Femtosecond laser-induced modifications in optical Properties and surface morphology of Copper Boron Oxide Thin Films
Abstract
We report on femtosecond (130 femtoseconds)
laser pulse removal and modification in Copper Boron Oxide thin films (CuBO2)
deposited on a silicon substrate. The samples were irradiated with 100 to 500
number of laser pulses at room temperature. Micro-size constructions on the
surface of exposed CuBO2 thin films were produced and were observed
using Scanning Electron Microscope (SEM). Self-prepared shapes like wrinkles,
cellular erections, groups of spots, and cracks are observed by SEM studies. The
small particles produced at initial laser pulses agglomerate into microspheres
of Copper Boron Oxide. The optical properties show the high transmission value
in (CuBO2) thin films when the number of laser pulses is increased.
The optical band gap energy also decreases as compared to as-deposited film up
to 300 pulses and then increases up to 500 pulses. The
results show that femtosecond laser irradiation considerably modifies the
optical properties and surface morphology of (CuBO2) thin films.
Keywords: Femtosecond laser interaction; Thin
films, Optical properties, Surface morphology.
1
Introduction
Femtosecond laser substantial treating is an
imperative means due to the least thermal effects. By exposing objectives to ultrashort pulses, several structures like wrinkles small hills,
shunts, cellular constructions, groups of particles, and holes be able to be shaped. Laser influences and conservational atmospheres play a vibrant part in the enlargement of these constructions.
The creation of these assemblies is logical on the basis of both thermal
and non-thermal devices counting plasma as well as parametric
variabilities, optical and interference effects, and external Plasmon’s. The
development of unlike surface erections on metals can enhance their numerous
properties e.g., optical absorption thermionic properties, photoelectron
emission, and super-hydrophobicity. The metals with enhanced properties are
useful for countless engineering, corporal, organic and compound submissions. Arrangement
of billows on Al, Si, CaF2, and CR-39 after light with Ti: Sapphire laser has
been accounted for. For all materials, surges with numerous periodicity are
inspected. While on the outside thin-film surges just as miniaturized scale
surges having a periodicity of about 1– 2 µm have additionally been exposed. The
fluency with respect to smaller-scale ripple development is higher than for
bigger ripples arrangement.
Major downsides of electron-beam and
photograph lithography are moderate procedure speed and high preparation expenses
because of the working circumstances and desires. Besides, the defilement of
the thin film with synthetic compounds is a preventive issue. When utilizing
pre-designed impulses, complex substrate readiness just as high temperatures
are required. Ultra-short laser pulse material removal is
an exact subtractive designing strategy with a few points of interest
contrasted with control pulses in the nano-second routine and nonstop wave
lasers. just as other subtractive designing methodologies. Laser hits in the
sub-picosecond routine demonstrate a little warmth-influenced zone because of
their ultra-short association with the subject. Consequently, exact material
removal without needless warming of the encompassing material can be performed.
Also, it is conceivable to do non-warm removal utilizing ultra-short laser
strokes. Besides, no specimen formation is required in laser communication
trials and laser removal can be connected utilizing encompassing conditions and
for materials that are delicate to high-temperature treatment. In the present
work, we first time account for the communication of femtosecond laser association
with copper boron oxide thin films.
2
Experimental details
Experimental work is involved two principle steps. The first is CuBO2 thin film deposition and the second step is the irradiation with a femtosecond laser. To grow the phase of copper boron oxide, the copper acetate and boric acid (in powder form) were taken in stoichiometric amounts according to the following chemical equation and separate solutions of both were made in deionized water.
The two
solutions were initially blended independently and then mixed for an attractive
stirrer for 20 minutes at 200 rpm to get the solution of copper boron oxide.
The solution was very agitated so citric acid is added to chill ate the
ions. With the addition of citric acid, the blue solution turned into greenish-blue which was again magnetically stirred for 20 minutes at 200 rpm. To
increase its viscosity, ethylene glycol was added. This solution was then used
in the dip coater to deposit the film. The film was deposited using the withdrawal
speed of 120 mm/min after submerging the ultrasonically cleaned n-type silicon
substrate (size: 2.5cm×2.5cm×0.4mm) into the
solution. The substrate was dipped five times in the solution to deposit the
thin film. In this way, a total of five films were deposited. Then Post deposition
annealing of all five samples was accomplished at 750 °C for one hour in an electrical furnace to get phase pure CuBO2 thin film.
