Processing 3.2.1. Structure of CdTe The minimal-energy state in

Processing
Techniques

 

 

3.1 Processing of Cadmium Sulphide and Cadmium
Telluride

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Cadmium and
Tellurium are commonly used for the manufacturing of polycrystalline compound
Cadmium telluride (CdTe). It is an II-VI semiconductor compound, which is very
suitable for thin film solar cell devices, infrared optical window, photo
detectors, and semiconductor devices. CdTe is specially known for its optical
properties.

3.2 Physical Properties of CdTe

 

Molecular formula

CdTe

Molar mass

240.01 g mol?1

Density

6.2 g/cm3

Melting point

1092 °C

Boiling point

1130 °C

Solubility in other solvents

insoluble

Band gap

1.44 eV (@300 K,
direct)

Refractive index (nD)

2.67 (@10 µm)

Table. 3.1 Physical properties of CdTe

3.2.1. Structure of CdTe

 

The minimal-energy state in the conduction band and the maximal-energy
state in the valence band are each characterized by
a certain crystal momentum (k-vector) in the Brillouin zone.

If k-vectors are same then
the gap in between them is “direct gap” and if they are different, then
the gap is “indirect gap”. It has a direct band gap of 1.45 eV

.

CdTe can absorb
more than 90% of the photons having energy greater than 1.45 eV that is why it
is well suited for solar cell applications. CdTe is stable up to 500 oC.

CdTe has Lattice constant of 0.68 nm at 300 K.  The specific heat capacity is about 210
J/(kg·K) at 293 K.  In solar cell fabrication,
CdTe is used as a P-Type semiconductor, which has a junction with CdS as an
N-Type semiconductor. Infrared detector material (HgCdTe) is manufactured when
(CdTe) is allowed with mercury.

                                                                                         Figure 3.1 .CdTe structure

3.3 Defects

  
    It has some native defects Cation vacancies
(Vcd) can behave as double accepters and Anion vacancies (VTe) can
behave as double donors. These vacancies can be formed with other extrinsic
impurities

3.4 Toxicity

 It is toxic if swallowed and if its powder is
inhaled. If properly processed, manufactured, and encapsulated then it may
harmless. It is observed that elemental Cadmium is more toxic than CdTe. It is
also studied that CdTe quantum dots causes extensive reactive oxygen damage to
cell membrane, mitochondria, and cell nucleus. If it is to be used at large
scale commercialization of solar panels then exposure and enduring safety of
CdTe will be serious matter. So attempts are being made to overcome all these
issues. 

3.5 Availability

Major issue for the CdTe PV modules is the
availability of Tellurium. Actually, Tellurium is also an tremendously rare
element (1-5) parts per billion in the crust of Earth.

3.6 CdTe Deposition Techniques

   CdTe has versatility because it
provides many ways for its deposition. The method of deposition should be inexpensive,
simply climbable, easy to handle, can give good conversion efficiency of the
device. The deposition technique will be preferred, which has maximum
conversion efficiency, low cost and have high deposition rate. Many techniques
have efficiency up to 12 % in laboratory but not at the industrial level. There
must be such kind of deposition technique that will give good conversion
efficiency in laboratory as well as at industrial level.

3.6.1 Electrode position

It is a technique in which electric current passes through a chemical
solution to produce ions and then these ions are deposit on the substrate. This process of electrolysis is done by the
deposition of a substance on electrode.

Figure
3.2. Apparatus of Electrode position

3.6.2 Physical Vapor
Deposition

 It is the process in which material is
sublimated from solid or liquid source and condensed upon the substrate, mostly
the whole process is done in a vacuum. The deposition rate for physical vapor
deposition is approximately from 1 to 10 nm per second. This deposition
technique is use for the deposition of alloys, elements and compounds by the
reactive deposition. The physical vapor deposition can be categorize as

1.                 
Vacuum evaporation

2.                 
 Sputter
deposition

3.6.2.1
Vacuum Evaporation

 It is the process, in which material is
evaporating thermally inside the vacuum and its vapors are condenses at the
substrate. The vacuum has an advantage of monitoring the adulterations and
reducing the melting temperature of the material. Vacuum also increases the
mean free path for the motion of deposited species. The vacuum required
pressure for deposition is about 10-5 torr to 10-10 torr.
Thermal evaporation is obtained by thermal heating sources such as tungsten
coils.

