Vertical cavity surface-emitting lasers (VCSELs) have advantages such as a single longitudinal mode, a low threshold, and ease of two-dimensional integration. VCSELs have been widely used in data transmission, optical communication, and three-dimensional sensing. Oxidation is the most common process for oxide-confined VCSELs. AlGaAs materials with high Al contents are oxidized via wet oxidation to form oxide apertures of aluminum oxide, and the structures of oxide apertures with different shapes and sizes have different effects on the optoelectronic characteristics of VCSELs. However, during the actual oxidation of the AlGaAs oxide confinement layer, the shape and size of the oxide aperture do not satisfy expectations because of various factors, which adversely affect the performance of the device in terms of the excitation mode, threshold current, and divergence angle. In this study, the dry etching and wet oxidation processes of VCSELs are experimentally investigated, and an optimized process scheme for oxidation pretreatment that combines dry etching and (NH4)2S passivation is developed. An (NH4)2S solution is used to passivate the table structure after dry etching, which achieves a stable oxidation rate and improves the quality of the oxide aperture shapes, further improving the optoelectronic characteristics of VCSELs and extending the applications of VCSELs in optoelectronics.
In this study, an (NH4)2S solution is used. Prior to oxidation, a cleaned VCSEL is passivated in a (NH4)2S (sulfur mass fraction >8%) solution in a heated water bath. After oxidation, the surface and sidewall microstructures of the VCSEL are observed using scanning electron microscopy (SEM). The shapes and sizes of the oxidation apertures of the VCSELs are observed separately using a microscope, and the oxidation rates of the oxidation apertures are determined. Based on this, the photoelectric properties of the unpassivated and passivated VCSELs are comparatively analyzed.
After wet oxidation, the layered structure of the unpassivated VCSEL undergoes fracturing and separation, and the VCSEL structure undergoes distortion [Fig.3(a)]. However, the passivation-pretreated VCSEL exhibits less significant fracture and delamination and good sidewall integrity [Fig.3(b)]. The passivated VCSEL [Figs.4(a1) and (a2)] has smoother oxide hole edges and more regular oxide aperture shapes than the unpassivated VCSEL [Figs.4(b1) and (b2)]. With an increase in the oxidation depth, the oxidation aperture of the passivated VCSEL has a somewhat diamond shape [Fig.4(a3)], whereas that of the unpassivated VCSEL has an irregular pentagonal shape [Fig.4(b3)]. The oxidation rate of the unpassivated VCSEL always exceeds that of the passivated VCSEL (Fig.5). The test results (Fig.6) show that the saturated output power of the passivated VCSEL is stable at 6.16 mW, whereas that of the unpassivated VCSEL varies between 5.18 mW and 6.14 mW. Moreover, the slope efficiency of the unpassivated VCSEL fluctuates within 0.40?0.42 W/A, and the slope efficiency of the passivated VCSEL is improved by 5% and stabilizes at 0.44 W/A. In conclusion, the passivated VCSEL exhibits improved device performance consistency, whereas the unpassivated VCSEL exhibits unstable device performance. Variability in the performance of both devices exists. In addition, the threshold currents of both VCSELs are close to 0.80 mA, but the threshold currents of the passivated VCSEL decrease to 0.72 mA. As shown in Fig.7(a), the side-mode rejection ratio of the passivated VCSEL reaches up to 36 dB at a driving current of 1 mA, whereas that of the unpassivated VCSEL is 22 dB, with the appearance of two excitation modes. When the current reaches eight times the threshold, the passivated VCSEL excites two modes, and a third mode gradually starts to appear but still manages to maintain a few mode outputs [Fig.7(b)]; in comparison, the unpassivated VCSEL appears with four or more modes [Fig.7(c)].
In this study, the effect of a preoxidation pretreatment process scheme that combines dry etching and (NH4)2S passivation on the sidewall integrity and oxide aperture of a VCSEL is investigated. The (NH4)2S passivation technology can effectively remove nontarget products, such as oxides, on the sidewall of the stage and minimize device delamination and fracturing during oxidation, improving the sidewall integrity and sample quality. The oxidation rate of the high-alumina component AlGaAs layer on the sidewall is more uniform and stable, and the oxide aperture shape is regular. Based on this, the passivation process is applied to prepare oxide-confined VCSELs with a 5-μm-diameter oxide aperture. Comparison experiments show that the maximum slope efficiency and threshold current characteristics of the VCSEL prepared by this process improve, and the device performance consistency is enhanced. The side-mode rejection ratio of the passivated VCSEL can reach 36 dB at a driving current of 1 mA in a single-mode excitation state. This study shows that the proposed oxide-optimized process scheme based on dry etching and (NH4)2S passivation is beneficial for the preparation of oxide aperture structures with regular shapes and good follow-through, which improves the structural stability of the device and the device performance of oxide-confined VCSELs.
