Si

Diamond & Related Materials 21 (2012) 24–27

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Large photoconductivity of Pd doped amorphous carbon film/SiO2/Si Qingzhong Xue, Ming Ma ⁎, Yuhua Zhen, Xiaoyan Zhou, Sheng Wang College of Physics Science and Technology, China University of Petroleum, Dongying, Shandong 257061, People's Republic of China

a r t i c l e

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Article history: Received 4 December 2010 Received in revised form 28 September 2011 Accepted 10 October 2011 Available online 17 October 2011 Keywords: Amorphous carbon Magnetron sputtering Photoconductivity Pd doping

a b s t r a c t The Pd doped amorphous carbon (a-C:Pd) films were deposited on n-Si substrates with a native SiO2 layer using direct current magnetron sputtering. The dark and photo current–voltage (I–V) characteristics of the a-C:Pd/SiO2/Si were investigated. It is found that under white light illumination of 20 mW/cm 2 at room temperature, the a-C:Pd/SiO2/Si fabricated at 350 °C has a large photoconductivity (the ratio of photocurrent to dark current) of 2000, which is much better than that of the a-C based junctions reported before. The large photoconductivity is attributed to the great increment of the reverse conductivity of the a-C:Pd/SiO2/Si under illumination, which is caused by the Pd doping. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Amorphous carbon films have attracted considerable attention for their potential applications as gas sensors [1], gas pressure sensors [2], hard coating [3], and microelectronic devices [4]. In the past, much work has been done on photovoltaic characteristics of carbon films doped with the elements (such as iodine, phosphorus, nitrogen and boron) [5–11]. Recently, photoconductivity of a-C has received much interest for its great potential in photonic sensors and other photoelectric devices [12–15]. Under white light illumination with power of 100 mW/cm 2 at room temperature (RT), it was reported that photoconductivity (the ratio of photocurrent to dark current) of a-C was magnitude of 5–20 [13,14]. The low photoconductivity is ascribed to confined band gaps and abundant localized states of a-C [15]. It is well known that the band gaps can be tunable by varying sp 2 and sp 3 content ratio [16] and the π bond in the sp 2 configuration is highly delocalized. In order to improve the photoconductivity of a-C, great efforts have been made. Recently, a high photoconductivity of 170–220 under white light illumination of 20 mW/cm 2 has been achieved from iron-doped a-C films deposited on Si substrates by pulse laser deposition [15]. In this paper, we deposit Pd doped a-C (a-C:Pd) films on n-Si substrates with a native SiO2 layer using magnetron sputtering, and find that the a-C:Pd/SiO2/Si shows a large photoconductivity of about 2000 under white light illumination of 20 mW/ cm 2, which is much better than that of the a-C based junctions reported previously [13–15]. This large photoconductivity of the a-

⁎ Corresponding author. Tel.: + 86 546 8392836; fax: + 86 546 8397900. E-mail address: [email protected] (Q. Xue). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.10.008

C:Pd films is also large compared with that of Si based films (such as SiO2/a-Si:H/p-Si and SiGe/Si) reported [17,18]. 2. Experimental details The a-C:Pd films were deposited on n-Si (100) substrates using direct current magnetron sputtering from Pd doped graphite target. The targets are cold-pressed composite disks, and the purities of the Pd and graphite are better than 99.9%. The silicon substrates are n-type materials with resistivity in the range of 2–3 Ω cm. Before deposition, the Si substrates were ultrasonically cleaned in ethanol and acetone and then rinsed in de-ionized water. The deposition took place inside a chamber where the argon pressure was kept at 3 Pa and the Si substrates were kept at 350 °C. A semitransparent copper surface electrode was deposited on top of the a-C:Pd films using direct current magnetron sputtering. Such semitransparency ensures the absorption of the light by both the a-C:Pd films and the underlying Si substrate. The current–voltage (I–V) characteristics of the a-C:Pd/SiO2/Si were measured by using two-probe method with a Keithley 2400 SourceMeter under illumination. The illumination was provided by a halogen tungsten lamp. 3. Results and discussions Fig. 1 shows the schematic illustration of the a-C:Pd/SiO2/Si structure. The scanning electron microscopy images (not shown in this paper) show that the thickness of the a-C:Pd films and the semitransparent copper surface electrode are about 20 and 30 nm, respectively. The thin native SiO2 layer on Si substrate is about 1.2 nm [19]. A semitransparent copper surface electrode was deposited on glass using direct current magnetron sputtering. The copper film with thickness of about 30 nm shows an average transmission of about 40% in the wavelength range

