Atomically uniform Sn-rich GeSn semiconductors with 3.0-3.5 μ m room-temperature optical emission

\u3cp\u3eThe simultaneous control of lattice strain, composition, and microstructure is crucial to establish high-quality, direct bandgap GeSn semiconductors. Herein, we demonstrate that multilayer growth with a gradual increase in composition is an effective process to minimize bulk and surface segregation and eliminate phase separation during epitaxy yielding a uniform Sn incorporation up to ∼18 at. %. Detailed atomistic studies using atom probe tomography reveal the presence of abrupt interfaces between monocrystalline GeSn layers with interfacial widths in the 1.5-2.5 nm range. Statistical analyses of 3-D atom-by-atom maps confirmed the absence of Sn precipitates and short-range atomic ordering. Despite the residual compressive strain of -1.3 %, the grown layers show clear room-temperature photoluminescence in the 3.0-3.5 μm wavelength range originating from the upper GeSn layer with the highest Sn content. This finding lays the groundwork to develop silicon-compatible mid-infrared photonic devices.\u3c/p\u3

Direct band bap wurtzite semiconductor nanowires

Growth of GaP and III-V GaP alloys in the wurtzite crystal structure by vapor phase epitaxy (VPE) is provided. Such material has a direct band gap and is therefore much more useful for optoelectronic devices than conventional GaP and GaP alloys having the zincblende crystal structure and having an indirect band gap

High refractive index in wurtzite GaP measured from Fabry-Pérot resonances

\u3cp\u3eWe investigate the optical emission of wurtzite GaP/Al\u3csub\u3e0.4\u3c/sub\u3eGa\u3csub\u3e0.6\u3c/sub\u3eP core/shell nanowires (NWs) transferred to a SiO\u3csub\u3ex\u3c/sub\u3e substrate to demonstrate a high degree of waveguiding of the emitted photoluminescence (PL) signal. By analysing the Fabry-Pérot mode spacing in combination with calculations of the guided modes in the NWs, we calculate a very high refractive index of bulk WZ GaP of 4.2 at a wavelength of 600 nm. The measured quality factors up to 600 indicate the excellent optical quality of the nanowire resonator.\u3c/p\u3

Recrystallization and interdiffusion processes in laser-annealed strain-relaxed metastable Ge0.89Sn0.11

The prospect of GeSn semiconductors for silicon-integrated infrared optoelectronics brings new challenges related to the metastability of this class of materials. As a matter of fact, maintaining a reduced thermal budget throughout all processing steps of GeSn devices is essential to avoid possible material degradation. This constraint is exacerbated by the need for higher Sn contents along with an enhanced strain relaxation to achieve efficient mid-infrared devices. Herein, as a low thermal budget solution for post-epitaxy processing, we elucidate the effects of laser thermal annealing (LTA) on strain-relaxed Ge$_{0.89}$Sn0$_{.11}$ layers and Ni-Ge$_{0.89}$Sn0$_{.11}$ contacts. Key diffusion and recrystallization processes are proposed and discussed in the light of systematic microstructural studies. LTA treatment at a fluence of 0.40 J/cm2 results in a 200-300 nm-thick layer where Sn atoms segregate toward the surface and in the formation of Sn-rich columnar structures in the LTA-affected region. These structures are reminiscent to those observed in the dislocation-assisted pipe-diffusion mechanism, while the buried GeSn layers remain intact. Moreover, by tailoring the LTA fluence, the contact resistance can be reduced without triggering phase separation across the whole GeSn multi-layer stacking. Indeed, a one order of magnitude decrease in the Ni-based specific contact resistance was obtained at the highest LTA fluence, thus confirming the potential of this method for the functionalization of direct bandgap GeSn materials

Atom-by-atom analysis of semiconductor nanowires with parts per million sensitivity

\u3cp\u3eThe functionality of semiconductor devices is determined by the incorporation of dopants at concentrations down to the parts per million (ppm) level and below. Optimization of intentional and unintentional impurity doping relies on methods to detect and map the level of impurities. Detecting such low concentrations of impurities in nanostructures is however challenging to date as on the one hand methods used for macroscopic samples cannot be applied due to the inherent small volumes or faceted surfaces and on the other hand conventional microscopic analysis techniques are not sufficiently sensitive. Here, we show that we can detect and map impurities at the ppm level in semiconductor nanowires using atom probe tomography. We develop a method applicable to a wide variety of nanowires relevant for electronic and optical devices. We expect that it will contribute significantly to the further optimization of the synthesis of nanowires, nanostructures and devices based on these structures.\u3c/p\u3

