Electrically pumped continuous-wave 1 . 3 μ m 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/cm 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/cm and single facet output power exceeding 130 mW at room temperature. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. OCIS codes: (230.5590) Quantum-well, -wire and -dot devices; (250.5960) Semiconductor lasers; (250.5300) Photonic integrated circuits. References and links 1. M. Asghari and A. V. Krishnamoorthy, “Silicon photonics: Energy-efficient communication,” Nat. Photonics 5(5), 268–270 (2011). 2. A. Rickman, “The commercialization of silicon photonics,” Nat. Photonics 8(8), 579–582 (2014). 3. D. Liang and J. E. Bowers, “Recent progress in lasers on silicon,” Nat. Photonics 4(8), 511–517 (2010). 4. S. Tanaka, S. H. Jeong, S. Sekiguchi, T. Kurahashi, Y. Tanaka, and K. Morito, “High-output-power, singlewavelength silicon hybrid laser using precise flip-chip bonding technology,” Opt. Express 20(27), 28057–28069


Introduction
Driven by cloud-based applications, 'big-data' services and enterprise data centers, silicon photonics has become of increased interest due to its potential prospects for integration of optical data transfer with data processing electronics on a single silicon die utilizing CMOScompatible IC technology [1,2].However, such integration is currently incomplete due to the lack of a reliable, directly integrable Si-based laser optical source [3].Although waferbonding and flip-chip-bonding have been extensively investigated for integrating III-V lasers into silicon photonics platforms, and impressive results have been achieved [4][5][6]; In the longer term, monolithic growth of III-V lasers on Si substrates remains the 'holy grail' for low-cost, high yield, and large-scale integration of complex optoelectronic circuits [7,8].However, direct epitaxial growth of III-V materials on Si substrates faces several significant challenges including large lattice mismatch, different thermal expansion coefficients and polar III-V versus non-polar Si surfaces, which induce the formation of different types of defects, such as threading dislocations (TDs), micro thermal cracks and antiphase boundaries (APBs), respectively.These defects all generate non-radiative recombination centers, which will dramatically reduce the quality of III-V materials as well as the operating performance and lifetime of devices fabricated from them [3,9].Quantum dots (QDs) have been becoming the main technology as active layers in semiconductor lasers due to their low threshold current density and temperature insensitive operation [10][11][12].In addition, QD structures have attracted increasing attention for the active element of III-V light emitting sources on silicon substrates [13][14][15][16][17][18], due to their enhanced tolerance to defects [19][20][21][22].
Pioneering work on 1.3 µm InAs/GaAs QD lasers monolithically grown on Si substrates has demonstrated impressive results [15,23,24].In order to prevent the emergence of the APBs while growing polar III-V materials on non-polar silicon substrates, silicon (001) or equivalent orientation substrates with an offcut of 4-6° [25,26] to the [110] plane have been used to form a perfect double silicon atom step.Although this approach is successful in overcoming the APB problem, it has the disadvantage of not being readily compatible with standard microelectronics fabrication, where wafers with nominally (001) silicon substrates are used.In general, the nominal silicon substrates, i.e., the so-called "exact" (001) silicon substrates with a miscut angle less than 0.5° [8,27], have been used in standard microelectronics fabrication.To this end, there has been notable progress made to achieve lasing operation from on-axis Si (001) substrates, including the demonstrations of electrically pumped 1 µm InGaAs/GaAs quantum well lasers grown on GaP/Si (001) substrates under pulsed operation [28], optically pumped cw 1.3 µm InAs QD microdisk lasers grown on patterned (001) silicon substrates [29] and electrically pumped cw 1.3 µm InAs QD edgeemitting lasers epitaxially grown on GaP/Si (001) substrates [30].However, for these techniques, either patterned Si substrates or an intermediate GaP buffer has to be used.Furthermore, QD lasers directly grown on GaAs/Si (001) substrates have not yet been reported.In this paper, we report on, to the best of our knowledge, the first electrically pumped cw 1.3 µm InAs/GaAs QD lasers directly grown on microelectronics standard nominal (001) Si substrates without any intermediate buffers.

