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Abstract
Directly grown III-V quantum dot (QD) laser on on-axis Si (001) is a good candidate for achieving monolithically integrated Si photonics light source. Nowadays, laser structures containing high quality InAs / GaAs QD are generally grown by molecular beam epitaxy (MBE). However, the buffer layer between the on-axis Si (001) substrate and the laser structure are usually grown by metal-organic chemical vapor deposition (MOCVD). In this paper, we demonstrate all MBE grown high-quality InAs/GaAs QD lasers on on-axis Si (001) substrates without using patterning and intermediate layers of foreign material.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
For next generation computing, silicon (Si) photonics is required to solve the problems of metal wiring such as low bandwidth density and high-power consumption [1–3]. However, group IV semiconductor based light sources are not fully integrated with Si photonics systems due to their low material gain [4]. III-V quantum dot (QD) lasers emitting in the 1.3 μm band have properties that satisfy the technical requirements of a Si photonics light source such as high efficiency, high operating temperature, temperature insensitivity of threshold current, low power consumption and high modulation speed [5–10]. Although flip-chip bonding [11] and wafer bonding [12–15] have been used to attach III-V light sources onto Si photonics platforms, direct growth is indispensable for a fully-integrated on Si light source which is scalable, has low cost and high yield. However, the growth of III-V semiconductors on Si produces high density crystal defects such as threading dislocations (TDs), antiphase boundaries (APBs) and cracks due to the differences in crystal structure and thermodynamic properties between III-V materials and Si [16,17]. Fortunately, individual QDs have independent energy barriers, the emission intensity from the QD active layer is less affected by the TD density compared to other structures [18].
In the past several years, InAs/GaAs QD lasers monolithically grown on offcut Si (001) were generally used to suppress the generation of APBs [19–21]. Although, the lasers fabricated by this approach had impressive device performance, this technology is not compatible with current CMOS technology using 'on-axis' Si (001). Since last year, InAs/GaAs QD laser technologies advanced significantly as evidenced by the various reported approaches such as using patterned Si substrates [22–24], an intermediate GaP buffer [24–26], and directly grown on GaAs/Si (001) substrates [27] have been reported. However, in these reports, metalorganic chemical vapor deposition (MOCVD) methods were used to grow the seed layers and buffer layers on Si [28–30]. Although MOCVD is capable of buffer film growth on Si, it is believed to be difficult to fabricate arsenic based high performance QD lasers due to inefficient dislocation filter layers, low QD luminescent efficiency and high growth temperature of AlGaAs cladding layers [31–33]. On the other hand, high crystal quality InAs/GaAs QD with AlGaAs cladding layer could be obtained by using the molecular beam epitaxy (MBE) method. Therefore, in order to form the QD laser structure on Si in a single epitaxial growth, it is necessary to grow the seed layers and buffer layers through MBE.
In this paper, we report on the first electrically pumped 1.3 μm InAs/GaAs QD lasers directly grown on on-axis Si (001) substrates with using only the MBE growth method.
2. Experimental procedure
2.1 Crystal growth
N-type doped Si (001) substrates were used for this research. The native oxide layer on the Si wafers were removed by diluted HF wet etching process. The wafers were then introduced into a standard solid source molecular beam epitaxy (MBE) chamber. After the normal outgassing process in the preparation chamber, the wafers were transferred into the deposition chamber and then heated to 950 °C and annealed for 600 s. Following this, a 40 nm-thick Al0.3Ga0.7As seed layer and an 800 nm-thick GaAs layer were grown directly on the Si substrate. To block the high-density TDs from reaching the QD layer, a 300 nm-thick GaAs layer followed by 5 layers of In0.15Ga0.85As (10 nm)/GaAs (10 nm) strained layer superlattice (SLS) were grown [19]. This growth step was then repeated twice. Above the SLS layers, a 700 nm-thick GaAs layer was grown. Then 8 layers of standard InAs/GaAs QD were grown between n- and p-1400 nm Al0.4Ga0.6As cladding layers. Finally, a 400 nm p-type doped GaAs contacting layer was grown. Figure 1 shows a cross-sectional schematic of the epitaxially grown QD laser structure on Si substrate.
Fig. 1 The schematic diagram of the InAs/GaAs QD laser structure grown on the on-axis Si (001) substrate.
