Low threshold GaInAsP lasers with semiconductor/air DBR fabricated by inductively coupled plasma etching

Abstract

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

Strong enhancement of light extraction efficiency in GaInAsP 2-D-arranged microcolumns

“…The lateral light is not extracted from a simple diluted semiconductor, while it is extracted from the microcolumns. In another study concerning a laser diode with a deep grating DBR, we carried out a finite difference time-domain simulation of light in a periodic structure of semiconductor vertical walls and air space, which has a similar side view to that of the formed columns [21]. We observed that the transmission of the lateral light through the grating was dependent on the design, e.g., the first-and the second-order grating conditions.…”