Annealing effects on the optical and morphological properties of ZnO nanorods on AZO substrate by using aqueous solution method at low temperature
© Hang et al.; licensee Springer. 2014
Received: 11 July 2014
Accepted: 12 November 2014
Published: 25 November 2014
Vertically aligned ZnO nanorods (NRs) on aluminum-doped zinc oxide (AZO) substrates were fabricated by a single-step aqueous solution method at low temperature. In order to optimize optical quality, the effects of annealing on optical and structural properties were investigated by scanning electron microscopy, X-ray diffraction, photoluminescence (PL), and Raman spectroscopy. We found that the annealing temperature strongly affects both the near-band-edge (NBE) and visible (defect-related) emissions. The best characteristics have been obtained by employing annealing at 400°C in air for 2 h, bringing about a sharp and intense NBE emission. The defect-related recombinations were also suppressed effectively. However, the enhancement decreases with higher annealing temperature and prolonged annealing. PL study indicates that the NBE emission is dominated by radiative recombination associated with hydrogen donors. Thus, the enhancement of NBE is due to the activation of radiative recombinations associated with hydrogen donors. On the other hand, the reduction of visible emission is mainly attributed to the annihilation of OH groups. Our results provide insight to comprehend annealing effects and an effective way to improve optical properties of low-temperature-grown ZnO NRs for future facile device applications.
ZnO is a promising II-VI compound semiconductor because of its excellent catalytic, optoelectronic, and piezoelectric properties. It has been demonstrated to have diverse applications in electronic, optoelectronic, and electrochemical devices, such as ultraviolet (UV) lasers, light-emitting diodes, high-performance nanosensors, and solar cells [1–6]. In addition to the low cost, ease of availability, and chemical stability, the wide direct bandgap of 3.37 eV and large excitonic binding energy (60 meV at 300 K) make ZnO a highly competitive material to GaN. It was also reported that textured ZnO films may have higher quantum efficiency than GaN films . Nowadays, ZnO thin films and nanostructures can be synthesized by using various deposition techniques, such as molecular beam epitaxy (MBE) , pulsed laser deposition (PLD) , metal-organic chemical vapor deposition (MOCVD) , chemical vapor deposition (CVD) [11–13], and aqueous solution deposition [14, 15].
High-temperature techniques such as CVD and thermal evaporation have been mainly employed to grow aligned ZnO nanostructures, for example, nanorods (NRs). These processes have disadvantages of high energy consumption and requirement of expensive infrastructure. Here, we adopt an inexpensive and simple method to prepare uniformly distributed and well-aligned vertical ZnO NRs, whereas no catalyst or seeding step is required to initiate controlled growth. This approach is based on a one-step electrochemical processing of reliably nontoxic and abundant materials in aqueous solution at low temperature (≤80°C). Moreover, it allows for large-scale processing at low cost and facile integration for complex devices. The substrate of our choice is aluminum-doped zinc oxide (AZO). Meanwhile, transparent and conductive AZO substrate is an alternative to the ITO glass. AZO has better lattice matching than ZnO, so the development of ZnO/AZO devices is now a hot pursuit.
It is known that due to oxygen vacancies (VO), inherent n-type ZnO is formed and its carrier concentration depends on post-growth annealing treatment. The intrinsic defects in ZnO are always associated with various deposition processes. The understanding of defect properties is very useful to improve the quality of ZnO. Generally, post-deposition annealing treatment is a convenient and appropriate way to modify intrinsic defects and improve the crystallinity of ZnO. Proper annealing is an effective way to obtain high-quality ZnO material. There are many reports on the thermal treatment of ZnO for different annealing conditions such as annealing temperatures and gas environments to improve the optical properties of ZnO. In this paper, ZnO NRs were synthesized by an aqueous solution deposition method and effects of post-growth annealing were studied. The structural, morphological, and optical characteristics have been studied after annealing processes. Mechanisms that are responsible for the annealing effects are investigated.
ZnO NRs were deposited on AZO substrates by an aqueous solution method. Zinc nitrate hexahydrate (Alfa Aesar, Ward Hill, MA, USA) was used as the zinc source. Ethanol amine (Merck, Whitehouse Station, NJ, USA) and hexamethylenetetramine (HMTA) were used as the stabilizer and base, respectively. Firstly, the AZO substrates were cleaned through sonication in a mixture of acetone and isopropyl alcohol (1:1), followed by cleaning with deionized water and drying in N2 atmosphere before use. The zinc precursor solution was prepared by dissolving equimolar zinc nitrate hexahydrate, HMTA, and ethanol amine in deionized water. The above solution was stirred by using a magnetic stirrer at 60°C for 10 min. NRs were grown by dipping the as-cleaned substrate horizontally into the prepared solution and were covered with a lid for 30 min at 80°C on a regular laboratory hot plate. The prepared NRs were annealed in air for 2 h by using a microprocessor-controlled furnace for different annealing temperatures ranging from 200°C to 600°C. X-ray diffraction (XRD) was conducted to examine the structure and orientation of ZnO. The surface morphology of the prepared NRs was investigated by scanning electron microscopy (SEM) (JEOL 6380, JEOL Ltd., Akishima-shi, Japan). For the photoluminescence (PL) investigations, the samples were excited by a chopped He-Cd laser beam working at 325 nm. The PL signal was dispersed by a Jobin Yvon Triax 550 monochromator (Jobin Yvon Inc., Edison, NJ, USA) equipped with a 2,400 rules/mm grating. A Hamamatsu R928 photomultiplier tube (Hamamatsu Photonics K.K., Iwata, Japan) equipped with a lock-in amplifier was used to record the optical intensity of the selected emission. A closed-cycle optical cryostat was used for low temperatures down to 10 K. Room-temperature (RT) Raman scattering measurements were performed in a backscattering configuration on a micro-Raman setup equipped with a Jobin Yvon iHR320 spectrometer and a multi-channel TE-cooled (−70°C) CCD detector.
