TiO2 nanoparticles composite

Microelectronic Engineering 119 (2014) 141–145

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Tailoring transport and dielectric properties by surface passivation of silicon nanowires with Polyacrylic acid/TiO2 nanoparticles composite Kamran Rasool a,b, M.A. Rafiq a,⇑, Z.A.K. Durrani b a Micro and Nano Devices Group, Department of Metallurgy and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences (PIEAS), P. O. Nilore, Islamabad 45650, Pakistan b Department of Electrical and Electronic Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK

a r t i c l e

i n f o

Article history: Received 14 October 2013 Received in revised form 22 January 2014 Accepted 29 March 2014 Available online 8 April 2014 Keywords: Silicon nanowires Composite devices Space charge limited current Dielectric properties

a b s t r a c t We have presented the composite device including silicon nanowires (SiNWs), Polyacrylic acid (PAA) and Titanium dioxide nanoparticles (TiO2 NPs). SiNWs and TiO2 NPs were synthesized by metal assisted electroless chemical etching (MACE) and co-precipitation method respectively. Solution containing PAA and TiO2 NPs in DI water was spun on already grown vertical SiNW arrays. We have investigated the transport and dielectric properties of p-type SiNWs/PAA/TiO2 NPs (p-SPT) and n-type SiNWs/PAA/TiO2 NPs (n-SPT) composite devices. Presence of PAA/TiO2 NPs on the surface of SiNWs have increased electrical current in p-SPT device than that of n-SPT device. Ohmic like conduction was dominant at lower bias voltages followed by space charge limited current (SCLC) with traps at intermediate voltages. The calculated values of trap densities (Ht) were 7.73  1011 cm3 and 5.34  1011 cm3 for p-SPT device and n-SPT device respectively. Similarly p-SPT device shows higher real part of dielectric constant (e0 ) and AC conductivity (rac) 15 times and 85 times respectively than that of n-SPT device. Increment in electrical and dielectric properties can be attributed to the presence of hydrophilic materials (PAA/TiO2 NPs) which may results in enhancement of acceptor like states. Ó 2014 Published by Elsevier B.V.

1. Introduction Silicon nanowires (SiNWs) are highly efficient for various applications in thermoelectric, photodetection and highly scaled information processing devices [1–3]. Different types of organic, inorganic and composite materials have been used for surface modification of nanostructures to tailor various properties e.g. sensing, electronic and optoelectronic [4–7]. Similarly carrier transport properties of SiNWs were modified using organic and inorganic materials [8–10]. Polyacrylic acid (PAA) is the key polymer for surface passivation because of its ionic nature. PAA can be easily diluted up to any extent in polar solvents e.g. acetone, isopropanol and DI water. PAA can absorb oxygen and sense gases even from environment [11]. TiO2 NPs is one of important oxide in semiconducting oxide family. Hybrid materials containing TiO2 NPs are significant for enhanced dielectric and photovoltaic properties [12–13]. Dielectric properties of TiO2 NPs can be enhanced drastically via surface capping with polymer [14]. PAA covered SiNWs [15] and TiO2 NPs ⇑ Corresponding author. Tel.: +92 512207381. E-mail addresses: maftabrafi[email protected], [email protected], [email protected]. pk (M.A. Rafiq). http://dx.doi.org/10.1016/j.mee.2014.03.040 0167-9317/Ó 2014 Published by Elsevier B.V.

decorated SiNWs [9] has been proved efficient for enhanced electrical transport and photodetection properties. Here we present composite devices including PAA, TiO2 NPs and SiNWs arrays. We have mixed TiO2 NPs in highly diluted PAA and spun on vertically grown SiNWs arrays. We have investigated the carrier transport properties of fabricated composite devices. Incorporation of PAA/TiO2 NPs had improved electrical and dielectric properties of p-SPT composite device in comparison to n-SPT composite device. The electrical and dielectric properties of SiNWs, ionic polymer and inorganic NPs based composite devices has not been investigated before according to best of our knowledge. 2. Experimental SiNWs were prepared by metal assisted electroless chemical etching (MACE) which is an anisotropic wet etching technique. In this technique immersion of silicon substrate in hydrofluoric acid (HF) and silver nitrate (AgNO3) solution forms silver (Ag) dendrites on the substrate. The substrate is then immersed in the solution containing HF and hydrogen peroxide (H2O2) for etching [15]. As a result long SiNWs are formed and length of SiNWs is highly dependent on etching time. Increase in etching time resulted in

