Enhancement of the light-scattering ability of Ga-doped ZnO thin films using SiOx nano-films prepared by atmospheric pressure plasma deposition system

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Enhancement of the light-scattering ability of Ga-doped ZnO thin

ﬁlms

using SiO

x

nano-

ﬁlms prepared by atmospheric pressure plasma

deposition system

Kow-Ming Chang

a

, Po-Ching Ho

a,

⁎

, Atthaporn Ariyarit

a

, Kuo-Hui Yang

b

, Jui-Mei Hsu

b

,

Chin-Jyi Wu

b

, Chia-Chiang Chang

b

a

Department of Electronics Engineering & Institute of Electronics, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30010, Taiwan, ROC

b

Industrial Technology Research Institute, Mechanical and Systems Research Laboratories, Hsinchu 31040, Taiwan, ROC

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 22 November 2012

Received in revised form 24 September 2013 Accepted 25 September 2013

Available online 2 October 2013 Keywords:

Transparent conductive oxide Light-trapping effect Atmospheric pressure plasma

To enhance the light-trapping qualities of silicon thin-ﬁlm solar cells, the use of transparent conductive oxide with high haze and high conductivity is essential. This study investigated an eco-friendly technique that used bi-layer Ga-doped zinc oxide/SiOxﬁlms prepared with an atmospheric pressure plasma jet to achieve high haze and

low resistivity. A minimum resistivity of 6.00 × 10−4Ω·cm was achieved at 8 at.% gallium doping. Examination of X-ray diffraction spectra showed that increasedfilm thickness led to increased carrier concentration in GZO bilayers. The optimal bilayer GZOfilm achieved considerably higher haze values in the visible and NIR regions, compared with Asahi U-typefluorine doped tin oxide.

1. Introduction

Transparent conductive oxide (TCO) has attracted substantial atten-tion because of the wide range of its applicaatten-tions. These include solar cells[1–3],flat-panel displays, and light emitting diodes. Two important absorber materials inside silicon thin-film solar cells are amorphous sil-icon (a-Si) and microcrystalline silsil-icon (μc-Si), both in the form of thin films. To reduce light-induced degradation, the a-Si thin film should be deposited as thinly as possible. By contrast,μc-Si is an indirect semi-conductor with a low absorption coefficient, and the μc-Si thin-film should be deposited thickly enough to completely absorb sunlight in a single path. To fulfill these requirements, TCO coatings with properties of high haze and high conductivity should be developed to enhance optical absorption in trapped light[4–7].

A simple technique can be used to produce high haze and high conductivity TCOfilms. This study developed TCO films using an atmo-spheric pressure plasma jet (APPJ). The properties of TCOfilms were compared with those of Asahi U-type TCO glass. The atmospheric pres-sure chemical vapor deposition (APCVD) process of commercial Asahi U-typefluorine doped tin oxide (FTO) glass requires a high deposition temperature and the use of expensive and toxic gas. Our method used eco-friendly precursors and a low deposition temperature. In addition, the APPJ deposition system offers the advantages of low cost, low

temperature processes, and applicability to large substrate processing

[8]. The inﬂuence of physical properties of SiOxon the growth of

Ga-doped zinc oxide (GZO)ﬁlms was investigated, and the results showed that the SiOxbuffer layers played an important role in improving the

light scattering ability of GZOﬁlms. 2. Experiment

Fig. 1shows a schematic diagram of the experimental apparatus for constructing SiOxand GZOﬁlms, and the route of scan. Silicon suboxide

ﬁlms were deposited on a 5 × 5 cm2 _{alkali-free glass sheet (Asahi,}

AN100) and scanned over an area of 81 cm2_{by APPJ. The plasma jet}

was stationary and the substrate was put on the x–y movable stage. The pitch controlled the uniformity of the deposited thinﬁlms and the optimal value was 2 mm, as shown inFig. 1c. The scan speed of y direc-tion was 250 mm/s. Hexamethyl-disiloxane [(CH3)3-Si-O-Si-(CH3)3,

Alfa Aesar], was used as the precursor and argon was used as the carrier gas to deposit SiOxthinﬁlms on glass substrates at 75 °C. The ﬂow rate

of compressed dry air main gas wasfixed at 40 SLM. To investigate the influence of the flow rate of argon carrier gas on SiOxbuffer layers, the

