In
this Letter we report on the exploration of axial metal/semiconductor (Al/Ge) nanowire
heterostructures with abrupt interfaces. The formation process is
enabled by a thermal induced exchange reaction between the vapor–liquid–solid
grown Ge nanowire and Al contact pads due to the substantially different
diffusion behavior of Ge in Al and vice versa. Temperature-dependent I–V measurements revealed the metallic
properties of the crystalline Al nanowire segments with a maximum
current carrying capacity of about 0.8 MA/cm2. Transmission
electron microscopy (TEM) characterization has confirmed both the
composition and crystalline nature of the pure Al nanowire segments.
A very sharp interface between the ⟨111⟩ oriented Ge
nanowire and the reacted Al part was observed with a Schottky barrier
height of 361 meV. To demonstrate the potential of this approach,
a monolithic Al/Ge/Al heterostructure was used to fabricate a novel
impact ionization device.
To fully exploit
the
potential of semiconducting nanowires for devices, high quality electrical
contacts are of paramount importance. This work presents a detailed
in situ transmission electron microscopy (TEM) study of a very promising
type of NW contact where aluminum metal enters the germanium semiconducting
nanowire to form an extremely abrupt and clean axial metal–semiconductor
interface. We study this solid-state reaction between the aluminum
contact and germanium nanowire in situ in the TEM using two different
local heating methods. Following the reaction interface of the intrusion
of Al in the Ge nanowire shows that at temperatures between 250 and
330 °C the position of the interface as a function of time is
well fitted by a square root function, indicating that the reaction
rate is limited by a diffusion process. Combining both chemical analysis
and electron diffraction we find that the Ge of the nanowire core
is completely exchanged by the entering Al atoms that form a monocrystalline
nanowire with the usual face-centered cubic structure of Al, where
the nanowire dimensions are inherited from the initial Ge nanowire.
Model-based chemical mapping by energy dispersive X-ray spectroscopy
(EDX) characterization reveals the three-dimensional chemical cross-section
of the transformed nanowire with an Al core, surrounded by a thin
pure Ge (∼2 nm), Al2O3 (∼3 nm),
and Ge containing Al2O3 (∼1 nm) layer,
respectively. The presence of Ge containing shells around the Al core
indicates that Ge diffuses back into the metal reservoir by surface
diffusion, which was confirmed by the detection of Ge atoms in the
Al metal line by EDX analysis. Fitting a diffusion equation to the
kinetic data allows the extraction of the diffusion coefficient at
two different temperatures, which shows a good agreement with diffusion
coefficients from literature for self-diffusion of Al.
With increasing interest in GaN based devices, the control and evaluation of doping are becoming more and more important. We have studied the structural and electrical properties of a series of Si-doped GaN nanowires (NWs) grown by molecular beam epitaxy (MBE) with a typical dimension of 2-3 μm in length and 20-200 nm in radius. In particular, high resolution energy dispersive X-ray spectroscopy (EDX) has illustrated a higher Si incorporation in NWs than that in two-dimensional (2D) layers and Si segregation at the edge of the NW with the highest doping. Moreover, direct transport measurements on single NWs have shown a controlled doping with resistivity from 10(2) to 10(-3) Ω·cm, and a carrier concentration from 10(17) to 10(20) cm(-3). Field effect transistor (FET) measurements combined with finite element simulation by NextNano(3) software have put in evidence the high mobility of carriers in the nonintentionally doped (NID) NWs.
Two approaches have been compared for the low temperature epitaxy of thick, partially relaxed GeSn layers on top of Ge strain relaxed buffers. The benefit of using step-graded instead of constant composition layers when targeting really high Sn contents (16%, here) was conclusively demonstrated. Digermane (Ge 2 H 6 ) and tin-tetrachloride (SnCl 4 ) were used as Ge and Sn precursors, respectively. The growth pressure (100 Torr) and the F(Ge 2 H 6 )/F(SnCl 4 ) mass-flow ratio being constant, it was through a temperature lowering that the Sn concentration in the graded structure was increased. X-ray diffraction, atomic force microscopy and transmission electron microscopy were used to gain access to the Sn concentration, the strain state, the surface morphology and thicknesses of the heterostructures. Using a step-graded approach allowed us to gradually relax the strain in the GeSn layers. It helped us obtain high crystalline quality and avoid Sn segregation/precipitation for high Sn contents.
Natural rubbers have extraordinary physical properties, typically the ability to crystallize under tension. Especially, they exhibit a high fatigue resistance. Furthermore, strain-induced crystallization (SIC) is a high thermo-sensitive phenomenon. Better understanding how SIC reinforces fatigue life and how temperature affects this property is therefore a key point to improve the durability of rubbers. The present study investigates temperature effects on the fatigue life reinforcement due to SIC for nonrelaxing loadings. After a brief state of the art that highlights a lack of experimental results in this field, a fatigue test campaign has been defined and was carried out. Results obtained at 23 • C were first described at the macroscopic scale. Both damage modes and number of cycles at crack initiation were mapped in the Haigh diagram. Fatigue damage mechanisms were then investigated at the microscopic scale, where the signature of SIC reinforcement in the crack growth mechanisms has been identified. Typically, fatigue striations,wrenchings and cones peopled the fracture surfaces obtained under non-relaxing loading conditions. At 90 • C, fatigue life reinforcement was still observed. It is lower than at 23 • C. Only one damage mode was observed at the macroscopic scale. At the microscopic scale, fracture surfaces looked like the ones of non-crystallizable rubbers. At 110 • C, the fatigue life reinforcement totally disappeared.
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