Cheng Buwen

Zero biased Ge-on-Si photodetector with a bandwidth of 4.72 GHz at 1550 nm ⁄

High quality Ge was epitaxially grown on Si using ultrahigh vacuum/chemical vapor deposition (UHV/CVD). This paper demonstrates efficient germanium-on-silicon p-i-n photodetectors with 0.8 mu m Ge, with responsivities as high as 0.38 and 0.21 A/W at 1.31 and 1.55 mu m, respectively. The dark current density is 0.37 mA/cm(2) and 29.4 mA/cm(2) at 0 V and a reverse bias of 0.5 V. The detector with a diameter of 30 mu m, a 3 dB-bandwidth of 4.72 GHz at an incident wavelength of 1550 nm and zero external bias has been measured. At a reverse bias of 3 V, the bandwidth is 6.28 GHz.

Substrate-induced stress in silicon nanocrystal/SiO2 multilayer structures

A Raman frequency upshift in the nc-Si phonon mode is observed at room temperature, which is attributed to a strong compressive stress in the Si nanocrystals. The 10-period amorphous-Si(3 nm)/amorphous-SiO2 (3 nm) layers are deposited by high-vacuum radio-frequency magnetron sputtering on quartz and sapphire substrates at different temperatures. The samples are then annealed in N2 atmosphere at 1100 °C for 1 h for Si crystallization. It is demonstrated that the presence of a supporting substrate at the high growth temperature can induce different types of stresses in the Si nanocrystal layers. The strain is attributed to the difference in the thermal expansion coefficient between the substrate and the Si/SiO2 SL film. Such a substrate-induced stress indicates a new method for tuning the optical and electronic properties of Si nanocrystals for strained engineering.

Evolution of Ge and SiGe Quantum Dots under Excimer Laser Annealing

We present different relaxation mechanisms of Ge and SiGe quantum dots under excimer laser annealing. Investigation of the coarsening and relaxation of the dots shows that the strain in Ge dots on Ge films is relaxed by dislocation since there is no interface between the Ge dots and the Ge layer, while the SiGe dots on Si0.77Ge0.23 film relax by lattice distortion to coherent dots, which results from the obvious interface between the SiGe dots and the Si0.77Ge0.23 film. The results are suggested and sustained by Vanderbilt and Wickhams theory, and also demonstrate that no bulk diffusion occurs during the excimer laser annealing.

Research progress of Si-based germanium materials and devices*

Si-based germanium is considered to be a promising platform for the integration of electronic and photonic devices due to its high carrier mobility, good optical properties, and compatibility with Si CMOS technology. However, some great challenges have to be confronted, such as: (1) the nature of indirect band gap of Ge; (2) the epitaxy of dislocation-free Ge layers on Si substrate; and (3) the immature technology for Ge devices. The aim of this paper is to give a review of the recent progress made in the field of epitaxy and optical properties of Ge heterostructures on Si substrate, as well as some key technologies on Ge devices. High crystal quality Ge epilayers, as well as Ge/SiGe multiple quantum wells with high Ge content, were successfully grown on Si substrate with a low-temperature Ge buffer layer. A local Ge condensation technique was proposed to prepare germanium-on-insulator (GOI) materials with high tensile strain for enhanced Ge direct band photoluminescence. The advances in formation of Ge n+p shallow junctions and the modulation of Schottky barrier height of metal/Ge contacts were a significant progress in Ge technology. Finally, the progress of Si-based Ge light emitters, photodetectors, and MOSFETs was briefly introduced. These results show that Si-based Ge heterostructure materials are promising for use in the next-generation of integrated circuits and optoelectronic circuits.

