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A semi-empirical model is compared with measurements to establish limiting factors in the performance of p-modulation doped InAs quantum dot (QD) lasers. Fitted absorption spectra allow identification of supposed factors and comparison of multiple samples isolates their origin, providing insights for future laser design.

sub-mA threshold currents are achieved for VCSELs using a simplified fabrication process which employs etched oxidation-vias for definition of the VCSEL aperture, as well as ease in formation of a bond pad without the need for bisbenzocyclobutane planarisation.

A systematic analysis of the performance of VCSELs, fabricated with a decreasing number of structural elements, is used to assess the complexity of fabrication (and therefore time) required to obtain sufficient information on epitaxial wafer suitability. Initially, sub-mA threshold current VCSEL devices are produced on AlGaAs-based material, designed for 940 nm emission, using processing methods widely employed in industry. From there, stripped-back Quick Fabrication (QF) devices, based on a bridge-mesa design, are fabricated and this negates the need for benzocyclcobutane (BCB) planarisation. Devices are produced with three variations on the QF design, to characterise the impact on laser performance from removing time-consuming process steps, including wet thermal oxidation and mechanical lapping used to reduce substrate thickness. An increase in threshold current of 1.5 mA for oxidised QF devices, relative to the standard VCSELs, and a further increase of 1.9 mA for unoxidised QF devices are observed, which is a result of leakage current. The tuning of the emission wavelength with current increases by ~0.1 nm/mA for a VCSEL with a 16 μm diameter mesa when the substrate is unlapped, which is ascribed to the increased thermal resistance. Generally, relative to the standard VCSELs, the QF methods employed do not significantly impact the threshold lasing wavelength and the differences in mean wavelengths of the device types that are observed are attributed to variation in cavity resonance with spatial position across the wafer, as determined by photovoltage spectroscopy measurements.

We report direct measurements of the optical gain profile for a vertical cavity surface emitting laser (VCSEL) epitaxial structure, by characterising the transverse electric (TE) in-plane net modal gain using the segmented contact method.

Initial age-related degradation mechanisms for InAs quantum dot lasers grown on silicon substrates emitting at 1.3 μ m are investigated. The rate of degradation is observed to increase for devices operated at higher carrier densities and is therefore dependent on gain requirement or cavity length. While carrier localization in quantum dots minimizes degradation, an increase in the number of defects in the early stages of aging can increase the internal optical-loss that can initiate rapid degradation of laser performance due to the rise in threshold carrier density. Population of the two-dimensional states is considered the major factor for determining the rate of degradation, which can be significant for lasers requiring high threshold carrier densities. This is demonstrated by operating lasers of different cavity lengths with a constant current and measuring the change in threshold current at regular intervals. A segmented-contact device, which can be used to measure the modal absorption and also operate as a laser, is used to determine how the internal optical-loss changes in the early stages of degradation. Structures grown on silicon show an increase in internal optical loss, whereas the same structure grown on GaAs shows no signs of increase in internal optical loss when operated under the same conditions.

InAs/GaAs quantum-dot (QD) lasers offer a promising method to realise Si-based on-chip light sources. However, the monolithic integration of III–V materials on Si introduces a high density of threading dislocations (TDs), which limits the performance of such a laser device in terms of device lifetime. Here, we proposed a kinetic model including a degradation term and a saturation term to simulate the degradation process caused by the TDs in the early stage of laser operation. By using a rate equation model, the current density in the wetting layer, where the TDs concentrate, is calculated. We compared the rate of degradation of QD lasers with different cavity lengths and of quantum-well lasers, where both are directly grown on Si substrates, by varying the fitting parameters in the calculation of current densities in the kinetic model.

This Letter reports on the temperature-dependent optical gain and absorption features, including the quantum confined Stark effect, of an InAs/InGaAs quantum-dash laser directly grown on a (001) Si substrate, with the lasing wavelength within the range of 1.5–1.6 μm. The maximum optical net gain was 22 cm−1, and the internal optical loss was ∼–17 cm−1 at 20 °C. Measurements as a function of injection level indicate that while the required current densities are still high, the intrinsic performance is significantly better than that of similarly operated InAs quantum dots operating at 1.3 μm, and further effort on growth could be made to reduce the internal optical losses and non-radiative current density. Optical modal absorption spectra were measured as a function of reverse bias from 0 V to 6 V, and a 40 nm redshift was observed in the absorption edge due to the quantum confined Stark effect, suggesting potential applications of these materials in electro-absorption modulators grown on silicon.

