Ultrashallow thermal donor formation in silicon by annealing in ambient oxygen

Czochralski-grown silicon wafers doped with phosphorus (similar to 10(14) c m(-3)) have been annealed in nitrogen, wet nitrogen, oxygen, argon, and vac uum ambients at 470 degrees C for times up to 500 h. Sample characterizatio n was made using predominantly electrical techniques such as admittance spe ctroscopy and thermally stimulated capacitance measurements but also second ary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscop y were employed. In all samples, an increasing concentration of free carrie r electrons is observed with increasing annealing time, reaching a maximum of similar to 10(16) cm(-3) at 100 h. For durations in excess of 100 h grad ual decrease of the free electron concentration takes place except for the samples treated in wet nitrogen and oxygen atmospheres, which display donor s stable even after 200 h. These stable centers are found to have shallower donor level positions in the energy band gap (similar to 25 meV below the conduction band edge E-c) than those of the centers formed in vacuum, argon , and nitrogen atmospheres (similar to 35 meV below E-c). The latter center s are associated with the well-established shallow thermal donors (STDs) wh ile the origin of the former ones, which are labeled ultrashallow thermal d onors (USTDs) is less known. However, on the basis of a wealth of experimen tal results we show that the USTDs are most likely perturbated STDs modifie d through interaction with fast-in diffusing oxygen species, possibly oxyge n dimers. Further, comparison between the electrical data and the SIMS meas urements reveals unambiguously that neither the STD nor the USTD centers in volve nitrogen, in contrast to recent suggestions in the literature. (C) 19 99 American Institute of Physics. [S0021-8979(99)06512-3].