ApplicationApplicationss

Until very recently silicate glasses Until very recently silicate glasses were the only type of materials were the only type of materials commonly used.commonly used.

AdvantagesAdvantages over their crystalline over their crystalline counterparts:counterparts:

Amorphous solids are relatively easy Amorphous solids are relatively easy to prepare i.e. large-area, to prepare i.e. large-area, homogeneous amorphous thin film homogeneous amorphous thin film can be prepared. For example a-Si:H can be prepared. For example a-Si:H for solar cells or thin-film transistors.for solar cells or thin-film transistors.

Near the glass transition Near the glass transition temperature, which is lower than temperature, which is lower than melting point, the materials remain melting point, the materials remain workable so that they can easily be workable so that they can easily be formed into various shapes.formed into various shapes.

Amorphous materials, particularly Amorphous materials, particularly bulk glasses are often structurally bulk glasses are often structurally homogeneous and isotropic and their homogeneous and isotropic and their physical properties are also physical properties are also homogeneous and isotropic, unlike homogeneous and isotropic, unlike crystalline materials.crystalline materials.

Various applicationsVarious applicationsBrief summaryBrief summary

Amorphous semiconductors are Amorphous semiconductors are promising electronic materials for promising electronic materials for wide range of applications such wide range of applications such as:as:

Solar cellSolar cellThin film transistors (TFT)Thin film transistors (TFT)Light sensorsLight sensorsOptical memory devicesOptical memory devicesElectro photographic applicationElectro photographic applicationX-ray image sensorsX-ray image sensorsEu-doped optical fiberEu-doped optical fiberDVD (digital video/versatile disc) DVD (digital video/versatile disc) Hard cover made from ta-C Hard cover made from ta-C

Various applicationsVarious applicationsDetailsDetails

Electro photographic applicationElectro photographic application:: one of the most common, everyday one of the most common, everyday used application is electro used application is electro photography or xerography (Greek photography or xerography (Greek word, meaning is “dry writing”).word, meaning is “dry writing”).

The first xerography was made by The first xerography was made by Carlson and Kornei in 1938(!) in Carlson and Kornei in 1938(!) in Astoria NY (USA).Astoria NY (USA).

The really first experiment was The really first experiment was made using sulfur, but later on made using sulfur, but later on Se was the basic material. Se was the basic material. Recently a-Si:H films have been Recently a-Si:H films have been utilized instead.utilized instead.

( I. Shimizu: 1985 J. Non-Cryst. ( I. Shimizu: 1985 J. Non-Cryst. Sol. Sol. 77-78,77-78, 1363 ). 1363 ).

Solar cells:Solar cells: Potentially the most Potentially the most important application of the important application of the amorphous semiconductors a-Si:H is amorphous semiconductors a-Si:H is in the direct conversion of sunlight to in the direct conversion of sunlight to electric power. electric power.

This is a cheaper raw material than This is a cheaper raw material than crystalline silicon. No structural crystalline silicon. No structural damage!damage!

For example: space shuttle use.For example: space shuttle use.

The conversation of solar light to The conversation of solar light to electric power is available electric power is available renewaable sources of energies.renewaable sources of energies.

The basic physical principle involved The basic physical principle involved is the absorption of photon resulting is the absorption of photon resulting in the creation of electron-hole pairs; in the creation of electron-hole pairs; the excess electrons in the the excess electrons in the conduction band, and holes in the conduction band, and holes in the valence band. valence band.

Internal junction field separates them Internal junction field separates them before recombination.before recombination.

There are several conditions that There are several conditions that a thin film solar cell must satisfy a thin film solar cell must satisfy in order to exhibit efficient in order to exhibit efficient photovoltaic energy conversion:photovoltaic energy conversion:

The optical absorption coefficient (The optical absorption coefficient (αα) ) must be large enoughmust be large enough

The photogenerated electrons and The photogenerated electrons and holes must be collected efficiently by holes must be collected efficiently by contacting electrodes on the both contacting electrodes on the both sides of the active film material sides of the active film material

Phase change memoryPhase change memory

BaseBase

crystalline form crystalline form

amorphous amorphous formform

rapidreversibletransition

Bit formationBit formation

~ 1 ns laser heating above melting ~ 1 ns laser heating above melting point, Tpoint, Tmm causes amorphous causes amorphous (polycrystalline) nanosize bit.(polycrystalline) nanosize bit.

~50 ns laser heating above glass ~50 ns laser heating above glass transition temperature, but below Ttransition temperature, but below Tm m

forms crytalline bit.forms crytalline bit.

RewriteableRewriteable

MonitoringMonitoring ResistivityResistivity Optical reflectivityOptical reflectivity

Phase change materials are the memoryPhase change materials are the memory materials of the future:materials of the future: 1. Fast (~ 10 ns)1. Fast (~ 10 ns) 2. Dense (bit diameter < 50 nm)2. Dense (bit diameter < 50 nm) 3. Stable (several years per lost bit)3. Stable (several years per lost bit) 4. Long-lived (> 104. Long-lived (> 101212 cycles per lost bit) cycles per lost bit) 5. Low manufactoring cost5. Low manufactoring cost 6. Low power consumption 6. Low power consumption

The endThe end