Degradation Studies of InAs / GaAs QD Lasers Grown on Si

Abstract

Lowering the threshold gain of InAs quantum dot lasers grown on Silicon, significantly extends device lifetime. Measurements on degraded devices show increased optical mode loss is responsible for degradation and a consequent shortening of lasing wavelength. Reliable and efficient electrically-pumped silicon-based lasers are currently required as sources in silicon photonic integrated circuits and ultimately to enable full integration of photonics and electronics. While wafer bonding of Compound Semiconductors (CS) and Silicon is the favoured immediate solution, in the longer term epitaxial growth of CS on silicon is seen as the ideal for large scale manufacturing. Recent results using Quantum Dots (QDs) show promising lifetimes [e.g. [1] [2]] even though epitaxially grown CS Quantum well (QW) laser on Si structures degrade within seconds. It is understood that both the number and size of defects present in a laser structure can increase during operation and this in-turn degrades performance and ultimately shortens the device lifetime. In QW lasers it has been shown that the primary mechanism for degradation is the increased non-radiative recombination rate that accompanies the defects, although an increased internal optical mode loss can also contribute. QDs bring two major advantages, firstly, they can effectively deflect defects or pin them preventing loop formation, and secondly, they localise charge carriers preventing them from diffusing laterally and recombining non-radiatively at a defect site. The latter, reduces the rate of degradation because it suppresses recombination enhanced defect reactions (REDR), a process which can cause dislocation climb, further reducing device performance. QDs are effective at isolating carriers and limiting the recombination at defect sites, however, the optical mode propagates along the entire length of the laser cavity and therefore will always suffer from the increase in dislocations via optical scattering, characterised by the internal optical mode loss. In this study, we test the reliability of deep-etched ridgewaveguide QD lasers grown directly on Si substrates and investigate how factors such as cavity length, and hence carrier density, affect degradation rate whilst running in continuous wave (CW) operation. Using a segmented contact structure, that can be run as either a laser or a single-pass gain structure, we show that measured optical mode loss increases for devices on life test and is a major contributing factor on the eventual degradation of performance in these QD-on-Si lasers. The samples examined here are similar designs to those used in reference [1], but with a larger dot size distribution and consequently somewhat higher threshold current density for similar cavity lengths. However, most importantly the approaches to minimise defects propagating towards the active region, comprising 5 layers of InAs dots each grown in a dot-in-a-well (DWELL) on In0.15Ga0.85As surrounded by a GaAs core, are similar. These include Si(100) wafers with 4° offcut to the [011...

Understanding the Bandwidth Limitations in Monolithic 1.3 μm InAs/GaAs Quantum Dot Lasers on Silicon

“…In addition to non-radiative recombination, the internal loss caused by dislocation-induced absorption and optical scattering can additionally compromise the performance of QD lasers on Si [39] especially with respect to the modulation speed, as optical loss effectively reduces the available gain. Whereas Wang et al have calculated waveguide loss on the order of 2.4 cm -1 to 5.5 cm -1 for the metamorphic epilayers of III/V QD lasers on Si in a similar configuration [39], Shutts et al and Jung et al have measured low internal losses of about 2.8 cm -1 [42] and 2.5 cm -1 [7], respectively, similar to what has been used in our simulations [18]. These values, even if possibly containing a residual dislocationinduced loss component, give clear evidence of the high crystal quality of the III/V lasers, and compare favorably with internal losses in QD lasers on native substrates [11], [32], This indicates that optical losses in monolithic QD lasers on Si are unlikely to limit their performance in a fundamental way.…”