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Post annealing effect on optical and electrical properties of

By George Arnold,2014-09-09 11:28
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Post annealing effect on optical and electrical properties of

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Post annealing effect on optical and electrical properties of

    Ga-doped ZnCdO thin films 1-xx

    DUAN Libing, ZHAO Xiaoru, LIU Jinming, GENG Wangchang, ZHANG Fuli,

    5 SHI Xiaolong, SUN Huinan

    (Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education of China and

    School of Science, Northwestern Polytechnical University, Xi’an 710072)

    Abstract: (Cd,Ga)-codoped ZnO thin films were prepared by sol-gel method. The codoping films retained wurtzite structure of ZnO, and showed preferential c-axis orientation. The effects of post

    10 annealing ambient (in vacuum and nitrogen) on the optical and electrical properties of the films were investigated. The transmittances of the films were obviously degraded by vacuum annealing to 60-70%, but enhanced to 80-90% after nitrogen annealing, which were about 10% higher than those of (Cd,Al)-codoped ZnO films. The carrier concentration increased, while resistivity decreased with narrowing band gap of Ga-doped ZnCdO, i.e. the conductivity is also improved by Cd codoping. 1-xx

    15 The resistivity of nitrogen annealing films is one order higher than that of vacuum annealing films, i.e. the transmittance and conductivity of the films seem irreconcilable, and the trade-off between transmittance and conductivity could be effectively controlled by post annealing ambient. Both the Cd doping (majority) and Burstein-Moss effect (minority) affect the band gap (Eg) modification. Due to 3+ 2+ 3+the ionic radius of Gais closer to that of Znthan Al, and hence less deformation of ZnO lattice

    20 after Ga doping, in view of transmittance and conductivity, Ga might be a more appropriate dopant for our band gap engineering transparent conducting oxide (TCO) films than Al.

    Keywords: Inorganic nonmetallic materials; ZnO films-based Transparent conducting; Band gap engineering; post annealing

    25 0 Introduction

    Transparent conducting oxide (TCO) films, which are characterized by a unique combination

    xtensively applied in several of low electrical resistivity and high optical transparency, have been e

    optoelectronic devices such as light emitting diodes (LEDs), solar cells, and flat panels etc.[1,2]. Indium tin oxide (ITO) is the most commonly used TCO films for these applications. However, a

    30 replacement for ITO is now required due to indium is rare and its supply is limited by the availability of natural resources [3,4]. ZnO has attracted increasing attention because of its abundance and relatively low cost. ZnO is a native n-type semiconductor with a wide band gap (E) g

    of 3.36 eV. To improve the conductivity, ZnO is typically doped with trivalent elements such as

    -4 Al, Ga, etc. [5-7]. Al-doped ZnO (AZO) film resistivity of less than 2×10Ωcm has been attained,

    35 which is comparable to those obtained in ITO films [3]. As an alternative donor, Ga should disrupt 3+ the ZnO lattice less than Al, as the ionic radius of Ga(0.47 Å, coordination number CN=4) is

    2+ 3+ smaller than that of Zn(0.60 Å, CN=4) but larger than that of Al(0.39 Å, CN=4) [8].

    3+ 3+ Furthermore, Gadopants are less reactive and more resistive to oxidation than Aldopants [9].

    Ga-doped ZnO (GZO) films prepared by Bhosle et al. [10] using pulse laser deposition (PLD)

    -4 40 displayed lowest resistivity of 1.4 × 10 cm at 5% Ga doping. Nayak et al. [11] also reported

    that the resistivity of 2 at. % Ga-doped ZnO film synthesized by sol-gel spin-coating method could

    -3 reach 3.3× 10 cm.

    Furthermore, the band gap of ZnO (E=3.36 eV) can be tailored by alloying MgO or CdO etc. g

    While Mg is known to enhance the band gap, Cd substitution leads to reduce in band gap, the

     45 resultant (Zn,Cd)O and (Zn,Mg)O alloys have allowed band gap covering a wide range of 2.8-4.5

    Foundations: Specialized Research Fund for the Doctoral Program of Higher Education (Grant No.

    20106102120051), NPU Foundation for Fundamental Research (NPU-FFR-JC201017), and National Natural Science Foundation of China (Grant No. 50872112, 51172186).

    Brief author introduction:DUAN Libing, (1981-), Male, Lecturer, Structure and physical properties of oxides films and nanoparticles. E-mail: lbduan@nwpu.edu.cn

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    eV in practice [12-15]. A TCO with modified band gap gives rise to many of its scientific and technical applications, such as improving the efficiency of different wavelength light emitting devices when used as a transparent electrode, and the realization of heterojunction and superlattice

    2+2+ 2+structures [16-18]. Theoretically, due to the isovalent ions of Mg, Cdand Zn, there is no

    50 contribution of extra free charge carriers for GZO from the Mg or Cd substitution. However, the incorporation of Mg or Cd may enhance electron scattering and grain boundary barrier effects and then destroy the conductivity of ZnO-based TCO [13,14]. Several investigators have examined the optical and electrical properties of Ga-doped ZnMgO films [19,20], Mg or Cd doping content 1-xx

    ight affect both of the optical and electrical properties of band gap engineered AZO. Meanwhile, m

    55 the effect of narrowing band gap by Cd doping on the electrical and optical properties of (Cd,Ga)-codoped ZnO films has been rarely reported.