High performance silicon waveguide germanium photodetector

High-performance Ge-on-SOI p?i?n waveguide photodetectors with different sizes were fabricated. The performances, in terms of dark-current, photo current responsivity and 3-dB bandwidth, were well studied. A responsivity of 0.842?A/W at 1550?nm and dark current of 70?nA was measured from this detector at -1?V. The detector with a size of 4??m?10??m demonstrated an optical band width of 19?GHz at -5?V for 1550?nm. Both the experimental results and the finite-difference time domain simulation show that, when the device size is above a certain threshold, the absorption is not sensitively dependent on such designing parameters as the width and length of the photodetector.

Effects of high temperature rapid thermal annealing on Ge films grown on Si(001) substrate

Tensile strain, crystal quality, and surface morphology of 500 nm thick Ge films were improved after rapid thermal annealing at 900 °C for a short period ( 20 s) rapid thermal annealing, tensile strain and crystal quality degenerated. This phenomenon results from intensive Si—Ge mixing at high temperature.

Enhanced light trapping in periodically truncated cone silicon nanowire structure

Light trapping plays an important role in improving the conversion efficiency of thin-film solar cells. The good wideband light trapping is achieved using our periodically truncated cone Si nanowire (NW) structures, and their inherent mechanism is analyzed and simulated by FDTD solution software. Ordered cylinder Si NW structure with initial size of 80 nm and length of 200 nm is grown by pattern transfer and selective epitaxial growth. Truncated cone Si NW array is then obtained by thermal oxidation treatment. Its mean reflection in the range of 300–900 nm is lowered to be 5% using 140 nm long truncated cone Si NW structure, compared with that of 20% using cylinder counterparts. It indicates that periodically truncated Si cone structures trap the light efficiently to enhance the light harvesting in a wide spectral range and have the potential application in highly efficient NW solar cells.

Thin film micro-scaled cold cathode structures of undoped and Si-doped AlN grown on SiC substrate with low turn-on voltage

The field emission characteristics of the AlN thin films with micro-scaled cold cathode structures are tested in the high vacuum system. The aluminum nitride (AlN) thin films with a thickness of about 100 nm are prepared on the n-type 6H-SiC (0001) substrate at 1100 degrees C by metal organic chemical vapor deposition (MOCVD) under low pressure. The I-V curves and surface micro-images of undoped and Si-doped AlN films are investigated. From the I-V and Fowler-Nordheim plots, it can be seen that the Si-doped AlN shows better field emission characteristics compared with the undoped AlN sample. The obtained turn-on field is 6.7 V/mu m and the maximum emission current density is 154 mA/cm(2) at 69.3 V for the Si-doped AlN film cathode after proper surface treatment. It is proposed that the relatively low electric resistivity of Si-doped AlN films is significant for electron migration to the surface region, and their rougher surface morphology is beneficial to a higher local electric field enhancement for the field emission.

High bandwidth surface-illuminated InGaAs/InP uni-travelling-carrier photodetector

Uni-traveling-carrier photodiodes (UTC-PDs) with ultrafast response and high saturation output are reported. A gradient doping layer and a narrow InP cliff layer were introduced to enhance the saturation and bandwidth characteristics. We measured the dark current, photo response, bandwidth, and saturation current of the fabricated UTC devices. For a 15-μm-diameter device, the dark current was 3.5 nA at a reverse bias of 1 V, and the 3-dB bandwidth was 17.2 GHz at a reverse bias of 5 V, which are comparable to the theoretically values. The maximum responsivity at 1.55 μm was 0.32 A/W. The saturation output current was over 19.0 mA without bias.

Room-temperature direct-bandgap photoluminescence from strain-compensated Ge/SiGe multiple quantum wells on silicon

Strain-compensated Ge/Si0.15Ge0.85 multiple quantum wells were grown on an Si0.1Ge0.9 virtual substrate using ultrahigh vacuum chemical vapor deposition technology on an n+-Si(001) substrate. Photoluminescence measurements were performed at room temperature, and the quantum confinement effect of the direct-bandgap transitions of a Ge quantum well was observed, which is in good agreement with the calculated results. The luminescence mechanism was discussed by recombination rate analysis and the temperature dependence of the luminescence spectrum.