Raman spectroscopy, x-ray diffractometry and atomic force microscopy have been used to characterize ZnO thin films grown by pulsed laser deposition as a function of the postgrowth annealing temperature. The results show substantial enhancement and broadening of certain Raman features which correlate excellently with the change in width of the xray diffraction peaks. The 570cm -1 Raman feature showed pronounced asymmetry and enhanced intensity in the unannealed sample. An increase in grain size observed after subsequent annealing produced a substantial reduction in both the asymmetry and intensity of this peak. Our experimental data suggest that electric fields, due to charge trapping at grain boundaries, in conjunction with localised and surface phonon modes are the cause of the intensity enhancement and asymmetry of this feature.

We present optically pumped emission data for ZnO, showing that high excitation effects and stimulated emission/lasing are observed in nanocrystalline ZnO thin films at room temperature, although such effects are not seen in bulk material of better optical quality. A simple model of exciton density profiles is developed which explains our results and those of other authors. Inhibition of exciton diffusion in nanocrystalline samples compared to bulk significantly increases exciton densities in the former, leading-via the nonlinear dependence of emission in the exciton bands on the pump intensity-to large increases in emission and to stimulated emission. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2174107͔ ZnO has recently emerged as a strong candidate for ultraviolet photonic devices, with attractive properties, including a large exciton binding energy of 60 meV enabling efficient excitonic emission at room temperature. Many groups have studied stimulated emission and lasing properties of bulk ZnO, under electron beam 1 and optical pumping, 2 but evidence for stimulated emission and lasing has only been seen at rather low temperatures. Recent studies of optically pumped nanocrystalline samples reveal stimulated emission and lasing even at room temperature, in contrast to the bulk studies, even in samples of similar or inferior quality compared to bulk crystals. [3][4][5][6] We have studied optically pumped emission at room temperature from bulk single-crystal material ͑Eagle-Picher Corporation͒ and samples grown by pulsed laser deposition ͑PLD͒ on c sapphire. Details of the sample growth are given elsewhere. 7 Some films were annealed in O 2 ͑0.3 mbar͒ between 400°C and 600°C in the growth chamber after deposition leading to a consistent increase in grain size ͑30 → 80 nm͒ with annealing temperature. 7 All films show evidence of electric-field damping of the free exciton ͑FE͒ and film thicknesses are in the range of 200-250 nm. 7,8 The samples were studied using continuous-wave photoluminescence ͑PL͒ and high excitation pumping with a frequencytripled Nd:YAG laser ͑355 nm 10 Hz 6 ns pulse width͒ focused on the sample to a diameter of ϳ1.2 mm ͑determined by the knife-edge method͒ enabling excitation intensities in the range of 500-7000 kW/ cm 2 . Full details are described in Refs. 7 and 9. Figure 1͑a͒ shows emission at 300 K from a PLD sample ͑annealed at 500°C͒ as a function of increasing Nd:YAG excitation power. The data from this sample are representative of all the PLD samples. With increasing excitation, one observes the growth first of the FE band at 3.33 eV and then the P band at 3.23 eV, corresponding to the well-known exciton-exciton collision process, ϳ100 meV below the FE. 2 The emission bands are significantly broadened due to the damping effects mentioned earlier. The plot of integrated emission intensity versus excitation power shown in Fig. 1͑b͒ exhibits a distinct threshold at a pump power slightly above 2000 kW/ cm 2 , which is not explained by a superlinear power-law dependence of emis...

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