  In spite of several articles, the origin of visible luminescence from
germanium nanocrystals in SiO$_2$ matrix is controversial even today. Some
authors attribute the luminescence to quantum confinement of charge carriers in
these nanocrystals. On the other hand, surface or defect states formed during
the growth process, have also been proposed as the source of luminescence in
this system. We have addressed this long standing query by simultaneous
photoluminescence and Raman measurements on germanium nanocrystals embedded in
SiO$_2$ matrix, grown by two different techniques: (i) low energy
ion-implantation and (ii) atom beam sputtering. Along with our own experimental
observations, we have summarized relevant information available in the
literature and proposed a \emph{Hybrid Model} to explain the visible
photoluminescence from nanocrystalline germanium in SiO$_2$ matrix.
