Lattice Mismatch Strain Research Articles

Determination of structural defects in Hg 1− xCd xTe multilayer materials

Lattice mismatch strain, intense strain fields (resulting from the substrate), and gradual and abrupt composition changes at heterojunctions in Hg 1− x Cd x Te multilayers, were nondestructively investigated by X-ray double crystal diffraction and X-ray topography. Some structural defects at heterojunctions, such as microtwins, stacking faults, mismatch dislocations, and misorientations between multilayer structures Hg 1− x Cd x Te/CdTe and CdTe/GaAs, were also studied by TEM (transmission electron microscopy). As to the heterojunctions of Hg 1− x Cd x Te/CdTe/GaAS (molecular beam epitaxy) method, it is clearly shown that the CdTe buffer layer acts as an effective barrier for the Hg 1− x Cd x Te epilayer for most structural defects, and the misorientations between multilayer structures Hg 1− x Cd x Te/CdTe and CdTe/GaAs were found to increase as the lattice mismatch increased.

Reduction of threading dislocation density in InP-on-Si heteroepitaxy with strained short-period superlattices

We applied multistrained short-period superlattices (SSPSs) and GaP buffer layers to the InP-on-Si heteroepitaxy, in order to suppress the generation of threading dislocations. As a result, it was found that the density of threading dislocations in an InP/SSPSs/GaAs/SSPSs/GaP/Si structure including (InAs)m(GaAs)n SSPSs and (GaAs)i(GaP)j SSPSs was remarkably reduced, compared with that in the InP/GaP/Si structure. Misfit dislocations lying along the 〈011〉 directions were observed at heterointerfaces in the InP/SSPSs/GaAs/SSPSs/GaP/Si structure. Therefore, the lattice mismatch strain was stepwise accommodated by the generation of misfit dislocations at the heterointerfaces. From these results, it was clarified that multi-SSPSs are effective for reducing the density of threading dislocations in heteroepitaxy with a large lattice mismatch.

Lateral composition modulation in InGaAsP strained layers and quantum wells grown on (100) InP by gas source molecular beam epitaxy

Lateral composition modulation (LCM) in InGaAsP layers (1500 Å) and quantum wells (50 Å) grown on (100) InP substrates by gas source molecular beam epitaxy were studied over a wide alloy range. Layers with lattice mismatches of −0.5% (compression), 0% (lattice-matched) and +0.5% (tension) were studied using photoluminescence, transmission electron microscopy and X-ray diffraction. Greater compositional fluctuations were observed for tensile material, compared with lattice-matched or compressively strained material. This is consistent with tensile material having nominal compositions that lie deeper within the calculated bulk spinodal isotherm. However, differences between the observed LCM structure and that predicted by bulk models, and the influence of growth conditions and lattice mismatch strain on the compositional fluctuations, suggest the need for a surface spinodal model.

Dislocation Interactions And Their Impact On Electrical Properties Of GeSi-Based Heterostructures

AbstractSubstantial improvements in the quality of relaxed, heteroepitaxial GeSi layers have occurred in the past few years due to both the development of compositionally-graded buffer layers and to an improved understanding of strain-relaxation in lattice-mismatched epitaxy, which have led to low dislocation densities within these layers. As a result, high performance GeSi devices have been achieved which has fueled yet greater device design flexibility through the use of relaxed GeSi layers over a wider range of alloy compositions and lattice constants. A complete understanding of the properties of all defects that result from strain-relaxation is essential to realize high performance and reliable devices, especially as the lattice-mismatch strain becomes more severe. While the structural properties of dislocations in relaxed GeSi layers have been studied in detail, the electronic properties of these defects and their consequences on the overall electrical properties of relaxed GeSi layers are not well understood. In this paper, we show that for high quality (low dislocation density) relaxed layers, clusters of intrinsic point defects formed by dislocation interaction, rather than the dislocations themselves, now dominate the deep level spectrum. We further show that these defects reduce the electronic material quality for films grown by UHVCVD at low temperatures (650 °C) in comparison to films grown at temperatures > 800 °C even though all films have similarly low threading dislocation densities. Using DLTS, spreading resistance, EBIC and Hall effect measurements, these defects are shown to introduce a low concentration of hole traps from Ev, + 0.05 to Ev + 0.30 eV that compensate and type convert nominally undoped GeSi layers grown at 650 °C to p-type. Annealing studies indicate that these defects anneal out at ∼ 750 – 800 °C(2w, hich is accompanied by conductivity type reversal to ntype, elimination of compensation and increased diffusion length. These observations, in addition to the precise position of each energy level, closely correlate with earlier results on plasticallydeformed bulk Si, suggesting that these defect complexes are of intrinsic origin. We conclude that strain-relaxation in graded heterostructures causes considerable dislocation interaction which introduces point defect complexes associated with dislocations that, in addition to threading dislocation density, must now be considered as an important factor for achieving high electronic quality, relaxed GeSi layers.

