Gallium arsenide (GaAs) is widely used in laser diodes, lasers, solar cells, high-frequency circuits, photodetectors, etc. owing to its high-energy bandwidth and electron mobility. However, when GaAs-based semiconductor devices are exposed to air, their electrical and optical properties deteriorate significantly, which seriously affects their performance. Therefore, the surface performance and stability of GaAs-based semiconductors are important for their application. Wet-etching passivation has considerable significance in the preparation of microdevices and nanodevices owing to its low cost and simple operation. 1-octadecanethiol (ODT) solution demonstrates a better passivation effect than other low-carbon chains. However, GaAs substrates must be etched with an acidic solution before passivation. The etching results directly affect the quality of the self-assembled monolayers (SAMs) formed on the GaAs substrate surfaces. According to theoretical analysis, the use of two different acid solutions can better control the etching speed and byproducts. However, there have been few reported studies on the effects of multistep wet-etching passivation on the surface of GaAs substrates. Therefore, in this study, two acidic solutions, H₂SO₄ (the volume ratio of H₂SO₄,H₂O₂,and H₂O is 1∶4∶x) and HCl ( the volume ratio of HCl,H₂O₂,and H₂O is 1∶1∶6), are used to etch the GaAs substrate. The ODT solution is used to passivate the GaAs substrate.

n-type doped GaAs (100) substrate with a Si volume fraction of 2.00×10¹⁸?3.12×10¹⁸ is used in the experiment. To remove organic pollutants from the surface, the GaAs substrate is ultrasonically cleaned with acetone, ethanol, and deionized water for 2 min, followed by N₂ drying. To remove the natural oxides and adsorbent atoms from the GaAs substrate surface, the sample is etched with a H₂SO₄ solution (the volume ratio of H₂SO₄,H₂O₂,and H₂O is 1∶4∶x ) and a HCl solution (the volume ratio of HCl,H₂O₂,and H₂O is 1∶1∶6), and then rinsed with anhydrous ethanol followed by N₂ drying. Finally, the electrode is passivated with an ODT in anhydrous ethanol solution.

The oxide layer on the GaAs substrate surface increases surface state density. Furthermore, the byproducts produced by different acid solutions have different effects on the GaAs substrate surface. Therefore, experimental etching schemes for different acidic solutions are designed in this study. The photoluminescence (PL) intensities of the samples after etching and passivation are higher than that of the GaAs substrate, proving that this process reduces the surface state density of the GaAs substrate. In addition, the etching time is found to be an important parameter in the acid solution etching process. The H₂SO₄ solution prepared in this study is highly corrosive, such that prolonged etching causes secondary damage to the surface of the GaAs substrate. Therefore, a comparative experiment without the ODT solution treatment is designed to verify the passivation effect of the ODT solution. The PL intensity is 2.5 times that of the GaAs substrate when the acid solution etching time is 90 s. As the etching time increases, the PL intensity is found to decrease. This is because over-oxidation occurs on the GaAs substrate surface during prolonged etching by the H₂SO₄ solution. The acid solution etching process involves simultaneous exchange of the surface chemical bonds HO—OH and Ga—As. When this process is repeated twice, Ga³⁺ and As(OH)₃ are formed. To optimize the etching process parameters, an experimental scheme in the absence of H₂O₂ in the acid solution under different H₂SO₄ solution concentrations is designed. When the concentrations of H₂SO₄ and H₂O₂ are reduced to 1/4 those in the original solution, the PL intensity is the highest, which is 3.02 times that of the GaAs substrate. The surface morphology of the GaAs substrate is optimized using an etching-passivation process. The surface is clean and smooth without obvious pits or impurities. To analyze the stability of the GaAs substrate after etching and passivation, the sample is exposed to an ultraclean environment for 60 d. Such sample surfaces still exhibit good uniformity. The PL intensity of the sample is 2.55 times that of the GaAs substrate. It is proven that the etching passivation process not only reduces the surface state density but also effectively improves its stability.

The effects of different acid solutions, etching time, acid solution concentrations, H₂O₂ in the acid solutions, and ODT solutions on the wet-etching passivation process are analyzed in detail. When the etching time of the H₂SO₄ solution (the volume ratio of H₂SO₄,H₂O₂,and H₂O is 1∶4∶15) and the etching time of HCl solution (the volume ratio of HCl,H₂O₂,and H₂O is 1∶1∶6) are both 90 s, the PL intensity is 3.02 times that of the GaAs substrate. When the sample is exposed to an ultra-cleaning environment for 60 d, the PL intensity is 2.55 times that of the GaAs substrate. This is consistent with the variation trend of peak intensity in the spectrogram by fast fluorescence spectrometer. After etching and passivation, the central wavelength homogeneity is improved and the full width at half- maximum is reduced according to the test results by fast fluorescence spectrometer. Raman analysis shows that the peak intensity ratio of transverse photon and longitudinal photon modes increases, and C and O elements are not detected in the energy dispersive spectra, proving that this process can effectively reduce the surface defects and surface state density. In addition, scanning electron microscope and atomic force microscope analyses shows that the etching passivation process can improve the large micrometer defects on the sample surface, but the surface roughness increases after etching passivation, which is consistent with the conclusions of existing etching passivation studies. In conclusion, this study provides an effective multistep acid solution etching theory and technology that can effectively reduce the GaAs defect density. The described etching passivation process is beneficial for expanding the applications of GaAs in the semiconductor field.