In the second step of the experiment, a Ti-sapphire laser was used to deliver the tripled
harmonic (200 nm) pulses of 130-fs duration. The repetition rate was fixed at 1
kHz and the energy per pulse of 0.3 μJ was adjusted. The beam was focused by a 0.25-NA objective lens.
Single pulses were selected by an electromechanical shutter and the sample was
x-y translated between each pulse to illuminate fresh material. The laser
was irradiated to 11×11 (total = 121) number of circular spots in the area of
0.5 mm×0.5 mm on the outward of CuBO2 thin film where 100 pulses were used to irradiate each circular spot. In this way, the other
four samples were irradiated with 200, 300, 400, and 500 laser pulses.
The center-to-center distance of each circular spot was selected in such a way
that a maximum area of 0.5 mm *0.5 mm became irradiated.
3
Results
and Discussion
The results have been analyzed by using a UV-Visible spectrophotometer
and Scanning Electron Microscopy (SEM). The details are given in the following:
3.1
Optical Properties
The optical properties of as-deposited and femtosecond laser irradiated copper boron oxide thin films with a different number of laser pulses are studied by using a UV-Visible spectrophotometer. The optical transmission spectra of pristine and femtosecond laser irradiated Copper boron oxide thin films in the range of 300 nm to 800 nm are shown in fig 1. A black line near the zero percent transmission indicates baseline. It can be seen that the percentage transmission decreases constantly for all films when the wavelength decreases from 800 nm to 300 nm. The absorption edge is observed near 350 nm. The percentage transmission becomes zero below 300 nm for all the thin film samples. It can also be seen that the percentage transmission increases as the number of pulses increase from 100 to 500. From the figure, the absorption coefficient of all films was calculated.
(αhν) n = A (hν – Eg)
Where α is the absorption coefficient, A is the absorption
and t is the thickness of the material. Using α, the (αhν)2 versus photon energy (E = hν) curves (known as
Tauc’s curves) are plotted using the Tauc’s relation as shown in
fig 2 for as-deposited and femtosecond laser irradiated thin films
Where α is the absorption coefficient, hν is photon
energy, A is the scaling factor and Eg is
the optical band gap energy and exponent n corresponds to the type of
transition. exponent n indicates the type of transitions. Thus n = 0.5 for
allowed direct transitions, n = 2 for allowed indirect transitions, n = 1.5 for
forbidden direct transitions and n = 3 for forbidden indirect transitions.
Considering allowed direct transitions (n = 0.5), optical band gap energy is
estimated through Tauc's curves [(αhν)2 versus hν] by considering
two portions (fig.2). The first portion is taken by considering the linear
portion of the curve (red line) and is denoted by Eg1. The second
portion of the curve is taken by considering the portion of the curve where the
transition from curve to linear portion occurs (green line) and is denoted by Eg2.
Eg1 and Eg2 are plotted as a function of a number of laser
pulses in fig 3. It can be seen that the optical band gap energy (Eg1)
increases from 3.68 eV to 3.72 eV by increasing the number of laser pulses from
0 to 200. Optical band gap energy decreases to 3.61 eV at 300 laser
pulses and then increases to 3.68 eV when the number of laser pulses is increased
to 500. If the trend of optical band gap energy (Eg 2) is
considered, then it is evidenced that the optical band gap energy shows almost
similar behavior as that of curve Eg1 except for the last point. The
increased optical band gap energy from 0 to 200 laser pulses is due
to the energy levels created within the conduction band and the decrease in Optical
band gap energy after 200 pulses is due to the formation of energy levels
between the valence band and conduction band.
