Photoluminescence and transport studies of boron in 4H SiC

Two distinct boron-related centers are known in silicon carbide polytypes, one shallow (ionization energy $\text{∼300\hspace{0.17em}}\mathrm{meV})$ and the other deep $\text{(∼650\hspace{0.17em}}\mathrm{meV}\mathrm{).}$ In this work, 4H SiC homoepitaxial films are 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 incorporation of boron onto the silicon sublattice, is verified by the temperature dependent Hall effect, admittance spectroscopy and deep level transient spectroscopy. In these samples a peak near 3838 Å appears in the low temperature photoluminescence spectrum. Further experiments support the identification of this peak with the recombination of a four particle (bound exciton) complex associated with the neutral shallow boron acceptor as follows: (1) The intensity of the 3838 Å peak grows with added boron. (2) Momentum conserving phonon replicas are observed, with energies consistent with other four particle complexes in SiC. (3) With increasing temperature excited states are observed, as for the neutral aluminum and gallium acceptor four particle complexes. However, the intensity of the shallow boron spectrum is quenched at lower temperatures than the corresponding spectra for Al and Ga, and the lineshapes are strongly sample dependent. These results may be related to the unusual configurational and electronic structure of this center inferred from recent spin resonance experiments by other groups. When the Si/C ratio is high, the optical signatures of the deep boron center, nitrogen-boron donor-acceptor pairs and conduction band to neutral acceptor free-to-bound transitions, are observed in the photoluminescence. At $\text{T=2\hspace{0.17em}}\mathrm{K}$ 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. Extrapolation of the temperature dependence of the free-to-bound peak to $\text{T=0\hspace{0.17em}}\mathrm{K},$ after correction for the temperature dependence of the exciton energy gap, leads to the value $E_A{\mathrm{(B)-E}}_X\text{=628±1\hspace{0.17em}}\mathrm{meV},$ where $E_A\mathrm{(B)}$ is the ionization energy of the deep boron center and $E_X$ is the binding energy of the free exciton which, for 4H SiC, can only be estimated at this time.