Objective 1.3-μm GaAs-based Ⅲ--Ⅴ quantum dot (QD) lasers have several advantages over commercial lasers of InP-based Ⅲ--Ⅴ quantum well lasers, such as low threshold current density, high quantum efficiency, high-temperature insensitivity, high optical feedback tolerance, and larger modulation bandwidth owing to the three-dimensional quantum confinement effect of carriers. This has made the 1.3-μm GaAs-based QD laser a very promising candidate as a light source for next-generation low-power-consumption, low-cost, small-footprint, and high-speed fiber-optical communication systems. However, the closely spaced energy levels of the confined holes and In-Ga interdiffusion during epitaxial growth for practical QD laser structures make the performance of current devices still far short of expectations. In addition, for high-speed lasers, a short cavity length is crucial because of the significantly reduced photon lifetime, but there is always a trade-off between cavity length and the saturation modal gain. In recent years, introducing p-doping in the active region to optimize the properties of QD materials has attracted extensive interest. p-Doping in Ⅲ--Ⅴ QD structures to compensate the thermal escape of carriers leads to better thermal stability. Modulation p-doping can significantly inhibit Ga vacancy propagation, leading to smaller interdiffusion and a reduced intermixing effect. In 2010, the ground state 25 Gbit/s operation of a 1.3 μm p-doped QD laser was first reported by Tanaka et al. Recently, a 15 Gbit/s high-speed 1.3 μm modulation p-doped QD laser has been demonstrated in a 500 μm long QD laser by Arsenijevic' et al. In this work, we further optimize the performance of QD lasers using subtle epitaxial growth and careful structure design.

Methods The InAs/GaAs multiple QD layer structures were grown using molecular beam epitaxy (MBE) on Si-doped GaAs(100) substrates. The QD active region consists of eight stacks of QD layers separated by 33 nm GaAs spacers (Fig. 1). Each QD layer comprises 2.7 monolayer InAs covered with a 6 nm InGaAs strain-reducing layer, and the active layers were sandwiched between the ≈2800 nm n-type Al_0.3Ga_0.7As lower-cladding layer and ≈1800 nm p-type Al_0.3Ga_0.7As upper-cladding layer. The p-doped QD sample was grown sequentially with identical structures, and the modulation p-doping was performed with Be in a 6 nm layer located in the GaAs spacer layer 10 nm beneath each InAs/InGaAs QD layer to obtain a concentration of 3×10 ¹⁷ cm ^-3. For the effective light excitation and photoluminescence (PL) signal collection, the upper p-side AlGaAs cladding layers were etched away using wet etching above the QD active regions. Ridge waveguide (3.5 μm ridge wide) lasers were fabricated using photolithography and dry-wet etching techniques. To prevent lasing from the first excited state of the QD, careful design and a facet-coating process were fully investigated. Finally, 1.3-μm ground state lasing has been found in p-doped InAs/GaAs QD lasers with a 300 μm ultrashort cavity length, which shows great potential in high-speed optical communication systems.

Results and Discussions The PL peak wavelength of the p-doped sample is longer than that of the undoped sample (Fig. 4). The difference in the emission wavelengths between undoped and p-doped samples is caused by the higher temperature needed for the growth of the AlGaAs cladding layer, which is equivalent to a rapid thermal annealing (RTA) process. The QD samples underwent an annealing effect at the higher growth temperature, in which a strong interdiffusion between QDs and surrounding barrier layers occurred with intermixing for undoped sample, resulting in a remarkable blue-shift of the peak position. Introducing the modulation p-doping can significantly inhibit the Ga vacancy propagation, which leads to smaller interdiffusion and a reduced intermixing effect. To be more specific, we employ the scheme depicted to illustrate the role of p-doping in a microscopic view (Fig. 5). As the result of intermixing, the potential profile of the undoped sample is severely altered, while that of the doped sample almost maintains its original profile due to the intermixing inhibition by p-doping. The excess of holes around the QDs leads to two results: more holes residing in the QDs and enhanced capture of holes in the carrier dynamics. Based on the outstanding performance of p-doped QD samples, continuous-wave (CW) ground state lasing has been realized in a 300 μm ultrashort cavity length laser with facet-coating design (Fig. 7). A shorter cavity length can reduce the photon lifetime, which is of great importance to improve the modulation bandwidth of high-speed lasers.

Conclusions In this work, undoped and p-type modulation doping eight-layer QD laser structures with 33 nm GaAs barriers were successfully fabricated using subtle epitaxial growth and careful structure design. By analyzing the PL spectrum, introducing p-doping can inhibit holes’ thermal broadening in their closely spaced energy levels and significantly suppress In/Ga interdiffusion between QDs and their surrounding matrix. Because of the superior features of the modulation p-doped QD materials, CW ground state lasing has been realized in p-doped QD lasers with a short cavity length (400 μm) without facet coatings. To prevent lasing from the first excited state of the QD, careful design and a facet-coating process were fully investigated. Finally, 1.3-μm GS lasing has been found in p-doped InAs/GaAs QD lasers with 300 μm ultrashort cavity length, which shows great potential in high-speed optical communication systems.