The ZnSe epitaxial layer with wide band gap have been grown by molecular beam epitaxy(MBE) at 280 ℃ to use light emitting devices in the blue spectral region.
GaAs, which has a small lattice mismatch to ZnSe(∼0.27%), has been the substrate of choice for the growth of ZnSe layer. The lattice mismatch and the difference in the thermal expansion coefficients are some of the sources of crystal defects generated at the epilayer/substrate interface in ZnSe/GaAs heterostructures. One of the main problems affecting the efficiency of ZnSe-based devices is the propagation of crystal defects associated with the heteroepitaxial growth of ZnSe on GaAs. Therefore, the analysis of degrading factors is essential in order to improve the quality of the heteroepitaxial ZnSe layer.
For the analysis, ZnSe film with 800 Å and 1 ㎛ were grown on GaAs(001) and characterized by TEM, HRXRD, AFM to study structural properties. And PL(photoluminescence) was measured for studying the optical properties with relation to the structural problems. Also, defect change was observed by application of annealing on behalf of various degradation conditions that real device could meet.
The thin film of high quality was observed by HRXRD and the initial island growth was characterized by AFM image with 800 Å and 1 ㎛ ZnSe thin film. This island growth caused the various defects and they were observed by TEM. It was turned out that most defects were stacking faults below the critical thickness and perfect dislocation above the critical thickness through the TEM observation. These differences were caused by the relaxation process during the growth. Those results showed the change of optical properties and they were proved by PL measurement. Yo peak with relation to perfect dislocation was observed above the critical thickness. Samples were annealed for 3 min in $N_2$ at as dep., 350, 450, 550 by RTA(rapid thermal annealing). FWHM of DCRC for each sample increased as the annealing temperature increase and the peak of thin film shifted to the peak of substrate. These changes showed that the quality of thin film was lowered. This phenominon was certified by TEM image. According to the annealing greater evaporation was observed around defects above the critical thickness. This result showed that there was relatively much defect and the evaporation was relatively active in that portion with many structural problems. According to the annealing these defect movements decreased the optical properties and that phenominon was observed by PL measurement. According to the annealing defects increased and this change increased Iv/Ex, Yo/Ex ratio in PL measurement.
In this reaserch, various defects were observed in the ZnSe thin film and the optical propeties that these defects affected was evaluated.
