Strain-relaxed Si Research Articles

Analytical strain relaxation model for Si1−xGex/Si epitaxial layers

An approximate but accurate analytical solution is presented for the system of differential equations used by Houghton to model kinetically limited strain relaxation in Si1−xGex alloys layers growing on Si substrates [J. Appl. Phys. 70, 2136 (1991)]. This solution makes it much easier to compare the relaxation model with experimental data. The analytical results are used to refit the Houghton model parameter n0 (representing the initial heterogeneous density of dislocation sources) to published relaxation data, including post-1991 experimental work. The fits, which include experiments in which the growth temperature ranged from 450 to 750 °C, show considerable scattering in n0, but suggests that n0 increases as the growth temperature is lowered. Since this trend was not apparent in the original Houghton work, a detailed analysis is carried out for samples grown and annealed at temperatures below 450 °C. For this purpose, the Houghton model is extended to include the reduction in effective stress as the strain relaxation advances as well as the effect of dislocation pinning. The analysis confirms that n0 increases as the growth temperature is lowered. Possible physical reasons are discussed, and an empirical fit to the temperature dependence of n0 is used to generate revised predictions of apparent critical thicknesses.

Promoting strain relaxation of Si 0.72Ge 0.28 film on Si (1 0 0) substrate by inserting a low-temperature Ge islands layer in UHVCVD

A flat, fully strain-relaxed Si 0.72Ge 0.28 thin film was grown on Si (1 0 0) substrate with a combination of thin low-temperature (LT) Ge and LT-Si 0.72Ge 0.28 buffer layers by ultrahigh vacuum chemical vapor deposition. The strain relaxation ratio in the Si 0.72Ge 0.28 film was enhanced up to 99% with the assistance of three-dimensional Ge islands and point defects introduced in the layers, which furthermore facilitated an ultra-low threading dislocation density of 5 × 10 4 cm −2 for the top SiGe film. More interestingly, no cross-hatch pattern was observed on the SiGe surface and the surface root-mean-square roughness was less than 2 nm. The temperature for the growth of LT-Ge layer was optimized to be 300 °C.

Paramagnetic point defects at interfacial layers in biaxial tensile strained (100)Si/SiO2

A comparative electron spin resonance study is reported on (100)Si/SiO2 entities, grown by thermal oxidation of biaxial tensile strained (100)Si (sSi) layers, epitaxially grown on a strain relaxed Si0.8Ge0.2 buffer layer, and standard (100)Si. In the as-oxidized state a significant decrease (∼50%) of inherently incorporated interface defects, Pb0 and Pb1, is observed, i.e., the sSi/SiO2 interface is found to be inherently significantly improved in terms of electrically detrimental interface traps (Pb0). After vacuum-ultraviolet irradiation two more SiO2-associated defects appear, namely Eγ′ (generic entity O3≡Si·) and EX. Interestingly, a decrease (∼50%) of Eγ′ defect density is observed compared to standard Si/SiO2. This reduction in inherent electrically active interface (Pb0) and near-interface (Eγ′) traps would establish sSi/SiO2 as a superior device structure for all electrical properties where (near)-interfacial traps play a detrimental role. For one, the reduction of detrimental (near)-interface defects may be an additional reason for the commonly reported mobility enhancement in sSi/SiO2 based metal–oxide–semiconductor structures over standard Si/SiO2 ones, and at the same time account for the reported reduction of 1/f noise in the former structures. The data also confirm the generally accepted notion that Pb-type defects are mismatch induced defects.

From rolled-up Si microtubes to SiO x/Si optical ring resonators

We present a fabrication technique to realize SiO x /Si optical ring resonators based on rolled-up Si microtubes. The strain gradient inside a partially strain-relaxed Si film on a relaxed Ge buffer is employed to roll up the released Si or SiO x /Si layers into microtubes by selectively etching away the Ge relaxed buffer. After annealing the SiO x /Si tubes at 850 °C, the hybrid microtubes emit light in the visible spectral range at room temperature. Optical resonant modes are observed in the freestanding hybrid tubes and are found to sensitively depend on the structural uniformity. Finally, we demonstrate positioning of freestanding SiO x /Si tubes on a Si substrate by using standard lithographic techniques.

Growth of strain relaxed Si 1−yC y films using SOI substrates

Strain relaxed Si 1− y C y films were grown on silicon on insulator (SOI) substrates. The Si 1− y C y films were deposited by a gas-source molecular beam epitaxy (GS-MBE) system using a gas mixture of Si 2H 6 and C 2H 2. The crystal structures and surface morphologies were investigated by X-ray reciprocal lattice space mapping and atomic force microscopy (AFM), respectively. The X-ray reciprocal maps showed that the strain relaxation started at a lower Si 1− y C y film thickness by using SOI substrates with thinner Si layer as compared to the case of strain relaxed Si 1− y C y films grown on bulk-Si substrates. As a result, we have obtained higher strain relaxation ratio. Surface roughness of strain relaxed Si 1− y C y films grown on SOI substrates was also evaluated and it was drastically suppressed by using SOI substrates. These results indicate the use of SOI substrates is effective for the growth of high-quality strain relaxed Si 1− y C y films.