ﬂow rate was set to vary at 0 sccm, 30 sccm, or 60 sccm. The plasma was generated by applying AC power at 20 kHz. The distance between plasma jet and substrate was 15 mm. GZOﬁlms were deposited on SiOx-coated glass at 150 °C using zinc nitrate (Zn(NO3)2, 99% purity,

J.T.Baker) and gallium nitrate (Ga(NO3)3, 99.9% purity, Alfa Aesar) as

precursors for Zn and Ga ions, respectively. The concentration of zinc nitrate dissolved in deionized water was 0.2 M and the atomic

⁎ Corresponding author.

http://dx.doi.org/10.1016/j.tsf.2013.09.082

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Thin Solid Films

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ﬁlms were analyzed using a scanning electron microscope (SEM; Hitachi, measurement (ACCENT, HL5500PC). The optical transmittance and haze values were measured using an UV–vis–NIR spectrophotometer (Jasco, V570). X-ray diffraction spectra of bilayer GZOﬁlms were measured using an X-ray diffractometer (XRD; Bruker, D8 Advance) with CuKα (_{λ = 1.54 Å) radiation source in Bragg–Brentano geometry.}

3. Results and discussion 3.1. SiOxbuffer layer effect

The APPJ was used to deposit rough morphology of SiOx [9]by

changing the distance between nozzle and substrate, substrate temper-ature and carrier gasflow rate. Preliminary experiments showed that carrier gasflow rate can precisely control the thickness and roughness of SiOxthinfilms. Based on this result, SiOxthinfilms were deposited

at argon carrier gasﬂow rates of 30 sccm and 60 sccm.Fig. 2a and b shows the cross-sectional and surface SEM images of SiOx buffer

layers deposited at an argon carrier gas_{ﬂow rate of 30 sccm. It can be} seen that the surface was rough and contains particles. At 30 sccm and 60 sccm, the root mean square (RMS) roughness of SiOx thin

ﬁlms was 12.20 nm and 34.87 nm. The thickness of sample of 30 sccm (60 sccm) ranged from 21.0 nm (36.2 nm) to 102 nm (242.9 nm). The elemental composition of SiOxﬁlms measured by XPS showed

that the atomic concentration of C1s, O1s and Si2p3 was 10.04 at.%, 59.48 at.% and 30.48 at.%, respectively. The O/Si atomic ratio was 1.95 and it revealed that oxidization was not fully promoted. The presence of carbon in the SiOxbuffer layer was attributed to Si-(CH3)xgroups

produced by the reaction of silicon with methyl groups[10]. The main difference between vacuum and non-vacuum deposition techniques is the mean free path (MFP) of radicals, which is associated with working pressure. The formation of rough surface of SiOxwas due to short MFP

and successive collisions between radicals inside plasma nozzle. The surface roughness of an SiOxbuffer layer is known to inﬂuence the

optical properties of GZOﬁlms subsequently deposited[10,11].Fig. 2c and d shows cross-sectional SEM images of GZO/SiOxbilayers and

mag-niﬁed images around the GZO/SiOxinterface. It was found that the grain

size of GZO thinﬁlm increased vertically because of rough surface of SiOxbuffer layers. The radicals generated by APPJ have low migration

on the rough SiOxsurface, which resulted in decreasing of nuclei

den-sity. After formation of critical size nuclei, small island formed sub-sequently on the SiOx surface. As growth proceeds, the island was

difﬁcult to contact with the other island edges because of few nucleation sites on SiOxsurface. Finally, this resulted in V-shaped GZO grain due to

difference of growth rate in vertical and horizontal direction.

Fig. 3shows the transmittance and haze spectra of conventional GZO ﬁlms and bilayer GZO ﬁlms deposited on various SiOxbuffer layers.

Silicon suboxide thinfilms were deposited at argon carrier gas flow rates of 30 sccm and 60 sccm. As the surface roughness increased, the transmittance of bilayer GZOfilms has a slight decrease in visible region (Fig. 3a). It is noteworthy that interference fringes of the sample of 60 sccm almost disappear. A possible explanation for this is that the rough morphology of bilayer GZO reduced the interference of light. As shown inFig. 3b, the haze values of bilayer GZOfilms in the visible region were significantly increased, compared with conventional GZO films (without SiOxbuffer layers). This result implies that bilayer GZO Fig. 1. Schematic diagram of the experimental equipment for (a) SiOxand (b) GZO; and

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films display more diffuse transmittance at the surface than do conven-tional GZOfilm. Data inTable 1show the Hall measurement results of conventional GZO and bilayer GZO. The sample of 30 sccm achieved the lowest resistivity and the highest Hall mobility. The sample of 60 sccm has similar electrical properties and higher haze value, but its surface was too rough to deposit thinfilms in subsequent process. Consequently, the argon carrier gas of 30 sccm was used to deposit SiOxbuffer layers.