Assessment of intervalley f-scattering time constants in [formula omitted] heterostructures

The conduction band edge of silicon consists of six minima located in k -space along 〈100〉 directions. Transitions of electrons between perpendicular valleys are induced by so-called f-type intervalley scattering processes. In this work we have studied f-type intervalley scattering times using a specific Si SiGe bipolar transistor test structure were (low effective mass) electrons are selectively injected into four of the six valleys split energetically apart from the other two valleys by lattice mismatch strain. From the observed dependence of the collector current on the base width f-scattering time constants are deduced.

Optical intervalence-subband transitions in strained p-type In1−xGaxAs/InP quantum wells

The lattice mismatch (strain) effects on the infrared absorption in p-type In1−xGaxAs/InP quantum wells are investigated systematically for both tensile (x≳0.47) and compressive (x<0.47) strains. The mismatch of the valence-band parameters in the well and barrier materials is taken into account in the optical matrix element calculations. We find that normal incidence optical matrix elements substantially increase in the case of the compressive strain (the ground state is heavy hole) and decreases in the case of the tensile strain (the ground state is light hole). The peak of the normal incidence absorption in the compressively strained QW is shown to reach a considerable value of 5000–6000 cm−1 for a sheet hole concentration of 1012 cm−2. For the z-polarization of the light we found a substantial enhancement of the optical matrix elements in the case of tensile strain (i.e., for a light-hole ground state).

Semiconductor molecular-beam epitaxy at low temperatures

Low-temperature molecular-beam epitaxy (MBE) in semiconductors is reviewed, with a focus on limited thickness epitaxy (LTE), the regime where crystalline growth over an epitaxial thickness hepi is followed by a transition to amorphous deposition. The goal is to summarize the main results on this phenomenon, make the connection to other results on low-temperature MBE, and present the large body of unpublished data on hepi. Since the problem is still not fully understood, all available data that have a bearing on the understanding of the effect are outlined. The scientific questions and practical problems that have driven interest in low-temperature growth are outlined, and the phenomenon of LTE and the dependence of hepi on the growth conditions are described. The LTE effect is apparently general, but Si(100) is the model system for which most data are available. Breakdown of epitaxy follows a universal curve that is inconsistent with continuous nucleation of the amorphous phase, implying that growth is truly thickness dependent. The epitaxial thickness is thermally activated in substrate temperature T as hepi=h0 exp(−Eact/kBT), with h0 following a weak ln(R) or R1/4 dependence on deposition rate R. hepi is also strongly influenced by lattice mismatch strain, residual H in the ultrahigh vacuum, and annealing during growth interrupts. Possible mechanisms for LTE are discussed, with particular emphasis on the roles played by H and kinetic roughening, and the key experiments distinguishing these mechanisms are described. Finally, an attempt is made to draw up the best current picture of the phenomenon. It is concluded that roughening provides the fundamental limit to epitaxy at low temperature, but with H contamination playing an important part in controlling surface diffusion: outstanding problems include the rate dependence and the details of the roughening behavior.