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longer SiNWs. TiO2 NPs are prepared by co-precipitation method [9,16] by using titanium tetrachloride (TiCl4), hydrochloric acid (HCl) and sodium carbonate (Na2CO3). Prepared precipitates of TiO2 NPs were cleaned with DI water. PAA having molecular weight 450 k and 15.8 wt.% in DI water was used as starting material. Highly diluted PAA 3.3 wt.% in DI water was mixed with TiO2 NPs under continuous stirring. Furthermore, ultrasonication was performed for 5 h. This solution containing PAA/TiO2 NPs in DI water was spun at 1000 rpm for 60 s on already grown SiNWs. This step was followed by heating the sample at 50 °C in air to dry PAA/ TiO2 NPs for 20 min. This process was repeated two more times. With this process whole area between SiNWs was not fully covered with PAA/TiO2 NPs and some pits and empty spaces might exist in the PAA/TiO2 NPs filling. However the empty spaces may not be down to the bottom of the substrate. The addition of PAA/TiO2 NPs on SiNWs can act as a source of interfacial doping by creating donor like or acceptor like states at the SiNW surface. In order to make better electrical contact on the top of the SiNWs, composite devices were exposed to oxygen plasma for etching the PAA left on the top of SiNWs. Parameters were set as Ar (2 sccm), O2 (98 sccm), pressure (50 Torr), temperature of chamber was 20 °C and power 200 W. The samples were etched for 60 min. The etching rate of polymer depends on the molecular weight and related functional groups. Oxygen containing polymer like PAA is more degradable as compared to other polymers (e.g. polystyrene and polyethylene) [15]. After oxygen plasma etching, the tips of SiNWs were clearly seen. Finally 10 nm Cr followed by 300 nm Au was sputtered to form top (SiNWs) and bottom (Substrate) metal contacts in two separate steps. All experimental conditions including etching time for SiNWs fabrication, spinning, oxygen plasma etching and metal contacts are identical for both p-SPT and n-SPT composite devices for comparison. Degradation and other properties of PAA can be influenced by its PH value and temperature. That’s why we have

(c)

PAA SiNWs TiO2 NPs

avoided further heating because temperature can influence the ionic character of PAA. 3. Results and discussion Fig. 1(a) shows the cross-sectional scanning electron microscope (SEM) image of composite device. Energy dispersive X-ray spectroscopy (EDS) was performed to confirm the presence of PAA and TiO2 NPs on the surface of SiNWs shown in the inset of Fig. 1(a). The length of SiNWs and diameter of TiO2 NPs were 40 lm and 50 nm respectively. Fig. 1(b) shows the planer SEM image of composite device and inset shows planer SEM image after plasma etching. Similar results were observed for both n and p-SPT composite devices. Schematic diagram of SiNWs/PAA/TiO2 NPs composite device with top and bottom metal contact is shown in Fig. 1(c). Synthesis of SiNWs and device fabrication process is approximately same as described elsewhere in case of PAA and TiO2 [9,15]. Further, we have performed the temperature dependent IV characteristics (290–77 K) to investigate and compare the transport properties of composite devices. Fig. 1(d) shows the comparison of electrical current in p-SPT and n-SPT composite devices at 290 K and 77 K. PAA is ionic polymer and can easily dissolve in ionic solvents like water, acetone etc. PAA shows high sensitivity to humid environment and gases that’s why, PAA can be used in humidity and gas sensing devices. SiNWs are of two types, hydrophilic and hydrophobic. Instant HF treatment on the surface of SiNWs resulted in hydrophobic groups but aged SiNWs are mostly hydrophilic in nature. Usually presence of Si–OH group makes SiNWs hydrophilic. The addition of highly diluted PAA/TiO2 NPs on the surface of SiNWs results in the enhancement of hydrophilic groups on SiNWs surface [17]. For comparison, we have compared the values of electrical current. The p-SPT composite device shows 94 and 103 times enhanced conductivity at 290 K and 77 K respectively than that

