Influence of grown-in defects on the optical and electrical properties of Si/Si_1-xGe_x/Si heterostructures

The optical, electrical, and structural quality of epitaxial Si/Si_1-xGe_x/Si layers has been studied as a function of growth temperature and layer deposition sequence. The epitaxial layers were grown by reduced pressure chemical vapor deposition in an ASM Epsilon-One reactor developed for production applications. The high material quality of both undoped and p-type modulation-doped Si/Si_1-xGe_x/Si heterostructures which were grown without a growth interruption of more than 30 s is indicated by sharp and intense Si_1-xGe_x photoluminescence with a FWHM of 6-10 meV for the no-phonon lines. Long growth interruptions, e.g. to change growth temperature between layer deposition, resulted in a reduced luminescence. However, this recovers after annealing the samples in a H₂-plasma at 400°C for 30 min, which indicates that non-radiative recombination centers are grown-in during epitaxial growth. The electrical quality of epitaxial Si/Si_1-xGe_x layers has been characterized by measuring the hole mobility of modulation doped samples at 4 K. Very high hole mobilities up to 8616 cm₂/Vs at hole densities of 6.5 × 10¹¹ cm^-2 were measured. The hole mobility did not depend on epitaxial growth temperature down to 625°C. Further, the growth sequence and subsequent thermal treatments after the growth did not have any negative influence on the high electrical quality of the epitaxial films.