The 2 μm spectral window is a host of a variety of applications in communication and sensing. However, the development of nanoscale emitters for this window has largely stagnated. Here, we present our growth, fabrication, and testing of InAs quantum dash (Qdash) lasers on InP for 2 μm photonics. The samples were grown by molecular beam epitaxy. Ridge waveguide lasers emitted at 1.97 μm, coinciding with the ground-state emission from the Qdash ensemble. A low threshold current density of 131 A/cm2 per Qdash layer was calculated under pulsed mode conditions. Good thermal stability was observed up to 50 °C, with a characteristic temperature of 44 K. InAs Qdash on InP is promising 0D laser gain materials for 2 μm communication and sensing.
We demonstrate the metamorphic growth of InAs1-xSbx (x = 0.08 – 0.68) layers on GaAs substrate with an optimized InAs-based intermediate buffer layer by molecular beam epitaxy. The broad range of group-V flux ratio is applied to investigate the effect between Sb incorporation and material quality. We find that high Sb compositions significantly roughen surface morphology, and that optimized growth temperature is crucial to prevent phase separation and surface segregation of Sb atoms. In addition, we achieve a high degree of strain relaxation (~94%) in the metamorphic InAsSb layers even with a 58% Sb composition. This result indicates that our InAs/GaAs virtual substrate is suitable for the growth of an almost fully relaxed InAsSb layer. Also, we investigate a threading dislocation density (TDD) trend with the broad range of Sb compositions, and a drastically increasing TDD trend (> 200 times) was observed. Finally, we report a narrow bandgap of 0.13 eV at 10 K of the InAs0.42Sb0.58 layer, which is promising for the detection of longwavelength infrared radiation. This InAsSb layer on GaAs substrate opens up possibility for mid-wavelength and long-wavelength infrared optoelectronics applications.
In this paper we review our recent progress on high performance mode locked InAs quantum dot lasers that are directly grown on CMOS compatible silicon substrates by solid-source molecular beam epitaxy. Different mode locking configurations are designed and fabricated. The lasers operate within the O-band wavelength range, showing pulsewidth down to 490 fs, RF linewidth down to 400 Hz, and pulse-to-pulse timing jitter down to 6 fs. When the laser is used as a comb source for wavelength division multiplexing transmission systems, 4.1 terabit per second transmission capacity was achieved. Self-mode locking is also investigated both experimentally and theoretically. The demonstrated performance makes those lasers promising light source candidates for future large-scale silicon electronic and photonic integrated circuits (EPICs) with multiple functionalities.
Direct epitaxial growth of III-V lasers on silicon provides the most economically favorable means of photonic integration but has traditionally been hindered by poor material quality. Relative to commercialized heterogeneous integration schemes, epitaxial growth reduces complexity and increases scalability by moving to 300 mm wafer diameters. The challenges associated with the crystalline mismatch between III-Vs and Si can be overcome through optimized buffer layers including thermal cyclic annealing and metamorphic layers, which we have utilized to achieve dislocation densities < 7×106 cm-2. By combining low defect densities with defect-tolerant quantum dot active regions, native substrate performance levels can be achieved. Narrow ridge devices with threshold current densities as low as ~130 A/cm2 have been demonstrated with virtually degradation free operation at 35°C over 11,000 h of continuous aging at twice the initial threshold current density (extrapolated time-to-failure >10,000,000 h). At 60°C, lasers with extrapolated time-to-failure >50,000 h have been demonstrated for >4,000 h of continuous aging. Lasers have also been investigated for their performance under optical feedback and showed no evidence of coherence collapse at back-reflection levels of 100% (minus 10% tap for measurement) due to the ultralow linewidth enhancement factor (αH < 0.2) and high damping of the optimized quantum dot active region.
We investigate the degradation processes that limit the long-term lifetime of 1.3 μm quantum dot lasers grown on silicon substrate. The analysis is based on combined optical and electrical characterization, carried out before and during accelerated ageing tests. Specifically, we demonstrate that: (i) when submitted to constant current stress, the analyzed devices show a monotonic increase in threshold current; (ii) degradation kinetics are strongly dependent on stress current; a power-law dependence of TTF on stress current was extrapolated (TTF proportional to J^-3.9). (iii) during stress time, a decrease in slope efficiency was detected, well correlated to the threshold current increase. This effect was ascribed to a decrease in injection efficiency of the devices. (iv) A detailed analysis of the degradation kinetics showed that the threshold current increase has a square-root dependence on stress time, indicating the presence of a defect-diffusion process, that degrades the properties of the active region. Finally (v), the analysis of the spectral characteristics plots indicates that stress is impacting quantum dots with high energy emission preferentially.
The results collected within this paper are explained by considering that stress promotes the diffusion of defects towards the active region of the devices. This mechanism results in a decrease in the SRH recombination lifetime, and in the subsequent increase in threshold current and drop in sub-threshold emission. An increase in the SRH rate next to the quantum dots can also reduce the injection efficiency into the QDs, thus inducing a drop in the slope efficiency of the lasers.