Crystal growth
In this work, microelectronics standard on-axis Si (001) 300 mm diameter wafers were deoxidized by the SiConi process using a NF 3 /NH 3 remote plasma.The wafers were then transferred into a 300 mm Applied Materials metalorganic chemical vapour deposition (MOCVD) reactor, where H 2 annealing was performed at 900 °C to promote structuring of the 2 × 1 surface using purified H 2 as the carrier gas.After the annealing process, the chamber was quickly quenched below 700 °C within 30 s to freeze the silicon surface structure.GaAs layers were then grown sequentially, with a standard two-step process [31]: a 40 nm GaAs nucleation layer was deposited at low temperature (400-500 °C); a 360 nm GaAs layer was then deposited at a higher temperature (600-700 °C).The typical V/III ratio was in the range of 5-30.The temperatures were monitored by an optical pyrometer.After completion of the epitaxial growth of 400 nm APB-free GaAs films on the 300 mm silicon (001) wafer, it was diced into smaller pieces and transferred into a 3-inch solid-source Veeco Gen-930 MBE system, where the QD laser structure was grown directly on the virtual GaAs/Si (001) substrate as shown in Fig. 1.The epitaxy starts with a 600 nm thick GaAs buffer layer, followed by five-repeats of InGaAs/GaAs strained layer superlattices (SLSs) separated by 300 nm GaAs spacing layers.The InGaAs/GaAs SLSs employed here were previously optimized [17] and play a critical role in blocking TDs [13,14,17,24].In situ thermal annealing [24,32] of SLS was also performed five times with the epitaxial growth paused and in an As-rich condition, by elevating the substrate temperature to 700 °C for 10 mins.Following the relatively low temperature growth of SLSs, the subsequent high temperature annealing process provides additional kinetic energy for the TDs increasing their in-plane movement, which promotes threading dislocations to meet and their annihilation.Above the InGaAs/GaAs SLSs is a 1.4 µm n-type lower Al 0.4 Ga 0.6 As cladding layer and 30 nm undoped Al 0.12 Ga 0.88 As guiding layer, followed by a standard undoped five-layer InAs/InGaAs/GaAs dot-in-well (DWELL) laser active structure [24] separated by 50 nm GaAs spacer layers.Above the active region is the second 30 nm undoped Al 0.12 Ga 0.88 As guiding layer and a 1.4 µm p-type upper Al 0.4 Ga 0.6 As cladding layer, and finally a 300 nm highly p-type doped GaAs contacting layer.

Device fabrication
The Si-based QD laser structure was fabricated into broad-area lasers with varying stripe widths of 25 µm and 50 µm following standard optical lithography and wet chemical etching techniques.The top mesa was etched to about 100 nm above the active region.The top ncontact layer was etched down to the highly n-doped GaAs buffer just below the n-type AlGaAs cladding layer.Ti/Pt/Au and Ni/GeAu/Ni/Au were deposited on top of the mesa and exposed highly n-doped GaAs buffer layer to form the p-and n-contacts, respectively.After thinning the silicon substrate to 120 µm, the laser bars were cleaved into the desired cavity lengths, which were then mounted on copper heatsinks and gold-wire bonded to enable testing.The final devices described in this paper were 25 µm in width and 3 mm in length, with no facet coating applied.

Measurements
The surface morphology was characterized by a Nanoscope Dimension 3100 SPM atomic force microscopy (AFM) system using a standard tapping mode.Optical properties were measured by photoluminescence (PL) measurements excited from a 635 nm diode-pumped solid-state laser.Laser device characteristics were measured under both cw and pulsed conditions of 1µs pulse-width and 1% duty-cycle.

