The Department of Physics and Astronomy at the University of Pittsburgh is proud to celebrate the 100th birthday of Professor Emeritus Dr. Wolfgang “Jim” Choyke, a pioneer in the physics of silicon carbide and a cherished colleague, educator, mentor, and friend.
Few scientists have the opportunity to watch the material to which they devoted their careers evolve from a research subject into one of the world’s most important semiconductors. Jim Choyke is one of them.
For more than six decades, Jim devoted his scientific career to understanding silicon carbide (SiC). In so doing, he had the opportunity to see the material to which he devoted his career evolve from research subject to one of the world’s most important semiconductors. Jim began studying SiC in the 1950s, before anyone imagined it would become key to modern power electronics. Today, SiC, a wide-band gap semiconductor, enables more efficient electric vehicles, renewable energy systems, solid-state transformers, and AI data centers, while it is also a promising platform for quantum sensing. Many of the fundamental optical and electronic properties that make these technologies possible were established through Jim’s research.
The history of silicon carbide is as remarkable as the material. SiC formed the basis of some of the first commercially important semiconductor electronic devices. SiC crystal detectors and rectifiers were used in radio receivers in the early 20th century. The earliest practical semiconductor light-emitting diodes (LEDs) were made from SiC in the 1920s. These early devices worked, but the microscopic physics that governed them remained largely unknown. The technique for growing high-quality crystals of SiC was developed in the 1950s. The Lely method produced mm-scale hexagons of SiC sufficiently pure for the characterization of the electronic properties of SiC.
It was into this scientific landscape that Jim Choyke entered. Working first at the Westinghouse Research Laboratories and later at the University of Pittsburgh, Jim and his collaborators (Dave Hamilton, Lyle Patrick, and others) carried out a series of spectroscopic experiments that established many of the electronic properties of SiC. Among their most influential discoveries was the demonstration that the band gap varies systematically among the SiC polytypes according to their crystal stacking sequence. This empirical relationship, now known as the Choyke–Hamilton–Patrick relation, revealed that changing only the stacking of identical silicon and carbon atoms could produce measurable changes in the electronic structure of the crystal. The Choyke-Hamilton-Patrick relation shows that band gap increases linearly with hexagonality. Their results , from Phys. Rev. 133, A1163 (1964), are shown in the accompanying figures. Three decades later, ab initio electronic-structure calculations by Käckel et al. (Phys. Rev. B, 50, 10761, 1994) provided the microscopic explanation for the relationship. Jim and his collaborators went on to help establish the indirect nature of the fundamental band gaps in the technologically important SiC polytypes and carried out measurements of excitons, phonon-assisted optical transitions, and impurity and defect states. These studies became references for experimentalists and theorists seeking to understand the electronic structure of SiC.
Equally important were Jim’s investigations into optical defects in SiC. Through the course of many careful spectroscopic studies, Choyke and his collaborators identified and characterized numerous defect centers that would later become central to quantum sensing and quantum information science. Today, researchers intentionally engineer silicon vacancies and divacancies into SiC crystals to create sensitive quantum sensors capable of exquisite measurements of magnetic fields, electric fields, strain, and temperature. Jim’s work is an example of the way in which careful fundamental measurements, performed decades before their technological significance was appreciated, can lay the foundation for future discovery.
Jim’s scientific style is characterized by patience and attention to detail. His measurements established standards against which theoretical calculations and experimental investigations have been compared. He co-authored numerous influential papers and co-edited the landmark reference volumes Silicon Carbide: A Review of Fundamental Questions and Applications to Current Device Technology (1997) and Silicon Carbide: Recent Major Advances (2004), works that serve as references for the SiC community.
Perhaps most remarkably, Jim’s is still going. At age 97 he published his historical chapter, Silicon Carbide: Presolar SiC Stardust Grains and the Human History of SiC from 1824 to 1974 (2023), a delightful reflection on both the scientific history of SiC and the extraordinary journey of a material whose electronic properties he helped reveal. Today, at 100 years old, Jim remains a regular presence in the Department of Physics and Astronomy, where colleagues and students continue to benefit from his curiosity, generosity, and enthusiasm for physics.
Jim’s centennial also inspired a thoughtful essay by our colleague Professor Jeremy Levy, One Hundred Years of Fortitude, which intertwines three stories born in 1926: the birth of quantum mechanics through Schrödinger’s equation, the construction of the Cathedral of Learning, and Jim Choyke’s own remarkable life. You can find that essay here.
On behalf of the Department of Physics and Astronomy at the University of Pittsburgh, we wish Professor Emeritus Wolfgang “Jim” Choyke a very happy 100th birthday and thank him for a lifetime of discovery.
Happy 100th Birthday, Jim!
