Jean-Marc Baribeau

Strain in coherent-wave SiGe/Si superlattices

We report measurements of the Raman spectra of Si12xGex=SiO0:4 # x # 0:6 and xa 1U wavy superlattices characterized by the presence of a coherent undulated alloy-layer structure. The strain induced shifts of the alloy Si‐Si, Si‐Ge and Ge‐Ge mode frequencies in the wavy superlattices from the corresponding unstrained bulk mode frequencies are compared with those found in Si12xGex=Si planar superlattices. Overall, the Si‐Ge and Ge‐Ge mode frequencies as a function of x are found to be reduced with respect to those in the planar superlattices, indicating a reduction of average strain in the wavy superlattices compared with the built-in strain in the planar superlattices. For the Si‐Si mode, the mode frequency surprisingly behaves the opposite way. We propose that a compositional fluctuation in the alloy layer of the wavy superlattices, caused by the outward diffusion of Ge into the Si layer during growth, can explain this inconsistency in the experimental results. q 2000 Elsevier Science Ltd. All rights reserved.

Heteroepitaxy of Ge on (100) Si substrates

The epitaxial growth of GaAs on Si is a subject of growing interest because of the possibility of integrating high‐speed and optical devices in Si‐based technology. The large lattice mismatch between Si and GaAs makes the growth of high‐quality epilayers very difficult. However, Ge and GaAs have similar lattice parameters and thermal expansion coefficients and thus a promising way to improve epitaxial growth of GaAs on Si is to introduce an intermediate Ge buffer layer. This technique will only be effective when the structural defects formed at the Si–Ge interface can be prevented from propagating into GaAs. A series of Ge epilayers have been grown on (100) Si by molecular‐beam epitaxy and several methods to reduce dislocation propagation have been examined. These include postgrowth annealing, graded layering, growth interruption, and/or superlattice buffers. The efficiency of these methods was assessed by transmission electron microscopy, Rutherford backscattering/channeling, and double‐crystal x‐ray dif...

Optical properties of pseudomorphic Si/sub 1-x/Ge/sub x/ for Si-based waveguides at the /spl lambda/=1300-nm and 1550-nm telecommunications wavelength bands

The index of refraction for pseudomorphic Si/sub 1-x/Ge/sub x/ layers grown on Si has been measured at wavelengths /spl lambda/=1310 mn and /spl lambda/=1550 nm. The refractive index values were obtained from waveguide mode profile measurements on a series of Si-Si/sub 1-x/Ge/sub x/-Si waveguides with Ge/sub x/ concentrations between x=0.01 and x=0.1. The index of refraction, n, is significantly larger for light polarized parallel to the growth direction than for light polarized in the plane of the epilayer. This birefringence is consistent with the anisotropic index change predicted using photoelastic theory, given the biaxial strain present in the pseudomorphic Si/sub 1-x/Ge/sub x/ layers. At all wavelengths and polarizations, n varies linearly with the Ge concentration. The pseudomorphic Si/sub 1-x/Ge/sub x/ waveguides layer are stable against lattice relaxation during short anneals at 950/spl deg/C, but exhibit partial relaxation after annealing at 1200/spl deg/C.

Type-I interband cascade lasers near 3.2 μm

Interband cascade (IC) lasers have been demonstrated based on type-I InGaAsSb/AlAsSb quantum well (QW) active regions. These type-I IC lasers are composed of 6-cascade stages and InAs/AlSb superlattice cladding layers. In contrast to the use of quinary AlGaInAsSb barriers for active region in previous type-I QW lasers, the type-I QW active region in each stage is sandwiched by digitally graded multiple InAs/AlSb QW electron injector and GaSb/AlSb QW hole injector. The fabricated type-I IC lasers were able to operate in continuous wave and pulsed modes at temperatures up to 306 and 365 K, respectively. The threshold current densities of broad-area lasers were around 300 A/cm2 at 300 K with a lasing wavelength near 3.2 μm. The implications and prospects of these initial results are discussed.

Influence of growth temperature on order within silicon films grown by ultrahigh-vacuum evaporation on silica

We study the role that the growth temperature plays in determining the amount of order present within silicon films deposited on fused silica substrates through ultrahigh-vacuum evaporation at growth temperatures ranging from 98 to 572°C. Through measurements of the Raman and optical absorption spectra, we quantitatively determine how the growth temperature influences the order present within 11 such films. We employ three disparate measures of order for the purposes of this study: the breadth of the transverse-optic phonon Raman peak, this being related to the amount of short-range order present; the area under the transverse-acoustic Raman peak divided by the area under the corresponding transverse-optic peak, this being related to the amount of intermediate-range order present; and the breadth of the optical absorption tail, which is a general measure of the overall amount of order present. All three measures of order indicate a dramatic increase in the amount of order present for growth temperatures a...

Organic monolayers detected by single reflection attenuated total reflection infrared spectroscopy

A single-reflection attenuated total reflection (ATR) technique has been used to study low index monolayers with a high index ATR hemisphere in an optical contact with the sample surface. A model calculation predicts a field enhancement with the present method substantially larger than that of conventional ATR. For the present method, a discontinuity of the field normal to the film is responsible for the field enhancement in the lower index thin film. We show theoretically and experimentally that the angle of incidence has a strong influence on the ATR signal strength. We demonstrate the present method by examining the infrared reflectance spectra of a undecylenic acid self-assembled on Si (111) and with the adsorption of a bovine serum albumin monolayer. We also apply the method to examine the oxidation and surface contamination of the functionalized Si samples stored in ambient conditions.

