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Sunday, July 24, 2016

#349 Electrical characterization of semiconductor surfaces

As mentioned in the previous blog methods of electrical characterization provide particularly relevant information regarding chemical/physical condition of semiconductor surfaces.


The methods of electrical characterization of semiconductor surface and near-surface region fall into two categories. The first is based on the use of test devices with permanent contacts (e.g. MOS capacitors or metal-semiconductor diodes) allowing I-V and C-V measurements from which key electronic properties of semiconductor surface can be obtained. Second category involves characterization methods which do not require permanent contact to the surface, and hence, allow measurements of selected electrical parameters of the as-processed surfaces, i.e. surfaces resulting from any given operation without wafer exposure to any additional processing steps.  Methods based on  non-contact Surface Photovoltage (SPV) and temporary contact based Photoconductance Decay (PCD) measurements fall into this category.



Posted by Jerzy Ruzyllo at 09:31 PM | Semiconductors | Link

Sunday, June 26, 2016

#348 Characterization of semiconductor surfaces

With wafer surfaces playing key role in defining characteristics of semiconductor devices and circuits, surface characterization and monitoring processes in semiconductor device engineering are increasingly important. This includes in particular wafer cleaning operations and other surface conditioning steps that need to be closely monitored.


Methods of semiconductor surface characterization that can be used in surface processing characterization and monitoring include (i) chemical/physical surface analysis involving methods such as SIMS, XPS, TXFR, SEM, AFM, (ii) optical methods such as spectroscopic ellipsometry, FTIR and others, and finally (iii)  electrical methods which allow identification of the  surface-related electrically active centers representative of the condition of the wafer surfaces. While not qualitative the last type of methods is of particular interest as they unravel surface characteristics which are directly affecting performance of the final device.  


Posted by Jerzy Ruzyllo at 09:14 AM | Semiconductors | Link

Sunday, June 5, 2016

#347 Semiconductor material engineering - final comments

The recent series of blogs on semiconductor material engineering (blogs #342-346) meant to emphasize how divers outcomes may result from the modifications of chemical make up of semiconductors.Versatility of materials displaying semiconductor properties in this regard can not be matched by any of conductors and only by some insulators.


Due these fundamental characteristics semiconductors will always be a foundation of progress in electronics and photonics.  It seems that this obvious truth needs to be brought to the surface once in a while.

Posted by Jerzy Ruzyllo at 01:50 PM | Semiconductors | Link

Sunday, May 29, 2016

#346 Semiconductor material engineering, part IV

Here is one more example of semiconductor material engineering. This time modifications of the chemical composition of some semiconductors are geared toward alteration of their magnetic properties.


Most of the mainstream semiconductors feature very low magnetic susceptibility, i.e. very small response to the magnetic field. Examples of non-magnetic semiconductors include silicon, germanium as well as gallium arsenide, indium arsenide and others. Some non-magnetic semiconductors when doped with transition metals (e.g. iron, manganese, chromium…) acquire well defined ferromagnetic properties, and hence, turn into magnetic semiconductors. For instance, originally diamagnetic gallium arsenide when doped with manganese (GaMnAs) features significantly increased magnetic susceptibility. Known as dilute magnetic semiconductors these materials will play key role in spintronics.

Posted by Jerzy Ruzyllo at 11:30 AM | Semiconductors | Link

Sunday, May 15, 2016

#345 Semiconductor material engineering, part. III

Besides affecting width of the energy gap, distribution of energy states within the energy bands of semiconductor affects also the mechanism of electron transition from the higher energy levels in the conduction band to the lower energy levels in the valence band in the process of charge carriers recombination.


In the case of semiconductors featuring direct bandgap the energy released in the process of electron recombination is in the form of light (photon) emitted by the material. In contrast, in the indirect bandgap semiconductors the same energy is released into semiconductor crystal mostly in the form of vibrational energy (phonon).


This inherent characteristic of the energy gap is predetermined for any semiconductor featuring set chemical composition. But when you start altering chemical compositions of some compound semiconductor the change of bandgap’s type from direct to indirect (or vice versa) is entirely possible.


Taking GaAs as an example, but not in combination with Al as in the previous blog, but with phosphorous (P). A peculiar property of the Ga-As-P material system is that at certain composition between GaAs and GaP the energy gap changes form direct to indirect making  material which is initially suitable for  light emitting devices  to the one which is entirely ineffective in this application.


And that’s yet another example of what manipulation of the chemical composition of semiconductor can bring about.


Posted by Jerzy Ruzyllo at 12:33 PM | Semiconductors | Link

Sunday, May 8, 2016

#344 Semiconductor material engineering, part II.

