Built-in Strain Research Articles

Experimental determination of local Strain effect on InAs/GaAs self-organized quantum dots

The energy barrier at InAs/GaAs interface due to the built-in strain in self-organized system has been determined experimentally. Such a barrier has been predicted by previous theories. From the deep-level transient spectroscopy (DLTS) measurements, we have obtained the electron and hole energy levels of quantum dots E-e(QD-->GaAs) = 0.13 eV and E-h(QD-->GaAs) = 0.09 eV relative to the bulk unstrained GaAs band edges E-c and E-v. DLTS measurements have also provided evidence to the existence of the capture barriers of quantum dots for electron E-eB = 0.30 eV and hole E-hB = 0.26 eV. The barriers can be explained by the apexes appearing in the interface between InAs and GaAs caused by strain. Combining the photoluminescence results, the band structures of InAs and GaAs have been determined.

Dutch Design Manual for Lightweight Pavements with Expanded Polystyrene Geofoam

The use of expanded polystyrene (EPS) geofoam instead of traditional “heavy” sand for pavement subbases can reduce or even eliminate the additional load on the subsoil, thus decreasing or eliminating the settlement of pavement structures on a compressible subsoil. Experience with EPS geofoam is very positive, but a uniform design procedure does not yet exist for this type of structure. Optimization of the existing EPS pavement design guidelines and their improvement has demanded materials research on EPS, the use of three-dimensional finite-element pavement models, and in situ full-scale measurement. Extensive materials research provided data for the stress-strain response of EPS under representative loading and environmental conditions. Three-dimensional modeling enabled critical evaluation of existing design methodologies by analyzing pavements with different road bases, different EPS types, and different asphalt thicknesses. In situ measurements by means of built-in strain transducers in asphalt provided data for verification of the three-dimensional modeling. Furthermore, evaluation and monitoring of Dutch projects with EPS geofoam were carried out to define problematic aspects from the practical point of view. On the basis of the research findings, the current Dutch design guidelines have been revised and optimized. In order to make this knowledge easily accessible, an official Dutch design manual for lightweight pavements with EPS geofoam was published under the auspices of the Center for Research and Contract Standardization in Civil and Traffic Engineering. An outline of the manual is presented.

Energy Band/Lattice Mismatch Engineering in Quaternary AlInGaN/GaN Heterostructure

We discuss structural, optical and electrical properties of AlxInyGa1—x—yN/GaN heterostructures grown on sapphire and 6H-SiC substrates. The incorporation of In reduces the lattice mismatch in a much stronger way than the energy gap discontinuity. An In to Al ratio close to 1:5 should result in nearly strain-free heterostructures. The incorporation of In and resulting changes in the built-in strain in AlInGaN/GaN heterostructures strongly affect the transport properties of the two-dimensional (2D) electron gas at the heterointerface. Using the incorporation of In in order to increase the Al molar fraction preserving the same value of strain should allow us to greatly enhance the sheet carrier density at the AlGaInN/GaN heterointerface. Further work on improving In incorporation techniques is needed in order to meet these expectations.

Correlated SiGe wires shaped by regular step bunches on miscut Si(113) substrates

Highly regular step bunches have been observed in appropriately strained SiGe/Si multilayers on miscut Si(113) substrates. Owing to the strong step-bunching properties on the Si(113) surface, both the Si layers and the SiGe layers of the multilayers exhibit a regular pattern of large steps. Considerable SiGe material in the SiGe layers accumulates at the edges of the underlying large Si steps (step bunches), forming well-shaped SiGe wires. The SiGe wires in the multilayers manifest an oblique correlation. A bending to vertical correlation is observed when the built-in strain in the multilayers is large enough. The oblique correlation is well described in a thermodynamic model of multilayer growth, based on the calculations of the in-plane strain fields of underlying step-modulated SiGe layers on the free surfaces of Si layers during growth. Si step bunches energetically form in the regions of maximum strain above the SiGe wires and cause lateral agglomeration of SiGe material at Si step edges, resulting in the oblique correlation. The bending to vertical correlation occurs, in contrast, if the in-plane strain field becomes large enough to cause the SiGe material to accumulate directly in the regions of maximum strain.