3.6.2.2
Sputter Deposition

 It is the process, in which material is not
heating thermally so it is a non-vaporizing process. Material from target is ejected
and then deposits on the substrate. When energetic particles like ions are
bombarded on the sputtering target a plume of material is released and deposits
onto a substrate just like a spray of sand when a golf ball lands in the
bunker. The attacking particles are usually gaseous ions accelerated from
plasma. The sputtering gas is often an inert gas like Argon gas. The spacing
between source and substrate is less than vacuum deposition. The plasma
pressure for sputter deposition is from 5 mtorr to 30 mtorr.

 

3.6.3 Atomic Layer Epitaxy

The term Epitaxy
comes from the Greek roots, meaning of word epi,
is “above”, and meaning of  taxis, meaning “in ordered manner”. It can be
translated as “to arrange upon”.
                   

 Atomic layer Epitaxy (ALE)
or Atomic Layer Chemical vapor deposition (ALCVD) now days it is known as
Atomic Layer Deposition (ALD).  This
technique used for the production of high quality, thin, solid films of
specific crystal structures. This method provides very well control of film thicknesses
to one atomic layer. It has a wide range of applications in areas such as thin
film ceramics, gas sensors, radiation detectors, optical/infrared filters,
surface hardening and fiber optical materials.  

      This method is based on sequential
use of gas phase chemical process. Mostly in (ALE) two chemicals are used
called precursors. Chemical reaction takes place between these precursors with
surface one at a time in sequential manner. Using ALE alternating monolayers of
two different elements can be deposited onto a substrate. In this technique
material quantity is deposited in every cycle is constant. This method provides
crystalline and uniform films especially if very thin film is required.

 

 3.6.4 Molecular Beam Epitaxy

It is a fabrication
of crystalline thin film Epitaxially above the other crystalline substrate with
the beam of molecules or atoms.  This method required an ultra-high vacuum
up to (10-8 pa) for the film deposition.

The rate of deposition is very slow
which is 1 ?m/hr. The materials are evaporated and reach the substrate
individually then on the wafer reaction take place between these vapors. This
method can give high-purity epitaxial layers of compound semiconductors.

The word “beam” shows that sublimated atoms of the material do not interact
with each other and with vacuum chamber until reach the wafer, due to long mean
free paths of atoms.

1.     
If crystalline film is fabricated on the substrate of
the same material is called    Homoepitaxy (This Epitaxy is done with
only single material)

2.     
If crystalline film is fabricated on the substrate of
the different materials is called    Heteroepitaxy (This Epitaxy is done
with different materials)

 

3.6.5 Close Spaced
Sublimation

          It is a process for a thin film
deposition of materials in a vacuum. The material is sublimated by heating and
its vapors condensed onto a substrate that is placed above the source material.
The basic phenomenon of thin film deposition based on dissociation at high
temperature. 

CdTe ? Cd + Te

Before fabrication of thin film by
this method, substrate needs a proper cleaning by Acetone, Isopropyl alcohol,
rinse with distilled water and ultrasonic cleaning. Quality of thin films,
material transport and deposition rate depends upon the following parameters.

1.     
Source and Substrate Temperature

   
It is observed that high substrate temperature of CdTe provides good
performance of solar cell devices. Resistivity of CdTe decreases by increasing
the substrate temperature and grain size reduces by increasing substrate
temperature. Deposition rate also improved at high source temperature 57.

2.     
Spacing between Source and Substrate

  
The spacing between source and substrate is inversely proportional to
the rate of deposition.

3.     
Vacuum Level

    Vacuum level for CdS and CdTe deposition
is from 10-3 to 10 -5 mbar.

4.     
 Source
Stoichiometry

   Composition
of material is an important for thin film quality.

5.     
 Annealing

Annealing of the
films provides improvement of surface morphology and it reduces roughness of
surface of CdS and CdTe films. Recombination centers reduce by annealing. Crystal
formation of film can improved by annealing. Open circuit voltage also
increased by increasing annealing temperature. Spectral response especially in
the range of 500 to 600 nm also improved by annealing temperature.  Hence, efficiency of solar cell can be
improved.

