Solution derived Al-doped zinc oxide films: doping effect, microstructure and

electrical property

J Sol-Gel Sci Technol (2009) 49:238–242

Keh-moh Lin, Yu-Yu Chen, Keng-Yu Chou

指導教授：林克默 博士報告學生：郭俊廷報告日期： 99/9/27

K.-m. Lin Y.-Y. ChenDepartment of Mechanical Engineering, Southern Taiwan

University, No.1, Nantai Str, Yung-Kang City, Tainan 710,Taiwan, ROC

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Outline

Introduction Experimental detail Results and discussions Conclusions

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Introduction

Recently, zinc oxide films have been widely investigated for new opto-electrical devices owning to their attractive electrical and optical properties [1, 2].

Techniques used to deposit pure and doped ZnO films include sputtering technique [6, 11–13], pulsed laser deposition [14], thermal plasma [15], MOCVD [16], spray pyrolysis [17] and sol–gel method [18–24].

Among them, the sol–gel method is not only a low-cost and simple deposition procedure to coat large area high quality TCO films

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Experimental detail

In our experiments, zinc acetate dihydrate was dissolved in isopropylalcohol, and aluminum nitrate was served as dopant sources.

The Al/Zn ratio in the solution varied from 0.25% to 4%. The solution concentration was 0.5 mol/L.

After being deposited on corning glass 1737 (some samples on silicon wafers) by dip-coating, the films were first dried at 70 for 10 min. Then, the films were heated in a tube℃ furnace at 600 for 1 h in air (pre-heat treatment).℃

The procedures from coating, drying, to annealing were repeated 2–5 times.

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Finally, these films were annealed in vacuum (1–10 mtorr) at 600 for 1 h (post-heat treatment).℃

The structural characteristics of the AZO films were studied by a thin-film X-ray diffractometry (Rigaku D/MAX 2500) with Cu K radiation.

The film resistivity, carrier concentration and mobility were obtained by Hall measurements (Ecopia HMS-3000).

The transmittances of the AZO films were measured by using a UV–Vis–NIR spectroscopy (Perkin Elmer Lambda 25).

The refractive indices of the AZO films were estimated by a spectroscopic ellipsometry (Woollam M-2000U).

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The PL spectra were obtained by a fluorescence spectrometer with a 150 W xenon lamp (Hitachi F-4500).

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Results and discussions

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The XRD pattern of the AZO films indicated that the crystal structure of the AZO films is wurtzite (Fig. 1).

The (002) peak was characterized by using the relative intensity

i(002) = I(002) /[I(100) + I(002) + I(101)]. With increasing aluminum

concentration, it became slightly stronger (Fig. 2).

Fig. 1 X-ray diffraction patterns of AZO films with different aluminum concentration, 0.5 mol/L, five-layer films

At the same time, the crystallite sizes d(002) which were calculated by using Scherrer’s equation [27] became distinctly smaller when Al concentration rose (Fig. 2).

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Fig. 2 The normalized I(002) and crystallite size in dependence on aluminum concentration, 0.5 mol/L, five-layer films

The additional aluminum dopant material promoted the nucleation behavior of the ZnO phase, which means, as the aluminum concentration rose, more ZnO nuclei emerged on the substrate and therefore the grown crystallites were smaller.

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Furthermore, the relative density of the coating films was calculated by using the equation [19]:

An average refractive index (maximal values between 370–380 nm) was applied to present the optical property of the graded films. All the refractive index values were smaller than that of the dense wurtzite-type ZnO (2.0) [19].

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1001-n

1-ndensity(%) Relative

2d

2f

It is found that the film resistivity decreased as the film became thicker and denser.

This implies that the film properties changed obviously after the third dip-coating process.

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Fig. 3 Film relative density and resistivity in dependence on layer number, 1.0 at.%, 0.5 mol/L, SiO2/Si substrate

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Fig. 4 Carrier concentration n in dependence on layer number, 0.5 mol/L

It can be clearly found in Fig. 4 that for all concentrations of aluminum, the carrier concentration n could only rise to a certain value (<1020 cm-3) while the film thickness was increasing.

The carrier concentration did not become higher along with the increasing aluminum concentration.

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Figure 5 showed that for all concentrations of aluminum, carrier mobility μH was slowly enhanced along with the increase in film thickness, with a few values of one-layer and two-layer as exceptions.

Results of SE measurements indicated this can be attributed to the increasing relative density as the film became thicker. Fig. 5 Carrier mobility l in dependence

on layer number, 0.5 mol/L

Furthermore, it can be found in Fig. 6 that generally, the mobility became lower while aluminum concentration was rising.

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Fig. 6 n, in dependence on aluminum concentration, 0.5 mol/L, five-layer films

Though the crystallite size reduced from 18.5 to 12.5 nm between 0.25 and 4.0 at.% (cf. Fig. 2), but it was still at least two times larger than the mean free path length Lf (0.5–3 nm), indicating the impurity scattering or other scattering centers inside the crystallite affected the film conductivity more than that of grain boundary scattering.

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All the samples are transparent in the visible region. Although the 0.25 at.% sample shows the highest conductivity, its transmittance is relatively lower than other samples.

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Fig. 7 Film transmittance index in dependence on wavelength, five-layer, 0.5 mol/L

PL measurement results (Fig. 8) show that, beside near band-edge emission, there are several emission centers between 425 and 550 nm (2.91–2.25 eV).

These are mainly caused by glass substrates and by interstitial zinc atoms or related defects.

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Fig. 8 Photoluminescence spectra in dependence on aluminum concentration, five-layer, 0.5 mol/L

Because interstitial zinc atoms will provide free electrons, this also indicates another source of charge carriers in the AZO films.

High PL peak value implies the good quality of the AZO films. This in turn means the numbers of potential barriers and scattering centers in the AZO films are also small. Thus, the carrier mobility increases.

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Conclusions

在這研究中，發現摻雜的濃度會影響晶粒尺寸的大小，也會增加在 (002) 方向的相對強度。

隨著塗佈層數的增加，載子濃度也會增加。但遷移率卻會在一定程度的載子濃度下減少。

由此可知，過度的摻雜無法提高載子濃度。同時，過多的載子濃度亦會形成載子的散射中心，提高電阻率。

另外，從 PL 的結果來看，過多的摻雜反而會惡化薄膜的品質。

本研究所獲得的最佳試片，為鋁的摻雜濃度為 1at.% ，其片電阻為 182 ，在可見光範圍下穿透率大於 80% 。

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Thanks for your attention

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