Strain in GaAs layers grown by liquid phase epitaxial lateral overgrowth

High resolution x-ray diffraction has been used to study strain in GaAs layers grown on GaAs substrates by the liquid phase epitaxial lateral overgrowth (ELO) technique. We show that the lattice and thermal expansion coefficient mismatch between the subsequent layers and the substrate, as well as the built-in strain in the SiO2 masking film, lead to long-range deformations (macroscopic bending) extending over the whole area of the sample. Moreover, we show evidences that microscopic bending of individual ELO stripes takes place due to adhesion of their laterally overgrown parts to the masking film.

Growth and thermal stability of pseudomorphic Ge1−yCy/Ge superlattices on Ge(001)

High quality Ge/Ge1−yCy superlattices with nominal carbon contents of 1.2% and 2.1% were grown by molecular beam epitaxy on Ge(001). In transmission electron microscopy the layers are planar and perfectly pseudomorphic without any extended defects observable. The infrared absorption line at 529 cm−1 is attributed to the local vibrational mode of substitutional carbon in germanium. However, in contrast to Si1−yCy alloys where almost 100% of the C is substitutional under optimized growth conditions, x-ray diffraction measurements indicate that the efficiency of carbon incorporation onto substitutional sites is only about 30% for low temperature growth at TS=200 °C. It reduces further for higher growth temperatures to only about 10% at TS=300 °C. Post-growth annealing experiments indicate thermal stability up to 450 °C. Annealing at higher temperature results in a reduction of substitutional carbon content. As in the case of Si1−yCy alloys the built-in strain is relaxed by C diffusion and not by nucleation of misfit dislocation.

Built-In Strain in Polysilicon: Measurement and Application to Sensor Fabrication

AbstractThe built-in strain in deposited polysilicon films depends on film morphology. The strain may be compressive or tensile. Its magnitude and uniformity may be measured by using surface micro machining with test structures which require a single photomask level and use Euler buckling. Anneal cycles may be used to convert compressive strain to tensile or zero strain. Deposition and anneal cycles may be optimized to produce fine grained polysilicon with isotropic mechanical behavior.Strain field control can improve yield by reducing surface sticking. This mechanical stiffening is used in, for instance, pressure transducer construction. Built-in and applied axial strain are used to change the resonant frequencies of clamped-clamped beams in a vacuum envelope where quality factors up to 300,000 have been measured.

Working equations for piezoelectric actuators and sensors

A new solution to the force, displacements, and charges developed in piezoelectric beams is derived. Differing from previous solutions, this development determines the neutral axis where the bending strains are zero and results in a closed form solution (without matrix inversion). With the closed form, simplifications become evident which increase understanding and facilitate calculations. These equations are than expanded to account for axial, built-in strains in the beam. A design example where axial forces exerted by the piezoelectric layer are important is presented.

III-V NITRIDES : IMPORTANT FUTURE ELECTRONIC MATERIALS

The present stage of development for the AlN-GaN-InN class of III-V nitrides is reviewed, with emphasis on the electronic properties of the materials. We also briefly cover the most important types of device structures presently under study, as well as application areas for III-nitride devices now foreseen. A brief overview of the main growth techniques is presented, since a further development of the growth of these materials is indeed the key towards a desired substantial improvement in material quality. This is needed both for the establishment of definite data for physical parameters, as well as for the proper functioning of most devices. The basic band structure as well as excitonic properties are briefly discussed. It is pointed out that these materials are in most cases excitonic at room temperature, for bulk as well as for quantum structures. Doping and properties of donors and acceptors are highlighted, here, in particular, the parameters for shallow acceptors, such as their binding energies, are far from well known. There is no complete picture of the identification of characteristic optical spectra (such as bound exciton spectra) with specific donors and acceptors. The properties of defects, in particular dislocations which are very abundant in the heteroepitaxial material studied today, are discussed. Dislocations may not themselves be severe recombination centers, and the degradation of active devices via dislocation climb seems to be absent in III-nitrides. The progress in the area of heterostructures and quantum wells based on the AlGaN/GaN and the InGaN/GaN systems is reviewed, in relation to the present strong development of related devices. An interesting observation is the importance of the piezoelectric effect, due to a strong built-in strain in most structures. Application areas cover a very broad range in optoelectronics, but also high frequency power devices, with applications for example in satellite communication systems.