2.2 Device fabrication
Figure 2 shows the scanning electron microscope image of a fabricated InAs/GaAs QD laser structure on Si (001) substrate. The laser material was fabricated into broad-area Fabry-Perot lasers. The mesa structures are stripes of widths of 80 μm, formed by using standard optical lithography and wet etching techniques. Then, the AuGeNi/Au p-contact and n-contact were formed on the top and the side of the mesa by using electron beam evaporation and lift off technique. After lapping the backside Si to 100 μm, the structures were cleaved to make 2.0 mm long devices. As the Si (001) substrate does not have an off-angle, it was easy to obtain a mirror surface perpendicular to the growth surface of the sample by cleaving. No high reflective (HR) coating was applied to the facets.
Fig. 2 Scanning electron microscope image of an InAs/GaAs QD laser structure on Si (001) substrate. The yellow regions indicate the AuGeNi/Au p- and n- electrode as labelled.
2.3 Measurements
The formation and elimination of the anti-phase boundaries were observed by cross-sectional scanning electron microscopy (SEM) of the wet etched samples. Due to the difference of etching rate between APB and single GaAs regions in NH4OH: H2O2: H2O solution, an APB appears as a dark line in the cross-sectional SEM image. After the cross-section of the sample was milled to 200 nm by using focused ion beam (FIB), the density of TD was evaluated by cross-sectional transmission electron microscopy (TEM) images. The surface morphology was characterized by an atomic force microscopy (AFM). The optical properties of InAs/GaAs QDs were measured by photoluminescence (PL) measurements. Laser oscillation was observed in a fabricated device under pulse current injection (2 kHz repetition frequency, 200 ns pulse-width, and 0.04% duty-cycle) at room temperature.
3. Results and discussion
3.1 Buffer layer and InAs/GaAs QD structure on GaAs/Si (001)
Figure 3 shows the GaAs layer grown on the Si substrate grown with the AlGaAs seed layer. The dark lines show that APBs are formed at the AlGaAs / Si interface and extends into the GaAs layer. By growing the Al0.3Ga0.7As seed layer on Si substrate at a relatively high temperature (500 °C) and a high growth rate (1.1 μm/h), the APBs are annihilated within GaAs buffer layer thickness of less than 400 nm. This method is thought to form low-density grain boundaries, because large nucleation grains grow in a short time under the longer migration length condition of adatom.
Fig. 3 Scanning electron microscope image of a sample grown on a Si (001) substrate. The arrows indicate the anti-phase boundaries in the GaAs buffer layer, showing clear termination of the anti-phase boundaries well within the layer. The bottom image shows a high magnification image. The white rectangle in upper image indicates the area of the bottom image.
We also grew the InAs/GaAs QDs on GaAs and GaAs/Si (001) substrates by adopting the same growth method and evaluated their sizes and densities. Figure 4 shows AFM images of the uncapped InAs/GaAs QDs grown on GaAs and GaAs/Si (001) substrates, respectively. From the AFM results, we observed the formation of InAs/GaAs QDs of almost the same size and density on both substrates. The average lateral size of the formed QDs were 30.1 nm and 30.2 nm, respectively, and the density was about 5.1 × 1010 cm−2 and 5.2 × 1010 cm−2, respectively. In addition, we found lower giant QD densities in on-GaAs/Si samples than in on-GaAs samples. Since the surface of GaAs/Si is rougher than that of GaAs substrate, it is thought that there is lower local concentration of stress and thus less likely to cause the formation of giant InAs QD.
Fig. 4 1 × 1 μm2 AFM images of uncapped InAs/GaAs QDs grown on (a) GaAs and (b)GaAs/Si (001) substrates, respectively.
Figure 5(a) shows a cross sectional bright-field TEM image of the buffer layer structure on GaAs/Si (001) substrates. The TEM cross-sectional images covers a total width of 22 μm. The TDD was estimated by counting the number of dislocations present along various growth thicknesses of 200 nm, 450 nm, 700 nm, 1200 nm, 1700 nm, 2200 nm, and 2700 nm, respectively. The numbers of threading dislocations were found to be 118, 64, 44, 24, 10, 4, and 1 along each value of the growth thickness listed above. As the GaAs buffer thickness increases, the density of threading dislocations decreases. Then, by introducing 3 groups of 10 nm In0.15Ga0.85As/10 nm GaAs SLS layer, the TDD was significantly reduced, down to 5 × 107 cm−2 in the QD region (see Fig. 5(b)). This density corresponds to 0.2% of the sheet density of the QD, and thus only a limited amount of QDs is directly affected by contact with the TD. While TDDs lower than 10−7 cm−2 have been reported [21, 34], in those work they applied in situ thermal cycling annealing (TCA) together with SLS filter layers. By applying this annealing process to this experiment, it will be possible to obtain higher quality GaAs, which could, in turn, reduce the TDD by up to 80% [34]. In this work, we managed to achieve a relatively low TDD within even TCA suggesting that we could lower the TDD even more by applying TCA.
