III-V Quantum Dot Lasers Monolithically Grown on Silicon

We review the direct growth of III-V quantum dot laser on Si substrates. A low threading dislocation density, on the order of 10<sup>5</sup> cm<sup>−2</sup>, for III-V epilayer on Si has been achieved.

Based on the pre-structure on the Si substrate, a standard p-i-n laser structure was grown at optimized conditions with the following order: a 1.4 μm n-doped AlGaAs cladding layer, followed by a 140 nm undoped AlGaAs, a fivelayer InAs/InGaAs/GaAs dots-in-well (DWELL) active region [10], another 140 nm undoped AlGaAs layer, a 1.4 μm p-doped AlGaAs cladding layer, and finally a 300 nm highly doped GaAs as the contact layer. A high resolution bright-field scanning transmission electron microscope (TEM) measurement was performed to characterize the QD active region [12]. An active region without noticeable defects can be observed in the TEM measurement, indicating the TDs generated at the III-V/Si surface have been annihilated efficiently due to the strategies employed during the pre-structure. A typical atomic force microscopy (AFM) was also used to characterize the uncapped InAs/GaAs QDs grown on the III-V epilayers. A dot density of ~3.0×10 10 cm -2 with good uniformity could be derived from AFM, which resulted in a strong room temperature photoluminescence (PL) emission at ~1300 nm with a narrow linewidth of ~29 meV has been achieved [12].
Broad-area lasers were fabricated following the standard lithography, wet etching and metallization techniques as described in [12]. The light-current (LI) characteristics of a typical InAs/GaAs QD laser under continuous-wave (c.w.) operation at room temperature was shown in figure 2(a). An extremely low threshold current density of 62.5 A/cm 2 and a high output power of over 52.5 mW has been achieved. The c.w. lasing spectrum was measured with an injection current density of 225 A was shown in figure 3(b). Moreover, the c.w. lasing in the ground state was maintained until 75 °C. As for the pulse mode, the silicon-based laser lased up to 120 °C [12].

III. QD laser on on-axis Si Substrate
Si (100) substrate with a 4° miscut-angle, mentioned in the previous works, has been introduced to suppress the formation of APBs when growing polar materials on unipolar materials [10][11][12]. The offcut Si substrate makes it possible to grow a high quality III-V epilayer directly on the cost of the full compatibility with standard microelectronics fabrication. In this part, a first electrically pumped c.w. InAs/GaAs QD laser directly grown on, CMOS compatible, on-axis Si (100) substrate has been demonstrated, through the cooperation of the Metal-Organic Chemical Vapor Deposition (MOCVD) and Molecular Beam Epitaxy (MBE) machines. The whole QD laser structure could be divided into two parts as shown in figure 3(a). The first part comprised a thin GaAs nucleation layer and a GaAs buffer layer. Both the nucleation layer and buffer layer were grown by MOCVD [13]. The second part was the InAs/GaAs QD laser grown by MBE at the following sequence under the optimized condition: a GaAs buffer layer with the thickness of 600 nm, 4 sets of InGaAs/GaAs SLSs DFLs, five layers of DWELL structure sandwiched by a 50 nm GaAs spacing layer, surrounded by 30 nm undoped AlGaAs guiding layer with the upper and lower 1.4 μm p-doped and n-doped AlGaAs cladding layer, finally a 300 nm pdoped GaAs contacting layer [14]. The GaAs buffer layer directly grown on the on-axis Si substrate by MOCVD and the InAs/GaAs QDs grown by MBE were characterized through AFM measurements as shown in figure 3(b) and (c). A small root-mean-square (RMS) roughness for the 400nm GaAs directly grown on Si (100) of 0.86 nm was calculated through a typical 5 × 5 μm 2 AFM image. Additionally, an APB-free GaAs film layer has been achieved by the MOCVD [12]. The AFM image of InAs/GaAs QD indicates a good dot uniformity has been achieved with a typical dot density of 3×10 10 cm -2 . The room-temperature PL spectrum of the InAs/GaAs QD is shown in figure 3(d), where the wavelength emission at ~1285 with a narrow linewidth of 32 meV.
A LI characteristic measurement of a typical InAs/GaAs QD laser grown on exact Si (100) substrate under c.w. operation at room temperature is shown in figure 3(e). A current threshold density of 425 A/cm 2 and a single facet output of 43 mW were observed with an injection current density of 1.3 kA/cm 2 . The c.w. lasing spectrum was measured with an injection current density of 533 A/cm 2 as the figure 3(f) shown, where the lasing peak located at 1288 nm.

IV Conclusion
We have reviewed our recent progresses in III-V quantum dot laser directly grown on Si substrates including the offcut Si substrate and on-axis Si substrate.