FIELD OF THE INVENTION
The present invention generally relates to forming a thin layer of wide band gap
semiconductor material by immersing a microwave absorbing substrate in
semiconductor precursor solution. In particular, the invention relates to a method
to deposite a thin layer of semiconductor material by microwave irradiation
technique. More particularly, the present invention relates to a method of
depositing a thin layer of semiconductor material by selective heating of a
microwave absorbing substrate to reduce deposition time.
BACKGROUND OF THE INVENTION
Research and development of the thin film semiconductor technology has been
aggressively promoted world-wide in recent years for its wide range of
applications in printed electronics, optical films, solar cells and coatings. A part of
the research and development has focused on solution processes which are
available to produce the films of semiconductors at comparatively lesser cost
than vacuum processes. Spin coating which is a representative solution process
is material-consuming especially on hydrophobic surfaces, where most of the
material is lost. It is also difficult to maintain uniformity in drop-cast and other
solution based techniques to deposit semiconductor thin films over large area.
Therefore, methods for deposition of thin film semiconductor material uniformly
over large area with high efficiency are of great interest for semiconductor
industries.
During last decade, photovoltaic (PV) industry has implemented different
methods to deposit thin film semiconductors for converting light energy into
electrical energy. Since thin film PV devices are lighter and have more radiation
resistance than conventional single crystal (silicon and gallium arsenide) devices,

these are potential candidate for space and terrestrial applications. Though the
present efficiencies of thin film PV devices are less than single crystal devices
[1], for space applications it can be compensated by its light weight as more
number of devices can be added without increasing much of pay-load. At
present, CdTe (Cadmium Telluride), CIS (Copper Indium Sulfide/Selenide) and
CIGS (Copper Indium Gallium Sulfide/Selenide) based thin film solar cells with
CdS (cadmium Sulfide) and ZnS (Zinc Sulpfide) buffer layers with different
deposition methods have reached the commercial level of manufacturing [2], [3],
[4], These thin film materials have better optical properties [5] than conventional
solar cell materials.
Defect-free deposition of thin film semiconductor materials is carried out
primarily by two techniques viz. vacuum-based techniques and solution-based
techniques. In vacuum-based techniques, target materials to be deposited are
either co-sputtered or co-evaporated onto the substrate. These methods can
produce a uniform and defect-free layer of semiconductor materials, however;
these methods are energy intensive and consume large amount of energy for
their high vacuum requirements. In solution-based non vacuum techniques,
reactants react either on the surface of substrate immersed in precursor solution
to form a layer semiconductor material or in precursor solution to form
semiconductor material which is then deposited over the substrate by spin
coating, doctor blade coating, inkjet printing etc. Generally, reactants are heated
by conduction using resistive heating in solution-based techniques, however,
other methods of heating like microwave irradiation [6] have also been used.
Heating chemicals by microwave energy has gained high popularity since it was
first reported by the groups of Gedye et. al [6] and Giguere et. al [7] in 1986. In
conventional heating methods, heat is transferred to reactants by conduction
through walls of reaction vessel, while in microwave assisted heating, microwave

fields increases the temperature of reactants directly through ionic conduction or
polarization. Hence, microwave assisted reactions can be used to reduce reaction
times, increase product yields and reduce unwanted side reactions compared to
conventional heating methods.
During ionic conduction heating, translation motion of charged particles in
microwave electric field causes increased collisions and this kinetic energy is
converted into heat. In polarization molecular dipoles align and oscillate with
microwave electric field and lose energy in form of heat through molecular
friction and dielectric loses. Microwave power absorbed by dielectric material is
given by
P = K. εr- tanδ f. E2
And penetration depth D, which is defined as distance from surface at which
fields are reduced by a factor of 1/e is given by
D=0.225 λ/ (√∙Εr √(√(1 + tan2δ)-1))
where K, εr tanδ, f, E, λ represents material dependent constant, specific
inductive capacity, dielectric power factor, frequency, electric field strength and
free space wavelength respectively.
In most of the cases, the reason for the observed rate enhancement of chemical
processes is purely a thermal (kinetic) effect. It means that high reaction
temperature achieved rapidly under microwave irradiation of polar materials
cause increase in the chemical reaction rate. In addition to thermal effects, there
are specific and non-thermal microwave effects that are caused by the unique
nature of the microwave dielectric heating. These effects include volumetric