Carbon doping effect on strain relaxation during Si1−x−yGexCy epitaxial growth on Si(1 0 0) at 500 °C

On strain relaxation during Si1−x−yGexCy epitaxial growth on Si(1 0 0) using an ultraclean hot-wall low-pressure chemical vapour deposition system at 500 °C, the carbon doping effect has been investigated. The depth profile of the strain in Si1−x−yGexCy films is measured by x-ray diffraction and Raman scattering spectroscopy combined with a wet etching technique. It is confirmed that the strain relaxation of Si1−xGex films (x = 0.45) occurs uniformly at every depth from the surface to the heterointerface. On the other hand, strain relaxation occurs partially in the surface part of Si1−x−yGexCy films (x = 0.45, y = 0.016), although the bottom part near the heterointerface is still strained.

Intense photoluminescence from Ge(Si) self-assembled islands embedded in a tensile-strained Si layer

Photoluminescence (PL) of structures with Ge(Si) self-assembled islands located between tensile-strained Si layers grown on smooth strain-relaxed Si1−xGex/Si (0 0 1) (x = 0.2–0.3) buffer layers has been studied. The peak observed in the PL spectra of the structures studied is associated with indirect optical recombination of electrons confined in strained Si layers on the heterojunction with islands and holes localized in Ge-rich islands. The redshift of the PL peak with an increase in the strained Si layer thickness confirms the validity of the proposed optical transition. The intensity of the PL signal from Ge(Si) islands located between tensile-strained Si layers at 77 K is one order of magnitude higher than the PL signal from Ge(Si) islands grown on unstrained Si (0 0 1) substrates. The substantial rise of the PL signal is associated with efficient quantum confinement of electrons in the strained Si layer on the heterojunction with Ge(Si) islands.

Defects and impurities in SiGe: The effect of alloying

As a fully miscible solid solution, silicon–germanium (Si1−xGex) alloy has attracted significant interest in recent years owing to its manifold applications in microelectronic and optoelectronic devices raised by the possibility of tailoring the band gap and lattice parameter. These additional degrees of freedom and the random distribution of Si and Ge atoms imply new phenomena not observed in elemental group-IV semiconductors. A fundamental understanding of the microscopic properties of the alloy is therefore essential for a good control of SiGe-based device performance. As a corollary, an ultimate understanding of the electronic properties of the most abundant point defects and impurities is needed. This review aims at describing recent results obtained with metallic impurities and irradiation-induced interstitial defects in strain relaxed Si1−xGex alloy in which two main aspects will be highlighted. Firstly, the electronic transitions between deep levels and the allowed bands as a function of Ge composition will be discussed with a special emphasis on practical consequences of the alloy-induced energy shifts. Secondly, the role of the atomic environment on the electronic transitions and diffusion mechanism will be highlighted.

Ultrathin amorphous Si layer for the growth of strain relaxed Si0.75Ge0.25 alloy layer

We propose a method for the growth of strain relaxed and smooth Si0.75Ge0.25 alloy layers on a Si(001) substrate. In this method, we have used an ultrathin amorphous Si (UTA-Si) layer as a buffer layer and implemented a two-step process to grow the top alloy layer. High-resolution x-ray diffraction studies show that the alloy layers are highly relaxed. Topographic studies by contact mode atomic force microscopy show that the surfaces are very smooth. UTA-Si works as a strain adjuster and helps to reduce residual strain introducing dislocation in the buffer and substrate regions. However, it was observed that the residual strain and the surface morphology depend on the thickness of the UTA-Si buffer layers and also on the growth mode of the alloy layer.

Phosphorus diffusion in the presence of threading dislocations in strain relaxed SiGe films

We have studied phosphorus diffusion in strain relaxed Si 1− x Ge x films ( x=0.1 and 0.2) by secondary ion mass spectrometry (SIMS). The relaxed films were grown with low-pressure chemical vapor deposition (LPCVD) on a Si substrate followed by a graded SiGe layer. Two sets of samples were prepared under different growth conditions, and by transmission electron microscopy (TEM) it was shown that these conditions resulted in one set of samples containing a high density of threading dislocations in the relaxed films, and one set with a low dislocation density. The SIMS profiles of the phosphorus distributions in the samples, after annealing in N 2-ambient in the temperature range of 700–950 °C, show that the phosphorus diffusion is significantly faster in the films with the high dislocation density. Furthermore, the data suggests that the fast diffusion is due to a higher mobility of the diffusing complex rather than an increase in the point defect concentration mediating the diffusion, a result which indicates that the threading dislocations may act as channels for the rapid dopant diffusion in SiGe.

