Interview: Chris G. Van de Walle
2013 Medard W. Welch Award Recipient
Paul Holloway and Dr. Jack Rowe, October 28, 2013
HOLLOWAY: Let me introduce myself. My name is Paul Holloway. I am with the AVS History Committee. Today is Monday, October 28, 2013. We’re at the 60th annual International Symposium of the AVS in Long Beach, California. I am joined today by Dr. Jack Rowe from the AVS History Committee and North Carolina State University as an interviewer, and we’re interviewing Chris G. Van de Walle from the University of California Santa Barbara who is the Medard W. Welch Award winner this year. His citation reads: “For seminal contributions to the theory of heterojunctions and its application to semiconductor technology, and for elucidating the role of hydrogen in electronic materials.” So Chris, congratulations on this well-deserved award.
VAN de WALLE: Thank you very much.
HOLLOWAY: Let me begin by asking you to give us your date and place of birth.
VAN de WALLE: I was born on May 10, 1959 in Ghent, Belgium.
HOLLOWAY: Good. Would you continue by giving us something of your educational background?
VAN de WALLE: Sure. I guess the first item there that somehow relates to my scientific career is the fact that in high school I had a very good math teacher…
HOLLOWAY: Wonderful.
VAN de WALLE: …who encouraged me as well as other people in the class to really perform at a very high level. In his eyes, and actually in the eyes of people in Belgium at the time, aspiring to an engineering career was the topmost thing you could do. So it wasn’t really a question for me of becoming a physicist. Doing engineering was really what it was all about. So he basically prepped us. His name was Walter Lievens, and he prepped us to do really well on the entrance exam which we needed to pass to get into engineering at the university. So that was a good starting point. Then once I had entered university in Ghent, at that point called the Rijksuniversiteit Ghent, I was studying electronic engineering, but I really developed an interest in the physics aspects of it. That was stimulated also by a physics professor, Professor Mortier who was really very inspirational. Also outside of class, I had a chance to meet with him. I think he really was instrumental in many ways to stimulate my interest in basic physics, which then during my five-year engineering degree, I had the chance to follow up. There were lots of elective courses to be taken. I took all of them in physics, and that proved to be excellent preparation to then go on to graduate school.
HOLLOWAY: What aspect of physics was he particularly interested in?
VAN de WALLE: Professor Mortier was teaching basic physics.
HOLLOWAY: Basic, general physics.
VAN de WALLE: What I was increasingly moving towards was quantum mechanics, solid state physics, condensed matter physics, and already taking some reasonably advanced courses even at an undergraduate level. There was one course in which we used Ashcroft and Mermin as the textbook, so it tells you something about the level.
HOLLOWAY: Yes. Very good. What year was that that you finished?
VAN de WALLE: I finished there in 1982. And just like my high school teacher had kind of set me on a track to go to university to study engineering, the influence from various professors at the University of Ghent steered me towards definitely going to graduate school and doing so in the United States, for which fellowships were available. In particular, there was an organization called the Belgian American Educational Foundation, which has been in the business of giving fellowships to Belgian students for study in the United States. They’ve been doing so since the 1920s.
HOLLOWAY: Is that right?
VAN de WALLE: It’s an interesting history. After the First World War, Belgium, along with many other countries in Europe, had suffered a lot of devastation, and Herbert Hoover, who later became president, was behind an effort to collect funds to help Belgium and other countries build up again after the First World War. That was a very successful effort—actually, so successful that not all the funds were used up for the rebuilding. They did a very smart thing. They invested the remaining funds in such a way that the proceeds from the investments would be used for fellowships for people to go and study in the United States. And vice versa, a certain number of American students every year can come to Belgium and study in Belgium. So this is an exchange program that has been going on for almost 90 years now and is extremely successful. So I benefitted from that.
HOLLOWAY: Wonderful! So you went to…
VAN de WALLE: I chose to go to Stanford University. The fellowship covered one year of study there, tuition and stipendOnce I was there, I got a master’s degree, but I became extremely interested in staying on for a PhD. And that also worked out. I met Walter Harrison, who has also been one of my mentors. Those who know Walter know that he always had an extremely small group—on the order of one student or less.