Theoretical Calculations of Shallow Acceptor States in Strained Quantum Well Structures

Acceptors confined in strained InxGa1—xAs/Al0.3Ga0.7As quantum well (QW) structures are theoretically investigated. The impurity states are calculated using a four-band effective-mass theory, in which the valence-band mixing as well as the mismatch of the band parameters and the dielectric constants between well and barrier materials have been taken into account. The binding energies of acceptors and the oscillator strengths of the active infrared electronic transitions between the acceptor ground states and the different excited p-like states are calculated with different indium fractions x and different well widths. The results show clearly, that a built-in strain strongly influences the transition energies and the relative oscillator strengths. Due to the splitting of the acceptor ground states, the absorption spectra of the acceptors also show a strong temperature dependence. The calculated spectra of the acceptor electronic transitions are ready to be compared with the infrared absorption measurements.

Identifying Impacts in Composite Plates with Piezoelectric Strain Sensors, Part I: Theory

This paper summarizes the theory used in a system that identifies foreign body impact on a composite plate using built-in strain sensors. The identification system consists of a model of the composite plate and an identification algorithm. The algorithm compares the measured response of the plate to the model response and estimates the impact location and force time history. The solution, which uses a smoother/filter optimization, includes a new computational algorithm that saves considerable computation time for systems with many degrees of freedom.

Theoretical study of shallow acceptor states under the influence of both a confinement potential and a deformation potential

Energy levels of the ground and the excited shallow acceptor states have been calculated for center-doped strain-free ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum wells (QW's) in the presence of an external pressure and in strained ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{y}{\mathrm{Ga}}_{1\ensuremath{-}y}\mathrm{As}$ QW's. The impurity states are calculated using a four-band effective-mass theory, in which the valence-band mixing as well as the mismatch of the band parameters and the dielectric constants between well and barrier materials have been taken into account. The acceptor binding energies and the internal electronic transitions are calculated. The results show that an applied external pressure in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ QW's or built-in strain in ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{y}{\mathrm{Ga}}_{1\ensuremath{-}y}\mathrm{As}$ QW's strongly influences the acceptor states. The oscillator strengths between the acceptor ground states and the different excited $p$-like states are also calculated for the ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ system with three different indium fractions $x.$ The results show clearly that a built-in strain strongly influences the transition energies and the relative oscillator strengths.

Electron-phonon interaction in anisotropic uniaxial double heterostructures

For a double semiconductor heterostructure having as interjacent layer an anisotropic uniaxial polar material with the optical axis directed along the normal to the interface, the eigenfrequencies and the eigenvectors of the field of the optical phonons are obtained in the context of the dielectric continuum model. The form of the operator describing the interaction between the phonon modes and an electron is derived. The results are particularized to the case of a heterostructure made of zincblende-type semiconductors InP and GaAs with the pseudo-morphic layer uniaxially distorted by the built-in strain distribution.

Cathodoluminescence spectra of tensile strained ZnSe/ MBE single quantum wells

This paper discusses the hydrostatic and shear deformation potentials of ZnSe by investigating cathodoluminescence (CL) spectra of biaxial tensile strained single quantum wells (SQW). The dependence of the shift of light- and heavy-hole exciton lines on the built-in strain of these MBE-grown strained layer heterostructures is discussed. We found a hydrostatic deformation potential of eV and a shear (tetragonal) deformation potential of eV. From the temperature dependence of the ground state exciton energy, an exciton gap meV at T = 297 K was deduced for unstrained bulk ZnSe.

Non-Markovian gain of strained-layer wurtzite GaN quantum-well lasers with many-body effects

A theoretical model for the optical gain of strained-layer wurtzite GaN quantum-well (QW) lasers is developed taking into account valence-band mixing, many-body effects and non-Markovian relaxation. The valence-band structure is calculated from a 6/spl times/6 multiband effective mass Hamiltonian for the wurtzite structure taking into account built-in strain due to lattice mismatch. The theoretical foundation for the optical processes is based on the time-convolutionless reduced-density operator formalism given in previous papers for an arbitrary driven system coupled to a stochastic reservoir. Many-body effects are taken into account within the time-dependent Hartree-Fock approximation and the optical gain with Coulomb (or excitonic) enhancement is derived by integrating the equation of motion for the interband polarization. It is predicted that the Coulomb enhancement of gain is pronounced with increasing magnitude of compressive strain in the QW.

