The fully epitaxial integration of IR laser sources into modern photonic circuits built on Si or SOI wafers is severely limited by the thermal- and lattice-constant mismatch between the substrate and the III-V layers that are required to achieve efficient solid-state lasing in the near infrared range. To overcome these limitations, modern commercially-available SiPh technologies employ selective wafer/device bonding techniques that allow to separately grow the III-V emitter and to integrate it into the photonic integrated circuit (PIC) at a later processing stage. Conversely, state-of-the-art devices leverage InAs quantum-dot (QD)-based active regions to highly reduce the sensitivity of the laser diode to the presence of the extended defects, which are generated as a consequence of the heteroepitaxial growth. In term of reliability, high levels of maturity have been demonstrated by both types of sources, either on the field or at laboratory level. Despite this, several degradation mechanisms still affect the long-term operation of such devices, thus limiting their useful lifetime. The aim of this paper is to discuss on the dominant degradation processes related to integrated laser sources for silicon photonics. This goal is achieved by summarizing some of the most recent results that have been obtained on two different, but well representative, classes of solid-state lasers: heterogeneously integrated quantum well (QW)-based emitters, such as vertical-cavity silicon-integrated lasers (VCSILs) emitting at 945 nm and III-V 1.55 m lasers bonded on silicon-on-insulator (SOI) substrates, and InAs QD laser diodes (LDs) epitaxially-grown on silicon, emitting in the 1.31 m window.
Silicon photonics is a technology that aims at improving state-of-the-art optical communication systems through high performance lasers, which currently rely mostly on III-As materials grown on silicon-on-insulator platforms and, in the future, on InAs quantum dots (QDs). In this work, we present an extensive investigation on the properties of defects in IIIAs layers as a function of the presence/absence of QDs layers and of dislocation density. By using deep level transient spectroscopy (DLTS), we analyzed two kinds of devices: GaAs diodes grown on Si (high dislocation density) and GaAs diodes grown on GaAs (low dislocation density) with QDs embedded. Our study showed that the device grown on Si exhibits four distinct traps (3 electron and 1 hole trap), whereas the sample grown on GaAs contains only one hole trap, which is in common among the two devices. These defects are placed in proximity of the semiconductor midgap, therefore they can act as efficient non-radiative recombination centers (NNRCs). The analysis of the capture kinetics showed that two of the traps are associated to point defects and that the remaining two seem to be related to point defects arranged along a dislocation. According our investigation, embedding QDs into the pin structure does not lead to the generation of additional defects, since the only measurable trap in the device with QDs is common to both samples. Finally, a tentative association of the detected traps with previous reports revealed that the dominant traps may be associated with native III-As defects or oxygen-related complexes.
This paper investigates the temperature dependence of the optical degradation of InAs quantum-dot (QD) lasers grown on silicon, and its relation with impurity diffusion processes. This goal was achieved by submitting a group of identical 1.3 μm QD LDs on Si to a series of constant-current stress experiments at baseplate temperatures ranging from 15 °C to 75 °C. The analysis of the threshold current (Ith) kinetics revealed that the optical degradation process i) is not activated by temperature for junction temperatures (Tj) lower than 60 °C, ii) becomes temperature activated with Ea ≈ 0.6 eV up to 80 °C, iii) is further accelerated for higher operating temperatures, and iv) resembles a diffusion process, due to the squareroot dependence of the Ith variation on stress time. This peculiar temperature activation was explained in terms of a recombination-enhanced diffusion process, driven be the escape of carriers from the InAs QDs toward nearby semiconductor layers. This process, which is strongly inhibited at low/room temperature, becomes relevant only above a specific temperature threshold. In this condition escaped carriers can be captured by extended defects, where they recombine and release their excess energy non-radiatively. This energy release contributes to the generation of additional defects, and/or to the diffusion of impurities, whose physical origin could be preliminarily attributed to the p-dopant Be, or to the native defects limiting its diffusivity (VGa or GaI).