    Researchers have employed numerous deposition techniques to prepare ZnO-based thin films, including PLD, molecular beam epitaxy (MBE), chemical vapor deposition (CVD), magnetron sputtering, etc.[10,19,20] However, the industrial production is limited due to the complex and 60 expensive vacuum technique. Furthermore, the preparation of homogeneous and large-area films is also an upfront challenge. The sol-gel method is a kind of cost-effective process and is helpful to realize the preparation of large-area homogeneous films [11]. More importantly, sol-gel method has the distinct advantages in excellent composition control and the ability to achieve atomic scale mixing of individual components [21]. In addition, post-annealing treatment by various 65 atmospheres, such as air, oxygen, hydrogen, nitrogen, or in vacuum for as-prepared TCO films is usually considered as an essential and effective technique to improve the electrical and optical properties [22]. The effects of atmospheres could be simply divided into two classes, (a) Annealing the TCO films in air, oxygen or nitrogen could improve the crystallinity and transmittance, but also would degrade the electrical properties due to the chemisorptions of Oor 2

    70 N; (b) Annealing in hydrogen environment was reported to be significant in improving the 2

    electrical conductivity, and the reason was considered to be both the production of additional oxygen vacancies and desorption of the absorbed oxygen at the grain boundaries. Treatment in vacuum, which was qualitatively similar to the effect of annealing in hydrogen, was also usually introduced to improve the conductivity of TCO films by enhancing oxygen vacancies [23,24]. In 75 our previous work, we have taken a research on the effect of annealing ambient on the structural, optical and electrical properties of the band gap modified (Cd,Al)-codoped ZnO thin films [24]. To make a comparison, here, we prepared 2 at.% Ga codoped ZnCdO (with nominal Cd content 1-xx

    x=0-8 at. %) thin films by dip-coating sol-gel method. Analogously, the effects of post-annealing in two representative ambient (in vacuum and nitrogen) on the optical and electrical properties of 80 the band gap modified (Cd,Ga)-codoped ZnO thin films are investigated.

1 Experimental procedure

    Analytical grade zinc acetate was firstly dissolved in a 2-methoxyethanol and monoethanolamine (MEA) solution at room temperature. The concentration of the sol was 0.75 mol/L and the molar ratio of MEA to zinc acetate was kept at 1.0. The solution was stirred at 60 o85 C for one hour until it became clear and homogeneous. Gallium nitrate and/or cadmium nitrate were added into some of the previous solutions in an appropriate ratio and then stirred vigorously

    oat 60 C for another one hour. The final solutions served for coating were aged for 36 hours at room temperature. The following dip-coating process and post annealing conditions (in air, vacuum and nitrogen) are exactly same as the preparation of (Cd,Al)-codoped ZnO thin films, 90 more details could be found in our previous work [24].

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     X-ray diffraction (XRD) patterns were collected from 20? to 80? using PANalytical Xpert

     radiation. Optical properties such as transmittance were measured by MPD PRO with Cu Kα UV-VIS spectrophotometer (Hitachi UV-VIS spectrophotometer U3010) in the wavelength of 300-800 nm. The thickness of the samples was carried out by spectroscopic ellipsometer (Spec

    95 EI-2000-VIS). The electrical properties such as resistivity, carrier concentration, and Hall mobility were detected by Hall effect measurements in the Van der Pauw configuration using an electrical

     transport property measurement system (Beijing Jingcheng, China, ET-9000) at room temperature. 2 Results and discussion Fig. 1 displays the XRD patterns of 2 at.% Ga-doped ZnCdO (x=0-8% with an increment 1-xx

    100 of 2 at. %) thin films treated by vacuum and nitrogen annealing, respectively. It is implied that all

     the films have a single phase which can be identified as the hexagonal wurtzite structure of ZnO (space group P6mc). No trace of other impurities is found within the detection limit of instrument. 3

     All the films show an extremely pronounced (002) texture with dominant peak 2θ?34.4?,

    indicating that the preferred orientation is along the crystallographic c-axis and perpendicular to

    105 the substrate. It also manifests a slight reduction of c-axis preferred orientation, which is revealed

     by the appearance of tiny (10l) (l=1,2,3) peaks [25]. From the better c-axis preferred orientation, especially for the samples after nitrogen annealing, it could be concluded that the disruption of

     (Cd,Ga)-codoped ZnO crystallinity is indeed less that of (Cd,Al)-codoped ZnO [24], due to the 3+ 2+ 3+ ionic radius of Ga(0.47 Å, CN=4) is closer to that of Zn(0.60 Å, CN=4) than Al(0.39 Å,

    CN=4) [8,9]. 110