I nvestigation into the InAs/GaAs quantum dot material epitaxially grown on silicon for O band lasers

Tianyi Tang, Tian Yu, Guanqing Yang, Jiaqian Sun, Wenkang Zhan, Bo Xu,†,Chao Zhao,†, and Zhanguo Wang

1Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

2Beijing Key Laboratory of Low Dimensional Semiconductor Materials and Devices, Beijing 100083, China

3College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Science, Beijing 101804, China

Abstract: InAs/GaAs quantum dot (QD) lasers were grown on silicon substrates using a thin Ge buffer and three-step growth method in the molecular beam epitaxy (MBE) system.In addition, strained superlattices were used to prevent threading dislocations from propagating to the active region of the laser.The as-grown material quality was characterized by the transmission electron microscope, scanning electron microscope, X-ray diffraction, atomic force microscope, and photoluminescence spectroscopy.The results show that a high-quality GaAs buffer with few dislocations was obtained by the growth scheme we developed.A broad-area edge-emitting laser was also fabricated.The O-band laser exhibited a threshold current density of 540 A/cm2 at room temperature under continuous wave conditions.This work demonstrates the potential of large-scale and low-cost manufacturing of the O-band InAs/GaAs quantum dot lasers on silicon substrates.

Key words: semiconductor laser; molecular beam epitaxy; quantum dots; III–V on Si; silicon photonics

1.Introduction

The rapidly increasing data volume in the digital age requires data centers with lower power consumption, higher transmission speed, and lower cost[1].Silicon-based optoelectronic integration technology is expected to meet this demand and attracts extensive attention[2−4].However, the major obstacle of silicon photonics is the lack of suitable on-chip light sources[5].Due to the indirect bandgap of the group IV materials, it is challenging to realize an efficient electrically pumped emitter[6].III–V compound semiconductor materials have a direct bandgap and high optical quality.It will have broad prospects if III–V lasers are combined with silicon substrates and applied to silicon-based optoelectronic integration[7−10].Compared with other technologies such as wafer bonding and flip-chip bonding, which have realized III–V devices on silicon[11−13], the monolithic growth technology has the advantages of large-scale, low-cost, and mass-production[14,15].However, the direct epitaxy of III–V materials on silicon substrates faces the problems of lattice mismatch,thermal mismatch, and polarity difference[16], resulting in a large number of defects such as threading dislocations(TDs)[17,18], antiphase domain boundaries (APBs)[19,20], and thermal cracks[21,22], acting as non-radiative recombination centers or electrical leakage paths, significantly reduce the performances of the optoelectronic devices grown on silicon substrates[23].Compared to quantum well, quantum dots (QDs)have remarkable dislocation tolerance.Besides, thanks to the three-dimensional quantum confinement of quantum dots on carriers, quantum dot lasers have lower threshold currents, higher temperature insensitivity than quantum well lasers, making them suitable for the room-temperature laser emitters for the silicon-based optoelectronic integration[24,25].

For InAs/GaAs QD lasers grown directly on silicon substrates, the quality of the GaAs buffer layer is important to the device performance.In recent years, a variety of methods have been proposed by different groups to reduce the defects of the GaAs buffer in the heteroepitaxy process.Bowers’group at the University of California, Santa Barbara (UCSB), realized a GaAs buffer with a density of 7.2 × 106cm–2on the GaP/Si substrate grown in MOCVD systems by using InGaAs/GaAs dislocation filter layers (DFL) and in-situ thermal cycle annealing (TCA), and the GaAs/AlGaAs heterostructure grown upon the GaAs buffer has a high photoluminescence intensity[26].Liu’s group at University of College London has also obtained APB-free GaAs buffers with low threading dislocations density by using 4°–6° miscut Si (001) substrate toward [011][14], Ge/Si virtual substrates[27], and high-temperature annealing of the silicon buffer in the dual-chamber MBE system[28].Arakawa’s group has used AlGaAs nucleation on the nominal Si (001) substrate and obtained a high-quality GaAs buffer with no APBs[29].And patterned silicon (001) substrates were also used by the groups of the Institute of Physics (IOP), Chinese Academy of Sciences[30], Hong Kong University of Science and Technology (HKUST)[31], and UCSB[32]for realizing a high-quality GaAs buffer on silicon.In addition,for the performance and fabrication yield of the on-chip light sources, the thickness of the GaAs layer on silicon needs to be controlled to less than 3μm to avoid the micro-cracks[33].

