  We report on amplified emission in polycrystalline ZnO below 100K. At 6K
emission is due to free exciton FXn=1 A (3.378 eV), bound exciton D1XA (3.347
eV), donor acceptor pair DAP (3.275 eV) and longitudinal phonon replica of free
excitons (FXn=1 A -mLO, m= 1,2,3) respectively. Peak intensity of D1XA and
FXn=1 A -1LO transition increases non-linearly with increase of excitation
intensity due to exciton-exciton scattering. Peak position of D1XA shows red
shift while FXn=1 A -1LO shows blue shift with increase of excitation
intensity. Fraction of exciton taking part in emission process and radiative
lifetime of exciton decreases with decrease of temperature .Threshold value of
excitation for D1XA decreases exponentially with decrease of temperature upto
100K. Whereas, below 100K no functional behaviour is observed. Threshold for
FXn=1A-1LO emission was observed up to 75K, at higher temperature it is mixed
with D1XA. Time delayed photoluminescence measurement reveals that amplified
emission decay faster than spontaneous emission.