heating, wall effect, selective substrate heating [8] and molecular radiators, and
cannot be achieved by conventional heating.
Selective heterogeneous heating is an interesting property of microwave assisted
reactions which has been exploited in present invention. In a sample containing
more than one component, only one which couples with microwaves is
selectively heated. This means that a strongly microwave-absorbing material or
heterogeneous catalysts can be selectively heated in a less polar reaction
medium.
Chen et al. has disclosed an aqueous solution growth method for CdZnS buffer
layer in thin film solar cells in their patent numbered US 5078804A (1992). The
CdZnS buffer layer comprised of two layers, a high Zn content CdZnS layer on
top of low Zn content CdZnS layer. Photovoltaic efficiencies of 12.5% at Air Mass
1.5 illumination conditions and 10.4% under AMO illumination were claimed.
However, time required for deposition time for two buffer layers of different
composition using conventional heating was high. Also, use of cadmium made
the process less attractive for commercial and environmental reasons.
Bjorkman et al. has disclosed in their patent numbered US 0180200A1 (2006) on
method of producing solar cells and process line for manufacturing of the cell
structure. Zn(0,S) buffer layer on top of CIGS layer was deposited by atomic
layer deposition (ALD) technique. Another buffer layer of ZnO was deposited
over first buffer layer using ALD technique. A major drawback of ALD is its low
deposition rate, making ALD less attractive for solar cell applications.
In another disclosure by R. N. Bhattacharya, ZnS/Zn(0,0H)S based buffer layer
deposition for solar cells, US0191359A1 (2009): CBD technique for deposition of
ZnS/Zn(0,0H)S was disclosed. Disclosed method used zinc salt, thiourea and

ammonium hydroxide dissolved in a non-aqueous/aqueous solvent mixture or
100% non-aqueous solvent. Significantly thick deposition and low oxides and
hydroxide formation were claimed. However, a significant amount of material is
wasted during deposition by CBD as reaction occurs in bulk of solution instead of
surface of substrate. Further, ZnS/Zn(0,0H)S layer get deposited on the walls of
reaction container which could contaminate the subsequent depositions.
B. M. Basol has disclosed his patent "Apparatus for continuous processing of
buffer layers for group IBIIIAVIA solar cells", US0223444A1 (2009) that CdS
buffer layer was deposited by heating the surface of solar cell absorber layer in
chemical solution. Novelty of disclosed method was that only a portion of
chemical solution near substrate got heated and chemical solution could be
cooled and reused, minimizing material wastage. However, time required to
increase the temperature of substrate dipped in chemical solution is significantly
high as heat is quickly transferred to the chemical solution.
In another disclosure by Ennaoui et al., Method of the application of a zinc
sulfide buffer layer on a semiconductor substrate, US7704863B2 (2010): CBD
method for deposition of ZnS buffer layer for high efficiency solar cell was
disclosed. Semiconductor substrate was dipped in solution of zinc sulfate (0.05 to
0.5 mol/1) and thiourea (0.2 to 1.5 mol/1) in distilled water at constant
temperature for 10 minutes. Deposition manner was claimed to be dependant on
the sequence in which chemicals were added to bath. Claimed efficiency of solar
cell uniform ZnS buffer layer was comparable or higher than conventional toxic
layer. To increase the thickness of buffer layer, CBD was carried out several
times. Multiple depositions increased both the time and material required for
deposition which hindered the large scale commercialization of the disclosed
method.

