Biexciton emission from high-quality ZnO films grown on epitaxial GaN by plasma-assisted molecular-beam epitaxy

We have investigated the optical and structural properties of high-quality ZnO films grown on epitaxial GaN (epi-GaN) by plasma-assisted molecular-bea m epitaxy employing low-temperature buffer layers. High-resolution x-ray di ffraction for both symmetric and asymmetric reflexes shows that crystalline defects in ZnO films have a similarity to epi-GaN used as a substrate. The quality of ZnO epilayers grown on epi-GaN is basically determined by epi-G aN. The photoluminescence (PL) spectrum at 10 K exhibits very sharp exciton emission with a linewidth of 1.5 meV, while deep-level emission is negligi ble, indicative of small residual strain. At 77 K, PL is dominated by a fre e-exciton emission line in the low-excitation regime, while it is overtaken by a new emission band due to biexcitons at its low-energy side as the exc itation intensity increases. This biexciton emission band emerges even unde r the intermediate excitation regime of 100 W/cm(2), which is 100 times sma ller than the previously reported threshold for bulk ZnO. The biexciton bin ding energy is estimated to be 15 meV, in agreement with previous results. At the higher excitation regime, the emission line due to exciton-exciton s cattering dominates the PL spectrum. (C) 2000 American Institute of Physics . [S0003-6951(00)01830-1].