Optical and photoelectrical properties of oriented ZnO films

ZnO films that have been used as active layers of optically pumped lasers are investigated by a number of optical and photoelectric techniques, such as transmission, reflectivity, photoluminescence and direct and alternating photocurrents. Homogeneity with respect to “bulk” and surface properties is evaluated. The spectral position of the lowest intrinsic exciton peak $E_x^{\mathrm{(A,B)}}$ is determined to be at $\text{(3.323±0.002)\hspace{0.05em}}\mathrm{eV}$ at 295 K. In spite of the rather small thickness of about 0.5 μm, the samples do not exhibit strain-induced modifications of the optical properties at the band edge. A deep level is found to be responsible for the room temperature photocurrent, and excitonic features appear as absorption lines. On the other hand, excitonic photocurrent peaks are observed at low temperatures; however, the conduction mechanism still remains defect related.