Ge island formation on Si(111) in solid phase epitaxy studied by medium-energy ion scattering

The formation of three-dimensional Ge islands in solid phase epitaxy is studied by medium-energy ion scattering and atomic force microscopy. When a six-monolayer Ge film on a Si(111) substrate is annealed below 580 °C, many small islands of various heights below 100 Å form on the two-dimensional layer strained by lattice mismatch. After annealing at higher temperatures such as 770 °C, the islands agglomerate to form large islands higher than 100 Å with definite facets. These three-dimensional islands do not relax completely from the lattice mismatch strain; they are compressed horizontally and expanded vertically compared with the ideal Ge bulk crystal. The vertical lattice length is 2.3% longer than the in-plane length.

Solid Phase Epitaxy of Implanted Si-Ge-C Alloys

AbstractSi1-x-yGexCy/Si heterostuctures were formed on Si (100) surface by Ge and C implantation with a high dose rate MEtal - Vapor Vacuum arc (MEVVA) ion source and subsequent Solid Phase Epitaxy (SPE). after thermal annealing in the temperature range from 600 °C to 1200 °C, the implanted layer was studied using Rutherford Back-scattering Spectrometry (RBS), cross-sectional High Resolution Transmission Electron Microscopy (HRTEM) and fourbounce X-ray Diffraction (XRD) measurement. Due to the small lattice constant and wide bandgap of SiC, the incorporation of C into Si-Ge can provide a complementary material to Si-Ge for bandgap engineering of Si-based heterojunction structure. Polycrystals are formed at temperature at and below 1000 °C thermal growth, while single crystal epitaxial layer is formed at 1100 °C and beyond. XRD measurements near Si (004) peak confirm the compensation of the Si1-x Gex lattice mismatch strain by substitutional C. C implantation is also found to suppress the End of Range (EOR) defect growth.

Interlayer correlations and helical spin ordering in MnTe/CdTe multilayers (abstract)

Mn-VI/II-VI superlattices belong to the family of artificial multilayered structures composed from zinc-blende II-VI semiconducting compounds and Mn chalcogenides using MBE and ALE. While all naturally existing crystals of MnTe have the NiAs (hexagonal) structure, the zinc-blende form is a fcc Heisenberg antiferromagnet with dominant nearest neighbor interactions. The magnetic properties of such frustrated systems are strongly influenced by lattice mismatch strain. For example, tensile strain produces an incommensurate helical antiferromagnetic phase with an in-plane axis. The strain and therefore helical period increases with thickness of CdTe, but in the presently studied samples is weak enough to allow investigation of the onset of incommensurate helical effects. In previously studied MBE-grown Mn-VI/Zn-VI systems, the nonmagnetic spacers were too thick to allow interlayer coupling. We report neutron diffraction studies of new MnTe/CdTe superlattices prepared by ALE with extremely thin CdTe spacers (from 2 to 6 monolayers). The results indicate the formation of spin helices, consistent with the tensile nature of strain in the MnTe layers. In addition, the widths of the AF diffraction peaks and the presence of satellite peaks clearly indicate that the magnetic interactions propagate through the CdTe spacers, introducing coherence between the helices in different MnTe layers. The nature of the interaction responsible for this transfer is not yet clearly understood. Some possible mechanisms are discussed.

Interlayer coupling in antiferromagnetic EuTe/PbTe superlattices (abstract)

We report neutron diffraction studies of magnetic correlations in MBE-grown (111) EuTe/PbTe multilayers. Bulk EuTe is a type-II fcc antiferromagnet. This structure consists of ferromagnetic (FM) sheets of spins on (111)-type planes which are coupled antiferromagnetically to one another. In EuTe/PbTe the lattice mismatch strain selects an arrangement of the FM sheets parallel to the multilayer plane. Thus, in the case of an odd number of EuTe monlayers, there should be a nonzero magnetic moment—in other words, such superlattices are expected to behave as ferrimagnets, not anitferromagnets. This has been confirmed by recent SQUID studies. As suggested by the above model, the dipolar fields arising from the lack of total moment compensation may introduce interlayer magnetic coupling, even though in this system there are no carriers that can transfer magnetic interactions across the nonmagnetic spacers through the RKKY mechanism, which is usually the case in coupled magnetic multilayered systems. Our experiments have indeed revealed clear magnetic superlattice peak patterns in several samples. The results of measurements in high external fields (up to 6 T) bring further support for the dipolar nature of the observed interlayer coupling. However, in some contrast to the expected scenario, the most pronounced coupling effects were seen in the case of an even number of monolayers. Possible explanations of this fact (e.g., higher-order terms in dipolar fields, or structural imperfections) are discussed.

