Zhongyunshen Zhu

Large negative magnetoresistance in quaternary Heusler alloy Ni50Mn8Fe17Ga25 melt-spun ribbons

Quaternary Heusler alloy Ni50Mn8Fe17Ga25 ribbons have been prepared by the melt-spun method. The ribbons exhibit large negative magnetoresistance (MR){=[R(H)−R(0)]∕R(0)} over a wide temperature region, particularly in the region during the martensitic phase transformation. The MR decreases significantly after annealing. The large MR is isotropic and is mainly attributed to the local magnetic disorders, magnetic clusters, and heterogeneity. The maximum MR at martensitic transformation may be due to the redistribution of electrons and the increase of phase boundary scattering. This feature adds a useful functionality to the already interesting Heusler alloys.

Structural properties of GeSn thin films grown by molecular beam epitaxy

GeSn thin films on Ge (001) with various Sn concentrations from 3.36 to 7.62% were grown by molecular beam epitaxy and characterized. The structural properties were analyzed by reciprocal space mapping in the symmetric (004) and asymmetric (224) planes by high resolution X-ray diffraction (XRD). The lateral correlation length (LCL) and the mosaic spread (MS) were extracted for the epi-layer peaks in the asymmetric (224) diffraction. With the increase of Sn concentration, the LCL reduces while the MS increases, indicating degrading crystalline quality. Dislocations were observed in the sample with 7.62% Sn concentration by transmission electron microscope, consistent with the strain relaxation found in XRD mapping. Besides, the surface morphologies were investigated.

Bi2Te3 photoconductive detectors on Si

The peculiar properties of the gapless surface states with a Dirac cone shaped energy dispersion in topological insulators (TIs) enable promising applications in photodetection with an ultra-broad band and polarization sensitivity. Since many TIs can be easily grown on silicon (Si) substrates, TIs on Si could make an alternative route for photon detection of Si photonics. We present good device performances of a Si-based single-crystal bismuth telluride (Bi2Te3) photoconductive detector. Room temperature photo responses to 1064 nm and 1550 nm light illumination were demonstrated. Linear dependences of the photocurrent on both the incident light power and the bias voltage were observed. The main device parameters including responsivity and quantum efficiency were extracted.

Large anisotropy of spin polarization in Heusler alloy Ni2MnGa induced by martensitic transformation

Spin polarization both in the cubic austenitic and tetragonal martensitic phases of the Ni2MnGa alloy has been investigated using first-principles calculations combined with classical Bloch‐ Boltzmann transport theory. It is shown that the degree of spin polarization, while decreasing from 42% in the 001 directions of the austenitic phase to 30% in the 100 direction of the martensitic phase, rises to 75% in the 001 direction of the martensitic phase, resulting from a preferential reconstruction of the spin-down Fermi surfaces upon martensitic transformation. With this finding, various recent intriguing electrical measurements upon Ni2MnGa across the martensitic transformation can find an explanation. This also opens a way of searching for giant magnetoresistance materials.

Effect of thermal annealing on structural properties of GeSn thin films grown by molecular beam epitaxy

GeSn alloy with 7.68% Sn concentration grown by molecular beam epitaxy has been rapidly annealed at different temperatures from 300°C to 800°C. Surface morphology and roughness annealed below or equal to 500°C for 1 min have no obvious changes, while the strain relaxation rate increasing. When the annealing temperature is above or equal to 600°C, significant changes occur in surface morphology and roughness, and Sn precipitation is observed at 700°C. The structural properties are analyzed by reciprocal space mapping in the symmetric (004) and asymmetric (224) planes by high resolution X-ray diffraction. The lateral correlation length and the mosaic spread are extracted for the epi-layer peaks in the asymmetric (224) diffraction. The most suitable annealing temperature to improve both the GeSn lattice quality and relaxation rate is about 500°C.

InPBi Quantum Dots for Super-Luminescence Diodes

InPBi thin film has shown ultra-broad room temperature photoluminescence, which is promising for applications in super-luminescent diodes (SLDs) but met problems with low light emission efficiency. In this paper, InPBi quantum dot (QD) is proposed to serve as the active material for future InPBi SLDs. The quantum confinement for carriers and reduced spatial size of QD structure can improve light emission efficiently. We employ finite element method to simulate strain distribution inside QDs and use the result as input for calculating electronic properties. We systematically investigate different transitions involving carriers on the band edges and the deep levels as a function of Bi composition and InPBi QD geometry embedded in InAlAs lattice matched to InP. A flat QD shape with a moderate Bi content of a few percent over 3.2% would provide the optimal performance of SLDs with a bright and wide spectrum at a short center wavelength, promising for future optical coherence tomography applications.

Suspended GeSn microstructure for light source on Si

A novel suspended GeSn microstructure is demonstrated by selective etching of GeSn thin film on Ge. XRD and μ-Raman measurements show that the compressive strain in the GeSn thin film is effectively relaxed, and furthermore, unexpected tensile strain was introduced in the suspended GeSn.

GeSn/Ge dual-nanowire heterostructure

A dual-nanowire heterostructure with a GeSn layer laterally laying on Ge nanowires is demonstrated by MBE. The strain field analyzed by FEM shows that the novel proposal can significantly release the compressive strain in GeSn for potential direct bandgap conversion as a Si-based light source.

Theoretical Investigation of Biaxially Tensile-Strained Germanium Nanowires

We theoretically investigate highly tensile-strained Ge nanowires laterally on GaSb. Finite element method has been used to simulate the residual elastic strain in the Ge nanowire. The total energy increment including strain energy, surface energy, and edge energy before and after Ge deposition is calculated in different situations. The result indicates that the Ge nanowire on GaSb is apt to grow along 〈100〉 rather than 〈110〉 in the two situations and prefers to be exposed by {105} facets when deposited a small amount of Ge but to be exposed by {110} when the amount of Ge exceeds a critical value. Furthermore, the conduction band minima in Γ-valley at any position in both situations exhibits lower values than those in L-valley, leading to direct bandgap transition in Ge nanowire. For the valence band, the light hole band maxima at Γ-point is higher than the heavy hole band maxima at any position and even higher than the conduction band minima for the hydrostatic strain more than ∼5.0%, leading to a negative bandgap. In addition, both electron and hole mobility can be enhanced by owing to the decrease of the effective mass under highly tensile strain. The results suggest that biaxially tensile-strained Ge nanowires hold promising properties in device applications.

Vapor-solid-solid grown Ge nanowires at integrated circuit compatible temperature by molecular beam epitaxy

We demonstrate Au-assisted vapor-solid-solid (VSS) growth of Ge nanowires (NWs) by molecular beam epitaxy at the substrate temperature of similar to 180 degrees C, which is compatible with the temperature window for Si-based integrated circuit. Low temperature grown Ge NWs hold a smaller size, similar uniformity, and better fit with Au tips in diameter, in contrast to Ge NWs grown at around or above the eutectic temperature of Au-Ge alloy in the vapor-liquid-solid (VLS) growth. Six growth orientations were observed on Ge (110) by the VSS growth at similar to 180 degrees C, differing from only one vertical growth direction of Ge NWs by the VLS growth at a high temperature. The evolution of NWs dimension and morphology from the VLS growth to the VSS growth is qualitatively explained by analyzing the mechanism of the two growth modes.