Wide tuning of electronic properties in strained III-V core/shell nanowires

The monolithic integration of III-V semiconductors on Si substrates is a part of a long-term technological roadmap for the semiconductor industry towards More-than-Moore technologies. Despite of the different lattice constants and thermal expansion coefficients, research efforts over the last two decades have shown that III-V crystals with a high structural quality can be grown epitaxially in the form of nanowires directly on Si using CMOS-compatible (Au-free) methods. Among other III-V compounds, InxGa1-xAs is of the special interest for the use in infrared photonics and high-speed electronics due to its tunable direct bandgap and low electron effective mass, respectively. For comparison, InxGa1-xAs thin films are typically grown on lattice-matched InP substrates with a limited range of compositions at around x=0.52. The realization of InxGa1-xAs nanowires on Si, though, has been proved challenging owing to the limited In-content when the nanowires are grown Ga-catalyzed or the high density of stacking faults when the nanowires are grown catalyst-free. In this work, the use of highly lattice-mismatched GaAs/InxGa1-xAs and GaAs/InxAl1-xAs core/shell nanowires on Si(111) substrates have been studied as an alternative to InxGa1-xAs nanowires. The core/shell mismatch strain and its accommodation within the nanowires plays an important role in the growth, the structural, and the electronic properties of the nanowires. A key parameter in this work was the unusually small diameter of 20 – 25 nm of the GaAs core. First, the strain-induced bending of the nanowires during the growth of the shell by molecular beam epitaxy was investigated. It was apparent that the nanowires bend as a result of a preferential incorporation of In adatoms on one side of the nanowires. To obtain straight nanowires with symmetric shell composition and thickness around the core, it was necessary to choose relatively low growth temperatures and high growth rates that limited the surface diffusivity of In adatoms. Second, the strain accommodation in straight nanowires was investigated as a function of the shell thickness and composition using a combination of Raman scattering spectroscopy and X-ray diffraction. For a fixed shell composition of x=0.20 and small enough shell thicknesses, the strain in the shell is compressive and decreases progressively as the shell grows thicker. On the other hand, the strain in the core is tensile with hydrostatic character and increases with shell thickness. Finally, for shell thicknesses larger than 40 nm, the shell becomes strain-free, whereas the strain in the core saturates at 3.2% without any dislocations. For a fixed shell thickness of 80 nm, the strain in the core was further increased by increasing the In-content in the shell, reaching values as high as 7% for x=0.54. A plastic relaxation via misfit dislocations was observed only for the next highest In-content of x=0.70. In agreement to theoretical predictions, the tensile strain in the core resulted in a large reduction of the GaAs bandgap (as measured by photoluminescence spectroscopy), up to approximately 40% of the strain-free value. A similar reduction in electron effective mass is also expected. The transport properties of electrons inside the strained GaAs core were assessed by optical-pump terahertz-probe spectroscopy. Quite high mobility values of approximately 6100 cm2/Vs at 300 K for a carrier concentration of 9×1017 cm−3 were measured, which are the highest reported in the literature for GaAs nanowires, but also higher than the values for unstrained bulk GaAs. The importance of the results in this work is two-fold. On the one hand, strain-free InxGa1-xAs nanowire shells were grown on Si substrates with x up to 0.54 and thicknesses well beyond the critical thickness of their thin film counterparts. Such shells could potentially be employed as conduction channels in high electron mobility transistors (HEMTs) integrated in Si platforms. On the other hand, highly tensile-strained GaAs cores with electronic properties like those of InxGa1-xAs thin films were obtained. In this case, the results demonstrate, that GaAs nanowires can be suitable for photonic devices across the near-infrared range, including telecom photonics at 1.3 and potentially 1.55 μm, as well as for high-speed electronics. GaAs as a binary material is expected to be advantageous compared to InxGa1-xAs due to the absence of structural imperfections typically present in ternary alloys. Finally, to explore the potential of the core/shell nanowires as HEMTs, self-consistent Schrödinger-Poisson calculations of two different modulation-doped heterostructures were performed. In the case of a strained GaAs core overgrown by an unstrained InxGa1-xAs shell and an additional unstrained Si-doped InxAl1-xAs shell, the possibility to form a cylindrical-like two-dimensional electron gas inside the InxGa1-xAs shell was found. In the alternative case of a strained GaAs core overgrown by an unstrained Si-doped InxAl1-xAs shell, it was found that it is possible to form a quasi-one-dimensional electron gas at the center of the core. Both structures are the subject of ongoing research.:1 Introduction 1 2 Fundamentals and state-of-the-art 7 2.1 Electronic and structural properties of III-V semiconductors 7 2.2 Growth of III-V nanowires on Si 20 2.3 Core/shell heterostructure nanowires 29 2.4 Strain in epilayers and core/shell nanowires 36 2.5 Strain engineering in core/shell nanowires and its effect on band parameters 46 2.6 Modulation-doped III-V semiconductor heterostructures 56 3 Methods 61 3.1 Optical and electron microscopes 61 3.2 X-ray diffraction 64 3.3 Raman scattering spectroscopy 65 3.4 Photoluminescence spectroscopy 75 3.5 Optical-pump terahertz-probe spectroscopy and photoconductivity in semiconductors 77 3.6 Device processing 82 3.7 Semiconductor nanodevice software “nextnano” 85 3.8 MBE for crystal growth and core/shell nanowire growth 86 4 Results and discussions 91 4.1 Structural, compositional analyses of straight nanowires and coherent growth limit 91 4.2 Bent nanowires 95 4.3 Strain analyses in core/shell nanowires 97 4.3.1 Dependence of strain on shell thickness 97 4.3.2 Dependence of strain on the shell chemical composition 102 4.3.3 Dependence of strain on the core diameter 105 4.4 Strain-induced modification of electronic properties 106 4.5 Strain-enhanced electron mobility of GaAs nanowires higher than the bulk limit 114 4.6 Towards high electron mobility transistors 123 5 Conclusion and outlook 129 Bibliography 131 List of abbreviations I List of Symbols III List of publications VII List of conference contributions VIII Acknowledgements