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Effect of growth conditions on zinc oxide nanowire array synthesized on Si (100) without catalyst
Materials Science in Semiconductor Processing 16 (2013) 171–178
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effect of growth conditions on zinc oxide nanowire array synthesized on Si (100) without catalyst M. Rajabi a, R.S. Dariani a,n, A. Iraji zad b a b
Department of Physics, Alzahra University, Tehran 1993893973, Iran Department of Physics, Sharif University of Technology, Tehran 1136511155, Iran
a r t i c l e i n f o
abstract
Available online 7 June 2012
A uniformly distributed ZnO nanowire array has been grown on silicon (100) substrates by catalyst-free chemical vapor transport and condensation. The effect of growth conditions including source heating temperature, substrate temperature, and gas flow rate on growth properties of ZnO nanowire arrays are studied. Scanning electron microscopy, X-ray diffraction, and room temperature photoluminescence are employed to study the structural features and optical properties of the samples. The results show a correlation among experimental growth parameters. There is a zone for substrate temperature, by controlling gas flow rate, that uniformly distributed and well aligned ZnO nanowire arrays can be grown. Also, experiments indicate that ZnO nanowire arrays with different diameter along their length have been formed under various growth conditions in the same distance from source material. It is found that supersaturation is a crucial parameter determining the growth behavior of ZnO nanowire arrays. The growth mechanism of ZnO nanowires is discussed. The room temperature photoluminescence spectrums of ZnO nanowire array show two emission bands. One is the exciton emission band (centered at 380 nm) and the other is a broad visible emission band centered at around 490 nm. As the substrate temperature decreases, the intensity of UV emission increases while the intensity of visible emission peak decreases. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Zinc oxide Nanowires Growth mechanism Photoluminescence
1. Introduction Zinc oxide (ZnO) is a wide band gap semiconductor with a direct band gap of 3.37 eV and a high exciton binding energy of 60 meV at room temperature. The large exciton binding energy and wide band gap make it a promising material for optoelectronic devices such as photo detectors, light emitting diodes (LEDs), laser diodes (LDs), and solar cells [1–6]. It is also applicable as a
n Corresponding author. Tel.: þ98 21 85692646; fax: þ 98 21 88047861. E-mail address: [email protected] (R.S. Dariani).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.05.004
piezoelectric, gas sensor, and transparent conducting material [7–9]. ZnO nanostructures can be grown in a wide variety of morphologies such as nanowires, nanorods, nanotetrapods, nanobelts, nanorings, and nanocombs [10–16]. Among them nanowires and nanorods grown by vapor deposition techniques as well as solution based methods have attracted much attention because they provide a direct path for charge transport [17] and have high surface to volume ratio (high surface area). Pulsed laser deposition (PLD) [18], chemical vapor deposition (CVD) [19], metal organic chemical vapor deposition (MOCVD) [20], and thermal evaporation are vapor phase deposition techniques for growth of ZnO nanowires and nanorods. Among thermal evaporation methods, chemical vapor
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M. Rajabi et al. / Materials Science in Semiconductor Processing 16 (2013) 171–178
transport and condensation (CVTC) is an extensively used, simple, and low cost method which can provide large quantity and high crystalline quality products. In this method, Zn and Zn suboxides (ZnOx, xo1) are generated by carbon thermal reduction of ZnO. Then the vapor is transported onto a substrate placed downstream of source material by carrier gas where it condenses and deposits through vapor–liquid–solid (VLS) [21] or vapor– solid (VS) [22] mechanisms. The VLS mechanism is used for making well aligned ZnO nanowires arrays via metal catalysts such as Au. However, catalysts usually affect purity and quality of products especially optoelectronic devices. Thus, there is an obvious interest in fabrication of ZnO nanowires and nanorods without the use of metal catalysts via VS mechanism. Although extensive work has been down on growing of ZnO nanowires and nanorods, but the effects of growth conditions on the growth mechanism of ZnO nanowires is still unclear [23–32]. A clear understanding of the nanowires growth process is necessary to reveal the growth mechanism to realize controllable synthesis of nanowires. In this work, we study the effect of growth conditions including source heating temperature, substrate temperature, and gas flow rate on structural features of ZnO nanowire array. The growth mechanism of ZnO nanowire array is presented. The crystal structure and optical properties of ZnO nanowire arrays grown under optimum conditions are studied.
which was placed at the central hot zone of the furnace was heated at a rate of 20 1C/min and was kept for 1 h under constant flow of high purity Ar gas. After the desired growth was reached, the furnace was turned off and cooled down to room temperature. The crystal structure of ZnO nanorods were characterized by X-ray diffraction (XRD; Philips X’ Pert) with CuKa radiation (l ¼0.1542 nm). The surface morphology of ZnO nanorods were indicated by scanning electron microscope (SEM; Philips XL30) and field emission scanning electron microscope (FESEM; Hitachi S-4166). The photoluminescence measurements were made at room temperature using a 325 nm excitation of a fluorescence spectrophotometer (Cary Eclipse). 3. Results and discussion It is believed that the formation of ZnO nanowire arrays is a complex process which involves different controlling parameters such as temperature, pressure, carrier gas flow, and oxygen availability. Also, there exists an optimum vapor environment which leads to the growth of well aligned ZnO nanowire arrays. In order to understand the growth process of ZnO nanowire arrays, a series of experiments are designed. These experiments were performed with the substrate temperature, gas flow rate, and source heating temperature for a constant deposition process.
