3.2 Fabrication processes
3.2.1 Fabrication processes of mesa structure
Fig. 3. 3: The fabrication processes of photonic crystal pattern.
3.2.1 Fabrication Processes of Mesa Structure
3.2.1.1 Photolithography the First Time and ICP Dry-Etching
At first, we will deposit SiNx layer of 140 nm on the InP by PECVD. The SiNx
layer here is served as hard mask for quantum wells beneath it. Then we will define the mesa pattern by photolithography process. Here we design the mesa sizes of 40, 50, and 60 μm in diameter, respectively, which are smaller than that of 70, 90, and 110 μm we used before due to the too large mesa size will easy to collapse, and we can shrink the device size at the same time. The mesa mask pattern is shown in Fig. 3.
4. The electrode is designed in ring-shape with the width of 10 μm, and have margin of 2 μm with the mesa edge to keep a tolerance in the second time photolithography.
The distance between mesas is 100 μm, and the rectangle and cross shape are alignment keys to help us align in photolithography steps.
The PR that we used is FH-6400. The parameters of PR spinning are 1,000 and 4,000 rev / sec for 10 and 30 second, respectively. The thickness of the PR in this recipe is about 1.6 µm. This thick PR layer will not be removed before the following undercut step, because it can prevent the MQWs collapse from the van der Waals force. The exposure and develop time in this recipe are 30 and 60 seconds, respectively. The developing dilute is FHD-5. Subsequently, we will etch the SiN and InP by a serious of ICP-RIE dry-etchig processes. We will transfer the patterns of mesas to SiN without cleaning PR. And then transferring patterns to QW and InP. The etching of InP recipe should be as short as possible or it will destroy the PR layer on the top. Now, the mesa without undercut has been fabricated.
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Fig. 3. 4: Photo mask design of mesa (40、50、60 µm in diameter) and electrode (10µm).
3.2.1.2 Undercut the First Time and PR Filling
After the mesa is formed, we then fabricate the undercut under MQWs by wet-etching process without remnant PR removing. The undercut is aim to produce the large post, which is the region we will write photonic crystal pattern on and prepared for current pathway fabrication. And we don’t remove the remnant PR from the first time photolithography for holding the MQWs, and prevent it bending from ven der Waals force. At the wet-etching step, the dilute HCl solution of HCl : H2O = 3 : 1 at room temperature is used for large etched region, the wet-etching rate and time are 10 μm and 67 second, respectively. The parameter of wet-etching time is crucial for the characteristics of device, which affect the size of the large post; too large post size will induce large leaky current and force the electrical property bad, as shown in Fig. 3. 5 (a) and (b) , however, too small post size will lead to MQWs slab bend and destroy the structure of quantum wells, as shown in Fig. 3. 5 (c) and (d) . Besides, the height of post is 800 nm as the same as p-InP sacrificial layer, and the height of filled PR would also be 800 nm without the MQWs slab bending. After numerous experiments, we then find the wet-etching time of 65 second with 40 μ m-diameter-mesa at room temperature will be appropriate for post fabrication.
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Fig. 3. 5 The tilted and cross-section SEM picture of the (a) (b) too large post size and (c) (d) too small post size. The remnant PR on the top of these SEM pictures have been cleaned in the following step.
After the undercut fabrication, we should fill it with PR instantly, otherwise, the MQWs layer will easy to bend for freestanding. The filled PR here not only to hold MQWs layer, but be a current block to isolate the MQWs and substrate, forcing injected current drift through the region of photonic crystal pattern. And we achieve the PR filling by spin coater with rotation of 1,000 and 4,000 rev / sec for 10 and 5 second, respectively. The tilted and cross-section SEM picture of the sample filled with PR successfully with appropriate size is shown in Fig. 3. 6 (a) and (b).
Fig. 3. 6: The (a) tilted and (b) cross-section SEM picture of the sample filled with PR under MQWs slab successfully with appropriate post size. And the remnant PR on the top of these SEM pictures have been cleaned in the following step.
We have fabricated the large post in two cases of 40 and 50 μm in diameter.
For mesa of 50 μm in diameter, the large post is fabricated with wet-etching time of 85 and 90 second, as shown in Fig. 3. 7. The post size and height of filled PR are about 33.3 μm and 450 nm with wet-etching time of 85 second, as shown in Fig. 3.7 (a) (b), and those are about 28 μm and 334 nm with wet-etching time of 90 second, as shown in Fig. 3.7 (c) (d). Due to the freestanding region of MQWs layer is seriously bending and the MQWs are destroyed, we then try to fabricate the large post with mesa of 40μm in diameter.
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Fig. 3. 7: The tilted and cross-section SEM picture of 50μm-diameter-mesa filled with PR under MQWs slab ,and the post size and height of PR are 33.3 μm and 450 nm for wet-etching time of 85 second (a) (b) , 28 μm and 334 nm for wet-etching time of 90 second (c) (d). And the remnant PR on the top of these SEM pictures have been cleaned in the following step.