Fig. 5 (a) Cross sectional bright-field TEM image of buffer layer structure on GaAs/Si (001) substrates and (b) dependence of dislocation density on the distance from III-V / Si hetero-interface.
Figure 6 shows the room temperature PL spectra from a single QD layer of grown structures on Si (001) and GaAs substrate. The integrated PL intensity of QDs on Si (001) is ~80% of that on GaAs sample. At room-temperature, we observed a PL emission of the ground state at ~1250 nm with a full-width at half-maximum (FWHM) of 31 meV. Additionally, the emission wavelength of excited level was at ~1150 nm. Due to the narrow emission linewidth and the large energy difference between the ground and excited levels (86 meV), the peaks could be clearly distinguished.
Fig. 6 PL comparison of an InAs/GaAs QD sample grown on GaAs/Si (001) to a reference sample grown on GaAs substrate at room temperature under the same pump conditions.
3.2 Device properties
Figure 7 shows the light output-versus-current (L-I) curve of InAs/GaAs QD laser grown on GaAs/Si (001) substrate under pulsed operation conditions at room temperature. Lasing was observed from the fabricated device under pulse current injection condition at room temperature. The lowest threshold current density (Jth) was 320 A/cm2 and the maximum output power from a single facet was more than 30 mW. Figure 8 shows the emission spectra of the InAs/GaAs QD laser on GaAs/Si (001) substrate at room temperature when excited below- and above the threshold current. Multi-mode lasing was observed from the sample, with the ground state lasing wavelength being ~1250 nm. Figure 9 shows the L-I curves of InAs/GaAs QD laser grown on GaAs/Si (001) substrate under pulsed operation conditions at various temperatures from 25 °C to 70 °C. Laser operation was observed up to 70 °C with a characteristic temperature (T0) of 51 K between 25 °C and 70 °C. The calculated slope efficiency was 0.052 W/A at 25 °C. We performed continuous current injection at room temperature, however, laser oscillation could not be detected up to 1000 mA.
Fig. 7 L-I curve of an InAs/GaAs QD laser grown on GaAs/Si (001) substrate under pulsed operation conditions at room temperature (25°C).
Fig. 8 Emission spectra of the InAs/GaAs QD laser on GaAs/Si (001) substrate at room temperature (25 °C).
Fig. 9 L-I curves of an InAs/GaAs QD laser grown on GaAs/Si (001) substrate under pulsed operation conditions at various temperatures.
The sample grown on the Si (001) substrate by the all MBE method shows degradation of several characteristics such as the output and thermal characteristics as compared with the QD laser grown on GaAs despite successful laser oscillation. This degradation is due to the quality of the GaAs buffer layer and the broad mesa width. Since this growth method is still in the incubation step, it is considered that the growth conditions suitable for the on-axis Si substrate can be obtained through optimization. Particularly, since the APD density is greatly changed by the growth conditions of the AlGaAs seed layer, it is considered that optimization of this layer is necessary. In addition, by adopting a narrow mesa design, the device will benefit from current constriction and heat dissipation, resulting in improved device performance.
4. Conclusion
We have successfully demonstrated an all MBE grown InAs/GaAs QD laser on on-axis Si (001) substrates. Using this growth method, we showed that a high-efficiency QD laser can be fabricated using only MBE without MOCVD or substrate patterning. High quality APB free GaAs buffer layer could be obtained by growing the seed layer growth a relatively high temperature and high growth rate. In addition, a GaAs layer with low dislocation density could be obtained by introducing well-designed InGaAs/GaAs SLS layers. On top of the GaAs buffer layer, a high density InAs/GaAs QDs with high emission intensity was obtained. The fabricated QD laser device successfully operated at room temperature. This direct growth method makes it possible to fabricate QD lasers monolithically on Si (001) substrates with a simple growth procedure. In addition, this approach demonstrates a way to integrate QD laser devices onto Si-based platforms.
Funding
New Energy and Industrial Technology Development Organization (NEDO).
Acknowledgments
The authors would like to thank Prof. Iwamoto and Dr. Chee for constructive criticism for this work.
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