Chapter 03 ProcessingTechniques 3.1 Processing of Cadmium Sulphide and CadmiumTelluride Polycrystallinecompound Cadmium telluride (CdTe) is formed from Cadmium and Tellurium. It isan II-VI semiconductor compound whichis very suitable for thin film solar cell devices, infrared optical window,photo detectors, and semiconductor devices. CdTe is well known for its optical propertiesespecially 39, 40.
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 It hasa direct band gap of 1.45 eV. The minimal-energy state in the conduction band and the maximal-energystate in the valence band are each characterized bya certain crystal momentum (k-vector) in the Brillouin zone. If thek-vectors are the same, it is called a “direct gap”. If they aredifferent, it is called an “indirect gap”. CdTe can absorb more than 90% of thephotons having energy greater than 1.
45 eV, that’s why it is well suited forsolar cell applications. CdTe is stable up to 500 oC. CdTe has Latticeconstant of 0.
68 nm at 300 K. Thespecific heat capacity is about 210 J/(kg·K) at 293 K. In solar cell fabrication CdTe is used as aP-Type semiconductor which has a junction with CdS as an N-Type semiconductor.Infrared detector material (HgCdTe) is manufactured when (CdTe) is alloyed withmercury. The very important application of CdTe is a radiation detector used todetect X-rays, Alpha, Beta and gamma rays, for this purpose CdTe is doped withChlorine.
CdTe has electron affinity of 4.3eV. Work function of 5.5 eV for P-TypeCdTe. It can be doped both as P-Type as well as NType semiconductors 41, 42.
Figure 3.1 .CdTe structure 3.3 Defects It has some native defects Cation vacancies (Vcd) can behave as doubleaccepters and Anion vacancies (VTe) can behave as double donors.These vacancies can be formed with other extrinsic impurities 43, 44.
3.4 Toxicity A problem of CdTe is that it istoxic if ingested and if its powder is inhaled. If properly processed andmanufactured and encapsulated then it may harmless. It is observed thatelemental Cadmium is more toxic than CdTe.
It is also studied that CdTe quantumdots causes extensive reactive oxygen damage to cell membrane, mitochondria,and cell nucleus. If it is to be used at large scale commercialization of solarpanels then exposure and long term safety of CdTe will be serious issue. Soattempts are being made to overcome all these issues. A document hostedby the U.S. National Institutes of Health dated 2003 discloses that:”Brookhaven National Laboratory (BNL) and the U.S. Department of Energy (DOE)are nominating Cadmium Telluride (CdTe) for inclusion in the NationalToxicology Program (NTP).
This nomination is strongly supported by the NationalRenewable Energy Laboratory (NREL) and First Solar Inc. The material has thepotential for widespread applications in photovoltaic energy generation thatwill involve extensive human interfaces. Hence, we consider that a definitivetoxicological study of the effects of long-term exposure to CdTe is anecessity.” The BNL has giventhe research that CdTe large-scale PV modules have not any health risks not anythreats for environment. These modules can be recycled. These modules do not produce any pollutantsduring their working for long term 45, 46.
3.5 Availability Major issue for the CdTe PV modules is the availability of Tellurium.Actually Tellurium is also an extremely rare element (1-5) parts per billion inthe crust of Earth. Manufacturing of CdTe PV solarpanels at large-commercial scale will cause depletion of Tellurium 47. 3.6 CdTe Deposition Techniques So many techniques can be usedfor the CdTe. It is the versatility of the CdTe because it provides many waysfor its deposition.
The method of deposition should be economical, easilyscalable, easy to handle, can give good conversion efficiency of the device.The method should be easily applicable at large industrial level. Thedeposition technique will be preferred which has maximum conversion efficiency,low cost and have high deposition rate.
Many techniques have efficiency up to12 % in laboratory but not at the industrial level. There should be such kindof deposition technique that will give good conversion efficiency in laboratoryas well as at industrial level 48. 3.6.1 Electrodeposition The deposition of a substance on an electrode by the process ofelectrolysis.
It is a technique in which electric current is passed through achemical solution and ionization occurs then these ions deposit on thesubstrate. R.K. Sharma et. al deposited CdTe thin films electrochemically froman acidic aqueous solution which is containing 2.5 M CdSO4·8H2O and 120 ppm TeO2.
Temperature was constant at 80 oC and electrolytic bath was atconstant stirring speed of (70-80) rpm (rotations per minute), pH was at 2. Inelectrochemical cell Platinum was anode saturated calomel as referenceelectrode and CdS coated substrate was working electrode. Figure 3.
2. Apparatus of Electrodeposition The deposition rate is about 0.86?m/hour. The function of saturated calomel electrode is to check the depositionconditions for the CdTe deposition it was maintained at 0.58 to 0.
