the fabrication and characterization of uv sensor … · abstract- a structure for ultraviolet (uv)...

International Journal of Electrical, Electronics and Data Communication, ISSN: 2320-2084 Volume-3, Issue-2, Feb.-2015

The Fabrication and Characterization of UV Sensor Based on TiO2 Nanorods Array on Silicon Substrate Heterojunction

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THE FABRICATION AND CHARACTERIZATION OF UV SENSOR BASED ON TIO2 NANORODS ARRAY ON SILICON SUBSTRATE

HETEROJUNCTION

1ABBAS M. SELMAN, 2Z. HASSAN

Nano-Optoelectronics Research and Technology Laboratory (N.O.R.), School of Physics, Universiti Sains Malaysia, Penang 11800, Malaysia.

Department of Pharmacology and Toxicology, College of Pharmacy, University of Kufa, Najaf, Iraq E-mail: [email protected]. 2nd Author Email: [email protected]

Abstract- A structure for ultraviolet (UV) photosensor based on rutile TiO2 nanorods (NRs) grown on p-type (111) - oriented silicon substrate seeded with a TiO2 layer synthesized by radio-frequency (RF) reactive magnetron sputtering has been fabricated. Chemical bath deposition (CBD) was carried out to grow rutile TiO2 NRs on Si substrate. The structural and optical properties of the sample were reported 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 366 nm compared with visible defect peaks centered at 436 and 716 nm was observed. Upon exposure to 365 nm light (2.3 mW cm -2) at 5V bias voltage, the device showed 62.8 × 102 sensitivity. In addition, the internal gain of the photosensor was 63.8, and the photoresponse peak was 467 mAW-1. Furthermore, the photocurrent was 6.7 × 10-4 A. The response and the recovery times were calculated to be 48 and 40 ms, respectively, upon illumination to a pulsed UV light (365 nm) at 5V bias voltage. All of these results demonstrate that this high-quality photosensor can be a promising candidate as a low-cost UV photodetector for commercially integrated photoelectronic applications. Keywords- CBD, Nanorods, Rutile TiO2, UV Photosensor. I. INTRODUCTION one dimensional structures (nanowires, nanonrods, nanobelts and nanotubes) of rutile TiO2 has attracted considerable attention during the last few decades due to its unique and excellent properties in optics, electronics, photochemistry and biology, as well as its variety of applications such as photocatalysis, dye-sensitized solar cell, photovoltaic, gas sensing, and UV detectors. A wide variety of strategies have been employed to grow TiO2. Fabricating TiO2 nanorods (NRs) is a representative example among the sustained efforts. Different deposition methods have been used to grow one-dimensional TiO2 nanostructures, such as template synthesis, sol–gel method, thermal evaporation, chemical vapor deposition, hydrothermal, electrochemical etching, and chemical bath deposition (CBD). 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 low cost to TiO2 nanostructure synthesis by strongly controlling their morphology. Recently, most research efforts have focused on heterojunctions in the study of UV photosensors 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. Lee and Hon 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. Xie et al. fabricated a self-powered UV photodetector (TiO2/water solid–liquid heterojunction UV detector) based on rutile TiO2 NRs that were deposited directly on Fluorine-doped tin oxide (FTO) coated glass through a low-temperature hydrothermal method. In this study, we report TiO2 photosensor based 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. The crystal structure and surface morphology of the deposited TiO2 NRs were investigated via X-ray diffractometry (XRD) and field-emission scanning electron