2. Experiments
3.1. Substrate temperature
The synthesis of ZnO nanowire arrays was carried out by a catalyst free chemical vapor transport and condensation method in a horizontal quartz tube furnace as schematically shown in Fig. 1. The source material was a mixture of zinc oxide powder (99%, Merck) and graphite powder (99%, LOBA chemie) in equal amounts (a weight ratio of 1:1). 1 g of source material and several pieces of Si substrates were placed into a small quartz tube (2.7 cm inner diameter and 40 cm length). This tube was transferred into the outer quartz tube (3.8 cm inner diameter and 95 cm length) of horizontal furnace. One side of the quartz tube was connected to an Ar gas inlet and the other side to the environment. The distance between the gas inlet and center of the furnace was about 55 cm. We used p-type Si (100) as the substrates. They were placed downstream in the inner tube along the direction of gas flow which results in different substrate (growth temperature) temperatures due to the natural temperature gradient along the furnace. The source material
It is proposed that the substrate temperature is the most effective parameter on the nanowires’ growth mechanism. Substrate temperature determines surface diffusion length and the amount of adatoms condensation on the surface [33]. To study the effect of substrate temperature on the growth process we put six Si substrates side by side away from the source material in different temperature zones as shown in Fig. 1. Source heating temperature was 1005 1C; gas flow rate was 150 sccm. These samples are labeled as S1, S2, etc. Morphology of the ZnO structures was analyzed using FE-SEM as shown in Fig. 2(a)–(f). The growth temperature and calculated average diameter of wires from SEM images are presented in Table 1. The images display that substrate temperature is an important parameter on the size, orientation, and density of the formed wires. At high temperatures (more than 900 1C, Fig. 2(a) and (b)), high rate of desorption and long diffusion length of adatoms leads to formation of low density and rather long length
Fig. 1. Schematic of the experimental set up for growth of ZnO nanowire arrays in a horizontal tube furnace, 1) source material and 2) substrate at different temperatures.
M. Rajabi et al. / Materials Science in Semiconductor Processing 16 (2013) 171–178
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Fig. 2. FE-SEM images of ZnO structures are grown on Si substrates at different substrate temperatures. The growth conditions are listed in Table 1: (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, (f) S6, and (g) variations of the wire’s average diameter versus growth temperature.
micro rods with average diameter higher than 2.5 mm as mentioned in Table 1. As the temperature of the substrates is reduced, the areal density of the wires increases and the average diameter decreases. At temperatures
lower than 800 1C they present nanosize wires and rods where a size distribution exists in diameter. Also the wires’ alignment is more normal on growth samples at low temperatures. Fig. 2(g) shows the average diameter
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Table 1 Characteristics of the ZnO wire arrays synthesized at different conditions and their average diameter calculated from SEM images. Sample
Source temperature (1C)
Gas flow (sccm)
Substrate temperature (oC) %
Average diameter
S1 S2 S3 S4 S5 S6 S7 S8 S10 S11 S12
1005 7 2 1005 7 2 1005 7 2 1005 7 2 1005 7 2 1005 7 2 1005 7 2 1005 7 2 960 7 2 1005 7 2 1060 7 5
150 150 150 150 150 150 210 210 210 210 210
945 715 915 715 8807 20 825 735 755 735 6807 40 6807 40 6007 30 7307 30 745 725 7307 40
3.00 730 mm 2.60 730 mm 1.10 70.10 mm 600 780 nm 360 770 nm 230 760 nm 200 750 nm 95 715 nm 1.90 70.10 mm 750 7100 nm 440 7100 nm
Fig. 3. (a) Top view SEM and (b) cross section FE-SEM images of a sample grown under 210 sccm Ar gas flow rate at 680 1C (S7). The scale bar in figures (a) and (b) is 1 mm and 5 mm, respectively.
(d) versus growth temperature (T) plot. By fitting this plot we can obtain the relationship between d and T 1 ¼ ABT d
ð1Þ
where A and B are constants. This equation is drawn by a solid line in the plot. This equation relates the substrate temperature (T) to the diameter of wires (d), this should decrease as the substrate temperature decreases; this is indeed observed. If we thought the diameter of ZnO wires are proportional to the size of critical nucleus then according to classical theory of nucleation the diameter of ZnO nanowire can be written as [34] 1 DHV T DHV DHV ¼ 1 ¼ T d T0 4gGL siny 4gGL siny 4T 0 gGL sin y ð2Þ where gGL is the surface energy of the interface between gas and liquid phases, y is the wetting angle between liquid phase and the substrate, DHV is the change in enthalpy per unit volume between gas and liquid phases, T0 and T are the temperature of equilibrium state and supersaturation state, respectively. If we suppose that [34] A¼
DHV 4gGL siny
then B is A/T0. These equations show the obvious agreement of experimental data with the classical nucleation theory.