In another disclosure by Uen-Ren Chen in patent numbered US0111129A1
(2011) on method of fabricating cadmium sulfide thin film: method for adhesive
CdS film deposition was disclosed. Chemical bath including sulfur containing
compounds and cadmium containing compounds was adjusted to alkaline by
addition of ammonia followed by addition of buffer salts. Glass substrate was
immersed in chemical bath and bath solution was heated such that thickness of
CdS film increased with increase in immersion time. Problems faced in processing
highly toxic cadmium waste made the disclosed method less attractive.
Recently, Kawano et al. in their disclosure, "Buffer layer and manufacturing
method thereof reaction solution, photoelectric conversion device, and solar cell"
numbered US8071400B2 (2011) have disclosed method for buffer layer
deposition by liquid phase method. Reaction solution of a zinc containing
compound, a sulfur containing compound (0.001 to 0.25M), a citrate compound
consisting of ammonium salt (0.001 to 0.4M) with pH of 9.0 to 12.0 at start of
reaction was used for deposition. As claimed, addition of citrate compound to
reaction solution assists the complex formation required for reaction; however,
reaction occurs in bulk of solution. Cost of raw material and cost incurred for
processing the waste material may increase as most of the reactants react in
bulk of solution and final product do not adhere to substrate.
OBJECTS OF THE PRESENT INVENTION
It is therefore an object of the invention to propose a method of depositing a
thin layer of semiconductor material by selective heating of a microwave
absorbing substrate to reduce deposition time.
Another object of the invention is to propose a method of depositing a thin layer

of semiconductor material by selective heating of a microwave absorbing
substrate to reduce deposition time, which reduces raw -material consumption.
A further object of the invention is to propose a method of depositing a thin layer
of semiconductor material by selective heating of a microwave absorbing
substrate to reduce deposition time, which increases productivity.
A still further object of the invention is to propose a method of depositing a thin
layer of semiconductor material by selective heating of a microwave absorbing
substrate to reduce deposition time, which improves the yield of the product.
SUMMARY OF THE INVENTION
The present invention discloses the method to deposit a semiconductor material
over a microwave absorbing substrate for solar cell and other thin film
applications. Accordingly, to the invention, a 10 nm to 2 ^im layer is deposited on
a substrate dipped in solution of semiconductor precursors by microwave
irradiation. Compared to conventional methods, the disclosed method heats the
substrate selectively to save the time required for deposition and produces
higher yield. Since, the substrate shows a good response to microwave
irradiation, a selective heating of the substrate immersed in solution of chemical
precursors is done. Reaction initiation sites are created on surface of the
substrate. A uniform layer of a semiconductor layer is formed on the surface of
the substrate and reaction in bulk of the chemical solution is minimized. In a
representative embodiment, zinc sulphide is deposited over indium doped tin
oxide coated glass slide which shows good microwave absorbance. Deposited
layer can be used as substitute for toxic cadmium compounds in buffer layer for
thin film solar cell application.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows an apparatus used for thin film semiconductor deposition by
disclosed method.
Figure 2 is a flow chart illustrating the steps involved in deposition by disclosed
method.
Figure 3 is SEM image of zn ZnS(0,OH) film deposited by the disclosed method.
DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention provides for the microwave assisted method for depositing
thin film semiconductor material. In a preferred embodiment, the substrate is
microwave absorbing material over which a 10 to 2000 nm layer of
semiconductor material is deposited by microwave irradiation technique.
Chemical precursors for deposition of semiconductor layer are dissolved in a
solvent such that the solution formed responds weakly to microwave radiation
relative to the substrate on which semiconductor layer is to be developed. A
microwave absorbing substrate or substrate having a coating of microwave
absorbing material is immersed in the growth solution and exposed uniformly to
microwave radiation. As the substrate absorbs more microwave radiation than
bulk of growth solution, it gets selectively heated. The growth solution around
the heated substrate gets energy required for reaction from the substrate and
form nucleation sites of the surface of the substrate. A continuous stirring of the
growth solution is maintained, to replace the used solution around the substrate
with fresh solution. To maintain uniform exposure of substrate to microwave
radiation, whole set up is rotated in microwave field. If desired deposition

