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Abstract
Direct epitaxial growth of O-band InAs/GaAs quantum-dot laser on Si substrates has been rapidly developing over the past few years. But most of current methodologies are not fully compatible with silicon-on-insulator (SOI) technology, which is the essential platform for silicon photonic devices. By implementing an in situ III-V/Si hybrid growth technique with (111)-faceted Si hollow structures, we demonstrate the first optically pumped InAs/GaAs quantum-dot microdisk laser on SOI substrates grown by molecular beam epitaxy (MBE). The microdisk laser on SOI is characterized with threshold pump power as low as 0.39 mW and a Q factor of 3900 at room temperature. Additionally, the compared device performance of InAs quantum-dot microdisk lasers on GaAs, Si (001) and SOI are simultaneously studied with identical epi-structures.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
There is a strong demand of fabricating energy-efficient silicon light sources and developing compact integrated light emitters over the past few years [1-5]. Microcavity lasers offer unique advantages in high quality factor, small footprint, and low power consumption [6–9]. Recently, there are many researches carried on the direct epitaxial growth of III-V quantum dot (QD) structures on Si [10–13], including Fabro-Perot (FP) lasers [14–17], distributed feedback (DFB) lasers [18] and microcavity lasers [19,20]. However, all the silicon photonic devices, including polarizers [21,22], filters [23,24], multiplexers [25], modulators [26,27], and detectors [28–30], are based on the silicon-on-insulator (SOI) platform rather than pure Si substrates. To the best of our knowledge, III-V lasers on SOI platform by direct epitaxy remain absent in the field, which could potentially participate as an essential step towards the silicon photonic integration. Here, we demonstrate optically pumped InAs/GaAs quantum dot microdisk laser on SOI substrates via the formation of (111)-faceted Si hollow structures, which are obtained by homo-epitaxial growth of Si via a U-shape patterned SOI substrate [31]. The in situ hybrid epitaxy growth of the InAs/GaAs quantum dot (QD) disk laser structures on such SOI substrate is performed by III-V/IV joint molecular beam epitaxy (MBE) system. The InAs/GaAs QD microdisk lasers on GaAs, Si (001) and SOI substrates with identical laser structure and disk diameter of 4 μm were fabricated and characterized. The air-cladded InAs/GaAs QD microdisk lasers on SOI exhibit sub-milliwatts threshold pump power, which has similar performance to those fabricated on GaAs and Si (001) substrates.
2. Material growth and device fabrications
Here, specially designed on-axis Si (001) substrate and SOI substrate with a 3 μm buried oxide layer and a 340 nm top Si layer are prepared under complementary metal-oxide semiconductor (CMOS) compatible patterning process. The standard 8-inch Si (001) and SOI substrates were initially patterned with grating-like U-shaped ridges along [110] direction with the processes of deep ultraviolet (DUV) photolithography (0.18 μm resolution) and subsequent dry etching [31]. The U-shape patterns on Si (001) have a period of 360 nm with 140 nm ridge width and trench depth of 500 nm. Due to the top silicon thickness (340 nm) of chosen SOI substrate, the U-shape patterns on SOI exhibit the same period and ridge width but with a shallow trench of approximately 270 nm. After cleaving the patterned 8-inch wafers into 3.2 cm × 3.2 cm pieces following by standard RCA cleaning process, the samples were dipped in a diluted HF solution to remove the native oxide and create a hydrogen terminated surface before loading into the group IV MBE chamber. After an in situ high temperature outgassing and de-hydrogen at 720 °C in the MBE chamber, the substrates were cooled down to 600 °C for the following homoepitaxy of Si buffer layer at a rate of 1.0 Å /s. Here, a 550 nm Si layer and a 450 nm Si layer were deposited, respectively, on patterned Si (001) substrate and on SOI substrate to construct (111)-faceted-sawtooth hollow structures as shown in Fig. 1. The Si and SOI samples with (111)-faceted-sawtooth hollow structures were then transferred into joint MBE chamber for in situ III-V growth. A low-temperature AlAs nucleation layer was first grown at 380 °C at a growth rate of 0.5 Å /s. The following GaAs buffers were grown using a two-step method, including a 30 nm low-temperature GaAs layer at 380 °C and a 560 nm high-temperature GaAs main layer at 580 °C at a growth rate of 0.5 Å /s and 1.0 Å /s, respectively. Here, a 600 nm thick GaAs layer is required to flatten the sawtooth structures following by 1.8 μm thick III-V buffer layer. The buffer structures consist of two repetitions of 10 nm-In0.15Ga0.85As/10 nm-GaAs quantum wells as dislocation filter layers (DFLs) grown at 480 °C separated by 200-nm GaAs. In addition, two periods of 10 nm-In0.15Al0.85As/10 nm-GaAs DFLs were also deposited at 480 °C to further reduce the threading dislocation density (TDD). For the final step, 5-period of 2-nm Al0.6Ga0.4As/2-nm GaAs superlattices (SLs) was grown at 580°C to smoothen the GaAs surface. The growth details have been previously reported in [31], W. Q. Wei et al.