ð3Þ
3.2. Gas flow rate In order to study the effect of gas flow rate, the experiment was carried out under different Ar gas flow rates while other growth parameters such as substrate temperature and source heating temperature were kept constant at about 680 1C and 1005 1C, respectively. We observed that when Ar flow rate is lower than 120 sccm no growth can be observed. So in the next experiment, the gas flow rate was increased to 210 sccm. Top view and cross section SEM images of the ZnO nanowire arrays grown under this condition are presented in Fig. 3. A comparison between the top view SEM image of Fig. 3 and Fig. 2(f; flow rate 150 sccm) indicates that the size distribution of the array decreases in the range 100– 250 nm with lower average diameter of 200 nm. Fig. 3(b) presents a cross section SEM image of aligned wires 6–9 mm in length. From these results the higher gas flow rate leads to the formation of well aligned ZnO nanowire arrays. At the same condition, ZnO nanowire arrays with diameters less than 100 nm, sharper heads, and lengths of 3–5 mm were grown at substrate temperature in a range 600 1C as is shown in Fig. 4.
M. Rajabi et al. / Materials Science in Semiconductor Processing 16 (2013) 171–178
3.3. Source temperature Since source temperature determines the amount of Zn vapor, in the present work we kept it at three temperatures 960, 1005, and 1060 1C while other growth parameters are kept constant. The growth conditions of these samples (S10, S11, and S12) are presented in Table 1. Fig. 5(a)–(c) shows top view SEM images of these arrays. The average diameter of wires calculated and listed in Table 1 revealing that the average diameter of wires decreases by enhancement of Zn vapor pressure. However, the cross section SEM images of Fig. 5(d) display that the narrowest ZnO wires are nucleated and grown on a thick ZnO buffer layer formed on Si substrate. The cross section SEM images of Figs. 3(b) and 5(d) indicate that
prior to wire growth a buffer layer is formed on the Si substrates. The crystal structure of ZnO nanowire arrays synthesized at two different substrate temperatures ( 745 1C (A) and 680 1C (B)) are investigated by the X-ray diffraction analysis in the 30–801 range of 2y. As shown in Fig. 6, the diffraction patterns indicate a hexagonal– wurtzite crystal structure. The lattice constants calculated and listed in Table 2 have good agreement with the standard data. All the peaks in the spectra can be indexed to the crystal structure of ZnO. The higher intensity of the (002) peak indicates the preferred orientation along their c-axis. The other ZnO peaks can be related to the thin ZnO layer formed at the interface of the Si substrate and the wires in the first stage of deposition. In order to determine the effect of the substrate temperature on the crystal structure of the wires, we calculate the texture coefficient, TC, of the (002) plane according to following expression [35]: TCðhklÞ ¼
Fig. 4. SEM image of ZnO nanowires grown on Si substrate at 600 1C (S8). The scale bar is 500 nm.
175
IðhklÞ =I3ðhklÞ P N IðhklÞ =I3ðhklÞ
N 1
ð4Þ
where IðhklÞ and I3ðhklÞ are measured and standard intensity of the ðhklÞ plane taken from the JCPDS data and N is the total number of diffraction peaks. When this value is larger than 1 a preferred orientation exists along the ðhklÞ plane. The presented results in Table 2 show that all the TC for the (002) plane is above 1 and depends on temperature. The reduction of growth temperature from 745 1C to 680 1C increases the intensity of XRD pattern and value of TC (002) from 2.6 to 3.1, revealing that the crystal structure of ZnO nanowire arrays is improved at lower substrate temperatures.
Fig. 5. Top view SEM images of ZnO wire arrays synthesized using different source heating temperature of (a) 960 1C (S10), (b) 1005 1C (S11), (c) 1060 1C (S12), and (d) cross section SEM image of sample S12. The scale bars in figures (a), (b) and (c) are 10, 5 and 2 mm, respectively.
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Fig. 6. X-ray diffraction of ZnO wire arrays grown at two different substrate temperatures. The growth conditions of samples A (S11) and B (S7) are presented in Table 1.
Table 2 Variations of lattice constants and texture coefficient with growth temperature. Labels A and B denote samples S11 and S7, respectively. Sample
˚ c (A)
˚ a (A)
c/a
TC (002)
Standard data (JCDS 80-0075) A B
5.2098 5.2086 5.2085
3.2539 3.2469 3.2510
1.6011 1.6041 1.6021
– 2.6 3.1
Fig. 7 shows the room temperature PL spectra of ZnO arrays that are grown on Si substrates at different growth temperatures while other growth parameters such as the source temperature and gas flow rate are similar. The PL spectra consist of two emission peaks: an ultraviolet (UV) emission at 380 nm and a broad visible emission at around 490 nm. The ratio of UV to visible emission intensity for growth temperatures of 690, 650 and 600 1C are equal to 0.5, 1, and 3, respectively which means it increases by reduction of substrate temperature. The UV emission band is generally ascribed to near-band emission of ZnO nanostructures. The origin of visible emission bands is related to deep level defect emission but has not been conclusively established [36]. A proposed hypothesis relates visible emission band to oxygen vacancy [37]. Fig. 7(d) shows PL spectra of a typical sample before (a) and after (b) annealing. The sample is annealed at 550 1C for 30 min while O2 gas passes through the furnace with a flow rate of 20 sccm. Our results show that the ratio of UV to visible emission intensity increases from 0.48 to 0.65 after annealing, indicating a reduction of oxygen vacancy concentration. Lowering the oxygen vacancy related defects increases the near-band emission observed in Fig. 7(d) and enhances the intensity ratio of UV to visible emission in photoluminescence. All these results show that an optimum condition is available in which low diameter and high density of ZnO nanowire arrays with good crystal structure and luminescence property can be grown.
