We fabricated GaInAsP/InP short cavity lasers with semiconductor/air distributed Bragg reflectors (DBRs) by inductively coupled plasma etching with pure Cl 2 gas. Nearly vertical sidewalls with low roughness of ∼10 nm were achieved, separated by air spaces of three quarter wavelengths. The lowest threshold current normalized by the stripe width was 3.2 mA/µm. From this value, the DBR reflectivity was evaluated to be 85%, which agreed with the theoretical value obtained from a finite-difference time domain (FDTD) simulation. We compared two types of devices with different DBR shapes, and observed that DBR reflectivity was affected more by the tilt of the DBR sidewalls than the sidewall roughness. This result also agreed well with the FDTD theory.
For GaInAsPDnP lasers used in access networks, low cost, simple process, and integration with other devices, e.g., photodetector and mode-size converter, are required. For this purpose, we have proposed and demonstrated a short cavity laser with semiconductor/air DBRs [l]. So far, this DBR has been fabricated by an ECR RIBE[ 13 and an anisotropic wet etching [2]. However, the reflectivity was reduced by the sidewall tilt, roughness and the diffraction of light in the air region. In this study, we fabricated high mesa stripe lasers having the DBR for one end and an etched facet (EF) for another end. We used an inductively coupled plasma (ICP) etching, which has demonstrated smooth and steep etching profiles for this material system [3]. We obtained a low threshold lasing of 3 mA/pm and evaluated a high reflectivity up to 85 %. By comparing these results with simulated ones, we discuss key issues of this type of device.We prepared an MOCVD-grown 1 S5-pm-GaInAsPAnP wafer with 0.3-pm-thick p-GaInAs contact layer, 1.7-pm p-InP cladding layer, 0.25-pm compressive-strained MQW GRIN-SCH active layer and 1.9-pm n-InP cladding layer. The DBR consisted of 3 -5 pairs of 3U4n wide semiconductor wall and air spacing. Both DBR and stripe patterns were drawn by an EB lithography. Fig. 1 shows the SEM view of a DBR fabricated by the ICP etching with CI, gas. The etch depth was 4.8 pm. By changing the etching condition, we obtained two device types. In type A, sidewall angle 8 around the active layer was almost 90 degrees, but the roughness cs was as long as 20 nm, while in type B, 8 = 80 -90 degrees (tapered sidewall) and CJ < 10 nm. We observed the pulsed lasing in these devices. Fig. 2 shows the relation of measured threshold current density J,,, and the inverse stripe length L-' for type A. When L = 200 pm, the minimum J, was 1.6 kA/cm2. The corresponding threshold current normalized by the stripe width was as low as 3.2 mA/pm. Thc maximum DBR reflectivities for A and B were evaluated to be 85 % and 50 %, respectively.To examine the difference of these two types, we precisely simulated the effective reflectivity for the waveguide mode using the 2-D FDTD method. Inverse stripe Length L-' [cm-'1Fig. 2 Threshold current density Jth versus inverse stripe length L-I. For theoretical lines, we assumed gain and loss parameters evaluated for cleaved lasers. Fig. 1 SEM view of a DBR fabricated by ICP etching. 0-7803-5661-6 / 99 / $10.00 0 1999 IEEE
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