J Nanopart Res (2012) 14:866 DOI 10.1007/s11051-012-0866-9
RESEARCH PAPER
Homogeneous ZnO nanostructure arrays on GaAs substrates by two-step chemical bath synthesis Chun-Yuan Huang • Tzung-Han Wu Chiao-Yang Cheng • Yan-Kuin Su
•
Received: 18 August 2011 / Accepted: 10 April 2012 / Published online: 13 June 2012 Ó Springer Science+Business Media B.V. 2012
Abstract ZnO nanostructures, including nanowires, nanorods, and nanoneedles, have been deposited on GaAs substrates by the two-step chemical bath synthesis. It was demonstrated that the O2-plasma treatment of GaAs substrates prior to the sol–gel deposition of seed layers was essential to conformally grow the nanostructures instead of 2D ZnO bunches and grains on the seed layers. Via adjusting the growth time and concentration of precursors, nanostructures with different average diameter (26–225 nm), length (0.98–2.29 lm), and density (1.9–15.3 9 109 cm-2) can be obtained. To the best of our knowledge, this is the first demonstration of ZnO nanostructure arrays grown on GaAs substrates C.-Y. Huang (&) Department of Applied Science, National Taitung University, Taitung 950, Taiwan e-mail:
[email protected] T.-H. Wu C.-Y. Cheng Y.-K. Su Institute of Microelectronics, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan e-mail:
[email protected] Y.-K. Su e-mail:
[email protected] T.-H. Wu C.-Y. Cheng Y.-K. Su LED Lighting and Research Center, National Cheng Kung University, Tainan 701, Taiwan C.-Y. Cheng Wafer Works Optronics Corporation, Chung-Li, Taoyuan, Taiwan
by the two-step chemical bath synthesis. As an antireflection layer on GaAs-based solar cells, the array of ZnO nanoneedles with an average diameter of 125 nm, a moderate length of 2.29 lm, and the distribution density of 9.8 9 109 cm-2 has increased the power conversion efficiency from 7.3 to 12.2 %, corresponding to a 67 % improvement. Keywords ZnO nanostructures Chemical synthesis Plasma treatment GaAs-based solar cells Energy conversion
Introduction As a functional material with unique optical and electrical properties, ZnO has attracted worldwide attention for its broad range of applications (Hoffman et al. 2003; Schrier et al. 2007; Tsukazaki et al. 2005). Besides the ordinary ZnO film deposition, numerous types of one- and two-dimensional nanostructures grown by physical or chemical routes have demonstrated that this material should be grouped with the most abundant family of nanostructures (Yavari et al. 2009). By precisely controlling the growth kinetics and local temperature, diverse nanostructures with different crystallinity, crystallographic orientation, crystallite size, morphology, geometry, and distribution density can be tailored (Yavari et al. 2009; Chen et al. 2009).
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Indeed, to make these nanostructures realistic and applicable, large-scale distribution of nanostructure arrays has been demonstrated to be deposited on silicon, GaN, glass, metal oxides, and flexible substrates respectively by cost-effective methods such as hydrothermal synthesis (Yavari et al. 2009; Lee et al. 2008; Zhang et al. 2009; Su et al. 2009). Regardless of these triumphs, large-scale ZnO nanostructures fabricated on GaAs substrates, the representative secondgeneration semiconductor materials utilized in most kinds of optoelectronic devices, have seldom been reported. As for those successfully grown, all were via expensive deposition equipment such as metal organic chemical vapor deposition (MOCVD) (Zhang et al. 2008). Though the vertically well-aligned one-dimensional ZnO nanostructures are easily obtained by MOCVD for short growth time (high growth rate), elevated growth temperatures ([800 °C) have greatly limited its applicability on novel substrates such as polydimethylsiloxane (PDMS) and polyethylene terephtalate (PET) for flexible electronic and optoelectronic applications. Nevertheless, large-scale and uniformly distributed ZnO nanostructures have been demonstrated on PDMS or PET substrates by chemical bath synthesis. Here, for the first time, homogeneous ZnO nanowire, nanorod, and nanoneedle arrays were respectively prepared on GaAs substrates by a simple two-step chemical bath synthesis (Yang et al. 2009), and their photovoltaic application was also investigated.
