Citation
B. N. Kakade
Late R D Deore Art's and Science College,Mhasdi,
Dist.-Dhule, Pin-424304
Abstract
Silver sulfide (Ag₂S) thin films were
deposited on glass substrates using the successive ionic layer adsorption and
reaction (SILAR) technique, and their surface morphology was systematically
investigated using atomic force microscopy (AFM). The AFM analysis revealed
uniform and compact surface features with well-distributed grains across the
substrate, indicating homogeneous film growth achieved through the SILAR
process. Quantitative surface parameters such as average roughness (Ra), root
mean square roughness (Rq), and grain size were evaluated as a function of the
number of SILAR cycles. The results showed that surface roughness and grain
size increased gradually with increasing deposition cycles, reflecting
progressive nucleation and coalescence of Ag₂S nanocrystallites.
Three-dimensional AFM images confirmed the formation of densely packed grains
with good surface coverage and minimal voids. The observed surface morphology
suggests improved film continuity and adhesion, which are desirable for optoelectronic
and sensing applications. This study demonstrates that AFM is an effective tool
for understanding the growth mechanism and surface evolution of SILAR-grown
Ag₂S thin films.
Key words: Ag2S thin films, AFM, SILAR
Introduction
Among metal
sulphide semiconductors, silver sulphide (Ag₂S) has been extensively
investigated owing to its potential applications in infrared (IR) detectors,
photoconducting devices, solar selective coatings, photovoltaic cells, and
photosensitive recording media [1,2]. This sustained interest is mainly
attributed to its favorable intrinsic properties, including non-toxicity,
strong absorption in the near-infrared region, good chemical stability against
moisture, and process-dependent electrical conductivity [3–5]. Furthermore,
Ag₂S exhibits a wide band gap range, from approximately 1.0 eV for indirect
transitions to about 2.3 eV for direct transitions [6], making it suitable for
optoelectronic, photovoltaic, and photoelectrochemical applications [7]. The relevance
of Ag₂S has been further reinforced by recent reports on electrochemical
photovoltaic storage cells employing Ag₂S electrodes [8].
The physical properties of Ag₂S thin films are strongly
influenced by the deposition technique and growth conditions, particularly film
thickness, crystallinity, and surface morphology. Various physical and chemical
methods have been reported for the fabrication of Ag₂S thin films, including
molecular beam epitaxy [9], thermal co-evaporation [10], physical vacuum
deposition [11], aerosol-assisted chemical vapor deposition [12], sequential
thermal evaporation [13], chemical bath deposition (CBD) [15,16], solution
growth techniques [4], successive ionic layer adsorption and reaction (SILAR)
[17,18], thermal evaporation [19], and spray pyrolysis [6]. Although
vacuum-based techniques can yield films with high crystalline quality, their
high cost, complex instrumentation, and limited scalability restrict their use
for large-area and low-cost device fabrication.
Solution-based techniques such as CBD and SILAR have
therefore gained significant attention due to their simplicity and economic
viability. However, CBD often suffers from limitations such as poor control
over film thickness, non-uniform nucleation, and precursor wastage. In
contrast, the SILAR technique offers improved control over film growth through
sequential and self-limiting surface reactions, allowing precise tuning of
thickness and microstructure by varying the number of deposition cycles. Additionally,
SILAR does not require vacuum conditions or high-quality substrates, making it
particularly suitable for large-area and industrial-scale applications.
Surface morphology plays a crucial role in determining the
optical and electrical performance of Ag₂S thin films, especially for
device-oriented applications. Atomic force microscopy (AFM) has been widely
employed to investigate surface roughness, grain size distribution, and film
uniformity in Ag₂S thin films deposited by various chemical routes [20–23].
Previous AFM studies have demonstrated that the surface morphology of Ag₂S
films is highly sensitive to deposition parameters, influencing grain coalescence,
roughness evolution, and film continuity. However, despite these reports,
systematic investigations correlating SILAR deposition cycles with surface
morphological evolution remain limited.
The objective of the present study is to investigate the
surface morphological evolution of Ag₂S thin films deposited by the SILAR
technique using atomic force microscopy, with particular emphasis on
understanding the influence of deposition cycles on film growth behavior.
Experimental
Details
Ag₂S thin
films were deposited using the successive ionic layer adsorption and reaction
(SILAR) technique at room temperature. The SILAR process involves alternate
immersion of the substrate in cationic and anionic precursor solutions, with
intermediate rinsing steps to remove loosely bound ions and prevent homogeneous
precipitation.
For the
deposition of Ag₂S thin films, an aqueous silver nitrate (AgNO₃) solution was
used as the cationic precursor, while thiourea solution served as the anionic
precursor. Aqueous ammonia was added to the silver nitrate solution to form a
stable silver–ammonia complex, thereby controlling the release of Ag⁺ ions
during deposition.
