formation of zno thinfilms consisting of nano-prisms and nano-rods with a high

Formation of ZnO thin films consisting of nano-prisms and nano-rods with a highYe Bin Kwona, Seung Wook Shinb, Hyun-Ki Leea, Jeong Yong Leeb, Jong-Ha Moona, Jin Hyeok Kima,* a Department

Formation of ZnO thin films consisting of nano-prisms and nano-rods with a high

Ye Bin Kwona, Seung Wook Shinb, Hyun-Ki Leea, Jeong Yong Leeb, Jong-Ha Moona, Jin Hyeok Kima,*

a Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong e Dong, Puk-Gu, Gwangju 500-757, Republic of Korea

b Photonics Technology Research Institute, Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea

a r t i c l e i n f o

Article history:

Received 6 September 2010

Received in revised form

22 November 2010

Accepted 26 November 2010

Available online 4 December 2010

Keywords:

ZnO thin film

Nano structure

Hydrothermal technique

Low growth temperature

a b s t r a c t

ZnO thinfilms that consist of elongated nano-prisms and nano-rods were successfully grown on 100 nm-thick ZnO seeded glass substrates by hydrothermal synthesis at 60
C and pH 10.3 in an aqueous solution containing Zn(NO3)$6H2O, Al(NO3)3$9H2O, Na3-citrate and NH4OH The effect of Al(NO3)$6H2O and

Na3-citrate, as surfactant chemicals, on the structural, morphological, optical and electrical properties of ZnO thinfilms were investigated X-ray diffraction results showed that all the deposited films were grown as a polycrystalline wurtzite hexagonal phase with a c-axis preferred, out-of-plane orientation and without unwanted second phase ZnO thin films deposited without any surfactant chemicals or deposited only with Al(NO3)$6H2O consist of elongated needle shaped nano-rods with a very rough surface morphology On the other hand, ZnO thin films prepared using Na3-citrate as a surfactant chemical consist of hexagonal nano-prisms with a very smooth surface morphology The thickness of the ZnO thin films with the very smooth surface morphology was increased remarkably using both

Na3-citrate and Al(NO3)$6H2O as surfactant chemicals, in which ZnO thinfilms consisted of elongated hexagonal nano-prisms These results show that relatively thick ZnO thinfilms with a good surface morphology can be grown easily by the appropriate use of surface modifying chemicals, such as

Na3-citrate and Al(NO3)$6H2O The photoluminescence results showed strong defect-related emission peak centered near 545 nm in the rough surfaced ZnO thinfilm grown without any surfactant chemicals and strong band-edge peak centered near 368 nm in the smooth surfaced ZnO thinfilm grown using both Na3-citrate and Al(NO3)$6H2O as the surfactant chemicals

Ó 2010 Elsevier B.V All rights reserved

1 Introduction

Zinc oxide (ZnO) is considered the most promising candidate for

a range of applications including room temperature ultraviolet

lasers transparent conducting electrodes, light emitting diodes,

sensors and surface-acoustic wave devices owing to its unique

binding energy (60 meV) and strong-chemical stability compared

scale ZnO crystals with various morphologies, such as

nano-rod, belt, wire and tubes are superior to those of the bulk-scale due

synthesized by several methods, such as metal organic chemical

hydro-thermal technique is one of the most attractive for industrial use

because industrial processes generally require rapid, low process costs, which are the main advantages of the hydrothermal

with a continuous, dense and smooth surface microstructure is

microstructure with varying process parameters

a hexagonal prism structure due to easily adjusting the surfactant

chemical species in the reaction solution, in which the surfactant

c-axis direction was poor, causing an increase in the growth processing time Therefore, it required a new approach to increase

* Corresponding author Tel.: þ82 62 530 1709; fax: þ82 62 530 1699.

E-mail address: jinhyeok@chonnam.ac.kr (J.H Kim).

Current Applied Physics

j o u r n a l h o me p a g e : w w w e l s e v i e r c o m/ l o ca t e / c a p

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the growth rate of ZnO crystal in the c-axis direction with keeping

hexagonal prism shape rather than needle likes shape

The purpose of this study is to grow dense and smooth ZnO thin

films with high growth rate We report that it is possible to grow

2 Experimental details

to control the shape of the ZnO crystals Four different solutions,

dissolved in 80 ml of DI water Solution C was formulated with

80 ml of DI water Solution D was formulated with 0.03 M of Zn

dis-solved in 80 ml of DI water 100 nm-thick ZnO seeded substrates,

which were deposited by RF magnetron sputtering at room

temperature, were cleaned sequentially in acetone and isopropyl

for 6 h The lid to the vessel was sealed to prevent evaporation At

the end of the period, the vessel was cooled to room temperature to

remove the specimens and the specimens were rinsed in DI water

The surface and cross-sectional morphologies of deposited thin

films were examined by field emission scanning electron

micros-copy (FE-SEM, JSM-6710F, JEOL, Tokyo, Japan) The crystallographic

Philips, and Eindhoven, Netherlands) The microstructure and

analyzed by transmission electron microscopy (TEM, JEM 2000EX,

JEOL, Tokyo, Japan) The optical properties of the deposited thin

films were determined by UVevis spectroscopy (Cary 100, Varian,

Mulgrave, Australia) at room temperature The photoluminescence

room temperature (RT) PL (APD, SH-4, USA) The electrical

3 Results and discussion

Fig 1shows XRD patterns of the ZnO thinfilms grown in

had a polycrystalline hexagonal wurtzite structure with a c-axis

preferred, out-of-plane orientation The samples grown in solutions

A and D showed a relatively strong (0002) peak intensity compared

to samples grown in solutions B and C The full width at half maximum (FWHM) values of the ZnO (0002) peak for the deposited

