简介概要

Zinc tin oxide thin films prepared by MOCVD with different Sn/Zn ratios

来源期刊：Rare Metals2017年第9期

论文作者：Ying Xu Lin-Yan Hou Xiao-Meng Zhang

文章页码：753 - 757

摘要：Zinc tin oxide（ZTO） thin films, with zinc acetate and tributyltin chloride as raw materials, were deposited on glass substrates by the method of metal organic chemical vapor deposition（MOCVD）. The crystallization, microstructure and optical properties were investigated by scanning electronic microscope（SEM）,X-ray diffraction（XRD） and ultraviolet-visible（UV-Vis）spectrophotometer. The results show that with the increase in Sn/Zn ratio, the crystal changes from wurtzite to rutile phase. When the ratio reaches 11:18,the intensity of Zn₂SnO₄ peaks appears to be the strongest and the optical band gap is about 3.27 eV. Calculated by the envelope method, the thickness of the ZTO thin films is 713.24 nm.Measured by UV-Vis spectrophotometer, the transmittance of the ZTO thin films reaches up to 80% in the wavelength range of 400-1000 nm when the Sn/Zn ratio is 7:18.

稀有金属(英文版) 2017,36(09),753-757

Zinc tin oxide thin films prepared by MOCVD with different Sn/Zn ratios

Ying Xu Lin-Yan Hou Xiao-Meng Zhang

Hebei Province Key Laboratory of Inorganic Nonmetallic Materials, Materials Science and Engineering College, North China University of Science and Technology

收稿日期：2 August 2014

基金：financially supported by the National Natural Science Foundation of China (No.51342006);

Zinc tin oxide thin films prepared by MOCVD with different Sn/Zn ratios

Ying Xu Lin-Yan Hou Xiao-Meng Zhang

Hebei Province Key Laboratory of Inorganic Nonmetallic Materials, Materials Science and Engineering College, North China University of Science and Technology

Abstract：

Zinc tin oxide(ZTO) thin films, with zinc acetate and tributyltin chloride as raw materials, were deposited on glass substrates by the method of metal organic chemical vapor deposition(MOCVD). The crystallization, microstructure and optical properties were investigated by scanning electronic microscope(SEM),X-ray diffraction(XRD) and ultraviolet-visible(UV-Vis)spectrophotometer. The results show that with the increase in Sn/Zn ratio, the crystal changes from wurtzite to rutile phase. When the ratio reaches 11:18,the intensity of Zn₂SnO₄ peaks appears to be the strongest and the optical band gap is about 3.27 eV. Calculated by the envelope method, the thickness of the ZTO thin films is 713.24 nm.Measured by UV-Vis spectrophotometer, the transmittance of the ZTO thin films reaches up to 80% in the wavelength range of 400-1000 nm when the Sn/Zn ratio is 7:18.

Keyword：

Sn/Zn ratio; MOCVD; ZTO thin films; Envelope method; Optical properties;

Author： Ying Xu,e-mail:yuyingdddd@sina.cn;

Received： 2 August 2014

1 Introduction

Transparent conducting oxide (TCO) thin films are important optical materials [ 1] ,which are widely used in photovoltaic industry with high visible light transmittance,high conductivity and high reflectivity.ZnO and SnO₂ are important semiconducting materials whose optical band gap is more than 3.1 eV at room temperature [ 2, 3] .The radius of Sn⁴⁺(6.9 nm) is very close to that of Zn²⁺(7.4 nm).Zinc tin oxide (ZTO) films,composited by ZnO and SnO₂,have the advantage of high stability,easy etch in hydrogen plasma of ZnO and high electrical stability of SnO₂.ZTO films are considered as a new generation of optoelectronic semiconductor materials due to the abundance of raw materials,nontoxic and low price [ 4, 5] ,which were widely applied in gas sensors,transparent electrodes,solar cells and other fields.

A wide variety of thin films preparation methods,including atomic layer deposition,radiofrequency magnetron sputtering deposition,chemical vapor deposition (CVD),sol-gel process,spray pyrolysis,molecular beam epitaxy (MBE) and pulsed laser deposition (PLD),were reported [ 6] .Although MBE and PLD are both used to grow high-quality ZnO films,these methods are limited to the laboratory because of their high costs and the difficulty in scaling up.CVD has many technical advantages for industrial applications,including being able to achieve high rate growths on large surface areas,a wide selection of metal organic source materials,easy purification by evaporation and low-cost production [ 7] .

