Spin coated (Mx, Zn1-x O), M= Sn, Al nanofibers investigation

(Mx, Zn 1-x O), M= Sn, Al nanofibers properties were investigated. Pure and M-doped zinc oxide films were fabricated onto a glass substrate by a facile and low cost spin coating route. X-rays analysis reveals that films crystallize with a wurtzite structure according to (002) orientation. The transmittance in the visible range was as high as 88 % at 550 nm. The doping increased slightly the transmittance, the as-grown films were high transparent in VIS and IR ranges. The optical band gap was a little bit changed by doping. AFM 3D-views revealed that grains were nanofibers with the size of 18.22, 18.23 and 44.27 nm respectively for pure, Al and Sn-doped ZnO films.


Introduction
ZnO is among a huge variety of transparent conducting oxides (TCO) which attract many researchers, due to the interesting properties such as a transparency in visible spectrum.ZnO is a wide band gap n-type semiconductor, E g =3. 3 eV at room temperature, with hexagonal wurtzite structure, and a large exciton binding energy of 60 meV [1][2][3].In order to improve the physical properties of ZnO, many elements such as aluminum (Al), tin (Sn), indium (In) and fluorine (F) and many others have been used as dopants [1][2][3][4][5].The synthesis of nanostructures has been of growing interest owing to their promising application in nanoscale optoelectronic devices, like the next generation solar cells [6][7].The morphology of ZnO has been demonstrated to be the richest among inorganic semiconductors, and the electrical and optical properties depend considerably on both the morphology and size.In the present work we have studied the effect of cationic substitution of Zn sites by aluminum (Al) and tin (Sn), based on a purely valence considerations.Further, we report the crystalline structure, surface morphology, optical and photoluminescence properties of pure, Al and Sn-doped ZnO films grown by spin coating route and the nanofibers fabrication.

Experimental procedures
The films of pure, aluminum and tin doped ZnO were deposited using a sol gel spin coating process onto a glass substrate as shown in figures 1 and 2. 0.5 Molar of dihydrated zinc acetate is dissolved in an appropriate amount of 10 ml of 2-methoxyethanol.Thorough mixing is done using a magnetic stirrer for 10 min at 60°C, and the dopants aluminum nitrate nona-hydrate, Tin (II) chloride dihydrate were added, with some addition of mono-ethanolamine (MEA) as stabilizer, until the homogeneous and clear solution is obtained, then the stirring continued for 1 hour, and followed by an ageing process of 24 h.The doping ratio Al/Zn, Sn/Zn is fixed at 2 and 3% in the solution.The samples were placed onto a spin coater and rotated for one minute at chuck rotation rate of 1000 rpm (rotate per minute).After this, the sample is heated at 150 °C for 10 min.The process is repeating 5       3.3.The optical band gap E g (eV) is systematically deduced from the following expression,

The tran
Where α is the absorption coefficient, h is Planck's constant, ν is the photon frequency, E g is the band gap energy.The direct energy gap of the coated films has been calculated by extrapolating the linear part of (αhν)² plot as a function of the photon energy to the energy axis as shown in figure 6.As can be observed from the recorded data in table 1 ZnO film reveals a wide band gap (E g ) of 3.74 eV, while E g decreases slightly with doping level.This result agrees with transmittance spectra.

03003-p.6
1st International Conference on Numerical Physics Fig. 6.Plot of (αhν)² against photon energy of pure and M-doped ZnO films, E g is determined by extrapolation.

Photoluminescence investigation
The photoluminescence (PL) of as-deposited films at room temperature is sketched in figure 7. Doping affects the photoluminescence intensity as seen in figure 7. Except weak UV emission for the Sn-doped ZnO no emission, originating from the near-band edge, was detected.While some PL emissions are revealed in visible band, among them we can observe the strong and narrow violet and yellow emissions (442 and 588 nm) respectively.Probably, both the preferential orientation and abundant grain boundaries in the as-grown ZnO film might cause the presence of the violet emission at 442 nm.The similar violet peaks have been detected by Liang et al. [13].In addition, the cause of 03003-p.7 EPJ Web of Conferences yellow emission in the range 570-610 nm in ZnO was due to extrinsic impurity or defects, some authors found the same behavior for Si-doped ZnO films [14].Furthermore, lesser emission peaks are respectively blue emission (501 and 508 nm) and green emission (533 and 538 nm).The green peaks ~ 538 nm presence is due to the concentration of defects responsible of deep level in ZnO asgrown films as reported in literature [13].The calculated energy (eV) of these PL peaks (A, B, C, D, E and F), by using 1240/λ (nm), is 2.81, 2.47, 2.44, 2.32, 2.30 and 2.1 eV respectively.No significant peak shift is observed but an important decrease in PL emission intensity, caused by doping level, is revealed particularly within the range 450-525 nm as indicated in fig 7a and 7b.It is noted that PL emissions reveal crystalline structure and presence of defects and anion vacancies in the lattice.However, blue emission correspond to surface defects in samples, green peaks might ascribe to oxygen vacancies.

Conclusion
In summary, nanofibers of pure and M-doped ZnO films are successfully synthesized by spin coating route @ 1000 rpm.A (002)-oriented hexagonal crystal structure was confirmed by X-ray patterns.Additionaly, X-ray pattern reveals that the nanofibers are preferentially growing along the c-axis orientation, which is in agreement with AFM views.ZnO nanofibers are wide band gaps > 3eV.A PL investigation reveals a strong VIS emission and a weak UV emission.By PL spectra, nanofibers present a strong violet (~440nm) and yellow (~588 nm) emissions, defects and vacancies presence is indicated.Owing these promising qualities, ZnO films are considered as the ideal materials for the 03003-p.8 1st International Conference on Numerical Physics microelectronic devices fabrication, such as the short-wavelength optoelectronic devices like UV and visible lasers, violet and yellow light-emitting diodes and photoconductive detectors.

Fig. 5 .
Fig. 5. Transmittance dependence on photon wavelength of pure and M-doped ZnO films (up), UV,VIS and IR bands are sketched (down).

Fig. 7 .
Fig.7.Photoluminescence plot of pure and M-doped ZnO films.Gaussian deconvolution is shown (orange curves) ( up).A plot of PL in restricted range of wavelength of 450-525 nm is sketched figures a and b (down).

Table 1 .
Optical band gap, high transmittance in visible range, RMS and grain size.