An interesting observed phenomenon is the formation of pencil like wires (wires with changed diameter along their length) as is shown in SEM image of Fig. 4 for samples located at 18–20 cm distance from the source and under different growth conditions. Thus, ZnO nanowires with changed diameter can be synthesized by carbothermal reduction of ZnO by controlling growth condition. The growth parameter that depends on source to substrate distance is supersaturation that is defined by [38,39]
s¼
p 1 p0
ð5Þ
where p and p0 are the actual vapor pressure at the surface and the equilibrium vapor pressure, respectively. The growth mechanism of ZnO wires at different substrate temperatures can be divided into different steps, as demonstrated in Fig. 8. First, when the source material is heated at temperatures above 900 1C, Zn and ZnOx (x o1) vapor are produced (according to the Ellingham diagram). The high supersaturation ratio of Zn vapor leads to formation of polycrystalline buffer layer on Si substrate [40,41]. Our results indicate that growth conditions (substrate and source heating temperatures) affect layer morphology and the film thickness reduces by reduction of substrate temperature. While by increasing the source temperature, which enhances Zn vapor pressure, the layer thickness increases. Thus, these growth parameters are correlated with supersaturation ratio [27]. The next step is nucleation and growth of hexagonal shaped wires on the buffer layer. Based on our analysis, the diameter of wires is determined by supersaturation ratio. The wires with large diameter are grown at high supersaturation ratio and low super saturation ratio would result in formation of nanowiers with small diameter [42]. Similar to what Ye et al. [27] showed in the supersaturation profile of ZnO vapor, the higher gas flow rate reduces and moves the maximum of supersaturation along the furnace. The higher supersaturation at lower substrate temperature leads to better alignment of ZnO nanowires.
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Fig. 7. Room temperature PL spectra of ZnO nanowire arrays on Si substrates grown at different growth temperatures (a) 690 1C, (b) 650 1C, and (c) 600 1C using the same gas flow ( 210 sccm) while the source temperature was 1005 7 2 1C. (d) The effect of annealing on PL intensity for a typical sample (a) before and (b) after annealing.
Fig. 8. Schematic illustration of the growth mechanism of ZnO wires at two different substrate temperatures.
The reduction of supersaturation ratio at the growing faces changes the diameter of the ZnO wires along its length (Fig. 4). This non equilibrium condition results from the Ehrilich–Schwoebel barrier that suppresses diffusion of adatoms to a lower terrace, decreases the rate of layer by layer growth and forms a cone like structure on top of the ZnO wires [43,44]. This potential barrier depends on the crystallographic structure of the materials and the growth temperature [29]. The last step is the
growth of nanorods on top of the cone like structures which can be a result of the new equilibrium condition between supersaturation and the growing face along the axis of the based wires. 4. Conclusion We have studied the catalyst free growth of ZnO wire arrays on silicon (100) substrate by the chemical vapor
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transport and condensation methods. We study an optimum condition for growth of ZnO nanowire arrays by investigating the effect of different growth parameters including substrate temperature, gas flow rate, and source heating temperature. Our results show that substrate temperature and gas flow rates are major parameters for controlling density, diameter, length, and alignment of ZnO nanowires. The growth mechanism of ZnO nanowire arrays is presented. The ZnO seed layer formation at the root of nanowires is observed. Diameter evolution of the ZnO nanowires along their length is related to supersaturation which depends on the distance from source materials. Substrate temperature has effect on crystal structure and room temperature photoluminescence properties of nanowire arrays. Annealing of the samples in oxygen ambient enhances the ratio of UV to visible emission intensity. References [1] L. Luo, Y. Zhang, S.S. Mao, L. Lin, Sensors and Actuators A 127 (2006) 201–206. [2] J.H. He, S.T. Ho, T.B. Wu, L.J. Chen, Z.L. Wang, Chemical Physics Letters 435 (2007) 119–122. [3] A. Nadarajah, R.C. Word, J. Meiss, R. Konenkamp, Nano Letters 8 (2008) 534–537. [4] X.W. Sun, J.Z. Huang, J.X. Wang, Z. Xu, Nano Letters 8 (2008) 1219–1223. [5] X.M. Zhang, M.Y. Lu, Y. Zhang, L.J. Chen, Z.L. Wang, Advanced Materials 21 (2009) 2767–2770. [6] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P.D. Yang, Nature Materials 4 (2005) 455–459. [7] T. Itoh, T. Suga, Applied Physics Letters 64 (1994) 37–39. [8] D.S. Ginley, C. Bright, MRS Bulletin 25 (2000) 15–18. [9] E. Comini, G. Faglia, G. Sberveglieri, Z. Pan, Z.L. Wang, Applied Physics Letters 81 (2002) 1869–1871. [10] S.H. Dalal, D.L. Baptista, K.B.K. Teo, R.G. Lacerda, D.A. Jefferson, W.I. Milne, Nonotechnology 17 (2006) 4811–4818. [11] V.A.L. Roy, A.B. Djurisic, W.K. Chan, J. Gao, H.F. Lui, C. Surya, Applied Physics Letters 83 (2003) 141–143. [12] X.Y. Kong, Z.L. Wang, Applied Physics Letters 84 (2004) 975–977. [13] X.Y. Kong, Y. Ding, R. Yang, Z.L. Wang, Science 303 (2004) 1348–1351. [14] C.S. Lao, P.X. Gao, R.S. Yang, Y. Zhang, Y. Dai, Z.L. Wang, Chemical Physics Letters 417 (2006) 358–362. [15] X.H. Wang, S. Liu, P. Chang, Y. Tang, Materials Science in Semiconductor Processing 10 (2007) 241–245.