In the present study,ZnO tin oxide (ZTO) thin films,with zinc acetate and tributyltin chloride as raw materials,were prepared on glass substrates by the method of metal organic CVD.The crystallization,microstructure and optical properties were investigated by scanning electronic microscope (SEM),X-ray diffraction (XRD) and ultraviolet-visible (UV-Vis) spectrophotometer.

2 Experimental

The experimental instrument of metal organic chemical vapor deposition [ 8] in laboratory is shown in Fig.1.Zinc acetate was used as Zn source and tributyltin chloride as Sn source.ZTO films [ 9, 10] were prepared with preheat air as carrier gas and ordinary glass as the substrate.Glass substrates were first cleaned with efficient detergent before experiment,then ultrasonic ally cleaned for 20 min with acetone and ethanol successively,and finally dried at 80℃in a drying oven.Detailed experimental process was depicted as follows:two raw materials were introduced into corresponding reactor pipe,and at the same time substrate device was placed in the corundum tube.Then,gas preheating device and substrate heating device were turned on.After a period of heating,two source regions started to be heated.When the temperature of source region reached the preset temperature,carrier gas preheated began to be output.Finally,ZTO thin films were prepared in air atmosphere at450℃for 30 min,and then,the films were annealed at450℃for 1 h.

Fig.1 Schematic diagram of experimental device

The surface morphology of the films was investigated by SEM (S-4800) of cold field emission.Secondary electron image resolution was 1.0 nm (15 kV),and the magnification was 20-80 times.Phase structure and growing tendency of the films were analyzed by XRD (D/MAX2500PC,Cu Kα,λ=0.1506 nm).The transmittance of ZTO thin films was measured with S-600 UV-Vis spectrophotometer.

3 Results and discussion

3.1 Analysis of optics properties

3.1.1 Analysis of transmittance

The influence of different Sn/Zn ratios on ZTO thin films is shown in Fig.2.When the Sn/Zn ratio is 7:18,films visible transmittance could reach more than 80%in the visible region.In addition,films visible transmittance could reach more than 90%in the range of 844-1000 nm.

As to optical properties of ZTO films [ 11, 12, 13, 14] ,peaks and troughs of the transmittance graph are obvious,the filmforming quality is excellent,and the film thickness is uniform.The transmittance has a downward trend with the increase in Sn/Zn ratio,but peaks and troughs in the transmittance curve are more obvious.When the Sn/Zn ratio reaches 11:18,peaks and troughs in the transmittance curve of films are the most obvious,which implies that the optical properties are excellent.

The relationship between optical band gap and absorption coefficient in semiconductor materials is shown in E_q.(1).

where E_g is optical band gap of the films,eV;αis absorption coefficient;h is the Planck constant,4.1357×10^-15 eV·s;v is the frequency of incident photon;and A is the constant of direct band gap transition.In Eq.(1),if the relationship between (αhv)² and hv was obtained,the band gap would be obtained whenα=0.According to the transmittance of ZTO thin films,a graph of (αhv)²～hv is shown in Fig.3.The band gap of the films,which could be obtained through the method of the middle portion in Fig.3,is extrapolated toα²=0,namely the hv value of (αhv)²=0.When the Sn/Zn ratios are 7:18,11:18,14:18 and 17:18,the optical band gap values of films are 3.30,3.27,3.29 and 3.22 eV,respectively.That is,with the increase in Sn/Zn,the optical band gap gradually decreases.

Fig.2 Optical transmittance on ZTO films with different Sn/Zn ratios

Fig.3 Relation of photon energy (hv) and (ahv)² of ZTO films prepared with different Sn/Zn ratios

3.1.2 Calculation of film thickness

The thickness of thin films can be calculated by envelope method according to Refs. [ 15, 16] .So the refractive index(n) is calculated by the following equation.

where n_s is the glass substrate refractive index (1.52),Yis a constant that represents the formula of Eq.(3).

where T_M and T_m are two envelope values of the transmittance in the same wavelength.

Film interference fringe is defined and shown in Eq.(4).

where L is integral number,λis wavelength,and d is film thickness.The film thickness can be calculated by Eq.(5)

where n₁ and n₃ are two adjacent film refractive indexes of the maximum and minimum andλ_l andλ₃ are the corresponding wavelengths.

