Celebrating notable advances in compound semiconductors: A tribute to Dr. Wladyslaw Walukiewicz

The Special Topic “Native Defects, Impurities and the Electronic Structure of Compound Semiconductors” is dedicated to Dr. Wladyslaw (Wladek) Walukiewicz, our esteemed colleague who passed away on November 9, 2022, at age 76. It is not often that a scientist is able to profoundly impact more than one to two scientific topics in their lifetime. Dr. Walukiewicz, a condensed matter theorist by training, arguably did that for at least four. While his scientific accomplishments tell the story of a successful career and valuable contributions, it does not convey the entire story. For the rest, one has only to look at the community of researchers he brought together across the globe. Dr. Walukiewicz conducted his work with curiosity, integrity, and respect for others that elevated those around him. Here, we pay tribute to Dr. Walukiewicz's ideas and the legacy he leaves behind.

Dr. Walukiewicz received his master's degree from Warsaw University in 1971 and went on to earn his Ph.D. from the Institute of Physics of the Polish Academy of Sciences in 1979. After coming to the U.S. twice as a visiting scientist (once at the National Magnet Laboratory in Boston and again at MIT), he moved there permanently in 1983 to take a position as a staff scientist at the Lawrence Berkeley National Laboratory, where he worked until his retirement in 2020. Toward the end of his career, Dr. Walukiewicz also served as an Adjunct Professor in the Department of Materials Science at the University of California, Berkeley and participated in the Singapore-Berkeley Research Initiative for Sustainable Energy.

The common thread in Dr. Walukiewicz's work was understanding the behavior of defects in semiconductors and their impact on material and device performance. In the late 1970s, he calculated carrier mobilities in InP and GaAs including all possible scattering processes such as ionized donors and acceptors, neutral impurities, deformation potential, piezo electric, polar- and nonpolar-optical and acoustic phonon scatterings, and tabulated the relationship between 77 K carrier concentration and mobility using compensation ratio as a parameter. The work was published in JAP in 1979 for GaAs¹ and 1980 for InP.² His table allows determination of compensation ratio in these materials by simply measuring carrier concentration and mobility at 77 K.³ The results provide a common, guiding principle to crystal growers for how to improve their crystal quality and doping efficiencies.

Dr. Walukiewicz's amphoteric defect model⁴ and the concept of a Fermi stabilization energy,^5,6 developed in the 1980s, has helped to explain doping limits in semiconductors, especially wide bandgap materials.⁷ Similarly, his band anticrossing (BAC) model provides an elegant conceptual framework for understanding the extreme evolution of the band structures in highly mismatched alloys (those with dilute concentrations of substitutional anions with much higher or lower electronegativity).^8–10 In 2002, Dr. Walukiewicz and co-workers' work additionally elucidated the important link between Mn interstitials and ferromagnetism in dilute magnetic semiconductors.^11–13 

Also in 2002, the discovery of the narrow band gap of InN by Dr. Walukiewicz and his co-workers made a tremendous impact to the semiconductors research society.^14–16 In 2015, his InN paper published in APL became one of the 50 most cited papers in APL's 50 years history.¹⁴ It continues to offer inspiration to the development of high efficiency solar cells,^17–19 broad-spectrum LEDs,²⁰ and full-color display based on In-rich InGaN alloys with much less lattice mismatch and high materials compatibility.

This “Native Defects, Impurities and the Electronic Structure of Compound Semiconductors” Special Topic includes a total of 32 papers (6 invited perspectives, 3 tutorials, and 23 research articles) covering a wide range of topics on native defects, impurities, and the electronic structure in various compound semiconductors. These articles highlight how Dr. Walukiewicz's ideas, models, and insights continue to shape our work on semiconductors, including newer classes of materials, such as metal halide perovskites²¹ and 2D transition metal dichalcogenides. In particular, several perspectives and tutorials feature topics that Dr. Walukiewicz helped to shape, including evaluating the electronic structure of highly mismatched alloys (Broderick et al.²² and Gladysiewicz and Wartak²³), exploring the behavior of Mn in III–V semiconductors (Furdyna et al.²⁴ and Piskorska-Hommel and Gas²⁵), understanding the role of defects in semiconductors (Cai and Wei,²⁶ Ko et al.,²⁷ and Li et al.²⁸) and probing the effects of defects and impurities in crystals (Makkonen and Tuomisto²⁹ and McCluskey.³⁰) Many more papers in this collection contribute new research results and analysis on topics that build on the concepts Dr. Walukiewicz helped to inform. His insights into the role of defects and dopants in semiconductors continue to serve an important function in the advancement of well-known and new materials.^31–34 These concepts are particularly timely in the development of a variety of wide bandgap nitrides, oxides, carbides and tellurides.^35–46 New research also adds to the foundational knowledge of highly mismatched alloys that Dr. Walukiewicz initially helped to establish.^47–52 

We close with our fond memories of Dr. Walukiewicz as a highly respected scientist, a thoughtful collaborator, a caring mentor, a devoted family-man, and an avid skier and runner. He will be greatly missed.

We are grateful to Dr. Kin Man Yu for his leadership in spearheading this special issue. This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.