Silicon in All Its Forms

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Silicon in All Its Forms Jim Chelikowsky Abstract The following article is based on an edited transcript of the Turnbull Lecture presented by Jim Chelikowsky (University of Minnesota), recipient of the 2001 Materials Research Society David Turnbull Lectureship, at the 2001 MRS Fall Meeting on November 27 in Boston. Chelikowsky was cited for “contributions to the fundamental understanding of electronic, optical, mechanical, surface, and interface properties of bulk and nanostructured semiconductors, ceramics, and metals through ab initio calculations; and for excellence in teaching, lecturing, and writing.” Keywords: electronic structure, pseudopotential calculations, semiconductors, silicon.

Introduction I am truly and deeply honored to be selected to give this talk. One of the highlights of my early career was to take a trek up to Harvard and visit with Professor David Turnbull. Although I only spent a small amount of time with him, it made a considerable impact on me. With respect to the matter at hand, I am going to tell you about my favorite material: element 14, silicon. As you know, we live in the age of silicon; it is all around us in terms of electronic devices. Silicon is the quintessential electronic material. It is often said that understanding silicon and its role in electronic materials is similar to understanding iron and its role in metallurgy and making steel. It has not always been that way, though. On May 10, 1954, a press release was issued on the first silicon transistor. Texas Instruments announced a “revolutionary new electronic product—long-predicted and awaited: the silicon transistor.” By using silicon instead of germanium, the initial commercial silicon transistor immediately raised power outputs and doubled the operating temperature.1 In 1954, we had a transistor made of silicon for the first time. In 1965, we had a chip containing 2000 transistors. In 2001, the Pentium 4 processor made by Intel consisted of 42 million transistors. Intel believes that by the year 2007, it will have created a processor containing one billion transistors.2 This is an amazing progression of technology, and we could spend days talking about the technical develop-

MRS BULLETIN/DECEMBER 2002

ments of silicon and devices made with silicon. However, I am going to discuss a different aspect of silicon technology, namely, the role that silicon has played in the development of theoretical tools for understanding materials. In the 1970s, approximately 30,000 papers were published with the word “silicon” in the abstract. In the 1980s, the number was up to 84,000 papers. By the time we got into the 1990s, the corresponding number was 135,652 (an average of one paper every 90 minutes).3 This amounts to a database of roughly a quarter of a million papers over the last 30 years directly or indirectly written about silicon. Most of these papers are focused on technological and experimental aspects of silicon, but this body of work has profoundly influenced the theoretical community. Specifically, theorists have capital