Optical transitions in strained In1−xGaxAsyP1−y quantum wells clad by lattice-matched barriers of InGaAsP

We extend the deformation potential model developed previously for InGaAs/InP quantum wells to include strained quaternary wells, under compressive or tensile strain, and lattice-matched (or also strained) quaternary barriers. This requires interpolation of the needed quaternary materials parameters in a plane bound by the four binary parent compounds. The calculated energies of heavy and light hole transitions are compared to measured values obtained on a set of compressively strained multi-quantum-well quaternary structures grown on (100) InP with the lattice mismatch strain Δa/a as large as 0.75%. Our experimental results are in good agreement with the extended model.

Strain effects on the intervalence-subband normal-incidence absorption in a p-type InGaAs/InP quantum well

The lattice mismatch (strain) effects on the normal-incidence infrared absorption in In1−xGaxAs/InP quantum wells is investigated systematically, both tensile (x≳0.47) and compressive (x<0.47) cases being considered. The difference of the valence-band parameters in the well and barrier materials is taken into account in the dipole matrix element calculations. For a constant hole sheet density, the compressive stress is found to enhance the infrared absorption substantially in the frequency range around 100 meV, corresponding to the H1–H3 type transitions, and the tensile stress is shown to decrease the normal-incidence intervalence-subband absorption.

A three wavelength infrared focal plane array detector element

We have demonstrated a novel three wavelength InGaAs focal plane array pixel element for detection at wavelengths from 0.9-2.6 μm, where each of three wavelength sensitive detectors are individually addressable. This device consists of successively smaller bandgap layers of In/sub x/Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> spl minus/x/As (x/spl ges/0.53), separated by compositionally graded layers of InAs/sub y/P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> spl minus/y/ to decrease defects induced by lattice mismatch strain with the InP substrate. The various layers were selectively removed so that p-n junctions with different wavelength response are formed. The three detectors have quantum efficiencies between 55 and 95% for front illumination and 15 and 60% for back illumination, and dark currents from 0.01 to 10 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Strained quaternary quantum well lasers for high temperature operation

We describe compressively strained separate confinement heterostructure 1.3 μm quantum well lasers optimized for high temperature operation. The active layer consists of ten GaInAsP wells, each 40–80 Å thick, grown under compressive lattice mismatch strain of Δa/a≤0.75%. Within the constraints of the well composition and thickness imposed on the active region, strain is necessary for efficient laser operation. Best results are obtained for Δa/a∼0.2%–0.3% with the laser threshold as low as 5 mA and slope efficiency of 42 mW/mA. In the temperature range of 25–85 °C a slope efficiency change as small as 30% was achieved. Power output of at least 20 mW can be maintained up to 100 °C at a current drive below 150 mA.

Effect of strain on confined optic phonons of highly strained InAs/InP superlattices

We have measured Raman scattering and high-resolution x-ray diffraction from highly strained [(InAs)4(InP)4]N short-period superlattices grown on InP substrates by atomic layer epitaxy at 355 °C. The InAs and InP confined phonons are observed in these highly strained short-period superlattices. The energy of the InAs confined longitudinal-optical phonon (LO) modes of a fully strained superlattice (with N=8) is blue shifted by about 10 cm−1 compared to the LO phonon of bulk InAs. This effect is explained by the large biaxial strain existing in the InAs layers. The observed frequency shift agrees with the lattice-mismatch strain given by elasticity theory and independently measured by high-resolution x-ray diffraction. No evidence of a frequency shift of the InP confined LO modes in the N=8 fully strained superlattice is observed, indicating that the strain is confined to the InAs layers. We show that in a partially relaxed superlattice (with N=20), the InAs layers are in compression, while the InP layers are in tension. In this case the InP confined LO1 phonons are red shifted by about 3 cm−1 compared to the InP LO1 phonons of the N=8 fully strained superlattice, and the shift of the InAs confined LO phonons with respect to the LO phonons of bulk InAs is also reduced.

