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In recent years, quantum dots have been successfully grown by self-assembling processes. For optoelectronic device applications, the quantum-dot structures have advantages such as reduced phonon scattering, longer carrier lifetime, and lower detector noise due to low-dimensional confinement effect. Comparing to traditional optoelectronic III-V and other materials, self-assembled Ge quantum dots grown on Si substrates have a potential to be monolithically integrated with advanced Si-based technology. In this paper, we describe the growth of self-assembled, guided Ge quantum dots, and Ge quantum-dot superlattices on Si. For dot growth, issues such as growth conditions and their effects on the dot morphology are reviewed. Then vertical correlation and dot morphology evolution are addressed in relation to the critical thickness of Ge quantum-dot superlattices. In addition, we also discuss the quantum-dot p-i-p photodetectors (QDIPs) and n-i-n photodetectors for mid-infrared applications, and the quantum-dot p-i-n photodetectors for 1.3-1.55 mum for communications applications. The wavelength of SiGe p-i-p QDIP can be tuned by the size as grown by various patterning methods. Photoresponse is demonstrated for an n-i-n structure in both the mid-infrared and far-infrared wavelength ranges. The p-i-n diodes exhibit low dark current and high quantum efficiency. The characteristics of fabricated light-emitting diode (LED) devices are also discussed, and room-temperature electroluminescence is observed for Ge quantum-dot LED. The results indicate that Ge dot materials are potentially applicable for mid-infrared (8-12 mum) detectors as well as fiber-optic (1.3-1.55 mum) communications.