors and optoelectronic devices [1]. Especially, this oxide material has been extensively studied for its application to the fabrication of ultraviolet (UV) photodetectors, due to its large direct bandgap (∼3.3 eV) [2], [3], [4]. Photoconduction in ZnO is primarily governed by the adsorption and desorption of oxidizing molecules. In the absence of UV light, oxidizing molecules adsorbed on the surfaces of the oxide materials create electron-depleted layers, which reduce their conductance. When UV irradiation is present, the photogenerated holes move to their surfaces and neutralize the negatively charged oxidizing molecules. This photodesorption process makes their surface conductivity increase significantly. When the UV light is turned off again, the adsorption of oxidizing molecules occurs and the conductivity is reduced again.

Photodetectors with channels made of various types of ZnO have been widely investigated [2], [5], [6], [7], [8], [9]. Nevertheless, there have been few studies on photodetectors based on zero-dimensional (0-D) ZnO nanoparticles (NPs). The structure of 0-D NPs endows them with better light absorption efficiency than other dimensional structures. Differing from one-dimensional nanowires and two-dimensional quantum well layers, the light absorption characteristics of 0-D NPs are independent of the light polarization [10]. Their superior light absorption characteristics are caused by the enhancement of the excitonic oscillator strength, due to the quantum size confinement effect [11], [12], as well as by the concentration of the densities of states near the conduction and valence band edges. When the channel layers of photodetectors are made of NPs, however, some problems related to surface defects still remain. The deep-traps present at the NP surfaces cause rise time delay when the light is illuminated. The chance of irradiative recombination between electrons and holes before charge conduction increases due to these surface defects. Also, the weak resistance of the NPs to chemical substances is another possible problem. In spite of these difficulties related to the surface defects of NPs, a variety of devices based on NPs have been reported [13], [14], [15], [16]. One of the most important reasons for developing NP-based devices is that, in addition to the merits originating from their 0-D structure, NPs have the characteristic of solution-processability. In other words, NP-based devices can be fabricated through solution processes. The solution-processability of NPs has some advantages in terms of the ease of fabrication, physical flexibility, large area, and most importantly, low cost.

The basic hypothesis of this study is as follows: if the channel of the photodetector is composed of NPs, the dark current may be dramatically reduced, due to the numerous junction barriers formed between them. Also, if the number of photogenerated charge carriers significantly increases, the number of charge carriers which jump over the barrier will increase, resulting in an enhancement of the photocurrent and, consequently, in an increase in the ratio of the photocurrent to dark current (the so-called on/off ratio). Since NPs have excellent absorption efficiency, the number of photogenerated charge carriers is maximized and, therefore, the above scenario can be realized. Based on this simple idea, UV photodetectors were fabricated using ZnO NPs as channel materials through the simple painting of the NPs across the pre-patterned electrodes, and their optoelectronic properties were investigated. ZnO NPs with an average size of about 70 nm were used in this work as the channel material, because the exciton Bohr radius of ZnO is estimated to be about 5–50 nm (or 10–100 nm in diameter) [11]. If the NPs are bigger than the exciton Bohr radius, the light is not efficiently absorbed, due to the weak quantum confinement effect, and if the NPs are too small, the transportation of charge carriers through the channel may be difficult, due to the increased scattering probability of the charge carriers at the boundaries of the NPs. Therefore, we chose ZnO NPs with an average diameter of 70 nm, which is in the range of the Bohr radius and not too small in size. After the painting of the ZnO NPs, their photoluminescence (PL) characteristics were examined using the 325 nm wavelength light obtained from a He–Cd laser. The photocurrent response spectrum, photo- and dark-current and photoresponse spectrum taken from the fabricated photodetectors were investigated with a He–Cd laser and a Xe lamp.