In addition to changing dopant concentration and by doing so changing electrical conductivity of semiconductor (see previous blog), chemical composition of semiconductor can be altered for the purpose of changing its bandgap Eg (width of the energy gap). This practice, known as “bandgap engineering” is commonly employed to form highly complex III-V compound semiconductors material systems used in engineering of high-speed transistors and light emitters.


Take for instance Al-Ga-As material system. By changing Al fraction x in AlxGa1-xAs, material transitions from gallium arsenide GaAs (x = 0) to aluminum arsenide AlAs (x = 1). The bandgap changes in the process from Eg = 1.42 eV to  Eg = 2.16 eV for AlAs.  

Posted by Jerzy Ruzyllo at 07:50 PM | Semiconductors | Link

Sunday, April 24, 2016

#343 Semiconductor material engineering, part. I

As a follow up to the previous post discussing the role of alterations of composition of solids a few remarks regarding specifically semiconductors (see also blog #341).


First and foremost it needs to be emphasized that with an exception of the very specialized p-n junctions which included intrinsic (undoped) “i” component (p-i-n junctions), all semiconductor devices are manufactured used doped material. In fact our ability to control concentration of alien elements in the host material is a foundation of semiconductor device technology. In short, our ability to process diodes and transistors is based on the choice of the dopant type, making semiconductor either p- or n-type, and dopant concentration which allows control over its electrical conductivity.


Importance of dopants in semiconductor device technology is such that unless semiconductor material has elements that can make it either p-type or n-type, its usefulness in the manufacture of diodes and transistor is close to zero. An example of such material is diamond which in theory is an excellent semiconductor, but in practice is hardly used to make commercial devices because no element in the periodic table can effectively make into a device-grade n-type material.


Posted by Jerzy Ruzyllo at 08:54 AM | Semiconductors | Link

Sunday, April 10, 2016

#342 Manipulating chemical composition of solids

Tinkering with chemical compositions of conductors such as metals will do us little good in terms of altering their physical properties to the point where new applications of the material will become possible. As an excellent conductor of electricity, metal can only up to certain point be converted into not-so-good conductor by altering its chemical makeup.


In contrast, fundamental physical properties of some insulators, most notably oxides, as well as semiconductors can be drastically altered by the modification of their chemical composition.


With proper alterations of their chemical composition some oxides can be made conductive, and even superconducting, can be converted from dielectrics to ferroelectrics, can acquire magnetic properties and so on, all under the banner of functional oxides.



Semiconductors story in this regard is equally exciting (see follow up blog).


Posted by Jerzy Ruzyllo at 05:50 PM | Semiconductors | Link

Sunday, April 3, 2016

#341 Different kinds of doping

For years, term "doping" in semiconductor nomenclature meant introduction of alien elements into a given semiconductor material in order to alter its electrical condutivity. For instance, introduction of boron makes silicon assume p-type conductivity while doping with phosphorus makes silicon n-type. And obviously higher the concentration of introduced dopant atoms, higher the electrical condictivity of semiconductor.


These days term "doping" takes on addditional meaning. With a growing interest in an effective control over the electron's spin state (see spintronics) the interest in magnetic semiconductors, i.e. semiconductors displaying  ferromagnetic properties, is also growing. The easiest way to make certain classic semiconductors display some ferromagnetic properties is to dope them with selected alien elements. For instance gallium arsenide, GaAs, will display ferromagnetic properties when manganese (Mn) atoms are introduced into its structure and change it into GaMnAs.


The difference between these two cases of "doping" is that in the former case it takes some 1 atom of dopant per milion atoms of silicon to make a diffference in its electrical conductivity, while  in the latter case manganese doping levels as high as 10% may be needed to see meaningful differences in ferromagnetic properties of GaAs.


Posted by Jerzy Ruzyllo at 12:45 PM | Semiconductors | Link

Sunday, March 20, 2016

#340 Reversing surface "aging"

Good news is that surface aging discussed in the previous blog can be relatively easily reversed by lamp cleaning. However, only light organic contaminants relatively weakly physi-sorbed at the wafer surface can be controlled this way. If allowed to remain on the surface for the long storage times, chemical reactions with moisture from the ambient will convert organic contaminants into heavy organic compound strongly chemi-sorged at the wafer surface. Then, only standard wet clean such as SPM or APM will be able to restore wafer surface to its original condition.

Posted by Jerzy Ruzyllo at 08:01 PM | Semiconductors | Link

‹‹ ›› is the personal blog of Jerzy Ruzyllo. With over 35 years of experience in academic research and teaching in the area of semiconductor engineering (currently holding position of a Distinguished Professor of Electrical Engineering and Professor of Materials Science and Engineering at Penn State University), he has a unique perspective on the developments in this progress driving technical domain and enjoys blogging about it.

With over 2000 terms defined and explained, Semiconductor Glossary is the most complete reference in the field of semiconductors on the market today.

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