Experimental Our two-step chemical bath synthesis can be described as follows: First, to provide sufficient wettability, the GaAs substrates were pre-treated with O2-plasma generated by applying an inductively coupled plasma (ICP) power of 100 W at 8 m Torr. The flow rates of Ar and O2 were maintained at 3.5 and 48.9 sccm, respectively (Cheng et al. 2010). The solution for the ZnO seed layer was prepared by the sol–gel method. It was formed by adding zinc acetate (1 g) into 90 % ethanol (10 mL) and stirring the solution at 80 °C for 30 min. A polycrystalline ZnO seed layer with controlled thickness of 80 nm was then obtained by spin-casting the gelled solution on O2-plasma-treated GaAs substrate and post-deposited annealing at 100 °C. Subsequently, the substrate was mounted on
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a glass slide and soaked in aqueous solution of zinc acetate dihydrate (Zn(OAc)22H2O) and hexamethylenetetramine (HMTA), the precursors, in a bottle with an autoclavable screw cap for chemical bath synthesis. Inside a conventional laboratory oven, the sealed bottle was baked at 90 °C for 1, 5, and 18 h, while the concentration of both Zn(OAc)22H2O and HMTA in mixed solution was varied as 0.01, 0.02, 0.03, 0.04, and 0.05 M. After reaction, the samples were washed repeatedly with deionized water and blow-dried with N2 for characterization. The synthesized ZnO thin films and nanostructures were then characterized by grazing incidence X-ray diffraction (GIXRD) using a Rigaku RINT2000 diffractometer operating in the h–2h scan mode, and scanning electron microscopy (SEM) using a JEOL 7100 FE-SEM operating at 10 kV. Based on the results of above experiments, ZnO nanostructure arrays were further grown on the front surface of GaAs-based solar cells with the device structure previously mentioned (Su et al. 2011) to be the anti-reflection layer. In device characterization, the illumination was calibrated by a calibration cell with a known one-sun AM1.5G short-circuit current measured at NREL.
Results and discussion Figure 1 shows the XRD spectra of ZnO seed layers with and without annealing. Polycrystalline ZnO films with well-defined diffraction peaks at 31.7°, 34.4°, and 36.2° corresponding to (100), (002), and (101) of ZnO wurtzite structure can be observed only after annealing. Low peak intensities were related to the formation of the amorphous phase in the thin film. It must be mentioned that increasing the annealing temperature improved the crystalline quality. However, a large discrepancy of thermal expansion between ZnO thin film and GaAs substrate also resulted in cracks on seed layer at higher temperatures (Huang and Lin 2008), as shown in the inset. Crack-free and smooth surface was only observed on the seed layer annealed at 100 °C, which was used for the subsequent growth of nanostructures. Typical SEM photographs of the geometry and distribution of ZnO nanostructures grown in 0.01 M solution are shown in Fig. 2a, b. The distribution of nanowires, those nanostructures with average
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o
annealing @ 100 C
(103)
GaAs (222)
(101)
(100)
(002)
Intensity (a.u.)
(a)
w/o annealing
30
35
40
45
50
55
60
65
70
2-theta (deg.)