Epitaxial InAs/GaAs QD laser structure on GaAs/Si (001)
Figure 2(a) shows a typical 5 × 5 µm 2 AFM image of a 400 nm thick GaAs film layer monolithically grown on a 300 mm industry-compatible Si (001) substrate by MOCVD based on the process described above.The measured AFM image indicates that a small RMS surface roughness of 0.86 nm has been achieved, and this very small surface roughness is comparable to the best reported values for 1 µm thick GaAs layers grown on Si (001) substrate with 4° -6° offcuts [33,34], despite the fact that only 400 nm GaAs has been deposited on the nominal (001) Si substrate in our structure.In addition, no obvious "V"groove feature can be observed indicating the realization of an APB-free surface due to the formation of perfect doubling of the height of all silicon surface steps thanks to the effective Si wafer preparation [31].The observations of APB-free heteroepitaxial GaP on Si (001) by MOCVD have been reported by W. Stolz's group [27,35].In their work, to achieve the APBfree GaP/Si (001) substrate, a homoepitaxial silicon buffer along with a high temperature annealing process is required prior to GaP heteroepitaxy.In comparison, the method presented in the present work is simpler and more compatible with standard industry fabrication processes, since our process does not require the additional Si buffer growth and annealing process or the use of a GaP layer prior to GaAs growth.
In addition, room temperature PL was employed to compare the optical properties of the QD laser structures grown on nominal GaAs/Si (001) and on native GaAs substrates.As shown in Fig. 2(b), the two samples present similar room temperature emission peaks at ~1285 nm and linewidths of ~32 meV.The PL intensity is quite comparable, with, the reference QD sample having an integrated PL intensity ~1.3 times greater than that of the sample grown on nominal GaAs/Si (001).An AFM comparison of uncapped InAs QDs grown on nominal GaAs/Si (001) and on native GaAs substrate is shown in the Figs.2(c) and 2(d), revealing similar morphologies.Both samples present good uniformity and the dot density determined by AFM is ~3.5 × 10 10 cm −2 and ~3 × 10 10 cm −2 for samples grown on GaAs/Si (001) and on native GaAs substrates, respectively.