6.     
 Diffusion

The particles movement from
higher concentration to lower concentration is called diffusion. The distance
particles can travel without any collision is called mean free path. Diffusion
of one kind of particles into other kind of material can change its
characteristics like a semiconductor material can be converted into N-

Type or a P-Type. The mathematical description can be
explained by the Fick’s law 

      According to Fick’s first
law   

 

 

 

   j is the atomic flux  per
unit area per unit time, D is the diffusion co-efficient   

 (dc/dx) is the concentration
gradient of the vapor 61.                                                

      

3.7 Processing of Cadmium
Sulphide

3.7.1 Chemical Bath
Deposition (CBD)

It is very
easy process for the fabrication of CdS thin films on ITO glass. For the
fabrication of thin film solar cell we need a very thin film up to (60 -80) nm,
which is not easily possible by using close spaced sublimation process. It is
very suitable process especially for solar cell point of view.  It is a process in which substrate is placed
in a hot chemical solution stirring vigorously for specific time, positive and
negative ions will reach and meet on the substrate and thin film is grown. The
advantage of this technique is that neither vacuum and nor very high temperature
is required for CBD 62.

3.8 Fabrication of CdS Thin
Film by Chemical Bath Deposition   

The process is very simple, these are steps given
below

1.       Cleaning

   
Substrate is cleaned by using detergent then rinsed in distilled water
then again clean with acetone and rinsed distilled water then clean with
isopropyl alcohol (IPA).

2.     
 Preparation of
the chemical solutions

                  The solution will be prepared
in distilled water according to the given molarities.

Molar mass of
CdCl2                          
183.32 g/mol

Molar mass of
KOH                                  56.0
g/mol

Molar mass of
NH4NO3                                80 g/mol

Molar mass
of  thiourea                                76 g/mol

Mass required making 1000 ml solution

Mass = (Mgiven X Vreq X Mol
mass)/1000

M = morality given

Vreq = required volume

Mass of CdCl2 required making 1000 ml

(0.02X500X183.22)/1000 = 1.832 grams

 Note: If hydrated cadmium chloride CdCl2
(2.5H2O) is available then mass of CdCl2 required for
making 1000 ml will be (2.283 g/1000 ml)

Mass of KOH required making 1000 ml solution

(0.5X1000X56)/1000 = 28 grams

Mass of NH4NO3 required making
1000 ml solution

(1.5X500X80)/1000=60 grams

Mass of Thiourea (NH2)2CS
required making 1000 ml solution

(0.2 X 76 X 1000)/1000 = 15.2 grams

3.     
 Procedure

The procedure is very simple for the CBD.
Clean beaker and substrate carefully

 Add chemical
solutions according to the amount given below

                 Cadmium chloride       CdCl2                     0.02M (80 ml)

                 Ammonium Nitrate      NH4NO3                 1.5M   (80 ml)

                Potassium Hydroxide    KOH                      0.5M  (200 ml)

These three
solutions are added into the beaker. The substrates are fixed inside the
solution with the help of a substrate holder. The beaker is placed on the hot
plate with magnetic stirrer inside. A pH meter and thermometer is also dipped
in the solution to measure pH and temperature respectively. Provide heat to solution
up to temperature of 75 oC. The solution is provided stirring
continuously throughout the experiment. 

When temperature reached at 75 oC add the thiourea about
0.2M (80 ml).

As thiourea is
added to the solution the reaction starts suddenly. So thiourea is the last
component which is added to the solution. So deposition of CdS film starts when
the thiourea is added. Films are retired from the solution according to the
specific deposition time and rinsed immediately with distilled water into an ultrasonic
cleaner.                          Figure
3.3.CBD Apparatus

 

 
CdS23

 
60

 
2.29

 
CdS 24

 
127

 
2.36

 
CdS 25

 
170

 
2.66

 
CdS 20

 
257

 
2.39

The CdS deposited films with
pale-yellow color are obtained. The CdS film is deposited on both sides of the
ITO glass. So film from glass side is cleaned by using 10% Hydrochloric acid
(HCl) solution. It needs much care when using HCl acid on glass side for
cleaning because drops of HCl can remove film from ITO side as well. After
deposition these CdS films will be annealed at 400 oC for 30 minutes
63, 64.                                                           Table 4.4 Thickness and energy gaps of CdS CBD