Critical Thickness For Strain Relaxation Research Articles

(Invited) Highly Doped Si1-XGex Epitaxy in View of S/D Applications

Modern CMOS devices are a fertile ground for innovation, as scaling and conceptual evolutions drive regular updates in process and material specifications. Requirements are usually set by device modelling, which defines the performance objectives for a new technology. Simulations recently carried out assuming fin [1] and nanosheet [2] field effect transistor geometries have for instance highlighted limitations due to the growing importance of contact resistance in devices with reduced dimensions. To meet the objectives, source/drain (S/D) contact regions receive a specific attention and aggressive targets are set to enable higher-performing generations of devices. Within this framework, contact resistivities (rc) lower than 1E-9 W.cm2 are specified for sub 5 nm technology nodes. Reaching this objective for n and p-type S/D is a challenging task, both from the perspectives of obtaining the right materials and of reliably measuring low rc values [3]. Several material parameters are known to affect the final contact resistivity. Novel epitaxial growth schemes are considered to maximize the active doping concentration to narrow the Schottky barrier at the metal - S/D interface. Additionally, the composition of and strain relaxation in the S/D layer affect band alignment and must be considered.This contribution focuses on S/D epitaxial layers grown and in-situ doped by chemical vapor deposition, and on the properties of fabricated Ti / p-Si1-xGex contacts. For a given Ge concentration, the Ti / Si1-xGex:B contact resistivity is minimized for an optimized range of Si1-xGe1-x:B layer thickness, and increases as the layer relaxes [4]. With increasing Ge content, the rise in contact resistivity begins for lower thicknesses. Observed trends are nevertheless similar (Fig. 1). Please note that the targeted boron concentration is constant within each series of samples but differs for different Ge concentrations. The variations in resistivity and contact resistivity as seen for different series are assigned to the different boron concentrations rather than the variation in Ge concentration. Indeed, lower resistivities and contact resistivities are measured for higher active boron concentrations.The correlation between the electrical properties and layer thickness reported in figure 1 confirms that preserving strain in the Si1-xGex:B S/D material is important. However, the increase in S/D layer and contact resistivities with increasing layer thickness is not caused by the (small) increase in electrical band gap. Instead, B incorporation during epitaxial growth is affected by the initiation of layer relaxation and decreases as the material relaxes (R) as can be concluded from Fig. 2. This figure shows the boron concentration as function of depth for two partially relaxed Si1-xGex with 25% and 65% Ge, respectively. A reduction in B incorporation when the S/D material thickness exceeds the critical thickness for strain relaxation (tc) is observed for the whole range of Ge concentrations. This phenomenon is supported by density functional theory results which indicate a reduced propensity of boron to occupy substitutional lattice sites in relaxed Si1-xGex.

Stretching Epitaxial La0.6Sr0.4CoO3−δ for Fast Oxygen Reduction

The slow kinetics of the oxygen reduction reaction (ORR) is one of the key challenges in developing high performance energy devices, such as solid oxide fuel cells. Straining a film by growing on a lattice-mismatched substrate has been a conventional approach to enhance the ORR activity. However, due to the limited choice of electrolyte substrates to alter the degree of strain, a systematic study in various materials has been a challenge. Here, we explore the strain modulation of the ORR kinetics by growing epitaxial La0.6Sr0.4CoO3-{\delta} (LSCO) films on yttria-stabilized zirconia substrates with the film thickness below and above the critical thickness for strain relaxation. Two orders of magnitude higher ORR kinetics is achieved in an ultra-thin film with ~0.8% tensile strain as compared to unstrained films. Time-of-flight secondary ion mass spectrometry depth profiling confirms that the Sr surface segregation is not responsible for the enhanced ORR in strained films. We attribute this enhancement of ORR kinetics to the increase in oxygen vacancy concentration in the tensile-strained LSCO film owing to the reduced activation barrier for oxygen surface exchange kinetics. Density functional theory calculations reveal an upshift of the oxygen 2p-band center relative to the Fermi level by tensile strain, indicating the origin of the enhanced ORR kinetics.