Metal Contact n-type p-type @ 290K

Si Substrate

Fig. 1. (a) Cross-sectional SEM image of SiNWs/PAA/TiO2 NPs composite device with EDS spectra (inset). (b) Planer SEM image of composite device and inset shows planer SEM image after plasma etching (similar images were seen in both n and p-SPT composite devices). (c) Schematic diagram of fabricated composite device. (d) Comparison of IV characteristics at 290 K and 77 K for n-SPT and p-SPT composite devices.

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of n-SPT composite device. The increase in conductivity of p-SPT composite device can be related to the presence of hydrophilic contents (PAA, TiO2 NPs) on the surface of SiNWs and this may result in conversion of hydrophobic bonds to hydrophilic. The presence of Si–OH bonds actually increases the amount of acceptor like states (holes), hence electrical conductivity increases in p-SPT composite device. But as expected, in n-SPT composite device current is smaller than that of p-SPT composite device. Independent of nature i.e. n-type or p-type, the nature of aged HF treated SiNWs surface is hydrophilic. Hence surface of n-SPT composite device may also act like p-type; this reduces the electrical current in n-SPT composite device as shown in comparison [18]. We have further investigated the transport properties from temperature dependent IV characteristics for both p-SPT and nSPT composite devices. Fig. 2(a) shows the double logarithmic plot of temperature dependent IV characteristics of p-SPT composite device for exploring conduction mechanism. The slope of double logarithmic graph directly gives m when current–voltage characteristics are of the form: JaVm. The Ohmic like conduction is dominant conduction mechanism at higher temperatures (290–230 K). Similarly the Ohmic (slope (m)1) conduction can be clearly seen at lower voltages in all temperature (290–77 K) range. After Ohmic like region the value of slope increases (m is in the range of 1.5– 1.9) in lower temperatures region named as charge injection region. When temperature further lowers (210–77 K), the value of m increases and the conduction followed space charge limited current conduction (SCLC) with exponential traps. This behavior is dominant at intermediate voltages. The SCLC region is followed by another region named as SCLC without traps. This region usually appeared at higher voltages and lower temperatures [19].

(a)

ln(I)

-6

Traps free SCLC -5

290K

SCLC

-9 -12

-4.5

-3.0

-1.5

-30

0.0

77K -4.5

-3.0

-1.5

0.0

ln(V) -4

290 K

Vc

-6.0

(b)

-8

ln(I)

ln(I)

SCLC

-25

(b)

-4.5

290K

-15

ln(V) -3.0

Ohmic

-20

77K

Ohmic

-15

Traps free SCLC

(a)

-10

ln(l)

-3

Fig. 3(a) shows the double logarithmic plot for n-SPT composite device. At lower voltages, again Ohmic like behavior is dominant followed by SCLC with traps having slope 2–9 in the temperature range (210–77 K). The large values of slope for n-SPT composite device in comparison to p-SPT composite device can be due to higher resistance and less mobility. Similarly at higher voltages again SCLC without traps can be observed. We have tried to explain the origin of charge injection mechanism which is then transformed to SCLC with and without traps. This type of mechanism is usually associated with low doped semiconducting crystals [20]. In case of semiconductors (n and p-type), the space charge region consists of free carrier’s concentration and also immobile ionized dopand atoms. The space charge density is zero in the neutral region, where n = ND and p = NA. Here n, p, ND and NA is free electron concentration, free hole concentration, donor doping concentration and the acceptor doping concentration respectively. The concentration of free carriers can be increased under bias condition. The space charge effect occurs, when the concentration of injected free carriers is greater than the thermal equilibrium value. We can also say that SCLC occurs when there is significant difference between the injected carrier concentration and concentration of carriers in bulk. So this results in creation of electric field or space charge layer near electrode region. The current produced due to the presence of a space charge effect is called the SCLC current [21,22]. Surface defects and the presence of localized electron traps can be created by the synthesis procedure and also from surface passivation. The presence of these traps can drastically hinder the