1 µm Fig. 1 XRD spectra of deposited ZnO seed layers with and without annealing. Inset shows the OM photographs of samples annealed at 100 (left) and 300 °C (right)
diameter (Dav) smaller than 100 nm, was very homogeneous on the whole substrate. Initially (Fig. 2a), wires were grown with diameters ranging between 20 and 30 nm and an average length (Lav) of 980 nm, and with a number density of 1.5 9 1010 cm-2. The same as those grown on silicon substrates (Greene et al. 2003), multiple nanowires were frequently grown from a single cluster, only a small portion of nanowires can be vertically developed, whereas others were inclined toward any direction. Since the cluster is directly related to the crystal size of seed layer and the oriented ZnO nanowires are preferentially grown on the same oriented seed layer, low alignment ordering of the nanowires at the beginning stage was attributed to the seed layer’s polycrystal nature (Yang et al. 2009). As shown in Fig. 2b, the average diameter and length of these nanowires increased with prolonging the reaction time from 1 to 18 h. However, the increase was not significant. The aspect ratio of nanowires thus decreased from 37.7 to 22.2. Accordingly, the decrease of dominant vertical growth rate can be associated with the consumption of precursors at low concentration. For comparison, the SEM photograph of ZnO bunches and grains grown on ZnO seed layers on the untreated substrates are displayed in Fig. 2c. Noticeably, O2-plasma treatment did not only modify the surface energy of GaAs substrates but also somehow improved the quality of the deposited seed layer. Without O2-plasma treatment, ZnO tended to nucleate on the specific area with a higher degree of crystallization, and with growth time further increasing, small nucleation sites coalesced together to be randomly distributed bunches.
(b)
1 µm
(c)
1 µm Fig. 2 SEM photographs of ZnO nanowire arrays prepared in 0.01 M solution for a 1 and b 18 h. The scale bars shown in the inset are 500 nm. c SEM photograph of ZnO bunches and grains grown on ZnO seed layers on the O2-plasma untreated substrates. The growth time was 1 h
Eventually, nanostructures in Fig. 2a, b were observed only on the seed layers grown on plasma-treated substrates. On the other hand, adjusting the concentration of precursors can dramatically change the
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Fig. 3 SEM photographs of ZnO nanostructure arrays prepared in solutions with concentration of a 0.02, b 0.03, c 0.04, and d 0.05 M for 1 h, and e 0.03 and f 0.04 M for 18 h
morphology of grown nanostructures. As shown in Fig. 3a, b, nanowires became gradually wider as the concentration was increased to 0.02 and 0.03 M. The increase of the wire’s diameter has led to the frequent impingement of wires with one another. When the concentration was increased to 0.04 M (Fig. 3c), Dav was developed beyond 100 nm and the nanowires turned out to be the nanorods. In this stage, those initially misaligned and impinged nanorods from adjacent clusters were forced to be developed along the vertical direction. Some closely contacted individuals began to merge together, and this phenomenon became more pronounced in the sample grown at concentration of 0.05 M, which is clearly observed in Fig. 3d. Consequently, the coalescence of nanorods
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increased the divergence of rod diameter and dramatically decreased the rod density from 5.6 9 109 to 1.9 9 109 cm-2. Regardless of the diameter and density of nanostructure array, the favored c-axis growth direction of nanowires and nanorods has been interpreted by the higher surface energy of the polar cplane. The flattened rod top was attributed to the dissolution effect, since the regrowth and dissolution of ZnO nanostructures happened simultaneously at different rates (Qiu et al. 2009). The dissolution effect can be verified when the growth time was extended to 18 h. As shown in Fig. 3e, f, more and more nanowires or nanorods became tapered with the reaction time increasing. Obviously, the tapering effect was proportional to the diameter of nanostructures, seldom taper-
J Nanopart Res (2012) 14:866
Dav (nm)
Lav (lm)
Density (109 cm-2)
26
0.98
15.3
31
1.02
11.7
50 52
1.11 1.21
9.3 9.1
Sample
Precursor Conc. (mM)
Growth time (h)
Feature
A
10
1
Nanowire
B
10
5
Nanowire
C D
10 20
18 1
Nanowire Nanowire
E
30
1
Nanowire
83
1.15
6.5
F
40
1
Nanorod
120
0.97
5.6
G
50
1
Nanorod
225
0.98
1.9
H
40
18
Nanoneedle
125
2.29
9.8
