# Oblique Deposition of Ti/Pt/Au Electrode on Photonic Crystal for Vertical Current Injection +
The GaAs core layer (red part in Figure 1) is sandwiched by two AlGaAs/AlGaO\({}_{\mathrm{x}}\) cladding layers, and they are covered by a GaAs contact layer. The cladding layers are selectively oxidized through air holes, and the AlGaAs at the cavity center is preserved as funnels for the current conduction because it is far from the outermost holes of the resonator. These cladding layers have such advantages as efficient vertical current injection, high thermal conductivity, and high mechanical stability, which together make RT-CW operations possible. Meanwhile, the oxidation width covers the WGM area perfectly, confining the light in the core layer so that the WGM can have a suitable quality factor \(Q\) around 5000-10,000. Since the CirD lasers can be simply tuned to operate at different wavelengths by adjusting the cavity's radius, we designed a monolithic optical module containing a CirD laser array, which is suitable for WDM Application (Figure 2a) [4]. As a benefit from the small size of the CirD cavity, this WDM device has a very small footprint. Twenty CirD cavities with different lasing wavelengths are arranged on both sides of a single PhC line-defect waveguide. Since the lattice constant \(a\) of the triangular PhC lattice is approximately 360 nm, the distance between two adjacent cavities can be set to 5 \(\upmu\)m. Each cavity is surrounded by more than ten PhC lattice periods except for the direction to the waveguide, which provides sufficient horizontal light confinement. This laser array handling 20 wavelengths needs a length of only 100 \(\upmu\)m. Because the top GaAs contact layer allows the current to diffuse, all of the cavities must be electrically isolated for independent operations. Around each cavity, a current blocking trench (CBT) is made by selectively etching the GaAs contact layer on the AlGaO\({}_{\mathrm{x}}\) cladding layer (Figure 2b) in order to obstruct the conductive path in the contact layer. The electrodes are deposited on each cavity. A 5-\(\upmu\)m-square area outside every cavity is used as the bonding pad, and thus the WDM device can be mounted to the circuits on the Si substrate by the flip-chip bonding method [4]. According to the rate equation calculations [4; 7; 17], a single CirD laser can be directly modulated at a rate of 50 Gbps under the injection current of 400 \(\upmu\)A, and thus a 20-channel CirD laser array can be expected to reach an extremely high bandwidth density of 10 Pb  
Figure 1: Schematic of a single Circular Defect (CirD) laser structure without electrodes. In our previous studies, the verticality of the profile of the 1.5-\(\upmu\)m-deep air holes was improved to ensure the quality of the PhC structure [18,19]. Selective oxidation of AlGaAs was studied to obtain the ideal oxidization parameters for good light confinement [20]. To block the current between the different cavities, a selective etching process was also studied to fabricate the CBT on the PhC pattern [21]. However, the operation of a single CirD laser by vertical current injection has not been accomplished successfully, due to the immaturity of the technology for electrode fabrication on a PhC structure. First, current leakage is often found during the test, which is believed to be due to the electrode material contacting the core layer. Current leakage must be eliminated to ensure that the current is injected into the resonator. Furthermore, the resistance of a single diode must be reduced to a very low level so that the internal temperature does not rise excessively when sufficient current is injected. Current methods of electrode deposition are too inefficient to meet the above requirements. As far as we know, no report has demonstrated the ability to deposit thick electrodes for current injection on PhC. During the deposition, it is hard to prevent the electrode materials from covering the side faces of air holes in PhCs, which may cause current leakage and degradation of PhC optical properties.