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Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effect of growth conditions on zinc oxide nanowire array synthesized on Si (100) without catalyst M. Rajabi a, R.S. Dariani a,n, A. Iraji zad b a b
Department of Physics, Alzahra University, Tehran 1993893973, Iran Department of Physics, Sharif University of Technology, Tehran 1136511155, Iran
a r t i c l e i n f o
abstract
Available online 7 June 2012
A uniformly distributed ZnO nanowire array has been grown on silicon (100) substrates by catalyst-free chemical vapor transport and condensation. The effect of growth conditions including source heating temperature, substrate temperature, and gas flow rate on growth properties of ZnO nanowire arrays are studied. Scanning electron microscopy, X-ray diffraction, and room temperature photoluminescence are employed to study the structural features and optical properties of the samples. The results show a correlation among experimental growth parameters. There is a zone for substrate temperature, by controlling gas flow rate, that uniformly distributed and well aligned ZnO nanowire arrays can be grown. Also, experiments indicate that ZnO nanowire arrays with different diameter along their length have been formed under various growth conditions in the same distance from source material. It is found that supersaturation is a crucial parameter determining the growth behavior of ZnO nanowire arrays. The growth mechanism of ZnO nanowires is discussed. The room temperature photoluminescence spectrums of ZnO nanowire array show two emission bands. One is the exciton emission band (centered at 380 nm) and the other is a broad visible emission band centered at around 490 nm. As the substrate temperature decreases, the intensity of UV emission increases while the intensity of visible emission peak decreases. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Zinc oxide Nanowires Growth mechanism Photoluminescence
1. Introduction Zinc oxide (ZnO) is a wide band gap semiconductor with a direct band gap of 3.37 eV and a high exciton binding energy of 60 meV at room temperature. The large exciton binding energy and wide band gap make it a promising material for optoelectronic devices such as photo detectors, light emitting diodes (LEDs), laser diodes (LDs), and solar cells [1–6]. It is also applicable as a
n Corresponding author. Tel.: þ98 21 85692646; fax: þ 98 21 88047861. E-mail address: [email protected] (R.S. Dariani).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.05.004
piezoelectric, gas sensor, and transparent conducting material [7–9]. ZnO nanostructures can be grown in a wide variety of morphologies such as nanowires, nanorods, nanotetrapods, nanobelts, nanorings, and nanocombs [10–16]. Among them nanowires and nanorods grown by vapor deposition techniques as well as solution based methods have attracted much attention because they provide a direct path for charge transport [17] and have high surface to volume ratio (high surface area). Pulsed laser deposition (PLD) [18], chemical vapor deposition (CVD) [19], metal organic chemical vapor deposition (MOCVD) [20], and thermal evaporation are vapor phase deposition techniques for growth of ZnO nanowires and nanorods. Among thermal evaporation methods, chemical vapor
172
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transport and condensation (CVTC) is an extensively used, simple, and low cost method which can provide large quantity and high crystalline quality products. In this method, Zn and Zn suboxides (ZnOx, xo1) are generated by carbon thermal reduction of ZnO. Then the vapor is transported onto a substrate placed downstream of source material by carrier gas where it condenses and deposits through vapor–liquid–solid (VLS) [21] or vapor– solid (VS) [22] mechanisms. The VLS mechanism is used for making well aligned ZnO nanowires arrays via metal catalysts such as Au. However, catalysts usually affect purity and quality of products especially optoelectronic devices. Thus, there is an obvious interest in fabrication of ZnO nanowires and nanorods without the use of metal catalysts via VS mechanism. Although extensive work has been down on growing of ZnO nanowires and nanorods, but the effects of growth conditions on the growth mechanism of ZnO nanowires is still unclear [23–32]. A clear understanding of the nanowires growth process is necessary to reveal the growth mechanism to realize controllable synthesis of nanowires. In this work, we study the effect of growth conditions including source heating temperature, substrate temperature, and gas flow rate on structural features of ZnO nanowire array. The growth mechanism of ZnO nanowire array is presented. The crystal structure and optical properties of ZnO nanowire arrays grown under optimum conditions are studied.
which was placed at the central hot zone of the furnace was heated at a rate of 20 1C/min and was kept for 1 h under constant flow of high purity Ar gas. After the desired growth was reached, the furnace was turned off and cooled down to room temperature. The crystal structure of ZnO nanorods were characterized by X-ray diffraction (XRD; Philips X’ Pert) with CuKa radiation (l ¼0.1542 nm). The surface morphology of ZnO nanorods were indicated by scanning electron microscope (SEM; Philips XL30) and field emission scanning electron microscope (FESEM; Hitachi S-4166). The photoluminescence measurements were made at room temperature using a 325 nm excitation of a fluorescence spectrophotometer (Cary Eclipse). 3. Results and discussion It is believed that the formation of ZnO nanowire arrays is a complex process which involves different controlling parameters such as temperature, pressure, carrier gas flow, and oxygen availability. Also, there exists an optimum vapor environment which leads to the growth of well aligned ZnO nanowire arrays. In order to understand the growth process of ZnO nanowire arrays, a series of experiments are designed. These experiments were performed with the substrate temperature, gas flow rate, and source heating temperature for a constant deposition process.
