N. Lovergine

Picosecond response times in GaAs/AlGaAs core/shell nanowire-based photodetectors

High-speed metal-semiconductor-metal (MSM) photodetectors based on Schottky-contacted core/shell GaAs/AlGaAs and bare GaAs nanowires were fabricated and characterized. The measured core/shell temporal response has a ∼10 ps full-width at half-maximum and an estimated corrected value less than 5 ps. The bare GaAs devices exhibit a slower response (∼35 ps) along with a slow decaying persistent photocurrent (∼80 s). The core/shell devices exhibit significantly improved dc and high-speed performance over bare nanowires and comparable performance to planar MSM photodetectors. The picosecond temporal response, coupled with picoampere dark current, demonstrate the potential for core/shell nanowires in high-speed imaging arrays and on-chip optical interconnects.

Size and shape control of GaAs nanowires grown by metalorganic vapor phase epitaxy using tertiarybutylarsine

Au-catalyzed self-assembly of GaAs nanowires on (1¯1¯1¯)B GaAs by metalorganic vapor phase epitaxy is reported between 375 and 500°C, using tertiarybutylarsine and trimethylgallium in H2. The nanowires are [1¯1¯1¯]B aligned and kink-free. Below 425°C the nanowires have narrow base diameter distributions, closely matching the size (∼60nm) of the Au nanoparticles at their tip (no tapering). Above 425°C the nanowires show a hexagonal-based pyramidal shape with base edges normal to the ⟨2¯11⟩ in-plane substrate directions and base diameters which increase exponentially with temperature, indicating a kinetics limited growth along the nanowire sidewalls. Activation energies in the range of ∼20–23kcal∕mol were estimated for growth along both the sidewalls and the [1¯1¯1¯]B direction.

Growth of ZnTe by metalorganic vapor phase epitaxy: Surface adsorption reactions, precursor stoichiometry effects, and optical studies

The metalorganic vapor phase epitaxy growth of ZnTe by di-isopropyl-telluride and di-methyl-zinc (Me2Zn) precursors is investigated by studying the epilayer growth rate as a function of both growth temperature and precursor transport rates. The ZnTe growth is a thermally activated process involving the heterogeneous pyrolysis of both Zn and Te alkyls onto the ZnTe surface. The growth rate dependence on growth conditions is explained in terms of surface adsorption-desorption reactions, assuming that the incorporation of Zn and Te atoms into ZnTe takes place through their selective adsorption on different surface lattice sites. There is also evidence that the occurrence of a competitive species for the surface adsorption of Zn atoms, which is identified as the CH3⋅ (methyl) radical, is produced by the pyrolysis of Me2Zn. Photoluminescence (PL) and absorption measurements performed on ZnTe allow to identify two new donor-acceptor pair (DAP) bands, originated from the recombination of a Ga donor with two acce...

Electron holographic tomography for mapping the three-dimensional distribution of electrostatic potential in III-V semiconductor nanowires

Electron holographic tomography (EHT), the combination of off-axis electron holography with electron tomography, is a technique, which can be applied to the quantitative 3-dimensional (3D) mapping of electrostatic potential at the nanoscale. Here, we show the results obtained in the EHT investigation of GaAs and GaAs-AlGaAs core-shell nanowires grown by Au-catalysed metalorganic vapor phase epitaxy. The unique ability of EHT of disentangling the materials mean inner potential (MIP) from the specimen projected thickness allows reconstruction of the nanowire 3D morphology and inner compositional structure as well as the measurement of the MIP.

MOVPE growth of wide band-gap II-VI compounds for near-UV and deep-blue light emitting devices

We report on the growth by metalorganic vapour phase epitaxy of high structural and optical quality ZnS, ZnSe and ZnS/ZnSe multiple quantum well (MQW) based heterostructures for applications to laser diodes operating in the 400 nm spectral region. High purity t BuSH, t Bu 2 Se and the adduct Me 2 Zn : Et 3 N were used as precursors of S, Se and Zn, respectively. The effect of the different MOVPE growth parameters on the growth rates and structural properties of the epilayers is reported, showing that the crystallinity of both ZnS and ZnSe is limited by the kinetics of the incorporation of Zn, S and Se species at the growing surface. Very good structural and optical quality ZnS and ZnSe epilayers are obtained under optimized growth conditions, for which also dominant (excitonic) band-edge emissions are reported. The excellent ZnS and ZnSe obtained by our MOVPE growth matches the stringent requirements needed to achieve high quality ZnS/ZnSe MQWs. Their structural properties under optimized MOVPE conditions are shown to be limited mostly by the formation of microtwins, a result of the intrinsic high lattice mismatch involved into the ZnS/ZnSe heterostructure. Despite the large amount of defects found, the optical quality of the MWQs turned out to be high, which made possible the full characterization of their electronic and lasing properties. In particular, photopumped lasing emission up to 50 K in the 3.0 eV energy region are reported for the present MQWs heterostructures under power excitation density above 100 kW/cm 2 .

