Molecular-beam epitaxial growth of tensile-strained and n-doped Ge/Si(001) films using a GaP decomposition source
Résumé
We have combined numerous characterization techniques to investigate the growth of tensile-strained and
n-doped Ge films on Si(001) substrates by means of solid-source molecular-beam epitaxy. The Ge growth
was carried out using a two-step growth method: a low-temperature growth to produce strain relaxed
and smooth buffer layers, followed by a high-temperature growth to get high crystalline quality Ge layers.
It is shown that the Ge/Si Stranski–Krastanov growth mode can be completely suppressed when the growth
is performed at substrate temperatures ranging between 260 °C and 300 °C. X-ray diffraction measure-
ments indicate that the Ge films grown at temperatures of 700–770 °C are tensile-strained with typical
values lying in the range of 0.22–0.24%. Cyclic annealing allows further increase in the tensile strain up to
0.30%, which represents the highest value ever reported in the Ge/Si system. n-Doping of Ge was carried
out using a GaP decomposition source. It is shown that heavy n-doping levels are obtained at low substrate
temperatures (210–250 °C). For a GaP source temperature of 725 °C and a substrate temperature of 210 °C,
a phosphorus concentration of about 10 19 cm −3 can be obtained. Photoluminescence measurements reveal
an intensity enhancement of about 16 times of the direct band gap emission and display a redshift of
25 meV that can be attributed to band gap narrowing due to a high n-doping level. Finally, we discuss
about growth strategies allowing optimizing the Ge growth/doping process for optoelectronic applications.