Electronic structure model for n- and p-type silicon quantum dots

We present effective mass, single-particle calculations of the electronic structure of n- and p-type silicon quantum dots. The structures investigated approximate silicon quantum dots fabricated on 〈 001〉-oriented SIMOX wafers. The effects of possible built-in strain are investigated in the framework of deformation potential induced splitting of the six degenerate conduction band valleys and the splitting of the degeneracy at the top of the bulk valence band. We present the energy levels and their degeneracies as functions of the dimensions of simple tetragonal model quantum dots. Our results are relevant for silicon quantum dots that are sufficiently small such as to lead to a predominance of the confinement energy over the Coulomb energy.

Optical anisotropy of excitons in strained GaN epilayers grown along the 〈101¯0〉 direction

We calculate the spectroscopy of $1s$ excitons in GaN epilayers grown with biaxial stress on 〈101\ifmmode\bar\else\textasciimacron\fi{}0〉-oriented $M$ plane. In contrast to the growth on conventional $C$-plane substrates, for which we predicted five radiative excitons, we now demonstrate the existence of nine radiative transitions. $A,$ $B,$ and $C$ excitons are now radiative for any polarization case, due to the crossed configuration of the wurtzite crystal field and the built-in strain one. In particular, the $A$ exciton is significantly coupled to the electromagnetic field in \ensuremath{\pi} polarization $(\mathbf{E}\ensuremath{\parallel}\mathbf{c}).$ Strong in-plane anisotropy of the optical response is predicted and computed.

Strain effects on optical phonons in 〈111〉 GaAs layers analyzed by Raman scattering

Strain and thermal effects on longitudinal and transverse optical (LO) and (TO) phonon frequencies and on associated electron–phonon interactions have been analyzed in GaAs. We focus separately on these effects to treat each one thoroughly. By using lattice-mismatched 〈111〉 Si or CaF2 substrates, GaAs layers with highly tensile or compressive strain have been analyzed. We show that the LO/TO Raman scattering efficiency ratio, i.e., the electro-optic versus the atomic displacement electron-phonon mechanisms, varies with the built-in strain and hence its induced piezoelectric field. The Raman selection rules are valid in this process. The increase and decrease of this ratio compared to a reference suggest an interference effect. We discuss this phenomenon on the basis of strain-induced Raman scattering.

Effect of strain on light emission from pseudomorphic Si/Si 1− xGe x/Si structures

Photoluminescence from Si/Si 1− x Ge x /Si under [110] uniaxial tensile stress was investigated at 4.2 K. The photoluminescence intensity shows a minimum value at a certain strain field (ε min), which shifts to a higher strain field with increasing alloy composition ( x). On the other hand, the deformation potentials obtained from the peak shift of Si 1− x Ge x under stress are consistent with the calculation assuming that the built-in strain in the Si 1− x Ge x is coherent. These results indicate that the local symmetry in the Si 1− x Ge x layer plays an important role on the light emissivity.

DAFS Study of Strained III-V Epitaxial Semiconductors

The effect of built-in strain on III-V epitaxial semiconductors has been investigated by Diffraction Anomalous Fine Structure (DAFS). We study two different systems in a different strain regime: a Strained Layer Superlattice of (GaP) 2 (InP) 3 grown on a GaAS(001) substrate, and a single epilayer of GaAs 1 - x P x , (x=0.225), also grown on a GaAs (001) substrate. In the first case the strain is accommodated by plastic deformation of the lattice, while in the second one it is partially relaxed by dislocations generation. The bond distances for the Ga-P and Ga-As pairs are obtained showing how they are affected by strain. The Ga-As pair shows to be softer' than the Ga-P pair, i.e. more available to accommodate strain by bond deformation, in good agreement with previous results obtained by different techniques. The DAFS provide a unique tool of studing systems that are out of the reach of the other X-ray techniques.