2. Experiments
3.1. Substrate temperature
The synthesis of ZnO nanowire arrays was carried out by a catalyst free chemical vapor transport and condensation method in a horizontal quartz tube furnace as schematically shown in Fig. 1. The source material was a mixture of zinc oxide powder (99%, Merck) and graphite powder (99%, LOBA chemie) in equal amounts (a weight ratio of 1:1). 1 g of source material and several pieces of Si substrates were placed into a small quartz tube (2.7 cm inner diameter and 40 cm length). This tube was transferred into the outer quartz tube (3.8 cm inner diameter and 95 cm length) of horizontal furnace. One side of the quartz tube was connected to an Ar gas inlet and the other side to the environment. The distance between the gas inlet and center of the furnace was about 55 cm. We used p-type Si (100) as the substrates. They were placed downstream in the inner tube along the direction of gas flow which results in different substrate (growth temperature) temperatures due to the natural temperature gradient along the furnace. The source material
It is proposed that the substrate temperature is the most effective parameter on the nanowires’ growth mechanism. Substrate temperature determines surface diffusion length and the amount of adatoms condensation on the surface [33]. To study the effect of substrate temperature on the growth process we put six Si substrates side by side away from the source material in different temperature zones as shown in Fig. 1. Source heating temperature was 1005 1C; gas flow rate was 150 sccm. These samples are labeled as S1, S2, etc. Morphology of the ZnO structures was analyzed using FE-SEM as shown in Fig. 2(a)–(f). The growth temperature and calculated average diameter of wires from SEM images are presented in Table 1. The images display that substrate temperature is an important parameter on the size, orientation, and density of the formed wires. At high temperatures (more than 900 1C, Fig. 2(a) and (b)), high rate of desorption and long diffusion length of adatoms leads to formation of low density and rather long length
Fig. 1. Schematic of the experimental set up for growth of ZnO nanowire arrays in a horizontal tube furnace, 1) source material and 2) substrate at different temperatures.
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Fig. 2. FE-SEM images of ZnO structures are grown on Si substrates at different substrate temperatures. The growth conditions are listed in Table 1: (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, (f) S6, and (g) variations of the wire’s average diameter versus growth temperature.
micro rods with average diameter higher than 2.5 mm as mentioned in Table 1. As the temperature of the substrates is reduced, the areal density of the wires increases and the average diameter decreases. At temperatures
lower than 800 1C they present nanosize wires and rods where a size distribution exists in diameter. Also the wires’ alignment is more normal on growth samples at low temperatures. Fig. 2(g) shows the average diameter
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Table 1 Characteristics of the ZnO wire arrays synthesized at different conditions and their average diameter calculated from SEM images. Sample
Source temperature (1C)
Gas flow (sccm)
Substrate temperature (oC) %
Average diameter
S1 S2 S3 S4 S5 S6 S7 S8 S10 S11 S12
1005 7 2 1005 7 2 1005 7 2 1005 7 2 1005 7 2 1005 7 2 1005 7 2 1005 7 2 960 7 2 1005 7 2 1060 7 5
150 150 150 150 150 150 210 210 210 210 210
945 715 915 715 8807 20 825 735 755 735 6807 40 6807 40 6007 30 7307 30 745 725 7307 40
3.00 730 mm 2.60 730 mm 1.10 70.10 mm 600 780 nm 360 770 nm 230 760 nm 200 750 nm 95 715 nm 1.90 70.10 mm 750 7100 nm 440 7100 nm
Fig. 3. (a) Top view SEM and (b) cross section FE-SEM images of a sample grown under 210 sccm Ar gas flow rate at 680 1C (S7). The scale bar in figures (a) and (b) is 1 mm and 5 mm, respectively.
(d) versus growth temperature (T) plot. By fitting this plot we can obtain the relationship between d and T 1 ¼ ABT d
ð1Þ
where A and B are constants. This equation is drawn by a solid line in the plot. This equation relates the substrate temperature (T) to the diameter of wires (d), this should decrease as the substrate temperature decreases; this is indeed observed. If we thought the diameter of ZnO wires are proportional to the size of critical nucleus then according to classical theory of nucleation the diameter of ZnO nanowire can be written as [34] 1 DHV T DHV DHV ¼ 1 ¼ T d T0 4gGL siny 4gGL siny 4T 0 gGL sin y ð2Þ where gGL is the surface energy of the interface between gas and liquid phases, y is the wetting angle between liquid phase and the substrate, DHV is the change in enthalpy per unit volume between gas and liquid phases, T0 and T are the temperature of equilibrium state and supersaturation state, respectively. If we suppose that [34] A¼
DHV 4gGL siny
then B is A/T0. These equations show the obvious agreement of experimental data with the classical nucleation theory.
ð3Þ
3.2. Gas flow rate In order to study the effect of gas flow rate, the experiment was carried out under different Ar gas flow rates while other growth parameters such as substrate temperature and source heating temperature were kept constant at about 680 1C and 1005 1C, respectively. We observed that when Ar flow rate is lower than 120 sccm no growth can be observed. So in the next experiment, the gas flow rate was increased to 210 sccm. Top view and cross section SEM images of the ZnO nanowire arrays grown under this condition are presented in Fig. 3. A comparison between the top view SEM image of Fig. 3 and Fig. 2(f; flow rate 150 sccm) indicates that the size distribution of the array decreases in the range 100– 250 nm with lower average diameter of 200 nm. Fig. 3(b) presents a cross section SEM image of aligned wires 6–9 mm in length. From these results the higher gas flow rate leads to the formation of well aligned ZnO nanowire arrays. At the same condition, ZnO nanowire arrays with diameters less than 100 nm, sharper heads, and lengths of 3–5 mm were grown at substrate temperature in a range 600 1C as is shown in Fig. 4.
