Elsevier

Applied Surface Science

Volume 305, 30 June 2014, Pages 445-452
Applied Surface Science

A high-sensitivity, fast-response, rapid-recovery p–n heterojunction photodiode based on rutile TiO2 nanorod array on p-Si(1 1 1)

https://doi.org/10.1016/j.apsusc.2014.03.109Get rights and content

Highlights

  • The TiO2 seed layers were deposited on silicon substrates by using an RF sputtering.

  • High-quality rutile TiO2 NRs were grown on seeded p-type Si (1 1 1) substrate via CBD method.

  • A high-sensitivity, fast-response, rapid-recovery p–n heterojunction photodiode was fabricated based on TiO2NRs/p-Si(1 1 1).

Abstract

The growth and characterization of a p–n heterojunction photodiode were studied. This photodiode was based on rutile TiO2 nanorods (NRs) grown on p-type (1 1 1)-oriented silicon substrate seeded with a TiO2 layer synthesized by radio-frequency (RF) reactive magnetron sputtering. Chemical bath deposition (CBD) was performed to grow rutile TiO2 NRs on Si substrate. The structural and optical properties of the sample were studied by X-ray diffraction (XRD) and field emission-scanning electron microscopy (FESEM) analyses. Results showed the tetragonal rutile structure of the synthesized TiO2 NRs. Optical properties were further examined by photoluminescence spectroscopy, and a high-intensity UV peak centered at around 392 nm compared with visible defect peaks centered at 527 and 707 nm was observed. Upon exposure to 395 nm light (2.3 mW/cm) at five-bias voltage, the device showed 2.9 × 102 sensitivity. In addition, the internal gain of the photodiode was 3.92, and the photoresponse peak was 106 mA/W. Furthermore, the photocurrent was 3.06 × 10−4 A. The response and the recovery times were calculated to be 10.4 and 11 ms, respectively, upon illumination to a pulse UV light (405 nm, 0.22 mW/cm2) at five-bias voltage. All of these results demonstrate that this high-quality photodiode can be a promising candidate as a low-cost UV photodetector for commercially integrated photoelectronic applications.

Introduction

Given the high stability, low cost, photo-active properties, and wide band gap (>3 eV for all crystalline phase) of titanium dioxide (TiO2)-based semiconductors, they are receiving significant interest and are considered as some of the most promising materials for optoelectronic devices [1], [2], [3]. TiO2 exists in three main crystalline structures (i.e., anatase, rutile, and brookite) and each crystalline form exhibits different physicochemical properties [4], [5]. Among these structures, the rutile exhibits higher dielectric constant, high chemical stability, high hardness, excellent mechanical strength, high refractive index, transparency in visible region, and ultraviolet (UV) ray absorption rate [3], [6]. Because of these properties, rutile phase of TiO2 has been one of the most attractive materials for investigation during the last few decades because of its variety of applications [3], [7], such as photovoltaic [8], [9], gas sensing [10], photocatalysis [11], [12], dye-sensitized solar cell [13], and UV detectors [14]. Apart from TiO2, the most common UV detectors that are currently used are fabricated from wide band gap inorganic semiconductors, such as ZnO [15], [16] and GaN [17]. A wide variety of strategies have been improved to enhance the UV photodetectors of TiO2. Fabricating nanostructures of TiO2, such as nanorods, is a representative example among the sustained efforts. Different deposition methods have been used to grow one-dimensional TiO2 nanostructures, such as chemical vapor deposition [18], [19], hydrothermal [20], [21], template synthesis [22], [23], [24], sol–gel method [25], [26], [27], thermal evaporation [28], electrochemical etching [29], [30], and chemical bath deposition (CBD) [31], [32]. Among these methods, CBD is a flexible technique and is a promising approach because this method does not require sophisticated instruments; the preparation parameters are easily controlled; the starting chemicals are commonly available and have low cost to TiO2 nanostructure synthesis by strongly controlling their morphology [32]. Recently, several studies have focused on TiO2 nanostructure fabrication as UV detectors because the range of photosensitive spectra is only within the UV region, as well as their high melting point (1855 °C), photostability, and high photoconversion efficiency [33], [34]. Most research efforts have focused on heterojunctions in the study of UV detectors because the heterojunction effect determines a device's ultimate performance. Moreover, when nanostructure materials are utilized, the heterojunction effects are increased and become even more critical. Therefore, further study in this field is necessary, especially in nanostructure deposition [33]. Lee and Hon [35] designed a TiO2/water solid–liquid heterojunction UV photodetector, demonstrating that the device can be operated in photovoltaic (PV) mode (self-powered ability) and exhibits a high photosensitivity, excellent spectral selectivity, linear variations in photocurrent, and fast response. More studies have focused on the fabrication of two heterojunction UV detectors. Chang et al. [33] reported that the two heterojunctions ITO/TiO2/Si show that the diode transitioned from TiO2–Si heterojunction-controlled to ITO–TiO2 heterojunction-controlled if the ITO electrode was used. Xie et al. [36] fabricated a self-powered UV photodetector (TiO2/water solid–liquid heterojunction UV detector) based on rutile TiO2 NRs that were deposited directly on (FTO) glass through a low-temperature hydrothermal method.

