Low-threshold epitaxially grown 1.3 μm InAs quantum dot lasers on patterned (001) Si

 Abstract —A three-fold reduction of threshold current, with a minimum threshold current density of 286 A/cm 2 , a maximum operating temperature of 80 °C, and a maximum 3 dB bandwidth of 5.8 GHz was achieved for 1.3 μm InAs quantum dot lasers on patterned, on-axis (001) Si. This was enabled by the reduced threading dislocation density (from 7×10 7 to 3×10 6 cm -2 ), and optimized probe design. The patterned Si produced antiphase domain free material in the coalesced GaAs buffer layer with reduced misfit/threading dislocation nucleation, without the use of Ge/GaP buffers or substrate miscut. Utilizing aspect ratio trapping, cyclic thermal annealing, and dislocation filter layers, high quality III-V on Si devices were grown, demonstrating the compelling advantages of this patterned Si template for a monolithic Si photonics integration platform.


I. INTRODUCTION
he explosive growth of Internet Protocol (IP) traffic is driving data centers into the Zettabyte Era.Consuming 1.5% of the total energy consumed in the U.S. at a cost of $4.5B, the data centers' energy consumption is predicted to triple by 2020 [1].Today, Si photonics supplies 100% of the 80 km dense wavelength division multiplexing (DWDM), >75% of the parallel single mode 4-channel (PSM4), and >50% of the 2 km coarse 4 × 25 Gbps coarse wavelength division multiplexing (CWDM4) for Microsoft [2].This technology is currently fueling high bandwidth density interconnection links and is primed to serve other growth markets such as light detection and ranging (LIDAR) and chip-scale wearable sensors [3].Although Si-based light guiding, modulation and detection technologies have been explored extensively with impressive achievements [4], the indirect bandgap has hampered the development of an on-chip light source.
Recently, InAs quantum dots (QDs) have demonstrated themselves as the most promising candidate for achieving high performance light emitters epitaxially grown on Si [5][6][7][8][9][10][11][12].The effective carrier localization in QDs considerably improves laser reliability over quantum well (QW) lasers on Si by suppressing growth of 〈1 1 0〉 dark-line defects [14].The extrapolated mean time to failure, as defined by doubling of the initial threshold current, are now more than 10,000,000 hours for state-of-art QD lasers on Si at 35 • C [15].In contrast, the most prolonged lifetime reported among GaAs-based QW lasers on Si is merely ~200 h after more than a decade of research [16].
To date, intensive research has been conducted to migrate this field to the use of complementary metal-oxidesemiconductor (CMOS) compatible on-axis Si substrates [17][18][19][20][21]. Various novel methods have been applied to solve the antiphase domain (APD) problem, obviating the need for miscut substrates.These approaches range from direct nucleation of a GaAs film with special Si wafer preparation [17,18], growing an Al0.3Ga0.7Asseed layer [19], the use of a thin GaP buffer layer [15], 0.15° misorientation in the [110] direction [20], Ushaped patterned Si (001) substrates to obtain (111)-facetedsawtooth Si hollow structures [21], and patterned growth on exposed {111} V-groove facets of silicon [22].Leveraging the aspect ratio trapping effects, we previously demonstrated electrically injected QD lasers grown on on-axis (001) Si with {111} V-groove facets [23].The "tiara"-like structures formed by the patterned Si simultaneously circumvent the issue of APDs and effectively filter out stacking faults in the coalesced GaAs buffer layer without additional Ge/GaP buffers or substrate miscut [24].A comprehensive comparison of the lasing characteristics in devices with the same active structure and geometry but fabricated on this V-groove Si template in conjunction with our simultaneously developed GaP/Si template has been made [25].Despite the fact that the GaP/Si possesses three-fold lower defect densities, devices on Vgroove Si substrate achieved continuous-wave (CW) lasing with thresholds as low as 36 mA for a 6×1200 µm 2 ridge laser with 95% reflectivity coating on one facet [23] and submilliamp threshold of 0.6 mA for a microring laser with an outer radius of 5 μm and a ring width of 3 μm [12].Furthermore, the V- Here, by combining the additional effects of thermal cyclic annealing and strained-layer superlattices, a record-low threading dislocation density (TDD) of 3×10 6 cm -2 on a Vgroove Si template was achieved.This is more than a 20-fold reduction over the previously reported TDD on V-groove Si [22], half of the value in the state-of-art GaP/Si template [10], and closes the gap relative to the best reported dislocation densities for lasers on Si [7,20].The significant decrement of TDD translates into a three-fold reduction of threshold currents (minimum value of 12 mA here, while the value is 36 mA in [22]), with a minimum threshold current density of 286 A/cm 2 , and a maximum operating temperature of 80 °C.Furthermore, by optimizing the probe design, a maximum 3 dB bandwidth of 5.8 GHz was achieved.This is larger than the theoretically predicted modulation bandwidth maximum (of 5GHz) in a recent report of monolithic 1.3 µm InAs/GaAs QD lasers on Si [27].