Q. Xue et al. / Diamond & Related Materials 21 (2012) 24–27

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exhibits a good linear relationship between photocurrent and illumination power. As shown in Fig. 3(d), the equivalent circuit of the a-C:Pd/SiO2/Si sample consists of a forward biased and a reverse biased a-C:Pd/ SiO2/Si junctions and the Si substrate. Because the resistance of reverse biased a-C:Pd/SiO2/Si junction is much larger than that of forward biased a-C:Pd/SiO2/Si junction [15], the I–V characteristics of the a-C:Pd/SiO2/Si structure can be expressed by Formula (1) I ¼ I Si ¼ IS f1− exp½−qðV−ISi RSi Þ=kT g:

Fig. 1. The schematic illustration of the a-C:Pd/SiO2/Si structure.

Intensity (a.u.)

of 350–1000 nm. From the energy dispersive X-ray spectroscopy of the cross-section of the sample, it is found that the atomic concentration of Pd into the a-C:Pd film is about 0.7%. Raman spectra of the a-C film and a-C:Pd film deposited at 350 °C is shown in Fig. 2. It is well known that the Raman spectrum of carbon materials can be fitted to the D band at 1350 cm − 1 and the G band at 1580 cm − 1. It can be seen that the a-C:Pd film deposited at 350 °C has one obvious D peak, which suggests that the a-C:Pd film deposited at 350 °C is a disordered graphite-like carbon system, which forms a lot of π–π* states [13]. Under illumination, the electron–hole excitation process between the π–π* states has an important effect on the photoconductivity [13]. Fig. 3(a) shows the I–V characteristics of the a-C:Pd/SiO2/Si fabricated at 350 °C under dark and white light illumination at RT. It can be seen that the device has an extremely low dark current without light illumination. Under white light illumination, with increasing applied voltage, the measured current increases first steeply and then increases very slowly and reaches saturation finally. The photocurrent saturation transition voltage increases with increasing the illumination power. In order to uncover the I–V properties under dark and illumination more clearly, the relation between the log (the forward I) and the forward V of the sample is shown in Fig. 3(b). We can find that the photocurrent under white light illumination of 1 mW/cm 2 is about 100 times higher than dark current. Furthermore, the sample shows a large photoconductivity of about 2000 under white light illumination of 20 mW/cm 2, which is much better than that of the a-C based junctions reported before [13–15]. The dependence of current on illumination power at a given forward voltage of 1 V is shown in Fig. 3(c). It is found that the sample

a-C:Pd film 350OC a-C film 350OC 1000

1200

1400

1600

1800

ð1Þ

where I, V, ISi and IS are the measured current, the measured voltage, the current transported in the Si substrate, and saturation current of the reverse biased junction formed between a-C:Pd film and the Si, respectively. The RSi, q, k and T are the resistance of the Si substrate, the electronic charge, Boltzmann constant and absolute temperature, respectively. Because of the fact that the silicon with resistivity in the range of 2–3 Ω cm and ISib1.26 mA, we can get ISiRSi b 46 mV. Thus, when V > > ISiRSi, the Formula (1) can be written as I ¼ I S ½1− expð−qV=kT Þ:

ð2Þ

When qV > > kT (for example, when V = 0.5V, exp(− qV/kT) ~ 4 × 10 − 9), Formula (2) can be rewritten as I ¼ IS :