3.2. Gallium doping concentration effect

Silicon suboxide thinfilms were deposited at argon carrier gas flow rates of 30 sccm and then GZO thinfilms were subsequently deposited on them using various Ga concentration precursors.Fig. 4shows the resistivity, Hall mobility, and carrier concentration of bilayer GZOfilms for Ga atomic ratio from 6 at.% to 10 at.%. The minimum mobility of 14.1 cm2/V·s was measured at 10 at.% gallium doping because of ionized impurity scattering[12]. The minimum resistivity of 6.00 × 10−4Ω·cm was obtained at approximately 8 at.% gallium doping. A possible expla-nation for thisfinding is the efficient occupation of Zn substitutional sites by Ga atoms in the wurtzite structure[13], which results in the highest carrier concentration of 6.58 × 1020_cm−3_{. The calculated}

opti-cal band gap in bilayer GZOﬁlms for a Ga doping concentration of 6 at.%, 8 at.% and 10 at.% was 3.71 eV, 3.82 eV and 3.74 eV, respectively. According to the Burstein–Moss effect, the optical band gap increases when carrier concentrations exceed the conduction band edge density of states. The relation between broadening of the optical band gap and carrier concentration was estimated using the following formula[14]:

ΔEopt¼ ℏ 2 2mcv ! 3π2 n 2 3

where is the reduced Planck constant, mcv⁎ is the reduced effective mass

and n is the carrier concentration. The sample of 8 at.% gallium doping had a maximum optical band gap of 3.82 eV and the result was consistent with the Hall measurements of carrier concentrations, as shown inFig. 4. A decrease in optical band gap was observed at 10 at.%; a possible

explanation is that excess dopants might have been trapped at the grain boundaries instead of contributing as free carriers[15]. The optical prop-erties of various Ga concentrations of GZO bilayers are summarized in

Table 2. As a result, the sample of 8 at.% has the highest transmittance of 82.2% at the wavelength of 550 nm.

In this study, the Ga content was higher than that reported in litera-tures (2 at.%–5 at.%) [16–18]. It can be speculated that the gallium doping efﬁciency of the APPJ was less efﬁcient than that of vacuum deposition techniques, such as magnetron sputtering. This may be due to the difference of working pressure between high vacuum and atmo-spheric pressure. The sample of 6 at.% has the highest Hall mobility and the lowest carrier concentration, which could reduce absorption of light in near infrared region, however, the larger deviation of its electrical properties indicated that it was non-uniform. Based on the electrical and optical results, the optimal gallium doping concentration was 8 at.%.

3.3. Effect of GZOﬁlm thickness

As mentioned above, silicon suboxide buffer layers were deposited at argon carrier gas flow rates of 30 sccm and the optimal Ga/ (Ga + Zn) ratio of precursor was 8 at.%. The thickness of GZO bilayers measured by SEM was 450 nm, 630 nm, and 775 nm, respectively, for deposition time of 16, 24, and 32 min.Fig. 5shows the XRD patterns of bilayer GZOfilms of varying thickness. The bilayer GZO films exhibit-ed two obvious (002) and (101) diffraction peaks. An increase infilm thickness led to an increase in intensity of (002) and (101) peaks, as shown inFig. 5. The full width at half maximum (FWHM) of (002) main peaks decreased from 0.262 to 0.241° as the thickness offilms in-creased from 450 nm to 775 nm. These results indicated a significant increase in the crystallinity of bilayer GZOfilms. Notably, the ionic radi-us of Ga is smaller than that of Zn; thradi-us, gallium incorporation yields a lower lattice constant[19]. A positive correlation was evident between the lattice constant and lattice plane d-spacing, and the Bragg angle was determined from d-spacing. The positive shift in 2theta values of (002) peaks for bilayer GZOfilms resulted from incremental increases in Ga doping concentration. The atomic percentage of gallium of bilayer GZOfilms measured by Auger electron spectroscopy was 2.85 at.%,

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2.96 at.%, and 3.25 at.%, respectively, for thickness of 450 nm, 630 nm, and 775 nm. As a result, an increase in crystallinity of GZO bilayers resulted in an increase in Ga at.%.