(0002) peak was observed in the sample grown in solution D, indicating the best c-axis preferred, out-of-plane orientation Fig 2(a,b,c,d) shows tilted-view FE-SEM images of ZnO thin films grown on the ZnO seeded glass substrates in solutions A, B, C and D, respectively The inset images show the corresponding

consisting of needle shaped nano-rods were observed in the

rough and discontinuous and the lengths of the ZnO nano-rods

(b,d)) A smooth and dense surface morphology was observed in those samples and the lengths of the ZnO nano-prisms were

Noticeable information was obtained from the aforementioned

contin-uous microstructure and reduced growth rate could be synthesized

surfactant chemical However, a dramatic decrease in the thickness

not observed in the sample grown in solution D, which contained

These characteristics were attributed to the surfactant ions It is well known that the oppositely charged ions produce positively charged Zn (0001) and negatively charged O (000l) surfaces, resulting in a normal dipole moment and spontaneous polarization

absor-bed on Zn-terminated (0001) planes of ZnO to slow down the

Table 1

Experimental conditions to prepare different solution of specimens.

Solution A Solution B Solution C Solution D

Fig 1 XRD patterns of the ZnO thin film grown on ZnO seeded glass substrate in different solutions at pH 10.3 for 6 h.

growth rate in the [0001] direction Especially, the morphology of

(Fig 2(b,c)) However, the morphology of ZnO thinfilm grown in

solution D was high aspect ratio nano-prism with a smooth surface

as compared to these grown in solution B and C This phenomenon

the growth of ZnO crystals in the c-axis direction, which results in

the formation of hexagonal prism-shaped ZnO rods with a low

nano-prisms with a smooth surface

Fig 3shows a cross-sectional bright-field TEM image of the ZnO

selected area electron diffraction (SAED) pattern obtained at the

substrate were very sharp without any indication of an interfacial

reaction or any formation of amorphous compounds The SAED

normal direction with a slight out-of-plane variation This TEM

and 2 Fig 4shows a plot of (ahy)2as a function of the photon energy of

four different solutions The band gap energy was derived by

Fig 2 The tilted-view FE-SEM images of ZnO thin film grown on a ZnO seeded glass substrate in solutions A (a), B (b), C (c), and D (d), respectively.

Fig 3 The cross-sectional bright-field TEM image of the ZnO thin films grown on ZnO seeded glass substrates in solution D at pH 10.3 for 6 h.

extrapolating the straight-line portion of the (ahy)2vs hyplot to

3.25 eV, 3.2 eV, 3.3 eV and 3.33 eV, respectively The abortion edges

Fig 5 shows the RT PL spectra of ZnO films grown in four

different solutions on the ZnO seeded glass substrates Two

attributed to recombination between electrons in the conduction

band and holes in the valence band and visible emission is related

shifted towards a lower wavelength compared to those grown in

Fig 6shows the electrical resistivity, carrier concentration and

grown in solution D compared to those grown in solution A was

chemicals These chemicals resulted in different morphology,

and 2) The Al(NO3)3$9H2O acted impurity of ZnO structure as well

films grown in solution D showed better crystallinity and

a smoother morphology than those grown in solution A with discontinuous morphology The surface morphology and

3.0 3.1 3.2 3.3 3.4 3.5

h υ ( eV)

Solution A Solution B Solution C Solution D

Fig 4 The plot of (ahy) 2 vs photon energy of the ZnO thin film grown on ZnO seeded

glass substrate in different solutions.

362 368 377

Wavelength (nm)

Solution C

Solution D

Solution A

Solution B

Fig 5 RT PL spectra of the ZnO thin film grown on ZnO seeded glass substrate in different solutions.

Solution D Solution A

2 4 6 8 10

-3 )

2 V

-1 s

Resistivity Carrier concentration Mobility

Fig 6 The electrical resistivity, carrier concentration and mobility of the ZnO thin film grown on ZnO seeded glass substrate in solutions A and D.

electron trap centers, which reduce the charge carrier

concentra-tion and show a high intensity of deep-level emission results from

of characteristics In addition, the small grain size and empty space

reduces the mean free path of the free electron, leading to an

struc-ture, which released the free electron, resulted in the improved

4 Conclusion

showed improved photoluminescence and electrical properties

Acknowledgements

This work was supported partially by Energy R&D program

(2008-N-PV08-P-08) under the Korea Ministry of Knowledge

Economy (MKE) and partially by the National Research Foundation

of Korea (NRF) grant funded by the Korean government (MEST) (No 2010-0007691)

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