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microscopy (FESEM). Photoluminescence (PL) and electrical characteristics of the UV-sensing device were also determined. II. EXPERIMENTAL DETAILS The rutile TiO2 nanorod synthesis has been described in our previous work. In brief, CBD was used to synthesize the TiO2 NRs by heating an acidic solution of titanium (III) chloride containing the immersed seed layer (TiO2)-coated Si substrate. Titanium (III) chloride liquid was mixed with deionized water (DI water) in appropriate quantities. Exactly 4 ml of TiCl3 (15 wt.% in HCl; Merck Sdn Bhd, Malaysia) was added to 50 ml distilled water. Urea (NH2CONH2; 0.1 M) was added to adjust the pH to approximately 0.7. After stirring at room temperature (27 °C) for 1 h, a homogeneous violet solution was obtained. The substrate was then vertically immersed in the aforementioned bath that was then heated at 55 °C. The substrate coated with TiO2 NRs were removed after 2 h, rinsed with distilled water, and dried with nitrogen gas. To improve crystallinity, the film was heat treated at 550 °C for 2 h in air. The fabrication of the UV sensor device was performed by depositing aluminum (Al) grid (approximately 100 nm thickness) as front contact on the top of TiO2 NRs using a shadow mask. Indium metallization (approximately 200 nm thickness) was deposited onto the back side of the p-Si. Both electrodes (back and front contacts) were deposited using RF reactive magnetron sputtering. The contacts were deposited at room temperature. The inset of Fig. 5 shows a schematic of the device structure. The active area of the Al/TiO2NRs/P-Si(111)/In heterojunction UV sensor was about 0.39 cm2. The structure and morphology of the prepared TiO2 NRs were characterized and analyzed by XRD (PANalytical X’Pert PRO MRD PW3040) with CuKα radiation (λ = 1.541 ˚A ) and FESEM (Leo Supra 50VP, Carl Zeiss, Germany) equipped with an energy-dispersive X-ray (EDX) system. Optical properties were measured at room temperature with a PL spectroscopy system (Jobin Yvon HR 800 UV, Edison, NJ, USA) having a He – Cd laser (325 nm, 20 mW). III. RESULTS AND DISCUSSION The XRD patterns of the prepared TiO2 NRs onto TiO2 seed layer-coated p-type (111)-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 (101) plane growth. Two diffraction peaks were noted to indicate the presence of the (111) and (211), planes of TiO2 material. This result is in agreement with JCPDS card No. 01-078-1508. The TiO2 NRs was polycrystalline

and can be indexed as tetragonal rutile phase based on the XRD patterns. Figure 2 shows the FESEM images with two different magnifications of the prepared sample. The FESEM images display uniform and high-density rutile NRs and nanoflowers films grown on silicon substrate. The seeded Si surface shows two kinds of distribution mode for NRs: most NRs are distributed individually on the surface, whereas some NRs are assembled into a nanoflower where the NRs are formed by preferentially oriented nucleation and surface growth. The diameter of most flowers ranges from 350 nm to 400 nm. Each flower is composed of more than 40 NRs. All NRs have a diameter of 18 nm to 22 nm and an average length of 85 nm. The optical properties of the as-grown rutile TiO2 NRs were determined at room temperature, using PL spectroscopy (Fig. 3), where the PL spectra of sample were excited by a He–Cd laser (wavelength=325 nm). Fig. 3 shows the PL spectra for TiO2 NRs grown on a p-type (111) silicon substrate. The dominant peak observed in the UV region is centered around 366 nm; these findings are attributed to the near-band edge UV emission (NBE) of the wide-bandgap TiO2, which results from the recombination of free excitons (electrons and holes). Furthermore, in the visible region, the TiO2 demonstrates two dominant PL emissions centered at 436 and 716 nm; as shown in Fig. 3.

Fig.1 XRD patterns of rutile TiO2 nanorods grown on silicon

(111) substrate.

Fig.2 FESEM image of the rutile TiO2 nanorods grown on

silicon (111) substrate.



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These emissions are the radiative recombination of the self-trapped excitons and the radiative transitions inside the sub-states initiated from the TiO2 surface. This result well agrees with those reported by Vishwas et al. The highest intensity of UV band compared with weak spectral bands in the visible region indicated that the TiO2 NRs have a high-quality structure with low structural and surface defects.