3.2
Scanning Electron
Microscopy
The
surface morphology of laser irradiated CuBO2 thin film is studied by
scanning electron microscope. Fig 4 shows the scanning electron micrographs of
laser irradiated spots on the surface of CuBO2 thin film with
changing the number of laser pulses from 100 to 500. The smaller micrographs on
the upper corner of each bigger micrograph are taken at smaller magnification
to observe the laser-irradiated circular spots for each sample. Fig 4 (a) shows
copper boron oxide thin film irradiated by 100 number of laser pulses. The
micrograph at 1000 magnification shows that laser irradiation changes the
surface morphology of thin films. The laser-irradiated circular spots with
surface corrugation are clearly visible along with very small holes of diameter
(60 μm). One of the circular spots is magnified and shown as a bigger micrograph
in fig 4 (a). which shows the formation of very small particles upon laser
irradiation. Fig 4 (b) shows the laser-irradiated spots on the surface of
copper boron oxide thin films irradiated at 200 laser pulses. The micrographs
at smaller magnification show the laser-irradiated circular spots when the
surface corrugations are more than the sample with 100 pulses. It can also be
seen that the holes are formed and bigger in size (63 μm) as compared to the
sample in fig 4(a). The micrographs at higher magnification in fig 4 (b)
confirm not only the production of new surface corrugation but the size of
particles also increased.
Fig 4 (c) shows the laser-irradiated spots
on the surface of copper boron oxide thin films irradiated at 300 number of
laser pulses. The micrographs at smaller magnification show the laser-irradiated circular spots when the surface corrugations are more than the
sample with 100 and 200 pulses. It can also be seen that the holes are formed
and bigger in size (69 μm) as compared to the sample in fig4 (a and b). In fig4
(d and e) the hole size increases to (75 μm) by irradiation of 400 pulses with
thin film and after 500 laser pulses hole size increases to (79 μm).
Similarly, when the number of laser pulses is increased then roughness, hole
size, and particle size are also increased. One interesting phenomenon can also
be seen which is the agglomeration of particles into a microsphere upon growing
the number of laser pulses.
The factor behind the development of surges is the excitation of Plasmons that produce occasional
enlargement in nearby fields of surface layers. At the point when the fs laser
beam in ultrashort time scale is made to fall onto the target, abundant
electrons become energized resulting in the formation of plasma. This will cause
the modification in surface physical and optical properties. The change in
optical properties will alter the electromagnetic force distribution and will
influence the laser-matter cooperation extremely. This force circulation will
impact restricted solid fields causing the phenomenon of Coulomb's Explosion
(CE) at the nanoscale, which is the essential purpose for the phase of nanostructures.
The holes and bulges are grown
arbitrarily over the laser removed area after laser light due to in-homogenous
vitality demonstration, started by the improved surface harshness. The elementary pulses extant surface
harshness and improve it. While the rest of the strokes cause the organizing
and variety in the surface organizations that are recognized with the
in-homogenous energy deposition. The surface unpleasantness, nearness of
contaminations, voids, in-homogeneities, and non-consistencies on the outside of
thin films are likewise in control of non-uniform laser vitality ingestion.
This variety in energy can be described based on spatial inhomogeneities,
surface imperfections, and laser properties. The incidence laser can be in part
dispersed from the laser-produced surface deformities in the tangential wave
that can go through the material surface. Along these lines, the arrangement of
billows can be clarified based on impedance between dispersed tangential waves
and incident laser light.
The hole size and density, particle size, and
sphere size as a function of a number of laser pulses are shown in fig 5 (a, b).
The hole size increases continually from 60 μm to 79 μm and hole density
decreases from 63679 holes/m3 to 39876 holes/m3 with the
growth in the number of laser pulses from 100 to 500 as shown in fig 5 (a). The
particle size increases from 0.8 μm to 3.6 μm with the increase in the number of
pulses from 100 to 500. Similarly, sphere size also increases from 0.27 μm to
24.43 μm as the number of laser pulses rises from 100 to 500 as shown in fig 5
(b).
4 Conclusions
Multi-pulse
femtosecond laser interaction with CuBO2 thin-film brings a remarkable change in the hole size and density, particle size, and sphere
size. By growing the amount of
femtosecond laser pulses, there is a rise in optical transmission. Optical band
gap energy also varies with respect to a different amount of laser pulses. As the
quantity of laser pulses rises both particle size and sphere size increase. So
it can be concluded that femtosecond
laser interaction with copper boron Oxide noticeably modifies the surface and
optical properties of CuBO2 thin films.
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