M. Berti

Lattice parameter in Si1-yCy epilayers: deviation from Vegard's rule

The precise C content of a series of Si1−yCy epilayer samples (0<y<0.012) was determined by resonant backscattering experiments using a 4He+ ion beam at 5.72 MeV. This beam energy is more suitable for the determination of the C content than the previously used 4.265 MeV. From the correlation of these investigations with x-ray diffraction experiments, a significant deviation of the lattice parameter variation in Si1−yCy from Vegard’s rule between Si and diamond or β-SiC was observed, which amounts up to 30% or 13%, respectively, for y<0.012. This negative deviation is in agreement with recent theoretical predictions by Kelires.

Growth and structure of titanium silicide phases formed by thin Ti films on Si crystals

The silicide formation with a titanium film deposited on 〈100〉 single‐crystal silicon, has been studied by using nuclear microanalysis, x‐ray diffraction, and transmission electron microscopy. The presence of interfacial defects and their possible role in the early stages of the reaction has been evidenced. The phase composition was dependent on the annealing temperature and time: at 550 °C only TiSi2 is observed; at higher temperatures (>600 °C), a thin TiSi2 layer at the interface is again observed, but Ti‐rich silicides grow on top of this layer by increasing the annealing time. For longer annealing times, all the reacted layer progressively transforms into TiSi2. The amount of reacted silicon grows with a (time)1/2 law; the activation energy of 1.8 eV reported for the growth of TiSi2 onto amorphous Si may be appropriate even in this case. The reaction proceeds at a rate of one order of magnitude higher than previously reported for reaction between silicon and an oxygen saturated titanium film. The kin...

Hydrogen-nitrogen complexes in dilute nitride alloys: Origin of the compressive lattice strain

Hydrogenation of GaAs1−xNx and GaP1−xNx epilayers grown on GaAs(001) and GaP(001) surfaces, respectively, is known to passivate the electronic activity of nitrogen through the formation of specific nitrogen-hydrogen complexes. The same epilayers also undergo a strain reversal from tensile (as grown) to compressive (fully hydrogenated). The authors show that the extent of strain reversal is determined exclusively by the nitrogen concentration. By performing in situ high resolution x-ray diffraction measurements during annealing and photoluminescence studies, the authors demonstrate that the lattice properties of fully hydrogenated GaAs1−xNx are ruled by a H complex, which is different and less stable than that responsible for electronic passivation of nitrogen in GaAs1−xNx.

Experimental evidence of two-dimensional–three-dimensional transition in the Stranski–Krastanow coherent growth

The two-dimensional (2D)–three-dimensional (3D) transition has been studied for InAs and InP layers grown on (001) GaAs substrates by metalorganic vapor pressure epitaxy at different growth temperatures and growth rates. The combined use of scanning force microscopy and of Rutherford backscattering spectrometry allowed the determination of the surface morphology of the samples and of their equivalent layer thickness with a precision better than 0.1 monolayers (ML). The critical thickness for the 2D–3D transition has been found to occur after a deposition of (1.2±0.1) ML for InAs/GaAs and after a deposition of (2.2±0.2) ML for InP/GaAs, independently of the temperature and of the growth rate. For coverages slightly exceeding 1 ML, for InAs, and 2 ML, for InP, the surface of the samples appears partially covered by small bi-dimensional structures (platelets). The dimensions of the platelets appear to be independent of the system under study and consequently of the misfit. Moreover, their distribution on the...

Mechanism of ion induced amorphization

By comparing results on ion induced amorphization of metals and metal alloys at liquid nitrogen temperature it is possible to separate the effects of atomic collisions from the effects due to the compositional changes. The crystal to amorphous transition in ion-implanted metals can be interpreted by a statistical model in which an amorphous cluster develops when a given concentration is reached inside a small critical volume ν c . The same transition occurs at much lower fluences in the case of metal alloys, demonstrating the overwhelming influence of chemical short range order. Quasi-universal amorphization curves are then obtained as a function of the concentration of displacement collisions whatever the incoming ion mass is. It is hence reasonable to assume that an amorphous cluster is formed when the number of displaced atoms inside a given volume ν c exceeds a critical value. Experimental results indicate that the size of ν c for both type of experiments corresponds to that over which structural correlations exist in a glass.

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.

Measurement of aluminum concentration in the Ga1−xAlxSb/GaSb epitaxial system

The composition of several Ga1−xAlxSb epitaxial layers of different thicknesses grown by molecular beam epitaxy on GaSb with x ranging between 0.1 and 0.8, has been obtained independently by high resolution x-ray diffraction, Rutherford backscattering spectrometry, and reflection high-energy electron diffraction. From the comparison between the results obtained by the different experimental methods, it has been possible to point out that the lattice constant of the layer increases nonlinearly with the Al content. A comparison with theoretical models has been done. A phenomenological equation has been derived for a correct analysis of the x-ray results.

Pulsed laser irradiation of GaAs under oxygen and silane atmosphere: Incorporation, losses, influence of native oxide

Gallium arsenide single crystals and thin GaAs amorphous deposits (in the 100‐nm range) on Si have been irradiated by pulsed (15 ns) ruby laser under various pressures of 16O2. All the samples were anodically preoxidized in order to obtain a 10‐nm‐thick 18O enriched oxide, the aim being to study the influence of native oxide on oxygen incorporation during irradiation. Nuclear microanalysis and channeling experiments provide information on (i) 16O incorporation, (ii) 18O losses and concentration profile, (iii) Ga and As atoms out of crystallographic sites for single crystal irradiations, and (iv) Ga and As losses for thin films irradations. Our results demonstrate that a surface oxide is formed with a composition near the Ga2O3‐As2O3 stoichiometry via oxygen penetration in the melted layer once the native oxide has been evaporated at irradiation energies above 1 J/cm2. Before complete evaporation,this latter oxide diffuses in GaAs and is responsible for defect creation at irradiation energies as low as 0.4...

Substitutional and clustered B in ion implanted Ge: Strain determination

The lattice strain induced both by substitutional and clustered B in B-implanted Ge samples has been investigated by means of high resolution x-ray diffraction (HRXRD). The main results can be summarized as follows: while substitutional (i.e., electrically active) B exhibits a negative strain, clustered (i.e., electrically inactive) B reverses the lattice strain from negative to positive values, the latter being much higher with respect to those found for clustered B in Si. In particular, the lattice volume modification for each B atom (ΔV) induced by substitutional (ΔVSub) and clustered (ΔVCl) B is ΔVSub=−12.4 A3 and ΔVCl=+14.8 A3, respectively. These unexpected results demonstrate the ability of HRXRD to quantitatively detect the amount of electrically inactive (and active) B.

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.