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We report a systematic study of time-resolved and power-dependent photoresponse in high-efficiency germanium quantum dot photodetectors (Ge-QD PDs), with internal quantum efficiencies greater than 100% over a broad wavelength, reverse bias, and incident power range. Turn-on and turn-off response times (τon and τoff) are shown to depend on series resistance, bias, optical power, and thickness (WQD) of the Ge-QD layer, with measured τoff values down to ∼40 ns. Two different photoconduction regimes are observed at low and high reverse bias, with a transition around -3 V. A transient current overshoot phenomenon is also observed, which depends on bias and illumination power. [ABSTRACT FROM AUTHOR]

The photon absorption in Si quantum dots (QDs) embedded in SiO2 has been systematically investigated by varying several parameters of the QD synthesis. Plasma-enhanced chemical vapor deposition (PECVD) or magnetron cosputtering (MS) have been used to deposit, upon quartz substrates, single layer, or multilayer structures of Si-rich-SiO2 (SRO) with different Si content (43–46 at. %). SRO samples have been annealed for 1 h in the 450–1250 °C range and characterized by optical absorption measurements, photoluminescence analysis, Rutherford backscattering spectrometry and x-ray Photoelectron Spectroscopy. After annealing up to 900 °C SRO films grown by MS show a higher absorption coefficient and a lower optical bandgap (∼2.0 eV) in comparison with that of PECVD samples, due to the lower density of Si–Si bonds and to the presence of nitrogen in PECVD materials. By increasing the Si content a reduction in the optical bandgap has been recorded, pointing out the role of Si–Si bonds density in the absorption process in small amorphous Si QDs. Both the photon absorption probability and energy threshold in amorphous Si QDs are higher than in bulk amorphous Si, evidencing a quantum confinement effect. For temperatures higher than 900 °C both the materials show an increase in the optical bandgap due to the amorphous-crystalline transition of the Si QDs. Fixed the SRO stoichiometry, no difference in the optical bandgap trend of multilayer or single layer structures is evidenced. These data can be profitably used to better implement Si QDs for future PV technologies. [ABSTRACT FROM AUTHOR]

Multilayers of Ge quantum dots (QDs, 3 nm in diameter) embedded in SiO2, separated by SiO2 barrier layer (3, 9, or 20 nm thick), have been synthesized by sputter deposition and characterized by transmission electron microscopy and light absorption spectroscopy. Quantum confinement affects the optical bandgap energy (1.9 eV for QDs, 0.8 eV for bulk Ge); moreover, the absorption probability greatly depends on the QD-QD distance. A strong electronic coupling among Ge QDs is evidenced, with a significant increase of the light absorption efficiency when the QD-QD distance is reduced. These data unveil promising aspects for light harvesting with nanostructures. [ABSTRACT FROM AUTHOR]

We report on high responsivity, broadband metal/insulator/semiconductor photodetectors with amorphous Ge quantum dots (a-Ge QDs) as the active absorbers embedded in a silicon dioxide matrix. Spectral responsivities between 1-4 A/W are achieved in the 500-900 nm wavelength range with internal quantum efficiencies (IQEs) as high as ∼700%. We investigate the role of a-Ge QDs in the photocurrent generation and explain the high IQE as a result of transport mechanisms via photoexcited QDs. These results suggest that a-Ge QDs are promising for high-performance integrated optoelectronic devices that are fully compatible with silicon technology in terms of fabrication and thermal budget. [ABSTRACT FROM AUTHOR]

In this work, the stationary and time-resolved electroluminescence (EL) properties of Si quantum dots embedded within a metal-oxide-semiconductor device are investigated. In particular, we measured the excitation cross section of Si nanocrystals under electrical pumping, finding a value of 4.7 × 10[sup -14] cm² which is two orders of magnitude higher with respect to the excitation cross section under 488 nm optical pumping. We also studied the radiative and nonradiative decay processes occurring in these devices by measuring the time evolution of the EL signal. We demonstrate that the mechanism responsible for the emission is the same under both electrical and optical pumping. The overall quantum efficiency of the electrical pumping is estimated to be two orders of magnitude higher than the quantum efficiency for optical pumping in all the studied temperature ranges. [ABSTRACT FROM AUTHOR]