HOLLOWAY: [Laughs] That’s small!
VAN de WALLE: And he already had a student when I approached him. So I couldn’t work with Walter directly, but Walter put me in touch with Richard Martin, who at that time was at the Xerox Palo Alto Research Center. Richard had expressed an interest in working with a Stanford student and actually had funding available for that. So effectively I did my PhD under the supervision of Richard Martin with Walter Harrison on my committee, and actually a third person who was very influential:Bill Spicer, William Spicer, who of course was an experimentalist and one of the originators of using photoemission spectroscopy to study surfaces. I think he was one of the people involved in the founding of the Stanford Synchrotron Radiation Lab (SSRL). By coincidence, he was my academic advisor in the Electrical Engineering Department at Stanford. Even though that was in principle only a formal role, it did actually inspire me to be very interested in the whole area of surface science, particularly interfaces, and experimental characterization, but then ultimately working with Richard Martin doing calculations for such interfaces and really studying the atomic and electronic structure of interfaces theoretically and computationally from first principles.
HOLLOWAY: So what was the emphasis in your Ph.D. dissertation?
VAN de WALLE: Well, it ended up being interfaces, semiconductor heterojunctions originally inspired by something that Richard Martin picked up on that was developing at that time—namely, the ability to grow germanium or silicon-germanium alloys on top of silicon. Of course, already at that time silicon was the workhorse material for electronics. But again, already at that time, 1983, it was also obvious that by combining silicon with other materials you might be able to do better—enhanced mobility, enhanced performance of transistors. There was a lot of experimental research starting on growing silicon-germanium alloys on top of silicon.
HOLLOWAY: Including yours truly, Jack Rowe.
VAN de WALLE: Indeed. What Richard thought would be highly interesting would be in parallel with the experiments to do computations to develop a fundamental understanding of the properties of such interfaces. We actually managed to predict band offsets, which apparently have been useful to the community because those papers are still gathering a lot of citations even now.
HOLLOWAY: So you finished your Ph.D. in 1986?
VAN de WALLE: That’s correct. Yes.
HOLLOWAY: And you went from there to a post-doc.
VAN de WALLE: Yes. I got a post-doc offer from the IBM Watson Research Center in Yorktown Heights and started working there with Sokrates Pantelides, who of course is another luminary in the area of first-principles calculations for solids and materials in general.
HOLLOWAY: And you continued to work with the silicon-germanium interfaces or silicon in general?
VAN de WALLE: Not really. I did continue on my own with some more work on band offsets for other heterojunctions, but the project that Sok (short for Sokrates) wanted me to work on was related to hydrogen in semiconductors.
HOLLOWAY: Is that right?
VAN de WALLE: One of the topics mentioned in the citation [of the Medard Welch Award] is hydrogen, and that’s really due to Sok Pantelides that I developed an interest in that. Again, there were tantalizing experimental results showing that hydrogen was playing an important role in the electronic properties of materials. Particularly the issue of the passivation of donors and acceptor impurities was something that had only recently been discovered, through some very clever detective work by C. T. Sah originally, who identified hydrogen as the culprit that was causing deactivation of dopants. But again, there were experimental observations. They seemed to some extent mutually inconsistent, contradictory, and Sok thought that by doing first-principles calculations we would be able to shed light on that issue. So again, that turned out to be a very fruitful project.
HOLLOWAY: And so you stayed there for a couple of years?
VAN de WALLE: I was at IBM for two years, and then I got an offer for a member of research staff position from Philips Laboratories in Briarcliff Manor, close to Yorktown Heights, New York. I actually did continue my interaction with IBM. I did continue collaborating with Sok, and in the meantime, Sok had taken on a student from Columbia University, David Laks, with whom I ended up collaborating closely together with the late Gertrude Neumark, who was a professor at Columbia University. So for the next three years, that was a very productive arrangement to keep collaborating with IBM and Columbia University, but simultaneously starting on a new track of research which was wide-band-gap semiconductors. Philips as a lighting company was extremely interested in semiconductors that could be used as light-emitting diodes, and at the time for blue light emission, zinc selenide was the candidate. Philips Labs was a leader in growing and fabricating zinc selenide light-emitting diodes and lasers. In my computational work, I started focusing on the properties of these wide-band-gap semiconductors: understanding their doping, interfaces, etc.
