Cross-Sectional Functional Scanning Probe Microscopy for In Situ and Post-Mortem 3D Mapping of Nanoscale Physical Properties of Internal Structure of Advanced Optoelectronic Devices

Modern optoelectronic devices are complex multilayer semiconductor structures with nanoscale features shaping charge transport to the active area, serving as an optical waveguides and providing heat management in the active devices. To enhance and tune the device performance, quantum structures such as quantum dots (QDs) and quantum wells (QWs) are often used, that are typically on the length scale from few nm to few hundreds of nanometres. In most cases, these are buried in the internal three-dimensional (3D) structure of the device, making nanoscale characterisation of these, especially when these operating extremely challenging. To study these structures via transmission electron microscopy (TEM) and focused ion beam (FIB) sectioning is a laborious and slow task, with material and the device properties affected by the FIB milling process. Moreover, the TEM investigates only a limited area of a few micrometres across at best, that is often insufficient to uncover sparse but highly detrimental defects. Finally, TEM cannot access vital physical properties of device structure such as mechanical moduli, workfunction or thermal conductivity.<br/><br/>Here we present a new concept for fast and efficient 3D characterisation of optoelectronic materials and devices via nano-cross-sectioning multifunctional scanning probe microscopy (xSPM) that can explore devices both <i>post-mortem</i> as well as <i>in-situ</i>. This approach uses sample sectioning and nano-polishing via variable energy Ar ion beam targeted at the edge of the sample to create a perfectly flat oblique flat section with sub-nm surface roughness of all layers of interest. Inert nature of Ar and glancing angle of polishing material results in near zero-damage, whereas open angle geometry makes it perfectly suitable for the material sensitive SPM study¹. The xSPM then reveals 3D morphology, composition, strain and crystalline quality via local physical properties mechanical and piezoelectric moduli, nanoscale heat conductance, workfunction and electrical conductivity. We apply this approach to explore the internal nanoscale structure of the vertical cavity surface emitting laser (VCSEL) devices² using material sensitive SPM to reveal the internal structure of the VCSEL across the whole stack of top and bottom distributed Bragg reflector mirrors (DBR) including MQW active area. We report for the first time the direct observation of local mechanical properties, electric potential and conductance through the energised 3D VCSEL stack. We used three different SPM measurement modes – nanomechanical local elastic moduli mapping via Ultrasonic Force Microscopy (UFM), surface potential mapping via Kelvin Probe Force Microscopy (KPFM) and mapping of injected current (local conductivity) via Scanning Spreading Resistance Microscopy (SSRM). By applying forward bias to the VCSEL to initiate laser emission, we were able to observe distribution of the potential in the working regime, paving the way to understanding the 3D current flow in the complete device. Finally, we use finite element modelling (FEM) that confirm the experimental results that of the measurements of the local doping profiles and charge distribution in the active area of the VCSEL around the oxide current confinement aperture. The new<i> in-situ </i>xSPM methodology allows advanced <i>in-situ</i> platform for highly efficient characterisation platform for a broad area of semiconductor materials and devices.<br/><br/>email: [email protected], [email protected]