Thickness dependent structural and magnetic properties of Nd2NiMnO6 epitaxial thin films

The structural and magnetic properties of epitaxial Nd2NiMnO6 (NNMO) thin films with varying thickness have been investigated. NNMO thin films of thickness ranging from 60 to 300nm were deposited onto (001) oriented SrTiO3 substrate by pulsed laser deposition technique. The X-ray diffraction and Raman scattering analysis confirm that all films have c-axis epitaxial growth with orthorhombic structure and films having smaller thickness (<180nm) were under tensile strain. The Raman spectra suggest that critical thickness for strain relaxation lies between 90 and 180nm. The magnetic measurements suggest a strong dependence of magnetic properties on the present strain in the films. The ferromagnetic transition temperature (TC) and saturation magnetization were found to increase with increasing film thickness. The film with thickness of 300nm exhibits the highest saturation magnetization (~3.5×105A/m) with a Curie temperature (TC) of 200K, which is higher than the bulk TC of NNMO compound.

Manipulating the metal-to-insulator transition ofNdNiO3films by orbital polarization

We investigate the film thickness dependent metal-to-insulator transition temperature $({T}_{\mathrm{MIT}})$ of $\mathrm{NdNi}{\mathrm{O}}_{3}$ films under tensile and compressive strain states. For the films exceeding the critical thickness for strain relaxation, ${T}_{\mathrm{MIT}}$ varies gradually with the film thickness caused by strain relaxation. The variation tendency differs dramatically for the films below the critical thickness: an increase (decrease) of ${T}_{\mathrm{MIT}}$ with increasing the film thickness for the case of tensile (compressive) strain, which is attributed to the decaying of orbital polarization. As the overlap of $\mathrm{O}\phantom{\rule{0.16em}{0ex}}2{p}_{x,y}$ orbits with $\mathrm{Ni}\phantom{\rule{0.16em}{0ex}}3d{x}^{2}\ensuremath{-}{y}^{2}$ orbits determines ${T}_{\mathrm{MIT}}$, a decrease of ${x}^{2}\ensuremath{-}{y}^{2}$ orbital occupation with increasing film thickness would reduce the orbital overlap and resultant enhanced ${T}_{\mathrm{MIT}}$ for tensile strained films, while their compressive counterparts do the opposite. Our findings identify the importance of orbital polarization in regulating the metal-to-insulator transitions, opening up a new perspective for orbital physics in transition metal oxides.

Crystalline Properties and Strain Relaxation Mechanism of CVD Grown GeSn

The crystalline properties of strained and strain-relaxed CVD-grown GeSn layers were investigated. A positive deviation from Vegard's law was determined, resulting in the extraction of a new experimental bowing parameter for GeSn: bGeSn = 0.041 Å (in excellent agreement with recent theoretical predictions). The GeSn critical thickness for strain relaxation as a function of Sn concentration was determined, resulting in significantly higher values than those predicted by equilibrium models. A composition-dependent strain relaxation mechanism was also found, with the formation of an increasing density of GeSn pyramidal islands in addition to misfit dislocations at lower Sn concentration.

Effects of strain relaxation on bare surface barrier height and two-dimensional electron gas in AlxGa1−xN/GaN heterostructures

A physics based analytical model is presented to describe the bare surface barrier height and the two-dimensional electron gas density in AlGaN/GaN heterostructures versus barrier layer thickness and Al content. Details of the model rely on the extraction of the critical thickness for strain relaxation based on the identification of a signature cusp in experimentally obtained bare surface barrier heights. This information is used to obtain the residual strain and total effective polarization when strain relaxation takes place. The model, which covers both unrelaxed and relaxed barriers, shows good agreement with reported experimental results and promises to become a useful tool in advanced design and characterization of GaN based heterostructures.

Crystalline Properties and Strain Relaxation Mechanism of CVD Grown GeSn

In this contribution, we discuss the crystalline properties of strained and strain-relaxed CVD-grown GeSn layers with Sn content in the range 6.4–12.6 at.%. A positive deviation from Vegard's law was observed and a new experimental bowing parameter was extracted for GeSn: bGeSn = 0.041 Å (in excellent agreement with recent theoretical predictions). The GeSn critical thickness for strain relaxation as a function of Sn concentration was determined, resulting in significantly higher values than those predicted by equilibrium models. A composition-dependent strain relaxation mechanism was also found, with the formation of an increasing density of GeSn pyramidal islands in addition to misfit dislocations at lower Sn concentration.

