A. M. Mancini

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...

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 .

Preparation and characterization of In2Se3 crystals

Abstract In2Se3 compound has been grown from the melt by using the Bridgman-Stockbarger method. The obtained ingots were polycrystalline with large layered single crystal regions from which samples for physical characterization were cut. Conductivity measurements as a function of temperature were made which indicated the α-β reversible phase transformation in In2Se3 at about 200 ° C.

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...

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...

Metalorganic vapour phase epitaxy growth of ZnS layers by (t-Bu)SH and Me2Zn:Et3N precursors

We report for the first time on the metalorganic vapour phase epitaxy of ZnS layers on (100)GaAs substrates by using tertiary-butyl-mercaptan ((t-Bu)SH) and dimethylzinc : triethylamine adduct (Me2Zn : Et3N) as sulphur and zinc precursors respectively. We demonstrate that the combination of Me2Zn : Et3N with (t-Bu)SH avoids the occurrence of pre-reactions even under atmospheric pressure conditions, allowing the growth of good quality single crystalline ZnS epitaxial layers. However, low pressures (∼ 300 mbar) were necessary to limit the gas phase depletion of the precursors in the reactor chamber, a consequence of the low thermal stability of (t-Bu)SH and Me2Zn : Et3N. ZnS growth rates as high as 2.4 μm/h were thus obtained at around 346°C, the growth process being kinetically limited at lower temperatures. The surface of the epilayers was invariably smooth and mirror-like, its average surface roughness being typically around a few nanometres. Finally, no apparent diffusion of Zn and S into the GaAs substrate was observed for samples grown at 350°C, whereas Ga and As were detected into regions of the ZnS epilayers close to the ZnSGaAs heterointerface.

Investigation of growth mode behavior and surface morphology evolution of metalorganic vapor phase epitaxy grown ZnTe layers on (001) GaAs

A systematic investigation on the mechanisms of nucleation and surface morphology evolution was performed on ZnTe epilayers, deposited on chemically etched GaAs(001) by metalorganic vapor phase epitaxy. A 2D–3D growth mode transition was observed at around two ZnTe equivalent monolyers (ML), which was ascribed to a Stransky–Krastanow growth mode. The 3D growth behavior was correlated to the development of {n11}-type planes, leading to a surface ridging effect along the [110] direction for 4000-ML-thick ZnTe epilayers. The use of a solid-on- solid kinetic roughening model allowed the identification of a mechanism that limits the self- organization of ZnTe nanosized islands, namely, the high density of kink sites found in non- atomically flat GaAs substrates.

Surface structural and morphological characterization of ZnTe epilayers grown on {100} GaAs by MOVPE

The surface structural and morphological characterization of ZnTe epilayers grown on {100}GaAs by atmospheric pressure MOVPE is reported. Scanning electron microscopy (SEM), reflection high energy electron diffraction (RHEED) and reflection electron microscopy (REM) have been performed on the as-grown ZnTe epilayer surfaces. RHEED pattern allowed to estimate the ZnTe crystalline quality, whereas the layer-to-substrate epitaxial relationships have been demonstrated from both RHEED and X-ray diffraction. SEM and REM observations in combination with alpha-step surface profiling measurements allowed to obtain a quantitative description of ZnTe surface irregularities, whose dimensions range from a few nm to just above 30 nm in height, being several hundred nm wide across the surface. Also, the ZnTe surface roughness depends linearly on the epilayer thickness and approaches the initial surface roughness of the GaAs substrates when the thickness tends to zero. Finally, no evident surface anisotropy has been revealed in the present samples, apart from a characteristic (011) elongation effect of some growth defect features at the epilayer surfaces.