290K

Vc

-12 -16

-7.5

-20

77K

-9.0 -2.1

-1.4

-0.7

0.0

ln(V) Fig. 2. (a) Double logarithmic graph to explain the conduction mechanism at all temperature range (290–77 K) for p-SPT composite device. (b) Power law fits to the data and the fits meet at single point named as cross over voltage Vc.

77K -2.1

-1.4

-0.7

0.0

ln(V) Fig. 3. (a) Double logarithmic graph to explain the conduction mechanism at all temperature range (290–77 K) for n-SPT composite device. (b) Power law fits to the data and the fits meet at single point named as cross over voltage Vc.

K. Rasool et al. / Microelectronic Engineering 119 (2014) 141–145

passage of injected current. This observation is more prominent especially at lower temperatures where the electrons are stable. Under lower bias and higher temperature condition, the SCLC transport is not evident because of the thermally generated electrons and lower carrier’s injection rate. But at intermediate voltage, we can observe charge injection region and SCLC with traps which can be attributed to creation of space charge layer as described earlier. When voltage increases further, the traps either become filled with enough input potential or carriers have now enough energy to depart the traps and hence shows trap free SCLC [19,23]. Hence the current density can be written as [20]

 lþ1  l 2l þ 1 l es e0 V lþ1 2lþ1 lþ1 l þ 1 Ht d

ð1Þ

Here Ht, e0, es, l, NDOS and d represents the trap density, permittivity of vacuum, dielectric constant of the material, mobility of carriers, density of states in the relevant band, and SiNWs thickness respectively. SCLC region in both p-SPT and n-SPT composite devices meets at single point known as cross-over voltage shown in Figs. 2b and 3b) respectively. Values of cross-over voltage are 0.925 V and 0.639 V for p-SPT and n-SPT composite devices respectively. From cross-over voltage we have calculated the trap density (Ht) which is 7.73  1011 cm3 and 5.34  1011 cm3 for p-SPT and n-SPT composite devices respectively by using equation. 2

Vc ¼

qN t d 2es e0

ð2Þ

In comparison, the values of slope in n-SPT composite device are greater than p-SPT composite device which can be related to lower conductivity and greater difference of injected carrier concentration and material bulk. At intermediate voltages, the SCLC region

-12000

(a)

-700 -600

n-type SiNWs/PAA/TiO2 NPs -6000 -3000

p-type SiNWs/PAA/TiO2 NPs

Z'' (Ω)

Z'' (Ω)

-9000

0 0

3000

6000

9000

12000

(b)

1.0

-400

0.8

-300

0.6

-200

0.4

-100

0.2

-5500

M''

2

-500

0.0

102

0.0016

n-type SiNWs/PAA/TiO NPs

1.2

0

Z' (Ω) 0.030

1.4

(a)

103

104

105

Frequency (Hz)

106

15

(b)

0.0008

-4400

p-type SiNWs/PAA/TiO NPs 2

10

0.0000 0.0000

0.015

0.0007

0.0014

-3300

Z'' (Ω)

M''

M'

-2200

5

-1100

0.000

0

0

0.000

0.015

0.030

M' Fig. 4. (a) Room temperature Nyquist plot for n-SPT and p-SPT composite devices (b) Modulus plane plot for both composite devices.

