COMPLETE SCHOOL PROJECT TOPICS & MATERIALS :
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CHAPTER ONE
INTRODUCTION
1.1 Background of the Study
In view of the rapid depletion of existing conventional energy resources, due to the high rate of consumption the world over, it’s obvious that mankind is heading towards an energy crisis. Today energy consumption per capita is synonymous with the standard of living of a nation and raise world population, standard of living of developed and developing nation and the trend towards higher technologies on the increases. It is evident that our oil wells, the source of the world well-known major energy resources petroleum will sooner or later dry up. To avert this impending energy crisis and further meet up with energy needs of nations, renewable energy technological revolution is the only answer (Cosmas, 1989).
Renewable energy source that can be developed include non-exhaustible energy source such as sunlight, ocean currents, waves, falling water, wind natural energy stocks whose replenishment is far greater than projected human use. Of this wide range of energy choice, sunlight, or solar energy is certainly one of the most attractive. Since the solar energy is more abundant in the poorer and less developed countries of the world, it is hoped that if this particular source of energy is researched into a developed the bridging of the technological gap between the third world countries and most of the developing countries is then feasible.
Semiconductor materials are always the focus in material science due to their outstanding electronic and structural properties and have potential application in various devices such as light emitting diodes, single electron transistors, and field effect thin film transistors. In principle, the electronic and structural properties of semiconductor materials are tunable by varying their shapes and sizes. So it is one of the desired goals in material science to have precise control of the morphology of semiconductor materials. As an important IV-VI group semiconductor, Lead sulfide (PbS) has attracted considerable attention due to its small direct band-gap (0.41eV at 300K) and a large excitation Bohr radius of 18nm. Lead sulfide (PbS) is an important direct narrow gap semiconductor material with a band gap of 0.4eV and has a cubic structure. Due to their suitable band gaps, PbS thin films are extensively used in IR detectors. Thin film of lead sulfide was establish to have very significant application in the manufacture of photoconductive infrared detectors, transistors, contact rectifiers, prisons, lenses, windows and other components of optical system. This material has also been used in many fields such as humidity, photography, solar absorption photo-resistance, diode lasers, and temperature sensors, decorative and solar control coatings.
The chemical bath deposition (CBD) method is attracting considerable attention, as it does not require sophisticated instrumentation. It is relatively cheap, simple to handle, convenient for large area deposition and capable of yielding good quality thin films (Uhuegbu, 2011)
Thin films of PbS have been prepared with various physical and chemical thin film deposition techniques, such as chemical bath deposition (CBD), electro-deposition (ED), chemical spray deposition (CSP), successive Ionic layer adsorption and reaction (SILAR), electrochemical atomic layer epitaxy (EC-ALE), atomic layer decomposition (ALD) and thermal evaporation technique. Among these different techniques, bath technique is advantageous on account of suitable method in the country. It also allow a uniform wide area deposition (Ahuome and Onimsi, 2016). As earlier mentioned, the availability of raw materials is an important factor in fabricating solar cells for solar energy conversion. Thin film devices will typically be about 5µm to 50µm thick, in contrast to bulk devices which are about 150µm to 250µm thick, it shall be pointed out.
1.2 Statement of Problem
Compound semiconductors provide a large range of materials with varying band gaps for application in solid-state devices. Generally these devices are made in the form of thin films, whose crystalline quality and stoichiometry play a key role in the stability and the performing characteristics of the device. Vacuum deposition of thin films is the most popular and reliable method of preparing thin films. However, it is a general problem that the compound semiconductors thermally dissociate into their constituent components during evaporation.
Among semiconducting materials, the II-VI sulphide semiconductors are of considerable interest for their efficient use in the fabrication of solid state devices. The structural, electrical and optical properties of vacuum deposited thin films of sulphide semiconductors are very sensitive to the deposition conditions [65]. The problem associated with thermal evaporation technique is maintaining the stoichiometry in the deposition of materials composed of elements having different vapor pressures such as Cd and S in CdS or Zn and S in ZnS [43,56]. On the basis of results reported by various researchers, it is observed that the vacuum evaporated thin films of compound sulphide semiconductors are in general non-stoichiometic and have deficiency of sulphur [65-68]. Stoichiometry can be restored by several techniques as by, co-deposition of sulphur together with CdS [66], annealing the film in CdS powder [67] and depositing the film in a controlled hydrogen sulphide atmosphere [68]. The properties of vacuum evaporated ZnS films deposited in H2S ambient have been reported recently [52]. In this present research work, we proposed that the compensation of sulphur deficiency by exposing the film to a hydrogen sulphide (H2S) atmosphere during evaporation inside the vacuum chamber is a better, convenient and cost effectiveness method for depositing sulphide semiconductors. So, in this research work, we optimize an alternative preparation method, wherein good quality stoichiometric films of sulphide semiconductors were grown by using a low ambient H2S atmosphere inside the vacuum chamber while thermally evaporating the sulphide semiconductor materials. From the point of view of vacuum evaporation, the thermal decomposition of thiourea [CS(NH2)2] is a convenient and most suitable source of hydrogen sulphide (H2S) which can be controlled by regulating the temperature of the electrically heated borocil test tube in the evaporation chamber.
Thin films of sulphide semiconductors will be deposited by evaporation of powdered semiconductor material in vacuum. The sulphide ions dissociate during evaporation will be thermally ejected out from the surface of the substrate which be kept at an elevated temperature. At the same time a low ambient atmosphere of hydrogen sulphide gas will be maintained inside the vacuum chamber during the evaporation process, which will help the dissociated metallic ion of the compound semiconductor material readily recombine with the H2S molecules and get deposition on the substrate in the sulphide form.
1.3 Research Objectives
The specific objectives of this thesis, which we have investigated during the course of this research work, are as follows:
- Synthesis of thin films of sulphide semiconductors (CdS, ZnS and PbS) both without and with H2S by vacuum evaporation technique and understand the growth mechanism for the same.
- Investigate the effect of H2S ambient atmosphere on the growth and properties of sulphide semiconductors thin films.