PbS PbS is an important binary IV-VI semiconductor


PbS thin ?lms were grown on
glass substrates by chemical bath deposition (CBD) using lead nitrate, thiourea
and sodium hydroxide in aqueous solutions for different deposition time (30-150
min). The microstructure and morphology evolution of the ?lms were investigated
using X-ray diffraction, scanning electron microscopy and atomic force
microscopy. As the deposition time increases, besides increasing the thickness
of the film from 100 to 600 nm, the ?shape of ?the ?particles changes
from round (with typical
size of 100 nm) to cubic (with
typical size of 500 nm).? Also simultaneously with complete
shift of particle shape, the roughness ?value of the
film increases sharply. The results indicate that deposition time is an
important parameter in determining the dominant mechanism of deposition and
consequently the characteristics of the film. The active deposition mechanism
changed from cluster to ion-by-ion mechanism during deposition reaction, and
consequently, ?lm properties such as shape, size, roughness and preferred
orientation changed completely.

PbS is an important binary IV-VI semiconductor material
with a rather small band gap (0.41 eV at 300K) and relatively
large excitation Bohr radius (18-20 nm) 1, which results in good quantum confinement of both holes and electrons in nanosized
structures 2. These inherent properties
make PbS one of the most important functional materials used in as thin films
for several applications such as IR detectors 3, photovoltaic cells 4, thin films transistors 5, LED 6, gas and biosensors 7-12 and photonic crystals 13.

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In recent years, various techniques have been used to
deposit PbS thin films including microwave
assisted chemical bath deposition 2, successive ionic layer
adsorption and reaction (SILAR) technique 14-17, atomic layer epitaxial process 18, pulse electro deposition 19, spray pyrolysis 20 and chemical bath deposition.

Chemical
bath deposition? ?method,
also called chemical solution deposition ?technique, has become an attractive method due to many
reasons, including ?low cost, no requirement of
sophisticated instruments, freedom? ?to
deposit ?materials on a variety of
substances, suitability for large scale deposition? ?areas, and ability of tuning thin film
properties? ?by
adjusting and controlling ?the
deposition experimental? ?parameters.
21 ?

It
was realized that changing CBD parameters such as temperature, ?deposition time  and solution composition leads to
nanoparticles with ?different sizes and shapes
22, which change the value of the band gap with ?respect to the effective mass model.?

CBD
process uses a controlled chemical reaction to achieve thin film ?deposition by precipitation. It is
necessary to eliminate spontaneous ?precipitation in order to form a thin film 23. Chemical
deposition of films ?on solid substrates can take
place via two major mechanisms.?

The
first mechanism is ion-by-ion mechanism, in which the film is formed ?by sequential ionic reactions. If the
reaction progresses in alkaline medium, ?a complex agent is required to prevent the formation of
hydroxide ?precipitates 24.?? ?If the complex concentration is
not adequate to completely prevent ?formation of metal hydroxides, cluster mechanism occurs. In this
case, a ?small amount of colloidal
hydroxide will be formed, which then reacts with ?anions generated in the bath and produces the final
product. On the other ?hand,
research has shown that the dominant mechanism of deposition is ?dependent on the reaction conditions and
changing the dominant ?mechanism
during deposition is possible 24.?

Each
mechanism ?leads to different particle
size and morphology, which affects films ?properties. Therefore, for synthesis of a film with
specific properties, we ?should
be able to predict the effect of deposition parameters on reaction ?mechanism. We have previously shown that
deposition temperature is ?effective
in determining dominant deposition mechanism 26.?

 

Materials and method

PbS thin films were deposited on clean,
spectroscopic glass substrates at ?different deposition time
(30 to 150 min). All the reagents were purchased ?from Merck Chemical Co. and
were used without further purification. ?

According to previous studies, the aging of
precursor solutions will affect ?deposition rate 27??.?? Therefore, fresh precursor
solutions were utilized to ?remove probable noise caused
by aging. Prior to deposition, substrates were ?cleaned with the cleaning
procedure of Obeid et al. In brief, substrates were ?washed with hot distilled
water, immersed in 20% HCl for 24 hr, and washed ?with acetone. Then the substrates
were cleaned ultrasonically with DI water for ??20 min 2. ?

