Dokument

Summary:
Our current telecommunication is based on the optical transmission of data using semiconductor lasers, which are typically fabricated based on indium phosphide substrates. The layers of material in which the emitted light is generated are typically a few nanometers thick and are therefore referred to as quantum wells. Although this approach enables the fabrication of semiconductor lasers emitting in the technologically important wavelength ranges around 1.3 μm and 1.55 μm, the investigation of alternative concepts is of great interest, because the efficiency of existing semiconductor lasers is limited by non-radiative loss processes. A possible alternative are gallium arsenide-based type-II heterostructures, in which electrons and holes are spatially separated. Therefore, the electronic properties of the two charge carrier species are dominated by different materials and can be adjusted independently of each other, which could enable a systematic reduction of non-radiative loss processes. In order to ensure that the spatial separation of electrons and holes in these systems does not lead to inefficient radiative recombination of the charge carrier species, they are often arranged as a so-called "W"-quantum well heterostructure. In this case, a hole quantum well is embedded in between two electron quantum wells resulting in an increased spatial overlap. The present thesis describes the fabrication of type-II "W"-quantum well heterostructures using metalorganic vapor phase epitaxy and their application as active medium in near-infrared lasers. The gallium arsenide-based (GaIn)As/Ga(AsSb)/(GaIn)As material system serves as a model system. Since any follow-up investigations of type-II heterostructures and lasers are based on the fabrication in a sufficiently high quality, the metalorganic vapor phase epitaxy-based growth of such heterostructures is studied first. The metalorganic compounds triethylgallium (TEGa), trimethylindium (TMIn), tertiarybutylarsine (TBAs), and triethylantimone (TESb) serve as precursors in this study. Due to the large number of possible composition and layer thickness combinations, the indium content, the (GaIn)As layer thicknesses, and the Ga(AsSb) layer thickness were held constant at 20 %, 6 nm, and 4 nm, respectively. Consequently, the antimony concentration remains as the last free parameter allowing for an investigation of the growth conditions of Ga(AsSb). The basis for this approach was a theoretical study in which the material gain of the present structures was optimized. The sample growth was carried out at a growth temperature of 550 °C with V/III gas phase ratios between 2.0 and 7.5. Furthermore, the TESb/V gas phase ratio was varied to determine the maximum achievable antimony concentration and thus, to determine the maximum achievable wavelength range. It was possible to demonstrate high-quality type-II heterostructures with antimony concentrations between 19.3 % and 30.2 % in this study. These concentrations corresponded to photoluminescence peak wavelengths between 1.22 μm and 1.47 μm implying a high spectral flexibility of these heterostructures. The results of the growth study are used to fabricate electrical injection lasers emitting at 1.2 µm. Electroluminescence measurements below laser threshold reveal a blue shift as a function of the injection current density of (93 ± 14) meV/(kA/cm2). This blue shift ends as soon as stimulated emission, which is indicated by a narrowing of the line width as well as a distinct threshold behavior of the laser characteristic, is observed. The evaluation of the laser characteristic yields a threshold current density of 0.4 kA/cm2, an optical efficiency of 0.35 W/A per facet corresponding to a differential efficiency of 66 %, and a maximum pulsed optical output power of 1.4 W, which is limited by the measurement setup. The temperature-dependent characterization of a single and a double “W”-quantum well laser shows that higher order type-II transitions can dominate the emission spectra. Higher order transitions are only observed in case of the single “W”-quantum well laser. This finding highlights that it is important to operate these devices at sufficiently low charge carrier densities. Furthermore, the temperature stability of the threshold current density as well as the differential efficiency are described within the framework of an exponential model. The so-called characteristic temperatures T0 and T1 are used as parameters which allow for an assessment of the temperature stability. The investigation yields characteristic temperatures of T0 = (56 ± 2) K and T1 = (105 ± 6) K in case of the single “W”-quantum well laser and T0 = (60 ± 2) K and T1 = (107 ± 12) K in case of the double “W”-quantum well laser. These relatively low T0 values in combination with the previously described blue shift result in a modification of the temperature-induced shift rate of the emission wavelength. Therefore, it is even possible to demonstrate negative shift rates. This modification can be considered as a fundamental difference with respect to the behavior of type-I lasers. As such, it enables the investigation of novel device concepts and may result in the optimization of existing device concepts. In addition to electrical injection lasers, it was also possible to demonstrate vertical-external-cavity surface-emitting lasers (VECSELs) based on “W”-quantum wells as active medium. These optically pumped devices exhibited a maximum optical output power of 4 W under continuous wave operation conditions. The characteristic blue shift also plays an important role in these devices resulting in the requirement of a positive detuning of the resonator with respect to the emission wavelength at low output powers. While the previously mentioned results are promising and highlight the application potential of type-II heterostructures, an important feature of semiconductor lasers was neglected so far. Their spectral flexibility is in important argument in favor of their application since it is possible to tune the emission wavelength for a certain application. An important wavelength range is the window around 1.3 µm, which is used for telecommunication applications. A theoretical optimization of the “W”-quantum well heterostructure for this emission wavelength range results in the in the successful demonstration of a double “W”-quantum well laser emitting at 1.3 µm. This device can be operated up to temperatures of at least 100 °C and electroluminescence measurements show that laser operation is based on the fundamental type-II transition up to 100 °C. The temperature-dependent characterization yields characteristic temperatures of T0 = (132 ± 3) K und T1 = (109 ± 12) K. Furthermore, a threshold current density of 1.0 kA/cm2, a differential efficiency of 41 %, and a maximum pulsed optical output power of 0.68 W per facet, which is once again limited by the measurement setup, are observed at a temperature of 20 °C.

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