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Abstract
Laser devices for silicon photonics are expected to be implemented in an integrated environment to complement CMOS devices. For this reason, quantum dot (QD) lasers with excellent thermal properties have been considered as strong candidates for Si photonics light sources. The direct growth of QD lasers on Si (001) on-axis substrates has been garnering attention owing to the possibility of monolithic integration on a CMOS-compatible wafer. In this paper, we report on the high-temperature (over 100°C) continuous-wave operation of an InAs/GaAs QD laser directly grown on on-axis Si (001) substrates through the use of only molecular beam epitaxy.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Since its first theoretical mention in 1982, the unique three-dimensional carrier-confined structure of the quantum dot (QD) has been demonstrated to enhance laser device performance [1–4]. The superior thermal properties of QD lasers, relative to semiconductor lasers with quantum wells, began garnering attention at an early stage [5,6], with research on combining QD capping materials and active layer p-doping techniques resulting in significant advances in the thermal properties of QD lasers [7,8]. In addition, InAs/GaAs QD lasers operating within a 1.3-μm wavelength range were demonstrated to achieve high-temperature operation up to 220 °C [9], and temperature-invariant operation (T0 = ∞) [4,10].
QD lasers are realistic candidates that can be monolithic fabricated on Si or Ge substrates. This is because the active QD layers maintain a high emission rate, even on the hetero-epitaxy layer, which has a high density of threading dislocations. In the active QD layer, each QD is spatially separated from other QDs, and the non-emission recombination due to defects is relatively small [11,12]. In the Si photonics platform, because the laser devices are integrated into CMOS devices with high heat dissipation, high operating temperature is a requirement for implementation in the laser devices [13]. Based on the characteristics mentioned above, direct-grown QD lasers have been deemed as suitable Si photonics light sources.
Previous research has demonstrated that InAs/GaAs QD lasers that implement p-doping techniques on an Si substrate can achieve a maximum operating temperature of 119 °C and a characteristic temperature (T0) above 200 K [14]. However, the reported substrate had a 6° offcut toward [111], which is not fully compatible with the type of substrate implemented in CMOS technology. In the last few years, there have been many reports on InAs/GaAs QD lasers on on-axis Si (001) substrates [14–20]. This method has revealed that defects such as anti-phase domains (APDs) and threading dislocation density (TDD) defects, which occur when directly growing III-V semiconductor materials on on-axis Si (100), are suppressed by adopting a V-groove structure and implementing epitaxial intermediate layers and dislocation filter layers [17–22]. Additionally, implementing the active-layer p-doping technique to fabricate on-Si QD lasers was demonstrated to improve the thermal properties of the device [18,21]. However, among the reported on-axis Si III-V QD lasers, the highest continuous-wave (CW) operation was below 100 °C [21–23].
In this paper, we show that molecular beam epitaxy (MBE) can be used to not only directly grow an InAs/GaAs QD laser on Si (001) on-axis substrates without modulation p-doping, but also to realize high-temperature CW operation (above 100 °C).
2. Experimental method
2.1 Crystal growth
All of the III-V crystal growth on Si substrate was achieved by using only a solid source MBE [24]. N-type Si (001) substrates with misorientation angles smaller than ± 0.2° forward [110] direction were used in this study [25]. The Si substrate was deoxidized via wet etching with a 1% (HF) solution. After loading the substrate onto the MBE chamber, the wafer was outgassed at 450 °C for 1 hour in an ultra-high vacuum environment. Prior to growth, the Si substrate was preheated to 950 °C for 600 s. Figure 1 details the design of the epitaxy structure of an InAs/GaAs QD laser grown on the Si (001) substrate. The epitaxial structure comprises the following three parts: a III-V buffer structure, a dislocation filter structure, and a QD laser structure.
Fig. 1 Detailed schematic diagram of the layers of the InAs/GaAs QD laser grown on the on-axis Si (001) substrate.
