Physics and applications of quantum dot lasers for silicon photonics

Direct epitaxial integration of III-V materials on Si offers substantial manufacturing cost and scalability advantages over heterogeneous integration. The challenge is that epitaxial growth introduces high densities of crystalline defects that limit device performance and lifetime. Quantum dot lasers, amplifiers, modulators, and photodetectors epitaxially grown on Si are showing promise for achieving low-cost, scalable integration with silicon photonics. The unique electrical confinement properties of quantum dots provide reduced sensitivity to the crystalline defects that result from III-V/Si growth, while their unique gain dynamics show promise for improved performance and new functionalities relative to their quantum well counterparts in many devices. Clear advantages for using quantum dot active layers for lasers and amplifiers on and off Si have already been demonstrated, and results for quantum dot based photodetectors and modulators look promising. Laser performance on Si is improving rapidly with continuous-wave threshold currents below 1 mA, injection efficiencies of 87%, and output powers of 175 mW at 20 °C. 1500-h reliability tests at 35 °C showed an extrapolated mean-time-to-failure of more than ten million hours. This represents a significant stride toward efficient, scalable, and reliable III-V lasers on on-axis Si substrates for photonic integrate circuits that are fully compatible with complementary metal-oxide-semiconductor (CMOS) foundries.

Chapter 6 - Reliability of lasers on silicon substrates for silicon photonics

Quantum dot (QD) lasers exhibit many useful properties such as low threshold current, temperature and feedback insensitivity, chirpless behavior, and low linewidth enhancement factor. The aim of this paper is to investigate the lasing spectra behaviour of InAs/InP(311B) QD lasers. In order to reach the standards of long-haul transmissions, 1.55μm InAs QD lasers grown on InP substrate have been developed. More particularly, it has been demonstrated that the use of the specific InP(113)B substrate orientation when combined with optimized growth techniques allows the growth of very small (4 nm high) and dense (up to 1011cm^-2) QD structures. Consequently, a model based on the multi-population rate equations (MPRE) taking into account many cavity longitudinal modes for the calculation of the entire emission spectrum has been developed. In order to include the inhomogeneous gain broadening of the QD ensemble, various dot populations, each characterized by a ground state (GS) and an excited state (ES) average energy level have been considered. It will be shown that the numerical results are in good agreement with the experimental ones, both for the case of the double laser emission and for the effects of the homogeneous broadening on the lasing spectra. This numerical investigation based on carrier dynamics is of prime importance for the optimization of low cost sources for optical telecommunications as well as for a further improvement of QD laser performances at 1.55-μm on InP substrate, as already demonstrated for InAs-GaAs QD lasers emitting at 1.3-μm.

Quantum dot (QD) lasers epitaxially grown on Si have already been demonstrated to show record low threshold, high temperature tolerance, and low feedback sensitivity. When grown on the silicon photonic chip and integrated with Si waveguides (WGs), QD lasers offer considerable economical and foundry‐scalable solutions to on‐chip light sources. Yet, a technology that enables both growth and integration of QD lasers on a silicon photonic chip has not been demonstrated. Herein, a novel device platform which enables integration of the QD active region with passive WG structures is designed. By doing so, complex and high‐performance lasers such as distributed Bragg reflector lasers, mode‐locked lasers, and sampled grating distributed Bragg reflector tunable lasers are demonstrated in this platform. The same laser epitaxial stack can be easily grown on the substrate of a silicon photonic chip to allow light coupling from QD laser cavities to the silicon WGs.

1.3 mum quantum dot (QD) lasers grown on GaAs substrates have recently shown promising performances. For the 1.55 mum wavelength range, QD structures grown on InP substrates have been proposed to produce lasers for long haul telecom. At this wavelength, the progress has been delayed by the difficulties to obtain high density of small QDs by molecular beam epitaxy. On (100) InP substrates, elongated structure named quantum dash (QDH) are achieved in the standard conditions. The growth on high index substrates allows the achievement of a higher density of smaller quantum dots. In this paper, we compare laser performance of devices elaborated on both substrates. After quantum dot elaboration optimization, on (311) B substrates, laser emission at 1.59 mum on the ground state transition is obtained at room temperature (RT), A very low threshold current density (Jth) of 21 A/cm2 for the best QD lasers is measured. This value can be compared to the Jth of quantum well (QW) laser, which are in the few hundred A/cm2. On (100) substrates laser emission is observed at 1.45 mum for a current density of 375 A/cm2 at RT. The evolution of the Jth and of the emission wavelength as a function of temperature is studied on both structures. The changes are interpreted in terms of density of states and of form of the gain curve