HOLLOWAY: So did you work with Bhargava during this period?
VAN de WALLE: Ramesh Bhargava at least for some of the time that I was there was actually my manager. He was instrumental in hiring me at Philips originally. But there was a transition during the time that I was there, and then ultimately Ramesh Bhargava went on to found his own company on phosphors.
HOLLOWAY: Yeah. He went into the quantum dot nanophosphor area.
VAN de WALLE: Indeed.
HOLLOWAY: Good. So what did you do? You worked on zinc selenide and hydrogen.
VAN de WALLE: Yes. At Philips, I kept the work on hydrogen going and actually with some interesting results on calculations of hyperfine parameters, which of course are quantities that experimentally can be determined withelectron spin resonance, but to interpret those values, being able to do first-principles calculations in parallel is highly useful. This has actually been a theme throughout my career: that doing calculations is—
HOLLOWAY: They can be interpreted then.
VAN de WALLE: … is good in its own right, but if you can calculate something that can also be experimentally measured, you can leverage both the theory and the experiment. So we did that with hyperfine parameters for the case of hydrogen and silicon, and then yes, we started doing a lot of work on zinc selenide, which, along with many other wide-band-gap semiconductors, suffers from doping problems. In this particular case, making p-type zinc selenide was difficult. For a long time people thought it was impossible. Then it was demonstrated that you could do it, but it was still very difficult. Understanding why that is became a strong motivation for doing calculations and developing a framework in which we can understand those problems.
HOLLOWAY: Yeah. But it goes back to hydrogen again, right?
VAN de WALLE: In many ways it goes back to hydrogen. Indeed, if you have hydrogen present during the growth of p-type material, it will passivate the acceptors, and you need an additional annealing step to remove the hydrogen and activate the acceptors and actually get p-type material. That was the case in zinc selenide, and, well, it’s still the case in many other materials, soyou need to be aware of it.
HOLLOWAY: Now in 1991 you rejoined PARC, I believe. Is that correct?
VAN de WALLE: That’s correct, yes. I had of course kept in touch with the Xerox Palo Alto Research Center, and when they had an opening in ’91, they contacted me and made me an offer. I was happy to move back to California.
HOLLOWAY: [Laughs] Yeah, the cold New York, huh?
VAN de WALLE: I did actually like living in New York. It was nice to be close to New York City with all its attractions. It was also nice to live in Westchester with its great natural beauty. Philips Labs actually is right on the Hudson River. I had a view of the Hudson from my office.
ROWE: It’s a beautiful location.
VAN de WALLE: I really enjoyed it. But from a career point of view, the Xerox Palo Alto Research Center had a great deal more basic research activity than Philips did, so I felt it was a good move to go back to Xerox.
HOLLOWAY: So you spent about 13 years at Xerox PARC and then went to the University of California Santa Barbara.
VAN de WALLE: That’s right. I was at PARC from 1991 to 2004.
HOLLOWAY: Good. Let’s talk about what you did at PARC. You continued to work in the area of hydrogen and interfaces?
VAN de WALLE: Yes. All of these themes that had been set early on did continue. The work on hydrogen came in very handy in the context of research on hydrogenated amorphous silicon in which Bob Street was a pioneer. I had the opportunity to work with him. We published several papers together on the properties of hydrogen in silicon and how that’s beneficial for amorphous silicon. I also had the wonderful opportunity to work with Noble Johnson and Conyers Herring. Conyers, one of the luminaries of condensed matter physics in general who had spent most of his career at Bell Labs, had retired to California, had an emeritus professor position at Stanford, and was also a consultant at Xerox PARC. So I had the opportunity to interact extensively with Conyers. That resulted also in a few papers; it was really a privilege to work with him. I also continued the theme of wide-band-gap semiconductors, but by that time (early ’90s), it was becoming clear that zinc selenide was not a great material to work with. Gallium nitride had been demonstrated to be a much better material for blue light emitters. So I had the chance to start working on that. There were some experimental projects starting at PARC, and I immediately jumped on the opportunity to start doing first-principles calculations on this exciting new material, gallium nitride.
