The traditional vertical-cavity surface-emitting laser (VCSEL) uses distributed Bragg reflectors (DBRs) to provide high reflectivity to conform to the lasing standard. However, due to the relatively small refractive index contrast of lattice-matching material systems, many pairs of DBRs are needed to achieve high reflection, which brings difficulties and limitations to the manufacturing of VCSELs. In addition, multilayer DBRs can cause problems such as high impedance and low conversion efficiency. To improve the performance of VCSELs, researchers introduce the high-index-contrast sub-wavelength grating (HCG) as a reflector in the VCSEL. By the adjustment of grating parameters, it can have extremely high reflectivity and can replace the traditional DBRs in VCSEL. Hence, VCSELs with the HCG will not suffer from the problems of high resistance and serious light absorption caused by DBRs.

In this paper, the HCG reflector for VCSELs is studied and fabricated. On the basis of the rigorous coupled wave analysis (RCWA), the polarization and reflection characteristics of a GaAs/AlO_x HCG reflector are analyzed. A TE-polarized HCG is designed to have the highest reflectivity of close to 1 near 940 nm when the incident light is perpendicular to the substrate. Moreover, the influences of topography error and the incident angle on reflectivity are investigated. Then, the device is prepared by mental-organic chemical vapor deposition technology, electron beam lithography (EBL), inductively coupled plasma (ICP) etching, wet etching, and wet oxidation. Since the GaAs/AlO_x HCG has the same material system as the half-VCSEL, it can be integrated with the VCSEL through one-time epitaxial technology, which is of great significance for obtaining high-quality wafers. Furthermore, the low stress between the HCG and half-VCSEL is crucial to keep the long-term stability of the device.

Fig. 1 shows the structure of the HCG, including the grating layer H₁, stress buffer layer H₂,_{and low index sub-layer H3, which are directly grown on the GaAs substrate. The HCG is composed of GaAs and AlOx, where the latter is obtained from AlAs by oxidation. The large index difference between the AlOx (refractive index n1≈1.6) and GaAs (refractive index n2≈3.538) grating layers is conducive to increasing the width of the reflection band. As the thickness of AlAs shrinks after oxidation, the GaAs grating layer is not completely etched to form a stress buffer layer to prevent delamination and fracture after oxidation.}

By the RCWA method, a TE-HCG mirror for the GaAs-based VCSEL is simulated. It can be seen from Fig. 2 that the TE-HCG has a large reflection bandwidth of up to 97 nm (Δλ/λ₀=10.3%), with its TE reflectivity of more than 99.5% and TM reflectivity of lower than 90%.

The simulation is based on the rectangular grating model, but the actual grating is usually trapezoidal. Therefore, we consider the influence of the grating shape on reflectivity. As shown in Fig. 3(a), although there is a 5% difference between the upper and lower fill factors, it has little effect on the high reflection band, which shows that the grating has great shape tolerance. Fig. 3(b) shows the impact of the incident angle on HCG performance. When the incident angle is greater than 5°, the reflectivity of the TE wave is significantly reduced. It is the sensitivity of HCG to the angle that makes the VCSEL integrated with HCG exhibit good single-mode performance.

The HCG is prepared given the above results. Fig. 4 shows the scanning electron microscope (SEM) images of the epitaxial structure, and the thickness of GaAs and AlAs layers is 370 nm and 220 nm, respectively. After epitaxial growth, the processes are followed by wet etching, wet oxidation, EBL, and ICP etching. As shown in Fig. 5, period Λ=750 nm, f=28%, thickness H₁=170 nm, thickness H₂=200 nm, and thickness H₃=200 nm, and they are all within the tolerance range.

Due to the limitations of test conditions, it is difficult to measure the reflectivity of the incident light from the substrate. Therefore, the reflectivity of the incident light perpendicular to the grating surface is measured. Fig. 6 shows the theoretical and measured results of the actual grating. The measured maximum reflectivity of TE-polarized light is 84.9%, which is close to the theoretical value of 86.5% under the same incident direction, while the reflectivity of TM-polarized light is lower than 40%. The test results are in good agreement with the simulations. The HCG can act as an ultra-thin reflector for VCSEL, with the advantages of a long period, a shallow etching depth, and great tolerance, which is easier to integrate with VCSEL. Meanwhile, the VCSEL integrated with the HCG features low loss, stable polarization, and single-mode operation.