| Summary: | 碩士 === 國立臺灣大學 === 電機工程學研究所 === 90 === The size distribution of self-assembled islands is critical to their application as quantum dots in novel devices. Strained islands formed in heteroepitaxy sometimes change shape during growth. Here we report a series of annealing experiments for Ge islands on Si(100) in order to clarify the shape transition of Ge islands. We employ identical deposition condition at 670。C by molecular beam epitaxy (MBE) but change the annealing condition to control island shapes. Brief annealing immediately following deposition allows island coarsening to take place. In situ non-contact atomic force microscope (NCAFM) and scanning tunneling microscopy (STM) imaging and cross-sectional analysis are used to document evolution of Ge islands. With increasing volume, a shape transition from pyramids to domes attribute to an abrupt change in chemical potential. We have observed bimodal size distribution will occur under specific condition, i.e. pyramids and domes will coexist. We also observed the distinct facets and very fast kinetics through pyramid to dome transition in our experiments.
Quantum dot infrared photodetectors (QDIPs) have some important advantages over quantum well infrared photodetectors (QWIPs) like lower dark current, higher internal gain and insensitive to the direction of polarization. Based on the knowledge of growing Ge islands, we designed the structure of our Ge quantum dot infrared photodetector sample. The sample was grown on P- Si(100) substrate with Ge quantum dots embedded in Si. We have investigated the photoresponse of our QDIP sample under different annealing time. For the sample annealing for 10 minutes, the broad excitation wavelength range is between about 2mm and 10mm. The absorption is attributed to transitions from hole states bound in Ge dots and wetting layer to continuum states. For sample annealing for 20 minutes, the excitation wavelength range between 2mm and 4mm appears. This may be caused by the wetting layer becomes SiGe alloy and larger Ge dots because of longer annealing time. The higher responsivity of the sample annealing for 10 minutes is consistent with this result, too.
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