Optimum Post-growth Rapid Thermal Annealing ...

Optimum Post-growth Rapid Thermal Annealing Temperature for the Structural and Optical Properties ofHydrothermal ZnO Nanorods Waqar Khan, Sam-Dong Kim*

Division of Electronics and Electrical Engineering, Dongguk University 100-715 Seoul, South Korea

Abstractinspected

In this study,

under

wide

Zinc oxide nanorods has been

range

of

rapid

thermal

annealing

temperatures to explore the optimum post growth annealing temperature, structural,

required

electrical

for

and

particular

optical

applications,

properties.

Vertical

under ZnO

Nanorods has been grown via low temperature hydrothermal route

on

Silicon

substrate,

then

the

as-grown

NRs

were

undergone through rapid thermal annealing process in nitrogen environment. The rapid thermal annealing temperature ranges from 300°C to 900°C. Full-width-half- maxima data, from X-ray diffraction peaks reveals that better crystallinity is achieved at 600°C. Scanning electron microscopy teils about the uniformity of

the

diameter

Photoluminescence

of

nanorods

spectra

gives

in

strong

the

same

emission

at

range. higher

temperature with the peak emission at 380nm wavelength, which confirms the wider direct bandgap of ZnO. Moreover, various aspects of ZnO nanorods have been thoroughly investigated under different annealing temperature, which can be helpful in choosing a specific annealing temperature for certain kind of applications.

Keywords- Zinc oxide (ZnO) nanorods (NRs), X-ray diffraction (XRD), rapid thermal annealing (RTA), photoluminescence (PL), Full-width-half- maxima (FWHM), hydrothermal process.

I.

INTRODUCTION

Zinc oxide (ZnO) as a transparent oxide semiconductor has been studied extensively in the last decades for its unique strength in various material properties, such as wide band gap of �3.37 eV, large exciton binding energy of 60 meV, chemical and mechanical stability, harmlessness to human body, and earthly abundancy [1]-[6]. One dimensional semiconductor nanorods (NRs) based on ZnO have been successfully integrated into various kinds of practical devices like photonics, nano-electronics, optoelectronics, piezotronics, nanogenerators, transparent sensors, and nanoactuators. A wide variety of techniques for the growth of high-quality ZnO nanostructures have been investigated to date, such as metalorganic chemical vapor deposition [7], pulsed laser deposition [8], electro-deposition [9], physical vapor This work (Project No.20l6006533) was supported by Mid-career Researcher Program through National Research Foundation grant funded by the Ministry of Education, Science and Technology, South Korea.

deposition method [10], and template-based method [11] but these methods require complex and expensive processes. Among various growth methods for ZnO nanorods (NRs), a synthesis technique in aqueous solution such as hydrothermal method is very promising because it does require complex process and they can proceed at relatively low process temperature which allows low-cost and large-scale fabrication [4], [5], [12],[13]. There are several factors which effect the structural, optical and electrical characteristics of NRs. The chemical composition can be change by changing the composition of colloidal solution which depends on the chemistry of the chemical reaction adopted for the crystal growth of the NRs. Another approach to improve the properties of NRs is temperature treatment, since ZnO NRs have high thermal stability at elevated temperature [3], [4]. Owing to a great amount of previous studies, it has been understood more in depth that the post-annealing can exhibit critical impact on the material properties of ZnO in various aspects. In this work, structural and optical properties of the hydrothermally-grown ZnO NRs have been examined under a wide range of RTA temperatures (300 - 900°C) to explore the optimum post-growth RTA temperature of NRs required for specific device applications. To investigate the structural evolution and crystalline quality of the thermally annealed NRs, we carried out various surface characterizations such as scanning electron microscopy (SEM) and X-ray diffraction as weIl as room-temperature PL to study the behavior of optical properties. Process steps of the hydrothermal ZnO NR-based devices are generally limited by the thermal stability up to �500 o e when glass is used as a transparent low-cost substrate. However, high-temperature treatments in a short time period can actually improve the structural properties of the NRs and therefore the device performance. Based on the experimental result of the best performance of polysilicon solar cell grown on glass rapid thermal annealed at a peak temperature of 900 o e for 200 s, we chose the annealing condition at a peak temperature of 900 o e for 3 min as a maximum thermal stress in this study. 11. EXPERIMENTAL PROCEDURE

The substrate which we used for the growth of ZnO nanorods is p-Si (100). Firstly, the substrate was thoroughly cleansed

*Corresponding author email: samdong@ dongguk.edu 978-1-4673-9073-6/17/$31.00 ©2017 IEEE Proceedings of 2017 14th International Bhurban Conference on Applied Sciences & Technology (IBCAST) Islamabad, Pakistan, 10th -14th January, 2017

