Study of amplified emission in polycrystalline ZnO below characteristic temperature

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.