Compositional analysis, XPS depth profile

Fig.3a shows XPS depth profile of a ZnO/ZnS/Silica bilayer, where the ZnS thin film has been obtained from the zinc acetate dehydrated and thiourea initial solution. In this figure is being represented the atomic concentration as a function of the sputtering time. It should be noted that the oxygen content in the bilayer decreases after 8 minutes of sputter, when the approximate depth is 24 nm (4 KeV Ar+) because it starts to appear sulphur, O1s and S2p peak are located at 530.04 eV and 161.0 eV respectively. The concentration of both elements change slowly inside the films and it is not until the 21 (1.5 KeV Ar+) minutes of bombardment and a estimated depth of 35.5 nm, when oxygen and sulphur present similar percentages, 24.35% oxygen located at 530.25 eV and 23.23% sulphur located at 162.06 eV. After this sputter time both elements follow the same trend, O1s profile decreases and S2p profile increases. This result may suggest the diffusion of ZnO in the ZnS film during the deposition of the outer layer.

A similar plot was obtained for a ZnO/ZnS/Silica bilayer (Fig 3b), where in this case the ZnS thin film has been prepared from zinc chloride and thiourea solution. The ZnO thin film is overcome after 6 minutes of Ar+ (4 KeV) sputtering, when 18.0 nm is the approximate depth, because 7.85% of sulphur is detected at 162.47 eV, O1s peak is located at 530.41 eV in 39.81%, the Zn2p peak has been used as a reference located at

Approximate depth (nm)

15.0 30.0 32.5 35.0 41.5 47.5

Approximate depth (nm)

15.0 20.0 22.5 25.0 27.5 42.5

1022 eV. The depth profile follows showing an oxygen decrease and a sulphur increase. After 16 minutes of sputtering (1.5 KeV Ar+) when the it has been sputtered 23 nm, it is found a similar concentration of ZnO and ZnS because there are a 23.69% oxygen detected at a binding energy of 529.91 eV and a 24.66% sulphur at the energy of 162.27 eV, 12.5 nm before than in the sample represented in Fig.3a, where it is also represented the approximate depth of the sputtering time, it has been stimated using the rates showed in the experimental method. These fact shows that the ZnS thin film deposited using zinc chloride and thiourea initial solution has a smoother surface, it is more dense and thicker.

Figure 3a.- XPS depth profile of a ZnO/ZnS/Silica bilayer where the ZnS thin film has been obtained using zinc acetate dehydrated and thiourea. Figure 3b.- XPS depth profile of a ZnO/ZnS/Silica bilayer where the ZnS thin film has been obtained using zinc chloride and thiourea.

Conclusions:

The ZnO and ZnS thin films and the bilayers ZnO/ZnS obtained by CSP present similar optical, structural and morphological properties to the films obtained from high vacuum methods. In general, the films present a smooth and homogenous surface without hollow or crack, and they show a good transmittance (80%-85%). However, for ZnS films, the initial precursor, ZnCl2 or acetate Zn, have influence in physical properties (optical, structural and morphological) of the films and, in general, the ZnS films obtained from ZnCl2 present better optical properties than the films obtained from acetate.

Neither carbon nor others by-products which could change the refractive index of bilayer have been found in any interface ZnO/ZnS, independent of the Zn precursor; however, there are some differences between the bilayer with the ZnS film obtained from ZnCl2 or Zn acetate. The bilayer with the ZnS film obtained from ZnCl2 show a depth

profile with a stronger decrease of oxygen and an increase of sulphur than the other interface; moreover, the sulphur appear at lower depth and the oxygen and sulphur concentrations become equals at lower depth. We can put these difference to the films obtained with ZnCl2 are smoother, denser and thicker than the other ZnS films.

Acknowledgements:

The authors are grateful to the European Union and CICYT (Spain) (grant MAT2000- 1505-CO2-O2) and the Junta de Andalucia trough the research group FQM-192. The bursary holders, M. C. Lopez wish to thank to the MEC (Spain) and the CSIC (Spain).

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