thickness is not achieved in single deposition, multiple depositions are carried out
on same substrate. Deposited layers are annealed in inert atmosphere to
improve the crystalline properties of the film.
The disclosed method deposits a uniform layer of semiconductor over the
substrate. Since microwave irradiation takes relatively less time to increase the
temperature of the substrate, time required for deposition is considerably
reduced as compared to conventional chemical bath deposition method. In the
disclosed method, heating of the entire growth solution at once is prevented and
only solution around the substrate gets heated and reacts to form
semiconductor. Reacted solution is then replaced by unused solution due to
stirring. Hence, yield of the process is maximized.
Flow chart illustrating the method described in present invention is shown in
Figure 2. In step 1, chemical precursors for semiconductor synthesis are
dissolved in a solvent to form a growth solution. Then, in step 2, a cleaned
substrate coated with a layer of microwave absorbing material is immersed in the
growth solution which shows relatively weak absorption of microwave irradiation.
Growth solution is irradiated with microwave energy of wavelength 2.45 GHz in
step 3. Duration of exposure depends upon the desired thickness, nature of
chemical precursors and semiconductor material deposited. Macro-temperature
of the substrate increases at much higher rate than increase in temperature of
the growth solution, therefore, a reaction is initiated on surface of the substrate.
These initiation sites are distributed uniformly over the surface of the substrate
and grow into a uniform film. Next, in step 4, this deposited film is removed from
the chemical solution, rinsed with solvent and blown with inert gas to clean its
surface. If desired thickness or morphology is not achieved, above steps can be
repeated for multi layer deposition in step 5. Subsequently, the deposited film is
annealed at 100-500°C in controlled atmosphere.

Example:
In preferred embodiment, Zn(0,0H)S layer was deposited on indium doped tin
oxide (ITO) coated glass. 0.34 gm of zinc chloride, 0.71 gm of thiourea, 1.47 gm
of trisodium citrate, 20 ml of 28-30% by wt, ammonium hydroxide and 20ml
deionized (DI) water were taken in 50 ml glass beaker. The mixture is
continuously stirred for 5 min at 200 rpm. The glass substrate coated with the
layer ITO was dipped vertically in the solution with the help of Teflon clamps.
Then, solution was irradiated with a microwave (2.45GHz) power of 300 W for 2
minutes while maintaining a constant stirring of 200 rpm using teflon coated
magnets. Next, solution was allowed to cool for 10 minutes at room
temperature. Substrate was removed slowly from the reaction solution and
rinsed with DI water to remove residual impurities and chemicals. The deposited
film was then dried by blowing nitrogen. Consequently, the sample is annealed at
200°C for 30 minutes. Figure 3 shows SEM image of deposited Zn(0,0H)S layer
deposited by disclosed method.
Although prsent invention has been descirbed with reference to preferred
embodiments thereof, the dislcosed method with few modifications wihtout
departing from scope of present invention cna be used fo rother applcations. The
claims of present invention are appended.