Fig.3 PL spectra at room temperature of rutile TiO2

nanostructures grown on silicon (111) substrate. Furthermore, the high ratio of the UV peak intensity to that of the visible peak intensity (IUV/IVis) is the most important indicator of the excellent crystal quality and main identifying feature of good optical nanostructure properties. The strong PL intensity for the TiO2 NRs was a result of the enhancement of the optical characteristic of the vertically aligned TiO2 nanostructures and less defect densities in the materials. The results showed that the used CBD method is capable of growing high-quality TiO2 nanostructures with few defects for future optoelectronic nanodevice applications, such as UV photosensor. Based on the above observations and structural analyses, this sample is fabricated as UV photosensor, and the UV photosensor device is found to demonstrate a high sensitivity to UV light at low-bias voltage, fast response, and fast recovery time. This result may be caused by the premise that TiO2 NRs grown at this condition can provide large and rough surface areas. This large and rough surface area is favorable in improving light absorption and enhancing the efficiency of charge carrier generation. The UV detection characteristics of the fabricated Al/TiO2NRs/p-Si(111)/In heterojunction photosensor illustrated in Fig. 4 were studied by measuring the current–voltage (I–V) relationships with rectifying ratio(If/Ir) of 17.6 and 32.6 at -5V and 5V bias voltages in the dark and under UV light illumination, respectively; i.e., the wavelength of the excitation light was 365 nm, and the incident optical power was 2.3 mWcm-2 at the distance of 3 cm from a UV lamp; where If is forward current and Ir is reverse current. The rectifying behavior indicates that a p-n junction is formed in the TiO2NRs/p-Si(111). From the experimental results, the obtained Id (without UV

light), which is generated from thermionic emission of carriers, was much smaller than the IPh gained under UV illumination. Notably, the current increases with increasing applied bias amount to 1.05 × 10-5 and 6.7 × 10-4 A at 5 V in dark condition and when the UV light is switched on, respectively. Under dark condition, the device showed current value which is six times of magnitude smaller than the current under UV illumination. This finding indicates that the photosensor of TiO2NRs/P-Si(111) exhibited excellent light response. The current stays nearly constant at small bias (Vbias < 0.5 V). These results indicate that the charge carriers created by the UV illumination at a high bias voltage (Vbias > 0.5 V) have a main function of determining the current, which causes the increase in current. However, when a small bias is applied, the current is not sensitive to the UV illumination since the current of the device is restricted by the depletion region between the n-TiO2 and p-Si. Therefore, the TiO2 NRs will absorb the UV light and generate electron–hole pairs in the depletion region. The presence of an electric field in the depletion region by applied bias voltage (Vbias > 0.5 V) results in the separation of the charge carriers, where the holes will move to the p-side and the electrons will move to n-side. Thus, a photocurrent will be generated at the external contact. The large difference between Iph and Id can be attributed to the short transit time of charge carriers with long lifetime. The presence of oxygen caused hole trap states at the TiO2 NRs surface which prevents the generation charge carrier pair (e--h+) recombination and extends the lifetime of holes [30]. The photosensor sensitivity (S) was calculated using the following relation:

100)((%)

d

dPh

IIIS (1)

Fig.4 Current-voltage characteristics of the Al/TiO2NRs /

p-Si(111)/In heterojunction photosensor under dark and UV illumination (365 nm, 2.3 mWcm-2).

The S value is 62.8 × 102 at 365 nm wavelength with the light intensity of 2.3 mWcm-2, and the applied bias



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voltage was 5 V. This result indicates that the device can be applied as an optical switch for ambient light. When the photosensor was illuminated, the photogenerated charges are created from the applied electric field. Consequently, the photocurrent that was added to the bias current efficiently increases the device conductivity. In general, the responsivity (R) and external quantum efficiency (η) are two critical parameters that determine the sensitivity for the photosensor device.

Fig.5 Room temperature responsivity spectra of the TiO2NRs/p-Si(111) heterojunction photosensor

R is defined as the device photocurrent (IPh) generated per unit power of incident light (Pinc) on the effective area of the device (A), and can be expressed as:

)()/()(

)()(

22 CmACmWEAI

WPAIR Ph

cin

Ph (2)

Where E is the irradiance of the UV light, which is measured by a standard UV power meter (Newport Power meter, Model 2936-C USA). The photoresponse spectrum of the fabricated photosensor was measured under front illumination within the range of 320 nm to 500 nm at 5V-bias voltage, as shown in Fig. 5. Figure 5 demonstrates the photocurrent increases with wavelength up to 320 nm, and then falls sharply in the wavelength up to 380 nm wavelength. When the wavelength decreases, absorption coefficient increases the penetration depth of UV light will be short. This process raises the charge carrier pair concentration near the TiO2 NRs surface. Consequently, lifetime of photogenerated carriers decline and leads to a decrease in responsivity. Under irradiation of 365 nm UV light, the peak photoresponse is 467 mAW-1, which is more than thirty times higher than those reported for TiO2 based Schottky-type UV light detector at around 330 nm. This higher R can be attributed to that TiO2 NRs provide high density with larger and rougher surface areas, as well as a good p-n junction was

formed in the TiO2NRs/p-Si(111). A sharp cut-off near 380 nm is observed, which matches the absorption edge of rutile TiO2 as reported in our previous work. The photoresponse decreases considerably across the cut-off wavelength implying that a photoconductive UV detector with high sensitivity has been fabricated. Quantum efficiency of a photodetector is as the number of charge carriers accumulated to produce the photocurrent (Iph) generated for every number of incident optical power power (Pinc), as shown in the following relation:

inc

Ph

Ph

qI % (3)

Equation (3) can be calculated by measuring the R via:.