II. EXPERIMENTAL PROCEDURE
The GaAs-on-v-groove Si template, shown in Fig. 1(c), was grown by the combined effort of both metal-organic chemical vapor deposition [23] (the first 400 nm coalesced GaAs) and solid-source molecular beam epitaxy (MBE).The GaAs nanowires were first selectively grown in the V-grooves, with openings of 70 nm, of the patterned (001) Si wafer on {111} planes.The SiO2 stripes were then removed using buffered oxide etch (BOE).The GaAs nanowires coalesced into a continuous thin film after growing to a total thickness of 100 nm at 600 ⁰C.Then, 300 nm of additional GaAs was deposited for planarization.Stacking faults nucleated at the GaAs-Si interface accommodate for the 4.1% lattice mismatch, which were then blocked by the Si ridges and thereby contained in the V-grooves.The high index growth plane ensured that no APDs would form at the interface.It is worth to mentioning that there is more than one strain relaxation mechanism for V-groove epitaxy under different growth parameters [28].The result from IMEC [29] shows the relaxation of III/V on {111} facets based on misfit and TD formation.
Here, by making use of a different strain relaxation mechanism (compared to blanket epitaxy in [16][17][18]) based on a highly twinned region with thickness <10 nm, the tip of the Si ridges ensures the stress-relaxing layer is confined within the V-shaped trenches.This phenomenon is shown in the crosssection bright-field transmission electron microscopy (TEM) image in Fig. 1(d) and (e).Thus, the V-groove approach eliminates APDs and moderately reduces TDD, while the main defect reduction was realized through the remaining structure of the GaAs-on-Si buffer by MBE [22].In the MBE buffer, four cycles of temperature annealing from 400 °C to 700 °C were applied after the initial 1200 nm GaAs growth, which provides additional kinetic energy for the TDs to increase their in-plane movement and promotes them to meet and annihilate.Next, 200 nm of In0.1Ga0.9Asand ten periods of In0.1Ga0.9As/GaAs(10 nm/10 nm) superlattices were employed to promote lateral movement of the TDs through the induced compressive stress.The buffer structure was finished with 800 nm n-doped GaAs with a concentration of 2×10 18 cm -3 .A root-mean-square roughness of ~2.8 nm was obtained across a scanning area of 5×5 µm 2 in the atomic force microscopy (AFM) image, shown in Fig. 2(a).The final TDD was measured to be 3 × 10 6 cm -2 by electron channeling contrast imaging (ECCI) shown in Fig. 2(b).The full laser structure with 5 layers of QD as the active region was grown on top of the buffer structure, as shown in Fig. 1(b).A GaAs/AlGaAs graded-index separate confinement heterostructure was used as the cladding layer for both optical and electrical confinement.Fig. 1(a) shows the room temperature photoluminescence (PL) spectrum of the InAs QDs, which has a peak intensity around 1290 nm for the ground-state transition and a full width at half maximum of about 30 meV.The nominal InAs thickness was 2.75 ML.The optimized growth temperature was 495 °C under As4 overpressure at a V/III of 35.The dots were grown in an In0.15Ga0.85Aswell with 2 nm below and 5 nm capping the dots followed by a 2.5 nm GaAs layer.The "In-flush" technique was employed at 580°C to improve dot uniformity [30].The bottom cladding layers were grown at 580 °C while the growth temperature for the top cladding layers was lowered to 560 °C to reduce the QD intermixing.All temperatures were measured by pyrometer.The inset of Fig. 1(a) is the AFM image of the as grown dots, showing that the areal density of the dots is about 6.4 × 10 10 cm -2 .
Fig. 3 summarizes the structure of the as-fabricated laser devices.Fig. 3(a) and (b) show the SEM and schematic image of the laser cross-section, respectively.The laser ridges were formed perpendicular to the V-grooves by inductively coupled plasma (ICP) etching down to the n-contact layer.Laser facets were formed by cleaving, with cavity lengths of 750, 1450, and 2000 μm and ridge widths ranging from 1 to 10 μm.Fig. 3(c) shows the top-down view of the as-cleaved laser bars.Sidewall scattering and surface recombination can be significant problems for the deeply etched devices.Thus, after ridge mesa formation, 12 nm of Al2O3 was applied by atomic-layer deposition (ALD) to passivate the side wall.An 800 nm thick SiO2 layer was then deposited by sputtering for electrical isolation and to isolate the optical modes from the Pd/Ti/Pd/Au (p) and Pd/Ge/Pd/Au (n) metal contact layers.The five layers of QDs are clearly visible in the cross-section bright-field TEM image in Fig. 3(d), taken with g = (002) under two beam condition.The individual dots revealed themselves as dark spots in the TEM image as they distort the surrounding lattice locally.