Structural and magnetic properties of La2NiMnO6 thin films on LaAlO3 substrate with varying thickness

In the present work, the effect of film thickness on the structural and magnetic properties of La2NiMnO6 (LNMO) ferromagnetic double perovskite thin films has been systematically investigated. LNMO thin films of varying thickness from 54nm to 256nm were deposited using the pulsed laser deposition technique onto single-crystalline LaAlO3 substrate. The X-ray diffraction pattern and Raman scattering observations reveal that all films have c-axis epitaxial growth and an orthorhombic structure. The Raman spectra of the films are dominated by two broad peaks at around 534cm−1 and 676cm−1 assigned to the antisymmetric stretching (AS) and symmetric stretching (S) vibrations of the octahedra, respectively. The Raman peaks of the LNMO thin films are blue shifted in comparison to those of bulk LNMO and the shift increases with decreasing film thickness, indicating the increased influence of strain. The critical thickness for strain relaxation, as determined from the Raman spectra, is between 110nm and 156nm. However, the strain is observed to have large effect on the magnetic properties of films. Ferromagnetic to paramagnetic transition temperature (Tc) and the saturation magnetization were found to increase with increase in the film thickness. The film with thickness of 256nm exhibits a saturation magnetic moment of 4.5μB/f.u. and coercive field of 630Oe at 10K, with a Curie temperature close to 280K.

Atomic Layer Engineering of Perovskite Oxides for Chemically Sharp Heterointerfaces

Atomic layer engineering enables fabrication of a chemically sharp oxide heterointerface. The interface formation and strain evolution during the initial growth of LaAlO(3) /SrTiO(3) heterostructures by pulsed laser deposition are investigated in search of a means for controlling the atomic-sharpness of the interface. This study shows that inserting a monolayer of LaAlO(3) grown at high oxygen pressure dramatically enhances interface abruptness.

Spectroscopic ellipsometry as an optical probe of strain evolution in ferroelectric thin films

Heteroepitaxial strain in ferroelectric thin films is known to have a significant impact on both their low and high frequency dielectric properties. In this paper, we use ex-situ spectroscopic ellipsometry to study the strain evolution with film thickness, and strain relaxation in ferroelectric Ba0.5Sr0.5<TiO3 epitaxial films grown on single crystal substrates. For films grown on MgO substrates, a critical thickness for strain relaxation is observed. In addition, studies of Ba0.5Sr0.5<TiO3 films grown on different single crystal substrates reveal that the strain relaxation rate can be inferred from changes in the optical properties. Using this information, we show that the optical constants of Ba0.5Sr0.5<TiO3 can be readily tuned via strain engineering.

Dependence on composition of the optical polarization properties of m-plane InxGa1−xN commensurately grown on ZnO

We have grown m-plane InxGa1−xN (x = 0.24-0.43) commensurately on m-plane ZnO by the use of a low temperature epitaxial growth technique and investigated its optical properties. We found that the critical thickness for strain relaxation in the InGaN films prepared by the present technique is at least one order of magnitude larger than those reported by the other techniques. Polarized optical absorption measurements revealed that the allowed optical transition between the conduction band and the topmost valence band in m-plane InxGa1−xN (x = 0.24−0.43) on ZnO is for light polarized along c-axis. We found that the valence band splitting energy strongly depends upon the In composition. The values of the deformation potentials of InN were determined as D3 = 2.4 eV, D4 = −6.3 eV, and D5 = −1.2 eV, by fitting the experimental results with theoretical calculations based on the k·p approach.

Epitaxial growth of La 0.7Sr 0.3CoO 3 thin films on SrTiO 3 substrates by metal–organic deposition

La 0.7Sr 0.3CoO 3 thin films have been epitaxially grown on SrTiO 3 (0 0 1) single-crystal substrates by metal–organic deposition process. Crystallinity and morphology of the obtained films were examined in detail using X-ray diffraction and transmission electron microscopy. The evolution of the out-of-plane lattice parameter with film thickness is investigated. The critical thickness for strain relaxation is found in the 60–80 nm range. The electrical properties of the obtained films in various conditions have been investigated. By increasing the annealing temperature to 1000 °C and the film thickness to 120 nm, the electrical resistivity was decreased by several orders of magnitude. We measured a resistivity of approximately 5 × 10 −4 Ω cm in a wide interval of temperature 77–320 K, making this material a promising candidate for a variety of applications.

