Antonio Drigo

Dissolution kinetics of boron-interstitial clusters in silicon

In this work, we have investigated the stoichiometry of boron-interstitial clusters (BICs) produced in a molecular-beam-epitaxy-grown B box by Si implantation and annealing, and their dissolution during further prolonged annealing cycles. Low-concentration B delta doping was used to quantitatively monitor the interstitial (I) flux. A stoichiometric ratio of about 1.2 between I and B was found for the BICs formed at 815 °C. The BIC dissolution kinetics was investigated by analyzing the concentration profiles at different times and temperatures (in the range 815–950 °C) with a simulation code able to deconvolve the processes of B diffusion and B release from clusters. We found that the main mechanism for cluster dissolution is the release of interstitial boron atoms, with a thermal activation energy of 3.2±0.4 eV. These data are discussed and compared with existing literature data.

Titanium and nickel silicide formation after Q‐switched laser and multiscanning electron beam irradiation

The use of Q‐switched ruby laser and multiscanning electron‐beam annealing to produce the reaction of thin Ti and Ni films deposited onto silicon single crystals has been studied. Rutherford Backscattering (RBS), 16O(d, p)17O* nuclear reaction, scanning electron microscopy (SEM) observation, and x‐ray diffraction were used to characterize the reacted layers. It was found that laser annealing produces a reaction only at the metal‐semiconductor interface: the reacted layers are not uniform in composition and more similar to a mixture than to a well‐defined phase. On the contrary, the silicide layers produced by multiscanning e beam result from the solid‐state reaction of the whole metal film and have a layered structure with well‐defined phase composition and sharp interfaces both between the silicide phases and the underlying semiconductor in Ti/Si system. It was observed that the TiSi2 growth mechanism during e irradiation cannot be explained with the parabolic ’’diffusion controlled’’ mechanism operating...

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 .

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.

Determination of lattice parameter and of N lattice location in InxGa1−xNyAs1−y/GaAs and GaNyAs1−y/GaAs epilayers

We have used an experimental strategy that, combining nuclear reaction analysis and Rutherford backscattering spectrometry both in random and channeling geometry, allowed an accurate quantification of the total amount of N in InxGa1−xNyAs1−y/GaAs and GaNyAs1−y/GaAs epitaxial systems (0.038<x<0.044, 0.015<y<0.045), and a precise localization of nitrogen atoms into the lattice. All N atoms were found on substitutional positions. This information was then exploited to correlate the relaxed lattice parameter of the epilayers obtained by high-resolution x-ray diffraction to the N concentration, by taking into account the elasticity theory, allowing a verification of the validity of Vegard’s rule in the whole range of investigated N concentrations for both alloys. The effect of N incorporation on the lattice parameter has been found to be the same both for ternary and quaternary alloys.

Mechanisms of strain relaxation in III-V semiconductor heterostructures

Abstract The known models describing the breakdown of coherency between layer and substrate in mismatched heterostructures are based on the isotropic elastic continuum approximation. As a matter of fact an internal contribution to the misfit accommodation, that is a deviation from the so-called “virtual crystal approximation”, is expected in ternary or more complex alloy structures. This effect is clearly seen in a set of In x Ga 1− x As / GaAs low misfit samples in the presence of misfit dislocations. The complete structural characterisation including the elastic distortion field and the dislocation density and distribution has been performed by means of Rutherford backscattering based techniques and double crystal X-ray diffraction.

Continuously graded buffers for InGaAs/GaAs structures grown on GaAs

We report on the preparation under optimized conditions and on the study of InGaAs buffers grown on GaAs, intended for MQW structures for 1.3 and 1.5 ~tm optical operation at 300 K; the buffers have linear, square-root and parabolic composition profiles. They were designed so that MQWs grown atop the buffers are virtually unstrained, unlike those prepared following the conventional approach that are under compressive strain. The results obtained by the concomitant use of TEM, HRXRD, AFM and PL show that, by carefully designing the buffers: (i) the misfit dislocation (MD) profiles and thicknesses of the MD-free regions in the buffers can be predetermined, (ii) active structures atop the buffers are virtually unstrained and have efficient 300 K photoluminescence in the 1.3 and 1.5 jxrn windows of photonic interest, (iii) the structures have threading dislocation concentrations in the low 106 cm -2 range and show smooth and symmetric cross-hatchings.

Optimization of the structural and optical properties of ZnS epilayers grown on (100)GaAs by MOVPE

Abstract The crystalline and optical quality of ZnS epilayers grown on (100)GaAs by MOVPE was investigated by channeling-RBS, 10 K photoluminescence and absorption spectroscopy for different growth conditions. The measurements point out that the crystalline and optical quality of the epilayers strongly depends on the VI II precursor vapor phase stoichiometry as well as on GaAs surface treatments before the growth. Optimized MOVPE growth conditions have been determined.

Charge storage and screening of the internal field in GaN/AlGaN quantum wells

The optical spectra of a set of high quality quantum wells, with well characterized structural parameters, are presented here. We propose a quantitative calculation for the measured emission energies which takes into account the spontaneous and piezoelectric polarization fields and the field-screening effect due to electron-hole photogenerated pairs; such an effect is especially effective in wide QW samples which in fact behave like mesoscopic capacitors.

Structural study of (100)CdTe epilayers grown by MOVPE on ZnTe buffered and unbuffered (100)GaAs

Abstract We report on the structural assessment of metalorganic vapour phase epitaxy grown (100)-oriented CdTe epilayers on both (100)GaAs and (100) ZnTe GaAs . Ion channelling Rutherford backscattering spectrometry and cathodoluminescence (CL) measurements are used to study the effect of inserting a proper ZnTe buffer layer [G. Leo et al., J. Vac. Sci. and Technol. B 14 (1996) 1739] between CdTe and GaAs. The insertion of a ZnTe buffer layer improves the surface crystalline and optical quality of the CdTe: CL images show that non-radiative recombination regions, associated with extended defects, strongly decrease when a ZnTe buffer layer is used. Also, enhanced excitonic emissions are observed in the case of CdTe/ZnTe/GaAs samples.