Development of a quick fabrication (QF) method for commercial wafer characterisation based on rapid feedback of VCSEL performance. We report on the design of the fabrication process including the systematic removal of time-consuming steps of planarization, oxidation and substrate lapping, and the associated impact on device performance and yield. We show comparable performance of the oxide-confined QF etched trench VCSELs and full process devices and we show that unoxidised devices behave as large aperture oxidised devices. Further, we demonstrate similar performance of substrate-lapped and -unlapped VCSELs between 1.0-1.2 Ith with a difference in current tuning typically 0.064nm/mA.

High-volume low-cost production of vertical cavity surface emitting lasers (VCSELs) will allow their exploitation in new commodity markets. We report the successful scaling up from research level fabrication to produce oxide confined VCSELs across a whole 150mm wafer. On-wafer light-current-voltage (L-I-V) and spectral measurements are analyzed to determine the cross-wafer variations in threshold current, threshold current densities and emission wavelength, which is compared with reflectivity measurements taken immediately after growth. We examine the dependence of VCSEL performance on fabrication parameters over a range of device dimensions to assess whether variation arises from non-uniformity of the epitaxial material or wafer processing.

We report temperature dependent optical modal gain and absorption features as function of injection current of 1.55 μm InAs Q-Dash laser with an InAs/InAlGaAs/InP structure monolithically grown on (001) silicon substrate. Board-area multi-segmented contact devices were fabricated and driven by pulsed current. Net modal gain and absorption was obtained by measuring amplified spontaneous emission using the variable stripe-length method. From 20˚C to 80˚C, the changes in maximum gain and gain bandwidth were studied and a redshift in peak gain wavelength was observed. Quantum confined Stark effect was measured under reversed bias from -1 V to -7 V.

The quantum confined Stark effect in InAs/InGaAs QDs using an undoped and p-modulation doped active region was investigated. Doping potentially offers more than a 3x increase in figure of merit modulator performance up to 100 °C.

InP/GaInP quantum dot laser structures exhibiting broad optical gain spectra suitable for mode-locking have been demonstrated. Two-section narrow ridge passive mode-locked lasers were fabricated from this material. Mode-locking conditions have been investigated for devices with different cavity lengths, with maximum frequency of 15.21 GHz.

Broad-area 1.55 μm InAs quantum dots (QDs) lasers were fabricated based on monolithic growth of InAs/InAlGaAs/InP active structures on nano-patterned (001) silicon substrates. Device optoelectronic properties and materials' optical gain and absorption features were studied to provide experimental support for further optimizations in laser design.

In this work, the feasibility of a monolithically integrated laser and electroabsorption modulator based on the same active quantum dot epistructure is studied. The net modal gain and the absorption in the modulator were measured using the segmented contact method from 25 °C to 125 °C. The maximum of the net modal gain active region of the laser decreases from 10 cm -1 at 25 °C to 3.9 cm -1 at 125 °C. The non-optimized maximum extinction ratio of the modulator, 4.1 dB·mm -1 , is almost constant until 25 °C. The wavelengths at which the net modal gain and the change in absorption are maximum shifts with temperature by 0.04 eV.

Compact coherent population trapping (CPT) based clocks require single-mode, low-power consumption, high-speed and polarization-stable laser sources. We report our progress in developing customized vertical cavity surface emitting laser (VCSEL) diodes designed for operating on the D1 transition of cesium, and specifically tailored for CPT-based atomic clocks. The VCSELs provide high power (>1 mW), narrow linewidths (15 dB). Preliminary spectroscopy has been observed.

Reliable, efficient electrically pumped silicon-based lasers would enable full integration of photonic and electronic circuits, but have previously only been realized by wafer bonding. Here, we demonstrate continuous-wave InAs/GaAs quantum dot lasers directly grown on silicon substrates with a low threshold current density of 62.5 A cm–2, a room-temperature output power exceeding 105 mW and operation up to 120 °C. Over 3,100 h of continuous-wave operating data have been collected, giving an extrapolated mean time to failure of over 100,158 h. The realization of high-performance quantum dot lasers on silicon is due to the achievement of a low density of threading dislocations on the order of 105 cm−2 in the III–V epilayers by combining a nucleation layer and dislocation filter layers with in situ thermal annealing. These results are a major advance towards reliable and cost-effective silicon-based photonic–electronic integration.

