Bistable Behaviour of the New Shallow Thermal Donor in Aluminum Doped Silicon

Abstract

Thermal donors (TDs) in Czochralski-grown silicon belong to the most studied defect systems in solids. They appear after prolonged thermal annealing of the oxygen-rich silicon in the temperature range of about 350-470°C. It is quite well established that the TDs are formed in a process of oxygen clustering, but the formation mechanism itself (simple diffusion cannot account for it), as well as, the influence of codoping remain still unclear. Also despite the extensive research done over last three decades no definitive models exist which could explain their microscopic structure [1-2]. The infrared absorption studies contributed significantly to our present knowledge on properties of TDs [2]. It has been established that two distinct spectral series can be related to TDs, proving them to be double donors (well described by the effective mass theory), with ground state energies of approximately 60 meV (TD0/TD+) and 150 meV (TD+/TD++) below the conduction band. Relatively recent investigations of photothermal ionization spectroscopy [3] revealed a series of lines being a signature of a new single donor species called shallow thermal donors (STDs). They are generated simultaneously to TD and they have ground state energies of about 30 meV.