Results and discussion
Figure 3(a) compares light-current-voltage (LIV) characteristics of InAs/GaAs QD lasers grown on GaAs/Si (001) and native GaAs substrates, respectively, under cw operation at room temperature.As expected, the measured series resistances extracted from IV characteristics for both samples were quite similar at round 1.9 ± 0.2 Ω, since the doping levels of the laser structure and the fabrication procedures were nominally identical.The measured lasing threshold current densities are 210 A/cm 2 and 425 A/cm 2 for GaAs-based and Si-based devices, respectively.The calculated slope efficiency and external differential quantum efficiency is 0.12 W/A and 12.7% for GaAs-based device, and 0.068 W/A and 7.2% for the Si-based laser.Compared with the laser device grown on native GaAs substrate, the degraded device performances for QD laser grown on the Si (001) substrate is related to the defects propagating into the QD active layers.Figure 3(b) compares the room temperature single facet output power against the current density for InAs/GaAs QD laser grown on GaAs/Si (001) substrate under cw and pulsed conditions.Under cw operation, a maximum single facet output power of 43 mW is obtained at an injection current density of 1332 A/cm 2 due to the limitation of the cw current source.However, a small thermal rollover of output power at high injection current densities is observed.In contrast, under pulsed operation, a maximum single facet output power of over 134 mW is achieved at an injection current density of 2 kA/cm 2 without any thermal rollover.Figure 4 shows the evolution of the emission spectra as a function of injected current densities under cw operation at room temperature.A spontaneous emission spectrum with a broad full width at half maximum (FWHM) of 49 nm is observed at a peak wavelength of ~1292 nm at a relatively low driven current density of 267 A/cm 2 .On increasing the injection current density to 425 A/cm 2 , a typical lasing oscillation behavior, evidenced by sudden spectrum narrowing to ~2.2 nm (as seen in the inset of Fig. 4) and a sharp increase in intensity, is observed.Further increasing the current density, an obvious multi-mode lasing spectrum appears.Figure 5(a) shows the LI characteristic of InAs QD laser grown on GaAs/Si (001) substrate at various heatsink temperatures under pulsed mode.This Si-based QD laser has a maximum heatsink temperature of 102 °C for lasing operation, with a characteristic temperature, T 0 , of ~32 K between 16 °C and 102 °C.To the best of knowledge, this is the rst demonstration of QD lasers monolithically grown on exact silicon (001) substrate that lase over to 100 °C.The temperature-dependent LI characteristics have also been measured under cw operation, as shown in Fig. 5(b).Under cw operation, this device has a maximum heatsink temperature of 36 °C for lasing operation.As seen from the inset of the Fig. 5(b), at a heatsink temperature of 36 °C, there is a strong thermal output power rollover due to the self-heating of the device, which severely limits its high temperature performance.It should be mentioned that the QD active region employed in this work is undoped; also the laser device was not hard soldered to a high thermal-conductivity heatsink.The relative poor T 0 value presented here is therefore mainly related to the hole excitation out of the lowest lasing state [36] and imperfect heat extraction strategy.A next step towards improving the temperature performance and maximum output power will be concerned with reducing the temperature sensitivity and optimizing the extraction of heat from the active region.The wellestablished strategies to achieve this is modulation p-doping of the QDs [12,36] or/and hard soldering the laser to a high-thermal-conductivity heatsink.
Comparing the present results grown on on-axis Si (001) substrates with the world-record results of InAs/GaAs QD lasers grown directly on offcut Si substrates [24], there has been observed degradation in laser performance for this work, including higher threshold current densities and poorer T 0 .We must emphasize that, the growth on Si (001) substrates is not fully optimized due to limitations on the wafer preparation and the MBE growth chamber was in sub-optimal condition with significant numbers of oval defects visible.Nevertheless, good c.w. performance has been achieved, and the threshold current density reported in the present work has been reduced by more than a factor of two, compared with the very recent achievement of an InAs/GaAs QD laser on on-axis Si (001) substrate with an intermediate GaP buffer layer [30].With further optimization on the wafer preparation and the growth of dislocation filter layers [17], even better results are to be expected.

Conclusion
In conclusion, we have demonstrated the first electrically pumped cw InAs/GaAs QD lasers directly grown on industrial-compatible nominal Si (001) substrates without any intermediate buffers.The devices exhibit cw lasing at ~1.3 µm with threshold current density of 425 A/cm 2 and output of 43 mW at room temperature.Under pulsed operation, we show significantly improved device performance including lasing operation at device temperatures exceeding 100 °C and output from a single facet exceeding 130 mW at room temperature by limiting self-heating.These results represent a major advance towards the commercial success of silicon-based photonic-electronic integration.Funding UK Engineering and Physical Sciences Research Council (EP/J012904/1, EP/K029118/1, and EP/P006973/1).

Fig. 1 .
Fig. 1.The schematic diagram of the QD laser structure grown on the on-axis Si (001) substrate.

Fig. 3 .
Fig. 3. (a) LIV characteristics comparison of InAs/GaAs QD laser grown on GaAs/Si (001) to the reference QD laser grown on native GaAs substrate at room temperature under cw operation.(b) LI comparison of InAs/GaAs QD laser grown on GaAs/Si (001) substrate under c.w and pulsed operation conditions at room temperature.

Fig. 4 .
Fig. 4. Emission spectra for InAs/GaAs QD laser on GaAs/Si (001) substrate at various injection current densities at room temperature.The inset shows the evolution of the FWHM as a function of injection current density.

Fig. 5 .
Fig. 5. Single facet light power verses current density for a InAs/GaAs QD laser grown on GaAs/Si (001) substrate at various heat sink temperatures under pulsed condition.The inset shows the natural logarithm of current density against temperature in the ranges of 16 -102 °C.(b) Single facet output power versus current density for the same Si-based InAs/GaAs QD laser as a function of temperature under cw operation.The inset shows the LI curve for this Sibased InAs/GaAs QD laser at a heat sink temperature of 36 °C.