In situ measurements of the critical thickness for strain relaxation in β-GaN/MgO structures

It is demonstrated by in situ reflection high-energy electron diffraction (RHEED) studies that the growth of cubic GaN on MgO occurs in a layer-by-layer mode under Ga-rich conditions, growth rates 0.25–0.30 ML/s and 700 °C substrate temperature. From streak-spacing analysis of the observed RHEED patterns, it is possible to infer that the GaN layer grows pseudomorphically on MgO up to 3 monolayers (ML). After that, a relaxation process begins and the layer-by-layer or 2-dimensional growth changes to a columnar or 3-dimensional growth, as evidenced by the transformation of the RHEED patterns, which change from streaky to spotty. The experimental critical thickness ( h c ) value of 3 ML is close to the prediction of the Frank–van der Merwe classical model of 2.5 ML. Other models predict h c values 3–6 times the experimental value experimentally observed.

Influence of defects on structural and magnetic properties of multifunctionalLa2NiMnO6thin films

Thin films of the double perovskite ${\mathrm{La}}_{2}\mathrm{Ni}\mathrm{Mn}{\mathrm{O}}_{6}$ (LNMO) have been grown on various lattice-matched substrates ($\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$, $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$, $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_{3}$, and MgO) by pulsed laser deposition under different oxygen background pressure $(25\char21{}800\phantom{\rule{0.3em}{0ex}}\mathrm{mTorr})$ conditions. The out-of-plane lattice constant of the LNMO film decreases with increasing pressure, which is likely caused by reduction in the defect concentration and improved structural ordering, before leveling off at higher oxygen concentrations. The scanning transmission electron microscopy results confirm that the films are epitaxial, and the interface is sharp and coherent. While few defects are observed in a film grown at a high oxygen pressure $(800\phantom{\rule{0.3em}{0ex}}\mathrm{mTorr})$, a film grown at a lower pressure $(100\phantom{\rule{0.3em}{0ex}}\mathrm{mTorr})$ clearly shows the formation of defects that extend throughout the thickness except for a very thin layer near the interface. The Raman spectra of the films are dominated by two broad peaks at around 540 and $685\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, which are assigned to the antisymmetric stretching and symmetric stretching modes of $\mathrm{Mn}{\mathrm{O}}_{6}$ and $\mathrm{Ni}{\mathrm{O}}_{6}$ octahedra, respectively. The Raman peaks of the LNMO thin films grown in the $800\phantom{\rule{0.3em}{0ex}}\mathrm{mTorr}$ background ${\mathrm{O}}_{2}$ are blueshifted in comparison to those of bulk LNMO, and the shift increases with decreasing film thickness, indicating the increased influence of strain. The critical thickness for strain relaxation, as determined from the Raman spectra, is between 40 and $80\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. However, the strain is observed to have negligible influence on the magnetic properties of films grown at high oxygen pressures. In contrast, films grown at low pressures exhibit degraded magnetic properties, which can be attributed to a combination of increased $B$-site cation disorder and the concentration of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Ni}}^{3+}$ Jahn-Teller ions caused by oxygen or cation related defects. With increasing oxygen pressure during growth, the paramagnetic-ferromagnetic transition temperature $(\ensuremath{\sim}280\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ becomes sharper and the saturation magnetization at low temperatures is enhanced. Based on the electron energy loss spectroscopy studies, the Mn and Ni ions in LNMO thin films are determined to be mixed-valent ${\mathrm{Mn}}^{3+}∕{\mathrm{Mn}}^{4+}$, and charge transition disproportionation of the ${\mathrm{Mn}}^{4+}+{\mathrm{Ni}}^{2+}\ensuremath{\rightarrow}{\mathrm{Mn}}^{3+}+{\mathrm{Ni}}^{3+}$ type likely occurs with increasing oxygen deficiency.

