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Mid-infrared photodetector based on sub-monolayer (SML) quantum dot quantum cascade structure monolithically grown on silicon substrate has been demonstrated in this work. Both the optical and electrical characteristics of the SML quantum dot quantum cascade photodetectors (QD-QCD) were analyzed quantitatively. The performances of these devices were compared with that on native GaAs substrate. A large resistance-area (R0A) product of 1.13×10 7 Ω.cm 2 is achieved at 77 K for the silicon based devices, which is only roughly one order less than that on GaAs substrate. The device shows a normal-incident peak responsivity of 0.59 mA/W under zero bias at the wavelength of 6.2 μm at 77 K, indicating a photovoltaic operation mode. Johnson noise limited specific detectivity is 3×10 10 cm•Hz 1/2 /W at 77 K, with photoresponse up to 100 K. These results suggest that the silicon-based QD-QCD in this work is a very promising candidate for large format mid-infrared focal plane array and mid-infrared silicon photonics applications.

An InGaAs quantum dot (QD) photodetector is directly grown on a silicon substrate. GaAs-on-Si virtual substrates with a defect density in the order of 10 6 cm −2 are fabricated by using strainedlayer superlattice as dislocation filters. As a result of the high quality virtual substrate, fabrication of QD layer with good structural properties has been achieved, as evidenced by transmission electron microscopy and x-ray diffraction measurements. The InGaAs QD infrared photodetector is then fabricated on the GaAs-on-Si wafer substrate. Dual-band photoresponse is observed at 80 K with two response peaks around 6 and 15 μm.

In this work, we demonstrate a sub-monolayer quantum dot quantum cascade photodetector (SML QD-QCD) grown on GaAs for photovoltaic mid-infrared photo-detection. The detector shows normal-incident peak responsivity of 1.90 mA/W at the wavelength of 6.05 μm and a resistance-area product of 1.54×10 8 Ω﹒cm 2 at 77 K. The corresponding specific detectivity is 3.22×10 11 cm•Hz 1/2 /W. Quantum cascade photodetector (QCD) has recently attracted substantial interests due to the advantage that no external bias voltage is required for operation, which leads to zero dark current, much lower noise, relatively high operating temperature, and low power dissipation 1. Quantum cascade photodetectors based on quantum well (QW-QCDs) have been demonstrated in short infrared wavelength 2 , mid infrared wavelength 3 4 and long infrared wavelength ranges 5. However, for QW-QCDs, there is no intrinsic response to normal incidence light, due to the inter-subband transition selection rules of quantum wells. Plasmonic waveguides 6 , grating 7 , and photonic crystal slab 8 have been used to make the QW-QCD sensitive to normal incident light. Quantum dot infrared photodetectors (QDIP) have been extensively studied due to the advantages of three dimensional confinement of carriers, which is expect to have lower dark current, long excited state lifetime and intrinsic sensitivity to normal incident light 9. In order to achieve normal incidence intra-band response in QCD, the quantum well layer used for light absorption is replaced by quantum dots, which is very similar to the case of quantum well infrared photodetector versus quantum dot infrared photodetector. Moreover, due to the fact that the carrier capture time in quantum dots is much _____________________________ a) Jian Huang and Daqian Guo contributed equally to this work.

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A nanocomposite Ni–B/Ga(As)Ox/GaAs photoanode fabricated by combining molecular beam epitaxy with in situ photoassisted electrodeposition enables efficient and stable photoelectrochemical water splitting.

Focused-ion-beam chemical vapor deposition (FIB-CVD) is a key technology to achieve the arbitrary three-dimensional (3D) nano-and microstructure fabrication. However, most of 3D nanostructure fabrications have focused on upward or lateral growth whereas few of them focused on downward growth. The downward growth characteristics of air nanowires were investigated in order to achieve the fabrication of more arbitrary 3D nanostructures. As a result, it was not able to fabricate a 1 m long downward growth nanowire with an angle lower than À10 . Also, we found that a nanowire with an angle lower than À18 cannot be fabricated. Furthermore, we demonstrated the fabrication of 3D nanostructure including downward growth by changing the scan speeds of Ga þ FIB during fabrication. With controlling the downward growth, the fabrication of more various 3D nanostructures can be achieved. #

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