S. Kugler: Lectures on Amorhous Semiconductors 1 Preparation

2 There are a lot of different methods that can be used to prepare materials in an amorphous state. There are a lot of different methods that can be used to prepare materials in an amorphous state. One lab. one preparation method. One lab. one preparation method. Vapour, liquid and even solid can be the starting raw materials. Vapour, liquid and even solid can be the starting raw materials. Do not forget: amorphous phase is not a thermodynamically equilibrium phase. Do not forget: amorphous phase is not a thermodynamically equilibrium phase.

S. Kugler: Lectures on Amorhous Semiconductors 3 Gas to solid preparation Thermal evaporation This technique is possible the most widely used method for producing amorphous thin film. This technique is possible the most widely used method for producing amorphous thin film. The staring compound is vaporized in vacuum (order of Torr) and the material is collected on a substrate. The staring compound is vaporized in vacuum (order of Torr) and the material is collected on a substrate. The starting compound in high melting point metallic “boat” is heated. The starting compound in high melting point metallic “boat” is heated.

S. Kugler: Lectures on Amorhous Semiconductors 4

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

S. Kugler: Lectures on Amorhous Semiconductors 6 Chemical Vapour Deposition Chemical vapour deposition uses a chemical reaction whose energy is supplied from Chemical vapour deposition uses a chemical reaction whose energy is 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).

S. Kugler: Lectures on Amorhous Semiconductors 7 Thermal CVD For example: a-Si:H preparation. For example: a-Si:H preparation. Silane (SiH 4 ) gas diluted with 99% H 2 is thermally decomposed at a temperature between 500 and 650 o C. Silane (SiH 4 ) gas diluted with 99% H 2 is thermally decomposed at a temperature between 500 and 650 o C. SiH 4 (gas) heat Si (solid) + 2 H 2 (gas) SiH 4 (gas) heat Si (solid) + 2 H 2 (gas) Recently, a new version (hot wire CVD) using heated tungsten filament at o C has been developed which provides high quality samples. Recently, a new version (hot wire CVD) using heated tungsten filament at o C has been developed which provides high quality samples.

S. Kugler: Lectures on Amorhous Semiconductors 8 Photo CVD Photo-CVD employs photochemical decomposition of SiH 4. Light source are, for instance, mercury lamp or laser. Photo-CVD employs photochemical decomposition of SiH 4. Light source are, for instance, mercury lamp or laser. Plasma CVD Gases are dissociated by r.f. or dc glow discharge and atoms deposited on the substrate. Gases are dissociated by r.f. or dc glow discharge and atoms deposited on the substrate.

S. Kugler: Lectures on Amorhous Semiconductors 9 Liquid to solid preparation If a liquid (melt) could be cooled sufficiently quickly (i.e. quenched) so that crystallization could be bypassed, than the disordered structure could be frozen-in. This disordered condensed phase is known as a glass. Such a glass-forming process involves supercooling of a liquid below its normal freezing point. If a liquid (melt) could be cooled sufficiently quickly (i.e. quenched) so that crystallization could be bypassed, than the disordered structure could be frozen-in. This disordered condensed phase is known as a glass. Such a glass-forming process involves supercooling of a liquid below its normal freezing point.

S. Kugler: Lectures on Amorhous Semiconductors 10 Definition: amorphous materials can be prepared only by gas to solid preparation methods while glasses can be prepared by gas to solid and by liquid to solid preparation methods. Definition: amorphous materials can be prepared only by gas to solid preparation methods while glasses can be prepared by gas to solid and by liquid to solid preparation methods. This is also a common, everyday used definition. This is also a common, everyday used definition.

S. Kugler: Lectures on Amorhous Semiconductors 11 Crystal to amorphous transition Irradiation Irradiation Implantation Implantation Pressure induced amorphization Pressure induced amorphization etc. etc.