QuantumATK Forum
QuantumATK => General Questions and Answers => Topic started by: pippin on March 9, 2010, 06:47
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A real-world device almost always has metal electrode. But if I am just interested in bulk transport, can I set my electrode material in DeviceConfiguration to be the same as my bulk material? Or, must I have some kind of metal material for my electrodes?
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Your electrode can be anything. Even a semiconductor.
In VNL 2010.02 there is a custom builder for setting up an "ideal bulk" device configuration, which is perfect for understanding transport properties of a bulk material.
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Do note, however, that converging a system with semiconducting electrodes might be harder.
Also, people often try "the simplest thing imaginable", which might be a perfect 1D chain of metal atoms, for instance. This has a very simple analytic solution (transmission T(E)=the number of bands crossing energy E), but for various reasons it's not a very simple system to compute using ATK, esp. at finite bias. Essentially, the ATK algorithms are designed to work best when there is a clearly defined place in the structure for the applied bias voltage to drop, i.e. there should be some resistance.
This is particularly noticeable for 3D systems, if one tries to compute the transmission spectrum of bulk gold, for instance. In fact, "conductance" in the Landauer picture (which is what ATK calculates) is not a well-defined quantity for a macroscopic bulk conductor, but rather it applies to some "quantum point contact", which should be understood in a broad sense but still clearly is related to microscopic (or nanoscopic) geometrical features.
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O.K. Suppose that the calculation with two semiconductor electrodes managed to converge. Also, suppose that the subsequent transmission calculations also converged. Are the resulting I-V (or conductance) characteristics not very well defined? I guess I am a bit confused. Could you clarify what I can and cannot draw from these transmission calculations if I have semiconductor (or insulator) electrodes as opposed to metal electrodes? Thanks!
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There are no difference between metallic electrodes and semi-conducting electrodes in the terms of how to understand and perform the analysis for your result.
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Thanks! One more small question, please. If I have semiconductor electrodes and if I apply a bias between them, is the Fermi level within (not between) the electrodes assume to be constant?
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It depends on what you mean by constant :)
If you apply a bias, the fermi level of the electrode is shifted by the applied voltage. This is the same as for metallic electrodes.
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Another way to ask is if the bias is applied between 1) the right side of the left electrode and the left side of the right electrode or 2) the left side of the left electrode and the right side of the right electrode...
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In 2008.10, the bias is applied between the "outer" edges of the electrodes.
In 2010.xx, the bias is applied across the central region.