Increased Modal Gain in 1.55 μm Quantum Dot Lasers Based on Improved Size Homogeneity Obtained by Comprehensive Growth Optimization

Abstract

InAs quantum dot (QD) lasers exhibit improved properties compared to quantum well lasers like higher temperature stability, lower threshold current density, and significantly reduced laser line width. This study aims to further optimize InAs-QD morphology and optical material properties in order to narrow the gain spectrum of the ground state transition, improving the laser performance. Self-assembled InAs QDs were fabricated on InAlGaAs layers lattice-matched to InP by using molecular beam epitaxy. Atomic force microscopy characterization and low-temperature photoluminescence spectroscopy were used to assess the size uniformity, emission wavelength, QD density, and reproducibility. In addition to optimizing growth parameters, the impact of an additional ultrathin nucleation layer (NL) on the QD formation process and related size homogeneity was investigated. High resolution scanning transmission electron microscopy confirmed the significant role played by the NL and the exact composition of the QDs and their surrounding layers. By optimizing the growth temperature, the V/III ratio, and by introducing a GaAs-NL, a photoluminescence line width of 17.6 meV at 10 K from a single QD layer and a record narrow line width of 18.4 meV for a stack layer of six QD layers were demonstrated. The structures with NL exhibit a blue shift in the peak wavelength by maintaining uniformity compared to QD reference structures. Additionally, processed broad-area lasers based on the optimized QD layers demonstrate a record high modal gain of 26.5 cm^–1 per QD layer.