COMPARISON OF CHEMICAL BEAM EPITAXY AND METALORGANIC CHEMICAL-VAPOR-DEPOSITION FOR HIGHLY STRAINED MULTIPLE-QUANTUM-WELL INGAASP/INP 1.5-MU-M LASERS

The performance and reliability of strained layer optoelectronic devices are in general limited by the integrity of metastable heterostructures. Misfit strain relaxation (and concomitant defects) can be avoided if the structural stability is optimised and elevated temperature exposure minimized. Chemical beam epitaxy (CBE) holds great promise in strained layer epitaxy, since by reducing growth temperature the overall thermal budget for epitaxy and processing can be significantly reduced. The design, epitaxial growth, fabrication and reliability issues related to strain and strain-compensated multi-quantum well lasers are first considered in order to determine the upper limits of compressive or tensile strain permissible in such structures. The benefits of strain (both tensile and compressive) on threshold current density are related to the amount of strain in the wells (via the reduction of the Auger recombination coefficient) and the well width (via the optical confinement factor). It is therefore the strain - well-width product for the active region which is of key interest. In this survey the practical upper bound to stability is defined theoretically using an energy balance model, where the effect of strain compensation from oppositely strained barrier layers, balances the strain in the quantum wells and renders the multilayer stack ''strain neutral''. The susceptibility of strained multilayers to defect injection through epitaxial growth and subsequent device fabrication is determined by growth simulation. Using this model as a design tool we have investigated the structural stability of a compressively strained multiple quantum well (MQW) laser through the concept of ''effective stress'' for misfit dislocation injection. The upper limits for quantum well strain incorporation with and without strain compensation are quantitatively defined in light of recent laser reliability data. The evolution of the driving force for misfit strain relaxation is mapped out through a typical epitaxial growth sequence highlighting the points in the growth process of highest vulnerability to defect injection. These design concepts were used to optimize structures for highly strained quantum wells (QWs) in strain compensated InGaAs/InP MQW lasers. The stability of strain-compensated MQW laser structures is demonstrated for devices grown by conventional metalorganic chemical vapour deposition.