In this letter, we conducted an in-depth and systematic investigation of the GaAs buffer and quantum dots material for the O band laser on silicon substrates with a thin Ge buffer of 310 nm, and most dislocations were confined in the interface between the Ge epilayer and Si substrate.A high-quality GaAs buffer with few dislocations and APBs was obtained on silicon substrates by the three-step growth method containing low-temperature migrate enhanced epitaxy (LT-MEE).Five layers of QDs with a density of 4 × 1010cm–2were grown on the GaAs buffer, which has excellent photoluminescence.Furthermore, an O-band InAs/GaAs QD laser with a threshold current density of 540 A/cm2at room temperature under continuous wave conditions was also demonstrated.

2.Experimental methods

The sample was grown on a Ge/Si virtual substrate in the Riber 32P MBE system, with a ～310 nm Ge buffer grown by a separate MBE system.The 6° offcut Si substrates were chosen to suppress the formation of APBs.The Ge/Si virtual substrate was degassed at 300 °C in the buffer chamber, then heated to 700 °C in the growth chamber for deoxidation and the formation of double atomic steps[34], which could suppress APBs.The substrate was then cooled to 300 °C for the growth of the GaAs layer subsequently.The temperatures mentioned are actual temperatures.

The schematic of the whole laser structure grown on the silicon substrate is shown in Fig.1(a); the corresponding scanning electron microscope (SEM) image of the laser structure is shown in Fig.1(b).The GaAs epitaxial layer was grown on the Ge/Si virtual substrate by the three-step growth method,including a 30 nm LT-MEE layer, 170 nm intermediate layer and 800 nm high-temperature layer growing at 300, 450, and 600 °C, respectively.The three-step growth method with LTMEE can effectively reduce the interdiffusion of Ge, Ga and As[35], and the initial Ga-prelayer can further promote the formation of the single domains.A 3-layers In-contained stained superlattice layer (SLS) followed by a 100 nm GaAs spacer was introduced as a dislocation filter layer (DFL).The SLS consisted of In0.18Ga0.82As/GaAs, In0.18Al0.82As/GaAs, In0.18Ga0.82As/AlAs,and In0.18Al0.82As/AlAs superlattices.The thickness of In-contained layer and In-free layer in the SLS was 7 and 13 nm, respectively.And the SLS and spacer layer was grown at 440 and 600 °C, respectively.This growth procedure was repeated ten times, which could bend dislocations and change their direction of propagation.Between the first six cycles and the last four cycles, additional 100 nm GaAs and 100 nm AlAs grown at 620 °C was introduced.After each SLS, in-situ thermal annealing at 660 °C was introduced to promote the in-plane movement of 60° dislocations and increase their meeting and annihilating[36].To avoid the In desorption of the SLS,an extra 10 nm (Ga/Al)As was deposited above the SLS before annealing.Above the DFLs, 30 cycles of n-doped Al0.35Ga0.65As (5 nm)/GaAs (5 nm) superlattice were grown at 620 °C to reduce the surface fluctuations.Finally, the QD laser structure contained separated confinement heterostructure(SCH) was grown, including 2μm n-doped GaAs contact layer, 100 nm n-AlGaAs (5%→60%) graded cladding layer, 1.3μm lower Al0.6Ga0.4As cladding layer, 100 nm lower undoped AlGaAs (60%→5%) graded-index layer, 50 nm lower GaAs waveguide, the active region contained 5-layers of InAs dot in the well (DWELL), 50 nm upper GaAs waveguide, 100 nm upper undoped AlGaAs (5%→60%) graded-index layer, 1.3μm upper p-doped Al0.6Ga0.4As cladding layer, 100 nm AlGaAs(5%→60%) graded cladding layer, and 300 nm p-doped GaAs contact layer.In the active region, 2.6 monolayers (MLs) InAs quantum dot was sandwiched between 2 nm In0.15Ga0.85As wetting layer and 5 nm In0.15Ga0.85As capping layer to form the InAs DWELL structure, with 5 nm LT-GaAs cap and 45 nm GaAs spacer layer between each DWELLs.The GaAs, Al-containing layer and In-containing layer in the laser structure was grown at 600, 620, and 495 °C, respectively.To characterize the morphology of quantum dots, we also grew uncapped surface InAs quantum dots.

Fig.1.(Color online) (a) Schematic of the QD laser structure grown on silicon substrates, and (b) the corresponding cross-sectional SEM image.

The epilayer on the Si substrate was fabricated to broadarea Fabry–Perot (FP) lasers with a width of 100μm by standard photolithography and wet etching techniques.Ti/Au and Ni/GeAu/Ni/Au were deposited on the top of the mesa and the n-GaAs buffer exposed by chemical etching for the formation of ohmic contact, respectively.Then the laser was cleaved to 2.5 mm without coating on the facets.