62 V. Electronic grade H2SO4 wasused to provide H+ ions during deposition 49. 3.6.2 Physical VaporDeposition It is the process in which material issublimated from solid or liquid source and condensed upon the substrate, mostlythe whole process is done in a vacuum. The deposition rate for physical vapordeposition is approximately from 1 to 10 nm per second. This depositiontechnique is used for the deposition of alloys, elements and compounds by thereactive deposition. In reactive deposition a gas environment is used which hasa reaction with the source material (depositing material) to form a compound,the gas environment may be a nitrogen gas etc.
The physical vapor deposition can be categorized as (i) Vacuumevaporation (ii) Sputterdeposition 50, 51 22.214.171.124Vacuum Evaporation In this process source material is evaporatedthermally inside the vacuum and its vapors are condensed at the substrate. Thevacuum has a benefit of controlling the contaminations and reducing the meltingtemperature of the source material. Vacuum also increases the mean free pathfor the motion of deposited species.
The vacuum required for deposition is from10-5 torr to 10-10 torr. Thermal evaporation is obtainedby thermal heating sources such as tungsten coils. The vacuum evaporationtechnique is used for fabrication of decorative coating, corrosion protectivecoatings and electrically conducting films 52.
126.96.36.199Sputter Deposition The sputter deposition is done by physicalsputtering.
It is the process in which material is not heated thermally so itis a non vaporizing process. Material from target is ejected and then depositsonto a substrate. When energetic particles like ions are bombarded on thesputtering target a plume of material is released and deposits onto a substratejust like a shower of sand when a golf ball lands in the bunker. The bombardingparticles are usually gaseous ions accelerated from plasma. The sputtering gasis often an inert gas like Argon gas. The spacing between source and substrateis less than vacuum deposition. The plasma pressure for sputter deposition isfrom 5 mtorr to 30 mtorr. The sputtered particles are thermalized by gas phasecollision before they reach substrate surface.
3.6.3 Atomic Layer Epitaxy Atomic layer epitaxy(ALE) or Atomic Layer Chemical vapor deposition (ALCVD) now a days it is known as Atomic LayerDeposition (ALD) is a technique used forthe production of high quality, thin, solid films of specific crystalstructures or orientations. This method provides very fine control of filmthicknesses to one atomic layer. It has a wide range of applications in areassuch as thin film ceramics, gas sensors, radiation detectors, optical/infraredfilters, surface hardening and fiber optical materials. The term epitaxy comes from the Greek roots, meaning of word epi, is “above”, and meaningof taxis,meaning “in ordered manner”.
It can be translated “to arrangeupon”. It is a method that is based on sequential use of gas phase chemicalprocess. Mostly in (ALE) two chemicals are used called precursors. Chemicalreaction takes place between these precursors with surface one at a time insequential manner. Using ALE alternating monolayers of two different elementscan be deposited onto a substrate. Dr. Tuomo Suntola inventedthis technique in 1977, at the University of Helsinki in Finland. In thistechnique material amount deposited in each cycle is constant.
This methodprovides crystalline and uniform films especially if very thin film isrequired. Laboratory of advanced energy systems, Finland used ALE to grow bothCdS and CdTe layers in a single process. ALE reactor with four individuallycontrolled sources to control inside solid reactants, and has number of sourcesfor external inert gas flow and liquid reactants.
The spacing between two substrates is 1-3 mm. CdS films were deposited by using elemental Cd and S powders asreactants. Temperature is (300-500) oC. Temperature for CdTe is (350-400) oC. For the fabricationof CdTe elemental Cd and Te is used.
All layers were deposited in one run atthe same reaction temperature 53. 3.6.4 Molecular Beam Epitaxy It is a fabricationof crystalline thin film epitaxially above the other crystalline substrate withthe beam of molecules or atoms. Molecular beam epitaxy (MBE) method wasinvented by J.
R Arthur and Alfred Y.Cho in 1960s at Bell Telephone LaboratoriesUSA. This method required an ultra high vacuum up to(10-8 pa) for the film deposition. The rate of deposition is very slowwhich is 1 ?m/hr. The materials are evaporated and reach the substrateindividually then on the wafer reaction take place between these vapors. Thismethod can give high-purity epitaxial layers of compound semiconductors.
The word “beam” shows thatsublimated atoms of the material do not interact with each other and withvacuum chamber until reach the wafer, due to long mean free paths of atoms Ifcrystalline film is fabricated on the substrate of the same material is calledHomoepitaxy. So this epitaxy is done with only single material. Heteroepitaxy is performed withdifferent materials. In Heteroepitaxy crystalline thin film is fabricated onthe substrate of a different material. Example gallium nitride (GaN) onSapphire, aluminium gallium indium phosphide (AlGaInP) on Galium arsenide(GaAs). The disadvantage of this methodis that it is very expensive. In MBE technique vapor phase, liquid phase and solidphase methods can be used 54. 3.