3.4. Comparison of GZO bilayers and Asahi U-type FTO

Asahi U-type FTO glass is produced by Asahi Glass Company (AGC), and it is used as the front electrode of a-Si solar cells. The process step of Asahi U-type is depositing FTO on glass substrates by using atmospheric pressure chemical vapor deposition (APCVD) and then proceeding wet etching. The biggest drawback of FTO thinﬁlm is not applicable to the process of hydrogen rich plasma, such as deposition of hydrogenated microcrystalline silicon (μc-Si:H) thin ﬁlms for a long time. Because tin

can be reduced by hydrogen plasma, which results in decreasing of transmittance of FTO. Consequently, it is necessary to deposit a ZnO or TiO2capping layer on FTO to protect against hydrogen plasma[20].

Con-versely, GZO material has high resistance to resist hydrogen plasma.

Fig. 6shows the SEM surface images of GZO/SiOxbilayers and Asahi

U-type FTO.Fig. 7shows a comparison for transmittance and haze be-tween Asahi U-type FTO_{films and bilayer GZO films with varying} thick-ness. The transmittance in the 750- to 1100-nm region was lower than 80% for all bilayer GZOfilms, as shown inFig. 7a; this was attributed to the high carrier concentration increasing the free carrier absorption. The 775-nm-thick bilayer GZO film achieved considerably higher haze values in the visible and NIR regions, compared with Asahi U-type FTO or other bilayer GZOfilms, as shown inFig. 7b.Table 3shows a compar-ison of Asahi U-type FTO and bilayer GZOfilms. The optimal bilayer GZO film was 775 nm thick and showed lower resistivity compared with Asahi U-type FTOfilm. The optimized bilayer GZO film was thinner

Table 1

Electrical properties of conventional GZO and bilayer GZO. Resistivity (Ω·cm) Hall mobility (cm2 V− 1 s−1) Carrier concentration (cm−3) Conventional GZO 6.19 × 10−4 10.6 9.74 × 1020 Bilayer GZO_30 sccm 6.00 × 10−4 15.8 6.58 × 1020 Bilayer GZO_60 sccm 7.18 × 10−4 15.2 5.72 × 1020

Fig. 4. Resistivity (ρ), Hall mobility (μ), and carrier concentration (n) of bilayer GZO ﬁlms deposited for various Ga/(Zn + Ga) atomic ratios.

Table 2

Optical (at 550 nm) properties of various Ga concentrations of bilayer GZOﬁlms.

6 at.% 8 at.% 10 at.%

Thickness (nm) 780 775 705

Transmittance (%) 80.90 82.20 80.35

Haze (%) 23.79 21.50 24.40

Fig. 3. (a) Transmittance and (b) haze spectra of conventional GZO and bilayer GZOﬁlms deposited on SiOxbuffer layers at various carrierﬂow rates. Conventional GZO and bilayer

GZOﬁlms were scanned 20 times by APPJ.

Fig. 5. XRD patterns of bilayer GZO of various thickness: (a) 450 nm; (b) 630 nm; and (c) 780 nm.

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than Asahi-U type FTO _{ﬁlm, but performed similarly. This result} suggested that APPJ deposition might entail relatively less material consumption than APCVD deposition.

4. Conclusion

In summary, the bilayer GZOﬁlm qualities of high haze and low sheet resistance is achieved by APPJ. The thickness and RMS roughness of SiOxbuffer layers increase as the argon carrier gasﬂow rates increase.

An increase in the roughness of SiOxbuffer layers is associated with a

significant increase in the haze values of bilayer GZO films. The mini-mum resistivity of 6.00 × 10−4was achieved at 8 at.% gallium doping, which had the highest carrier concentration. An increase in thickness offilm led to an increase in intensity of (002) main peaks, Hall mobility, and carrier concentration. The bilayer GZOfilm was thinner than Asahi-U type FTOfilm but achieved similar electrical and optical performance. Furthermore, the 775-nm-thick bilayer GZOfilm had a considerably higher haze value in the visible and NIR regions compared with Asahi U-type FTOfilm. These results suggest that APPJ is an effective method for preparing textured TCO.

Acknowledgment

The authors are grateful for research support provided by the Mechanical and Systems Research Laboratories, Industrial Technology Research Institute (ITRI).

