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We investigate the material properties of GaAsP graded buffers on both GaP and Si substrates in order to determine limitations to dislocation glide in GaAsP. Phase separation is not observed in these GaAsP buffers, but CuPtB atomic ordering is present, and has an impact on the Burgers vector distribution of gliding dislocations. Tellurium surfactant eliminates ordering in GaAsP, allowing control over the glide plane distribution while highlighting the importance of the growth surface and choice of dopant. The impact of single-variant CuPtB ordering of GaAsP buffers grown on GaP substrates is investigated by monitoring the threading dislocation density throughout the buffer in sequential steps. The dislocation density rises steadily throughout the graded buffer, implying that factors other than ordering-induced glide plane switches play a dominant role in the final dislocation density. We observe an increase in surface roughness throughout the buffer and speculate that a dislocation nucleation source on the surface has a low activation energy and may lead to increased threading dislocation density. On Si substrates, the GaAsP buffer displays dual-variant CuPtB ordering rather than single-variant ordering due to the surface step geometry. We discuss how dual-variant ordering leads to a variable glide force, and thus has a different impact on dislocation dynamics than single-variant ordering. Because the III-V on Si nucleation procedure determines the step geometry, it also influences the dislocation dynamics.

The quality of heterostructures greatly depends on the accurate placement of dopants. However, doping transients remain as drawback in vapor-phase epitaxy. In this work starting transients of doping profiles of Si and Mg in GaN layers grown by low pressure metal-organic vapor-phase epitaxy (MOVPE) on sapphire have been investigated. Here we restrict the investigation to concentrations typically used for electrical or electro-optical devices which are in the range of 2×10 17 cm −3 to 1×10 19 cm −3 for Si and 3×10 19 cm −3 to 1×10 20 cm −3 for Mg. In the case of Mg possible diffusion was excluded by growing the doped layers at a lower growth temperature, far too low for device structures, where the profiles show essentially no sign of Mg back-diffusion. In the case of lower Si concentrations we did not observe diffusion at usual growth temperatures. The profiles were measured by secondary ion mass spectrometry. By comparing the transients from two different reactor types we show and quantitatively explain the influence of adsorption at the internal surfaces of the reactor on the dopant incorporation.

Molecular beam epitaxial (MBE) growth and lasing operation of quaternary GaInAsSb/AlGaAsSb-based optically-pumped vertical-external-cavity surface-emitting lasers (VECSEL) emitting at a wavelength of 2.0 μm are reported. MBE growth of such structures is particularly challenging as it requires, apart from the large total thickness of the epitaxial layer stack of 10–12 μm a change of group-III fluxes for the growth of AlAsSb and low Al-content AlGaAsSb. Two different growth schemes are compared. The first one is the conventional growth procedure in which the wafer remains in the growth chamber and the growth is interrupted at the interfaces before and after the active region to adjust the group-III cell temperatures for different flux settings. At the interfaces where the growth had to be interrupted, secondary-ion mass-spectrometry revealed the unintentional incorporation of In at a concentration equivalent to 1–2 monolayers. Here we introduce a new growth procedure—the sequential growth scheme where each section of the VECSEL is grown separately, and after the growth of a section, the sample is taken out of the growth chamber and stored in the buffer chamber while the group-III cell temperatures are adjusted. After the new group-III fluxes have been stabilized, the wafer is transferred back into the growth chamber and the next section of the laser is grown. This way the unintentional incorporation of In at interfaces between different sections of the VECSEL structure can be avoided. A comparison of nominally identical VECSEL structures grown within the same growth campaign using the two different growth procedures reveals an increase of the maximum output power of nearly 100% for a 2.0 μm emitting VECSEL structure grown with the sequential growth scheme accompanied by an improvement of the optical-to-optical power conversion efficiency from 14.4% to 21.5%.

Molecular beam epitaxial growth and operation of quaternary GaInAsSb/AlGaAsSb-based optically pumped vertical-external-cavity surface-emitting lasers (VECSEL) emitting at wavelengths 2.X mu m are reported here. The epitaxial layer sequence of a VECSEL consists of three different sections: a distributed Bragg reflector (DBR), an active region, and a window layer terminated by a capping layer. The Al content of the AlGaAsSb layers Varies between 100% in the DBR and 30% in the barriers separating the compressively Strained GaInAsSb quantum wells (QW) in the active region. While the active region was grown without growth interruption, growth had to be interrupted at the interfaces between the above-mentioned sections to adjust the group-HI cell temperatures for different flux settings. At the growth interruption interfaces secondary-ion mass-spectrometry revealed the unintentional incorporation of In at a concentration equivalent to 1-2 monolayers. Inspire of the formation of InAsSb-like interfacial layers between the DBR and the active region on the one hand, and the active region and the window layer on the other, high-performance VECSELs emitting in the 2.0-2.3 mu m wavelength range have been grown. A maximum output power of 3.4 W at 2.25 mu m and -10 degrees C heat-sink temperature has been achieved with a maximum optical-to-optical power conversion efficiency of 24%.

The optimization of MBE growth conditions and layer structures for room temperature operation of 2.26 mu m AlGaAsSb/GalnAsSb laser structures is investigated. Index guided triple quantum well large optical cavity diode lasers with 64 mu m x 1000 mu m cavities and high reflection/antireflection coated facets reveal a cw output power of 350 mW at T = 280 K. An internal quantum efficiency eta(i) of 69 %, internal losses chi(i) of 7.7 cm(exp -1) and a threshold current density for infinite cavity length of 144 A/cm2 are obtained for this structure.