s. kugler: lectures on amorhous semiconductors 1 preparation

S. Kugler: Lectures on Amorhous SemiconS. Kugler: Lectures on Amorhous Semiconductorsductors

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PreparatioPreparationn


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There are a lot of different methods There are a lot of different methods that can be used to prepare that can be used to prepare materials in an amorphous state. materials in an amorphous state.

One lab. one preparation method. One lab. one preparation method.

Vapour, liquid and even solid can be Vapour, liquid and even solid can be the starting raw materials.the starting raw materials.

Do not forget: amorphous phase is Do not forget: amorphous phase is not a thermodynamically equilibrium not a thermodynamically equilibrium phase.phase.


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Gas to solid Gas to solid preparationpreparation

Thermal evaporationThermal evaporation This technique is possible the most widely This technique is possible the most widely used method for producing amorphous thin used method for producing amorphous thin film.film.

The staring compound is vaporized The staring compound is vaporized inin vacuumvacuum (order of 10 (order of 10- 6- 6 Torr) and the Torr) and the material is collected on a substrate. material is collected on a substrate.

The starting compound in high melting The starting compound in high melting point metallic “boat” is heated. point metallic “boat” is heated.


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SputteringSputtering

It consists of the bombardment of a It consists of the bombardment of a target by energetic ions from a low target by energetic ions from a low pressure plasma, causing erosion of pressure plasma, causing erosion of material. The radio frequency (at material. The radio frequency (at typically 13 MHz) is applied between typically 13 MHz) is applied between the target and the substrate the target and the substrate electrode. Usual sputtering gas is Ar. electrode. Usual sputtering gas is Ar. Sputtered atoms are deposited on a Sputtered atoms are deposited on a substrate forming thin film. substrate forming thin film.


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Chemical Vapour DepositionChemical Vapour Deposition

Chemical vapour deposition uses a Chemical vapour deposition uses a chemical reaction whose energy is chemical reaction whose energy is supplied from supplied from

thermal energy (thermal CVD), thermal energy (thermal CVD), optical energy (photo-CVD) and optical energy (photo-CVD) and plasma processes (plasma CVD).plasma processes (plasma CVD).


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Thermal CVDThermal CVD

For example: a-Si:H preparation.For example: a-Si:H preparation.

Silane (SiHSilane (SiH44) gas diluted with 99% H) gas diluted with 99% H2 2 is is thermally decomposed at a thermally decomposed at a temperature between 500 and 650 temperature between 500 and 650 ooC. C.

SiHSiH4 4 (gas) (gas) heat heat Si (solid) + 2 HSi (solid) + 2 H22(gas)(gas)

Recently, a new version (hot wire CVD) Recently, a new version (hot wire CVD) using heated tungsten filament at 1500-using heated tungsten filament at 1500-1900 1900 ooC has been developed which C has been developed which provides high quality samples. provides high quality samples.


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Photo CVDPhoto CVD

Photo-CVD employs photochemical Photo-CVD employs photochemical decomposition of SiHdecomposition of SiH44. Light source . Light source are, for instance, mercury lamp or are, for instance, mercury lamp or laser. laser.

Plasma CVDPlasma CVD

Gases are dissociated by r.f. or dc Gases are dissociated by r.f. or dc glow discharge and atoms deposited glow discharge and atoms deposited on the substrate. on the substrate.


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Liquid to solid Liquid to solid preparationpreparation

If a liquid (melt) could be cooled If a liquid (melt) could be cooled sufficiently quickly (i.e. quenched) so sufficiently quickly (i.e. quenched) so that crystallization could be bypassed, that crystallization could be bypassed, than the disordered structure could than the disordered structure could be frozen-in. This disordered be frozen-in. This disordered condensed phase is known as a glass. condensed phase is known as a glass. Such a glass-forming process involves Such a glass-forming process involves supercooling of a liquid below its supercooling of a liquid below its normal freezing point.normal freezing point.


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Definition: amorphous Definition: amorphous materials can be prepared materials can be prepared only by gas to solid only by gas to solid preparation methods while preparation methods while glasses can be prepared by glasses can be prepared by gas to solid and by liquid to gas to solid and by liquid to solid preparation methods.solid preparation methods.

This is also a common, everyday This is also a common, everyday used definition.used definition.


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Crystal to amorphous Crystal to amorphous transitiontransition

Irradiation Irradiation Implantation Implantation Pressure induced amorphizationPressure induced amorphizationetc.etc.

s. kugler: lectures on amorhous semiconductors 1 preparation

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