T. Tite

Percolation behavior in the Raman spectra of ZnBeTe alloy

Zone-center longitudinal (LO) and transverse (TO) optical phonons of Zn1−xBexTe epilayers are identified by Raman spectroscopy. On top of the expected BeTe- and ZnTe-like modes, which obey the modified-random-element-isodisplacement model, we observe an extra BeTe-like (TO, LO) doublet at intermediate x values. It has the same atypical characteristics as its BeSe-like counterpart in ZnBeSe alloy. Its activation validates our percolative picture for multimode description in Be–chalcogenide alloys that open the attractive class of mixed crystals with a sharp contrast in the bond stiffness. Also, the local modes of Be in ZnTe and of Zn in BeTe are identified at 411 cm−1 and 195 cm−1, respectively.

Percolation-based vibrational picture to estimate nonrandom N substitution in GaAsN alloys

The number of N atoms in N-rich regions mostly due to nonrandom N incorporation in GaAsN (N∼4%), referred to as the Nr rate, is studied using a nonstandard Raman setup that addresses transverse symmetry. The Ga–N optical range shows a two-mode signal which discriminates between the N-poor (Np) and N-rich (Nr) regions. This is discussed via a percolation-based picture for Be-chalcogenide alloys, which exhibit mechanical contrast with regard to the shear modulus. This applies to GaAs–GaN even though the contrast is in the bulk modulus. The balance of Nr/Np strength provides a Nr rate of ∼30%, i.e., much larger than the corresponding Be rate of ∼4% in random Be-based alloys.

Percolation picture for long wave phonons in zinc-blende mixed crystals: from (Zn, Be) chalcogenides to (Ga, In)As

We propose a simplified version of the one-bond two-mode percolation model originally developed for the long wave phonons related to the stiff Be–VI bond in (Zn, Be) chalcogenides, which open the class of random mixed crystals with contrast in the bond stiffness. This is deduced from the comparison between the Raman responses from the stiff Be–VI bond and the soft Zn–VI one in these systems. The simplified version is tested on (Ga, In)As, made of soft-like bonds only and taken here as a representative challenging system. This results in a successful reinterpretation of the puzzling multi-phonon behaviour in the Raman/infrared spectra of this alloy, that has been a subject of debate. The discussion is supported by contour modelling of the TO and LO Raman lineshapes by applying the Hon and Faust treatment to a version of the modified-random-element-isodisplacement model generalized to multi-oscillators. Also, the assignment of the long wave phonons in (Ga, In)As is supported by atomistic calculations of the bond length distributions of the minority bond species in large (Ga, In)As supercells corresponding to alloy compositions close to the In–As (In ~0.19) and Ga–As (In ~0.81) bond percolation thresholds. The configurations are analysed to distinguish between isolated and connected bonds, not in the usual terms of next nearest neighbours.

Investigation of the nanomechanical properties in relation to the microstructure of Zn1−xBexTe alloys

A study by nanoindentation has been performed to investigate the variation of the mechanical properties of Zn1−xBexTe alloys as a function of the beryllium content in the range 0≤x≤0.84. Our data show a very marked increase of hardness (H) and a significant increase of the elastic modulus (E) with Be concentration. These trends are related to an increase in the stiffness of the BeTe bond relative to that of the ZnTe bond. Furthermore, discontinuities are unambiguously revealed in the variations of both H and E at the critical Be content of ∼20%. We relate these to Raman studies of the layers’ microstructure and propose an explanation in terms of a percolative behavior.

Does In-bonding delay GaN-segregation in GaInAsN? A Raman study

The longitudinal (LO) and transverse (TO) optical Ga–N phonons of thick as-grown Ga1−yInyAs1−xNx∕GaAs (001) layers with x∼3%–4%, i.e., just above the N-solubility limit xs∼2% in GaAs, and y up to 30% are studied by Raman scattering. We observe a three-mode behavior which we associate with Ga–N modes from the isolated Ga4N complexes (∼460cm−1), the Ga3InN ones (∼480cm−1) and the GaN-segregated region (∼425cm−1). The corresponding N fractions are derived via combined contour modeling of the three-mode TO and LO Ga–N Raman lineshapes. We find that substantial In-bonding minimizes GaN segregation only in a very narrow x range of ∼1% just above xs.