Kinetics of strain relaxation in Si1−xGex thin films on Si(100) substrates: Modeling and comparison with experiments

We report the results of a theoretical analysis for the kinetics of strain relaxation in Si1−xGex thin films grown epitaxially on Si(100) substrates. The analysis is based on a properly parametrized dislocation mean-field theoretical model describing plastic deformation dynamics due to threading dislocation propagation and addresses strain relaxation kinetics during both epitaxial growth and thermal annealing, including post-implantation annealing. Theoretical predictions for strain relaxation as a function of film thickness in Si0.80Ge0.20∕Si(100) samples annealed after epitaxial growth either unimplanted or after He ion implantation are in excellent agreement with experimental measurements [J. Cai et al., J. Appl. Phys. 95, 5347 (2004)].

Effect of strain on p-channel metal-oxide-semiconductor field-effect-transistor current enhancement using stress-modulated silicon nitride films

The stress in chemical-vapor-deposited silicon nitride films (∼100nm thick) is modulated by an ion implantation technique. The tensile-strained silicon nitride film is deposited on the back and front sides of a Si wafer. After implantation of P+, As+, Sb+, or BF2+ ion species, the stress of the silicon nitride film becomes compressive on the front side. It can be explained by strain relaxation of Si–N bond on the front side and highly tensile-strained Si–N bond on the back side of wafer. The compressive stress is increased with a double-implantation process because of more atomic collisions in the silicon nitride film, as well as more strain relaxation of Si–N bond on the front side of wafer. After a thermal annealing process, the compressive stress in the silicon nitride film with P+, As+, or Sb+ implantation is reduced, while compressive stress is increased for BF2+ ion implantation due to formation of the B–N bond in silicon nitride film. To justify the stress-modulated silicon nitride film by the ion implantation technique, the p-channel metal-oxide-semiconductor field-effect transistor (p-MOSFET) is fabricated. The drive current of a p-MOSFET is improved by 7–13% for implanted silicon nitride films, due to the compressive strain-induced effective mass lowering in the channel.

Strain relaxation by stripe patterning in Si/Si 1 − x Ge x/Si(100) heterostructures

The relaxation of strain in Si/Si 1 − x Ge x/Si(100) heterostructure by stripe patterning in [110] direction has been investigated using Raman scattering spectroscopy. It is found that the strain is relaxed by stripe patterning and that the degree of strain relaxation becomes larger for a narrower stripe, higher Ge fraction, and thinner capping Si layer. Even for the 5 μm-width line, the strain relaxation was observed at 0.4 μm inside of the stripe-patterned heterostructure. Therefore, it is suggested that strain relaxation in Si 1 − x Ge x layer proceeds in the region from the stripe edge to the inner position of 0.4 μm away from the edge.

Growth of strain relaxed Si 1− yC y on Si buffer layer by gas-source MBE

We studied the growth of strain relaxed Si 1− y C y films on Si(001) substrates. The critical thickness of Si 1− y C y on Si was calculated using People's equation. The strain relaxed Si 1− y C y films on Si substrates were grown by gas source MBE (GS-MBE) and it was found from cross sectional TEM analysis that the use of Si buffer layers was very important in order to obtain high quality Si 1− y C y films. Lattice relaxation was clearly observed when the thickness of the Si 1− y C y layers exceeded their critical thickness. The relaxation ratio of Si 1− y C y films with a carbon concentration of 1.0% was 40% in the vertical direction when the film thickness was 1800 nm.

The effect of biaxial strain on impurity diffusion in Si and SiGe

Results from diffusion studies of different impurities in biaxially strained Si and Si 1 − x Ge x for low x-values will be presented. The structures are all molecular-beam epitaxy (MBE) grown on strain-relaxed Si 1 − x Ge x layers, and the impurity profiles are introduced during growth. We have in particular been concerned with the effect of biaxial strain (compressive and tensile) on the diffusion of pure vacancy-assisted diffusers (Sb and, partly, Ge) and pure interstitial-assisted diffusers (B and P). It is found that compressive biaxial strain retards the diffusion of the interstitial-assisted diffusers, whereas tensile biaxial strain enhances the diffusion of these impurities. The opposite is the case for the vacancy-assisted diffusers.