Cracking the Si shell growth in hexagonal GaP-Si core-shell nanowires

\u3cp\u3eSemiconductor nanowires have increased the palette of possible heterostructures thanks to their more effective strain relaxation. Among these, core-shell heterostructures are much more sensitive to strain than axial ones. It is now accepted that the formation of misfit dislocations depends both on the lattice mismatch and relative dimensions of the core and the shell. Here, we show for the first time the existence of a new kind of defect in core-shell nanowires: cracks. These defects do not originate from a lattice mismatch (we demonstrate their appearance in an essentially zero-mismatch system) but from the thermal history during the growth of the nanowires. Crack defects lead to the development of secondary defects, such as type-I\u3csub\u3e1\u3c/sub\u3e stacking faults and Frank-type dislocations. These results provide crucial information with important implications for the optimized synthesis of nanowire-based core-shell heterostructures.\u3c/p\u3

Direct band gap wurtzite GaP nanowires for LEDs and quantum devices

\u3cp\u3eCommercially available light-emitting diodes (LEDs) suffer from low-efficiency in the green region of the visible spectrum. In order to solve this issue III-V materials such as Gallium phosphide (GaP) can be investigated. GaP in the zinc blende (ZB) crystal structure has an indirect band gap, limiting the efficiency of the green emission. However, when the material is grown with wurtzite (WZ) crystal phase a direct band gap is predicted. Here, we show the fabrication and the characterization of wurtzite GaP nanowires, together with the demonstration of the direct band gap. The strong photoluminescence signal observed at 594 nm with a lifetime in the order of 1ns matches with the expectation for a direct band gap material. Furthermore, the emission wavelength can be tuned across a wide range of the visible spectrum (555-690 nm) by incorporating aluminum or arsenic in the WZ GaP nanowires.\u3c/p\u3

Pseudodirect to direct compositional crossover in wurtzite GaP/InxGa1-xP core-shell nanowires

Thanks to their uniqueness, nanowires allow the realization of novel semiconductor crystal structures with yet unexplored properties, which can be key to overcome current technological limits. Here we develop the growth of wurtzite GaP/In(x)G(x)G(1-x) core shell nanowires with tunable indium concentration and optical emission in the visible region from 590 nm (2.1 eV) to 760 nm (1.6 eV). We demonstrate pseudodirect (Gamma(8c)-Gamma(9v)) to direct ((Gamma 7c-Gamma 9v)) transition crossover through experimental and theoretical approach. Time resolved and temperature dependent photoluminescence measurements were used, which led to the observation of a steep change in carrier lifetime and temperature dependence by respectively one and 3 orders of magnitude in the range 0.28 +/- 0.04 <= 0.41 +/- 0.04. Our work reveals the electronic properties of wurtzite In(x)G(1-x)P

New opportunities with nanowires

Summary form only given. Light emission from Si, would allow integration of electronic and optical functionality in the main electronics platform technology, but this has been impossible due to the indirect band gap of Si. This talk will discuss 2 different approaches, using unique properties of nanowires, to realize light emission from Si-based compounds. In the first route, the paper focuses on the fabrication of defect-free GeSn compounds. The growth mechanism is discussed, the structural properties are investigated by electron microscopy and atom probe tomography and the temperature dependent optical properties are studied. The second route concentrates on Si and Ge with a different crystal structure. Here, crystal structure transfer is employed, in which wurtzite GaP is used as a template to epitaxially grow SiGe compounds with the hexagonal crystal structure. With this method, defect free hexagonal SiGe shells and branches with tunable Ge concentration are gorwn. The structural and optical properties of these new crystal phases will be discussed. The author believes that these new 3-dimensional epitaxial nanostructures have great potential to integrate optical functionality in Si technology

Impurity and defect monitoring in hexagonal Si and SiGe nanocrystals

Silicon-Germanium in a hexagonal crystal-structure is a candidate material for a direct band-gap group IV semiconductor that can be integrated into the CMOS process. It has recently been synthesized as a crystalline shell grown epitaxial around a nanowire core of hexagonal Gallium-Phosphide. In order to study the optical properties of this newly generated material and evaluate its potential for building optical devices it is necessary to grow defect and impurity free hexagonal Silicon-Germanium. Impurity detection and mapping in nano-structures is however challenging as most bulk and thin film characterization methods cannot be used. Here we show that Atom Probe Tomography can be used to map the impurities in hexagonal shells of Silicon-Germanium and Silicon. This will allow to optimize growth of hexagonal Silicon-Germanium nanocrystals towards impurity free, optically active crystals