Effect of copper on composition, structural and optical properties of ...

where T0 is the transmittance and t the thickness <strong>of</strong> the film.
( h) 2 (eV/cm) 2
( h) 2 (eV/cm) 2
( h) 2 (eV/cm) 2
1.0x10 16
5.0x10 15
t = 6 5 0 Å
0.0
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8
4x10 15
3x10 15
2x10 15
1x10 15
5x10 14
4x10 14
3x10 14
2x10 14
1x10 14
(a)
(b)
h ( e V )
t = 1 0 0 0 Å
0
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8
(c)
h ( e V )
t = 3 3 0 0 Å
0
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8
h ( e V )
Fig. 4. The dependence <strong>of</strong> (αhυ) 2 vs hυ for <strong>copper</strong> doped ZnTe films <strong>of</strong>
thickness (a) 650 Å, (b) 1000 Å and c) 3300 Å.
Fig. 3 shows the variation <strong>of</strong> extinction coefficient
with wavelength for <strong>copper</strong> doped ZnTe thin films. A
notable increase in extinction coefficient is observed near
the fundamental absorption edge in the <strong>copper</strong> doped films
whereas it decreases in pure films. This may be due to the
presence <strong>of</strong> <strong>copper</strong> in those films. It is also noted that the
extinction coefficient <strong>of</strong> the film decreases with decrease in
R. Amutha
<strong>copper</strong> composition. The electronic transition between
valence and the conduction bands, is given by [35]-
P
αhυ = A hυ-E g
where the magnitude <strong>of</strong> the exponent ‘p’ characterizes the
type <strong>of</strong> transition and takes the values 1/2, 3/2, 2 and 3 for
direct allowed, direct forbidden, indirect allowed and
indirect forbidden transitions respectively. In the above
equation ‘A’ is a constant, ‘Eg’ the optical band gap and
‘ hυ ’ the energy <strong>of</strong> photon.
Fig. 4 shows (αhυ) 2 against the photon energy (hυ) for
<strong>copper</strong> doped ZnTe films, indicating the possible optical
transition is <strong>of</strong> direct-allowed type and the band gap
energies are given in Table 2.
The direct band gap values these <strong>copper</strong> doped films
decreased appreciably when compared to pure ZnTe films,
which may be attributed to the segregation <strong>of</strong> impurities at
the grain boundaries. Similar observation has been made by
Pal et al for In doped ZnTe films [36]. The appreciable
decrease in the band gap <strong>of</strong> <strong>copper</strong> doped films may also be
caused by the presence <strong>of</strong> internal electric fields associated
with the defects and by the changes in composition <strong>of</strong> the
films.
Conclusion
Copper doped ZnTe thin films were deposited onto wellcleaned
glass substrates by thermal evaporation. The
decrease <strong>of</strong> atomic percentage value <strong>of</strong> <strong>copper</strong> with
increase <strong>of</strong> the ZnTe film thickness is confirmed by EDX
analysis. The X-ray diffraction analysis indicates that the
films undergo a phase change from hexagonal to cubic
structure. Copper doped ZnTe films have very low
transmittance when compared to pure ZnTe films. A sharp
increase in extinction coefficient is observed near the
fundamental absorption edge. The optical transition <strong>of</strong>
these films is found to be direct allowed.
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