On top of the GaAs/Si (001) and GaAs/SOI platforms, the microdisk laser structures were grown, which consist of a 600 nm Al0.65Ga0.35As sacrificial layer and a 700 nm disk region (Fig. 2). In the disk region, a 7-layer InAs/GaAs dot-in-well (DWELL) active structure was employed. Each DWELL layer consisted of a 3.1-monolayer InAs QD layer sandwiched by the 2-nm In0.15Ga0.85As wetting layer and 6-nm In0.15Ga0.85As capping layer, all grown at 450 °C. The InAs/GaAs dot-in-a-well (DWELL) structures were separated by 50 nm thick GaAs spacer layers, which were grown at an optimized temperature of 560 °C. The active region was enclosed by outer 50 nm thick Al0.35Ga0.65As barriers. For comparison purpose, a reference sample grown on standard GaAs substrate is also prepared with identical growth conditions.
Fig. 2 (a) Cross-sectional schematic diagram of the InAs/GaAs QD microdisk laser on SOI, Si, or GaAs substrate. (b) Three-dimensional schematic diagram of the InAs/GaAs QD microdisk laser on SOI substrates. Inset shows the QD active layer.
By standard inductive coupled plasma (ICP) etching and subsequent wet etching, microdisk lasers (MDLs) on three different substrates with diameters of 4 μm were defined and fabricated as shown in the tilted scanning electron microscope (SEM) image of Fig. 3. All devices show straight vertical etching profile, smooth sidewall, and great circular shape.
Fig. 3 Tilted and top-view SEM images of disk laser on GaAs substrate (a), Si substrate (b) and SOI substrate (c).
3. Material characterizations
By implementing the growth technique above, GaAs buffer layer on the patterned SOI substrate with a root-mean-square (RMS) surface roughness of ~0.6 nm across atomic force microscopy (AFM) scanning area of 5 × 5 μm2 is achieved, as shown in Fig. 4(a), no anti-phase boundaries (APBs) were observed on the GaAs surface. In contrast, a reference sample was also prepared with identical growth parameters on planar SOI substrate without (111)-faceted Si grating structure, which exhibit a rough surface with a RMS of approximately 60 nm as shown in Fig. 4(b). Furthermore, surface SEM images of identical III-V structures directly grown on (111)-faceted-sawtooth Si on SOI (Fig. 4(c)) and standard SOI substrates (Fig. 4(d)) are compared here. Extremely rough surface with high density APBs is observed on standard SOI, while GaAs on SOI substrate with (111)-faceted Si (001) hollow structure exhibits an ultra-flat surface. The surface TDD of GaAs buffer on SOI is approximately 9 × 106 cm−2 measured by electron channeling contrast image (ECCI) method, which is two times larger those on Si (001) substrate as report in [32], W. Q. Wei et al.
