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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Tuesday, May 18, 2004

[ODCAD] Junction and Vacuum Level in Organic Devices

[ODCAD] Junction and Vacuum Level in Organic Devices
The electrical performance of a device is effected by the junctions present. The energy barrier is a major factor to consider in modeling the junction. In device physics, the vacuum level is regarded as the common reference point to calculate the energy barrier. In inorganic semiconductors based on Si material, it is always regarded as truth that the vacuum level is the same for all of contacting layers (materials). This is called Vacuum level alignment.

In organic devices, it is common that function layer is organic semiconductor, and electrode layers are inorganic materials. Scientists found that this vacuum level alignment may not be applicable in the junction between organic layer and inorganic layer. For example, the junctions Ag/Alq3, Ag/Almq3 have been observed that the vacuum levels are not aligned for difference about 1.1 eV [1].

This non aligned vacuum level can happen for system with or without chemical bond, and doped and undoped. It means that it is not due to chemical reaction in the junction. Scientists explained that there is dipole behaving like internal field across the junction. The source of the dipole is due to significant difference between two materials. The molecules in the junction try to arrange their position to have minimum free energy of the whole system. This junction and the dipole are different from p-n or Schottky junction in terms of size and field. They will be discussed in the other topic.

The consequence of different vacuum levels is critical for the energy barrier. For example, the vacuum level of organic layer is usually reduced by a amount (say 0.5 eV). For hole injection from the junction into the organic layer, the energy barrier is increased by that amount, while the energy barrier is reduced by that amount for electron injection. That amount of energy barrier change can result in significant effect to electrical performance of the device.

1. I. G. Hill, Appl. Phys., Vol. 84, 3236 (1998)

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Monday, May 17, 2004

[ODCAD] US Defence Dept.- Organic Photovotaics (PV)

Effective solar cell device definitly has great application in any electronic products that require power supply. Inorganic semiconductors like Si currently rule the market. Organic materials potentially have advantage of smaller size (thinner), light weight, effcient, flexible (bend without breaking), and low cost. Many experts, and labs bet the future of photovotaics (PV) on organic materials.

US Dept. of Defence may be the biggest investor in this technology. Lynne Samuelson, a researcher at the US Army's Natick Soldier Center, in Massachusetts, claimed that his lab is starting to make proto-type PV devices to try out in the field. That Lab teamed up with a chemistry lab at Univ. of Massachusetts Lowell. The structure and materials used in their cell may contain Titania (? it may mean Ti)-TiO2 particles/Polymer. Titania particle can be as small as 20nm. The polymer is polyethylene terephthalate. Its effciency has not been disclosed. It is estimated that it may be >4%. This lab is expecting 20% effciency in five years.

Part of Information is from Spectrum of IEEE.
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Monday, May 03, 2004

[ODCAD] Coming Events in May, 2004

May 4, Singapor: Semicon 2004
Location: Singapore International Convention and Exhibition Centre (SICEC)
When :May 4-6, 2004
More Info:

May 4 US: The 15th Annual IEEE/SEMI Advanced Semiconductor
Manufacturing Conference
Location: Seaport Hotel,Boston, Massachusetts, USA
Mre Infor:

Ma 7 Bay Area, CA, US:IEEE EDS/SCV Evening Seminar in Bay area, CA
Location: National Semiconductor, Building 31, 955 Kifer Rd.
Sunnyvale, CA
Topic: Compact Modeling
When: 6-8 PM
Admission: Free

May 17 NY, US: Nanobusiness Conference
Location: Marriott Financial Center, NY
More Info

May 23-28 Seattle, US: SID Display 2004
Location: Washtington Sate Convetion and Trade Center
More info :

May 25 Pennsylvania, US:Pennsylvania Nanotechnology Conference 2004
Location: The Pennsylvania Convention Center, Philadelphia, PA
More Info:

To check events in June or later, visit, chect its calendar, and
add interesting event to yours.

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Saturday, May 01, 2004

[ODCAD] Schottky Effect: Junction with Organic Semiconductor
Electric current vs voltage of an organic device such as OLED is important factor to decide the device performance. When field is at medium strength and the device is at junction control, one factor in device model is Schottky effect.

This effect reduces energy barrier for charge carrier injection from the electrode. This is due to the image force while charge carrier leaving electrode. The equation can be expressed as

[qE/(4 Pi e0)]^1/2 where e0 is dielectric constant of the semiconductor.