We investigate the temperature and pressure dependence of a series of intrinsic and modulation p-doped InAs-based dot-in-well (DWELL) laser diodes grown on silicon substrates. Temperature dependence of the threshold current density (Jth) and pure spontaneous emission spectra provide an insight into inhomogeneity and non-radiative recombination mechanisms within the devices. Initial investigations showed that the intrinsic devices exhibited low temperature sensitivity in the range 170-200K. Above this, Jth increased more rapidly consistent with Auger recombination. P-doping increased the temperature at which Jth(T) started to increase up to 300K with a temperature insensitive region close to room temperature. P-doping delays the onset of carrier thermalization, leading to a high T0 but with an associated higher Jth. Temperature dependence of gain spectrum broadening was investigated by measuring the spontaneous emission spectral width parameter (1/e2) just below Jth (T). A strong direct correlation is found between the temperature dependence of peak width with the temperature dependence the radiative component of threshold, Jrad(T). At low temperature the correlation is consistent with strong inhomogeneous broadening of the carrier distribution. As temperature increases Jth reduces associated with carriers thermalizing to lower energy states. At higher temperatures homogeneous thermal broadening coupled with non-radiative recombination causes Jth to increase. Inhomogeneous broadening is more pronounced in the p-doped devices due to coulombic attraction between acceptor holes and injected electrons. A detailed analysis of recombination processes using high hydrostatic pressure and spontaneous emission in these lasers as a function of doping density will be presented and discussed at the conference.
This paper reports on a preliminary investigation of the gradual degradation processes that may affect the lifetime of InAs quantum dot (QD) lasers epitaxially grown on silicon substrates. To this aim, a series of identical Fabry-Pérot lasers emitting at 1.31 μm have been subjected to current step-stress and constant-current aging experiments at an ambient temperature of 35°C. With the adopted stress conditions, the optical characteristics of the devices exhibited an increase in the threshold-current and a decrease in the slope efficiency. This latter process was found to be well correlated with the variation in the threshold current, suggesting that this specific degradation mode may be ascribed to a stress-induced reduction in the injection efficiency. Moreover, the linear dependence of the threshold-current variation on the square root of time observed for longer stress time highlighted the possible role of a charge/defects diffusion process in the optical degradation of the devices. Consistent with this hypothesis, the electrical characteristics of the devices exhibited an increase of the forward leakage current in the bias regime dominated by defect-assisted current conduction mechanisms. The degradation process was found to be heavily accelerated for bias values allowing excited-state operation: this peculiar behavior was ascribed to the higher rate of carriers escaping from the quantum dots that undergo Recombination Enhanced Defect Reactions (REDR) in proximity of the active region of the device.
A common way of extracting the chirp parameter (i.e., the α-factor) of semiconductor lasers is usually performed by extracting the net modal gain and the wavelength from the amplified spontaneous emission (ASE) spectrum. Although this method is straightforward, it remains sensitive to the thermal effects hence leading to a clear underestimation of the α-factor. In this work, we investigate the chirp parameter of InAs/GaAs quantum dot (QD) lasers epitaxially grown on silicon with a measurement technique evaluating the gain and wavelength changes of the suppressed side modes by optical injection locking. Given that the method is thermally insensitive, the presented results confirm our initial measurements conducted with the ASE i.e. the α-factor of the QD lasers directly grown on silicon is as low as 0.15 hence resulting from the low threading dislocation density and high material gain of the active region. These conclusions make such lasers very promising for future integrated photonics where narrow linewidth, feedback resistant and low-chirp on-chip transmitters are required.
The integration of optical functions on a microelectronic chip brings many innovative perspectives, along with the possibility to enhance the performances of photonic integrated circuits (PIC). Owing to the delta-like density of states, quantum dot lasers (QD) directly grown on silicon are very promising for achieving low-cost transmitters with high thermal stability and large insensitivity to optical reflections. This paper investigates the dynamical and nonlinear properties of silicon based QD lasers through the prism of the linewidth broadening factor (i.e. the so-called α-factor) and the optical feedback dynamics. Results demonstrate that InAs/GaAs p-doped QD lasers epitaxially grown on silicon exhibit very low α-factors, which directly transform into an ultra-large resistance against optical feedback. As opposed to what is observed in heterogeneously integrated quantum well (QW) lasers, no chaotic state occurs owing to the high level of QD size uniformity resulting in a near zero α-factor. Considering these results, this study suggests that QD lasers made with direct epitaxial growth is a powerful solution for integration into silicon CMOS technology, which requires both high thermal stability and feedback resistant lasers.
Silicon photonics promises scalable manufacturing of integrated photonic devices through utilization of established CMOS processing techniques and facilities. Unfortunately, the silicon photonics platform lacks a viable light source, which has historically been overcome through heterogeneous integration techniques. To further improve economic viability, the platform must transition to direct epitaxy on Si to bypass the scaling limits imposed by the small sizes and high cost of III-V substrates in heterogeneous integration. InAs quantum dots have demonstrated themselves as the most promising candidate for achieving high performance light emitters epitaxially grown on Si. Using molecular beam epitaxy, we have grown quantum dot lasers composed of InAs dot-in-a-well active layers on industry-standard, on-axis (001) Si substrates. In this report, we utilized p-doping of the quantum dot active region to increase gain for improved dynamic performance and reliability. These devices have been subjected to accelerated aging conditions at 60°C and a bias multiple of twice threshold current density. After 2,750 hours of continuous aging, an extrapolated lifetime of more than 100,000 hours has been calculated.
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