)(24.1%

mgR

gqhcR

(4)

Where q, λ, v, h, g, and c are the electric charge, the UV light wavelength, the frequency of incident light, the Plankconstant, the gain, and the speed of light, respectively. The current gain g has been calculated in this study, and is defined as the ratio between the number of electrons collected per unit time in the dark and the number of absorbed photons to generate photoelectron per unit time (ratio of photo current to dark current). The current gain is measured from the I–V characteristics using (5);

d

hp

II

Gain (5)

The calculated gain g was 63.8 at wavelength 365 nm with the light intensity of 2.3 mWcm-2, and the applied bias voltage was 5 V.

Fig.6 The TiO2NRs/p-Si(111) heterojunction photosensor

photoconductivity versus light wavelength. The photosensor device photoconductivity was measured at wavelength scanned from 320 nm to 500 nm with 5 V applied voltage as shown in Fig. 6. The photocurrent increase gradually from 320 nm to the highest photocurrent value observed at 325 nm



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wavelength which was 6.7 × 10-4 A; this value is more than four order of magnitude higher than those reported in previous study for UV sensor based on TiO2 NRs arrays on FTO thin film at the same wavelength. Furthermore, the reason for high photocurrent generated when photosensor is illuminated is that the rutile TiO2 has high crystallinity structure; this finding demonstrates the reduced boundaries and structure defects that act as trap preventing the charge carrier transports. By contrast, oxygen desorption at the TiO2 rod surface also increases the photosensor photoconductivity. To test photosensor repeatability, the optical response has been studied using dynamics response time measurement by illuminating photosensor to a pulsed UV light (365 nm, 2.3 mW/cm2) under different bias voltages. Figure 7 shows the corresponding increase in the photocurrent as a function of time at different bias voltages, while the UV light was repeatedly switched on and off 17 times (partially show in Fig. 7). The results show acceptable difference with time. All of these curves in Fig. 7 reveal rectangular-shaped profiles, and the magnitudes of the photocurrent for every on/off cycle are steady and repeatable. The response time (i.e., defined as the time for the current

to increase from 10% to 90% of its saturation value) and recovery time (i.e., defined as the time for the current to decrease from 90% and back to 10% of its saturation value) for photosensor were measured at various bias voltages as shown in Fig 7. The response time and recovery time is clearly by far the shortest time achieved in the various TiO2 UV detectors reported in the literature. The very fast photosensor photoresponse in the current study can be related to high quality and large photoactive surface areas of the TiO2 NRs grown on p-type Si substrate. The faster response indicates that the vertical TiO2 NRs photosensor is useful for high-speed operation. Moreover, the high value of photocurrent and responsivity can be ascribed to more carriers collected under illumination in the TiO2 NRs films. This finding is in agreement with previous study, which shows that the exceptional photoactivity of nanostructures, which may explain its sensitivity and stable performance in UV light detection. These results reveal that Al/TiO2NRs /p-Si(111)/In heterojunction photosensor exhibits potential applications in UV detector.

Fig.7 The repeatability property (ON/OFF) of the TiO2 NRs / p-Si(111)heterojunction photosensor under pulsed UV light (365 nm, 2.3

mWcm-2 ) at various bias voltages.



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CONCLUSIONS High-quality rutile TiO2 NRs were grown on p-type Si (111) substrate via chemical bath deposition method. The photosensor 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 photosensor can be a promising candidate as a low-cost UV photodetector for commercially integrated photoelectronic applications. ACKNOWLEDGEMENTS This work was supported by FRGS grant (203/PFIZIK/6711353), PRGS grant (1001/PFIZIK/846073) and Universiti Sains Malaysia. REFERENCES

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