Optimierung Ga(NAsP)-basierender Heterostrukturen auf Si-Substrat für Laseranwendungen

It was the objective of this work, to optimize Ga(NAsP)-Laser structures on Si-substrate and the materials required for their growth. An improvement of optical efficiency as well as laser operation at higher temperatures were expected. Up to now, laser operation for Ga(NAsP)/Si has been limited to temperatures below 160 K, but operation at room-temperature is desired. The inspection of laser processing showed a potential for optimization. An improved procedure for processing yields structures with an enhanced homogeneity of electrical and optical characteristic lines across an entire laser bar. The quality of laser facets was improved by application of an advanced cleaving procedure. The homogeneity of the electrical characteristic lines across a laser bar turned out to be not satisfactory. The contact resistivity suffered from poor reproducibility due to heavy lateral fluctuations in thickness of the (BGa)P layer. An improved etching procedure has been introduced, so that the n-contacts are deposited directly onto silicon and the homogeneity of electrical characteristic lines across a laser bar has been increased. As the determination of composition and growth rate of the active material is not a trivial task, an estimation of the accuracy of the characterization methods available has been attempted. Ga(NAsP) was characterized by means of high resolution x-ray diffraction (XRD) and transmission electron microscopy (TEM). As it is known that material composition and quantum well (QW) thickness both have an influence on the photoluminescence (PL)-signal, a theoretical approach for the prediction of PL for various material composition has been discussed. The error in determination of the material compositions has been estimated. It was possible to quantify the resulting deviations between thickness determination using TEM and XRD. In order to optimize the electrical injection, an extensive characterization of all the conducting layers has been performed. Especially for (BGa)(AsP):Te layers the van-der-Pauw method yielded a surprisingly low conductivity. A significantly positive effect of thermal annealing on conductivity and carrier concentration has been observed on tellurium doped boron containing layers. As an alternative method for determination of layer resistivity, the transfer length method (TLM) was used. The results allowed to estimate the overall electric resistivity of a laser structure. An optimized annealing procedure using a rapid thermal annealer (RTA) has been defined by comparison of a set of different procedures. A clearly positive effect of the RTA procedure in comparison to a procedure using the MOVPE reactor has been observed. Using TEM analysis, it was possible to directly observe annealing induced defects in the active material for the first time. Together with additional characterization results, an diffusion of As out of the Ga(NAsP)-layer has to be assumed. The previously developed approaches for optimization of laser structures were tested using electroluminescence (EL) measurements on appropriate laser structures. There was no evidence for additional radiative loss mechanisms that have a negative influence on current laser structures. It has been demonstrated that an improved annealing procedure boosts the electroluminescence by a factor of 2.6. The voltage U0, which is the minimum voltage required for a current flow through a laser structure, was also subject of investigation. It has been proven that both the thickness as well as the composition of boron containing layers in the structure have a significant influence on U0. By variation of thickness and composition of the (BGa)(AsP) layers, the electrical charac­teristic curve has been substantially improved. U0 has been decreased by up to 7 V and Rdiff has been decreased by up to 2.6 Ω. Compared to non-optimized structures, an enhancement of the optical efficiency by a factor of 1.6 has been demonstrated. This finding gives another evidence for the existence of an additional depletion region within the laser structures, introduced by unexpected doping of a (BGa)(AsP) layer. To eliminate poor carrier confinement as a source of poor optical efficiency under electrical injection, structures with modified hetero-offsets have been developed and characterized. The interpretation of PL and EL spectra provided no evidence for the presence of a poor electrical confinement. Altogether, the electroluminescence of the laser structures under examination has been improved significantly and loss processes have been minimized. The enhancement of the according characterization methods yields reliable statements about the quality of the structures observed. In addition to that, it was possible to give proposals for a further improvement of the electrical characteristic line. Especially the proper n-doping of (BGa)(AsP) could lead to a major improvement of this predicate.