High efficiency low threshold current 1.3 μm InAs quantum dot lasers on on-axis (001) GaP/Si

Over the past decade, silicon photonics has emerged as a promising candidate for high performance, chip-scale interconnects to meet the rapidly growing demand for data bandwidths.^1,2 Although impressive progress has been achieved for various passive silicon photonics devices, realization of a monolithically integrated, efficient light source on a Si substrate has yet remained as a challenging task.³ Self-assembled InAs quantum dot (QD) lasers show promise as an efficient light source for silicon photonic integrated circuits because the lateral carrier confinement of individual QDs makes them less sensitive to the high density of threading dislocations generated from the inherent lattice mismatch between GaAs and Si.⁴ 

Since the first demonstration of 1.3 μm InAs QD lasers on Si substrates in pulsed operation by Wang et al. in 2011,⁴ much effort has been made to lower the threshold current density and to increase the output power.^4–6 Liu et al. presented impressive results on high performance InAs QD ridge-waveguide lasers on Ge/Si substrates with low threshold current (16 mA), high output power (176 mW), and high continuous-wave (CW) operating temperature (119 °C).⁶ Also, Chen et al. reported efficient broad-area QD lasers on Si with threshold current densities as low as ∼60 A/cm² at room temperature.⁷ All of these QD lasers were grown on offcut (4°–6°) Si substrates to avoid the formation of anti-phase domains (APDs), which are known to be electrically active and can be detrimental for laser diode efficiency.⁸ Unfortunately, offcut Si substrates are not fully compatible with standard CMOS processing foundries, and this hinders the viability of InAs QD lasers as light emitters for silicon photonics.

Recently, we reported the first demonstration of 1.3 μm QD lasers epitaxially grown on on-axis (001) GaP/Si substrates.⁹ The GaP layer, grown by metal-organic chemical vapor deposition (MOCVD), terminates APDs within ∼40 nm of the interface between GaP and Si, removing the need for offcut Si.¹⁰ GaAs on V-groove Si (001) templates also enable QD laser growth on Si without offcut because the KOH etched Si trenches allow APD-free III-V layers by growing on {111} surfaces.¹¹ Chen et al. also reported QD lasers on on-axis (001) Si by growing an APD-free initial GaAs layer using MOCVD with a sophisticated Si surface preparation step.¹² However, all the QD lasers grown on on-axis Si substrates showed relatively diminished performance in terms of threshold currents and output powers compared to the previous high performance QD lasers on offcut Si.

In this work, we demonstrate highly efficient 1.3 μm QD lasers epitaxially grown on on-axis GaP/Si (001) substrates enabled by improved GaAs templates with a threading dislocation density (TDD) of 7.3 × 10⁶ cm⁻². Narrow ridge waveguide lasers with as-cleaved facets achieve CW threshold currents as low as 9.5 mA at 20 °C. The lowest CW threshold current density is 132 A/cm², and the highest output power is 175 mW from both facets of a device. The InAs QD lasers also show a high CW wall-plug-efficiency (WPE) of 38.4%. The CW ground-state operation persists up to 80 °C, producing an output power of 7 mW. Applying a high reflectivity (HR) coating to one facet lowers the CW threshold currents even further to 6.7 mA, which is the lowest value to date for any Fabry-Perot lasers on Si.

Figure 1(a) shows a schematic of the epitaxial QD laser structure grown on an on-axis (001) GaP/Si substrate. The GaP/Si (001) on-axis wafer is commercially available in 300 mm size from NAsP_III/V GmbH. We first grew a 3 μm thick GaAs buffer layer with In_0.1Ga_0.9As/GaAs strained superlattice dislocation filter layers in a solid-source molecular beam epitaxy (MBE) chamber. Thermal cycle annealing was performed four times at temperatures between 320 °C and 700 °C under As₂ overpressure to reduce threading dislocations via dislocation annihilations. Then, 10 periods of In_0.1Ga_0.9As/GaAs strained superlattice layers were grown at 500 °C at a growth of approximately 1 μm/hr.^13,14 The root-mean-square roughness of the surface was reduced from 5.45 nm to 2.48 nm from the atomic force microscopy (AFM) measurement (not shown here). More details about the GaAs template growth on GaP/Si and TDD characterization will be reported in a separate study. After the GaAs buffer growth, we removed the sample from the MBE chamber and conducted electron channeling contrast imaging (ECCI) measurements to investigate the TDD in the GaAs template.¹⁵ 

Figures 1(b) and 1(c) show representative ECCI images from before and after the optimization of the GaAs template layer. Threading dislocations are shown in either bright or dark spots. The electron channeling condition used here is the cross point of the (220) and (⁠$4¯00$⁠) channeling conditions to increase the contrast. The ECCI measurement revealed that the TDD from the optimized GaAs buffer is 7.3 × 10⁶ cm⁻² by surveying a total area of 2017 μm². Compared to the non-optimized GaAs buffer as shown in Fig. 1(b), the TDD is reduced by a factor of ∼20. After the ECCI measurement, we loaded the sample back to the MBE chamber to grow a GaAs/AlGaAs graded-index separate confinement heterostructure laser with four stacks of QD layers as the active region. The QD layer consists of 2.55 monolayers of InAs embedded in a 7 nm In_0.15Ga_0.85As well, both grown at a substrate temperature of 495 °C measured using an optical pyrometer. The lower half of the laser structure including the GaAs/AlGaAs separate-confinement-heterostructure was grown at 580 °C, while the top half was grown at 560 °C. In addition to the laser sample, we also grew a photoluminescence structure with a single layer of InAs QDs on a GaAs/GaP/Si substrate to assess their optical and structural properties.