To prepare the reactive solution, 40 mL of
0.146 M NaOH and 100 mL of DI ?water were mixed. After drop
wise addition of 8 mL of 0.175 M lead nitrate to ?the stirring mixture, pure
N2 was passed through the reaction solution for 1 hr ?in order to
diminish levels of dissolved O2 and CO2. Then 8 mL of 1 M ?thiourea solution
was then added to reaction mixture. Finally, the clean glass ?substrates were
placed in the solution at 70ºC with respect to the horizon using ?the Plaxi holder
to prevent large particles from adhering to the growing film. ?The samples were
taken out after deposition time (30, 60, 90,120 and 150 min) ?rinsed with DI
water and then air dried. The grayish obtained films were well ?adherent to the
substrate and homogenous. The reactions process for synthesis ?of lead sulfide
films through ion by ion and cluster mechanisms have been ?previously
reported28, 29.?

Structural characterizations of the films
were determined by X-ray diffraction ?method using a Philips
PW3710 at room temperature with Cu K? radiation (? ??= 1.5405 ?A,
Time/step=0.5S, Step Size=0.02). In order to determine crystallites ?size from XRD,
the Scherrer formula was used. Field emission gun scanning ?electron
microscopy (FE-SEM) studies were carried out using a HITACHI S-??4160 microscope,
in order to determine the morphology of the films. Film ?thickness was
measured from cross sections while surface topography was ?observed in plans
view. The surface morphology of the thin films was ?characterized with an Auto
probe CP (Park Scientific Instruments) scanning ?electron microscope. AFM
imaging was performed under ambient conditions ?using commercial Si3N4
cantilevers in contact mode at a scan rate of 1 Hz. The ?optical
transmittance and reflectance spectrum were recorded on a Perkin Elmer ?Lambda950
spectrophotometer in the wavelength range of 200-3100 nm. ?

Results and Discussion

Figure 1 shows the XRD pattern of a PbS film deposited
on a glass substrate ?at room temperature for 30 to 150
minutes. As shown in Fig. 1, with ?increasing
deposition time, the intensity of the peaks and the crystallinity of ?the films increased. This issue can be attributed to
increased thickness and ?increased particle size with
increasing reaction time.?? ?

According to the
identification with X’pert HighScore software, all reflections ?corresponding to rocksalt phase of PbS (JCPDS powder
diffraction file #5-??0592). The absence of any other
diffraction peaks indicates that no other ?crystalline
phases, such as oxides or carbonates of Pb, exist with detectable ?concentration within the layers.?

The XRD spectra indicate an
increase in grain size with increasing deposition ?time,
and a gradual transition to texture, which likewise strengthens ?with deposition time.

Fig. 1. XRD pattern of film PbS deposited on glass substrate at
room temperature and 30 to 150 ?minutes

?

The evolution of the film
topography with deposition time is illustrated by ?AFM
surface plot images shown in Fig.2??.? Fig.
2a displays the initial nucleation stage, whereas the subsequent images ??(Fig. 2b–d) show films which gradually developed with increasing
?deposition time. The plot of the surface roughness vs.
deposition time for ?layers deposited at room
temperature shown in Fig. 2f indicates that simultaneously with complete shift
of particle shape, the roughness ?value of the film
increases sharply from 20 nm to 65 nm;?? which falls
back to around 30 nm?, with further increasing
deposition time. Similar behavior ?has been reported
for samples deposited at lower deposition temperatures (10 ??ºC)
on the GaAs (100) substrate, however, due to the lower deposition ?temperature, these changes occurred over longer periods of
time30. ?

In the period from 90 to 120 minutes, island growth has occurred,
resulted in a ?significant increase surface roughness
of the film, but over a period of 120 to ??150 minutes,
the growth process has progressed through layer by layer growth ??(Frank–van der Merwe) resulted in a significant reduction
in RMS.?