102

M'' x 10-3

J ¼ q1l lNDOS

is more visible and clear in n-SPT composite device than that of p-SPT composite device. But as voltage increases these injected carriers may fill the traps and shows the trap free SCLC as explained above. We have also measured the dielectric properties of both p-SPT and n-SPT composite device at room temperature from impedance spectroscopy in the frequency range 100 Hz to 3 MHz. Nyquist plot shows the typical semicircular behavior for both composite devices. Both semi circles are depressed as shown in Fig. 4(a). This shows the deviation from ideal Debye behavior [24]. Nyquist plot shows one semicircular arc for both cases. Impedance of p-SPT composite device is smaller than that of n-SPT composite device. This result is in agreement with DC data explained above. Experimental data was fitted with Zview software. Where RC circuit represents interface of SiNWs and R is modeled as resistance of bulk SiNWs explained elsewhere [15]. Here instead of ideal capacitor we are using constant phase element (CPE) due to inhomogeneity of interface between SiNWs and PAA/TiO2 NPs [25]. The proposed circuit fits our data well. For further investigation of electro-active regions we have also plotted the complex modulus plot as shown in Fig. 4(b). In comparison to Nyquist plot, modulus plane plot gives more information. Here n-SPT composite device shows one semicircle in the given frequency ranges. But p-SPT composite device shows clearly two well resolved semicircles in the given frequency range [26]. Fig. 5(a) and (b) shows the comparison between M00 vs. Z00 for nSPT and p-SPT composite devices. The p-SPT composite device shows two peaks of M00 appeared at lower and higher frequencies but n-SPT composite device shows one peak. We have also calculated the values of capacitance from peak position of M00 by using this relation, M00 = e0/2C as shown in Fig. 5(a) and (b). The value of capacitance for smaller peak which is appeared at lower frequency is 1011 F may be due to interface effect. Other peak

M'' x 10-3

144

103

104

105

Frequency (Hz)

106

Fig. 5. Comparison of Z00 and M00 with frequency for (a) p-SPT composite device (b) n-SPT composite device.

K. Rasool et al. / Microelectronic Engineering 119 (2014) 141–145

1350

(a) p-type SiNWs/PAA/TiO NPs 2

900

145

posite device shows higher e0 and rac 15 times and 85 times respectively than that of n-SPT composite device. This increment in electrical and dielectric properties can be attributed to the presence of high dielectric materials such as PAA and TiO2 NPs on the surface of SiNWs.

ε'

4. Conclusion

450

n-type SiNWs/PAA/TiO2 NPs 0 103

0.10 0.08

σac(S/m)

0.06

104 105 Frequency (Hz)

106

(b) n-type SiNWs/PAA/TiO2 NPs p-type SiNWs/PAA/TiO2 NPs

0.04 0.02 0.00 103

104 105 Frequency (Hz)

106

Fig. 6. (a) Dependence of e0 on frequency for n-SPT and p-SPT composite device (b) Dependence of rac on frequency for n-SPT and p-SPT composite device.

appeared at higher frequency is not fully resolved due to frequency limitations of our system. The value is 1012 F, which may be associated to the bulk SiNWs. Similarly n-SPT composite device shows one peak in M00 and have capacitance value 1012 F. Interface effect for n-SPT composite device is not clear in this frequency range. We have tried to explain the possible reason for one type of relaxation in n-SPT composite device. This may be attributed to the utilization of majority carriers from n-type SiNWs via surface passivation with PAA/TiO2 NPs. The creation of acceptor like states on n-type SiNWs can reduce majority carriers and can even convert ntype SiNWs to p-type. When peaks of M00 vs. Z00 overlap each other the conduction is usually associated with non localized or ionic conduction. But as seen from both cases our peaks of M00 vs. Z00 are separated so the dielectric response depends on localized conduction or due to defect relaxation. This type of defect relaxation can be due to surface passivation or presence of traps which can be due to variety of reasons [9]. Fig. 6(a) and (b) shows the dependence of e0 and rac on frequency for n-SPT and p-SPT composite devices. Again p-SPT com-