III. RESULTS AND DISCUSSION
110 ridge lasers were tested at room temperature under CW operation.Fig. 4(a) shows the typical light-current-voltage (LIV) characteristics of a laser with a 1450 μm cavity length and 10 μm ridge width.The measured 1 V turn-on voltage and 2.7 Ω differential series resistance from the I-V curve indicate good metal contacts for efficient current injection.A clear knee behavior in the LI curve is observed at the lasing threshold current of 39 mA.This corresponds to a threshold current density (Jth) of 270 A/cm 2 , or 54 A/cm 2 per QD layer.No output saturation was observed when the output power reached 75 mW per facet at an injection current of 600 mA, which corresponds to a total output power exceeding 150 mW assuming equal output from both facets.Power roll over occurs until the injection reaches 650 mA.A maximum wall-plug efficiency of 15.6% was achieved.As a comparison, in Fig. 4(b), for a laser with the same cavity length of 1450 μm, but a smaller ridge width of 2 μm, the maximum power of 33 mW per facet is obtained at an injection current of 318 mA, with a threshold current of 16 mA.
Figure 5 depicts a threshold current density histogram for all measured lasers, with the peak located around 400 to 500 A/cm 2 and a minimum of 270 A/cm 2 .As a comparison, our previously reported lasers on the V-groove Si template had a threshold current density distribution peaked around 700 to 1000 A/cm 2 .The main difference between the two is the threading dislocation density at the surface of the buffer.The inset of Fig. 5 shows a continuously decreasing trend of threshold current as the ridge width narrows down to 1.5 μm.This suggests that the side wall recombination is greatly suppressed by the well-  optimized Al2O3 passivation layer as well as the reduced carrier diffusion length from the QD active region.The higher threshold current with shorter cavities is due to the increase in the mirror loss.The typical carrier diffusion length of a QW active region is about 3 to 5 μm, and this value is reduced to about 1 μm with the QD active region [31].The reduced carrier diffusion length is the reason for QD's insensitivity to internal defects as well as device side walls.
The spectra of a device with 1450 μm cavity length and 2 μm ridge width are shown in Fig. 6.The primary lasing peaking is located around 1284 nm.A slight red-shift of the lasing wavelength with increasing injection current was observed.It was interesting to notice that at low injection levels, namely less than 35 mA, an apparent single mode operation was obtained with side-mode-suppression ratio of about 35 dB.There was a crack perpendicular to the ridge direction, suggesting that the realization of single-mode operation is due to the laser effectively operating as a coupled-cavity device with large cavity length ratio [32].The inset of Fig. 6 shows the measured laser linewidth is about 14 MHz using self-heterodyne method.
We also tested the same device at elevated temperatures as shown in Fig. 7.The laser was able to function under CW operation up to 80 °C for ground state lasing, still producing an output power of ~4 mW.This demonstrates that the QD lasers on the V-groove Si template can meet the temperature requirement in a realistic datacenter environment.This result compares favorably to the best heterogeneously integrated quantum dot lasers on Si which operate CW up to 100°C [33].
The natural logarithm of threshold current versus stage temperature for the ridge laser in Fig. 7 and other lasers with the same length but different widths are plotted in Fig. 8. Using an exponential function of  ∝ exp , the characteristic temperature T0 was extracted to fall into the range of 30-35 K, as shown in the inset of Fig. 8.Even though lasers with narrower ridges have smaller heat extraction area through the bottom Si to the heat sink, they are as insensitive, if not more, to temperature change as the lasers with wider ridges.There are two possible reasons.Firstly, a considerable amount of heat was extracted from the side walls, where narrower lasers had advantages with their higher area to volume ratio.Secondly, the residual thermal stress in GaAs on Si (~250 MPa) could be relieved with the higher aspect-ratio laser structures [34].Fig. 9 depicts the small-signal modulation response, S21, of type A laser and type B laser with the same dimension (length of 1450 μm and a width of 5 μm) biased from 24 to 88 mA, respectively.For the type A laser in Fig. 9(a), the probe metals were designed in a way that there was no vertical overlap between the p-and n-contact metal due to the cross-over of p n probe metal.This is contrary to that of the type B laser in Fig. 9(b), where a portion of p-probe was placed upon the n-metal area with an 800 nm SiO2 separation layer.Otherwise, the two structures are  nominally the same in terms if epi structure and device fabrication.The 3 dB bandwidth, f3dB, increased with increasing bias current and saturated at 5.8 GHz for type A laser and 3.6 GHz for type B laser when biased at 88 mA.The measurement procedure was similar to that described in [35].The inset in Fig. 9 shows the plots of f3dB as a function of square root of bias current above threshold.The modulation efficiencies of f3dB for type A laser and type B laser are 0.58 GHz/mA 1/2 and 0.39 GHz/mA 1/2 .The reduced parasitic capacitance of type A laser gives rise to a decent f3dB similar to that measured from the state-of-art result (6.5 GHz f3dB and 0.58 GHz/mA 1/2 modulation efficiency) on GaP/Si where 8 pairs of SiO2/Ta2O5 films are coated on one facet to achieve a low mirror loss (99% reflection) and allow for lasing in small cavities (cavity length of 580 μm and ridge width of 3 μm) [35].Note that the device presented here has a much length of 1450 μm and a width of 5 μm, with both facets formed as cleaved.As a comparison, the recent reported maximum 3dB bandwidths and modulation current efficiency of QD laser grown on Si with as-cleaved facet is only 1.6 GHz and 0.4 GHz/mA 1/2 (cavity length of 2500 μm and ridge width of 2.2 μm) [36].With an optimized QD laser design readily based on the current growth process by scaling the number of QD layers and the cavity length, their modelling suggests that theoretical modulation bandwidth is only 5 GHz [27].

