Preparation of Semiconducting Thin Films

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  • Topic: Semiconductor, Band gap, Photon
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  • Published : October 8, 2008
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Abstract:

One is to determine the gap energy of cadmium sulfide semiconductor thin film using the equation: Eg = h.c/
is the signal of the long wavelength limit of the absorption peaks (measured in the lab). The reaction between cadmium sulfide and NH3, in the presents of thiourea yield a  of 579(nm). The % error of the reaction is 1.24%.

Introduction:

There are 3 types of metals for electricity conducting: metallic conductor, semiconductor, and superconductor. Metallic conductors allow the free flow of ions and electrons through a sample; and its conductivity decreases as the temperature increases. Semiconductors allow the flow of both ions and electrons through the sample but not completely free; and its conductivity increases with increasing temperature. Superconductors are a class of metallic conductor whose resistance drops to zero suddenly below a certain critical temperature.

The energy levels in semiconductors are divided into two bands, valance bands and conduction bands. At absolute zero, the semiconductors’ valance bands are fully filled with electrons and partially filled with electrons in the conduction bands. For the conductors, the valance bands are partially filled with electrons and completely filled with them in the conduction bands. A band gap is the value of the energy for which there is no molecular orbital (the gap size between the valance and conduction bands). Electrons are the available conduction carriers which can get excited and go cross the band gap if they have enough thermal energy. The semiconductors’ band gaps are usually thinner than the conducting ones.

In this experiment, one is to prepare a cadmium sulphide semiconducting film on the glass slides. The transition of electrons from the valence band to the conduction band can be achieved by the absorption of electromagnetic radiation, if this energy is equal to or larger than Eg. The excited electrons reaching its new energy level can absorb light according to the equation: hc /  >= Eg (indicates the absorption of light).

Results:

Table 1: Cadmium Sulfide Semiconductor Thin Film Experimental Results compared to Theoretical Result Semiconductor filmsWavelengths(nm)Eg (eV)%error
Cadmium sulfide Theoretical5132.42
Cadmium sulfide Experimental5072.451.24

Reaction equation of Cadmium sulfide semiconductor:

CH4N2S+H2O+CdSO4+2NH3 CH4N2O+CdS+(NH2)2SO4

Sample calculations:

Eg theoretical is 2.42eV

From the equation: E = h.c / where measured experimentally was 507 nm

Eg = (6.63x10-34 J.s) * (3.00x108 m/s) / (507×10-9 m)
Eg = 3.92×10-19J
Knowing that 1eV = 1.602×10-19 J
Eg = 2.45 eV

%error = (2.42eV-2.45eV)/2.42eV*100% = 1.24%

Discussion:

Preparing thin film semiconductor using cadmium sulphide requires the mix of cadmium sulphate, thiourea, and NH3 in hot water bath held at 80oC for 15 minutes. The solution was under constant stirring. A bright orange-yellow precipitate was observed on half of the glass side. The slide was rinsed with deionized water and then air dried. The obtained wavelength for the reaction was 507 nm from the absorption spectrum. Preparation of zinc sulphide and bismuth sulphide semiconducting films were not required in this experiment.

Questions:

1.What is the purpose of using thiourea of thioacetamide? Write balanced equations for the reactions that occurs when aqueous NH3 is added to them, and the mixture is heated.

To form zinc and cadmium sulfides, thiourea was used. The formation of the bonds between zinc & cadmium atoms and the sulfur atom is through the production of urea, the sulfur atom will be replaced by oxygen atom (aqueous solution, water).

Reactions:

CH4N2S+H2O+ZnSO4+2NH3 CH4N2O+ ZnS+(NH4)2SO4

CH4N2S+H2O+CdSO4+2NH3 CH4N2O+CdS+(NH2)2SO4

2.ZnS, CdS and Bi2S3 films absorb in the range of 350, 500 and 950 (nm), respectively....
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