Effects of heat-up procedure and grating depth on the erosion of InGaAs grating

Effects of heat-up procedure and grating depth on the erosion of InGaAs absorptive layers and the quality of strained multiple quantum wells (MQWs) were investigated using transmission electron microscopy (TEM) and double crystal X-ray diffraction (DXRD) analysis, in the fabrication of loss-coupled distributed feedback laser diodes (DFB-LDs) that have absorptive grating on the side of substrate. Decomposition of an InGaAs grating depends on the growth starting temperature and grating depth. Even before starting InP growth, a part of the InGaAs was decomposed during the heat-up process. These decomposed adatoms of indium, gallium, and arsenic seem to migrate to the concave region of the grating and then form a new material phase at the bottom of the concave region. At the initial stage of InP planarization layer growth, InP growth on the InGaAs mesa does not take place until the concave region of grating is fully filled, while a part of InGaAs is decomposed. Desorbed adatoms seemed to migrate to the upper part of the growing InP on the concave region and form another new material phase in this region. The growth technique of n-InP planarization layers over shallow gratings with the growth starting temperature of 550°C allows strained MQWs of good crystalline quality, leading to the successful fabrication of very low threshold loss-coupled DFB-LDs.