Quantum Dot Laser Structures Research Articles

Continuous-wave operation of Si-based 1.31 μm InAs/GaAs quantum-dot laser grown by molecular beam epitaxy

Excellent performance III-V quantum-dot (QD) lasers grown on Si substrates by molecular beam epitaxy (MBE) are the most promising candidates for commercially viable Si-based lasers. This makes coveted chip-to-chip and system-to-system optical interconnections feasible. This paper reports the realization of high performance 1.31 μm InAs/GaAs QD lasers on a Si substrate with all-MBE The transition from Si to GaAs was realized using Ge as the intermediary layer, and the InAs/GaAs QD laser structure was grown on the GaAs/Ge buffer. Under continuous wave (CW) operation mode, a low threshold current density of 375 A cm−2, high output power of 63 mW, and high operating temperature of 80 °C, have been achieved using Si-based InAs QD lasers with a narrow ridge structure. It has great potential for application in the development of Si-based photonic integration circuits.

Ultra-high thermal stability InAs/GaAs quantum dot lasers grown on on-axis Si (001) with a record-high continuous-wave operating temperature of 150 °C.

Direct epitaxial growth of group III-V light sources with excellently thermal performance on silicon photonics chips promises low-cost, low-power-consumption, high-performance photonic integrated circuits. Here, we report on the achievement of ultra-high thermal stability 1.3 µm InAs/GaAs quantum dot (QD) lasers directly grown on an on-axis Si (001) with a record-high continuous-wave (CW) operating temperature of 150 °C. A GaAs buffer layer with a low threading dislocation density (TDD) of 4.3 × 106 cm-2 was first deposited using an optimized three-step growth method by molecular beam epitaxy. Then, an eight-layer QD laser structure with p-type modulation doping to enhance the temperature stability of the device was subsequently grown on the low TDD Si-based GaAs buffer layer. It is shown that the QD laser exhibits the ultra-high temperature stability with a characteristic temperature T0=∞ and T1=∞ in the wide temperature range of 10-75 °C and 10-140 °C, respectively. Moreover, a maximum CW operating temperature of up to 150 °C and a pulsed operating temperature of up to 160 °C are achieved for the QD laser. In addition, the QD laser shows a high CW saturation power of 50 mW at 85 °C and 19 mW at 125 °C, respectively.

High optical gain in InP-based quantum-dot material monolithically grown on silicon emitting at telecom wavelengths

We describe the fabrication process and properties of an InP based quantum dot (QD) laser structure grown on a 5° off-cut silicon substrate. Several layers of QD-based dislocation filters embedded in GaAs and InP were used to minimize the defect density in the QD active region which comprised eight emitting dot layers. The structure was analyzed using high resolution transmission electron microscopy, atomic force microscopy and photoluminescence. The epitaxial stack was used to fabricate optical amplifiers which exhibit electroluminescence spectra that are typical of conventional InAs QD amplifiers grown on InP substrates. The amplifiers avail up to 20 dB of optical gain, which is equivalent to a modal gain of 46 cm−1.

Impact of Built‐In Electric Field on the Emission Characteristics of InAs/GaAs Quantum Dot Laser Structure

The impact of the built‐in electric field on the emission characteristics of an InAs/GaAs quantum dot (QD) laser structure is investigated by performing systematic spectroscopic measurements. Photoluminescence (PL) features related to the ground state (GS) and excited state (ES) transitions of a QD ensemble are clearly observed in the temperature range of 8–300 K. The cross‐sectional transmission electron microscopy image and a systematic analysis of temperature‐dependent PL data confirm the excellent crystalline and optical quality of the QD laser structure. It is found that the values of the activation energy associated with GS and ES transitions of the QD sample reasonably match with the energy splitting of PL features. This successfully explains the trends observed in temperature‐dependent PL spectra, where thermal transfer of carriers to the wetting/barrier layer via QD excited states is proposed to be a major decay channel. An interesting observation is made where the integrated intensity of PL features increases up to 75 K and falls thereafter during the heating cycle. Such a peak‐like behavior of the integrated intensity is explained by invoking the temperature dependence of the built‐in electric field, which is estimated from the analysis of Franz–Keldysh oscillations observed in photoreflectance spectra and is important in the design of QD lasers.