Fig. 4 5 × 5 μm2 AFM image of the GaAs film deposited on patterned SOI (a) and standard SOI (b) substrates. Top-view SEM image of GaAs grown on patterned SOI substrate (c) and standard SOI substrate (d).
To further characterize the material quality, X-ray diffraction (XRD) measurements and cross-sectional transmission electron microscopy (TEM) were performed. Figure 5(a) shows the XRD omega-2theta curve of III-V buffer layer on the SOI substrate. The peaks of Si layer (top Si) and Si-substrate layer are indicated in the XRD, the angular separation is typical due to the tilt between the bond top Si layer and substrate Si planes as discussed in [32]. The rocking curve peak of GaAs buffer can be observed clearly which indicates a high crystalline quality of GaAs layer. Even more, the noticeable multiple-peaks from the InGaAs/GaAs and InAlAs/GaAs DFLs can be distinguished clearly, which confirms a high-quality of III-V buffer layer. To further measure the asymmetric distribution of the defects on the (111)-facetted Si sawtooth structures, the scans of (004) ω-rocking curves across the GaAs peak were performed, with the incident x-ray beam perpendicular to the stripes (shown in inset of Fig. 5(b)) and parallel to the stripes, respectively. As the curves show in Fig. 5(b), a smaller full-width-at-half-maximum (FWHM) can be obtained from the ω-rocking measurement with the incident x-ray beam perpendicular to the stripes (309.7 arcsec) than the other direction (369.4 arcsec). The orientation-dependent FWHM of ω-rocking curves may result from an unequal distributions of defects in [1–10] and [110] directions on the (111)-facetted Si sawtooth structures, as previously reported in [33], W. Q. Wei et al. The cross-sectional TEM image in Fig. 5(c) shows that most of the defects are confined and annihilated at the interface between the GaAs and the (111)-facetted Si (001). The DFLs are utilized to further suppress the propagation of threading dislocations (TDs). Figure 5(d) shows a zoomed-in TEM image of the region marked in Fig. 5(c), which displays an annihilating phenomenon of the stacking faults (SFs) lying on the equivalent Si (111) planes. Those results experimentally prove the effectiveness of our designed structures for high quality III-V/SOI hybrid growth.
Fig. 5 (a) XRD ω-2θ curve of III-V buffer layer on the SOI substrate. (b) (004) ω-rocking curves across the GaAs peak by the incident x-ray beam perpendicular and parallel to the trenches. (c) Cross-sectional bright field TEM image of GaAs on sawtooth-hollow-structured SOI substrate, taken along the [110] axis. (d) Zoomed-in cross-sectional TEM images of the interface between GaAs and Si.
In Fig. 6, we compare the room-temperature photoluminescence (PL) of seven-layer InAs/GaAs DWELLs structure grown on GaAs (001), GaAs/Si (001) and GaAs/SOI substrates. The PL strength of the InAs QDs on SOI and Si (001) is almost the same as that on GaAs substrate, indicating the high quality of GaAs film on both Si and SOI substrate via (111)-faceted Si grating structures. The PL spectra of InAs QDs on GaAs, GaAs/Si and GaAs/SOI substrate are peaked at 1282 nm, 1297 nm, and 1291 nm, respectively. The peak wavelength shifts among three samples are induced by variations of surface temperature for different substrates. Uniform InAs QDs are obtained with QD density of 3.1 × 1010 cm−2 on GaAs substrate, 3.3 × 1010 cm−2 on Si (001) substrate and 2.52 × 1010 cm−2 on SOI substrate, respectively, as shown in the inset AFM images of Fig. 6.
Fig. 6 Room-temperature photoluminescent spectra of InAs/GaAs QDs grown on GaAs, Si (001) and SOI substrates. Insets are the 1 × 1 μm2 AFM images of the surface InAs QDs on GaAs, Si (001) and SOI.