Surface morphology and quality of strained InGaAs grown by molecular-beam epitaxy on GaAs

InGaAs layers of varying epilayer thickness grown at different temperatures on GaAs substrates by molecular-beam epitaxy are analyzed by atomic force microscopy. The surface roughness and morphology of InGaAs grown at substrate temperatures ranging from 450 to 530 °C are analyzed as the epilayer thickness is increased from 100 to 800 Å. Over an area of 100 μm2, an abrupt increase in the surface roughness probably indicates an acceleration of surface roughening mechanisms related to the process of strain relaxation. At higher InGaAs thicknesses, surface corrugations were observed. The results indicate that thicker pseudomorphic InGaAs films on GaAs substrates can be grown at lower temperatures. Raman spectroscopy data of strain-induced shifts in the GaAs-like mode longitudinal optic phonon indicate that the GaAs has tensile strain along the interface. The strain calculated from the observed frequency shift agrees with the lattice-mismatch strain given by the elasticity theory.

Epitaxial cubic and hexagonal crystallites in titanium thin films evaporated onto sodium chloride substrates

Abstract Ti films of 10 nm thickness prepared by evaporation onto NaCl substrates at 250°C have been studied by high-resolution electron microscopy. The films have two characteristic regions: CaF2-type TiHx crystallites predominated, while h.c.p. Ti crystallites containing (1101) and/or (1011) twin boundaries grew in the other region. This is the first observation of {1101} twins in evaporated h.c.p. Ti films. Furthermore, misfit dislocations and a gentle bending of the TiH x lattice occurred at the interface between TiHx and h.c.p. Ti, owing to lattice mismatch strains. The orientation relationships and growth mechanism of the epitaxial films, especially of the h.c.p. Ti films, is discussed.

Stability analysis of quaternary alloys including the lattice mismatch strain energy

A thermodynamic analysis is given on the unstable region behavior for type A 1- x B x C 1- y D y quaternary solid solutions, including the effect of the lattice mismatch between the substrate and the epitaxial layer. The elastic free energy expression has been derived in the case of the biaxial plane stress. The thermodynamic model for stability analysis takes into account the additive strain free energy, a function of the elastic coefficients of the materials and the excess free energy of mixing of the quaternary solid solution. Computer simulation results have shown that in the case of In 1- x Ga x P 1- y As y epitaxial layers grown on GaAs and InP both the epitaxial stabilization and destabilization can occur within the unstable areas.

Effects of thermal strain on the optical properties of heteroepitaxial ZnTe.

Optical measurements have been used to study the biaxial tensile strain in heteroepitaxial ZnTe grown by molecular-beam epitaxy on both GaAs and GaSb substrates, and its effect on the low-temperature photoluminescence (PL) spectrum of the material. The observed strain (0.92\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ for ZnTe/GaAs and 0.45\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ for ZnTe/GaSb) agrees with that expected for differential thermal contraction from the growth temperature to low temperature, based on the difference in thermal expansion coefficients. The amount of strain increases with growth temperature, as expected, but decreases slightly in thin layers on GaAs. The latter effect is due to incomplete relaxation of the large (7.6%) lattice mismatch strain between ZnTe and GaAs, the unrelieved part of which constitutes a biaxial compressive strain. Reflectance and variable temperature PL measurements show that the free exciton splits into heavy-hole (${\mathit{X}}_{\mathrm{hh}}$) and light-hole (${\mathit{X}}_{\mathrm{lh}}$) components, which both shift to lower energy. The J=1 (allowed) and J=2 (forbidden) components of the oxygen isoelectronic center bound exciton are mixed and shifted to lower energy by the strain. Temperature-dependent PL measurements show that the oscillator strengths of the two components are strongly redistributed by the strain. We calculate the strain-induced splittings in the O-bound exciton and in the excitons bound to neutral acceptors and neutral double acceptors, and find good agreement with the experimentally observed peak positions. Further confirmation of our assignments of the strain-split and shifted bound exciton peaks is obtained using magnetospectroscopy in fields up to 12 T. The diamagnetism, g factors, and splitting patterns of the free and bound excitons in the magnetic field are discussed.