References

[1] C.H. Huang, H.L. Cheng, W.E. Chang, M.S. Wong, J. Electrochem. Soc. 158 (2011) H510.

[2] B. Sang, K. Kushiya, D. Okumura, O. Yamase, Sol. Energy Mater. Sol. Cells 67 (2001) 237.

[3] E. Fortunato, L. Raniero, L. Silva, A. Gonc-alves, A. Pimentel, P. Barquinha, H. A´guas, L. Pereira, G. Goncalves, I. Ferreira, E. Elangovan, R. Martins, Sol. Energy Mater. Sol. Cells 92 (2008) 1605.

[4] J. Muller, B. Rech, J. Springer, M. Vanecek, Sol. Energy 77 (2004) 917.

[5] M. Kambe, M. Fukawa, N. Taneda, Y. Yoshikawa, K. Sato, K. Ohki, S. Hiza, A. Yamada, M. Konagai, Proceedings of 3rd World Conference on Photovoltaic Energy Conver-sion, vol. 2, IEEE, Osaka, Japan, 2003, p. 1812.

[6] O. Kluth, B. Rech, L. Houben, S. Wieder, G. Schope, C. Beneking, H. Wagner, A. Lofﬂ, H.W. Schock, Thin Solid Films 351 (1999) 247.

[7] J. Krc, M. Zeman, O. Kluth, F. Smole, M. Topic, Thin Solid Films 426 (2003) 296.

[8] K.M. Chang, S.H. Huang, C.J. Wu, W.L. Lin, W.C. Chen, C.W. Chi, J.W. Lin, C.C. Chang, Thin Solid Films 519 (2011) 5114.

[9] S.H. Yang, C.H. Liu, C.H. Su, H. Chen, Thin Solid Films 517 (2009) 5284.

[10] S.H. Yu, P.C. Ho, T.W. Yang, C.C. Bi, C.H. Yeh, C.Y. Chang, ECS Solid State Lett. 1 (2012) 48.

[11] B.D. Ahn, Y.G. Ko, S.H. Oh, J.H. Song, H.J. Kim, Thin Solid Films 517 (2009) 6414.

[12] H. Makino, Y. Sato, N. Yamamoto, T. Yamamoto, Thin Solid Films 520 (2011) 1407.

[13] D.W. Kang, S.H. Kuk, K.S. Ji, S.W. Ahn, M.K. Han, Jpn. J. Appl. Phys. 49 (2010) 031101.

[14] F. Wu, L. Fang, Y.J. Pan, K. Zhou, Q.L. Huang, C.Y. Kong, Phys. E. 43 (2010) 228.

[15] T. Yamada, H. Makino, N. Yamamoto, T. Yamamoto, J. Appl. Phys. 107 (2010) 123534.

[16] L. Gong, J. Lu, Z. Ye, Sol. Energy Mater. Sol. Cells 94 (2010) 1282.

[17] D.H. Kim, S.H. Lee, G.H. Lee, H.B. Kim, K.H. Kim, Y.G. Lee, T.H. Yu, J. Appl. Phys. 108 (2010) 023520.

[18] H. Akazawa, Thin Solid Films 520 (2011) 2418.

[19] K.T. Ramakrishna Reddy, H. Gopalaswamy, P.J. Reddy, R.W. Miles, J. Cryst. Growth 210 (2000) 516.

[20] J. Poortmans, V. Arkhipov, Thinﬁlm solar cells fabrication, Characterization and Applications, Wiley, 2007. 209.

Fig. 6. SEM surface images of (a) GZO/SiOxbilayers and (b) Asahi U-type FTO.

Fig. 7. (a) Transmittance and (b) haze of Asahi U-type FTOﬁlm and bilayer GZO ﬁlms of various thickness: 450 nm, 630 nm and 780 nm.

Table 3

Comparison of bilayer GZOﬁlms and Asahi U-type FTO.

Bilayer GZO Asahi U-type Structure GZO/SiOx/glass Textured FTO/glass

Technique APPECVD APCVD + etching

Substrate temperature (°C) 140 450 Thickness (nm) 775 900 RMS (nm) 40.73 33.10 Transmittance (%) at 550 nm 82.2 83.0 Haze (%) at 550 nm 21.50 13.50 Resistivity (Ω·cm) 6.00 × 10−4 _{7.38 × 10}−4 Hall mobility (cm2 V−1s−1) 15.8 46.2 Carrier concentration (cm−3) 6.58 × 1020 1.83 × 1020