The present study reports on the fabrication of p–n heterojunction from n-type TiO2 and p-type silicon substrate, with the aim of manufacturing UV sensor device with high sensitivity to UV light at low-bias voltage, fast response, fast recovery time, uncomplicated low-cost fabrication, and environment-friendly feature. The CBD was performed to grow high-quality rutile TiO2 NRs arrays on the Si substrate. This process is simple and the instruments used are low cost as compared to other wet chemical processes. Therefore, more studies in this field are required, particularly in nanostructure systems. The crystal structure and surface morphology of the deposited TiO2 NRs were investigated via X-ray diffractometry (XRD) and field-emission scanning electron microscopy (FESEM). Photoluminescence (PL) and electrical characteristics of the UV-sensing device were also determined.

Section snippets

Preparation of the TiO2 seed layer on silicon wafer

TiO2 seed layer was deposited onto p-type (1 1 1)-oriented silicon wafer for 80 min by radio-frequency (RF) reactive magnetron sputtering. A high-purity TiO2 disk (99.99%) 3 in diameter was used as a target. Prior to deposition, silicon substrate was cleaned by wet chemical etching using the RCA cleaning method. The chamber was evacuated below 2 × 10−5 mbar with an RF power of 150 W. High-purity argon was used as a sputtering gas at a fixed ratio of 17%. Deposition was then performed under a total

Morphological and structural characterization of the TiO2 nanostructures

The X-ray diffraction of the prepared TiO2 nanostructures onto TiO2 seed layer-coated p-type (1 1 1)-oriented silicon substrate is depicted in Fig. 1. The scanning Bragg angle was within the 2θ range from 20° to 80°. Within this range, dominant sharp peak was noted in the XRD pattern at 27.4° that confirms (1 1 0) plane growth. Eight diffraction peaks were noted to indicate the presence of the (1 1 0), (1 0 1), (1 1 1), (2 1 1), (2 2 0), (0 0 2), (3 1 0), and (2 2 1) planes of TiO2 material. This result is in

Conclusions

High-quality rutile TiO2 NRs were grown on p-type Si (1 1 1) substrate via chemical bath deposition method. The photodiode that was fabricated based on the prepared rutile TiO2/Si was tested in the UV light region, which demonstrated excellent stability over time, high photocurrent, good sensitivity, and high responsivity. Furthermore, the device showed fast response and recovery time. All of these results demonstrate that this high-quality photodiode can be a promising candidate as a low-cost UV

Acknowledgements

This work was supported by ERGS grant (203/PFIZIK/6730046), PRGS grant (1001/PFIZIK/846073) and Universiti Sains Malaysia.

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