X-RAY DIFFRACTION OF FERROELECTRIC NANODOMAINS IN PBTIO3 THIN FILMS

ABSTRACT X-ray diffraction constitutes a powerful technique with which to characterise ferroelectric domains. Here we describe the principles of ferroelectric nanodomain diffraction and present some results for PbTiO3 thin films grown under tensile strain on two different substrates, with thicknesses below and above the critical thickness for strain relaxation. The combination of conventional and grazing incidence diffraction and the analysis of the scattering between Bragg peaks allowed the identification of a new polar symmetry in ultra-thin films with only anti-parallel 180° domains. Thick films showed tetragonal 90° ferroelectric/ferroelastic domains instead, with a depressed TC and a domain periodicity largely independent of temperature.

Calculation of critical dimensions for wurtzite and cubic zinc blende coaxial nanowire heterostructures

We employ a methodology, based on established approaches for determining the critical thickness for strain relaxation in planar films, to determine critical dimensions for coherently strained coaxial nanowire heterostructures. The model is developed and executed for various specific core-shell heterostructures in [111] zinc blende and [0001] wurtzite geometries. These calculations reveal that critical dimensions in such heterostructures can be quantified by a unique critical core radius and a critical shell thickness, which is dependent on the core radius. It is anticipated that this work will serve as a guide to determine the feasibility of specific coherently strained nanowire heterostructure designs.

Growth and nonlinear characterization of AlN/GaN structures

In this work we present a preliminary study on the influence of the epitaxial growth mode and defect density on second harmonic generation carried out on AlN/GaN structures. The analysed AlN/GaN structures represent the basic unitary cell of a AlN/GaN multilayer structure designed for the enhancement of second harmonic generation. The individual layer thickness is beyond the critical thickness for strain relaxation. The structures were grown on sapphire (Al2O3) substrates using an HT-AlN crystallization layer and morphologically characterized by AFM measurements. Second harmonic generation was observed only in a structure presenting a 3D-like growth mode, obtained for an intermediate value of the V/III ratio and a crystallization layer growth rate of 500 nm h−1.

High In content pseudomorphic InGaAs layers for high-mobility heterostructures on InP (0 0 1)

In this work, we study the molecular beam epitaxial growth of strained InGaAs alloys on InP in order to increase as far as possible the In content of the active layer of heterostructures. We show that a reduction of the growth temperature with respect to the 500 °C usual one could be efficient to increase the critical thickness for strain relaxation but does not result in an electron mobility improvement. At 500 °C, the maximum In content compatible with an active layer thickness of 10–15 nm is 0.75. Within this framework, the electron mobility rises to 16 000 and 139 000 cm 2/V s at 300 and 77 K, respectively, for a heterostructure with a 140 Å thick channel layer and a 400 Å thick spacer layer. Both values are among the best ones ever reported for a InGaAs/InAlAs heterostructure.

In situ measurements of the critical thickness for strain relaxation in AlGaN∕GaN heterostructures

Using in situ wafer-curvature measurements of thin-film stress, we determine the critical thickness for strain relaxation in AlxGa1−xN∕GaN heterostructures with 0.14⩽x⩽1. The surface morphology of selected films is examined by atomic force microscopy. Comparison of these measurements with critical-thickness models for brittle fracture and dislocation glide suggests that the onset of strain relaxation occurs by surface fracture for all compositions. Misfit-dislocations follow initial fracture, with slip-system selection occurring under the influence of composition-dependent changes in surface morphology.

A Physically Based Analytical Model for the Threshold Voltage of Strained-Si n-MOSFETs

A physically based analytic model for the threshold voltage V/sub t/ of long-channel strained-Si--Si/sub 1-x/Ge/sub x/ n-MOSFETs is presented and confirmed using numerical simulations for a wide range of channel doping concentration, gate-oxide thicknesses, and strained-Si layer thicknesses. The threshold voltage is sensitive to both the electron affinity and bandgap of the strained-Si cap material and the relaxed-Si/sub 1-x/Ge/sub x/ substrate. It is shown that the threshold voltage difference between strained- and unstrained-Si devices increases with channel doping, but that the increase is mitigated by gate oxide thickness reduction. Strained Si devices with constant, high channel doping have a threshold voltage difference that is sensitive to Si cap thickness, for thicknesses below the equilibrium critical thickness for strain relaxation.