In the following discussion, we assume organic device has structure of Electrode/Organic semiconductor/electrode. Also, we assume one junction is ohmic.

In terms of equation, there is nothing new for organic semiconductor (compared with Si technology). A few points we have to pay attention when you use the model to describe your device behavior.

1. The derivation of the equation assumes that the field in the semiconductor is uniform. This may not be true if the organic material has high impurity such as ions that can redistribute under field effect. In such case, the field E may need to be carefully related with external electric voltage.

2. This is effect for junction control. Do not apply it when the device is at bulk control. It is easy to be confused with Poole Frenkel Model.

3. It is applicable when the field is relatively strong say E>10^3 V/cm, but not too strong say E~10^6V/cm. At very strong field, see if it is tunneling in control.

4. Compared with Si material, Schottky effect is much stronger for organic semiconductor. This is because the organic material has much smaller dielectric constant (ususally it is ~3,much less than ~11 of Si).

This article is from Organic Device group

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Tuesday, April 27, 2004

ODCAD] A play of Material in Next Century

In next century, what do we expect for the hi tech industry? The
industry from tele-comunication in wire or wire less, to hand held
electronic device. A lot of expectation is there. Well, one important
factor to meet the expectations is the electronic materials. Si based
technology may not meet all of the exepectation such as flxible
display device. Organic material, epecially polymer play important
role. A good article on has a
detailed discussion about the electronic materials and its
application in hi tech.

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Wednesday, April 14, 2004

[ODCAD] Mobility Effect :Junction of Organic Semiconductor, Electrode

The charge carrier mobility is critical information for the simulation (modeling) of electronic devices such as OLED, TFT etc. Its value of organic semiconductor is usually much lower than crystal Si material. This low mobility has impact to the transistor (see "Organic TFT Transistor: Interface and Performance"). Also, it has impact to the electrical behavior of the other device.

For a layered structure device, say a simple three layer DIODE device: bottom electrode, middle semiconductor, top electrode. Assume the bottom junction is ohmic, then the diode is due to the top junction. One popular approximate equation is Schottky junction model. The reversed current J0 measured for the junction is usually 6 order (or higher) less than what the model predicts (see "Reversed Current in Schottky Junction"). What is the reason to cause this?

There are quite few reasons for this. One important effect is due to the slow mobility of the charge carrier. A complete model considering charge injection and charge diffusion is Thermionic Emission-Diffusion model (Sze 2nd Edition). In this model, the mobility effect (drift velocity) is trivial if it is large enough compared with thermal charge carrier velocity. Otherwise, the injected current is proportion to the drift velocity that is the product of mobility and field. Dr. Scott from IBM lab in San Jose, CA) has done a set of experiments and the results have confirmed the mobility effect.

This does tell us that the current can be dependent on the mobility even it is at junction control. For device engineer, he (she) has to design (choose) the material to ensure the current obtained from the device can meet the requirements.

More related articles can be found in Electronic Device Group (click the link to join the group).

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Thursday, April 08, 2004

[ODCAD] Organic TFT Transistor: Interface and Performance

The transistor based on organic semiconductor may not be able to replace Si technology in fast response device product such as CPU. However, for many other applications such as in address cell circuit for display device such as OLED (PLED, LED), its performance is good enough.

The current from drain Id and transconductance gm are proportion to the charge carrier mobility. Both Id and gm are two important characters to show the performance of the device. Large Id means low resistance of the device, which result in small size of the device. Large gm means high amplifying capability that also result in the reduction of device size.

For organic semiconductor, its mobility is usually much smaller than crystal Si material. The best value obtained so far is 5 cm2/V-S (some lab claims that 10 cm2/V-s). This organic material is at crystal phase. For polymer semiconductor, the mobility is even lower. The best charge carrier mobility in polymer is 0.05 cm2/V-s.

The main reason for the low mobility of the organic semiconductor is due to lack of ordered material structure like Si crystal. For TFT transistor, the important area to decide the device performance is the interface between the dielectric and the organic semiconductor. It is about 10 A thickness of the interface whose charge carrier mobility is critical.

This gives a good task for engineers. They should design the method to manufacture the device in the way that can ensure highly ordered organic semiconductor at the junction (interface). For polymer semiconductor, the direction of the order (the conjugated bond direction) should be along the charge carrier transportation direction.

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Wednesday, April 07, 2004

All info is currently posted on Electronic Device Group. Click the link at the right to visite the site.