High-Power Dilute Nitride Lasers Grown by Molecular Beam Epitaxy

Semiconductor lasers are the most widely used type of lasers. This is due to many beneficial properties including compact size, wavelength coverage, and high efficiency. Different semiconductor laser architectures and gain materials can be used to fulfill requirements of different applications. Semiconductor gain materials are easy to tune to emit at desired wavelengths by changing the composition of the material and they can cover a wide range of wavelengths from ultra-violet to mid-infrared. Still, there are some important gaps in the wavelength coverage. Two of these gaps are located at ~600 nm and ~1200 nm, i.e. just below and above the wavelength coverage of traditional GaAs-based semiconductors. Especially the yellow–red (580–620 nm) part of the visible spectrum is important for applications in the fields of medicine, spectroscopy, astronomy and laser projection.This work targeted to cover both of the mentioned wavelength gaps by using dilute nitride GaInNAsSb/GaAs quantum well gain material in novel high-power lasers. This thesis discusses especially the fabrication of the dilute nitride gain materials using plasma-assisted molecular beam epitaxy. Incorporating few percent of nitrogen into InGaAs/GaAs QWs can increase the upper wavelength limit of GaAs-based semiconductors up to 1550 nm by reducing band gap and lattice strain. Using this dilute nitride material system, we fabricated the first multi-watt semiconductor disk lasers (SDLs) emitting at 1180 nm and 1230 nm. The output powers exceeded 10 W at both wavelengths. Although frequency doubling is out of the scope of this thesis, it should be mentioned that these lasers emitted multi-watt powers also at the corresponding frequency doubled wavelengths of 590 nm and 615 nm. In addition, this thesis reports a GaInNAsSb/GaAs SDL emitting at 1550 nm, which is the longest wavelength demonstrated for a monolithic GaAs-based SDL.SDLs, unlike other semiconductor lasers, can emit high-powers (up to 100 W) in nearly diffraction-limited beams and can be efficiently frequency doubled. However, not all applications require multi-watt output powers but would rather benefit from smaller size of the laser source. For this reason we studied also another laser architecture, namely edge-emitting laser diodes. A single-mode laser with record-high output power of 340 mW at 1180 nm, corresponding to yellow (590 nm) frequency-doubled wavelength, was demonstrated. The laser showed also excellent temperature stability, which is important for miniaturization of frequency-doubled lasers.The laser demonstrations could not have been realized without good understanding of the basic properties of the GaInNAs(Sb) gain material and its fabrication. Studies related to these aspects and to calibration of PA-MBE reactors form an important part of this thesis. Especially, effects of growth temperature and As/III beam equivalent pressure ratio on the grown semiconductor structures were studied.In summary, this work is concerned with plasma-assisted molecular beam epitaxy of GaInNAsSb/GaAs gain materials. The fabricated materials were used in novel lasers emitting at wide range of technologically important wavelengths that are difficult to reach otherwise.