Compound-semiconductor-based photonic devices, including lasers and modulators, directly grown and on-chip integrated on Si substrates provide a promising approach for the realization of optical interconnects with CMOS compatibility. Utilizing quantum dots as efficient dislocation filters near the GaAs-Si interface, for the first time, we demonstrated high-performance InGaAs/GaAs quantum dot (QD) lasers on silicon with a relatively low threshold current density (J, h = 900 A/cm 2 ), large small-signal modulation bandwidth of 5.5 GHz, and a high characteristic temperature (T 0 = 278 K). The integrated InGaAs QD lasers with quantum well (QW) electroabsorption modulators, achieved through molecular beam epitaxy (MBE) growth and regrowth, exhibit a coupling coefficient greater than 20% and a modulation depth ∼100% at 5 V reverse bias. We achieved the monolithic integration of amorphous and crystalline silicon waveguides with quantum dot lasers by using plasma-enhanced-chemical-vapor deposition (PECVD) and membrane transfer, respectively. Finally, preliminary results on the integration of QD lasers with Si CMOS transistors are presented.

A high-growth-temperature step used for the GaAs spacer layer is shown to significantly improve the performance of 1.3-μm multilayer InAs/GaAs quantum-dot (QD) lasers. The high-growth-temperature spacer layer inhibits threading dislocation formation, resulting in enhanced electrical and optical characteristics and hence improved laser performance. The combination of high-growth-temperature GaAs spacer layers and high-reflectivity (HR) coated facets has been utilized to further reduce the threshold current and threshold current density ( J th ) for 1.3-μm InAs/GaAs QD lasers. Very low continuous-wave room-temperature threshold current of 1.5 mA and a threshold current density of 18.8 A/cm 2 are achieved for a 3-layer device with a 1-mm long HR/HR cavity. For a 2-mm cavity the continuous-wave threshold current density is as low as 17 A/cm 2 at room temperature for an HR/HR device. An output power as high as 100 mW is obtained for a device with HR/cleaved facets. The high-growth-temperature spacer layers have only a relatively small effect on the temperature stability of the threshold current above room temperature. To further increase the characteristic temperature ( T 0 ) of the QD lasers, 1.3-μm InAs/GaAs QD lasers incorporating p-type modulation doping have been grown and studied. A negative T 0 and J th of 48 A/cm -2 at room temperature have been obtained by combining the high-growth-temperature GaAs spacer layers with the p-type modulation doping of the QDs.

In this paper, a theoretical model is used to investigate the lasing spectrum properties of InAs-InP(113)B quantum dot (QD) lasers emitting at 1.55 mum. The numerical model is based on a multipopulation rate equations analysis. Calculations take into account the QD size dispersion as well as the temperature dependence through both the inhomogeneous and the homogeneous broadenings. This paper demonstrates that the model is capable of reproducing the spectral behavior of InAs-InP QD lasers. Especially, this study aims to highlight the transition of the lasing wavelength from the ground state (GS) to the excited state (ES). In order to understand how the QD laser turns on, calculated optical spectra are determined for different cavity lengths and compared to experimental ones. Unlike InAs-GaAs QD lasers emitting at 1.3 mum, it is shown that a continuous transition from the GS to the ES is exhibited because of the large inhomogeneous broadening comparable to the GS and ES lasing energy difference.

The goal of this research work is to characterize and to suggest a method to improve the ten-layer InAs/InGaAs quantum dot (QD) laser performance by using post-growth thermal annealing. The approach of post-growth annealing is initially performed on the passive dots-in-a-well structure in this thesis. Interdiffusion of In and Ga atoms caused by thermal annealing has been proven from the photoluminescence measurements. The results show that p-doped QD structures are more resistant to intermixing. The post-growth thermal annealing technique is further applied to the active QD laser structure. Significant improvements in the performance of p-doped ten-layer InAs/InGaAs QD laser are demonstrated after annealing at 600 oC for 15 s. Defect reduction is thought to be the most likely mechanism contributing to the improved performance. The competition between the ground state (GS) and excited state (ES) lasing is investigated from the as-grown and thermally annealed QD lasers. Our results show that by adjusting the rapid thermal annealing temperature, the competition between the GS and ES lasing can be modified.