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3.3. Source temperature Since source temperature determines the amount of Zn vapor, in the present work we kept it at three temperatures 960, 1005, and 1060 1C while other growth parameters are kept constant. The growth conditions of these samples (S10, S11, and S12) are presented in Table 1. Fig. 5(a)–(c) shows top view SEM images of these arrays. The average diameter of wires calculated and listed in Table 1 revealing that the average diameter of wires decreases by enhancement of Zn vapor pressure. However, the cross section SEM images of Fig. 5(d) display that the narrowest ZnO wires are nucleated and grown on a thick ZnO buffer layer formed on Si substrate. The cross section SEM images of Figs. 3(b) and 5(d) indicate that
prior to wire growth a buffer layer is formed on the Si substrates. The crystal structure of ZnO nanowire arrays synthesized at two different substrate temperatures ( 745 1C (A) and 680 1C (B)) are investigated by the X-ray diffraction analysis in the 30–801 range of 2y. As shown in Fig. 6, the diffraction patterns indicate a hexagonal– wurtzite crystal structure. The lattice constants calculated and listed in Table 2 have good agreement with the standard data. All the peaks in the spectra can be indexed to the crystal structure of ZnO. The higher intensity of the (002) peak indicates the preferred orientation along their c-axis. The other ZnO peaks can be related to the thin ZnO layer formed at the interface of the Si substrate and the wires in the first stage of deposition. In order to determine the effect of the substrate temperature on the crystal structure of the wires, we calculate the texture coefficient, TC, of the (002) plane according to following expression [35]: TCðhklÞ ¼
Fig. 4. SEM image of ZnO nanowires grown on Si substrate at 600 1C (S8). The scale bar is 500 nm.
175
IðhklÞ =I3ðhklÞ P N IðhklÞ =I3ðhklÞ
N 1
ð4Þ
where IðhklÞ and I3ðhklÞ are measured and standard intensity of the ðhklÞ plane taken from the JCPDS data and N is the total number of diffraction peaks. When this value is larger than 1 a preferred orientation exists along the ðhklÞ plane. The presented results in Table 2 show that all the TC for the (002) plane is above 1 and depends on temperature. The reduction of growth temperature from 745 1C to 680 1C increases the intensity of XRD pattern and value of TC (002) from 2.6 to 3.1, revealing that the crystal structure of ZnO nanowire arrays is improved at lower substrate temperatures.
Fig. 5. Top view SEM images of ZnO wire arrays synthesized using different source heating temperature of (a) 960 1C (S10), (b) 1005 1C (S11), (c) 1060 1C (S12), and (d) cross section SEM image of sample S12. The scale bars in figures (a), (b) and (c) are 10, 5 and 2 mm, respectively.
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Fig. 6. X-ray diffraction of ZnO wire arrays grown at two different substrate temperatures. The growth conditions of samples A (S11) and B (S7) are presented in Table 1.
Table 2 Variations of lattice constants and texture coefficient with growth temperature. Labels A and B denote samples S11 and S7, respectively. Sample
˚ c (A)
˚ a (A)
c/a
TC (002)
Standard data (JCDS 80-0075) A B
5.2098 5.2086 5.2085
3.2539 3.2469 3.2510
1.6011 1.6041 1.6021
– 2.6 3.1
Fig. 7 shows the room temperature PL spectra of ZnO arrays that are grown on Si substrates at different growth temperatures while other growth parameters such as the source temperature and gas flow rate are similar. The PL spectra consist of two emission peaks: an ultraviolet (UV) emission at 380 nm and a broad visible emission at around 490 nm. The ratio of UV to visible emission intensity for growth temperatures of 690, 650 and 600 1C are equal to 0.5, 1, and 3, respectively which means it increases by reduction of substrate temperature. The UV emission band is generally ascribed to near-band emission of ZnO nanostructures. The origin of visible emission bands is related to deep level defect emission but has not been conclusively established [36]. A proposed hypothesis relates visible emission band to oxygen vacancy [37]. Fig. 7(d) shows PL spectra of a typical sample before (a) and after (b) annealing. The sample is annealed at 550 1C for 30 min while O2 gas passes through the furnace with a flow rate of 20 sccm. Our results show that the ratio of UV to visible emission intensity increases from 0.48 to 0.65 after annealing, indicating a reduction of oxygen vacancy concentration. Lowering the oxygen vacancy related defects increases the near-band emission observed in Fig. 7(d) and enhances the intensity ratio of UV to visible emission in photoluminescence. All these results show that an optimum condition is available in which low diameter and high density of ZnO nanowire arrays with good crystal structure and luminescence property can be grown.