ð3Þ

In a word, when V > > ISiRSi the current almost is the reverse saturation current of either a-C:Pd/SiO2/Si junction. And the reverse saturation current IS increases with increasing the illumination power [Fig. 3(a)]. The SiO2 layer of the a-C:Pd/SiO2/Si can increase the barrier height [20], which can increase the resistance of reverse biased a-C:Pd/SiO2/ Si. The enhanced resistance of reverse biased a-C:Pd/SiO2/Si can help the device to generate the low dark current without light illumination. For comparison, we deposited pure a-C films on n-Si substrates with a native SiO2 layer at 350 °C. Fig. 4(b) shows that the photoconductivity of the a-C/SiO2/Si sample has a low photoconductivity of about 20 under illumination of 20 mW/cm 2, which is much lower than that (~2000) of the a-C:Pd/SiO2/Si sample under the same light condition [Fig. 4(a)]. Therefore, the Pd doping can greatly improve the photoconductivity (or reverse saturation current under illumination) of a-C based junctions. As shown in Fig. 2, due to Pd doping, the intensity of D peak of the a-C:Pd film deposited at 350 °C is stronger than that of the a-C film deposited at 350 °C. Therefore, the a-C:Pd film is a more disordered graphite-like carbon system, which forms more π–π* states [13]. Under illumination, the electron–hole excitation process between the π–π* states plays an important role for the photoconductivity [13]. According to the Raman spectrum of the a-C:Pd film deposited at 350 °C, we can get the positions of D and G peaks and the ratio of their intensities, which demonstrate that the a-C:Pd film deposited at 350 °C is a graphite-like a-C film with a relatively small energy gap (Eg b 1 eV) [21–23]. Under illumination, the small Eg makes electron transition easier, which can help the a-C:Pd film to generate much more carriers. Besides, Pd doping itself can help the a-C:Pd film to generate much more carriers and enhance the reverse saturation current of either a-C:Pd/SiO2/Si junction under illumination [15]. 4. Conclusions

2000

Raman shift (cm-1) Fig. 2. Raman spectra of the a-C film and a-C:Pd film deposited at 350 °C.

In summary, the Pd-doped a-C films were deposited on n-Si substrates with a native SiO2 layer. The results show that the a-C:Pd/SiO2/ Si fabricated at 350 °C has a large photoconductivity of about 2000 under light illumination of 20 mW/cm2 at RT, which is much better

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Q. Xue et al. / Diamond & Related Materials 21 (2012) 24–27

1.5

0.5 0.0 I V

-0.5

Cu a-C:Pd SiO2 Si

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0.1

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0 mW/cm2 1 mW/cm2

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1E-3

5 mW/cm2 10 mW/cm2

1E-4

15 mW/cm2 20 mW/cm2

1E-5 0.0

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1V

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Illumin ation (mW/cm2) Fig. 3. (a) I–V characteristics of the a-C:Pd/SiO2/Si structure under dark and illumination at room temperature. The inset shows the schematic illustration of the electrical measurement. (b) Replots of (a) with log(the forward I) and the forward V. (c) The dependence of current on illumination power at a given forward voltage of 1 V. (d) The equivalent circuit diagram of the a-C:Pd/SiO2/Si sample.

than that of the a-C based junctions reported before. The photoconductivity is attributed to the reverse I–V characteristics of the a-C:Pd/SiO2/ Si junction with eminent light sensitivity caused by the Pd doping. This

2500

Acknowledgments

a) Photoconductivity

study shows that the a-C:Pd/SiO2/Si structure has great potential application as photoelectric sensors for its large photoconductivity, good linear relationship between photocurrent and illumination power, low cost, and it is highly chemical and environmentally stable.

This work is supported by the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (708061), the Natural Science Foundation of China (10974258), the Program for New Century Excellent Talents in University (NCET-080844), the Natural Science Foundation of Shandong Province (ZR2010AL009), and the Fundamental Research Funds for the Central Universities (10CX05001A).

2000 1500 (Iphoto/Idark)=2000

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b) 80 60 40 (Iphoto/Idark)=20

20 0

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Voltage (V) Fig. 4. The dependence of photoconductivity on applied voltage under illumination of 20 mW/cm2: (a) the a-C:Pd/SiO2/Si sample and (b) the a-C/SiO2/Si sample.

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