The Fabrication and Stability Study of Bismuth Doped Zinc Oxide Prepared by Spray Pyrolysis

YuHsuan Huang, WenHow Lan, MingChing Shih, ChunYi Lee, YenWen Wang, WeiHsuan Hsu

Department of Electrical Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan

Correspondence should be addressed to WenHow Lan: whlan@nuk.edu.tw

Abstract

The bismuth doped zinc oxide (ZnO:Bi) was prepared by spray pyrolysis with zinc acetate and bismuth nitrate precursors. The surface morphology and crystalline quality were characterized. For films without and with low bismuth nitrate content, n-type conduction was observed. The p-type zinc oxide film can be achieved with the bismuth nitrate concentration doping more than 0.3 at.%. The film conductivity, concentration and mobility were characterized. A stable hole concentration as 4x1017cm-3 can be achieved with 5% bismuth nitrate doping.

Introduction

Zinc oxide (ZnO) thin film shows the proven fabrication

technology in recent years applied in solar cell [1], LED [2] and transparent electrode [3] etc.. In order to improve the conduction properties of this high bandgap material[4], thin film fabricated with variety of dopants are required.

For the unintentionally doped ZnO, as oxygen vacancy and zinc interstitial may incorporate in the film during fabrication process, n-type ZnO is generally be observed. For ZnO doped with group III elements such as Al [5], In [6], Ga [7] etc., element substitution results in n-type ZnO films also. For group V element doping such as N [8], P [9], As [10] in ZnO, as the oxygen substitution may occurred, p-type ZnO can be achieved. For the largest group V element, Bi, less study was carried on [11]. A lot of deposition method such as spray pyrolysis [12], sputtering [13], sol - gel [14] methods etc. were applied in the fabrication of ZnO films. In this study, we report the work regarding the Bi doped ZnO film prepared by spray pyrolysis method, which is a high efficient deposition technology for large area process. The surface morphology, crystalline quality and electrical conduction analysis were applied to characterize the films.

Experimental details

The bismuth doped ZnO thin film was deposited on n-Si substrate at 450oC by spray pyrolysis. Before deposition, the Si substrate was cleaned by RCA cleaning process followed by rinse in buffer oxide etchant 20 s to remove the oxide layer. The 0.2M znic acetate and bismuth nitrate with null to 7 atomic concentrations (at.%) were used as precursors. The SEM, XRD and Hall measurement with van der Pauw four point methods were applied to characterize the films.

Results and discussion

Figure 1 shows the surface morphology characterized by

SEM for the ZnO thin film doped Bi at 0, 1, 5, 7 at.% ((a) to (d)). It is observed that the surface morphologies of the doped ZnO thin films were considerably affected by bismuth doping.

With the increasing of bismuth content, the petal structures were observed clearly and the grains become larger

Figure 2 shows the X-ray diffraction (XRD) patterns of the undoped and bismuth doped ZnO thin films. The undoped ZnO film shows obvious (002) (101) (102) and (103) peaks. With the increasing of bismuth doping, the (101) and (103) peaks decreases and the (102) peak almost vanish. The ZnO thin film with bismuth at 7 at.% showed a primary (002) peak.

According to XRD spectrum in Fig.2, the full-width-half-maximum (FWHM) values of (002) peak were evaluated in Fig.3. Based on Scherrer’s formula, the averaged grain size D for the film can be calculated as [15]

(1)

, where is the Bragg diffraction angle, is the wavelength of the incident radiation, and is FWHM of the diffraction ray. The FWHM of (002) decreases with increasing concentration. With minimum FWHM for samples with 5 at.% doping, better crystal quality can be expected.

Figure 4 shows the carrier concentration and mobility and resistivity characteristics as a function of bismuth content. As the bismuth doping less than 0.3 at.%, n-type ZnO with concentration around 1016 cm-3 can be observed. As the bismuth doping higher than 0.5 at.%, p-type concentration were characterized. With the increasing of bismuth doping in p-type region, the carrier concentration increases and resistivity decreases with mobility remains. A concentration saturation like behavior can be observed as the doping reaches 5 at.%. As bismuth doping of 5 at.%, the carrier concentration is 4.4x1017 , the mobility is 5.5

, resistivity is 3.10 -cm for a better electrical properties.

Conclusion

In conclusion, the bismuth doped ZnO thin films prepared

by spray pyrolysis method were studied. With bismuth doping increases, electrical conduction type caries from the n-type to p-type. By the XRD analysis, better crystallinity film was achieved for the 5 at.% doped ZnO thin film. The bismuth doping shows an effective method in the fabrication of p-type ZnO.

References

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Fig.1 The surface morphology characterized by SEM of the ZnO thin films doped Bi at (a) 0, (b) 1, (c) 5 , (d) 7 at.%.

Fig.2 The XRD spectra for the ZnO thin films with different bismuth nitrate doping ratios.

-1 0 1 2 3 4 5 6 7 8

0.33

0.34

0.35

0.36

0.37

0.38 FWHM D

Bi concentration (at.%)

FWH

M (d

egre

e)

27.0

28.5

30.0

31.5

D (n

m)

Fig.3 The FWHM for the ZnO thin films with different bismuth ratios.

0 1 2 3 4 5 6 70

5

100

20

401E16

1E18

(c)

(b)

PN

Res

istiv

ity(Ω

-cm

)

Bi concentration (at.%)

Resistivity(a)

Mob

ility

(cm

2 V-1

sec-1

)

Mobility

Con

cent

ratio

n (c

m-3

)

Concentration

Fig.4 The carrier concentration and mobility and resistivity characteristics of ZnO thin films with different bismuth ratios.