A high-performance III–V quantum-dot (QD) laser monolithically grown on Si is one of the most promising candidates for commercially viable Si-based lasers. Great efforts have been made to overcome the challenges due to the heteroepitaxial growth, including threading dislocations and anti-phase boundaries, by growing a more than 2 µm thick III–V buffer layer. However, this relatively thick III–V buffer layer causes the formation of thermal cracks in III–V epi-layers, and hence a low yield of Si-based optoelectronic devices. In this paper, we demonstrate a usage of thin Ge buffer layer to replace the initial part of GaAs buffer layer on Si to reduce the overall thickness of the structure, while maintaining a low density of defects in III–V layers and hence the performance of the InAs/GaAs QD laser. A very high operating temperature of 130 °C has been demonstrated for an InAs/GaAs QD laser by this approach.

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.

InAs quantum-dot (QD) lasers emitting at 1300nm with nominally undoped and modulated p-type doping are studied. Modal-gain measurements indicate a higher gain can be achieved from the ground-state for a given Fermi-level separation with p-doping and a reduced temperature-dependence of threshold current for short-cavity lasers.

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.

We investigate factors affecting device performance of deep-etched InAs/GaAs quantum-dot ridge-lasers grown directly on silicon substrates, emitting close to 1.3-μm. Continuous-wave reliability measurements show there is a significant reduction in degradation, in terms of threshold current and slope efficiency, as the cavity length is increased.

We present a study of metalorganic vapour phase epitaxy of ternary InAsP quantum dots on AlGaInP/GaAs. The properties of InAsP QD laser structures were compared with reference samples containing binary InP QDs. Based on X-ray diffraction, the molar fraction of As in InAsP QDs was estimated to be ∼25%. Room temperature liquid contact electro-luminescence measurements revealed a long wavelength shift of the InAsP QD emission to ∼775 nm as compared with the InP QD emission at 716 nm and an increased full width at half maximum of the spontaneous emission (71 meV vs 50 meV). As cleaved, 4 mm long and 50 µm wide InAsP QD lasers operated in a pulsed regime at room temperature at ∼770 nm with a threshold current density of 155 A/cm2 and a maximum output optical power of at least ∼200 mW. The maximum operation temperature was at least 380 K.

The addition of elevated temperature steps (annealing) during the growth of InAs/GaAs quantum dot (QD) structures on Si substrates results in significant improvements in their structural and optical properties and laser device performance. This is shown to result from an increased efficacy of the dislocation filter layers (DFLs); reducing the density of dislocations that arise at the Si/III-V interface which reach the active region. The addition of two annealing steps gives a greater than three reduction in the room temperature threshold current of a 1.3 μm emitting QD laser on Si. The active region of structures grown on Si have a room temperature residual tensile strain of 0.17%, consistent with cool down from the growth temperature and the different Si and GaAs thermal expansion coefficients. This strain limits the amount of III-V material that can be grown before relaxation occurs.

Three InAs quantum dot (QD) samples with dislocation filter layers (DFLs) are grown on Si substrates with and without in-situ annealing. Comparison is made to a similar structure grown on a GaAs substrate. The three Si grown samples have different dislocation densities in their active region as revealed by structural studies. By determining the integrated emission as a function of laser power it is possible to determine the power dependence of the radiative efficiency and compare this across the four samples. The radiative efficiency increases with decreasing dislocation density; this also results in a decrease in the temperature quenching of the PL. A laser structures grown on Si and implementing the same optimum DFL and annealing procedure exhibits a greater than 3 fold reduction in threshold current as well as a two fold increase in slope efficiency in comparison to a device in which no annealing is applied. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We explore the origins of the threshold current temperature dependence in InP quantum-dot (QD) lasers. While the internal optical mode loss does not change with temperature, the peak gain required to overcome the losses becomes more difficult to achieve at elevated temperature due to the thermal spreading of carriers among the available states. In 2-mm-long lasers with uncoated facets, this effect is responsible for 66% of the difference in threshold current density between 300 and 360 K. Spontaneous recombination current only makes up at most 10% of the total recombination current density over this temperature range, but the temperature dependence of the spontaneous recombination in the QD and quantum-well capping layers can be used, assuming only a simple proportional nonradiative recombination process, to explain the temperature dependence of the threshold current density.

We report on InAsP quantum dot lasers grown by MOVPE for 730-780 nm wavelength emission and compare performance with InP dot samples grown under similar conditions and with similar structures. 1-4 mm long, uncoated facet InAsP dot lasers emit between 760 and 775 nm and 2 mm long lasers with uncoated facets have threshold current density of 260 Acm(-2), compared with 150 Acm(-2) for InP quantum dot samples, which emit at shorter wavelengths, 715-725 nm. Pulsed lasing is demonstrated for InAsP dots up to 380 K with up to 200 mW output power. Measured absorption spectra indicate the addition of Arsenic to the dots has shifted the available transitions to longer wavelengths but also results in a much larger degree of spectral broadening. These spectra and transmission electron microscopy images indicate that the InAsP dots have a much larger degree of inhomogeneous broadening due to dot size variation, both from layer to layer and within a layer.

