PHOTOREFLECTANCE TEMPERATURE-DEPENDENCE OF GRADED INALAS INGAAS HETEROJUNCTION BIPOLAR-TRANSISTOR LAYERS/

Photoreflectance(PR) spectra of the graded InAlAs/InGaAs heterojunctio n bipolar transistor layers were investigated at various temperatures between 8 K and 300 K. The energy features of the PR spectra were fitt ed and identified as band-to-band transitions in the graded layers whi ch were grown by pulsed molecular beam epitaxy (pulsed-MBE) and InGaAs as well as InAlAs layers. The temperature variation of energy gaps ca n be described by the Varshni and Bose-Einstein equations. A linear va riation relationship of band gaps with Al composition (z) was observed and approximated to be E(0)(z)=0.809+0.769z eV at T=0 K. However, the parameters a(B) and Theta(B) derived from the Bose-Einstein expressio n do not change meaningfully in the whole range of Al composition. Fro m the observed Franz-Keldysh oscillations (FKOs) we have evaluated the built-in de electric fields in the i-InGaAs collector, i-InGaAs space r and n-InAlAs emitter regions. The electric fields are in good agreem ent with the continuity condition of electric displacements in the int erfaces between emitter and base. (C) 1995 American Institute of Physi cs.