Raman study of the random ZnTe-BeTe mixed crystal : Percolation model plus multimode decomposition

The Verleur and Barker-like multiphonon description proposed for the refined understanding of the two-mode transverse optical (TO) and longitudinal optical (LO) Raman responses from the stiff∕short Be–Se bond in the percolation range (0.19⩽Be⩽0.81) of the random BeSe–ZnSe mixed crystal [O. Pages et al., Phys. Rev. B 70, 155319 (2004)] is extended to the similar Be–Te bond in the percolation range of the random BeTe–ZnTe mixed crystal. Moreover this description is shown to remain valid just outside the percolation range, in spite of appearances. We deduce that it is not activated by the close intermixing of the stiff and soft continua at the mesoscopic scale but relies directly on the contrast in the bond properties at the microscopic scale. The whole study is supported by extensive ZnBeTe Raman data, and full contour modeling of the TO and LO Raman line shapes by using our phenomenological 1‐bond→2‐mode percolation model. Care is taken that our ZnBe(Se,Te) epitaxial layers undergo large substrate-induced ...

LO phonon–plasmon coupling and mechanical disorder-induced effect in the Raman spectra of GaAsN alloys

Abstract Mechanical and chemical disorders are investigated in GaAsN alloys with N content close to the solubility limit, ∼2–3 %. One attractive characteristic, yet unexplored, of N-based III–V ternary alloys is that they consist in mixed crystals with a sharp contrast in the bulk modulus of the two constituents. Raman scattering is well-suited for its investigation because it addresses the force constant of the bond, which depends on the mechanical properties of the host matrix. In the Ga–N region, we observe a mode at 425 cm −1 , i.e. below the local mode of N in GaAs at 470 cm −1 . This mode has degenerated LO and TO components and seems to grow at fixed frequency with N content. From a direct comparison with Zn–Be chalcogenides, which exhibit a similar mechanical contrast, it is attributed to GaN vibrations in hard-like N-rich bounded regions, dispersed within the soft-like Ga-rich matrix. In the GaAs region we use Si-doping as a sensitive probe to investigate the structural quality close to the solubility limit. At low N content ( n ⩾10 18 cm −3 ), we observe similar LO phonon–plasmon (LOP) coupling as in GaAs:Si. Unexpectedly LOP coupling is disorder-cancelled at higher N contents while the pure LO and disorder-activated theoretically forbidden TO modes substitute for it. Disorder-induced red-side LO asymmetries are fairly accounted for by the spatial correlation model with Gaussian distribution and the fitted value of correlation length is a quantitative indication for the disorder rate.

Percolation context in mixed crystals with mechanical contrast

Abstract At intermediate Be-content, random Zn1−xBexTe and Zn1−xBexSe alloys, which open the class of mixed crystals with contrasted shear moduli, can be described in terms of composite systems made mainly of a hard-like (Be-rich) and a soft-like (Zn-rich) intermixed continua. This results in an atypical three-modes Raman response. A straightforward model based on percolation is used to simulate the spectra; no adjustable parameter is required. Preliminary results obtained with low-N content GaNxAs1−x alloy suggest that our picture extends to arriving III–V alloys with contrast in the bulk modulus.

Percolation effect in the vibrational spectra of mixed crystals with highly contrasted bond stiffness

We present a detailed investigation by Raman spectroscopy of the zone-center longitudinal (LO) and transverse (TO) optical phonons from Zn1−xBexTe epilayers with special emphasis on samples with low-medium Be content. On top of the expected ZnTe- and BeTe-like modes, which obey the standard modified-random-element-isodisplacement (MREI) model, we observe an additional BeTe-like mode at intermediate frequency. At medium Be-content (0.2<x<0.61), this extra mode is a real (TO, LO) doublet. We show that its frequency dependence versus x obeys a ‘restricted’ MREI description. On the strength side, it is remarkable that the extra mode grows at the cost of the other BeTe-like mode when x increases; the balance is equilibrated at x∼0.5. At low Be content (x≤0.2), the Raman response from the extra mode changes in nature; it turns from a restricted MREI-like (TO, LO) doublet into a TO–LO degenerated mode with fixed frequency. One key point is that the latter stays below the local mode of Be in ZnTe. The present results enrich our ZnBeSe-based percolative picture for multimode description in Be–chalcogenide alloys that open the attractive class of mixed crystals with a sharp contrast in the bond stiffness.