HOLLOWAY: Did PARC do much work in experimental growth and processing of zinc selenide and devices from them?
VAN de WALLE: In zinc selenide, there had been a project well before my time at PARC—I think maybe late ’70s, early ’80s. If I remember correctly, somebody by the name of Wolfgang Stutius spent time at PARC and was involved in zinc selenide research. But that had been discontinued. So in the area of wide-band-gap semiconductors, it was really in the early ’90s when a new project was initiated which was on gallium nitride. The main drivers behind that were David Bour and Ross Bringans, if I recall correctly—David Bour being an excellent MOCVD grower who in a very short time started producing very high-quality gallium nitride light-emitting diodes at a time when very few places in the world, either universities or companies , were capable of producing good nitride diodes. So in principle, Xerox could have picked up on this and gone into that business, but they issued rather firm orders that we were not supposed to work on light-emitting diodes, but only on lasers which would be used in laser printers.
HOLLOWAY: Is that right? So the laser printer was a product they had and they didn’t see an obvious product for the light-emitting diodes and so they ignored it then.
VAN de WALLE: That must have been the logic. Indeed. Now they were right about blue lasers being good for laser printers for the same reason why we have Blu-ray DVDs. You get the smaller spot size with blue light, and so you can have higher resolution. So that was good logic. But to ignore the commercial opportunities that could have been pursued with light-emitting diodes, yeah, that was a bit sad.
HOLLOWAY: I have to insert at this point that the University of Florida was the organization that Bob Park split the nitrogen molecule and incorporated it into p-dopant nitrogen and with 3M produced the first blue p-n junction LED that I’m aware of.
VAN de WALLE: Yeah, that’s right. That was in zinc selenide. Right.
HOLLOWAY: In addition to that, continuing with the gallium nitride, Nakamura was a post-doc at the University of Florida and learned MOCVD from Ramu Ramaswamy and Tim Anderson during that time period as well. So we were mixing it up with the right people.
VAN de WALLE: Definitely. Yes.
HOLLOWAY: I don't want to delay your story, though. I just wanted to get that.
VAN de WALLE: Well, these stories intermingle of course…
ROWE: It’s a small world.
VAN de WALLE: …because I did get to meet Shuji Nakamura. I forget exactly what year, but in the early ’90s right after he was getting noted for his achievements in nitride light-emitting diodes. I knew he was going to be in the United States. I invited him to give a talk at Xerox PARC, and he kindly accepted that invitation. So I got to know him early on, and of course now we are colleagues at the University of California Santa Barbara.
HOLLOWAY: Along those same lines in a small world, Bob Street is going to come to the University of Florida and give us a seminar this spring on his work with flexible electronics and solar cells. So it is a small world. Let’s continue with your story. You’ve mentioned several mentors at PARC in that second period. Are there others that you can think of that you’d like to mention?
VAN de WALLE: Well, there is one person who I’ve never directly collaborated with, and we’ve never been at the same institution. But I do have to acknowledge Eugene Haller, who contacted me, if I remember correctly, while I was a post-doc at IBM. He had noticed some of the work I did during my PhD on calculations of band offsets, and he was interested in particular aspects of that. He invited me to give a seminar in Berkeley if I would be on the West Coast, and I gladly took him up on that. Ever since, Eugene has been a tremendous mentor and has provided me with a lot of great scientific insights, scientific advice, and also a lot of support throughout my career quite unselfishly because as I said, we never really worked closely together.