In conclusion, we have successfully fabricated n-SPT and p-SPT composite devices. The presence of PAA and TiO2 NPs on SiNWs was confirmed by EDS. Temperature dependent IV characteristics shows the dominant conduction mechanism at intermediate voltage is SCLC with traps in both cases. Further, dielectric properties have also been investigated. The p-SPT composite device shows higher electrical current and superior dielectric properties than that of n-SPT composite device. This increment can be attributed to the creation of acceptor like states via PAA/TiO2 NPs which favors the conduction in p-SPT composite device than that of nSPT composite device. References [1] K.-I. Chen, B.-R. Li, Y.-T. Chen, Nano Today 6 (2011) 131. [2] A.I. Hochbaum, R. Chen, R.D. Delgado, W. Liang, E.C. Garnett, M. Najarian, A. Majumdar, P. Yang, Nature 451 (2008) 163. [3] S.J. Park, J.G. Eden, J.J. Ewing, Appl. Phys. Lett. 81 (2002) 4529. [4] C.-H. Lin, T.-T. Chen, Y.-F. Chen, Opt. Express 16 (2008) 16916. [5] V. Dobrokhotov, D.N. McIlroy, M.G. Norton, A. Abuzir, W.J. Yeh, I. Stevenson, R. Pouy, J. Bochenek, M. Cartwright, L. Wang, J. Dawson, M. Beaux, C. Berven, J. Appl. Phys. 99 (2006) 104302. [6] K. Rasool, M.A. Rafiq, M. Ahmad, Z. Imran, S.S. Batool, M.M. Hasan, AIP Adv. 3 (2013) 082111. [7] M. Ahmad, K. Rasool, M. Rafiq, M. Hasan, Appl. Phys. Lett. 101 (2012) 223103. [8] A.R. Abramson, W.C. Kim, S.T. Huxtable, H. Yan, Y. Wu, A. Majumdar, C.-L. Tien, P. Yang, J. Microelectromech. Syst. 13 (2004) 505. [9] K. Rasool, M. Rafiq, M. Ahmad, Z. Imran, M. Hasan, Appl. Phys. Lett. 101 (2012) 253104. [10] M. Ahmad, K. Rasool, M. Rafiq, M. Hasan, C. Li, Z. Durrani, Phys. E (2012). [11] B. Ding, M. Yamazaki, S. Shiratori, Sens. Actuators, B 106 (2005) 477. [12] C. Wu, X. Huang, L. Xie, X. Wu, J. Yu, P. Jiang, J. Mater. Chem. 21 (2011) 17729. [13] K. Takanezawa, K. Hirota, Q.-S. Wei, K. Tajima, K. Hashimoto, J. Phy. Chem. C 111 (2007) 7218. [14] A. Dey, S. De, A. De, S. De, J. Nanosci. Nanotechnol. 6 (2006) 1427. [15] K. Rasool, M. Rafiq, C. Li, E. Krali, Z. Durrani, M. Hasan, Appl. Phys. Lett. 101 (2012) 023114. [16] K. Rasool, M. Usman, M. Ahmad, Z. Imran, M.A. Rafiq, M.M. Hasan, A. Nazir, Effect of modifiers on structural and optical properties of Titania (TiO2) nanoparticles, in: Electronics, Communications and Photonics Conference (SIECPC), 2011 Saudi, International, 2011, p. 1. [17] M. Grundner, H. Jacob, Appl. Phys. A 39 (1986) 73. [18] J. Jie, W. Zhang, K. Peng, G. Yuan, C.S. Lee, S.T. Lee, Adv. Funct. Mater. 18 (2008) 3251. [19] K. Rasool, M. Rafiq, M. Ahmad, Z. Imran, M. Hasan, J. Appl. Phys. 113 (2013) 193703. [20] M. Rafiq, Y. Tsuchiya, H. Mizuta, S. Oda, S. Uno, Z. Durrani, W. Milne, Appl. Phys. Lett. 87 (2005) 182101. [21] A. Rose, Phys. Rev. 97 (1955) 1538. [22] M.A. Lampert, Phys. Rev. 103 (1956) 1648. [23] P. Mark, W. Helfrich, J. Appl. Phys. 33 (1962) 205. [24] A. Jonscher, Phys. Status Solidi A 32 (1975) 665. [25] G. Brug, A. Van Den Eeden, M. Sluyters-Rehbach, J. Sluyters, J. Electroanal. Chem. Interfacial Electrochem. 176 (1984) 275. [26] D. Almond, A. West, Solid State Ionics 11 (1983) 57.