Wang Qi-ming

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.

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.

High-speed 2 £ 2 silicon-based electro-optic switch with nanosecond switch time ⁄

A 2 £ 2 electro-optic switch is experimentally demonstrated using the optical structure of a Mach{Zehnder interferometer (MZI) based on a submicron rib waveguide and the electrical structure of a PIN diode on silicon-on-insulator (SOI). The switch behaviour is achieved through the plasma dispersion efiect of silicon. The device has a modulation arm of 1 mm in length and cross-section of 400 nm£340 nm. The measurement results show that the switch has a V…L… flgure of merit of 0.145 V¢cm and the extinction ratios of two output ports and cross talk are 40 dB, 28 dB and i28 dB, respectively. A 3 dB modulation bandwidth of 90 MHz and a switch time of 6.8 ns for the rise edge and 2.7 ns for the fall edge are also demonstrated.

Nonlinear Optical Response of nc-Si-SiO2 Films Studied with Femtosecond Four-Wave Mixing Technique

Nonlinear optical properties of silicon nanocrystals (nc-Si) embedded in SiO2 films are investigated using time-resolved four-wave mixing technique with a femtosecond laser. the off-resonant third-order nonlinear susceptibility chi((3)) is observed to be 1.3 x 10(-10) esu at 800 nm. The relaxation time of the film is fast as short as 50 fs. The off-resonant nonlinearity is predominantly electronic in origin and enhanced due to quantum confinement.

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.

Design of Waveguide Integrated Ge-Quantum-Well Electro-Absorption Modulators *

We present two designs for a waveguide Ge-quantum-well electro-absorption modulator. In our designs, the strip SOI waveguides are butt-coupled and evanescent-coupled to the modulator, respectively. The proposed Ge-quantum-well electro-absorption modulator is based on quantum-confined Stark effect (QCSE), having a 3-dB bandwidth above 50 GHz, as well as a low switching power (around 60 fJ/bit at 1435 nm). In the butt-coupled design, the optimized extinction ratio is up to 11.4 dB, while the insertion loss is only 6.74 dB. For the second one, which utilizes evanescent coupling, the extinction ratio and insertion loss are 9.18 dB and 6.72 dB, respectively.