ZnxCd1-xS thin films were characterized to obtain high quality films deposited by RF magnetron sputtering system. This is the first time report of the influence of substrate temperature of ZnxCd1-xS thin films prepared by co-sputtering of CdS and ZnS. Investigations show that higher substrate temperatures result in films of better crystallinity, higher optical transmission, larger grains and lower sheet resistances. On the other hand, energy band gap decreases slightly with temperature which might have been due to the improved crystalline nature of the material.

Introduction

Md. Sharafat Hossain1, Md. Aminul Islam2, T. Razykov2, K. Sopian2 and N. Amin1,2 1 Faculty of Engineering and Built Environment, The National University of Malaysia, 43600 Bangi, MALAYSIA.

2Solar Energy Research Institute (SERI) Universiti Kebangsaan Malaysia, 43600 Bangi, MALAYSIA*Corresponding email: nowshad@ukm.edu.my

Effect of Substrate Temperature on ZnxCd1-xS Thin Film Growth by RF Magnetron Sputtering

ZnxCd1-xS thin films at low zinc content for a possible usage as window layer in CdTe thin film solar cells were deposited on bare soda lime glass substrates by RF magnetron co-sputtering of CdS and ZnS at different substrate temperatures. The substrate temperature was varied against several parameters, until best film characteristics were obtained. The composition of the thin films was controlled by varying the RF power of CdS and ZnS in such a ratio so that the zinc content in the thin films remained low. The crystallinity of the films increased with increasing deposition temperature. The optical absorption and transmission spectra were recorded within the range 300-900 nm. The band gap of thin films showed slight decreasing trend for higher deposition temperature. From SEM, it was found that the thin films were formed by different clusters of grains which were changed to isolated grains as the deposition temperature increased. Moreover, electrical sheet resistance of the thin films was also investigated as a function of deposition temperature. This work confirms that deposition temperature has significant influence on the Zn xCd1-

xS thin film properties grown by RF magnetron co-sputtering, which can be optimized for window layer to be suitable for CdTe thin film solar cells.

1.The RF power of CdS and ZnS was varied in such a ratio so that ZnxCd1-xS was deposited in low Zn concentration (≤0.2) and this concentration was achieved by varying the RF powers of CdS and ZnS. The desired composition (x=0.17) was found with RF power 40 Watt for CdS and 8 Watt for ZnS.2. The ZnxCd1-xS thin films deposited at 100 0C showed four diffraction peaks of (002), (102), (110) and (103) belonging to strong wurtzite structure. The peak intensities of (002) diffraction line were increased with the increasing of deposition temperature which indicated that the thin films deposited at low temperature possessed poor crystallinity compared to the films deposited at high temperature. 3. The band gap of thin films showed slight decreasing trend for higher substrate temperature. 4. The transmittance of the films significantly increased in longer wavelength regions when the films were deposited at high temperature. 5. SEM analysis showed that that the thin films were formed by different clusters of grains which were changed to isolated grains as the substrate temperature increased. 6. The value of sheet resistance decreased from 2.4×105 to 1.4×102 Ω/□ with deposition temperature variation from 100 0C to 400 0C.

Major Findings and Discussions

Conclusion

Bare soda lime glass (SLG) substrates were cleaned in an ultrasonic bath using acetone-ethanol-DI water solvents, sequentially for five minutes each followed by drying in a stream of dry nitrogen gas. After evacuation of the magnetron sputtering chamber to around 10 -5 Torr, CdS and ZnS targets (99.999% purity) were then co-sputtered on these cleaned bare glasses by varying the substrate temperatures from 100 0C to 400 0C by non-reactive RF magnetron sputtering in an argon gas ambient. The Kurt J. Lesker 4-gun model sputtering machine was used for sputtering of all the samples. Before starting deposition, the sputtering chamber was flushed by evacuation and purged with pure nitrogen gas for 3 times. Likewise, the targets (CdS and ZnS) was covered by the shutter and pre-sputtered for between 10-15 minutes in order to remove any oxide on the target surfaces. The RF power of CdS and ZnS was varied in such a ratio so that ZnxCd1-xS was deposited in the desired composition (x=0.17) in working pressure of 16 mTorr.

Experimental Procedures

EUPVSEC-2015, 14th to 18th September 2015 at Hamburg, Germany.

As-deposited

3DV.3.13

EDX spectrum of ZnxCd1-xS thin films SEM image of ZnxCd1-xS thin films XRD spectra of ZnxCd1-xS thin films

Transmittance and bandgap of ZnxCd1-xS thin films for different substrate temperature