Inhomogeneous Strain Relaxation and Defect Distribution of Znte Layers Deposited on (100)Gaas by Metalorganic Vapor-Phase Epitaxy

The structural characterization of ZnTe epilayers grown on (100)GaAs by metalorganic vapor‐phase epitaxy is reported. A detailed study of the ZnTe/GaAs heterostructure based on both high‐resolution and conventional electron microscopy and ion channeling Rutherford backscattering spectrometry allows correlation of the type and spatial distribution of the extended defects occurring at or close to the ZnTe/GaAs interface with the amount of residual lattice strain into the ZnTe epilayers. Both pure edge Lomer and 60°‐mixed misfit dislocations were identified at the interface along with partial dislocations bounding stacking faults, their overall density and distance distribution indicating the occurrence of a residual compressive strain at the heterostructure interface. By comparing this interface strain to the corresponding surface value of the same samples the occurrence of an inhomogeneous strain relaxation along the growth direction is clearly demonstrated. It is shown that such a strain gradient should b...

Polarization anisotropy of individual core/shell GaAs/AlGaAs nanowires by photocurrent spectroscopy

We investigate the photodetection properties of individual core/shell GaAs/AlGaAs nanowires (NWs) and, in particular, their behavior under linearly polarized light. The NWs are grown by Au-assisted metalorganic vapor phase epitaxy and electrical contacts are defined on NWs by electron beam induced deposition. The spectral photocurrent of the single NW is measured and the dependence of the polarization anisotropy ρ (varying from ∼0.1 to ∼0.55) on the absorption wavelength is found to be clearly affected by the core/shell structure. High quantum efficiency values (10% at 600 nm) are obtained which are attractive for a wide range of optoelectronic devices.

On direct-writing methods for electrically contacting GaAs and Ge nanowire devices

The electronic transport and gating characteristics in GaAs and Ge nanowires (NWs) are altered significantly following either indirect or direct exposure to a focused Ga+ ion beam (FIB), such as that used to produce Pt electrical contacts to NWs. While these results challenge the assumptions made in some previously reported work relating to the electronic properties of semiconductor NWs using FIB-assisted production of contacts and/or their leads, local electron beam induced deposition is shown to be a reliable and facile route for producing robust electrical contacts to individual vapor phase-grown NWs in a manner that enables study of their actual carrier transport properties.

Determination of surface lattice strain in ZnTe epilayers on (100)GaAs by ion channeling and reflectance spectroscopy

We report on the direct measurements of surface lattice strain in ZnTe epitaxial layers on {100}GaAs substrates by ion channeling Rutherford backscattering spectrometry and low‐temperature (10 K) reflectance spectroscopy measurements. The measured ZnTe strain is the superposition of the expected thermal (tensile) strain and a thickness‐dependent residual compressive strain. Our data indicate that the removal of this residual strain is slower than the rate predicted by the equilibrium theory, following an apparent h−1/2 power‐law dependence on the epilayer thickness h, above ∼100 nm.

Influence of a ZnTe buffer layer on the structural quality of CdTe epilayers grown on (100)GaAs by metalorganic vapor phase epitaxy

The influence of a ZnTe buffer layer on the structural quality of CdTe epilayers grown on (100)GaAs substrates by metalorganic vapor phase epitaxy has been investigated by both x‐ray diffraction and ion channeling Rutherford backscattering spectrometry measurements. Single‐crystal (100) oriented CdTe epilayers of good structural quality have been obtained after inserting a ZnTe buffer layer of a thickness ranging between 300 and 500 nm. The influence of the buffer layer thickness on the crystalline quality and the morphology of the CdTe epilayer has been related to the defect distribution and the surface roughness of the ZnTe buffer layer. The crystalline quality and the surface strain have been thus studied as a function of the CdTe thickness on samples having optimal ZnTe layer thickness. The initial compressive mismatch between CdTe and ZnTe, f=−5.8%, appears to be almost fully relaxed for a CdTe thickness around 200 nm. A residual compressive in‐plane strain (about −0.02%), independent of the CdTe epi...