Results and discussion
Evaluation of the as-grown sample
Effects of annealing on structural and optical properties
Low-temperature PL characterization
where E(0) is the transition energy at zero temperature and α and β are fitting parameters referred to as Varshni's coefficients . Both α and β are material-dependent. The β value is expected to be correlated with the Debye temperature, but a range of values were reported for ZnO [15, 20, 25]. The fitting results, which are denoted by the solid line in Figure 6, yield E(0) = 3.364 eV, α = 5.5 × 10−4 eV/K, and β = 250 K.
Origin of enhanced optical properties
Based on the results above, we discuss annealing effects on the emission properties of our low-temperature-grown ZnO NRs. The enhancement of NBE by annealing might be explained by two previously proposed mechanisms. One is the elimination of unwanted functional groups acting as nonradiative centers on the surface of ZnO, and the other is the improvement of the crystal quality resulting from removal of intrinsic defects . But the anomalous behavior after 400°C treatment, which shows reduced NBE, cannot be explained by the mechanisms above. In our aqueous solution growth of ZnO, generation of interstitial H is highly possible. Hydrogen defects can be introduced as a result of incomplete dehydration during the formation of ZnO [27, 28]. Most of these initial hydrogen states, interstitial H or H complex (hydroxyl group and complex with other defects), are not active donors. During annealing at 400°C, intrinsic defects are passivated and initially trapped hydrogen is released. The interstitial H has high mobility and can move around the lattice. Since there are oxygen vacancies in the ZnO NRs, the interstitial hydrogen can be trapped inside oxygen vacancies, forming HO. Substitutional hydrogen at the oxygen site, from many experimental and theoretical investigations, is believed to be an important shallow donor in ZnO [23, 28]. The existence of HO exactly accounts for the observed activation energy in our PL study . The decrease of NBE intensity under prolonged annealing or annealing higher than 400°C can then be attributed to the dissociation of hydrogen donors. Therefore, we attributed our NBE enhancement to the hydrogen donor formation activated by thermal annealing. The inset of Figure 3 shows illustrative diagrams for recombination processes that have taken place in (1) NBE in the as-grown sample and (2) NBE in the annealed sample. From the schematic diagrams, we can understand the redshift of NBE after annealing treatment, which is due to the annealing-induced activation of hydrogen donors.
The visible emission drops exactly in the opposite way as NBE enhances with annealing temperature, that is, it decreases with increasing annealing temperature up to 400°C and then it again increases at 600°C. Figure 3 also shows that visible emission decreases with increasing annealing time as well, but it is noted that after 120 min, further reduction of visible emission is insignificant. The chemical origin of the DE is intriguing. Our result shows that the broad DE is already reduced even with a low-temperature annealing at 200°C. It is indicative of the presence of OH groups whose desorption temperature is at approximately 150°C . The assignment of this visible emission to the presence of hydroxyl groups is in agreement with a previous report on visible luminescence in ZnO nanocrystals . Moreover, a previous study on the O-H local vibrational modes in ZnO also confirmed that such a low-temperature annealing results in the removal of OH groups . Therefore, we attribute the corresponding reduction of visible (defect-related) emission to the annihilation of OH groups.
Finally, we carried out RT Raman scattering measurements to understand the light-scattering properties. Raman spectra for the nonannealed and 400°C-annealed samples are shown in the inset of Figure 6. Two dominant peaks have been clearly resolved. One peak is observed around 446 cm−1, which is attributed to the nonpolar optical phonon E2 (high) mode of wurtzite ZnO , and the other peak at 588 cm−1, known as E1 (LO), which is ascribed to the defect formation of oxygen vacancies [32–34]. No other mode related to defect-induced local vibration mode is observed. The weak intensity of E1 (LO) suggests that there are relatively low oxygen vacancies. Since both spectra contain E1 (LO) contribution, the low oxygen vacancies should be responsible for the residue visible emission after annealing treatment.
In conclusion, we present the investigation of annealing effect on the optical and structural properties of vertically aligned ZnO NRs on AZO substrates by a single-step aqueous solution method at low temperature. The annealing temperature strongly affects both the NBE and visible emissions. We found the optimum annealing temperature to be 400°C. It yields a sharp and intense NBE emission and effectively suppressed visible emission. The enhancement of NBE is due to the activation of radiative recombinations associated with hydrogen donors while the reduction of DE is ascribed to the annihilation of OH groups. These results are useful to understand and optimize ZnO NRs grown in a low-temperature solution. Our approach has the advantages of low cost, fine quality, and straightforward one-step synthesis, which is promising to realize facile and controlled ZnO-based nanodevice fabrication.
This work was supported by the Ministry of Science and Technology of the Republic of China under Grant No. MOST 103-2112-M-110-001.
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