We discuss in detail a new mechanism of nonlinearity of the light-current characteristic (LCC) in heterostructure lasers with reduced-dimensionality active regions, such as quantum wells (QWs), quantum wires (QWRs), and quantum dots (QDs). It arises from: 1) noninstantaneous carrier capture into the quantum-confined active region and 2) nonlinear (in the carrier density) recombination rate outside the active region. Because of 1), the carrier density outside the active region rises with injection current, even above threshold, and because of 2), the useful fraction of current (that ends up as output light) decreases. We derive a universal closed-form expression for the internal differential quantum efficiency /spl eta//sub int/ that holds true for QD, QWR, and QW lasers. This expression directly relates the power and threshold characteristics. The key parameter, controlling /spl eta//sub int/ and limiting both the output power and the LCC linearity, is the ratio of the threshold values of the recombination current outside the active region to the carrier capture current into the active region. Analysis of the LCC shape is shown to provide a method for revealing the dominant recombination channel outside the active region. A critical dependence of the power characteristics on the laser structure parameters is revealed. While the new mechanism and our formal expressions describing it are universal, we illustrate it by detailed exemplary calculations specific to QD lasers. These calculations suggest a clear path for improvement of their power characteristics. In properly optimized QD lasers, the LCC is linear and the internal quantum efficiency is close to unity up to very high injection-current densities (15 kA/cm/sup 2/). Output powers in excess of 10 W at /spl eta//sub int/ higher than 95% are shown to be attainable in broad-area devices. Our results indicate that QD lasers may possess an advantage for high-power applications.

We report the device characteristics of stacked InAs-GaAs quantum dot (QD) lasers cladded by an Al/sub 0.4/Ga/sub 0.6/As layer grown at low temperature by metal-organic chemical vapor deposition. In the growth of quantum dot lasers, an emission wavelength shifts toward a shorter value due to the effect of postgrowth annealing on quantum dots. This blueshift can be suppressed when the annealing temperature is below 570/spl deg/C. We achieved 1.28-/spl mu/m continuous-wave lasing at room temperature of five layers stacked InAs-GaAs quantum dots embedded in an In/sub 0.13/Ga/sub 0.87/As strain-reducing layer whose p-cladding layer was grown at 560/spl deg/C. From the experiments and calculations of the gain spectra of fabricated quantum dot lasers, the observed lasing originates from the first excited state of stacked InAs quantum dots. We also discuss the device characteristics of fabricated quantum dot lasers at various growth temperatures of the p-cladding layer.

Laser diodes (LDs) and other optical devices using quantum dots (QDs) are promising devices, as have already been reported. Compact and highly functional photonic integrated circuits (PICs) are desired to establish advanced photonic networks. Moreover, characteristics such as high temperature stability are essential if the PICs are integrated with large-scale integration (LSI), because the heat diffused from that causes the surrounding temperature to increase, which is detrimental to the operating conditions. We have investigated the QD intermixing (QDI) technique [1] to apply it to the fabrication of QD integrated PICs [2]. This is advantageous in that it is easy to fabricate PICs without any epitaxial re-growth. In this study, we found that the temperature stability of intermixed QD-LDs, fabricated using the Ar-implantation-induced QDI technique, could be improved, with a characteristic temperature (T0) greater than 570 K in the high temperature region.

This paper highlights quantum dot (QD) lasers and QD optoelectronic device integration. The emission spectrum, room temperature photoluminescence, and electroluminescence spectra of a 3-stacked InAs QD laser are presented. On the other hand, monolithic integration of QD devices is desirable for practical applications, as this would lead to low loss, high speed modules, operating at lower currents.

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.

The spectral dependence of the modal gain and linewidth enhancement factor is measured in an InAs/GaInAs/AlGaAs/GaAs quantum dot (QD) laser and a GaInAs/AlGaAs/GaAs quantum well laser of the same design lacking only the quantum dots. The material differential gain and material differential carrier induced refractive index are found to be about three times smaller in the quantum dot laser than in the quantum well laser. The linewidth enhancement factor is smaller in the QD laser and exhibits considerably less dispersion.