VAN de WALLE: Unselfishly. I just realized that. Without there being anything in it for him really. We don’t collaborate. We don’t work at the same institution, but Eugene has really been a tremendous mentor.
HOLLOWAY: He’s been a mentor for many students at Berkeley, I believe.
ROWE: That’s right, and his work connects to your thesis work because he was, for many, many years, the person that grew the purest germanium in the world. That was very valuable for many applications, including heterojunctions that you worked on.
VAN de WALLE: And there is another connection because in the quest for ultra-pure germanium, Eugene Haller along with Leo Falicov actually identified hydrogen as one of the few remaining impurities that would be electrically active in germanium when you eliminate most everything else. We are talking about doping levels below 1013 per cm3, and it turns out that whatever is left, hydrogen is one of those impurities. For that reason actually, Eugene has been a pioneer in stimulating research on the fundamentals of hydrogen in semiconductors.
HOLLOWAY: In your bio that you helped put together, you mentioned Scheffler and Ploog as well. Would you say a few words about those two?
VAN de WALLE: Yes. Matthias Scheffler, as well as Klaus Ploog actually, I have both known them for more than 25 years—since the late 1980s. We always had great scientific interactions. They nominated me for a Humboldt Research Award for senior U.S. scientists that enabled me to cumulatively spend one year of research in Germany in Berlin, which I split between the Fritz Haber Institute where Matthias Scheffler is and the Paul Drude Institute where at that time Klaus Ploog was the director. So that was a great opportunity. I spent most of that year during the calendar year of 1999 in Berlin, and it was great from many points of view, but I will mention two things in particular. It enabled me to renew my collaboration with Jörg Neugebauer, who was my first post-doc at Xerox PARC. We worked on studying gallium nitride. But ever since, I have kept collaborating with Jörg, and he is an amazing person to collaborate with. We have many high-impact papers together, both in the area of nitrides but also in the area of hydrogen in semiconductors, so spending time in Berlin where Jörg was at that time was a great opportunity. By the way, Jörg is now a director at the Max Planck Institute for Steel Research in Düsseldorf. The other thing I want to mention about that Humboldt year in Germany was that it enabled me, as a sabbatical should, to start working on some things that didn’t directly fit into a research program at Xerox. What I chose to do was to work on zinc oxide, which I had thought about for some time because I was doing a lot of work on gallium nitride, a III-V compound material. I looked in the periodic table. You move outwards towards the II-VIs. You go from gallium to zinc. You go from nitrogen to oxygen. So I figured, well, zinc oxide maybe has similar properties to gallium nitride. Maybe it’s an interesting material to pursue. Of course I was not the first person to think this, but there were lots of intriguing reports in the literature that provided inspiration for me to start doing new calculations. I had the opportunity to do that during that time in Berlin, and that has developed into a very fruitful research area in oxides in its own right.
HOLLOWAY: Zinc oxide is a magic material in many cases.
ROWE: It is.
HOLLOWAY: We use it in some of our devices for a variety of reasons. But a very interesting material.
VAN de WALLE: Yeah. Well, another person whom I should acknowledge at this point is the late Jim McCaldin from Caltech because he was instrumental in encouraging me to work on zinc oxide. He approached me at some point to discuss some results that had originated at Bell Labs in the 1950s about what happens when you expose zinc oxide to hydrogen. Jim asked me, “Is it possible that hydrogen acts as a shallow donor in zinc oxide?” and my first reaction was, “No,” because by that time, I had already done a lot of work on hydrogen in semiconductors. I thought I had seen a pattern develop to indicate that hydrogen would always passivate impurities, would always counteract the prevailing conductivity, but not act as an active dopant in its own right. But to make a long story short, Jim McCaldin was right. Zinc oxide is an exception, one of the very few exceptions in which hydrogen behaves differently. It really acts as an active shallow donor by itself. That was basically what I worked on in Berlin in ’99 and was able to write a paper about that. It turns out to be very heavily cited.
HOLLOWAY: Good. What energy level is the hydrogen in zinc oxide [unintelligible]?