The AlGaAs nucleation layers were grown under relatively high growth rate of 1.1 μm/h and high substrate temperature of 500 °C. A 40-nm-thick Al0.3Ga0.7As nucleation layer and an 800-nm-thick GaAs layer were directly grown on the substrate. This growth was followed by 5 layers of a 10-nm-thick In0.15Ga0.85As / 10-nm-thick GaAs strained layer superlattice (SLS), then a 300-nm-thick GaAs layer. This growth step was then repeated two more times. After the growth of a 700-nm-thick n-type GaAs contact layer, 8 layers of InAs/GaAs QD [26] were then grown between two 1170-nm Al0.4Ga0.6As cladding layers. The InAs QD was partially capped with GaAs by the height of the QD (~7 nm), followed by the In-flush method. The total GaAs interlayer spacer thickness of 39.3 nm was grown. Finally, the structure was capped with a 400-nm-thick p-type GaAs contact layer.
2.2 Device fabrication
Figure 2 is a schematic illustration of the fabricated InAs/GaAs QD laser device structure. Laser mesa with a 7-μm width were fabricated by using photolithography and wet etching. AuGeNi/Au was deposited onto the p- and n-contact layers (see Fig. 3). After lapping the backside silicon substrate to 100 μm, the structures were cleaved to make 1.1-mm-long devices. Note that no high-reflection coatings were applied to the cleaved facets.
Fig. 2 Schematic illustration of the InAs/GaAs QD laser device structure on on-axis Si (001) substrate.
Fig. 3 Cross-sectional view of a scanning electron microscope image of an InAs/GaAs QD laser device fabricated on a Si substrate.
2.3 Measurement
Scanning electron microscopy (SEM) was used to obtain a cross-sectional view of the substrate, and thus observe the laser mesa structures. After milling the sample at 200 nm by using focused ion beam (FIB), the formation of threading dislocations was observed via cross-sectional bright-field transmission electron microscopy (TEM). Because of its low density, the dislocation density in the QD layer was measured via plane-view TEM. Additionally, all laser device characteristics were measured under CW electrical injection, and a thermocouple was used to measure the heat-sink temperature.
3. Results and discussion
3.1 Crystal growth
APDs disappear in the GaAs lower buffer layer stage [24]. The average lateral size of the uncapped QDs was approximately 30 nm, and the density of the QD was approximately 5 × 1010 cm−2 (see Fig. 4.). Figure 5(a) shows a cross sectional bright-field TEM image of the buffer layer structure on a GaAs/Si (001) substrate. The density of dislocations at the interface between the Si substrate and the GaAs epi-layer is decreased through the three groups of InGaAs / GaAs SLS. We reduced the thickness of AlGaAs clad layers from previous work [24], to suppress the generation of threading dislocations due to thermal expansion coefficient. However, there was no significant change in the generation of threading dislocation. Plane-view TEM was used to accurately estimate TDDs in the sample below 1 × 108/cm2. Figure 5(b) shows a plane-view bright-field TEM image of the GaAs layer on an n-clad layer. This is the layer that immediately precedes that on which the InAs QD layers are grown. The white arrows in Fig. 4(b) indicate threading dislocations. The TDD was 4.7 × 107/cm2, which is higher than the lowest reported TDD, but still corresponds to only one threading location per 1000 QDs. Since a buffer layer was not applied to reduce the number of dislocations via a technique such as thermal cycling annealing, it is suggested that incorporating this technique can reduce the TDD to below 107/cm2 [21,27,28]. Stacking faults were not observed in the plane-view TEM images.
Fig. 5 (a) Cross-sectional TEM image of buffer layer and dislocation filter layer. Purple areas indicate InGaAs/GaAs strained layer superlattice. (b) Plane-view transmission electron microscope image of a GaAs layer directly under the InAs QD layer.
