OPTICAL PROPERTIES OF CVD GROWN DILUTED Ⅱ-Ⅵ MAGNETIC SEMICONDUCTOR NANOSTRUCTURES

摘要

Optical properties of diluted magnetic semiconductor (DMS) are not well understood so far, especially relationship to their ferromagnetism. Here we prepared Mn ion doped ZnO, CdS and ZnSe nanostructures by CVD method, studied their optical properties via microphotoluminescence techniques, found many very interesting properties, which are all related to the exciton magnetic polaron (EMP), itinerant or partially itinerant, their energy levels are agree well with the AB initio calculations. In ZnO:Mn nanowires, the EMP can show up with free exciton together for very diluted doping(＜0.001%), this EMP can form condensate to produce single mode lasing line at fs pulse excitation along with the disappearing of free excitons. With a little bit large amount of Mn doping, the nanowire show EMP lasing mode with background at fs laser pulse excitation, but at even high power, some electron-hole plasma induced lasing modes could be observed due to the carrier effect. The time-delay photoluminescence by ns laser pulse are also studied, only free EMP and localized EMP(d-d transition) show up in the emission spectra, we gave the clear assignments for all the d-d transitions of Mn in ZnO, which have been argued for a long time. It is more interesting that these d-d transitions exhibit clear enhanced coherent relaxation behaviors with increasing excitation power, like that by free excitons, behave a collective spin-dependent coherent radiation. We also observed the Mn-O-Mn cluster peak in the long wavelength range, which may be related to the ferromagnetic properties. In CdS:Mn nanowires, we found many peaks above the single Mn ion emission band (575nm) when increasing the Mn concentration, we used a simple Hydrogen-like cloud theoretical model to describe them well, in this model, the Mn-S-Mn- cluster with variable Mn ion number and their ferromagnetic coupling are considered. The SQUID detection proved its ferromagnetic behavior, and MFM result indicated its cluster nature in nanobelt. Ab initio calculation results also support our cluster assignments. In ZnSe:Mn nanobelt, doping often produce many optical domains which can works as optical cavities to produce cavity modes in a wide spectral range, this can be easily observed in the CW laser excitation. If we turn to the ns laser, we can detect bandedge emission at low power, but they goes to stimulated emission by EMP at high power excitation. This stimulated emission is usually limited by the magnitude of optical domain size. All above experimental results indicate that the d-d transition is not absolutely localized within Mn ion, it is excited by light to become localized EMP. The Mn-O(or S)-Mn aggregate can be detected by the microphotoluminescence technique with their emission and ferromagnetism, this may be used to study more DMS substances with wide band. These findings facilitate to get better understanding of DMS ferromagnetism and find novel applications in booptics and magnetics.