THE STUDY OF STRUCTURAL AND ELECTRICAL PROPERTIES OF LEAD SULPHIDE (PbS) THIN FILM DEPOSITED THROUGH CHEMICAL BATH DEPOSITION

THE STUDY OF STRUCTURAL AND ELECTRICAL PROPERTIES OF LEAD SULPHIDE (PbS) THIN FILM DEPOSITED THROUGH CHEMICAL BATH DEPOSITION

ABSTRACT

Lead sulfide (PbS) thin films were prepared on glass substrates at room temperature by chemical bath deposition (CBD) technique. The influence of dip times on the lead sulphide (PbS) thin films deposited on glass slide substrates via chemical bath deposition (CBD) technique using chemical precursors, nitrate Pb(NO3)2, and thiourea SC(NH2)2, and sodium hydroxide (NaOH) at room temperature was investigated. The thicknesses of the layers were measured as the structural property, the thickness of , and were obtained for the time deposits of 60, 80, and 100 minutes respectively. The charge carrier mobility μ was studied as the electrical properties and the charge carrier mobility (μ) of , and for the time deposits of 60, 80 and 100 minutes respectively.

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 I have 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

that the ultimate lower limit of the cost of bulk devices is defined by the cost of the wafer itself and thus cost lowering below the price of wafers is thus not possible. Hence, even for Si, a thin film technology needs to be developed to meet the cost goals (Cosmas, 1989).

Apart from saving cost as given above conversion efficiencies of these cells have been improved upon. In recent years, efficiencies of 10 to 17% or more in crystallized and/or epitaxial thin Si and GaAs solar cells respectively, have been made possible.

In comparison to thin film CuS/CdS cells for example, the production of bulk Si (primarily single crystal) solar cells is in the range of 5MW internationally for large scale applications, Si production capability does not exist and the trend today is toward ribbon technology.

However, single crystal ribbon technology is energy intensive and costly and requires a large material input. Although the scope for immediate application exists, the long range potential is low. On the other hand, thin film a Silicon solar cells offer a very promising future, at present Sanyo, Fuji, and Sharp of Japan are producing 2 to 3 MW/year of such cell for low power electronic applications.

The simple production involved and the simple module/panel fabrication make thin film solar cells the only viable system. To further stress the importance, I quote Ehrenrich, “the potential payoff would be immense if truly inexpensive technology based on thin film system were to be developed” (Cosmas, 1989).

Finally, the Pbs thin film is a p-type semiconductor with a crystal structure of centered cubic structure, looking at the uses, benefits and applications of the thin films in different areas of technology to be given shortly, one does not have any other alternative for technological breakthrough than the „thin film technology'.

1.2 Applications of Thin Films.

Metallic films are used as electrical conductors in numerous micro-electronic devices. For most applications of silicon semiconductor devices the conducting films used (usually referred to as metallization) are 1-2μm thick. The films can be as narrow as 1 or 2μm but occasionally they are several millimeters wide.

Thin films can also be used in information recording systems based on light or electron recording. The recorded data appears as a change in electrical or optical properties of the film. Reflective films have been used in photo-plastic recording. This is a method of recording light images in the form of surface deformations in a photoconductive thermoplastic film. Also ferroelectric films are used in display devices like the microwave capacitors, thermistors, bolometer, pyro-electrics, piezoelectric transducers, and optical display devices.

Thin films of silver and lead halides as well as arsenic sulphide are made use of as photographic information storage devices.

It is easy to carry out absorption, reflection, emission, and generally optical studies for visible or near ultraviolet radiation because of availability of good filters, compensators, or transmission polarizers. Studies below the 2000Ȧ wavelength are not possible because of lack of equivalent filters, compensators, and polarizers but the use of thin film in ultraviolet spectroscopy eliminates some of the difficulties encountered in determining the optical constants of materials. With the thin films, the design of good filters, polarizers, and compensators is possible for optical studies under high intensity synchrotron radiation and for the use of the hydrogen laser which operates at 1600Ȧ by solid state physicists and biophysicists i n research centers and laboratories.

A practical application of Yttria (Y203) films as a dielectric for thin film capacitors has been reported from Japan. Dielectric thin films have attracted attention as convenient media for

surface propagation of optical waves, the technique of which is used in two dimensional integrated optical circuits like in the two dimensional processors. Wave guide characteristics of the films (refractive index and attenuation) permit coloration between the optical and electrical properties of thin film to be made.

It is very interesting and exciting to observe that superconducting transition temperatures, of some materials are raised after depositing these materials on substrates. For instance the transition temperatures of Cd and Zn have been raised from 0.3K to 0.9K, and 0.8K to 1.5K respectively. The for Cd condensed on a substrate held at that 0.3K is almost twice the one of the bulk Cd which is 0.53K. Though the increase in the is small, it can be noted that if efforts by solid state physicists are geared towards this direction, definitely there should be a remarkable breakthrough in superconductivity studies in no distant future. Then the problem of electric power crisis will have been solved and man will therefore live to enjoy.

TOPIC: THE STUDY OF STRUCTURAL AND ELECTRICAL PROPERTIES OF LEAD SULPHIDE (PbS) THIN FILM DEPOSITED THROUGH CHEMICAL BATH DEPOSITION

Chapters: 1 - 5
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Number of Pages: 58

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