2.1 Black GaAs nanoarrays fabrication process
All samples were cut into 1.5 cm × 2 cm pieces of bulk GaAs, the samples were pre-cleaned with conventional solvent and rinsed in deionized (DI) water. Then the experiments were performed in an Oxford System100 etching reaction chamber, equipped with a maximum available power 900 W, 13.56 MHz RF coil generator. The gases employed in this study were BCl3, Cl2, Ar, N2 and O2. During all the experiments, the temperature of the electrode was fixed at 25℃. A 5-min-long oxygen clean procedure was performed between each run to remove any polymer from the reactor sidewalls, minimize contamination, and preserve process repeatability. The samples were loaded into the reactor by mounting them on an SiO2 carrier wafer,since the sample was etched at low temperature, silicone grease was unnecessary before etching process. As part of the optimization of the etching parameters, different etching time for measuring the process outcome was employed, as shown in Fig. 1.
Figure 1 showed SEM images of GaAs substrate under different etching time. From the picture we can see that etching depth increases with the increment of etching time, but the morphology of the sample does not change greatly. After etching, the surfaces of GaAs samples become flocculent, relatively uniform in height but scattered around. This kind of flocculent structure greatly increases the specific surface area of the device and can be applied in the fields of supercapacitors and sensors.
We also tested the reflectivity of the prepared structure with Agilent's Cary 7000 spectrophotometer and found that the flocculent structure of GaAs sample had a very low reflectivity, as shown in Fig. 2. In the wavelength range of 590–800 nm, the reflectivity is 3 min < 5 min < 4 min. In the wavelength range of 400–590 nm, the reflectivity is 5 min < 4 min < 3 min. In the meantime, we can see that the reflectivity of the samples under different etching time is very low, with a difference of less than 1%. Considering the time and cost in the actual process, we choose 3 min as the fixed etching time in the subsequent experiments. We attribute the decrement of reflectivity to the rough structure formed on the GaAs surface. The sample formed a cluster structure after etching and the roughened surface will limit the reflection of light and reduce the scattering of light, thus reducing the reflectivity of light. To verify our conclusion, AFM images were performed on the surface of the etched sample and the unetched sample, as shown in Fig. 3. The results show that the surface roughness of the etched sample is much larger than that of the unetched sample.
Then we investigate the effect of etching gas flow rate on the surface morphology and reflectivity of the sample when etching time is fixed at 3 min, and the oxygen flow was controlled. Here the role of oxygen is to form oxides during the etching process, and because of the different volatilization temperatures during etching process, oxygen reacts with base atoms to form a micro-mask, thus affecting the etching result. Here, the oxygen flow ratio is set as 2:3:4, and the SEM images after etching are shown in Fig. 4. It can be seen from the figure that when the oxygen flow ratio is 3, the etched GaAs surface presents a neat columnar shape, while when the oxygen flow ratio increases to 4, the GaAs surface becomes bright, and the sample surface is found to be smooth and without any pattern, as shown in Fig. 4c. The main reason for this phenomenon is that as the increment of oxygen flow, the etching rate will be accelerated. When the oxygen flow increases to a certain extent, the structure formed during etching process on the substrate surface will be etched away, thus obtaining the sample shown in Fig. 4c. Cary7000 spectrophotometer was used to test the reflectivity of the three samples with different morphometry, and we found that the reflectivity increased gradually with the increase of oxygen flow. When the oxygen flow ratio is 2, the reflectivity is extremely low, nearly close to 0 within the GaAs absorption range, as shown in Fig. 5. This low reflectivity is caused by the excessive oxygen flow. Sufficient oxygen oxidized the surface structure formed by previous etching and made the surface of the sample smooth and flat, as shown in Fig. 4c. The etched sample with smooth surface presented high reflectivity. When the oxygen flow ratio is 2, the etched samples has the lowest reflectivity, because the flocculent surface of black GaAs greatly increase the propagation path of photons and reduce the reflection of light. The structured GaAs sample also presented hydrophobicity,while the contact angle is 125°, as shown in the enlarged SEM images of Fig. 4(d), broadening the application range of black GaAs.