Abstract
Recently, scientists have devoted to the study of visible light emission of Si quantum dots (SQD). Although the SQD has the highest efficiency among the Si-based luminescence materials, it is difficult to apply to the integrated circuit from the available semiconductor techniques. In this experiment, the PL mechanism for the isolate Si quantum dots (ISQD) was studied in the first part and an “ambient state model” was proposed. In the second part, a novel structure, coalesced Si quantum dots (CSQD), was prepared, and the PL characteristics were studied. In the first part, ISQD was prepared by the thermal evaporation method. The size distribution and average size of the ISQD were estimated by TEM and XRD, respectively. FTIR spectroscopy was employed to analyze the surface composition. The PL and UV-visible light absorption spectroscopy were used to investigate the PL mechanism for the ISQD. With the particle size larger than 9 nm, the PL mechanism could be ascribed to the “surface state model”. When the particle size was smaller than 9 nm, the mechanism could be described by the “core/surface state model”. Besides, aged ISQD with particle size of 3 nm or 5 nm displayed a blueshift in the PL spectra, which could be attributed to the quantum confinement effect (QCE). On the other hand, the gases such as H2, N2, and Ar were found to affect the PL spectra of ISQD. The effect demonstrated that the absorbed gas molecules could modify the surface states and thus the PL characteristics for ISQD. The second part concerns with the discovery of a new structure of the SQD, i.e., CSQD. Both TEM and SEM indicated that the CSQD was composed of 8-nm quantum dots. From the XRD pattern, the average crystallite size was about 7 nm. In addition, FITR and ESCA analyses indicated that hydrogen, oxygen, and nitrogen were adsorbed on the CSQD surface. The PL from the CSQD measured in vacuum exhibited much stronger white light. The intensity was larger than that of ISQD of 11 nm by a factor of ~10. The PL spectrum showed two luminescence bands：one was in the blue-green range (1.82∼3.54 eV) that was attributed to the effect of typical Si:O:H bonds, and the other was in the near-IR range (1.24∼1.82ev) that could be ascribed to the QCE. The PL intensity was enhanced by a factor of 2~3 when testing in hydrogen, nitrogen, or air. However, it was reduced in Ar. This phenomenon was ascribed to the change in the surface state by the adsorbed gas molecules. The PL spectrum of the CSQD was similar to that of nanocrystalline Si film, but the intensity was much larger. Moreover, this coalesced structure has a mixed characteristics of quantum dot and a-Si film, and can be applied to the Si-based integration technique. It has a potential application in the optical devices. Key words：、、ISQD、、CSQD、、、XRD、TEM、SEM、