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ScienceWeek
SEMICONDUCTORS: ON SURFACE EFFECTS IN THIN SILICON FILMS
The following points are made by John J. Boland (Nature 2006 439:671):
1) Silicon-based electronics continues to obey the dictum known as Moore's law: that the density of transistors on an integrated circuit -- a rough measure of the attainable processing power --doubles about every 18 months. As the size of the smallest features of a device approaches the nanoscale, the electronic properties of the constituent materials are increasingly affected by the surrounding surfaces and interfaces. This can sometimes have deleterious effects on the transport of charge carriers --and so on device performance. New work[1] shows how surface effects can in fact be used to control conduction in silicon membranes. These results potentially provide a new route to nanoscale materials and devices.
2) The continued drive towards higher device densities on silicon chips requires not only lithographical techniques that can define ever-smaller device features, but also the ability to manipulate precisely the Fermi energy of electrons in silicon, conventionally achieved by the process of doping. But surfaces and interfaces are known to affect the electronic properties of silicon too. For instance, electrons from surface atoms can establish themselves within silicon's band gap, in which there are normally no allowed electronic states. There they can trap or deplete charge carriers, and so affect the position of the Fermi energy. Having the correct surface passivation chemistry -- that is, the right sort of chemical bonds on the surface to eliminate mid-gap states -- minimizes the effect of these rogue states. For silicon, an exceptionally low density of interface traps is achieved by an interface with an insulating layer of silicon oxide, SiO2. In this case, provided the dopant level is high enough, at most only a few silicon layers adjacent to the interface are depleted of carriers. The position of the Fermi energy is still controlled by the dopant concentration in the bulk of the material.
3) This interplay between doping and interface states is a fundamental aspect of device physics, and becomes even more important when small structures are involved[2]. Then, the lower total number of atoms, the increasing fraction of surface and interface atoms and the dopant population combine to provide an interesting mix. One possible consequence is fluctuation in the number of dopant atoms incorporated into the silicon, resulting in unacceptable variations in device performance across the wafer.
4) A possible solution to this problem is the development of high-resolution techniques that enable dopant ions to be implanted into the semiconductor material one by one[3]. But even if the dopant density could be controlled so precisely, the nanoscale membranes would eventually become so thin that even the low density of traps afforded by the SiO2 interface would deplete the device of carriers, rendering it unworkable. Such effects have long been regarded as a fundamental limitation on the minimum size and thickness of silicon-based devices. Zhang et al[1] use a combination of scanning tunnelling microscopy and modelling to show that this conventional analysis cannot fully explain electron transport in nanoscale silicon films. Using silicon-on-oxide membranes, already employed as the basis of low-capacitance, fast-switching devices, they demonstrate that transport is feasible even with very thin membranes, and for doping levels at which the Si-SiO2 interface effectively depletes all carriers within the semiconductor. Electronic conduction is seemingly, in this case, not controlled by dopants in the bulk of the solid.[4]
References:
1. Zhang, P. et al. Nature 439, 703-706 (2006)
2. Erwin, S. et al. Nature 436, 91-94 (2005)
3. Shinada, T. , Okamoto, S. , Kobayashi, T. & Ohdomari, I. Nature 437, 1128-1131 (2005)
4. Hamers, R. J. et al. Acc. Chem. Res. 33, 617-624 (2000)
Nature http://www.nature.com/nature
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Related Material:
MATERIALS SCIENCE: ON OXIDES AS SEMICONDUCTORS
The following points are made by J. Mannhart and D.G. Schlom (Nature 2004 430:620):
1) By 2007, the information age will have hit a fundamental roadblock. Without major changes in technology, the spectacular improvements in computer performance that we have enjoyed for decades will cease, because transistors based on silicon and silicon dioxide will no longer be able to keep up with Gordon Moore's famous law(1,2) -- that the number of transistors per unit area in an integrated circuit doubles every couple of years. But these limitations might be overcome if Si and SiO2 were complemented in these devices by other materials. The candidates of choice are oxides, which are already assuming a vital role in semiconductor electronics. Muller et al(3) have demonstrated that it is possible to control the electronic properties of these materials with the nanoscale precision necessary for the information industry.
2) Oxides offer a broad spectrum of properties -- some are excellent insulators, others are superconductors. Some oxides have flippable electric or magnetic dipoles, suggesting myriad device possibilities. Indeed, oxides such as hafnium dioxide are forecast to replace SiO2 in the transistors of laptop computers within only three years(1). Another oxide known as "Lustigem" --alias strontiun titanate (SrTiO3) -- was a popular diamond substitute in the 1960s. If some of its oxygen atoms are removed, the glittering gem turns a deep blue, and changes from insulating to conducting. This change in color and conductivity is due to electrons that are left behind: because there is a difference in charge between an oxygen ion (O2-) and an oxygen atom, for each oxygen atom removed two electrons are added to the SrTiO3 matrix. Oxygen vacancies thus function as electron-donating dopants -- an effect commonly achieved in semiconductors by replacing some atoms with others that contain more or fewer electrons than the atoms for which they substitute. But can doping through vacancies be implemented and monitored in a controlled way on the atomic scale?
3) It seems so. Muller et al(3) have made an unexpected double breakthrough. They have measured the quantity and location of oxygen vacancies in films consisting of layers of fully oxidized SrTiO(sub3) and of SrTiO(sub3-delta), in which some oxygen atoms are missing. Their first major advance is to have grown alternating layers of doped (delta not 0) and undoped (delta=0) SrTiO(sub3-delta), where a layer may be as thin as three unit cells. Analogous "superlattices" are used in conventional semiconductor technology to enhance the lifetime of charge carriers(4); in oxide superconductors, they are used to increase the supercurrent density(5). Muller et al(3) grew their superlattices using pulsed laser ablation -- a popular research technique for depositing thin films of oxide materials. Deposition occurs when a laser beam hits a SrTiO3 target inside a vacuum chamber, vaporizing its surface into a plasma. Some of the vaporized atoms condense on a nearby substrate, again of SrTiO3, heated to 750 deg C. Adjusting the oxygen pressure in the chamber controls the delta of the single crystalline SrTiO(sub3-delta) layers deposited.