Optimization of 1.3 µm InAs/GaAs quantum dot lasers epitaxially grown on silicon: taking the optical loss of metamorphic epilayers into account

Taking the optical loss caused by the metamorphic layers into account, we have proposed an optimization strategy for 1.3 µm InAs/GaAs quantum dot (QD) laser structures directly grown on silicon. We have investigated the effects of the QD layer number, the thickness and the composition of AlGaAs cladding layers on QD laser performance. The results demonstrate, with respect to the net modal gain and the differential quantum efficiency, that the optimized QD layer number is 7 for lasers grown on silicon, which is different from the optimized number of 5 for the counterparts grown on native substrates. The optimized thickness is obtained for the cladding layers of Al0.4Ga0.6As, Al0.6Ga0.4As and Al0.8Ga0.2As. Further, the optimized QD layer number for the net modal gain is nearly independent of the material gain of active regions. More importantly, the optimization strategy provides a comprehensive method to understand the differences in design between QD laser structures on silicon and those on native substrates.

Performance of quantum-dot-based tunnel-injection lasers: A theoretical analysis

Tunnel-injection lasers promise advantages in the modulation bandwidth and temperature stability in comparison with conventional laser designs. In this paper, we present results of a microscopic theory for laser properties of tunnel-injection devices and a comparison with a conventional quantum-dot laser structure. In general, the modulation bandwidth of semiconductor lasers is affected by the steady-state occupations of electrons and holes via the presence of spectral hole burning. For tunnel-injection lasers with InGaAs quantum dot emitting at an telecom wavelength of 155 μm, we demonstrate that the absence of spectral hole burning favors this concept over conventional quantum-dot based lasers.

13 μm InAs/GaAs quantum dot lasers on silicon with GaInP upper cladding layers

We report on the first electrically pumped continuous-wave (CW) InAs/GaAs quantum dot (QD) laser grown on Si with a GaInP upper cladding layer. A QD laser structure with a Ga0.51In0.49P upper cladding layer and an Al0.53Ga0.47As lower cladding layer was directly grown on Si by metal–organic chemical vapor deposition. It demonstrates the postgrowth annealing effect on the QDs was relieved enough with the GaInP upper cladding layer grown at a low temperature of 550°C. Broad-stripe edge-emitting lasers with 2-mm cavity length and 15-μm stripe width were fabricated and characterized. Under CW operation, room-temperature lasing at ∼1.3 μm has been achieved with a threshold density of 737 A/cm2 and a single-facet output power of 21.8 mW.

Development of Modulation p-Doped 1310 nm InAs/GaAs Quantum Dot Laser Materials and Ultrashort Cavity Fabry–Perot and Distributed-Feedback Laser Diodes

Multiple-layer InAs/GaAs quantum dot (QD) laser structures were etched to remove the p-side AlGaAs cladding layers to investigate the temperature-dependent photoluminescence (PL) characteristics. Four QD samples, including undoped as grown QDs, p-doped as grown QDs, undoped annealed QDs, and p-doped annealed QDs, were prepared by molecular beam epitaxy (MBE) and a postgrowth annealing process for comparison. Among them, modulation p-doped QD samples exhibit much less temperature-dependent characteristics of PL spectra and notable insensitivity to intermixing compared to undoped ones. This is attributed to the effects of modulation p-doping, which can inhibit holes’ thermal broadening in their closely spaced energy levels and significantly suppress In/Ga interdiffusion between QDs and their surrounding matrix. These results provide greater freedom in the choice of MBE growth for high-quality active regions and claddings of QD laser diodes. The superior features of the modulation p-doped QD materials have b...

Electrically pumped continuous-wave 1.3 µm InAs/GaAs quantum dot lasers monolithically grown on on-axis Si (001) substrates.