IV. CONCLUSION
In conclusion, we report a significant improvement of QD laser performance grown on V-grooved Si.The utilization of In0.1Ga0.9Asstrained layers and cycles of temperature annealing reduces the TDD from 7×10 7 to 3×10 6 cm -2 .This translates into a reduced minimum threshold current density from ~700 A/cm 2 to ~286 A/cm 2 , with minimum threshold current of 12 mA and a maximum operating temperature of 80 °C.By optimizing the probe design, a maximum 3 dB bandwidth of 5.8 GHz has been achieved, which exceeds the previous theoretical prediction of modulation bandwidth [27].Commercial use will require stable long-term operation above room temperature without an additional active cooling system.It has been reported that the main cause of device failure during prolonged operation is the growth of dislocation networks in the laser material [37].Despite the QD's insensitivity to defects, the in-plane misfit dislocations interacting with multiple QDs in a row can be detrimental to device performance.Such misfit dislocations are thought to nucleate from existing TDs.Since reducing the number of TDs can effectively increase the energy barrier to nucleate misfit dislocations [38], lasers grown on GaAs-on-Si buffer with lower TDD are expected to possess higher reliability during device operation.According to the investigation of the impact of threading dislocation density on the reliability [39], reducing the dislocation density from 2.8×10 8 to 7.3×10 6 cm -2 has improved the laser lifetime from a few thousand hours to more than 10×10 6 h at 35 °C and at twice the threshold current density.With the record low TDD (3×10 6 cm -2 ) buffer on patterned Si, one can envision further improved lifetime.