VAN de WALLE: Well, it’s a shallow donor. The ionization energy I think is about 60 milli-electron volts. It’s unusual for hydrogen to do this, but it’s very important to be aware of such behavior because hydrogen of course is a ubiquitous impurity. I already mentioned the case of germanium where if you work hard at it, meaning decades of research, to purify everything, one of the elements that still remains is hydrogen, right?
HOLLOWAY: Right.
VAN de WALLE: Same thing in ultra-high vacuum. It ties in with the theme of AVS, right? In ultra-high vacuum, no matter how hard you bump, the predominant background impurity that you are left with is still hydrogen. And combined with the fact that hydrogen tends to have a high solubility in most materials, it’s no wonder that materials will incorporate hydrogen. If hydrogen actually has a significant effect on the atomic structure or on the electronic properties, you’d better be aware of that.
HOLLOWAY: Absolutely. You worked a number of years in interfaces, and you have developed the universal alignment model for those interfaces in semiconductor materials. Bill Spicer had the mid gap impurity states, I believe. I’ve forgotten exactly what he called his. Then there were some in Europe. How do those models—are they converging or are they still…different conditions lead to different interface models?
VAN de WALLE: That’s a very interesting question. I would say that this is actually still an active area of research. I think there is a general consensus that it is possible to make a diagram in which you put the band structures of various materials, semiconductors and insulators, on an absolute energy scale, (i.e., refer to the vacuum level), and that by then taking differences between valence-band positions of two of those materials, you can predict a valence-band offset. Now exactly how you do this alignment on an absolute energy scale, there are many models for that. Deep down, I believe that all of these models, even though superficially they seem to be based on very different assumptions or very different ways of calculating things, deep down I believe that they are all related to each other. One of these relationships we pointed out in this universal alignment model—namely, that the electronic level of hydrogen in these various materials actually turns out to be remarkably consistent across the various materials and can actually offer you a way to predict band lineups. Or conversely, if you know the band lineup, then you will find that the electronic level of hydrogen is aligned between the different materials. That of course is not a property of hydrogen. It’s a property of the way hydrogen interacts with the materials which then relates to these concepts of charge neutrality levels, etc, that various people have pursued.
HOLLOWAY: I guess Bill Spicer drug it out of my memory. Metal-induced gap states, MIGS, was what he called his.
VAN de WALLE: Yes, and then Winfried Mönch in Germany picked up on that and has published a lot.
ROWE: And extended that quite a bit.
HOLLOWAY: So you’re now at the University of California Santa Barbara.
VAN de WALLE: Yes.
HOLLOWAY: You’re leading the group of Computational Materials. Tell us a little bit about the breadth of the Computational Materials Group. Is it all electronic materials or is it structural materials, etc.?
VAN de WALLE: Not so much structural materials, but it does go beyond what we normally think of as electronic materials. I’ll give you one example of that. Based on the work I had done previously on hydrogen in semiconductors, I figured we had a pretty good understanding of the way hydrogen interacts with materials. Of course one way in which such interactions are technologically exploited is for hydrogen storage materials, which have been heavily pursued. It’s a very difficult research area because you want to identify materials that can store a lot of hydrogen, but at the same time, they should be capable kinetically of releasing the hydrogen very fast and be recharged very fast. I figured that the sort of interactions that we had come to understand for semiconductors could actually also be playing a role for these hydrogen storage materials, so I built it up as a research area within the group, supported by the Department of Energy. We actually have been successful in identifying some fundamental mechanisms that seem to be really important for governing the kinetics of storage materials.
HOLLOWAY: So how many people are in your Computational Materials Group there?
VAN de WALLE: Well, I have about 15 people right now. Probably half of those are graduate students and then five or six post-docs, and then I have a couple of more senior people at the level of project scientist who also help me run the group and work with the students, etc.
HOLLOWAY: Now at Florida, the computational materials people are Susan Sinnott and Simon Phillpot. Materials collaborate with the physics and the chemistry people. Is that the same case for your group?
