Influences of preferred orientation growth on electrical properties

of ZnO:Al films by sol–gel method

教授 : 林克默博士學生 : 董祐成

日期 :2010/10/18

Keh-moh Lin • Hsin-Cheng Chen • Yu-Yu Chen •Keng-yu Chou

J Sol-Gel Sci Technol (2010) 55:369–376DOI 10.1007/s10971-010-2252-3

ORIGINAL PAPER

Received: 18 February 2010 / Accepted: 14 May 2010 / Published online: 27 May 2010 Springer Science+Business Media, LLC 2010

K. Lin H.-C. Chen Y.-Y. ChenDepartment of Mechanical Engineering,Southern Taiwan University, Tainan, TaiwanK. Lin (&) K. ChouInstitute of Nanotechnology, Southern Taiwan University,No.1, Nantai Str., Yung-Kang City, Tainan 710, Taiwan, ROCe-mail: kemo@mail.stut.edu.tw

Outline

Introduction Experimental Result and Discussion Conclusion

Introduction

In recent years, wide band-gap transparent conducting oxides have received great attention because they can be applied on UV Laser diodes, transparent electrodes of LCDs, touch panels.

Among them, zinc oxide arouses great expectations, not only because it has large exciton binding energy, high breakdown strength, good resistance to radiative damage, but also because the techniques for large area single crystal and epitaxial growth of zinc oxide have been well developed.

In this study, our investigations will focus mainly on two points: (1) the effects of different heat treatments, MEA amount and initial layers on the preferred orientation of the AZO films, (2) the influences of the micro-structure, different nucleation behaviors, and film growth behaviors on the films’ conductivity. Furthermore, in order to enhance the accuracy of film thickness measurement, we intend to use a multi-angle spectroscopic ellipsometry (SE) combined with a multi-layer model.

Experimental

In our experiments, zinc acetate dihydrate (ZnAc) was dissolved in methanol. Aluminum nitrate as well as gallium trichloride were served as dopant sources. Based on our earlier study , the best conductivity was achieved with an Al/Zn ratio of 1.0 at 88%. The solution concentration was 0.3 mol/L. Symbol A represents the sol solution without any MEA as stabilizer.

Symbol B means that the molar ratio of ZnAc:MEA was 1:1 in the sol solution.

The molar ratio of sol solution C was 1:4. The pH values were 6.3, 7.4, and 10.5, respectively.

Several fabricating procedures were carried out to obtain AZO films: Using corning glass (corning 1737, hereafter referred as ‘‘P2-G’’ to differentiate from the process in our earlier works) and silicon wafers (hereafter referred as ‘‘P2-S’’) as substrates, the films were deposited by spin-coating and were then pre-heated in a rapid thermal annealing (RTA) furnace at 600 ℃ for 10 min in air.

In the third procedure (P3-G), we used a 3-inch AZO (ZnO with 2wt.% Al2O3) target in the sputtering process to create an initial layer on the corning glass.

The distance between the target and the corning glass was 5 cm.

Ar was introduced at a flow rate of 40sccm. The working pressure was set at 8 × 10-3 torr.

The power for sputtering was set at 70 W; the substrate temperature was 270 ℃

The thicknesses of the initial layers were 50 and 343 nm.

Finally, all the films of these three deposition processes were annealed in vacuum (~150 mtorr) at 600 for 1 h.℃

Result and Discussion

For this reason, the microstructure was rather loose, indicating that the growth of the films began inside the sol-layer. However, the pores were smaller.

The reason could be that we prepared the films layer by layer. Thus, every time was a new beginning and the growth process was not continuous. As a result, the effect of the initial layer was limited.

The reason can be that MEA retarded the condensation of the Zn2+ ions and promoted the formation of zinc monoacetate as well as of ZnO later on.

That means, the combination effects of adding MEA and the strong heat flux additionally enabled the crystallites to grow towards the direction of the normal lines of the substrate, i.e., to enhance the (103) reflection .

At the same time, the sputtering layer was not only favorable for the growth of AZO films in the (002) direction, but also reduced the (103) intensity.

Table 2 that the strong heat flux made the crystallite size of the P2-S samples become the biggest.

i(002) also increased as the MEA amount increased.

one can see that the sputtering layer could enhance the i(002). The thicker the sputtering layers were, the stronger the i(002) became.

The conductivities of the P2-S and P2-G samples reached their highest values when the MEA:ZnAc ratio was 1:1.

Adding a proper amount of MEA made the zinc monoacetate to evenly distribute in the sol solution, which was favorable for the growth of the ZnO phase in direction (002) and thus improved the crystallinity as well as the carrier mobility. However, the sol would become base when too much MEA was added. Under such situation, the film density would become loose causing the carrier mobility to drop.

P3-G-D that the sputtering process was favorable for the doping effect, i.e., the carrier concentration of this sample was almost the highest in this study. However, when the number of the sol–gel layers was small, their relative densities were low. Consequently, the carrier mobilities were also low.

Shows that the compared SE data and SEM fitting results are in good accordance. The deviation caused by their errors in the Hall measurement is also within the acceptable range.

Shows that all the P2-G and P3-G samples are highly transparent within the visible light range (the transmittance reaches 90% at wavelength 550 nm). As the wavelength of the incident light moved to the short wavelength range, the transmittance dropped dramatically. This is the so-called optical absorption edge (wavelength at about 380 nm). As the film layers increased, the absorption edge tended to move toward the long wavelength region.

Conclusion

結果發現， MEA 影響溶膠溶液裡的離子複合結構而促進了（ 002 ）方向增長。由於結晶度提高，載流子遷移和導電率也增強。

應用高品質的初始層雖能提高晶體性質、載子遷移率、 AZO 薄膜的電導率，但並不多。

另一方面，均相成核所造成的直接紅外加熱，增強優選取向增長，這是由於摻雜增強了活化率而使載子濃度上升。因此，最明顯的是導電率的改善。

提高導電性對溶膠凝膠 AZO 薄膜的關鍵因素就是摻雜使活化率提高。

THANKS FOR YOUR ATTENTION