# Oblique Deposition of Ti/Pt/Au Electrode on Photonic Crystal for Vertical Current Injection +
## 2 Process and Measurement System for Electrode Process Verification  
Prior to the actual manufacture of the WDM device containing multiple CirD lasers, the conductive property of a single CirD diode structure must be verified to establish a reliable fabrication process. Therefore, in this study, we simplified the structure by placing the CBT outside of the PhC pattern with a CirD cavity, and dry etching was performed only one time. Different from the 5-\(\upmu\)m-square bonding pad for the flip-bonding in the proposed device, a 90-\(\upmu\)m-square electrode pad was designed for contact with the probe in the electrical measurement system. In the fabrication of the device in Figure 2, another selective etching must be performed for the CBT [21]. The CBT pattern would then be etched to the top of the upper AlGaOx layer to maintain the integrity of the waveguide. However, in this study, the dry etching process only needs to be carried out one time, making the process simpler. The epi-wafer used in this section was grown by molecular beam epitaxy (MBE). A 300-nm-thick n-GaAs buffer layer was grown on a 450-\(\upmu\)m-thick n-GaAs substrate. The thickness of each of the two Al\({}_{0.95}\)Ga\({}_{0.05}\)As cladding layers is 500 nm. The GaAs core layer, including three stacked self-assembled InAs QD layers, has a total thickness of 220 nm. The photoluminescence (PL) peak of these QDs was designed at around 1290 nm (PL experimental data showed 1288.5 nm) to reduce the absorbing loss of output light (1300-1320 nm) propagating in the line defect waveguide. A 180-nm-thick p-GaAs contact layer is placed above the cladding layer. Figure 3 shows the fabrication process flow of the CirD Laser and its electrode for current injection measurement. First, the PhC structure was patterned on the resist (ZEP520A) by electron-beam (EB) lithography. An inductively coupled plasma reactive-ion etching (ICP-RIE) process was used to make the air holes and the CBT surrounding the PhC pattern (Figure 3b). This etching ends at the n-GaAs substrate layer. Then, the resist was removed using the ZDMAC remover. The AlGaAs layers were oxidized to AlGaOx through the air holes by H2O/N2 steam at a temperature of 390 \({}^{\circ}\)C (Figure 3c). The process shown in Figure 3d is an oblique electrode deposition. We spin-coated the resist again and drew the electrode pattern. The resist enters the CBT, but this can be ignored. The resist in the CBT is removed in the step of lifting off. We deposited the electrode on the PhC pattern after performing another overlaid EB process. As a result, the electrode only remained inside the PhC pattern area surrounded by the CBT. The top electrode is made in the order of 30 nm of Ti, 10 nm of Pt, and 150 nm of Au. Then, we deposited the bottom electrode in the order of 20 nm of AuGe and 200 nm of Au by the resistive thermal evaporation method and annealed the sample to make the electrodes adhere tightly. With these processes, we can prevent the current from leaking outside of the PhC pattern. The only conductive path is in the unoxidized funnels. This is important for the independent operation of each  
Figure 3: **(a–e) Fabrication process of CirD laser and its electrode for current-voltage characteristics measurement. (f) Top view of the sample. The size of the electrode pad is 90 \(\upmu\)m square. **CirD cavity in the WDM device. In order to prevent metal atoms from entering the cladding layer or the core layer through the air holes, we considered depositing the Ti/Pt/Au metal atoms at an angle of \(60^{\circ}\) in the top electrode fabrication. EB evaporation is applied in making the top electrode so that the direction of material deposition can be controlled. As shown in Figure 4a, we use a holder to set the sample in the EB evaporation equipment (ULVAC, UEP-2000 OT-H/C) to make an \(\alpha=60^{\circ}\) angle between the direction of the sample and the vapor flux.