VAN de WALLE: Absolutely. We have a lot of interactions at Santa Barbara, a lot of interdisciplinary work, a lot of centers in which such collaborations are thriving. At the risk of omitting relevant examples, I will mention the Materials Research Lab, which is supported by NSF. It’s a MRSEC in which Susanne Stemmer and I are leading an interdisciplinary research group on complex oxides. Susanne is an excellent experimentalist both in the area of transmission electron microscopy and MBE growth of oxides. So that’s an example of a very successful interaction. Then another center is the Center for Energy Efficient Materials, which was one of the Department of Energy’s Energy Frontier Research Centers. John Bowers and Dave Auston are the directors of that, and I’m involved in a project along with Jim Speck, Shuji Nakamura, Steve DenBaars, and Claude Weisbuch on enhancing the efficiency of nitride light emitters.
HOLLOWAY: Good. Well, that’s quite a career. Jack, do you have some questions you would like to ask?
ROWE: I’d like to go back to this theme of the small world and point out that I knew your advisor from Xerox PARC when he was a student and finished his Ph.D. at Bell Labs where I spent most of my career. I probably got to know Richard [Martin] after he moved to California through a topical conference that’s been co-sponsored by AVS, certainly in recent years. Della Miller has been very important to that conference, and that’s the PCSI conference which originally was on compound semiconductors like gallium arsenide and other materials and interface band offsets was something that was not understood and was studied experimentally and theoretically by a number of people at this small meeting. The first one of those meetings that I attended was in January of 1974, and it was held—I think that was about the third one of the meetings—it was always held in San Diego, California because the naval electronics school was very important in founding that. Then later in 1996, this conference returned to San Diego, and I believe you were the co-chair of that conference along with Karen Kavanagh.
VAN de WALLE: That’s correct.
ROWE: That was the year that I had chosen to leave Bell Labs and move to North Carolina. I remember that was a very, very good conference, and it holds a special place for me. That conference also connects to Paul Holloway here because in 1990, that conference was held in Clearwater Beach, Florida. Paul Holloway was co-local chair along with myself. There were many people studying these same topics.
I want to turn to Santa Barbara, though, because I believe you were at Santa Barbara when Herbert Kroemer won the Nobel Prize also for semiconductor heterojunction effects. Maybe you weren’t.
VAN de WALLE: I was not at UCSB yet. I think Herb won in the year 2000.
ROWE: Maybe it was. Maybe it was earlier.
VAN de WALLE: I only joined in 2004.
ROWE: Yeah, okay. So my memory isn’t as good as I thought it was. But it’s interesting that there’s a connection other than yourself between semiconductor heterojunctions and UC Santa Barbara, and here we are in Long Beach, California. So California certainly has played a key role in these kinds of scientific topics.
VAN de WALLE: Right. This afternoon in my talk, I will actually acknowledge Herbert Kroemer because the topic that I’m going to be talking about, which is complex oxide interfaces, in some ways has parallels to semiconductor interfaces, which is of course the area in which Herb is an absolute expert. I have been discussing with him how we should revisit some of the physics of interfaces such as germanium gallium arsenide or silicon gallium phosphide now that we have the new insights that have resulted from complex oxide research.
HOLLOWAY: Let me ask a question. I am particularly interested in colloidal nanoparticle II-VI materials that are grown in an aqueous environment or a solvent of one variety or another with lots of hydrogen around. How much do you think hydrogen influences those types of materials?
VAN de WALLE: Probably a lot. [Laughter] I think that’s a safe answer.
HOLLOWAY: There’s another lifetime of work there for somebody, huh? [Laughter]
VAN de WALLE: Unfortunately, the experimental characterization of such particles, I’m aware, is very difficult.
HOLLOWAY: Very difficult.
VAN de WALLE: Right. So to know exactly what you have in the particles and what role the various impurities play I’m sure is very challenging. But I’m also sure that hydrogen is playing some role.