3.2 Device properties
Figure 6(a) shows the temperature dependence of the light-current (L-I) characteristics under the CW condition. The threshold current at room temperature was 27.6 mA, and the corresponding threshold current density was as low as 370 A/cm2. Additionally, CW operation was observed at up to 101 °C in a fabricated device. The slope efficiencies were 53.2 mW/A at 20 °C, 49.5 mW/A at 50 °C, 38.5 mW/A at 80 °C and 26.3 mW/A at 100 °C. Figure 6(b) shows the temperature dependence of the threshold current density. T0 was nearly constant at approximately 50 K between operating temperatures of 20 °C and 90 °C. Figure 7(a) shows the dependence of the ground-state lasing spectrum on the heat-sink temperature. Under the bias current of 210 mA, the ground-state lasing wavelengths at 25 °C and 101 °C were 1224.6 nm and 1272.0 nm, respectively, and the wavelength shift was 0.62 nm/K (see Fig. 7(b)). We measured the sub-threshold electro-luminescence spectrum and evaluated the net modal gain of QD lasers by the Hakki-Paoli method at 25 °C using a 450-μm-long Fabry-Perot (FP) laser (see Fig. 8). The maximum net modal gain was 20 cm−1 at around 1225 nm. This gain is smaller than the reports on QD lasers on GaAs substrate [9,29], but it is higher than reported QD lasers on the on-Si substrate [12].
Fig. 6 (a) Temperature dependence of L-I characteristics under CW operation for an InAs/GaAs QD laser directly grown on a Si (001) on-axis substrate. (b) Temperature dependence of the threshold current density for an InAs/GaAs QD laser directly grown on a Si (001) on-axis substrate.
Fig. 7 (a) Temperature dependence of the emission spectra for the InAs/GaAs QD laser directly grown on a Si (001) on-axis substrate. (b) Peak wavelength shifts due to the heat-sink temperature change.
Fig. 8 (a) Amplified spontaneous emission (ASE) spectrum of InAs/GaAs QD laser on Si and (b) Gain characteristics of InAs/GaAs QD laser on Si by the Hakki-Paoli method.
By narrowing the mesa width, we were able to achieve room-temperature and high-temperature CW operation, which was not achieved in our group’s previous study [24]. This is because narrow mesa structures result in more efficient heat dissipation and current constriction. Additionally, the maximum operating temperature and T0 of our samples were lower than those for previously reported QD lasers on-GaAs without p-doping [30]. It is believed that this is owed to the increased non-radiative recombination associated with the relatively high TDD, and the reduced modal gain caused by the surface roughness-induced uniformity of the QD structure. However, implementing various approaches to reduce the TDD of GaAs on Si may have improved the thermal properties via optimization of the crystal quality of the GaAs buffer layer.
In addition, our samples have demonstrated a buffer-layer TDD value that is approximately eight times higher than that for previously reported QD lasers on on-axis Si (001) substrates [22], although the modulation p-doping that is commonly purposed to improve the high-temperature characteristics was not applied here. Conversely, the maximum operation temperature of our sample was comparable to that of previously reported devices. It is believed that the adoption of a GaAs capping structure and the high gain of the 8-layer stacking QD structure improved the thermal characteristics of the proposed device. Although it was not achieved in this study, we believe that a lower TDD in the buffer layer and modulation p-doping could serve to improve the thermal properties of the laser devices fabricated in this study.
4. Conclusion
We have successfully demonstrated the high characteristic temperature and high-temperature CW operation of an InAs/GaAs QD laser that was directly grown on Si (001) on-axis substrates by only using MBE. Despite the relatively high TDD of the buffer layer and the absence of modulation p-doping, high-temperature operation was achieved by the fabricated laser device. Considering these results, this study suggests that the QD laser is a realistic candidate for integration into silicon CMOS technology, which requires high-temperature operation.
Funding
New Energy and Industrial Technology Development Organization (NEDO).
Acknowledgments
The authors would like to thank Dr. Takeo Kageyama, Mr. Joohang Lee, and Prof. Satoshi Iwamoto for their constructive criticism of this work.
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