3, Karlsruhe Institute of Technology, Karlsruhe, , Germany
2, University of Siegen, Siegen, , Germany
The integration of III-V semiconductors on silicon is of great interest regarding a possible combination of direct band-gap material systems with the standard CMOS platform. Despite the large lattice mismatch, an epitaxial connection of both materials can be achieved by growing III-V nanowires on Si substrates using e.g. molecular beam epitaxy (MBE). MBE deposition chambers are typically equipped with Reflecting High-Energy Electron Diffraction (RHEED) systems, whose main purpose is in the field of thin film deposition. For a quantitative interpretation of RHEED during NW growth however, the high scattering cross-section and the grazing incidence angle require the consideration of shadowing effects.
We developed an approach for simulating RHEED intensity patterns from polytypic NWs, taking into account the incidence angle and divergence of the electron beam, the number density and shape of the NWs, and the phase distribution of the NW ensemble. In addition the model allows for simulation of the impact of tapering, facet growth and parasitic interstitial growth. As an important effect we considered the occurance of shadowing in the recorded RHEED intensities, i.e. that NWs are not illuminated because of the electron absorption by another NW.
Our approach has been applied for modeling experimental data obtained during the nucleation phase of self-catalyzed GaAs NWs by simultaneous time-resolved in-situ XRD and RHEED. The in-situ experiment has been performed at beamline P09 at DESY, Germany. We show that the phase composition at the NW base is affected by a variation of the Ga-droplets acting as seed particle in the nucleation process.
Due to the different sensitivity of RHEED and XRD to very small objects, the combination of both in-situ methods can be used to acquire a detailed understanding of the processes happening at the onset of NW growth and allows for investigating the crystal phase evolution with high accuracy. This is a fundamental step of monitoring growth and towards the fabrication of defect free NW based devices.