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Edge-emitting electrically pumped room-temperature spin laser

Subject Area Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
Term from 2017 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 392782903
 
Final Report Year 2023

Final Report Abstract

Within the funding period, edge emitting spin laser samples were grown, processed and characterised. Because of the reduced funding duration of only 2 years and due to the pandemic-related delays not all of our goals could be reached. The laser structure is a GaAs/AlGaAs double heterostructure. This was chosen to avoid a preselection of the polarisation of the emission by the semiconductor structure, as it appears, for example in quantum well structures. For spin injection, a 15 nm thick iron layer was implemented, which injects spin polarised electrons through a 3 nm thick MgO tunnel barrier. First, the semiconductor structure was grown in the Wieck group by GaAs MBE. Then the sample was cooled down to room temperature under As flow in order to achieve an As cap layer that is thick enough to protect the surface of the sample of pollution and H2O-contamination when it is exposed to air. After transfer of the sample in air to Duisburg, the sample was introduced into the MBE system at the Wende group and heated up to 300°C in UHV in order to remove the As cap layer and possible contamination on its top by sublimation. Thus, a clean surface was prepared for the following epitaxial steps as verified by RHEED. Then the spin injector was deposited in the Wende group. The processing was done in the Hofmann group in cooperation with the ZHO cleanroom in Duisburg. The process development for definition of the waveguide was challenging because of the unusual n-injection contact with ferromagnetic layer and tunnel contact. After processing of the waveguide, the laser facets have to be cleaved. Because of the large sample thickness, only one clean facet could be cleaved. For future processing, a thinning of the substrate down to about 100 µm is mandatory to achieve also a clean second facet for definition of the laser resonator. Because of the still missing second facet, the sample could only be electrically and optically characterised in LED operation. First, the samples were contacted but due to a thin top gold layer, bonding was not possible. Therefore contacting was done with a tip. Then current-voltage and current-light power characteristics were recorded. They document correct electrical and electro-optical operation of the diode, but a laser thresold cannot be observed because of the missing second laser facet. In the following, the sample was analysed for spin injection. The polarisation was characterised with a Stokes polarimeter consisting of a rotating quarter wave plate and a linear polarizer. We found a small but significant circular polarisation degree of the emission which was current dependent and changed sign with the magnetisation direction of the ferromagnetic contact. Thus, spin injection was clearly confirmed. In conclusion, we can state that concepts for growth and processing of the samples were successfully elaborated while for the realisation of a spin laser the processing still has to be completed by thinning of the substrate for clean cleaving of the two laser facets. The electrooptical characterisation confirms the correct function of the injector and the characterisation with the Stokes polarimeter proves the spin injection.

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