Figure 2 displays photoluminescence spectra at room temperature with a peak wavelength at 1285 nm. The full-width at half-maximum is ∼28 meV, indicating an excellent QD size homogeneity. The AFM image in the inset of Fig. 2 confirms the highly uniform self-assembled QDs grown on the GaP/Si substrate. The QD density is ∼5.2 × 10¹⁰ cm⁻², which is comparable to the density of the QD grown on a GaAs native substrate.¹⁶ The energy separation between the ground state and excited state is 84 meV. In addition to the low TDD in the GaAs template, the superior optical and structural qualities of the QDs enable high performance QD lasers grown on GaP/Si substrates.

Ridge waveguide lasers with various ridge widths were fabricated using standard dry etching techniques. The ridge widths range from 2.5 to 8.0 μm, and the laser cavity length is defined by cleaving after thinning the silicon substrate down to 150–200 μm. All laser performance measurements described in this report were carried out in the CW mode. Figure 3(a) shows representative light-current-voltage (LIV) curves from a 1485 × 2.5 μm² device with as-cleaved facets at 20 °C. The threshold current is 9.5 mA, which is significantly lower than the previous results from QD lasers grown on-axis and offcut (4°–6°) Si substrates. We believe that the significant threshold reduction is attributed to the increased internal quantum efficiency in the devices grown on the low TDD GaAs templates. The output power of the laser is more than 71 mW per facet for ground-state lasing. Excited state lasing begins at 400 mA. The lasing wavelength is measured using an optical spectrum analyzer, and Fig. 3(b) displays the onset of lasing at 1270 nm with numerous Fabry-Perot longitudinal modes as the injected current increases from 5 mA to 15 mA.

Figure 4(a) shows a maximum single-side output power of 175 mW from an as-cleaved 2600 × 8 μm² device. The threshold current of this device is 27.5 mA (current density of 132 A/cm²), and the WPE is 29.5% at an injection current of 175 mA. It should be mentioned that appropriate dielectric coatings (anti/high-reflectivity) and facet passivation can further increase the output power of the device by avoiding possible catastrophic optical damage on the facets as the power density approaches ∼1 MW/cm².^17,18 We also tested the same device at elevated temperatures as shown in Fig. 4(b). The ground-state lasing persists up to 80 °C, still producing an output power of ∼7 mW. This demonstrates that the QD lasers on GaP/Si can operate in a harsh environment such as those found in datacenter and high performance computing applications. The characteristic temperature of the device between 20 and 80 °C is 32 K. This relatively low characteristic temperature is due to the undoped active region in the QD laser, and we expect to improve the temperature performance by employing p-modulation doping in the active region.^19,20

Figure 5(a) reveals a clear trend of lower threshold currents with smaller laser widths due to the reduced current injection area. The scattering in the threshold current at a given width is due to imperfect facet cleaving and device processing. It should be noted that the threshold current decreases almost linearly with the laser width down to 2.5 μm, indicating negligible sidewall recombination effects due to the excellent lateral carrier confinement of our InAs QD material. We expect that lasers with smaller ridge widths may achieve even lower threshold currents. The lowest CW threshold current density among the as-cleaved devices (device lengths = 1343–1633 μm) is 147 A/cm² (36.75 A/cm² per layer) at 20 °C, which is considerably lower than the previously reported value of ∼425 A/cm² (60.71 A/cm² per layer) from HR-coated QD lasers grown on-axis (001) Si substrates. We attribute this to the high quality GaAs template with low TDD in addition to the low transparency current from using a reduced number of QD layers (four instead of five to seven) in the active region. WPEs and single-side output powers of 161 as-cleaved devices are presented in Figs. 5(b) and 5(c). The highest WPE value of 38.4% is measured at an injection current of 74 mA from a 1366 × 4 μm² QD laser, producing an output power of 18.6 mW. Figure 5(c) shows ground-state output power versus device width at 20 °C. The relatively large scattering in WPE and optical power of our QD devices on GaP/Si is from a combination of the device fabrication and imperfectly cleaved laser facets.

We have coated one facet of the 1106 μm laser bar with a 95% (Ta₂O₅/SiO₂) HR coating, and Fig. 6(a) shows a CW threshold current of 6.7 mA from a device with a width of 3 μm. We believe that this value is the lowest threshold current density to date for any kind of Fabry-Perot lasers on Si substrates. Its corresponding threshold current density is 205 A/cm², and the maximum single-side WPE is 23.55%. Reliability tests on these devices are underway to investigate the lifetime at various aging conditions. We expect that QD lasers grown using the high quality GaAs templates will exhibit improved reliability relative to previous results due to their reduced threshold current densities and defect-driven degradation.²¹ Also, further laser characterizations such as extracting gain parameters will be investigated.

In this work, we have demonstrated the significantly improved performance of 1.3 μm InAs quantum dot lasers epitaxially grown on on-axis (001) GaP/Si substrates. The high quality quantum dot active region and reduced threading dislocation density lead to a threshold current of 9.5 mA, an output power of 175 mW, and a wall-plug-efficiency of 38.4% from as-cleaved devices in the continuous-wave mode. High-reflectivity coating to one facet of the device further reduces the threshold current to 6.7 mA with a single-side wall-plug-efficiency of 23.55%. We believe that this work is very promising for highly efficient light sources for photonic integrated circuits on Si.

This research was supported by Advanced Research Projects Agency-Energy (ARPA-E) DE-AR000067. We are also grateful to Kurt Olsson and John English for their assistance in MBE maintenance and Alan Liu, Kei May Lau, Chris Palmstrøm, and Kunal Mukherjee for fruitful discussions.