structures (Lee et al. 2008). If the solar spectrum is taken into account, a solar-weighted reflectance of 12 % can be calculated for our nanoneedle arrays on GaAs substrates over the entire spectral region. It is not as low as that grown on Si substrates (6.6 %) (Lee et al. 2008), and the light-scattering due to the needle’s mis-orientation should be responsible. Furthermore, once the nanoneedle arrays are attached onto the solar cells, the incident photon-to-current efficiency (IPCE) spectrum will be improved, as shown in Fig. 4. The interface and surface recombinations of carriers from incident photons with shorter wavelengths are significantly suppressed. Because of the increase of incident light intensity by attaching the nanoneedle arrays, the short-circuit current density (Jsc) of the solar cells is increased from 12.75 to 18.46 mA/cm2. It turns to be 65
80
Bare GaAs E F G H
60 55
with NNA
50
w/o NNA
70 60
45 50 40 35
40
30 30 25
E
20
20
Quantum efficiency (%)
shaped nanowires were observed on the samples grown at 0.01 or 0.02 M for 18 h (not shown). Besides the tapering, the extension of growth time was unhelpful to redirect the growth orientation of nanowires in Fig. 3e. In contrast, those taper-shaped nanorods eventually evolved to be directional nanoneedles in Fig. 3f. As a result, the ordered nanoneedle arrays can thus be obtained. Compared with the MOCVD-grown ZnO nanoneedle arrays, the nanoneedles in our case cannot be synthesized within few hours due to their very different growth mechanisms. For better understanding of the variations, statistical results including the average diameter, length, and density of the ZnO nanostructures are listed in Table 1. It is anticipated that textured surface traps light and leads to a broadband suppression in light reflection. To be the AR layer, the reflectivity spectra of above ZnO nanostructure arrays including samples E, F, G, and H were measured; accordingly, the structures were grown on the standard solar cells. As shown in Fig. 4, samples F and G exhibited similar spectra due to their similar aspects of nanostructures. However, the coalescence effect of ZnO nanorods was not beneficial for light penetration, therefore, sample F showed a lower reflectance. As sample F was compared with sample H, it was clearly found that the elongation and tapering of nanoneedles in sample H has significantly suppressed the light reflection. The GaAs substrate with nanoneedle array exhibited very low reflectances (\15 %) at the wavelength longer than 470 nm. It can be concluded that the tapering of nanoneedles provides the effective graded-index for light incidence. Theoretically, this smooth transition of refractive index is even more efficient than the stepwise graded refractive index in multilayer
Reflectance (%)
Table I Statistics of ZnO nanostructures grown at different precursor concentration for different reaction time
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F
15
10 10
H
5 0 400 450 500 550 600 650 700 750 800 850 900 950
Wavelength (nm)
Fig. 4 Reflectance spectra of the bare GaAs substrate and samples E, F, G, and H. Also shown are the IPCE spectra of standard solar cells with and without the nanoneedle arrays (NNA) in sample H
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the boost of power conversion efficiency (PCE) from 7.3 to 12.2 %, which corresponds to a 67 % improvement.
Conclusions In conclusion, our experiments demonstrated nanowire, nanorod, and nanoneedle arrays grown on GaAs substrates by a two-step chemical bath synthesis for the first time. The O2-plasma treatment of GaAs substrates before the seed layer deposition played an important role to conformally grow the nanostructures instead of two-dimensional ZnO thin films on seed layers. By increasing the precursor concentration, the diameter of the nanostructures could be effectively increased. However, the length elongation by increasing the reaction time was significant only when the diameter was large enough, due to the dissolution effect. With the nanoneedle array attaching, Jsc and PCE of GaAs-based single-junction solar cells could be improved from 12.75 to 18.46 mA/cm2 and from 7.3 to 12.2 %, respectively. Acknowledgments The authors are grateful to the National Science Council of the Republic of China, Taiwan, for funding this research under contract no. NSC 100-2221-E-143-005MY2 and the LED Lighting Research Center of NCKU for the assistance of device characterization.
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