HOLLOWAY: Yeah. In fact, I’ll show in my talk tomorrow how the structure can be very complex, and the reactions you get out of it depend upon what you’re looking at. It’s like feeling the elephant. It depends upon what part of the elephant you’re feeling as the results you get out of that. Let me ask a general question, and that is your career has been very successful. You’ve accomplished a lot to be congratulated, and an award is well-deserved. What advice do you have for young people as to how to proceed to have something as successful as you have been able to accomplish?
VAN de WALLE: I would say you have to be passionate about what you do, and that means really thinking about the problems you’re working on all the time. Sok Pantelides once gave me some advice when we started working on hydrogen in semiconductors. There were so many puzzles, and we couldn’t really make sense originally of the results. He told me, “Well, to find out what’s really going on, don’t think of it as sitting at your desk from 9 to 5 and you’ll find a solution during working hours. No. You have to live and breathe this problem and think about it all the time. You should be thinking about it in the shower.” I distinctly recall Sok’s advice. I really think that that’s a valuable way of looking at it also in the sense that if you’re not willing to do that, maybe a scientific career is not the best choice for you. It’s much more than a 9 to 5 job. You should feel a real passion for it.
HOLLOWAY: There are some times when people do wake up in the morning with a fresh idea in these, so their mind has been working at night as well.
VAN de WALLE: Absolutely. I’ve had some of my best ideas in the gym while working out on a stationary bike and keeping thinking about some problem that bugs me and well, all of a sudden, there it is. I stop and grab a pen to write down some notes. I guess getting the blood flowing to the brain during exercise is productive!
HOLLOWAY: [Laughs] Let me ask another question along those lines, and that is that you’ve interacted with a number of people in a number of different places. You’ve moved from Europe to the United States and back again. What has networking and what have the mentorships, the role that you would see, and what advice would you give to young people in that area?
VAN de WALLE: Interactions are tremendously important, one area being the choice of research topics. I think during our interview here, it emerged at various points that I started working on a given topic because somebody commented to me about there being an interesting puzzle and maybe I should look into it. Well, if your mind is open to such suggestions, if you follow up on it, it can be tremendously productive. Very often, that results from interactions with people. Also, once you have results in a certain area, being able to talk to other people about it, having colleagues whom you know you can trust because you have a network of trusted colleagues. You can turn to them either in the area, in my case, of theory or computation because I know that I can trust the work that these people do and their advice will be reliable, or in the area of experiment where I can make suggestions for things to be tried. That really provides tremendous leverage.
HOLLOWAY: Yeah. A lot of people become scared and say they don’t want to share their best ideas. But Eugene Haller is an example of a person who has shared with you ideas. I think a good person has more good ideas than they can possibly approach or attempt to understand in their lifetime.
ROWE: That’s right.
VAN de WALLE: Indeed.
HOLLOWAY: So they share and everybody benefits from that.
VAN de WALLE: Yes.
HOLLOWAY: That completes the topics that I thought I would like to touch on today. Do you have any that you would like to add?
VAN de WALLE: I think you’ve been pretty thorough in your range of questions. I think I had a chance to acknowledge most of the people at the risk, of course, of leaving people out. Maybe one last person who I do want to mention and who is actually a previous Welch Award winner is Jerry Tersoff, whom I’ve also known for almost 30 years. Of course I interacted with him while we were both at the IBM Watson Research Center. Jerry is just tremendously smart—scarily smart almost. He also has no fear when confronted with an interesting experimental observation or problem. He just digs in and explains it. I wish I could do that. I’m not nearly as good as he is at doing that, but it’s certainly inspirational to see somebody like him performing research at that level. I think we all need role models, right? We all need people who we can look to and say, “Gosh, I wish I could do that!” Well, even if you can’t quite do it yourself, to aspire to doing it motivates us.
HOLLOWAY: I think Jerry’s giving a talk at the symposium later this week.
ROWE: He is. I’ve met with him earlier today. I forget exactly when that talk is.
HOLLOWAY: But anyhow, we’re looking forward to your talk this afternoon.
ROWE: We sure are.
HOLLOWAY: So let me thank you again and congratulate you on a well-deserved Welch Award.
VAN de WALLE: Thank you.