Metalorganic chemical vapor deposition of high performance GaAs based quantum dot lasers

In this work, Metalorganic Chemical Vapor Deposition (MOCVD) of novel GaAs-based semiconductor laser structures with self-organized In-GaAs/GaAs Stranski-Krastanow quantum dots (QDs) as active medium was advanced with regard to the laser characteristics. The three-dimensional morphology of self-organized QDs leads to a significant roughening of thin cap layers on top of QD sheets. Smoother QD cap layers are required, however, to reduce the distance between stacked QD layers and thus to increase the QD volume density for larger modal gain of QD lasers. Hence, the growth of QD lasers was complemented by an in-situ annealing step flattening such corrugated surfaces. The strain of lattice-mismatched selforganized QDs and the untypically low QD deposition temperatures around 500°C lead to dislocations and point defects in QD heterostructures. The density of such defects was strongly reduced by in-situ annealing. Lasers with in-situ annealed QDs exhibit room-temperature transparency current densities around 6 A/cm2 per QD sheet at emission wavelengths between 1.14 and 1.16 µm. The internal quantum efficiency was increased to beyond 90 %. Lasers based on 6-fold stacks of such in-situ annealed QDs show room-temperature peak output powers of 11.7 W in quasi-continuous-wave mode and 4.7 W under continuous-wave operation. This was the first demonstration of optical output powers of QD lasers beyond 10 W. The characteristics of such QD lasers did not exhibit significant changes during lifetime measurements of more than 3000 h at 50°C and output powers of 1.0 - 1.5 W. Arsine, widely used as arsenic precursor in MOCVD, is strongly toxic and was therefore replaced in the course of this work by the alternative precursor tertiarybutylarsine (TBAs). The growth of QDs had to be recalibrated as the physical and chemical properties of TBAs differ from those of arsine. The worldwide first QD laser grown using alternative-precursor MOCVD could be demonstrated. Different techniques to grow QDs emitting at the commercially important data communication wavelength of 1.3 µm were developed and evaluated. Such QD structures were investigated using photoluminescence spectroscopy and transmission electron microscopy. Using InGaAs QDs overgrown with gallium-rich InGaAs quantum films, the room-temperature lasing wavelength could be extended to 1.24 µm. The growth of laser structures for the fabrication of QD-based vertical-cavity surface emitting lasers (VCSELs) with Al(Ga)Ox/GaAs oxide mirrors and nine-fold stacks of InGaAs/GaAs QDs as active region was implemented. A VCSEL with a 3.5 µm aperture and four top-DBR pairs exhibited a maximum output power of 0.68 mW at 1.1 µm, a threshold current of 280 µA, and a differential efficiency of 43 %.