This paper reports on the impact of the quality of the epitaxial structure of InAs Quantum Dot (QD) lasers grown on silicon substrates on lifetime. To this aim, a series of current step-stress and constant-current aging experiments were carried out on two sets of Fabry-Pérot QD lasers, emitting at 1.3 μm and featuring two different Threading Dislocations Densities (TDDs) within the epilayers, nominally 6E7 cm-2 (high TDD) grown on Si substrates and 500 cm-2 (low TDD) grown on lattice matched GaAs substrates. The results of the step-stress procedure indicate that i) the high-current optical performance of the lasers is limited by TDD, which reduces the bias range for useful Ground State (GS)-only operation and lowers the roll-off point of the optical output characteristic; ii) TDD contributes to the acceleration of the dominant optical degradation mechanism at high stress current levels, represented for this family of devices by the recombinationenhanced generation of defects in device regions close to the active layers; iii) dislocation density also accelerates optical degradation in GS regime. This process is primarily driven by the diffusion of Non-Radiative Recombination Centers (NRRCs) toward the active region of the devices, as demonstrated by the dependence of threshold current variation on the square root of time. These experimental results show that the presence of TDDs is the main limiting factor for the reliability of QD lasers for epitaxially-integrated silicon photonics applications, further confirming the outcome of previous statistical lifetime analyses carried out on devices featuring similar epitaxial structures.
Quantum dot lasers directly grown on silicon are excellent candidates to achieve energy and cost-efficient optical transceivers thanks to their outstanding properties such as high temperature stability, low threshold lasing operation, and high feedback tolerance. In order to reach even better performance, p-type doping is used to eliminate gain saturation, gain broadening due to hole thermalization and to further reduce the linewidth enhancement factor. Optical transceivers with low relative intensity noise are also highly desired to carry broadband data with low bit-error rate. Indeed, the intensity noise stemming from intrinsic optical phase and frequency fluctuations caused by spontaneous emission and carrier noise degrades the signal-to-noise ratio and the bit-error rate hence setting a limit of a highspeed communication system. This paper constitutes a comprehensive study of the intensity noise properties of epitaxial quantum dot lasers on silicon. Results show minimal values between - 140 dB/Hz and - 150 dB/Hz for doping level between 0 and 20 holes/dot in the active region. In particular, the intensity noise is insensitive to temperature for p-doped QD laser. Modulation properties such as damping, carrier lifetime, and K-factor are also extracted from the noise characteristics and analyzed with respect to the doping level. We also provide numerical insights based on an excitonic model illustrating the effects of the Shockley-Read-Hall recombination on the intensity noise features. These new findings are meaningful for designing high speed and low noise quantum dot devices to be integrated in future photonic integrated circuits.
This work reports on the optical feedback dynamics of InAs/GaAs QD lasers epitaxially grown on silicon operating in both the short and long delay regimes. Both undoped and p-doped QD lasers are considered. Whatever the external cavity length, no chaotic oscillations are observed on both samples as a result of the small α-factor observed in the silicon QD lasers. Despite that, experiments conducted in the short-cavity region raise period-one oscillation for the undoped QD laser. In addition, the transition from the short to long delay regimes can be finely covered by varying the external cavity length from 5 cm to 50 cm, and the boundaries associated to the appearance of the periodic oscillation are identified. In the short-cavity region, boundaries show some residual undulations resulting from interferences between internal and external cavity modes; whereas in the long-delay regime, the feedback ratio delimiting the boundaries keeps decreasing, until it progressively becomes rather in- dependent of the external cavity length. Overall, our results showed that the p-doped device clearly exhibits a much higher tolerance to the different external feedback conditions than the undoped one, seeing that its periodic oscillation boundaries are barely impossible to retrieve at the maximum feedback strength of -7 dB. These results show for the first time the p-modulation doping effect on the enhancement of feedback insensitivity in both short- and long-delay configurations, which is of paramount importance for the development of ultra-stable silicon transmitters for photonic technologies.
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.
Passively mode-locked InAs/InGaAs quantum dot on silicon lasers emitting at 1310nm are promising sources for high-speed high-capacity communication applications. Optical self-injection stabilization of a monolithic passively mode-locked quantum dot on Silicon laser with an absorber section length to total length ratio of 18% is investigated experimentally. A repetition rate tuning range of 24MHz around the free-running repetition rate of 9.4 GHz and a pulse-to-pulse timing jitter reduction by a factor of 2.5 from 150 fs to 59 fs are achieved for an external optical cavity length of 5.8m with fine-delay control. Obtained experimental results are in good quantitative agreement with simulation results obtained by a stochastic time-domain model.
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.
The performance of single photon sources based on single quantum dot emitters coupled to microcavities is analyzed with respect to different conditions of polarization. Electro-optic tuning is shown as a method to tune microcavities with distributed Bragg reflector mirrors into polarization degeneracy. Typically, for large cavity polarization splitting, excitation in the linearly polarized cavity modes is the only viable method for resonantly driving a single photon source. However, polarization degenerate cavities allow for arbitrary polarization conditions. A semi-classical model is used to analyze the performance of single photon sources under different polarization conditions. Further, the effect of residual cavity polarization splitting is analyzed under pulsed excitation.
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