Optical gain and absorption of 1.55 μm InAs quantum dash lasers on silicon substrate

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 1.5 - 1.6 μm range. 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 similarly operated InAs quantum dots operating at 1.3 μ m and further efforts 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 material in electro-absorption modulators grown on silicon. Published under license by AIP Publishing. https://doi.org/xx.xxxx/x.xxxxxxx

Employing quantum dot (QD) and/or quantum dash (QDash) based active regions is considered a promising approach for the realization of III-V laser structures directly grown on (001) silicon (Si) substrates.This is primarily because of the tolerance to material defects afforded by carrier localization.For monolithic integration of these lasers with other components, the relative insensitivity to operating temperature and optical feedback and the broad optical gain bandwidth provide further advantages.2][3][4] These lasers comprise of InAs QDs incorporated into layers of GaAs-based alloys which have ~4% lattice mismatch to Si.It is highly desirable to access the 1.55 μm lasing wavelength range to meet growing demands in siliconphotonics for long-haul optical communication and sensing technologies.The essential requirement to realize such a QD laser is to achieve high-quality growth of InAs QD active regions with the overall laser structure, utilizing InP-based materials.However, a larger lattice mismatch exists between InP-based materials and Si (~8%), thus inevitably introducing more defects and causing degradation in device performance.
Only recently, 1.55 μm Si-based QDash lasers driven by This is the author's peer reviewed, accepted manuscript.However, the online version of record will be different from this version once it has been copyedited and typeset.
PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0043815constant currents have been reported. 5These lasers have high operating currents and in this paper we assess the reasons for this and, by characterizing the underlying optical properties, the potential for further significant improvement in laser performance.
We have used Si-based 1.55 μm InAs QDash laser material the same as published in Ref. 5 for this investigation.
Broad-area edge-emitting segmented-contact devices are used to study the temperature dependent optical properties, including direct measurements of gain-current characteristics and optical absorption using the variable stripe-length method, 6 from 20 °C to 80 °C.We also explore the change in absorption characteristics of the structure with temperature, while inducing the quantum confined Stark effect (QCSE) under a range of reverse bias conditions, to understand the potential of the material for other integrated photonic functions.
The Si-based 1.55 μm InAs QDash laser structure was grown by metal organic chemical vapor deposition (MOCVD).The (001) silicon substrate was fully patterned with V-shaped nanogrooves, which have been proven to effectively inhibit anti-phase domains when growing polar molecules (e.g.III-V materials) on non-polar surfaces (e.g.silicon). 7,8As represented in respectively.At the long-wavelength range all spectra converge to the value of αi of 17 ± 3 cm -1 .This value, while better than previous work, 12 is significantly higher than the typical values for the shorter wavelength InAs dot structures grown at a GaAs lattice constant and will have a significant impact on the threshold current density that can be achieved.While the gain spectra of Fig. 2 (a) are useful when plotted as a function of current, they depend upon the quantity of non-radiative recombination much of which may be associated with extrinsic factors such as defect density, which could be improved in the future.Therefore, to better understand the underlying intrinsic performance we make use of the difference between the transparency point (TP), which is indicated in the graph, and the transition energy, as a measure of the degree of population inversion. 13The TP is related to the quasi-Fermi level separation (ΔEf), which must exceed the photon transition energy (hν) to achieve positive gain. 14 pumping levels (e.g.0.33~1.0kA/cm 2 , 20 °C), there is a rapid increment in the net gain (from -8.5 cm -1 to 10.5 cm -1 ), whereas at high current density the net gain magnitude increases more slowly.
At higher temperatures, the gain magnitude is lower at the same current density.For example, the highest net gain is 22.2 cm -1 under an injection level of 1.67 kA/cm 2 at 20 °C, but this peak gain magnitude drops to 9.4 cm -1 and to 0.2 cm -1 when the temperature increases to 50 °C and to 80 °C respectively.The current densities required to achieve positive net gain at all temperatures are very high, as might be expected from laser results, 5 and we now consider whether this is due to extrinsic or intrinsic factors.
The peak net gain is plotted as a function of injection level (TP-AE) in Fig. 3 (b).This removes extrinsic factors such as non-radiative recombination.The net gain is negative at low injection level (TP-AE) because the modal gain has not overcome the relatively high internal optical loss.The positive net-gain which contributes to lasing appears with further increase the injection level.However, to maintain a same net gain, it requires a higher level of injection at higher temperatures.This effect has been explained for InAs QD material previously and is attributed to the increased thermal distribution of carriers among the available energy states, leading to a lower population at any one energy.InP-based material using the same definitions of TP-AE and under very similar operating conditions.Here, the definition of AE can introduce a rigid shift between the relative x-axis positions of the two datasets.It is clear that the magnitude of gain and differential gain that can be obtained from the QDash material is significantly larger and that the intrinsic performance of the InAs QDash material grown on InP is superior to that of the more developed InAs on GaAs QDs, in this respect.It is highly likely that the significantly higher threshold current densities observed for the former materials are due to extrinsic factors such as defect density.Significant further improvement on reducing these extrinsic factors is required and, as the data of Fig. 4 shows, very worthwhile.

FIG.4, Peak modal gain versus the injection level (|TP-AE|)
of InAs QDash grown on InP material system and InAs QDs grown on GaAs material system.
To provide an initial assessment of the suitability of these QDash materials grown on Si for modulators, utilizing the electro-absorption effect we measure the evolution of the modal absorption spectra as a function of reverse bias.
The voltage was applied to the first (absorbing) section of the segmented contact device with ASE spectra measured from the second section.Fig. 5 (a) shows absorption spectra measured at 20 °C with the reversed voltage varied from 0 V to 6 V.A clear red-shift in the absorption edge (AE) can be observed due to the QCSE which had been demonstrated in quantum well and QD structures already. 17,18The potential of external electrical field was assumed to be dropped on the un-doped area comprising of the active region and SCH layers, This is the author's peer reviewed, accepted manuscript.However, the online version of record will be different from this version once it has been copyedited and typeset.

FIG. 1 ,
FIG.1, (a) upper: schema of layer stack.(b) lower: schema of segmented contact structure showing sections used and direction of light collection.Absorption spectra are obtained by taking the ratio of the emission spectra when pumping L2 or L1 and Gain by taking the ratio of emission spectra when pumping both L1 and L2 and L1 alone as per reference 6.

Fig. 3 (FIG. 2 ,
Fig.2 (b), where the device area is determined from its measured length and the width derived from the near-field profile, which

FIG. 3 ,
FIG.3, Temperature-dependent characteristics of (a) peak net gain versus current density, 20 °C to 80 °C, (b) peak net gain versus injection level which is the difference between transparency energy and transition energy, at 20 °C, 50 °C and 80 °C.

Fig. 4
Fig.4 shows a comparison of peak modal gain as a function of injection level between InAs QDs grown in GaAsbased material and this work which is InAs QDash grown in

FIG. 5 ,
FIG.5, (a) modal absorption spectra under a reversed bias of 0 V to 6 V measured at 20 °C, the arrow indicating the redshift with increasing the voltage, (b) the calculated ER from 20 °C to 80 °C.