[ODCAD] Cu(TCNQ) as active Layer in Memory Device

Cu(TCNQ) is organometallic material. Organic compound TCNQ has CN group that is good electron acceptor, and form charge transfer complex (CT) with Cu.  There is a set of this type material. They are Cu(TCNE), Cu(TCNAP), Cu(DM-TCNQI).   The conductivity of the material can be manipulated by applying voltage. A structure Cu/Cu(TCNQ)(5~10um)/Cr can be switched from off to On with conductivity change over 100 times when V>Vt~3V. This switch is independent of voltage polarity.  The "On" state stays even after removing  the field.  This memory effect has data retention up to days.  The "on" state can be switched "off" after applying opposite voltage on the device. There are two models to descibe the switching mechanism. Model 1 is Field Induced Electrochemical Reaction.     Cu(TCNQ)  <=> Cu + TCNQ Off state has more Cu(TCNQ) that is thermodynamically more favored.  With the help of field, Cu gains electron back and the mixture Cu+TCNQ is more conductive. This state is less stable and can change back to Cu(TCNQ) either after certain time or with opposite field. A  recent work [1] suggests that the switch is due to Phase change.  CU(TCNQ has two phases. Phase 1 has Cu bonded with two sets of TCNQ in two perpendicular planes.   Phase II has Cu bonded with all TCNQ in almost on teh same plane. Therefore, this allows more dilocalized space for conductive electrone.  Concequently, Phase II has higher conductivity.  This work reports that the phase II has conductivity upto 1000 S/cm, and Phase 1 has conductivity 1.0E-5 S/cm.  The phase I is more stable and can be switched to phase II by external field, and this state can be changed back by opposite field.   1. R. A. Heintz, H. Zhao, X. Ouyang, G. Grandinetti, J. Cowen, and K. R. Dunbar; Inorg. Chem.; 38, 144 (1999) 2. R. S. Potember, T. O. Poehler; Semiconducting Organic Thin Films; 17, 233 (1982) 3. R. S. Potember, T. O. Poehler; Appl. Phys. Lett.; 34, 405 (1979)

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[ODCAD] Sharp-Blue Laser Diode

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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[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 www.mrs.org/publications/bulletin has a
detailed discussion about the electronic materials and its
application in hi tech.

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[ODCAD] Organic Semicondutors and Devices

Organic material from small molecule like Alq3 to large molecule like conjugated polymer are semi conductor. They have advantage of low cost, low energy consumption, unique electric and optical properties. This type of material can be used to make Diode, Memory, Transistor, and Display device (OLED) etc.

The reader can visit the electronic device group to get more articles.

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[ODCAD] Mobility Effect :Junction of Organic Semiconductor, Electrode
The charge carrier mobility 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.


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