Structural and optical properties of ZnO nanorods grown on MgxZn1xO buffer layers

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Structural and optical properties of ZnO nanorods grown on Mg

x

Zn

1 x

O

buffer layers

Liang-Wen Ji

a

, Chih-Ming Lin

b

, Te-Hua Fang

a,

*

, Tung-Te Chu

c

, Huilin Jiang

c

, Wei-Shun Shi

a

,

Cheng-Zhi Wu

a

_{, Tian-Long Chang}

a

_{, Teen-Hang Meen}

a

_{, Jingchang Zhong}

c

a

Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan b

Department of Applied Science, National Taitung University, Taitung 950, Taiwan c

National Key Lab of High Power Semiconductor Laser, Changchun University of Science and Technology, Changchun 130022, PR China

1. Introduction

ZnO is a promising material for optoelectronic applications because of its wide band gap (3.37 eV) and large exciton binding energy (60 meV), which is considerably greater than conventional semiconductor materials[1,2]. In addition, many researchers have studied one-dimensional ZnO nanostructures, such as nanorods, nanowires, and nanoribbons, which can be applied to sensors[1,2], ﬁeld emission[3,4], and piezoelectric devices[5,6].

The vertical ZnO nanorod arrays can be prepared by evapora-tion[7], aqueous solution method[8], and epitaxial growth[9]and have been fabricated into hetero-junction light emitting diodes, nanogenerators, and vertical ﬁeld-effect transistors[10–12]. The highly oriented growth of ZnO nanorods is required for fabricating nanorod devices. For ZnO nanorod arrays grown on GaN, AlN,

Al1 xGaxN, and ZnO buffer layers, the lattice match between buffer

layers and ZnO nanorods can affect the orientation and density of nanorods[13–15].

In this paper, the effect of MgxZn1 xO buffer layers with

different Mg concentrations on the morphology and crystallization of ZnO nanorod arrays grown on them is investigated.

2. Experimental

The poly-crystalline MgxZn1 xO (x = 0, 0.07 and 0.15) ﬁlms were

prepared on glass substrates by sol–gel method and thermal annealing. Zn(CH3COO)22H2O (0.02 moles), HN(CH2CH2OH)2

(0.02 moles), and different moles of magnesium chloride hexahydrate (MgCl26H2O) (0, 0.002, and 0.004 moles,

respec-tively) were added into 2-propanol solution. The resultant solution was stirred by spinning at 60 8C for 1 h to become a sol. The obtained sol was left for several days at room temperature until a gel was formed. Several drops of the gel were coated on corning glass substrate by spin coater with a constant speed of 3000 rpm for 30 s and then dried at 300 8C in oven. The last procedure as

Applied Surface Science 256 (2010) 2138–2142

A R T I C L E I N F O Article history: Received 8 April 2009

Received in revised form 14 September 2009 Accepted 18 September 2009

Available online 25 September 2009 PACS: 81.07. b 68.65. k 78.67. n 71.55.Gs 81.15.Lm Keywords: MgxZn1 xO Sol–gel ZnO nanorods Chemical-liquid deposition Photoluminescence A B S T R A C T

ZnO nanorod arrays were synthesized by chemical-liquid deposition techniques on MgxZn1 xO (x = 0,

0.07 and 0.15) buffer layers. It is found that varying the Mg concentration could control the diameter, vertical alignment, crystallization, and density of the ZnO nanorods. The X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) data show the ZnO nanorods prefer to grow in the (0 0 2) c-axis direction better with a larger Mg concentration. The photoluminescence (PL) spectra of ZnO nanorods exhibit that the ultraviolet (UV) emission becomes stronger and the defect emission becomes weaker by increasing the Mg concentration in MgxZn1 xO

buffer layers.

* Corresponding author at: No. 64, Wenhua Rd., Huwei, Yunlin 632, Taiwan. Tel.: +886 5 631 5395; fax: +886 5 631 5397.

E-mail address:fang.tehua@msa.hinet.net(T.-H. Fang).

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and 0.15 by the chemical-liquid deposition technique. Our results show the ZnO nanorod arrays with good vertical alignment and crystallization could be attained for the MgxZn1 xO ﬁlm with x = 0.15 because of the small grain size

and surface roughness of the ﬁlm. We also show the MgxZn1 xO

ﬁlm with a higher Mg concentration has higher band gap energy up to 3.45 eV. The high quality ZnO nanorods with a crystalline hexagonal wurtzite structure and a preferred orientation in the (0 0 2) c-axis direction are conﬁrmed and can be applicable to the nanodevice fabrication.

Acknowledgment

This work was supported by National Science Council of Taiwan under contract number NSC-95-2221-E-150-077-MY3.

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