Scientists at the University of California, Santa Barbara (UCSB), have finally resolved a 30-year old mystery: why carbon, commonly believed to be a shallow acceptor in GaN, can give rise to luminescence at 2.2 eV, in the yellow region of the spectrum.
Yellow luminescence is almost universally observed in GaN, irrespective of growth technique. The first reports on the subject, dating back to the 1980s, attributed the luminescence to carbon; however, an explanation for the microscopic source of the emission never emerged.
Indeed, almost everyone in the community believed that carbon is a shallow acceptor, though it has never been observed to make GaN p-type. No plausible configuration that would lead to deep-level emission was ever proposed.
In a paper published in Applied Physics Letters [APL 97, 152108 (2010)], the group lead by Chris Van de Walle at UCSB now explains the role played by carbon. Using cutting-edge first-principles calculations, the researchers find that substitutional carbon on a nitrogen site is not a shallow acceptor at all; it is a very deep acceptor with an ionization energy of 0.90 eV.
John Lyons, the lead researcher on the project, also calculated the optical properties of this center, finding a broad emission centered at 2.14 eV, fully consistent with the experimentally observed luminescence.
Carbon is frequently intentionally added to GaN to achieve semi-insulating layers in transistor structures; the deep-level nature of the carbon acceptor now explains why this works so well.
Van de Walle also noted, “Carbon is also almost unavoidable as an unintentional impurity, particularly in MOCVD-grown GaN. Knowing its effects on electronic and optical properties is therefore essential!” The UCSB team pointed out that carbon is not the only source of yellow luminescence. Indeed, gallium vacancies have been confirmed to produce such luminescence in numerous experiments. But yellow luminescence is also observed in samples that contain no gallium vacancies, but do contain carbon; the mechanism for this emission has now been revealed at last.