It has been reported by one of the authors (A.H.) of the present study that ultrashallow thermal donors (USTDs) with a hydrogen-like electronic structure are formed in C- and N-doped O-rich Si crystals at approximately 450–600 °C. In this paper, it is reported that there are two types of USTDs: USTDs comprising C, H, and O [D(C,H,O)s] and USTDs comprising C, N, and O [D(C,N,O)s]. The thermal stability of these two types of donors was different; D(C,N,O)s were stable at 600 °C, while D(C,H,O)s were not stable at approximately 600 °C.

We previously reported on ultrashallow thermal donors (USTDs) in carbon-doped oxygen-containing monocrystalline silicon (Czochralski-grown, CZ-Si) crystals that were preannealed to introduce hydrogen at 1300°C, and then annealed at 480°C. In this study, the formation mechanism of the USTDs was evaluated. It was observed that an increase in the intensity of UTSDs leads to a reduction in that of hydrogen-related shallow thermal donors [STD(H)s], and the sum of the area intensities of the lines in the transmission spectra of USTDs and STD(H)s is nearly constant when the silicon crystals are annealed for longer than 10 h at 480°C. We also found some thermally activated processes linked to the formation of USTDs. We thus conclude that the mechanism is composed of the high-speed formation of STD(H)s in the first stage and carbon modulation of the electronic structure of STD(H)s in the second stage.

Ultrashallow thermal donors (USTDs), which consist of light element impurities such as carbon, hydrogen, and oxygen, have been found in Czochralski silicon (CZ Si) crystals. To the best of our knowledge, these are the shallowest hydrogen-like donors with negative central-cell corrections in Si. We observed the ground-state splitting of USTDs by far-infrared optical absorption at different temperatures. The upper groundstate levels are approximately 4 meV higher than the ground-state levels. This energy level splitting is also consistent with that obtained by thermal excitation from the ground state to the upper ground state. This is direct evidence that the wave function of the USTD ground state is made up of a linear combination of conduction band minimums.

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