6.5 Close SpacedSublimation It is a process for a thin filmdeposition of materials in a vacuum. The material is sublimated by heating andits vapors condensed onto a substrate which is placed above the sourcematerial. The basic phenomenon of thin film deposition based on dissociation athigh temperature. CdTe ? Cd+ Te Before the fabrication of thin filmby close spaced sublimation the substrate need a proper cleaning by Acetone,Isopropyl alcohol, rinse with distilled water and ultrasonic cleaning. Qualityof thin films, material transport and deposition rate depends upon thefollowing parameters 55, 56. 3.6.
5.1Source and Substrate Temperature It is observed that high substrate temperature of CdTe provides goodperformance of solar cell devices. Resistivity of CdTe decreases by increasingthe substrate temperature and grain size reduces by increasing substratetemperature. Deposition rate also improved at high source temperature 57. 188.8.131.52Spacing between Source and Substrate The spacing between source and substrate is inversely proportional tothe rate of deposition.
184.108.40.206 Vacuum Level Vacuum level for CdS and CdTe depositionis from 10-3 to 10 -5 mbar.
4Source Stoichiometry Compositionof material is an important for thin film quality. 220.127.116.11 Annealing Annealing of thefilms provides improvement of surface morphology and it reduces roughness ofsurface of CdS and CdTe films. Recombination centers reduce by annealing.Crystallinity of film also improved by annealing. Open circuit voltage alsoincreased by increasing annealing temperature.
Spectral response especially inthe range of 500 to 600 nm also improved by annealing temperature. Hence efficiency of solar cell can beimproved 58-60. 18.104.22.168Diffusion Theparticles movement from higher concentration to lower concentration is calleddiffusion. The distance particles can travel without any collision is calledmean free path.
Diffusion of one kind of particles into other kind of materialcan change its characteristics like a semiconductor material can be convertedinto N-Type or a P-Type. The mathematicaldescription can be explained by the Fick’s law Accordingto Fick’s first law j is the atomic flux per unit area per unit time, D is thediffusion co-efficient (dc/dx) is theconcentration gradient of the vapor 61. 3.7 Processing of CadmiumSulphide 3.
7.1 Chemical BathDeposition (CBD) It is veryeasy process for the fabrication of CdS thin films on ITO glass. For thefabrication 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 isvery suitable process especially for solar cell point of view. It is a process in which substrate is placedin a hot chemical solution stirring vigorously for specific time, positive andnegative ions will reach and meet on the substrate and thin film is grown.
Theadvantage of this technique is that neither vacuum and nor very hightemperature is required for CBD 62. 3.8 Fabrication of CdS ThinFilm by Chemical Bath Deposition The process is very simple, these are steps givenbelow 3.8.1 Cleaning Substrate is cleaned by using detergent then rinsed in distilled waterthen again clean with acetone and rinsed distilled water then clean withisopropyl alcohol (IPA). 3.
8.2Preparation of the chemical solutions The solution will be preparedin 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 Molmass)/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 making1000 ml solution (1.5X500X80)/1000=60 grams Mass of Thiourea (NH2)2CSrequired making 1000 ml solution (0.2 X 76 X 1000)/1000 = 15.
2 grams 3.8.3 Procedure The procedure is very simple for theCBD.
Clean beaker and substrate carefully Add chemicalsolutions 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 tothe beaker. The substrates are fixed inside the solution with the help of asubstrate holder.
The beaker is placed on the hot plate with magnetic stirrerinside. A pH meter and thermometer is also dipped in the solution to measure pHand temperature respectively. Provide heat to solution up to temperature of 75 oC.The solution is provided stirring continuously throughout the experiment.
Whentemperature reached at 75 oC add the thiourea about 0.2M (80 ml). As thiourea is added to the solution thereaction starts suddenly. So thiourea is the last component which is added tothe solution.
So deposition of CdS film starts when the thiourea is added.Films are retired from the solution according to the specific deposition timeand rinsed immediately with distilled water into an ultrasonic cleaner. Figure 3.3.CBD Apparatus The CdS deposited films withpale-yellow color are obtained. The CdS film is deposited on both sides of theITO 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 cleaningbecause drops of HCl can remove film from ITO side as well. After depositionthese CdS films will be annealed at 400 oC for 30 minutes 63, 64.
CdS23 60 2.29 CdS 24 127 2.36 CdS 25 170 2.66 CdS 20 257 2.39 Table 4.4 Thickness and energy gaps of CdSCBD