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Fig. 1 .
Fig. 1.(a) Photoluminescence spectrum of the as-grown sample.Inset: atomic force microscopy image of QD with a density of 6.4× 10 10 cm -2 .The image has a size of 1 × 1 μm 2 .(b) The full stack of the laser structure.(c) Schematic image of the buffer structure.Materials above the dashed line are grown by MBE.(d) and (e) Cross-sectional TEM images of the V-grooved structure, showing stacking faults (indicated by the white arrows) trapped by the Si pockets.

Fig. 2 .
Fig. 2. (a) AFM and (b) ECCI of the buffer structure.In the AFM, a rootmean-square roughness of ~2.7 nm was obtained across a scanning area of 5×5 µm 2 .The vertical bar is 22 nm.In the ECCI, TDD is as low as 3 × 10 6 cm -2 .The circles show the only threads in the image.

Fig. 3 .
Fig. 3. (a) SEM image of the cross-section of an as-cleaved laser, tilted at 75⁰.(b) Schematic image of the laser cross-section, revealing the geometry of the contact and probe metals.(c) Top-down view of the laser bar.Each tick mark on the metal pad indicates a length of 100 μm.(d) Bright-field TEM image of the active region cross-section, showing the five layers of QDs.

Fig. 4 .
Fig. 4. Typical L-I-V characteristics of an as-cleaved laser with a cavity length of 1450 μm and a ridge width of (a) 10 μm and (b) 2 μm, together with the wallplug efficiency.

Fig. 5 .Fig. 6 .
Fig. 5. Threshold current density distribution histogram for all measured lasers.Most of the devices fall between 300 and 500 A/cm 2 .The inset shows a continuous decrease of threshold current with reduced ridge width, indicating good suppression of side-wall recombination.

Fig. 7 .Fig. 8 .
Fig. 7. High temperature measurements of the same device show lasing up to 80 °C under CW operation.

Fig. 9 .
Fig. 9. Frequency response of a (a) type A laser and (b) type B laser with the same dimension (length of 1450 μm and a width of 5 μm) biased from 24 to 88 mA.The maximum f3dB saturated at 5.8 GHz for type A laser and 3.6 GHz for type B laser.The upper inset shows the microscopic image of the corresponding laser.(c) 3dB bandwidth versus square root of the bias current above threshold of the two types of laser in (a) and (b) The dashed lines represent linear fitting to the experimental data.