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Photocurrent in a 4H-SiC p–n junction diode under illumination with sub-bandgap light was investigated. Under a high reverse bias condition, the photocurrent significantly increased with an increase in the reverse bias voltage. We calculated the photocurrent taking into consideration the phonon-assisted optical absorption due to the Franz–Keldysh effect. The calculated photocurrent showed good agreement with the experimental results. The photocurrent also increased at elevated temperatures, which could be quantitatively explained by the redshift of the 4H-SiC absorption edge (the shrinkage of the bandgap) and the increase in the phonon occupation number with rising temperature.

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Electric-field dependence of optical absorption induced by Franz–Keldysh (FK) effect strongly depends on the value of the reduced effective mass along electric field. In this study, reverse current–voltage characteristics of 4H-SiC{11 0} p–n junction diodes under sub-bandgap illumination were investigated. Under a reverse bias condition, a photocurrent induced by FK effect was observed and increased with the reverse voltage. We calculated a photocurrent with consideration of phonon-assisted optical absorption induced by FK effect in a depletion region using the reduced effective mass perpendicular to the c-axis (  = 0.26 ), and the calculated values showed good agreement with the experimental values. This result indicates that the anisotropy of optical absorption induced by FK effect in 4H-SiC is small, since the reduced effective mass perpendicular to the c-axis (  = 0.26 ) is close to that parallel to the c-axis (  = 0.28 ).