MBE Growth Of GaAs-based Pseudomorphic Lasers: Key Growth Trade-offs Between The InGaAs MQWs And The AlGaAs Cladding

Pseudomorphic InyGal-yAs/GaAs/AlxGal.xAs MQW lasers are receiving increasing attention for shorthaul, high-speed data transmission, because they have much larger modulation bandwidths1 and smaller linewidth enhancement factors2 than GaAs/AlGaAs lasers. Although pseudomorphic InGaAs/GaAs technology is relatively young, lasers fabricated in this material system have already achieved larger direct modulation bandwidths (33 GHz at 65 mA 3, than any other material system. The key processes influencing the MBE growth of pseudomorphically strained InGaAs/GaAs/AlGaAs structures are only now being identified4p5. Thus, the growth of such structures has followed a "trial and error" approach, based on experience from the growth of GaAs/AIGaAs heterostructures. For GaAs/AlGaAs lasers, it was shown that higher substrate temperatures improved the AlGaAs quality and reduce the threshold current densities6. For pseudomorphic MQW lasers, it became necessary to grow two different ternary alloys, whose independently optimized growth parameters are quite different. As a result, it has become common practice to optimize the InyGal.yAs, GaAs and AI,Gal.,As growth parameters independently7. Such independent optimization presumes that the growth of each layer does not significantly influence the other layers a presumption which becomes questionable for the larger strains and stresses associated with the newest generation of pseudomorphic diode lasers. In this paper, we present the most recent results of our on-going investigation into the MBE growth processes of pseudomorphically strained InyGal.yAs. We present compelling evidence showing that the quality of InGaAs/GaAs MQWs in such lasers depends strongly on the growth parameters of the other epitaxial layers particularly on the AlGaAs growth temperature. This interdependence forces a complete re-evaluation of the appropriateness of I n G W G a A s growth protocols based on unstrained GaAs/AlGaAs MBE growth processes. As-grown and annealed In0.35Ga0.65As/GaAs test structures and diode lasers have been extensivel! characterized with resonant Raman scattering, photoluminescence (PL), PL microscopy (PLM) and ion channeling. We observe a gradual onset for strain relaxation through the formation of -oriented dislocations and oriented line defects. The formation and propagation of both misfit dislocations and line defects depends strongly on i) the growth temperature, ii) the substrate quality, iii) the total number of QWs. iv) the thickness of the QW barrier layers, and v) the impurity (e.g dopant) concentration in these barriers. Ion channeling measurements and annealing studies reveal increasing structural instability of the MQW structures as the number of QWs increases. The formation of misfit dislocations and line defects ispreceded by the incorporation of as yet unidentified point defects. Resonant Raman measurements (Fig. 1) show an increase in the intensity of 1-LO phonon scattering from the GaAs barriers with increasing number of QWs, attributed to an increasing point defect density. Above -100K. temperature dependent PL measurements (Fig. 2) show a faster decrease in PL intensity as the number of QWs increases. Thus, these new strain-induced point defects are probably active nonradiative recombination centers. These defects become increasingly important as the strain relaxation limit is approached and we suspect that these point defects may also play a role in facilitating the nucleation and propagation of misfit dislocations and line defects. The nature of these point defects is currently being intensively investigated and will be discussed. Optimization of the active region of high-speed InCaAsGaAs lasers requires highly stressed InGaAs/Gai\s MQWs, close to the misfit strain relaxation limit5. TO reduce the aforementioned point defect concentration. the growth temperature of the AlGaAs cladding layers must be reduced well below the optimal AIGaAs growth temperatures. Previous investigations have shown improved luminescence efficiencies, reduced interface roughness and reduced impurity incorporation in GaAs/AlGaAs MQW lasers grown with GaAs/AIAs short-period supperlattice (SPSL) pseudoalloys than in comparable devices grown with ternary AlGaAs alloys7. SPSL pseudoalloys allow sufficient flexibility in the MBE growth parameters for high quality lasers to be grown at substantially reduced temperatures'. We present results from In0.35Gao.65As/GaAs 4 QW lasers (Fig. 3) whose SPSL AI0,8Ga0,2As cladding was grown at either 700°C or 620°C. The threshold current densities are -3 times smaller for the laser5 whose AlGaAs cladding was grown at 620°C (Fig. 4). At the same time. the internal quantum efficiency increases from 60% to 70%. We attribute the lower threshold currents and increased internal quantum efficiencies to a dramatic decrease in the point defect concentration in the MQW region. Ridge lasers (3x100pm2) were also fabricated from these structures. Ridge lasers grown with low temperature SPSL AlGaAs have achieved 3dB modulation bandwidths of 24 GHz at record low bias currents of only 25 mA, with a single-facet output power of 5 mW (uncoated mirrors),