4) To image the oxygen vacancies, Muller et al(3) used a scanning transmission electron microscope (STEM). As the tightly focused electron beam of the STEM is scanned across a cross-sectional slice of the deposited superlattice, a map is made of the positions where electrons are scattered slightly by oxygen vacancies and related defects; simultaneously, the energy loss of the transmitted electrons is measured, revealing the electronic effects of the missing oxygen atoms on the surrounding atoms (that is, changes in their oxidation state). This powerful technique offers outstanding sensitivity in resolving and identifying columns of atoms in crystalline samples, and has been used to image individual impurity atoms in silicon.
References (abridged):
1. International Technology Roadmap for Semiconductors, 2003 Edn, Front End Processes, 2 & 23-33 (Semiconductor Ind. Assoc., San Jose, 2003)
2. Moore, G. E. Electronics 38, 114-117 (1965)
3. Muller, D. A., Nakagawa, N., Ohtomo, A., Grazul, J. L. & Hwang, H. Y. Nature 430, 657-660 (2004)
4. Tsu, R. & Esaki, L. Appl. Phys. Lett. 22, 562-564 (1973)
5. Hammerl, G. et al. Nature 407, 162-164 (2000)
Nature http://www.nature.com/nature
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CONTROL OF SEMICONDUCTOR MAGNETISM BY EXTERNAL ELECTRIC FIELDS
Notes by ScienceWeek:
In general, "ferromagnetism" is a property of certain materials subjected to a magnetic field, the magnetic field causing induced magnetism which combines with the applied field to increase the local field. Ferromagnetic materials are strongly attracted to a magnetic pole and have high effective magnetic permeabilities that are greatly dependent on the applied magnetizing field. Iron, cobalt, nickel, and certain alloys are typical examples of ferromagnetic materials.
During the past five decades, several ionically bound compounds have been discovered to be ferromagnetic. Some of these compounds are electrical insulators, but others have the conductivity of semiconductors.
Above its Curie point (Curie temperature), the spontaneous magnetization of a ferromagnetic material vanishes and the material becomes "paramagnetic", i.e., it remains only weakly magnetic. This evidently occurs because the thermal energy becomes sufficient to overcome the internal aligning forces of the material.
The term "spintronics" refers to a relatively new field that aims to combine ferromagnets with semiconductors to develop electronic devices that exploit the quantum mechanical "*spin" of electrons as well as their charge. One aim is to integrate information storage with information processing, but a broader goal is to develop new functionality that does not exist separately in a ferromagnet or in a semiconductor. To this end, investigators are searching for "emergent behavior" in combined ferromagnetic semiconductor structures.
The following points are made by H. Ohno et al (Nature 2000 408:944):
1) The authors point out that it is often assumed that it is not possible to alter the properties of magnetic materials once they have been prepared and put into use. For example, although magnetic materials are used in information technology to store trillions of bits in the form of magnetization directions established by applying external magnetic fields, the properties of the magnetic medium itself remain unchanged on magnetization reversal. The ability to externally control the properties of magnetic materials would be highly desirable from fundamental and technological perspectives, particularly in view of recent developments in *magnetoelectronics and spintronics. In semiconductors, the conductivity can be varied by applying an electric field, but the electrical manipulation of magnetism in such materials has proved elusive.
2) The authors report experiments that demonstrate electric-field control of ferromagnetism in a thin-film semiconduction alloy [(In,Mn)As], using an *insulating-gate field-effect transistor structure. By applying electric fields, the authors were able to vary isothermally and reversibly the transition temperature of *hole-induced ferromagnetism.
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
spin: See related background material below.
magnetoelectronics: See related background material below.
insulating-gate field-effect transistor: The "field effect transistor" (FET) is a transistor consisting essentially of a channel of semiconductor material, the resistance of which can be controlled by the voltage applied to one or more input terminals (gates). It is a 3-terminal device in which current flow through one pair of terminals, the "source" and the "drain", is controlled or modulated by an electric field that penetrates the semiconductor, with this field introduced by the voltage applied at the third terminal, the "gate". The controlling field applied to the gate must be isolated somehow from the current flow in the channel, and there are two general methods of accomplishing this isolation: a) in the "junction field-effect transistor" (JFET), invented by Shockley, the isolation is provided by a special junction barrier across which current flow from gate to channel is very small; in the "insulated gate field-effect transistor" (IGFET), first proposed in the 1930s but not realized until 1960, an insulating layer is placed between the gate electrode and the conducting channel, preventing any current flow between them. The insulated-gate field-effect transistor is sometimes called a "surface field- effect transistor", since the effective conducting channel is the semiconductor surface. (In contrast, the JFET, in which the bulk of the semiconductor is the current carrier, is sometimes called a "bulk field-effect transistor".)
hole-induced ferromagnetism: In this context, a "hole" is an independently translocatable positively charged virtual particle produced by a translocated electron in a crystal semiconductor lattice, and the conductivity of the semiconductor is based on the mobility of both electrons and holes. In the alloy used in the Ohno et al experiments, manganese substitutes for indium at a number of loci in the alloy and simultaneously provides a localized magnetic moment and a hole, owing to its electron-acceptor nature. These holes apparently mediate magnetic interaction, resulting in so-called "hole-induced ferromagnetism".
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