An interesting observed phenomenon is the formation of pencil like wires (wires with changed diameter along their length) as is shown in SEM image of Fig. 4 for samples located at 18–20 cm distance from the source and under different growth conditions. Thus, ZnO nanowires with changed diameter can be synthesized by carbothermal reduction of ZnO by controlling growth condition. The growth parameter that depends on source to substrate distance is supersaturation that is defined by [38,39]
s¼
p 1 p0
ð5Þ
where p and p0 are the actual vapor pressure at the surface and the equilibrium vapor pressure, respectively. The growth mechanism of ZnO wires at different substrate temperatures can be divided into different steps, as demonstrated in Fig. 8. First, when the source material is heated at temperatures above 900 1C, Zn and ZnOx (x o1) vapor are produced (according to the Ellingham diagram). The high supersaturation ratio of Zn vapor leads to formation of polycrystalline buffer layer on Si substrate [40,41]. Our results indicate that growth conditions (substrate and source heating temperatures) affect layer morphology and the film thickness reduces by reduction of substrate temperature. While by increasing the source temperature, which enhances Zn vapor pressure, the layer thickness increases. Thus, these growth parameters are correlated with supersaturation ratio [27]. The next step is nucleation and growth of hexagonal shaped wires on the buffer layer. Based on our analysis, the diameter of wires is determined by supersaturation ratio. The wires with large diameter are grown at high supersaturation ratio and low super saturation ratio would result in formation of nanowiers with small diameter [42]. Similar to what Ye et al. [27] showed in the supersaturation profile of ZnO vapor, the higher gas flow rate reduces and moves the maximum of supersaturation along the furnace. The higher supersaturation at lower substrate temperature leads to better alignment of ZnO nanowires.
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Fig. 7. Room temperature PL spectra of ZnO nanowire arrays on Si substrates grown at different growth temperatures (a) 690 1C, (b) 650 1C, and (c) 600 1C using the same gas flow ( 210 sccm) while the source temperature was 1005 7 2 1C. (d) The effect of annealing on PL intensity for a typical sample (a) before and (b) after annealing.
Fig. 8. Schematic illustration of the growth mechanism of ZnO wires at two different substrate temperatures.
The reduction of supersaturation ratio at the growing faces changes the diameter of the ZnO wires along its length (Fig. 4). This non equilibrium condition results from the Ehrilich–Schwoebel barrier that suppresses diffusion of adatoms to a lower terrace, decreases the rate of layer by layer growth and forms a cone like structure on top of the ZnO wires [43,44]. This potential barrier depends on the crystallographic structure of the materials and the growth temperature [29]. The last step is the
growth of nanorods on top of the cone like structures which can be a result of the new equilibrium condition between supersaturation and the growing face along the axis of the based wires. 4. Conclusion We have studied the catalyst free growth of ZnO wire arrays on silicon (100) substrate by the chemical vapor
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transport and condensation methods. We study an optimum condition for growth of ZnO nanowire arrays by investigating the effect of different growth parameters including substrate temperature, gas flow rate, and source heating temperature. Our results show that substrate temperature and gas flow rates are major parameters for controlling density, diameter, length, and alignment of ZnO nanowires. The growth mechanism of ZnO nanowire arrays is presented. The ZnO seed layer formation at the root of nanowires is observed. Diameter evolution of the ZnO nanowires along their length is related to supersaturation which depends on the distance from source materials. Substrate temperature has effect on crystal structure and room temperature photoluminescence properties of nanowire arrays. Annealing of the samples in oxygen ambient enhances the ratio of UV to visible emission intensity. References [1] L. Luo, Y. Zhang, S.S. Mao, L. Lin, Sensors and Actuators A 127 (2006) 201–206. [2] J.H. He, S.T. Ho, T.B. Wu, L.J. Chen, Z.L. Wang, Chemical Physics Letters 435 (2007) 119–122. [3] A. Nadarajah, R.C. Word, J. Meiss, R. Konenkamp, Nano Letters 8 (2008) 534–537. [4] X.W. Sun, J.Z. Huang, J.X. Wang, Z. Xu, Nano Letters 8 (2008) 1219–1223. [5] X.M. Zhang, M.Y. Lu, Y. Zhang, L.J. Chen, Z.L. Wang, Advanced Materials 21 (2009) 2767–2770. [6] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P.D. Yang, Nature Materials 4 (2005) 455–459. [7] T. Itoh, T. Suga, Applied Physics Letters 64 (1994) 37–39. [8] D.S. Ginley, C. Bright, MRS Bulletin 25 (2000) 15–18. [9] E. Comini, G. Faglia, G. Sberveglieri, Z. Pan, Z.L. Wang, Applied Physics Letters 81 (2002) 1869–1871. [10] S.H. Dalal, D.L. Baptista, K.B.K. Teo, R.G. Lacerda, D.A. Jefferson, W.I. Milne, Nonotechnology 17 (2006) 4811–4818. [11] V.A.L. Roy, A.B. Djurisic, W.K. Chan, J. Gao, H.F. Lui, C. Surya, Applied Physics Letters 83 (2003) 141–143. [12] X.Y. Kong, Z.L. Wang, Applied Physics Letters 84 (2004) 975–977. [13] X.Y. Kong, Y. Ding, R. Yang, Z.L. Wang, Science 303 (2004) 1348–1351. [14] C.S. Lao, P.X. Gao, R.S. Yang, Y. Zhang, Y. Dai, Z.L. Wang, Chemical Physics Letters 417 (2006) 358–362. [15] X.H. Wang, S. Liu, P. Chang, Y. Tang, Materials Science in Semiconductor Processing 10 (2007) 241–245.
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