Photoinduced excess carrier dynamics in PLD-grown ZnO

Abstract

Abstract ZnO shows a number of similarities with other wide gap semiconductor materials as, e.g., GaN as far as photoluminescence and photoconductivity are concerned. Depending on film quality a broad luminescence band is found in the yellow and/or green spectral region apart from a narrow excitonic line. This study focuses on the observation of non-exponential photoinduced carrier density decay in ZnO. We have deposited thin polycrystalline ZnO films on sapphire by a cyclic pulsed laser deposition process. We extracted a room temperature band gap of 3.31 eV from absorption spectroscopy measurements, and found evidence for strong sub gap Urbach tails. Photocurrent transients were measured upon pulsed laser excitation at 532 and 266 nm and compared with transient microwave conductivity decay upon excitation at 355 nm. Both measurements yield power-law decay with an exponent from −0.3 to −0.4. In addition, ps-pulses were used to monitor the initial photoluminescence decay near the bandgap. We have already observed similar power-law behaviour in polycrystalline GaN films prepared by the same PLD reactor. The interpretation will consider the hypothesis of minority carrier capture and a model invoking thermalization in broad band tail distribution with delayed subsequent recombination during the decay.