Using the Atomistically Informed Device Engineering (AIDE) Method to Identify Experimentally "Invisible" Defects—The Mystery of the Ga Vacancy in Gallium Arsenide

Atomic defects play a major role in the electrical properties of semiconductor materials and devices. As devices shrink, their impact will magnify which makes their identity and formation increasingly important. In gallium arsenide (GaAs), electron irradiation displaces atoms from their lattice sites producing mobile interstitial and immobile vacancy defects on both sublattices. For more than 40 years, defects in GaAs have been investigated with deep level transient spectroscopy revealing five defect centers labeled E1-E5. These were firmly identified only decades later when density functional theory associated the divacancy (v_Gav_As) with E1-E2 and the As vacancy (v_As) with E3. Mysteriously, the Ga vacancy is absent from experimental observation despite theory predicting several charge states including a (3-/2-) transition at mid-gap which should be readily observed. This implores the question: Where is the missing Ga vacancy? To unravel this mystery, we developed a multiscale Atomistically Informed Device Engineering (AIDE) method to study the dynamical behavior of defects in an irradiated Si-doped GaAs material. Capable of probing the rapid evolution of defects, the AIDE method uses reliable DFT data (e.g., defect levels) and available experimental measurements (e.g., capture cross-sections) as input parameters. After the initial radiation creates defects, electrons flow to swiftly equilibrate charge. Charge equilibration (charge carrier reactions) fills the (3-/2-) Ga vacancy nearly instantaneously and pulls the Fermi level from the Si-doping level to mid-gap. This Fermi level shift changes the charge state for each defect and produces positively charged, highly mobile interstitials (As_i¹⁺ and Ga_i¹⁺) and negatively charged immobile vacancies (v_As^1-, v_Gav_As^2-, and v_Ga^3-) as the initial population of defects. Experiment and theory indicate that the As_i¹⁺ is a fast athermal diffuser and will dominate the short-time early defect reactions. Focusing our simulations on these fastest As_i¹⁺ reactions, we discover that the As_i¹⁺ preferentially annihilates the v_Ga^3- in a Coulomb-driven defect reaction. The Ga vacancy population plummets below detectable concentration limits before it can be measured. Therefore, the Ga vacancy’s absence is not due to some experimental blindness or theoretical inaccuracies but because they have been annihilated by the initial wave of fast As interstitials. These findings resolve the longstanding mystery of the missing Ga vacancy, and the AIDE method brings a new dynamical approach to understanding defect physics. The AIDE method is generalizable and can provide insight into the rich defect physics that occurs in these experimentally unobservable regimes.
--This work was supported by a Laboratory Directed Research and Development (LDRD) Project (No. 229430). Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.