Vertical emitting PbTe Lasers and mid infrared luminescence of Eu implanted in Si.

Semiconductor sources emitting in the spectral range between 2m m and 20 m m are important tools for ambient pollution monitoring, ultra fine gas spectroscopy, the analysis of trace gases, and infrared heterodyne experiments in the astronomy. Lead salt lasers are the only semiconductor sources spanning over the whole mid infrared range. For shorter wavelength the advantages of vertical emitting laser devices were demonstrated already since a long time. However, up to now there are no electrical driven vertical emitting mid infrared semiconductor lasers. In this project we plan to fabricate for the first time vertical emitting lead salt laser diodes. An aim of this project is in addition to increase the maximum possible operation temperature of these devices. In contrast to the previous work, lead salt laser devices will be grown on BaF2 substrates. This should give a better mechanical stability and thus a longer lifetime of the lasers devices compared to those grown on PbTe substrates. Furthermore, lead salt micro cavity light emitting diodes will be synthesised. The use of micro cavities allows to achieve high efficiencies of the light emitting device at rather low driving currents. Rare earth ions implanted in semiconductors cause sharp absorption and emission spectra. These sharp lines are due to optical transitions within the closed 4f shell of the ion. In particular Er 3+ ions were used to achieve room temperature 1.5 m m electroluminescence from Si diodes. To achieve emission with longer wavelength we propose to use EU 3+ ions instead of Er. Especially, we want to achieve long wavelength luminescence from a transition taking place below the optical phonon edge of the host semiconductor and thus having an elongated lifetime. The luminescence will be driven by both, by optical excitation and by electrical pumping across a pn-junction.

In this project, we fabricated optoelectronic devices like lasers and detectors of light for the mid infrared spectral range, i. e. wavelengths between 3 and 6 microns. This wavelength range is of special interest due to the many molecular absorption lines in this range, which are used to identify a particular molecule like a fingerprint identifies a human. Such "molecular fingerprints" in the mid infrared range are found, e. g., for carbon dioxide and carbon monoxide, for nitrogen oxides as well as for many volatile substances containing C-H compounds. Many of these molecules are known as pollutants in the atmosphere. Therefore, electro-optic devices for this spectral range are employed not only for chemical analysis of trace gases but also for monitoring of pollutants. Each device developed within this project consists of lead salts, which are semiconductors with favorable properties in the mid infrared spectral range. One of these properties is an especially high refractive index allowing to obtain efficient high-reflectivity mirrors with only a few semiconductor layers with a particular thickness. Between two such mirrors, only a distance comparable to the optical wavelength apart, light can be confined efficiently. Thus one gets a microresonator, which is used for amplification of light in vertical emitting lasers, for improving the efficiency of detectors of light as well as for filtering out particular wavelengths. Within this work, concepts employing such microcavities were applied for the first time to lead salt structures for the technically important mid infrared spectral range. As a starting point, a microcavity with the highest quality factor ever obtained was demonstrated. Based on these microcavities, we demonstrated optically pumped lasers for emission wavelengths between 3 and 6 microns operating in pulsed mode not only at cryogenic temperatures but also at temperatures as high as 60 C. 6 microns is still the longest wavelength for all semiconductor lasers with a vertical resonator. For our laser devices, we used also nanostructures like quantum wells and quantum dots with dimensions of a few millionth millimeters as laser active materials. Especially lead salt quantum dots are of high interest because in contrary to other quantum dots they can be fabricated in a way to form highly ordered artificial dot crystals. Within the project, we also obtained the first optical measurements on these ordered quantum dots, indicating an outstanding homogeneous size distribution of the dots. Furthermore, we demonstrated novel detectors of light based on our lead salt microcavities for the mid infrared, which are sensitive to only one particular wavelength. Therefore, they can be used combination with a broadband light source like a lamp for molecular spectroscopy. The success of our work on lead salt optoelectronic devices for the mid infrared wavelength range is also reflected by our many publications in renowned scientific journals as well as by the independent reports in various periodicals of semiconductor industry. Subsequently, our devices will not only be further improved, but they will also be integrated to form small and compact units for highly sensitive monitoring of pollutants in the atmosphere.