The material quality of the epilayer was characterized by high-resolution X-ray diffraction (XRD) using a Bruker D8 Discover Plus diffractometer.To study the interface between GaAs/Ge/Si, high-resolution cross-sectional transmission electron microscopy (HRTEM) was done using an FEI Talos 200S microscope with super EDS.Atomic force microscopy (AFM)was done using an MFP 3D scanning probe microscope to characterize surface InAs QDs, and photoluminescence spectroscopy of the QDs was done by Horiba IHR320 with a 532 nm laser.The fabricated lasers were tested by light–current–voltage (L–I–V) measurement systems and electroluminescence spectroscopy (YOKOGAWA AQ6375B).

3.Results and discussion

The quality of GaAs epilayers was first examined by XRD.As shown in Fig.2(a), the rocking curve (RC) was obtained around GaAs (004), and the full-width-of-half-maximum(FWHM) was about 252 arcsec, which is closed to the reported value for GaAs on Si[37], indicating a high quality of the GaAs epitaxy layer.Clear Pendellόsung fringes up to the fourth-order are observed in Fig.2(b) for the epilayers grown on Si, which confirmed the high crystalline quality and abrupt interfaces of the epilayers.The peak from Si substrate is also observed in Fig.2(b).XRD reciprocal space mapping(RSM) around (004) and (224) reflection was used to further examine the crystalline quality and residual strain inside the layers, as shown in Figs.2(c) and 2(d).Both figures show clear fringes around the (004) and (224) reciprocal lattice points, indicating coherent interfaces between the substrate and the epilayers.The anisotropic elongation of the GaAs diffraction pattern suggests mosaic spread, attributed to the residual defects in the layers[38].The vertical line between the Si and GaAs spot in (004) RSM in Fig.2(c) shows there is a lattice tilt due to the epilayers grown on the offcut Si substrate[39].Fig.2(d) shows the (224) RSM of the structure where the patterns representing GaAs and Si follow the full-relaxation line as indicated in the figure, implying that the epilayer is fully relaxed with respect to the Si substrate and no residual strain is present.The XRD result demonstrates that the GaAs epilayer we grew on the Ge/Si virtual substrate has an excellent crystal quality.

Fig.2.(Color online) (a) XRD RC around GaAs (004).(b) High resolution XRD (004) scan of GaAs grown on Si.High resolution XRD RSM results taken from (c) (004) and (d) (224) reflections.

We further examine the GaAs/Ge/Si interface by cross-sectional HRTEM to study the defect formation and propagation.Figs.3(a) and 3(b) show the high crystal quality with no APBs in both GaAs and Ge layers.There is a very sharp interface between the 310 nm Ge layer and GaAs layer, as shown in Fig.3(a), and with no element intermixing between the interface of GaAs layer, Ge layer, and Si substrate, as shown in energy-dispersive X-ray (EDX) elemental mapping of Fig.3(b).Fig.3(c) shows the periodic lattice arrangement of the Ge/Si interface, which reveals the mismatch strain between them was relaxed by forming misfit dislocations[40], and few threading dislocations are propagating into the growth direction.The interface between Ge and GaAs layer is abrupt with no visible defects shown in Fig.3(d), benefit from their lattice mismatch of merely 0.08%[41].Most defects are confined to the Ge/Si interface, making a high-quality GaAs buffer with few defects possible.

Fig.3.(Color online) (a) Cross-sectional TEM high-angle annular dark field image (HAADF) of GaAs grown on the Ge/Si virtual substrate.(b) Elemental mapping of the interface between GaAs layer, Ge layer and Si substrate.High resolution cross-sectional TEM image of the interface between (c) Ge layer and Si, and (d) GaAs layer and Ge layer, respectively.

InAs QDs on the surface and in the active region grown on the above-mentioned high-quality GaAs buffer were evaluated by AFM and cross-sectional HRTEM.A typical AFM image of the uncapped surface InAs QDs with few large-sized dots grown on the GaAs/Ge/Si buffer is shown in Fig.4(a),from which a QD density of ～4 × 1010cm–2is obtained.From the high-resolution TEM image of the uncapped dot in Fig.4(b), it is estimated that the dot size is about ～25 nm in diameter and ～9 nm in height.Fig.4(c) shows the cross-sectional TEM image of the active region of the laser, with five DWELL structures separated by 45 nm GaAs spacer layers; no obvious defect is observed, and the abrupt interface between the well and barrier is observed.It is clear that the antiphase domains and threading dislocations are suppressed effectively using the growth scheme we developed.Fig.4(d) shows a typical cross-sectional TEM image of a single InAs QD in the active region.The pyramid-shaped quantum dots shown in Fig.4(b) become disc-shaped due to the intermixing process and the additional stress during the InGaAs capping layer growth, which is common for InAs QDs on GaAs substrates[42].

