PHOTOLUMINESCENCE AND TRANSPORT STUDIES OF BORON IN 4H SIC

Two distinct boron-related centers are known in silicon carbide polyty pes, one shallow (ionization energy similar to 300 meV) and the other deep (similar to 650 meV). In this work, 4H SiC homoepitaxial films ar e intentionally doped with the shallow boron center by controlling the silicon to carbon source gas ratio during chemical vapor deposition, based on site competition epitaxy. The dominance of the shallow boron center for samples grown with a low Si/C ratio, favoring the incorpora tion of boron onto the silicon sublattice, is verified by the temperat ure dependent Hall effect, admittance spectroscopy and deep level tran sient spectroscopy. In these samples a peak near 3838 Angstrom appears in the low temperature photoluminescence spectrum. Further experiment s support the identification of this peak with the recombination of a four particle (bound exciton) complex associated with the neutral shal low boron acceptor as follows: (1) The intensity of the 3838 Angstrom peak grows with added boron. (2) Momentum conserving phonon replicas a re observed, with energies consistent with other four particle complex es in SiC. (3) With increasing temperature excited states are observed , as for the neutral aluminum and gallium acceptor four particle compl exes. However, the intensity of the shallow boron spectrum is quenched at lower temperatures than the corresponding spectra for Al and Ga, a nd the lineshapes are strongly sample dependent. These results may be related to the unusual configurational and electronic structure of thi s center inferred from recent spin resonance experiments by other grou ps. When the Si/C ratio is high, the optical signatures of the deep bo ron center, nitrogen-boron donor-acceptor pairs and conduction band to neutral acceptor free-to-bound transitions, are observed in the photo luminescence. At T=2K well resolved, detailed nitrogen-boron pair line spectra are observed in addition to the peak due to distant pairs. As the temperature is raised, the donor-acceptor pair spectrum decreases in intensity while the free-to-bound no-phonon peak appears. Extrapol ation of the temperature dependence of the free-to-bound peak to T=0 K , after correction for the temperature dependence of the exciton energ y gap, leads to the value E-A(B) - E-X = 628 +/- 1 meV, where E-A(B) i s the ionization energy of the deep boron center and E-X is the bindin g energy of the free exciton which, for 4H SiC, can only be estimated at this time. (C) 1998 American Institute of Physics.