We report on the first electrically pumped continuous-wave (cw) InAs/GaAs quantum dot (QD) lasers monolithically grown on on-axis Si (001) substrates without any intermediate buffer layers. A 400 nm antiphase boundary (APB) free epitaxial GaAs film with a small root-mean-square (RMS) surface roughness of 0.86 nm was first deposited on a 300 mm standard industry-compatible on-axis Si (001) substrate by metal-organic chemical vapor deposition (MOCVD). The QD laser structure was then grown on this APB-free GaAs/Si (001) virtual substrate by molecular beam epitaxy (MBE). Room-temperature cw lasing at ~1.3 µm has been achieved with a threshold current density of 425 A/cm2 and single facet output power of 43 mW. Under pulsed operation, lasing operation up to 102 °C has been realized, with a threshold current density of 250 A/cm2 and single facet output power exceeding 130 mW at room temperature.

Negative activation energy and dielectric signatures of excitons and excitonic Mott transitions in quantum confined laser structures

Mostly, optical spectroscopies are used to investigate the physics of excitons, whereas their electrical evidences are hardly explored. Here, we examined a forward bias activated differential capacitance response of GaInP/AlGaInP based multi-quantum well laser diodes to trace the presence of excitons using electrical measurements. Occurrence of “negative activation energy” after light emission is understood as thermodynamical signature of steady state excitonic population under intermediate range of carrier injections. Similar corroborative results are also observed in an InGaAs/GaAs quantum dot laser structure grown by molecular beam epitaxy. With increasing biases, the measured differential capacitance response slowly vanishes. This represents gradual Mott transition of an excitonic phase into an electron-hole plasma in a GaInP/AlGaInP laser diode. This is further substantiated by more and more exponentially looking shapes of high energy tails in electroluminescence spectra with increasing forward bias, which originates from a growing non-degenerate population of free electrons and holes. Such an experimental correlation between electrical and optical properties of excitons can be used to advance the next generation excitonic devices.

A hybrid silicon evanescent quantum dot laser

We report the first demonstration of a hybrid silicon quantum dot (QD) laser, evanescently coupled to a silicon waveguide. InAs/GaAs QD laser structures with thin AlGaAs lower cladding layers were transferred by direct wafer bonding onto silicon waveguides defining cavities with adiabatic taper structures and distributed Bragg reflectors. The laser operates at temperatures up to 115 °C under pulsed current conditions, with a characteristic temperature T0 of 303 K near room temperature. Furthermore, by reducing the width of the GaAs/AlGaAs mesa down to 8 µm, continuous-wave operation is realized at 25 °C.

Quantum Dot Laser for a Light Source of an Athermal Silicon Optical Interposer

This paper reports a hybrid integrated light source fabricated on a silicon platform using a 1.3 μm wavelength quantum dot array laser. Temperature insensitive characteristics up to 120 °C were achieved by the optimum quantum dot structure and laser structure. Light output power was obtained that was high enough to achieve an optical error-free link of a silicon optical interposer. Furthermore, we investigated a novel spot size convertor in a silicon waveguide suitable for a quantum dot laser for lower energy cost operation of the optical interposer.

1.3-μm InAs/GaAs quantum-dot lasers monolithically grown on Si substrates using InAlAs/GaAs dislocation filter layers.

We compare InAlAs/GaAs and InGaAs/GaAs strained-layer superlattices (SLSs) as dislocation filter layers for 1.3-μm InAs/GaAs quantum-dot laser structures directly grown on Si substrates. InAlAs/GaAs SLSs are found to be more effective than InGaAs/GaAs SLSs in blocking the propagation of threading dislocations generated at the interface between the GaAs buffer layer and the Si substrate. Room-temperature lasing at ~1.27 μm with a threshold current density of 194 A/cm(2) and output power of ~77 mW has been demonstrated for broad-area lasers grown on Si substrates using InAlAs/GaAs dislocation filter layers.

Characterization and Analysis of 1.3-μm InAs/InGaAs Self-Assembled Quantum Dot Lasers

High performance 1.3-μm GaAs-based InAs/InGaAs quantum dot (QD) lasers have been fabricated. The QD lasers have demonstrated low threshold current, high output power as well as high temperature operation. Temperature-dependent (20–100 °C) and excitation power-dependent (12–700 mW) photoluminescence (PL) measurements have been carried out on the QD laser structures. Both ground state (GS) and excited state (ES) luminescence has been observed in the PL spectra as well as in the lasing spectra. Rate equations were used to interpret the PL behavior of the QD structures. High radiative recombination efficiency in the QD structure has been verified even at 100 °C. The excitonic modal gain has been calculated and compared with the experimentally obtained modal gain value by the Hakki–Paoli method from the QD laser with good agreement.