For mesa of 40 μm in diameter, the large post is fabricated with wet-etching time of 60, 65, and 70 second, as shown in Fig. 3. 8. The post size and height of filled PR are about 24.6 μm and 771 nm with wet-etching time of 60 second, as shown in Fig. 3.8 (a) (b), and those are about 21.6 μm and 750 nm with wet-etching time of 65 second, as shown in Fig. 3.8 (c) (d), 17.1 μm and 253 nm with wet-etching time of 70 second, as shown in Fig. 3.8 (e) (f). The wet-etching time of 60 second can successfully fill the PR in the wet-etched region and not induce slab bending, but the post size is a little bit large for the case of 40μm-diameter-mesa. On the other hand, the wet-etching time of 70 second could fabricate a small post size to prevent the leaky current through, however, the MQWs layer bending is still serious, such as in 50
μm-diameter-mesa case. Consequently, we will choose the wet-etching time of 65 second due to the acceptable post size and undestroyed MQWs layer in 40μ m-diameter-mesa case.
Fig. 3. 8: The tilted and cross-section SEM picture of 40μm-diameter-mesa filled with PR under MQWs slab ,and the post size and height of PR are 24.6 μm and 771 nm for wet-etching time of 60 second (a) (b) , 21.6 μm and 750 nm for wet-etching time of 65 second (c) (d), 17.1 μm and 253 nm for wet-etching time of 70 second (e) (f). And the remnant PR on the top of these SEM pictures have been cleaned in the following step.
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3.2.1.3 Remnant PR Clean, and Hard Bake
For the following lithography and electrode deposited steps, we need to clean the remnant PR and SiNx on the top of MQWs slab. First, we use O2 plasma to remove the most remnant PR, and then the less remnant PR will be cleaned clearly after removing SiNx by BOE wet-etching process. In Fig. 3. 9 (a) , we apply O2
plasma of 2 min, and the mesa is still with few remnant PR. In Fig. 3. 9 (b), the remnant PR is removed completely with O2 plasma for 3 min, and we can see the post shape exactly in the top view. The SEM pictures from Fig. 3. 5 to Fig. 3. 8 are all processed with remnant PR and SiNx removing.
Considering the filled PR may be dissolved by acetone at the lift-off step, we will hard bake the sample at high temperature of 300 ℃ for 2 min by hot plate before the second time lithography. And the filled PR will be harden that undissolvable by acetone.
Fig. 3. 9 : The tilted SEM picture with O2 plasma of (a) 2 min and (b) 3 min. The former is still with few PR, and the later is cleaned clearly.
3.2.1.4 Photolithography the Second Time and Electrode Deposited
Then we will define the pattern of electrode by the second photolithography.
Although the pattern may not in the center of mesa correctly due to the alignment is not east to perform, we can still compensate it by aligning at E-beam lithography step.
And we will deposit the electrode of Au/Ge/Ni for 200/35/35 nm by E-gun evaporator.
The electrode is designed for ring-shape for effectively current injected. After lift-off, the mesa structure with ring-shape metal contact is successfully fabricated, as shown in Fig. 3. 10 (a) (b) .
Fig. 3. 10 : (a) (b) The SEM pictures of mesa structure successfully fabricated with ring-shape Au/Ge/Ni metal contact.
After fabrication process of mesa structure, we measure the I-V curve by HP-4156B. The measured result of 40μm-diameter-mesa structure is shown in Fig. 3.
11 (a). We also fabricate 50μm-diameter-mesa structure without PR filling for comparison in following step, the measured I-V curve is shown in Fig 3. 11 (b). We can know the mesa structure that is a p-n diode from Fig. 3. 11 and we can also
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confirm that the Au/Ge/Ni electrode is ohmic contact. The turn-on voltage and resistance of 40μm-diameter-mesa are about 4.51 V and 523 Ω, and that of 3.88 V and 550.9 Ω for 50μm-diameter-mesa.
Furthermore, we also measure the PL (photoluminescence) spectrum of original sample (unprocessed), 40 and 50μm-diameter-mesa structure, as shown in Fig. 3. 12, the intensity of emission peak are 826.4, 672.2, and 569 pW, respectively, with pump power of 1.7 mW. And we find the MQWs layer is degraded for a little.
Fig. 3. 11 : The measured I-V curve of device (a) with PR and (b) without PR, the turn-on voltage of each are 4.51 V and 3.88 V, and the resistance of each are 523 Ω and 550.9 Ω, respectively.
Fig. 3. 12 : The measured PL spectra of (a) original sample (unprocessed), (b) mesa with PR and (c) without PR, with intensity of emission peak of 826.4, 672.2, and 569 pW, respectively, with pump power of 1.7 mW.