REFERENCES
Non-oatent literature
1. M.A.Green, K. Emery, Y. Hishikawa, W.Warta, E.D. Dunlop, "Solar cell
efficiency tables (version 39)", Progress in Photovoltaics, 20, (1), 12-20,
January 2012.
2. C.S.Ferekides, D. Marinskiy, V Viswanathan, B Tetali, V Palekis, P Selvaraj,
D.L. Morel, "High efficiency CSS CdTe solar cells", Thin Solid Films, 361-
362, 520-526, 21 February 2000.
3. K. Ramanathan, M.A. Contreras, C.L. Perkins, S.Asher, F.S. Hasoon, J.
Keane, D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda,
"Properties of 19.2% efficiency ZnO/CdS/CulnGaSe2 thin-flim solar cells",
Progress in Photovoltaics, 11, (4), 225-230, June 2003.
4. M.A. Contreras, T. Nakada, M. Hongo, A.O. Pudov, J.R. Sites,
"Proceedings 3rd World conference of Photovoltaic Energy Conversion",
Osaka, Japan, 2003.
5. H.Z.Xiao, L.-Chung Yang, A. Rockett, "Structural, optical, and electrical
properties of epitaxial chalcopyrite Culn3Se5 films", Journal of Applied
Physics, 76, (3), 1503-1510, 11 April 1994.
6. R.Gedye, F.Smith, K. Westaway, H.AIi, L. Baldisera, L. Laberge, J. Rousell,
"The use of microwave ovens for rapid organic synthesis", Tetrahedron
Letters, 27, (3), 279-282, 1986.

7. R.Giguere, T.L. Bray, S.M. Duncan, G. Majetich, "Application of
commercial microwave ovens to organic synthesis", Tetrahedron Letters,
27, (41), 4945-4948, 1986.
8. B. Reeja-Jayan, K.L. Harrison, K. Yang, c. Wang, A.E. Yilmaz, A.
Manthiram, "Microwave-assisted low temperature growth of thin films in
solution", Scientific Reports, Volume:2, Article number: 1003, 19
December, 2012.


WE CLAIM
1. A method for depositing a thin film of semiconductor material over a
microwave absorbing substrate, comprising the steps of:
• Adding chemical precursors for semiconductor deposition in a solvent;
• Immersing a cleaned microwave absorbing substrate in the chemical
bath;
• continuous stirring of the growth solution;
• irradiating microwave on the substrate;
• cleaning of the deposited film with the solvent and blowing with an
inert gas; and
• Annealing the substrate in inert atmosphere.

2. The method as claimed in claim 1, wherein the solvent is one of polar and
non-polar and dissolves chemical precursors.
3. The method as claimed in claim 1, wherein the substrate is formed of
microwave absorbing material or coated with microwave absorbing
material.
4. The method as claimed in claim 1, wherein the solution is continuously
stirred at 10 to 500 rpm during deposition.
5. The method as claimed in claim 1, wherein power delivered by microwave
radiation of frequency 2.45 GHz varies from 100 W to 800 W depending
upon the microwave absorbing capacity of the coating on the substrate.

6. The method as claimed in claim 1, wherein the bath container and the
substrate are rotated in microwave field during deposition for uniform
exposure to microwave.
7. The method as claimed in claim 1, wherein the deposited film is cleaned
with the solvent and by blowing inert gas.
8. The method as claimed in claim 1, wherein the deposited film is annealed
at 100-500°C in controlled atmosphere.

ABSTRACT

The thin film semiconductors ranging from few nanometers to several
micrometers find a variety of applications in sensors, field effect transistors, light
emitting diodes and solar cells. These materials have properties different from
bulk material which depend upon method of synthesis and other interrelating
parameters. Various methods like sputtering, co-evaporation, chemical bath
deposition, atomic layer deposition etc. have been used for deposition of thin
film semiconductor material. These methods are constrained by complex
processing, long deposition time and material wastage. A new method for
deposition of thin semiconductor films by microwave irradiation over microwave
absorbing material has been invented. The disclosed method deposits zinc sulfide
(ZnS), indium sulfide (In2s3) and other semiconductor material over microwave
absorbing substrate. The microwave absorbing substrate is immersed in solution
of semiconductor precursors and exposed to microwave energy to react at the
surface of substrate. The substrate is selectively heated by microwave
irradiation, creating reaction initiation sites on substrate instead of bulk of
growth solution and a uniform film is formed. Thus, disclosed method enhances
the product yield and reduces time required for deposition.