Envelop curves of the transmission spectrum for films are given in Fig.4.Through the transmittance curves,T_M(maximum) and T_m (minimum) under the same wavelength could be known in the region of the weak absorption of transmittance spectra.

From seven consecutive extremum points (1-7) as shown in Fig.4,d₁-d₇ can be got by Eq.(5),and then,their average value can be calculated,which can be shown as D₁.Because of the large discrepancy of d_i (i=1,2...7),the results should be further optimized.The method is shown as follows:first,the interference order L_i can be worked out from Eq.(4) according to d_i.Then,it is rounded on L_i to acquire a series of[L].Finally,a series of thickness can be acquired according to Eq.(4),and their average value can be written as D₂.Then,the eventual average value D is obtained,which is the average value of D_i and So the film thickness is calculated to be about713.24 nm by envelope method.Mostly the values of[L]corresponding continuous extreme points of transmittance are 11,10,9,8,7,6 and 5,respectively.Obviously,the interference series are consecutive integers as the continuous extreme points.Thus,the calculation accuracy is high.

Fig.4 Envelope curves of film transmission spectrum at Sn/Zn ratio of 11:18

3.2 XRD analysis

XRD patterns of ZTO films with different Sn/Zn ratios are shown in Fig.5.As can be seen,the crystal phases translate from wurtzite ZnO to rutile SnO₂ with the increase in Sn/Zn ratio and Zn₂SnO₄ spinel structure could be detected.When the ratios of Sn/Zn are 7:18 and 9:18,ZnO wurtzite structure is the main crystal type in ZTO films.Usually,(002) of ZnO performs the lowest density of surface energy,(002)plane is growing,and other planes are restrained in the process of crystal growth.So (002) plane is the preferred orientation in ZnO films [ 17] .It is interesting to note the increase in (311) peak of Zn₂SnO₄ and the decrease in (002)peak of ZnO with the increase in Sn/Zn ratios.The intensity of Zn₂SnO₄ diffraction peak is the highest when the ratio is11:18,while the spinel structure of Zn₂SnO₄ is weakened with the increase in Sn/Zn ratio.As the main crystal phase,the spinel structure inhibits the preferred orientation of(002) plane and increases the intensity of (100) and (101)diffraction peaks with the Sn/Zn ratio of 11:18.When the ratio of Sn/Zn reaches 17:18,rutile structure of SnO₂ is the main crystal phase in the ZTO films.

3.3 SEM analysis

SEM images of ZTO films with different Sn/Zn ratios are shown in Fig.6.It is obvious that ZTO films crystallize well and that the grain size is uniform when the Sn/Zn ratios are 9:18 and 11:18,respectively.While the crystallization of films is relatively poor and the film surface is rough with the ratio of 7:18,the grain size reaches about200 nm,larger than those of other groups when the Sn/Zn ratio is 11:18.The reason is that Zn₂SnO₄ crystal is the main crystal phase.With a ternary compound crystal,Zn₂SnO₄ crystal could form a continuous growth of spinel crystal structure and suit large grain growth.This interpretation is in good agreement with the XRD results.

Fig.5 XRD patterns of ZTO films with different Sn/Zn ratios:a 7:18,b 9:18,c 11:18,d 14:18 and e 17:18

Fig.6 SEM images of ZTO films with different Sn/Zn ratios:a 7:18,b 9:18,c 11:18,d 14:18 and e 17:18

4 Conclusion

ZTO films were deposited on glass substrates by MOCVD.When Sn/Zn ratio is 7:18,film visible transmittance reaches up to 80%in the visible region.And film visible transmittance reaches up to 90%in the wavelength range of 844-1000 nm.When the Sn/Zn ratio is 11:18,the filmforming quality is excellent and the film thickness is uniform.The optical band gap of ZTO thin films is about3.27 eV,and the film thickness is calculated to be713.24 nm by envelope method.

When the Sn/Zn ratios are 7:18 and 9:18,wurtzite structure is the main crystal phase in ZTO films and Sn exists in Zn₂SnO₄ structure.Spinel structure of Zn₂SnO₄ is the main crystal phase,and rutile structure of SnO₂ appears when the Sn/Zn ratio is 11:18.The rutile structure of SnO₂is the main crystal phase in ZTO films with the increase in Sn/Zn ratio.

Acknowledgments This study was financially supported by the National Natural Science Foundation of China (No.51342006).