We report on 1.3-µm emitting InAs/GaAs quantum-dot ridge lasers grown directly on a silicon substrate. Using an optimised dislocation filter design, continuous-wave operation is achieved with power outputs exceeding 20-mW and operation up to 39°C.

We employ a device which exploits the properties of InP quantum dots (QD), (emitting from 650-730 nm), to produce simultaneous dual-λ lasing from a single ridge-waveguide comprising two sections. Due to the effects of state-filling in an inhomogeneously broadened QD ensemble, the wavelength is strongly dependent on magnitude of the gain (or cavity loss). Therefore, by altering the loss of each section of the device we are able to demonstrate a large range of difference-wavelengths, up to 63 nm. Here, we test the performance of the device and measure effects of temperature and difference-wavelength on the stability of the two lasing modes.

We demonstrate dual-wavelength lasers operating with simultaneous emission for a range of difference-wavelengths by accessing the various in homogeneously broadened dot states. We investigate temperature tuning and quantify factors affecting mode stability, including temperature and difference-wavelength.

We describe the effect of growth temperature on the optical absorption, gain, and threshold current density of 730-nm emitting, metal-organic vapor phase epitaxy (MOVPE) grown, InP-AlGaInP quantum-dot lasers. Decreasing the growth temperature from 750°C to 690°C leads to an increase in ground state absorption, while sufficient optical gain and low 300 K threshold current density is obtained in the growth temperature window between 710°C and 730°C . Wider (16 nm compared to 8 nm) interlayer barriers lead to lower threshold current density with 300 K values as low as 165 Acm-2 for 2-mm-long lasers with uncoated facets.

Intensity stability with respect to pumping level and temperature is examined in electrically injected dual-λ lasers with separations between 8 and 63 nm that utilise different quantum dot states or quantum dot and quantum well states.

We quantify the mechanisms that govern the lasing wavelength in edge-emitting InP/AlGaInP quantum dot (QD) lasers, by characterising the constituent factors controlling the temperature dependence of the gain peak wavelength. We show that a regime exists where the temperature coefficient of the bandgap can be compensated by the increasing wavelength-shift associated with state-filling in the QD ensemble, necessary to recover the gain peak magnitude. We demonstrate cleaved-facet edge-emitting lasers with a wavelength temperature dependence of 0.03 nm/K, similar to the temperature dependence of a Bragg stack fabricated in this material and approximately a sixth of the dependence of the bandgap.

Dual-mode monolithic lasers emitting in the range of 655-720nm have been demonstrated, which were designed to exploit the properties offered by quantum dot material. The criteria determining wavelength selection and mode stability are discussed.

The facets of InP/(Al)GaInP/GaAs quantum dot laser active regions offer superior resistance to catastrophic optical mirror damage at high facet power densities. These structures degrade by bulk damage. We have used a new range of techniques to identify changes occurring during damage in working devices: thermography through windows in the nmetallization, photoluminescence via p-metallization windows and photocurrent studies. Devices were aged with single very high current pulses or pulses of increasing size and monitored during this process with these techniques. Previous investigation with panchromatic cathodoluminescence revealed dark non-radiative spots throughout the plane of the active region. The dark spots, which were present even in unprocessed material, increased in size in the pumped regions only during lasing action. The spots and background regions darkened throughout the pumped stripe area only for the whole duration of the current pulse. Thermography after successive pulses confirmed damage originating from a point in the bulk rather than at the facet. p-windows observations of light and dark regions showed a blue shift in the photoluminescence spectra of the dark regions. Photocurrent studies of more gently aged devices showed a greater decrease in signal in the region associated in previous work with defective very large dots. Identification of such spectral regions, which were previously found to be influenced by changes in structure design and growth conditions offer a route to control degradation mechanisms by this means.

Electrically injected dual wavelength lasers are demonstrated with wavelength separation as large as 66 nm with temperature tuning of the difference wavelength at 0.11 nm / K (61 GHz / K).

By optimizing the inhomogeneous properties of a QD structure we have created a monolithic dual-wavelength laser, sourcing spatially coherent emission between 650 and 730 nm, with independent control of the light output at each wavelength.