4. Device properties
The microdisk lasers (MDLs) on GaAs, Si (001) and SOI substrates were pumped by 532 nm laser and measured under continuous-wave optical pumping using a micro-photoluminescence (μPL) system. The focused laser spot size was estimated to be approximately 3 μm. The laser emission was detected by a liquid nitrogen cooled InGaAs line array detector. Considering multiple reflections/absorptions in microdisk, the effective incident power was estimated by the formula of (1-R)[1-exp(-αd)]/ [1-R × exp(-αd)], where R is the surface reflection of the microdisk and α is the material absorption coefficient [34]. Optimized optical alignment was optimized to ensure that the maximum emission intensity of MDLs is obtained in each laser measurement. The plot of output intensity versus pump power (L-L curve) and the room-temperature power-dependent spectra for representative devices of the 4 μm MDLs on GaAs substrate and Si substrate are presented in Fig. 7(a) and Fig. 7(b), respectively. At low pumping power, broad spontaneous emission spectra were observed in the Figs. 7(c) and 7(d) with weak mode modulation. As the pumping power gradually increased, sharp peaks occurred and became dominant beyond lasing threshold. For the MDL on the GaAs substrate, the dominant lasing wavelength was peaked at 1279 nm, with a low threshold pumping power of approximately 0.33 mw at room temperature. The log-log scale plot of the L-L curve is presented in the inset of Fig. 7(a), revealing a typical ‘S-shaped’ nonlinear transition with all three regimes of operation: spontaneous emission, amplified spontaneous emission, and laser oscillation [35]. The cold cavity quality factor (Q) was calculated with Q = λcav /△λcav, where △λcav is the linewidth below threshold. The Q value of 3550 was obtained for InAs QD MDL on GaAs. With pumping power increasing, fundamental lasing mode at 1296 nm were identified. The spacing between the adjacent modes in the same radial-order is measured as 17 nm, which is in well agreement with the calculated free spectral range (FSR) of the first-order wisper gallery modes (WGMs) for the 4 μm disk.
Fig. 7 Integrated intensity of microdisk lasers as a function of wavelength (L-L curve) for devices on the (a) GaAs substrate and (b) Si substrate at room temperature. Inset: shows the log-log plot of the L-L curve, indicating an “S-shaped” transition. μPL spectra of the 4-μm-diameter microdisk laser at different pump powers for devices on the (c) GaAs substrate and (d) Si substrate.
For the identical MDL on the Si (001) substrate, the first-order WGM at the O-band wavelength of ~1304 nm was identified with a Q factor of 3674 as shown in Fig. 7(d). The FSR of this device is extracted from the spectrum with a value of 18 nm. The double log plot of the L-L curve is also presented in the inset of Fig. 7(b), to show the typical nonlinear transition in laser operation.
For the InAs QD MDL on SOI substrate, the lasing wavelength is slightly shifted to 1312 nm as shown in Fig. 8(a). Importantly, a threshold pump power of approximately 0.39 mW was measured at room temperature, which is similar to those on the Si (001) substrates. The power dependent spectra, L-L curve and power dependent linewidth plot are displayed in Fig. 8. By increasing the optical pump power, another strong lasing mode at 1326 nm was identified, as shown in Fig. 8(a). From the spectra, relatively higher Q factor of 3900 was obtained with FSR of 17 nm, which is larger than the Q factor of MDLs on Si (3674) and GaAs (3550). Here, the higher Q factor of InAs QD MDL on Si (001) and SOI could be caused by the grating-like Si patterns, which lead to the enhancement of reflected pumping beam. The L-L curve and its double log plot of InAs QD MDL on SOI are shown in Fig. 8(b). The lasing mode is pronounced with narrowing spectral linewidth as the pump power increases beyond the lasing threshold, as shown in Fig. 8(c). At a higher excitation power, the higher order WGMs appear with slightly widen spectral linewidth, which is attributed to the thermal accumulation on the laser device.
Fig. 8 (a) Lasing characteristics of a 4-μm-diameter microdisk on SOI. Photoluminescent spectra as a function of pump power. Inset: Zoom-in spectrum of the microdisk pumping in 0.64mw. (b) Linear plot of integrated output intensity versus effective input power (L-L curve). Inset: Double-logarithmic plot of the L-L curve. (c) FWHM evolution with the pump power.
