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Refining Germanium

June 22, 2017

One of my brothers and I had the experience of working with Ernest ("Ernie") Buehler, he at Bell Labs, and I at a corporation whose name has been expunged by a merger and typical corporate chessmanship. Ernie enjoyed repairing small appliances for his elderly lady friends, and he was famous for his potato bread. He was a crystal grower, also famous for growing the germanium crystal used in the first transistor, invented at Bell Labs in December, 1947, by John Bardeen (1908-1991), William Shockley (1910-1989), and Walter Brattain (1902-1987). This trio was awarded the 1956 Nobel Prize in Physics for this work. Buehler also grew the crystal for the first silicon transistor.[1]

IEEE Commemorative plaque for first transistor

Commemorative plaque marking the demonstration of the transistor. This plaque was dedicated on December 8, 2009, during a ceremony at Bell Labs.

(My photo, via Wikimedia Commons. Click for larger image.)


While the semiconductor of choice in today's world is silicon because of its better temperature stability, germanium crystals are easier to grow, and this fact was especially important in the 1940s. Both silicon and germanium are prepared using the Czochralski process in which a seed crystal is dipped into a crucible of molten semiconductor just at the melting point, and the seed grows larger when rotated and pulled away at a slow rate. This method is used not just for semiconductors, but also for laser materials such as yttrium aluminum garnet (YAG, Y3Al5O12).

The Czochralski process is named after its inventor, Polish chemist, Jan Czochralski. Czochralski wasn't interested in growing crystals; rather, he developed this process in 1916 in an attempt to produce metal wire without drawing. Gordon Teal of Bell Labs adapted the process to the growth of germanium crystals that were used to fabricate high frequency diodes during World War II.

The first transistor wasn't a junction transistor or field effect transistor, both of which are now common. It was a point-contact transistor, a type that was easy to demonstrate with the crude laboratory equipment of the time. As shown in the figure, it was created by the contact of two closely-spaced gold electrodes on a slice of germanium crystal mounted on another electrode for ohmic contact. The metal contacts form shallow p-type regions in the overall n-type germanium.

First transistor, Bell Labs, 1947

The first transistor was a point-contact transistor, improvised by Walter Brattain using a bent paper clip as a spring. In an amplifier circuit, a small current at the emitter generates a higher current between the base and collector. Once you have the germanium, the rest is relatively easy, a fact that shows the importance of materials science to progress in technology. (Left photo by Windell H. Oskay, via Wikimedia Commons; right image created using Inkscape. Another image of the first transistor can be found at archive.org)


When growing a crystal of germanium, or another element or compound, you need high purity starting materials. The typical standard is "five-nines;" that is, 99.999% pure, which is quite a step above 99 44/100% pure. Often, it's important that the 1000 ppm of impurities in such materials do not contain certain elements, such as sodium.

Germanium is obtained as a byproduct of zinc refining from the chief ore of zinc, sphalerite, which is mostly zinc sulfide (ZnS) with quite a few metal impurities substituting on the zinc lattice sites in the crystal. One of these impurities is germanium, which generally exists at about 1000 ppm, but as high a concentration as 3000 ppm in some ores.[2]

The extraction process to separate germanium from zinc is not environmentally friendly, since it uses chlorine and hydrochloric acid. Now, a team of chemists from McGill University (Montreal, Quebec, Canada), the University of Western Ontario (London, Ontario, Canada), and Soochow University (Suzhou, China) has developed an alternative process that uses a recyclable quinone/catechol redox reaction that uses oxygen, rather than chlorine, as an oxidizer to produce an air- and moisture-stable Ge(IV)-catecholate that is converted to the high-purity germanium organometallic compound, germane.[3-4]

Germane molecule

Germane.

This molecule is like methane, but with germanium as the central atom in place of carbon.

(Via Ref. 3.[3] Licensed under the Creative Commons Attribution-NonCommercial license.)


The inspiration for the process came from biology. McGill University had been researching melanin, the chemical that gives color to skin and hair and will also bind to metals.[4] The chemists decided to synthesize a molecule that mimics such qualities of melanin to extract germanium at room temperature without the use of solvents.[4] Germanium is a good target for such research, since it's a useful element without a suitable substitute that exists as just a trace element in various ores.[3] Such compounds can be used to extract other metals, such as zinc, copper, manganese and cobalt.[4]

Says Jean-Philip Lumb, an associate professor in the McGill University Department of Chemistry.
"At a time when natural deposits of metals are on the decline, there is a great deal of interest in improving the efficiency of metal refinement and recycling, but few disruptive technologies are being put forth... That's what makes our advance so important... Applications of green chemistry lag far behind in the area of metals, yet metals are just as important for sustainability as any organic compound. For example, electronic devices require numerous metals to function."[4]

Their final process is green in more than one sense, since it also consumes far less energy than the conventional techniques.[3-4] One aspect of the process is the use of mechanochemistry; that is, using mechanical energy to promote chemical reactions. The chemicals are shaken at high speed in milling jars containing stainless steel balls.[4-5] Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, the National Natural Science Foundation of China, and other agencies.

References:

  1. Oral-History:Goldey, Hittinger and Tanenbaum, Interview No. 480, IEEE History Center, The Institute of Electrical and Electronics Engineers, Engineering and Technology History Wiki, September 25, 2008.
  2. Nigel J. Cook, Barbara Etschmann, Cristiana L. Ciobanu, Kalotina Geraki, Daryl L. Howard, Timothy Williams, Nick Rae, Allan Pring, Guorong Chen, Bernt Johannessen, and Joël Brugger, "Distribution and Substitution Mechanism of Ge in a Ge-(Fe)-Bearing Sphalerite," Minerals, vol. 5 (March 24, 2015), pp. 117-132;, doi:10.3390/min5020117.
  3. Martin Glavinović, Michael Krause, Linju Yang, John A. McLeod, Lijia Liu, Kim M. Baines, Tomislav Friŝĉić, and Jean-Philip Lumb, "A chlorine-free protocol for processing germanium," Science Advances, vol. 3, no. 5 (May 5, 2017), Article e1700149, DOI: 10.1126/sciadv.1700149. This is an open access article with a PDF file available here.
  4. A more sustainable way to refine metals, McGill University Press Release, June 7, 2017.
  5. Solvent-free mechanochemical techniques..., YouTube Video by Michael Brand (University of Cardiff) and Jean-Louis Do (McGill University), June 6, 2017.

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