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with acetone and isopropyI alcohol (IPA) sequentially for 5min each and washed with de-ionized (DI) water and then dried with nitrogen purge. The c1eaned substrate was put in buffer oxide etchant for 5min, to remove the native oxide, formed on the silicon surface. Zinc oxide seed layer was deposited from a colloid seed solution which was prepared from dissolving 0.06 g of zinc acetate dehydrate [Zn(CH3COO)2.H20] in 30ml n-propanol [C3H80] to form a 0.2M concentrated solution. The colloid seed solution was sonicated for 30min at room temperature. Then the colloid seed solution was coated on the substrate by spin coating. The colloid seed solution was deposited on the substrate by rotating it at 3000 rpm for 30 sec and heated at 100 °C for 60 sec. This process was repeated 10 times to get a thick ZnO colloid seed layer. After that the substrates were heated on hot plate at 300 °C for 1 hour to remove the organic salts and residuals [12], [13]. The seeded substrates were then transferred into the aqueous solution for ZnO nanorods (NRs) growth. The aqueous solution is a mixture of zinc nitrate hexahydrate (Zn(N 03)2.6H20) and hexamethylenetetramine (HMT, C6H12N 4) in 200 ml DI water, having molarity of 25 mM. This solution has been stirred thoroughly at room temperature for 25 min. After putting the sampies in aqueous growth solution the container was fully covered and heated at 90 °C for 6 hours. As-grown nanorods were washed thoroughly with DI water to remove the residual salts and then dried with nitrogen blow [14].

ZnO seed layers and nanorods were examined by PL spectroscopy (MFP-3D Bio, Asylum Research) excited at 266 nm at room temperature (RT). The preferred orientation of the ZnO nanorods was measured by XRD (8-28) scan (D8 ADVAN CE, Bruker AXS GmbH) using a CuKa. radiation (A 0.15406 nm). =

III.

RESULTS AND DlseUSSI0N

The top view of ZnO NRs at different temperature is give in Fig 2. The NRs are vertically aligned, closely packed and have hexagonal structure. The variation in the diameter of NRs with respect to change in temperature has been given in Fig 3.

Vertical ZnO NRs was grown using a low-temperature hydrothermal method on (100) silicon substrate followed by the post-annealing by RTA process under nitrogen environment. The peak RTA temperature ranged from 300 to 900 °C with a holding time for 3 min. The temperature ramp up is iIIustrated in Fig 1. As shown, the temperature is increased from RT to 100 °C rapidly (�3 sec) and then temperature is kept constant for 27 seconds (total 30 sec). In next step temperature in increased by 100 °C suddenly (�3 sec) and then kept constant for more 27 seconds (total �30 sec) and so on, until it reaches the target temperatures (300 °C - 900 °C) and then temperature is kept constant for 180 sec for the target temperature.

Fig. 2. FE-SEM image of ZnO NRs annealed for the temperature ranging from 300 oe to 900 oe with a step of 100 oe. The scale bar is 100 nm.

Fig. 1. Temperature ramp-up for annealing ZnO NRs, the temperature ranges from 300 oe to 900 oe with a step of 100 oe per 30 sec.

Morphology of ZnO nanorods were investigated by FE-SEM (S-4800, Hitachi) operating at 25 keV. Optical properties of

It is depicted from Fig 3, that as we increase the temperature, the diameter of NRs is decreased and the range of diameter contracts to smaller values. Moreover, the hexagonal facets are obvious at temperature lower than 600 °C while the diameter of NRs becomes more round beyond 600 oe. Finally at highest temperature of 900 °C, the NRs seems to be melted with an unclear view of diameter.

Proceedings of 2017 14th International Bhurban Conference on Applied Sciences & Technology (IBCAST) Islamabad, Pakistan, 10th -14th January, 2017

41

Fig. 3. Variation of diameter of ZnO nanorods annealed at temperature ranging from 300°C to 900 oe.

To check the crystallinity of ZnO NRs we performed XRD analysis as shown in Fig 4. The XRD plot shows typical pattern of ZnO NRs with the pronounced peak of (002) at an angle (2-9) of 34.4 ° as reported for the hydrothermally grown NRs [4], [6], [9], [13].

Fig. 5. Full·width·half·maxima of ZnO nanorods annealed at temperature ranging from 300°C to 900 oe.

Fig. 6. PL plot of ZnO nanorods annealed at temperature ranging from 300°C to 900°C.

IV. Fig. 4. XRD 9-29 scan of ZnO nanorods annealed at temperature ranging from 300°C to 900 oe.

X-ray diffraction 9-29 scans revealed that full-width-half maximwn (FWHM) was decreased from 0.1601° to 0.l416° with the increase ofRTA peak temperature from 300 to 600 °C, while there was an opposite trend of FWHM broadening from 0.1401° to 0.2111° with further increase of peak temperature from 600° to 900° as shown in Fig 5. Optical properties are justified by RT PL for ZnO NRs grown on p-Si substrate under different RTA temperatures is shown in figure 6. As evident from the graphs there is a major sharp peak at 370 nm (3.35 eV) wavelength corresponds to UV emission [13], [14]. The UV emission is actually, near-band edge emission and is connected with radioactive band to band recombination. By elevating the RTA temperature from 300 to 900 °C, we also observed a red shift of �5 nm with the significant increase in peak intensity from the band edge emission.

CONCLUSION

In summary, the hydrothermal route was adopted to grow weH aligned ZnO NRs. The as-grown nanorods were annealed at different temperature to observe the changes, which occur in structural and optical properties. After thorough analysis of various aspects of NRs, guidelines of optimwn RTA temperature for different device applications are presented. V. [I]

[2]

[3]

[4]

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