Table 1 summarizes device parameters of previously reported 1.3 μm InAs/GaAs QD microdisk lasers grown on GaAs and Si and this work on GaAs, Si and SOI substrates. In previous research works, the GaAs buffer was grown on Si by MOCVD in order to benefit from the V-groove pattern based aspect ratio trapping (ART) mechanism [38], but the InAs QD microdisk laser structures were grown by post growth in MBE system for high quality formation of InAs QDs. The sample surface would be easily polluted and oxidized during the sample transfer from MOCVD chamber to MBE chamber, which leads to the material degradation of subsequently growing III-V structures. In addition, the electron beam lithography patterned V-groove structures are not fully compatible with large scale CMOS process, while the direct growth of III-V on etched V-groove Si patterns generally introduce additional dislocations, due to the surface vacancies created during pattern etching process. In our work, the direct growth of the GaAs buffer and MDL structure on the Si and SOI substrate by in situ hybrid epitaxy via joint III-V/IV MBE system, could strongly boost the material quality of heterogenous growing III-V structures and wafer yield. The whole growth process is carried out in ultra-high vacuum conditions, and the homoepitaxially formed (111)-faceted Si sawtooth structure exhibit highly homogeneous surface conditions, which ensures the interfacial quality between III-V and Si. Most importantly, our work is the first time to realize O-band QD MDLs on SOI substrate. Compared with the results of the MDLs on Si (001) and GaAs substrates in our work, the threshold pump power is slightly increased by less than 20%, but a relatively higher Q factor (3900) is achieved in contrast with those on Si (001) and GaAs substrates.
Table 1. Comparison of various InAs QD microdisk lasers on GaAs, Si and SOI substrates
For on-chip integration purpose, we propose two individual approaches to couple the InAs QD MDL and electrically pumped fabry-perot laser to SOI based silicon waveguides as shown in Fig. 9. Figure 9(a) shows the evanescent coupling of SOI embedded InAs QD MDL to silicon waveguides, where the laser light is coupled by bringing single mode Si arm waveguides close to the disk cavity with optimized gap width. In the case of electrically pumped fabry-perot lasers, butt coupling are used for selective-area grown InAs QD lasers with well-adjusted epi-structure height. Therefore, electrically pumped SOI embedded InAs QD laser can be directly coupled into pre-fabricated silicon waveguides at SOI top silicon layer as shown in Fig. 9(b). To note, the pre-fabricated silicon passive waveguides are required to be protected by SiO2 hard mask, where the III-V/IV interfaces have to be wet etched for creating smooth sidewalls. Most importantly, tapered silicon waveguides need to be properly designed in order to mode match the embedded InAs QD lasers with relatively larger dimensions (~2 - 4 μm in width). The described above coupling designs are currently under technical process, which should be expected in the near future.
Fig. 9 Schematics of proposed on-chip integration design for InAs QD MDLs and electrically pumped fabry-perot laser.
5. Conclusion
In this work, O-band InAs QD microdisk lasers on GaAs, Si (001) and SOI substrates via III-V/IV hybrid epitaxial growth are fabricated and characterized for experimental comparisons. By implementing (111)-faceted Si hollow structures, the lasing threshold and Q factor of microdisk lasers on Si (0.38 mW, 3674) and SOI (0.39 mW, 3900) substrates exhibit similar performance in comparison with GaAs based devices (0.33 mW, 3550). The successful demonstration of the first microdisk laser on SOI substrate with high performance shows a great potential of merging SOI technology with III-V photonic devices for future silicon photonic integration.
Funding
National Natural Science Foundation of China (11574356, 11434010, 61635011, 61804177 and 11804382); National Key Research and Development Program of China (2016YFA0300600 and 2016YFA0301700); Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-JSC009); Youth Innovation Promotion Association of CAS (2018011).
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