Monday, June 28, 2004

[ODCAD] Spin on inorganic semiconductor

Did I make mistake in typing "inorganic"? No, it is true. Spin techlogy is usually regarded as a cheap method to deposite organic material because organic material can be dissovled in popular solvent. Si based inorganic semiconductor is covalent bond, and it has very low slubility in virtually any solvent. However, some inorganic material such as chalcoginide has relatively weak bond compared with Si. IBM's T.J. Watson Resaerch Center has discovered that this type of material may be dissolved in solvent hydrazine (N2H4) with other adding chemical. for example, Sn(S)y(Se)x can be dissovleved when extra solfur is added. The solution then is spin on substrated. After baking, the solvent and extra sulfur is evaporated. This results in very thin semiconductor Sn(S)y(Se)x.

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Friday, June 25, 2004

[ODCAD] Sharp-Blue Laser Diodes

Blue-violet Laser Diode is one of the components used in next generation of DVD. This high density (27 gigabits/disc) DVD requires Blue laser light to record digital information. The lab of Sharp in UK has developed a method to make this diode. The material is Indium-Gallium-Nitride (InGaN). This lab used a technique of molecular beam epitaxy (MBE). Sharp has already used this technique to manufacture Red laser diodes. This new method can compete with the others[1] that have been protected by patents.

1. Blue-laser using metal organic chemical vapor deposition developed by Shuji Nakamura, Nichia Corp, Tokushima, Japan

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Wednesday, June 16, 2004

[ODCAD]PEDOT-Various Electronic Structure

PEDOT is commercial name (trade name) for mixture of organic semiconductor PEDT with dopant PSS. Polystyrene Sulfonic acid (PSS) functions as dopant that oxidizes Polyethylenedioxy-thiophene (PEDT) resulting in PEDT+ as p-type doped semiconductor.

Hewlett-Packard (HP) and Princeton university are working together to develop WORM memory with PEDOT material. They found[1] that doped material PEDT+ has energy levels with LUMO=4.4eV, and HOMO=5.0eV. These structure allows hole to be injected easily from ITO (work function=4.8eV) or other high work function material such as Au (work function=5.0eV). Therefore, device ITO/PEDOT/AU is conductive because PEDT+ has high hole concentration, and the hole injection energy barrier is trivial. The resistivity of the device is about 20 ohm/sq.

This doping reaction between PSS and PEDT can be reversed when the external field is applied.
Electrical energy + PEDT(+) + PSS(-) > PEDT + PSS
The neutral PEDT in this resulting mixture PEDT+PSS has different energy levels with LUMO=3.9 and HOMO=5.4V. This results in much higher energy barrier for hole injection from anode. Also, the reduced PEDT has much few hole charge carrier. Therefore, the device ITO/PEDOT/Au has much low conductivity because theses two effects.

1. S. Moller, S. R. Forrest, C. Perlov, W. Jackson, C. Taussig; J. Appl. Phys.; 94, 7811(2003)

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