Sample deposited for 30 min
(Fig. 3a) showed a discontinuous nano-crystalline ?film
consisting of round particles with typical size of 100 nm. Increase of the ?deposition time to 60 min (Fig. 3b) resulted in relatively
continuous and dense ?film. In addition nuclei with
typical 20-30 nm have appeared on the primary ?film.
Within 90 minutes a well adherent, dense compact layer which covers the ?entire substrate surface was achieved (Fig. 3c) the first
signs of change in ?particle shape have appeared in
this stage. Due to the compactness of the film, ?distinguishing
of particle boundaries and determination of particle size are ?difficult.
Further increase in deposition time to 120 min (Fig. 3d) results in ?complete transition to faceted cubic particles with typical
size of 500 nm. The ?boundaries of particles are quite
distinctable, it can be attributed to ?the ?dominance of columnar growth (versus layer by layer growth)
at this stage. ?Further increase in deposition time to
150 minutes, was not varied the film ?morphology
significantly.?

Fig. 2. AFM surface plot images of PbS films deposited at
room temperature for (a) 30 min, (b) 60 ?min, (c) 90
min, (d)120 min, (e) 150 min, (f) surface RMS roughness as a function of ?deposition time ?

Figure 3f shows the thickness
the film as a function of the deposition time. ?Change
in the growth rate with the reaction time is illustrated by this curve. ?Different slope of the graph represent the different stages
of the reaction. The ?initial slope can be attributed
to the nucleation stage or incubation time; at this ?stage,
as ?the time increases, the thickness increases
slightly because the ?primary ?nuclei
are forming. The formation of these nuclei provides fast growth ?rate of the film during the next stage. In the third stage,
due to the depletion of ?the reaction solution from the
reactants, the deposition rate is less than the ?previous
stage.? Each deposition mechanism has a characteristic
growth rate, grain size and shape, which directly affect the nature and
properties of the films 21. Hence, it can be concluded
that the changes in the deposition rate and particle shape is due to the
transition in deposition mechanism. On the other hand, it is well understood
previously that cluster mechanism has higher growth rate; so, the high rate of
deposition in the second stage (60-90 min) can be attributed to the dominance
of cluster mechanism.

The deposition rate declines,
tendency to form larger particles and columnar ?growth
in the third stage of deposition are evidences to transition from the ?cluster growth mechanism in the initial stages of growth to
ion-by-ion growth.?

Fig. 3. Field Emission Scanning Electron Microscopy (FESEM) Images
of PbS films ?de?-?
posited on glass at room temperature (a) for 30 min, (b) for 60 min , (c) for
90 min, (d) for 120 min  (e) for 150 min,
(f) film thickness as a function of deposition time.

 

In fact, this time-dependent
transition from cluster to ion-by-ion mechanism is ?expected
due to depletion of lead ions in solution (e.g. increase in complex-to-?metal ion concentration ratio) as the reaction proceeds.?

The gradual change in film
morphology, accompanied by enhancement of (200) ?preferred
orientation, occurs with increasing film thickness.? This
observation consistent with the AFM results; with increasing deposition ?time from 90 to 120 minutes, roughness of the samples
increases sharply (from ?about 20 to about 65 nm),
which can be attributed to the columnar growth, ?whereas
the columnar growth is characteristic of the ion-by-ion mechanism, it ?would be suggested than after 90 min from the beginning  of the reaction active ?mechanism
altered to ion-by-ion.?

Deposition mechanism depends
on the reaction conditions and specifies the ?product
characteristics. Previous studies on the PbSe films deposited via CBD, ?revealed that texture development which observed with
increasing thickness is ?also related to the change of
the dominant mechanism 31. The results of this ?study
show that increasing deposition time leads to (200) texture ?developments.?

Conclusions

The thin film of
lead sulfide was deposited on a glass substrate using the CBD ?method for
different deposition times. It was observed that morphology of the ?samples
depends on deposition time. As the deposition time increases, the ?shape of the ?particles
changes from round to cubic, texture ??(200) ?develops Furthermore the roughness of the film changes ?during the
deposition. These ?changes
are attributed to the change in the dominant deposition mechanism. ?This study
showed that deposition time is an important parameter in ?determining the dominant mechanism of
deposition and consequently the ?characteristics of the film.?

References

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