Fig.4.(Color online) (a) A2 ×2 μm2 AFM image of surface InAs QDs grown on Si substrates.(b) Cross-sectional TEM images of surface InAs QDs on Si substrates.(c) Cross-sectional TEM images of the active region of the laser grown on Si substrates.(d) High-resolution TEM images of InAs QDs in the active region.

Fig.5 shows the PL spectrum of the DWELL active region of the laser structure.The room temperature PL spectrum in Fig.5(a) shows an emission wavelength of 1270.9 nm with a FWHM of 56 meV under an laser power density of 35.5 W/cm2, which corresponds to the ground state of the InAs QDs.The PL peaks at shorter wavelength derived from the excited state and wetting layer of QDs are observed under high power of the excitation laser.The typical temperature-dependence of the integrated PL intensity extracted from the temperature-dependent PL spectra (see the inset of Fig.5(b)) for the 5-layers InAs DWELL is shown in Fig.5(b), from which we can estimate the thermal activation of the QDs in the active region.The integrated PL intensity remains approximately constant up to 200 K and then decreases 100 times up to 270 K.Using the Arrhenius equation at high temperature, the estimated activation energy is about 283.5 ± 22 meV, which is close to the 290 meV energy difference between the bound states in the InAs DWELL and the quasi-continuum states in the barrier[43].It indicates that the thermal quenching at high temperature is attributed to the thermal escape of carriers.The PL peak wavelength has a slight red shift shown in the temperature-dependent PL spectra, which could be attributed to carriers’ transfer from the small size quantum dot to the large size quantum dot and the decrease of the InAs bandgap as the temperature increasing[44,45].

Fig.5.(Color online) (a) PL spectrum of InAs DWELL grown on Si substrates.(b) Temperature-dependences of the integrated PL intensity.Inset:temperature-dependent PL spectra of the InAs QD grown on Si substrates.

Fig.6(a) shows the typicalL–I–Vcurve of the QD laser grown on silicon substrate under a pulse width of 10μs and 0.1% duty cycle at room temperature.The SEM image of the fabricated InAs QD laser on silicon substrate was shown in the inset in Fig.6(a).The p-contact and n-contact were taken on the top of substrates.The laser with a wider ridge can reach a maximum output power of 50 mW when working at pulsed mode.Fig.6(b) shows theL–Icurve of the devices operated at room temperature under continuous-wave (CW)mode.The threshold current of the InAs quantum dot laser grown on Si substrate was 1.35 A, the corresponding threshold current density was 540 A/cm2.Fig.6(c) shows the emission spectrum of the QDs laser on the Si substrate at room temperature under CW mode when excited above the threshold current.The emission wavelength from the ground state was 1265.6 nm in the O band, which could be adjusted by tuning the growth parameters of InAs QDs in the active region.More works have to be done to improve the output power and reduce the threshold current density of the laser.

Fig.6.(Color online) (a) L–I–V curve of InAs QD lasers grown on Si substrates under pulsed mode at room temperature.Inset: SEM image of the whole InAs QD laser structure on the Si substrate.(b) L–I curve of InAs QD lasers grown on Si substrates under CW mode.(c) Emission spectrum of InAs QD laser grown on Si substrates under RT and CW modes.

4.Conclusion

In conclusion, we demonstrated the high-quality GaAs buffers on the Si substrate using a thin Ge buffer in the MBE systems.The stress of the epilayer was released by forming misfit dislocations in the Ge/Si interface.The characterization results indicate that the growth scheme we developed was very effective for the direct epitaxy of high-quality GaAs film on the Si substrates.Lasers containing 5-layer InAs DWELL were grown on the GaAs/Ge/Si buffer with no obvious dislocations in the active region by the cross-sectional TEM results.The broad-area edge-emitting lasers with 2.5 mm cavity length and 100μm ridge width were fabricated and characterized.Under continuous wave conditions, a threshold current of 540 A/cm2was obtained at room temperature.Our works demonstrate the large-scale and low-cost manufacturing potential of the O-band InAs/GaAs quantum dot lasers on silicon substrates.More works are in progress, such as optimizing buffer growth conditions (optimize dislocation filter layer or reduce the buffer thickness to avoid microcracks, etc.) and device fabrication process to further improve device performance.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (Grant No.2018YFB2200104), the “Strategic Priority Research Program”of the Chinese Academy of Sciences (Grant No.XDB43010102), and the Frontier Science Key Research Program of CAS (Grant No.QYZDB-SSW-SLH006).Prof.Bo Xu thanks Prof.Jinsong Xia from Huazhong University of Science and Technology for growing Ge layers on Si.