The PL spectrum was measured using the 514.5 nm line of
an argon-ion laser. The average excitation power and the spot
diameter of the laser beam were 50 mW and 0.5 mm,
respectively. The PL spectrum was collected by aspheric lens
group with three groups and three elements (F/ 1.25) and
analyzed with a 1 m focal length single monochromator (Jobin
Yvon THR-1000) and detected with a liquid nitrogen-cooled
Ge p-i-n photodiode (Edingburg Instruments) and amplified by
the lock-in technique.