Zhongchao Fan

Directional emission InP/GaInAsP square-resonator microlasers

InP/GaInAsP square-resonator microlasers with an output waveguide connected to the midpoint of one side of the square are fabricated by standard photolithography and inductively-coupled-plasma etching technique. For a 20-mum-side square microlaser with a 2-mum-wide output waveguide, cw threshold current is 11 mA at room temperature, and the highest mode Q factor is 1.0x10(4) measured from the mode linewidth at the injection current of 10 mA. Multimode oscillation is observed with the lasing mode wavelength 1546 nm and the side-mode suppression ratio of 20 dB at the injection current of 15 mA.

AlGaInAs–InP Microcylinder Lasers Connected With an Output Waveguide

AlGaInAs-InP microcylinder lasers connected with an output waveguide are fabricated by planar technology. Room-temperature continuous-wave operation with a threshold current of 8 mA is realized for a microcylinder laser with the radius of 10 μm and the output waveguide width of 2 μm. The mode Q-factor of 1.2 x 104 is measured from the laser spectrum at the threshold. Coupled mode characteristics are analyzed by 2-D finite-difference time-domain simulation and the analytical solution of whispering-gallery modes. The calculated mode Q-factors of coupled modes are in the same order as the measured value.

Equilateral-Triangle-Resonator Injection Lasers With Directional Emission

Equilateral-triangle-resonator (ETR) lasers with an output waveguide jointed at one vertex of the resonator are fabricated on (100) GaInAsP-InP wafers using photolithography and a two-step inductively coupled plasma (ICP) etching technique. Distinct peaks with the mode spacing of longitudinal mode intervals are observed in the luminescence spectra at room temperature. Furthermore, some minor peaks appear in the middle of the main peaks, which can be attributed to the first-order transverse modes as predicted in the theoretical results. CW directional lasing emissions are achieved for ETR lasers with side lengths ranging from 15 to 30 mum up to 200 K. The temperature dependences of the threshold current and lasing wavelength are measured for an ETR laser with the side length of 20 mum from 80 to 200 K. The observed threshold current rapidly increases as temperature increases over 170 K

The thermal stability study and improvement of 4H-SiC ohmic contact

The thermal stability of the standard Ni/SiC and a TiW/Ni/SiC Ohmic contacts was investigated and compared after being aged at 400 °C in the N2 atmosphere. The Ohmic contact was characterized using a combination of I-V measurements, the optical microscopic imaging, X-ray diffraction (XRD), and Auger electron spectroscopy (AES) techniques. It is shown that the standard Ni/SiC Ohmic contact failed after being aged at 400 °C for 20 h in the N2 atmosphere, while the TiW/Ni/SiC Ohmic contact could stand after 100 h. The TiW/Ni/SiC Ohmic contact was found kept a smooth surface morphology during the rapid thermal annealing and aging process, while the standard Ni/SiC Ohmic metal surface was found rougher. Both the Ohmic contact deteriorations after high temperature aging could be attributed to the formation of graphite which is confirmed by the XRD results. The XRD and AES results reveal that the better thermal stability of the TiW/Ni/SiC could be explained by the formation of CW3 and TiC, which deter the C atom diffusion to form graphite.

Fabrication and optical optimization of spot-size converters with strong cladding layers

A silicon-on-insulator (SOI) optical fiber-to-waveguide spot-size converter (SSC) overlaid with specially treated silica is investigated for integrated optical circuits. Unlike the conventional process of simply depositing the hot silica on silicon waveguides, two successive layers of silicon dioxide were grown on etched SSC structures by PECVD (plasma-enhanced chemical vapor deposition). The two layers have 0. 8% index contrast and supply stronger cladding for an incident light beam. Additionally, this process is able to reduce the effective refractive index of the input mode to less than 1.47 (extremely close to that of the fiber), substantially weakening the unwanted back reflection. Exploiting this technology, it was demonstrated that the SSC showed a theoretical low mode mismatch loss of 1.23 dB for a TE-like mode and has an experimental coupling efficiency of 66%.

Fully lithography independent fabrication of nanogap electrodes for lateral phase-change random access memory application

A nanogap electrode fabrication method was developed and nanogap electrode as small as 17 nm was achieved based on sacrificial spacer process and conventional lithography. We have transferred this method to lateral phase-change random access memory (PCRAM) device fabrication. The electrical characterizations of 4.6 μm gap width using conventional lithography and 88 nm width based on this technology are shown. It is found that the threshold voltage and the dc power consumption are remarkably decreased due to nanogap electrode process. Our method cannot only improve the fabrication efficiency of PCRAM but also be easily transferred to other nanoelectronics applications.

Design, Fabrication, and Characterization of an Ultracompact Low-Loss Photonic Crystal Corner Mirror

An ultracompact, low-loss, and broad-band corner mirror, based on photonic crystals, is investigated in this paper. Based on the theoretical analysis of the loss mechanism, the boundary layers of the photonic crystal region are revised to improve the extra losses, and the transmission characteristics are evaluated by using the 3-D finite-difference time-domain method. The device with optimized structure was fabricated on silicon-on- insulator substrate by using electron-beam lithography and inductively coupled plasma etching. The measured extra losses are about 1.1plusmn0.4 dB per corner mirror for transverse-electronic polarization for the scanning wavelength range of 1510-1630 nm. Dimensions of the achieved PC corner mirror are less than 7times7 mum2, which are only about one tenth of conventional waveguide corner mirrors.

A Compact and Highly Efficient Silicon-Based Asymmetric Mach-Zehnder Modulator with Broadband Spectral Operation

An asymmetric Mach?Zehnder electro-optic modulator is demonstrated by using a silicon-based p-i-n diode embedded in compact 200 ?m long phase shifters. The measured figure of merit V?L = 0.23 V ?mm shows highly efficient modulation by the device, and an open eye-diagram at 3.2 Gbit/s confirmed its fast electro-optic response. Integrated with the grating coupler, the device exhibits a broad operational wavelength range of 70 nm with a uniform 18 dB extinction ratio covering the C-band and part L-band of optical communication.

Selective and lithography-independent fabrication of 20 nm nano-gap electrodes and nano-channels for nanoelectrofluidics applications

A new method has been developed to selectively fabricate nano-gap electrodes and nano-channels by conventional lithography. Based on a sacrificial spacer process, we have successfully obtained sub-100-nm nano-gap electrodes and nano-channels and further reduced the dimensions to 20 nm by shrinking the sacrificial spacer size. Our method shows good selectivity between nano-gap electrodes and nano-channels due to different sacrificial spacer etch conditions. There is no length limit for the nano-gap electrode and the nano-channel. The method reported in this paper also allows for wafer scale fabrication, high throughput, low cost, and good compatibility with modern semiconductor technology.

Pseudo-Rhombus-Shaped Subwavelength Crossed Gratings of GaAs for Broadband Antireflection

Holographic lithography coupled with the nonlinear response of photoresist to the exposure is adopted to fabricate porous photoresist (PR) mask. Conventional dot PR mask is also generated, and both patterns are transferred into a underlying GaAs substrate by the optimal dry etching process to obtain tapered subwavelength crossed gratings (SWCGs) to mimic the moth-eye structure. In comparison of the experiment and simulation, the closely-packed pseudo-rhombus-shaped GaAs SWCGs resulting from the porous mask outperforms the conical counterpart which comes from the dot mask, and achieves a reported lowest mean spectral reflectance of 1.1%.