Fermi-dirac and random carrier distributions in quantum dot lasers

Using experimental gain and emission measurements as functions of temperature, a method is described to characterise the carrier distribution of radiative states in a quantum dot (QD) laser structure in terms of a temperature. This method is independent of the form of the inhomogeneous dot distribution. A thermal distribution at the lattice temperature is found between 200 and 300 K. Below 200 K the characteristic temperature exceeds the lattice temperature and the distribution becomes random below about 60 K. This enables the temperature range for which Fermi-Dirac statistics are applicable in QD laser threshold calculations to be identified.

Carrier Dynamics in Self-Assembled InAs QD Laser Structures and Broad-Area InAs QD Lasers Grown by Molecular Beam Epitaxy

ABSTRACTWe investigated carrier dynamics in both proton-irradiated InAs-GaAs quantum dot laser structures and in high power broad-area InAs-GaAs quantum dot lasers with windowed n-contacts using time-resolved PL (TR-PL) techniques.

Optical injection enables coherence resonance in quantum-dot lasers

We demonstrate that optically injected semiconductor quantum-dot lasers operated in the frequency-locked regime exhibit the counterintuitive effect of coherence resonance, i.e., the regularity of noise-induced spiking is a non-monotonic function of the spontaneous emission noise, and it is optimally correlated at a non-zero value of the noise intensity. We uncover the mechanism of coherence resonance from a microscopically based model of the quantum-dot laser structure, and show that it is related to excitability under optical injection and to a saddle-node infinite period (SNIPER) bifurcation occurring for small injection strength at the border of the frequency locking regime. By a model reduction we argue that the phenomenon of coherence resonance is generic for a wide class of optically injected lasers.

Absorption, Gain, and Threshold in InP/AlGaInP Quantum Dot Laser Diodes

We study self-assembled InP quantum dot (QD) laser structures grown at two temperatures (690°C and 730 °C) each with three different quantities of deposited quantum dot material (2, 2.5, and 3 mono-layers). The absorption spectra of these structures show features associated with the QD distributions and the magnitude of the absorption increases for samples where more material is deposited and for lower growth temperature. The 690°C growth temperature structures exhibit nonradiative recombination and internal optical mode loss that increase with the quantity of material deposited; we suggest that the laser performance is limited by the presence of defects. The higher growth temperature samples have lower threshold current density and are limited by gain saturation. For these samples and for 2-mm long lasers with uncoated facets, the threshold current density is as low as 150 A cm-2, emitting in the wavelength range around 730 nm.

Reduced linewidth enhancement factor due to excited state transition of quantum dot lasers

The carrier induced refractive index change and linewidth enhancement factor α due to ground-state (GS) and excited-state (ES) transitions have been compared by measuring the optical gain spectra from an InAs/GaAs quantum dot (QD) laser structure. It is shown that the ES transition exhibits a reduced α-factor compared to the value due to the GS transition. This result can be explained by the α-factor due to the ES transition having a smaller increase from the non-resonant carriers in the combined state of the wetting layer and InGaAs strain reducing layer than the α-factor increase due to the GS transition, since the relaxation time for carriers from the combined state of the wetting layer and InGaAs strain reducing layer to the ES is shorter than to the GS. The result reported here shows another advantage of using ES QD lasers for optical communication, in addition to their higher modulation speed.

Temperature dependence of the gain peak in p-doped InAs quantum dot lasers

Gain peak shifts with injection in undoped and p-doped InAs quantum dot laser structures between 200 K and 350 K are measured. The blue-shift with increasing injection, due to state-filling of the inhomogeneous distribution, is temperature independent for a fixed peak gain in the undoped sample, but temperature dependent in the doped sample. This is due to the wide electron state distribution and lowering of the electron quasi Fermi level by p-doping relative to the undoped device. While p-doping reduces the temperature dependence of the threshold current, it comes at the expense of increasing the temperature sensitivity of the wavelength.