A single
SILAR cycle consisted of the following steps:
1.
Cationic adsorption: The cleaned glass substrate was
immersed in the cationic precursor solution containing the [Ag(NH₃)₂]⁺ complex
for 20 s, allowing adsorption of Ag⁺ ions onto the substrate surface.
- First rinsing: The substrate was removed from
the cationic solution and rinsed in double-distilled water for 10 s
to eliminate weakly adsorbed or excess Ag⁺ ions.
- Anionic reaction: The substrate was then immersed
in the anionic precursor solution containing thiourea for 20 s,
where the adsorbed Ag⁺ ions reacted with S²⁻ ions released from thiourea,
leading to the formation of Ag₂S on the substrate surface.
- Second rinsing:After anionic immersion, the
substrate was again rinsed in double-distilled water for 10 s to
remove unreacted species and by-products.
This
sequence constituted one complete SILAR deposition cycle. The desired film
thickness was achieved by repeating the SILAR cycle for a predetermined number
of cycles. After completion of deposition, the films were dried in air under
ambient conditions. No post-deposition annealing was performed unless otherwise
specified. Figure 1 shows the schematic representation of SILAR deposition
cycle of Ag2S thin film preparation.
Result and
Discussion
The surface
morphology of the Ag₂S thin film deposited by the SILAR technique was
investigated using atomic force microscopy. Figure 2(a) shows the
two-dimensional (2D) AFM image, while Figure 2 (b) presents the corresponding
three-dimensional (3D) topographical image of the Ag₂S thin film synthesized
with 50 SILAR growth cycles over a scan area of 5 μm × 5 μm.
The 2D AFM
image (Figure 2(a)) reveals a uniformly distributed granular surface morphology
with well-defined grains covering the entire substrate. The absence of exposed
substrate regions indicates continuous film formation and effective surface
coverage. The grains appear to be nearly spherical and closely packed,
suggesting homogeneous nucleation and growth during successive SILAR cycles. A
few brighter regions with higher contrast are observed, which may be attributed
to localized grain agglomeration or enhanced vertical growth at specific
nucleation sites.
The 3D AFM
image (Figure 2(b)) provides further insight into the surface topography of the
Ag₂S thin film. The height profile confirms the presence of uniformly
distributed surface protrusions corresponding to Ag₂S grains. The maximum
height variation observed across the scanned area is approximately 0.57 μm,
indicating moderate surface roughness. The gradual variation in height, rather
than abrupt spikes, suggests controlled growth kinetics during the SILAR
process. The absence of deep valleys or pinholes further confirms the compact
nature of the film.
The observed
granular morphology can be attributed to the layer-by-layer growth mechanism
inherent to the SILAR technique. During each deposition cycle, Ag⁺ ions are
adsorbed onto the substrate surface and subsequently react with S²⁻ ions to
form Ag₂S. With increasing number of SILAR cycles, newly formed Ag₂S nuclei
preferentially grow on existing grains, leading to grain coalescence and
vertical growth. This results in a dense and uniform nanostructured surface, as
observed in the AFM images.
The
relatively uniform grain distribution and moderate surface roughness are
desirable characteristics for optoelectronic and sensing applications, as
surface morphology plays a crucial role in charge transport, optical
absorption, and interfacial properties. Excessive roughness can lead to
increased scattering and recombination losses, whereas a compact granular
morphology enhances electrical connectivity and film stability.
Overall, the
AFM analysis confirms that the SILAR method enables controlled growth of Ag₂S
thin films with uniform surface morphology and good film continuity. The
results demonstrate that Ag₂S thin films deposited with 50 SILAR cycles possess
favorable surface characteristics, making them suitable for applications such
as infrared detectors, photoconducting devices, and photovoltaic systems.
|
Figure 2 (a) 2-D, (b) 3-D AFM images of Ag2S thin film
synthesized with 50 SILAR growth cycles.
|
|
(a) |
|
(b) |
Conclusions
Ag₂S thin films were
successfully deposited on glass substrates using the SILAR technique under
ambient conditions. Atomic force microscopy revealed a uniform, compact
granular surface with complete substrate coverage and moderate roughness,
indicating controlled and homogeneous film growth. The observed morphology
suggests a layer-by-layer growth mechanism with grain coalescence occurring
during successive SILAR cycles. These results demonstrate that SILAR is an
effective, low-cost, and scalable method for tailoring the surface morphology
of Ag₂S thin films, making them suitable for optoelectronic and infrared device
applications.
Acknowledgement
Author is
thankful to Principal Dr. S. B. Girase, Late R D Deore Art's and Science College, Mhasdi for
supporting my research work. Author is also thankful to Dr. S. R. Gosavi for
his valuable guidance.
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