Syntheses, Phase Behavior, Supramolecular Chirality, and Field‐Effect Carrier Mobility of Asymmetrically End‐Capped Mesogenic Oligothiophenes

A novel series of asymmetrically end-capped mesogenic oligothiophenes, with various oligothiophene core lengths, alkoxy tail lengths, and molecular polarities through introducing alkylsulfanyl or alkylsulfonyl functionalities as the terminal group, have been synthesized by palladium-catalyzed Suzuki cross-coupling and Kumada cross-coupling reactions as key steps. For the single end-capped oligothiophenes, C(m)O-Ar-OT(4)-H in which m=10, 12, 14, 16, and 18, all of these oligomers exhibited a broad temperature range of highly ordered smectic E and enantiotropic nematic phases, apart from the one with the longest octadecyloxy tail. For the double end-capped series C(10)O-Ar-OT(n)-R, R=Ph-SC(6) or Ph-SO(2)C(6) in which n=1, 2, 3, and 4, oligomers with more than one thiophene ring exhibited smectic A and smectic C phases, various crystal polymorphs and/or unusual low-temperature condensed phases. In the nonpolar, alkylsulfanylphenyl-substituted oligothiophene series, both the crystal/solid melting point and mesogenic clear point increased significantly with an increasing oligothiophene conjugation length. In the polar, alkylsulfonylphenyl-substituted oligothiophene series, all the oligomers showed increased melting points, but decreased mesogenic temperature intervals than those of their corresponding alkylsulfanyl counterparts. Remarkably, two different helical structures showing distinct striated textures or striped patterns were observed with a pitch of several to tens of micrometers under a polarized optical microscope upon cooling from their preceding fluidic smectic phases. The unusual twisted smectic layer structures in the thin solid films exhibiting distinct supramolecular chirality of both handednesses, revealed by circular dichroism measurements, were further confirmed by XRD analyses characterized by a sharp layer reflection together with its higher orders and diffuse wide-angle scatterings. In addition, initial studies showed that the highly ordered smectic phase of the single end-capped oligothiophenes can be utilized to improve field-effect charge mobility. C(10)O-Ar-OT(4)-H showed a hole mobility of 0.07 cm(2) V(-1) s(-1) when deposited on octyltrichlorosilane-treated substrates at 140 degrees C and the on/off current ratios reached 5 x 10(5); on the other hand, its mobility was only 8 x 10(-3) cm(2) V(-1) s(-1) on the same substrate when deposited at room temperature.

Si1−x−yGexCy alloy growth by electron cyclotron resonance plasma-assisted Si molecular beam epitaxy

Abstract We report the growth of Si 1− y C y and Si 1− x − y Ge x C y alloys by electron cyclotron resonance (ECR) plasma-assisted Si MBE using an argon/methane gas mixture. No significant carbon incorporation was measured in films grown under a high methane partial pressure without plasma excitation, whereas C incorporation up to ≈2 at.% has been achieved upon plasma excitation. Various Si/Si 1− y C y and Si/Si 1− x − y Ge x C y multilayers have been grown and characterized. The structures show good structural properties and sharp interfaces, with carbon being essentially substitutionally incorporated up to concentrations of ≈1%. Raman scattering spectroscopy of Si 1− y C y revealed two bands near 480 and 606 cm −1 that are indicative of substitutional incorporation. Strong strain compensation and enhanced thermal stability were observed in Si 1− x − y Ge x C y alloys. These results suggest that ECR plasma-assisted Si MBE may be an interesting alternative to more conventional methods for producing lattice matched epitaxial Si 1− y C y and Si 1− x − y Ge x C y thin films on Si.

Compositional Redistribution in Coherent

Coherent Si1-xGex island growth by molecular beam epitaxy is studied for a fixed growth temperature but for different Ge concentrations in the range 0.37 lesxles 0.56. A combined transmission electron microscopy, X-ray diffraction, and Raman spectroscopy characterization of the samples showed that during growth the Ge migrates towards the center of the large islands to minimize the strain energy, while maintaining epitaxial growth, and that the most uniform structures are obtained at higher Ge composition when the built-in strain is also higher

{\rm Si}_{1\hbox{-}x}{\rm Ge}_{ x}

Abstract Strained layer superlattices (SLSs) of Si1−xGex/Si, consisting of four pairs of layers, each about 40 nm thick with x = 0.16-0.25, have been grown on Si(100) by molecular beam epitaxy (MBE). The SLSs were characterized by Rutherford backscattering (RBS)/channeling, double crystal diffractometry (DCD), and cross-sectional transmission electron microscopy (XTEM) techniques. Tetragonal distortion in the Si1−xGex layers, caused by the lattice mismatch between Si and Si1−xGex, was measured by RBS/channeling and DCD. The strain measured by these two techniques agreed well with that calculated from the theory of elasticity and Monte Carlo simulation. XTEM investigation of the SLSs showed a dislocation-free structure and smooth interfaces.