Contents 1. Introduction
Light sources that operate at high temperature are attracting a lot of interest owing to their ability to reduce the power consumption of laser module temperature controllers. Metamorphic growth enables us to use materials whose lattice constant is between those of GaAs and InP [1]–[3]. Therefore, the conduction band offset between a quantum well and a cladding layer can be enlarged and the temperature characteristics improve [3]. The metamorphic buffer has generally been at least about thick in order to obtain an unstrained (completely relaxed) quasi-substrate with a low dislocation density. However, this thick buffer causes the wafer to warp considerably and induces high thermal resistance. So, we need a thin buffer providing an unstrained quasi-substrate with a low dislocation density and a small wafer curvature (curvature is defined as the inverse of curvature radius). In this paper, we describe a newly developed method for reducing metamorphic buffer thickness using an in-situ wafer curvature measurement. The method enables us to fabricate a metamorphic buffer that is only 240 nm thick and an unstrained quasi-InGaAs substrate with a low threading dislocation density and little wafer curvature. Moreover, we achieved the first directly modulated operation using fabricated laser diodes (LDs) at 25 Gb/s with a high characteristic temperature on a metamorphic buffer.