Coupling atomistic and continuum length scales in heteroepitaxial systems: Multiscale molecular-dynamics/finite-element simulations of strain relaxation inSi∕Si3N4nanopixels

A hybrid atomistic-continuum simulation approach has been implemented to study strain relaxation in lattice-mismatched Si/ Si3N4 nanopixels on a Si111 substrate. We couple the molecular-dynamics MD and finite-element simulation approaches to provide an atomistic description near the interface and a continuum description deep into the substrate, increasing the accessible length scales and greatly reducing the computational cost. The results of the hybrid simulation are validated against full multimillion-atom MD simulations. We find that strain relaxation in Si/ Si3N4 nanopixels may occur through the formation of a network of interfacial domain boundaries reminiscent of interfacial misfit dislocations. They result from the nucleation of domains of different interfacial bonding at the free edges and corners of the nanopixel, and subsequent to their creation they propagate inwards. We follow the motion of the domain boundaries and estimate a propagation speed of about 2.5 10 3 m / s. The effects of temperature, nanopixel architecture, and film structure on strain relaxation are also investigated. We find: i elevated temperature increases the interfacial domain nucleation rates; ii a thin compliant Si layer between the film and the substrate plays a beneficial role in partially suppressing strain relaxation; and iii additional control over the interface morphology may be achieved by varying the film structure.

Investigation of hydrogen implantation-induced blistering in SiGe

A systematic investigation of the hydrogen implantation-induced blister formation on the surface of Si 0.78Ge 0.22 when annealed in the temperature range of 300–700 °C was carried out. The strain-relaxed Si 0.78Ge 0.22 layers were grown epitaxially on 8-inch Si(1 0 0) substrates by reduced pressure chemical vapour deposition (RPCVD). These wafers were implanted with hydrogen, H 2 +, ions at 240 keV with a dose of 5 × 10 16 cm −2. The formation of optically detectable blisters on the surface was observed using an optical microscope in Nomarski contrast mode. The width and depth of the blisters was determined with a profilometer. The characterization of the hydrogen implantation-induced damage and defects inside the Si 0.78Ge 0.22 layer was performed using cross-sectional transmission electron microscopy (XTEM). The Arrhenius plot of blistering time versus annealing temperature revealed two different activation energies for the formation of blisters. In the low temperature regime (300–400 °C) an activation energy of 1.2 eV was obtained, while in the high temperature regime (400–700 °C) the activation energy was 0.38 eV. In analogy to the case in Si, the lower activation energy can be associated with the free atomic diffusion of hydrogen in Si 0.78Ge 0.22 and the higher activation energy can be related to the hydrogen diffusion limited by trapping-detrapping phenomena. The depth of the broken blisters and the depth of the hydrogen-induced microcracks inside Si 0.78Ge 0.22 came out to be around 1.05 μm, which matches quite well with the calculated (by Stopping and Ranges of Ions in Matter, SRIM code) concentration peak of the hydrogen distribution inside Si 0.78Ge 0.22 for the case of 240 keV H 2 + implantation.

Study of recombination and transport characteristics in strain-relaxed Si–SiGe layers

A set of direct techniques were combined to resolve the carrier recombination, trapping and diffusion parameters in thin strain-relaxed SiGe buffer (SRB) layers in a contact-less way. The excess carrier density relaxation in a wide time scale from picoseconds to tens of microseconds was examined by the microwave absorption/reflection and light-induced reflection/grating transients. A decrease of recombination lifetime in nanosecond-time scale as a function of threading dislocations (TD) density was determined. The asymptotic hyperbolic-like carrier decay in the SRBs was ascribed to multi-trapping processes characterized by trapping coefficients Ktr > 10. Relaxation of multi-trapping in the disordered structure of the dislocation-rich SRBs was generalized by the stretched-exponent approach. The time-stretching index of 0.15 was found to be the same for dislocation-rich SRBs, containing different densities of TDs. Value of the lateral carrier ambipolar diffusion coefficient Da ⩽ 0.13 cm2 s−1 was estimated. Peculiarities of the J–V characteristics of diodes fabricated in the SRBs were examined and attributed to the space charge limitation effects. These features correlate with carrier trapping and slow diffusion unveiled by transient techniques.

Determination of local lattice tilt in Si1−xGex virtual substrate using high resolution channeling contrast microscopy

Strain relaxed Si1−xGex virtual substrates with low densities of threading dislocations are grown by molecular beam epitaxy or chemical vapour deposition using the compositional grading technique. They have a wide range of applications in microelectronic and optoelectronic devices. A common surface feature of such virtual substrates is a cross-hatch pattern that is oriented along orthogonal [110] directions and is believed to be associated with strain relaxation processes. Such cross-hatch roughening affects carrier mobility and the quality of the subsequent layers grown on the virtual substrates. Accurate measurements of the material properties of these virtual substrates are essential for the optimization of the growth processes and conditions. Channeling contrast microscopy, which uses a focused ion beam to obtain laterally resolved channeling yield information, is an ideal technique to determine the microstructural characteristics of such samples. Here, we determine the extent of the local lattice tilt from channeling contrast measurements and investigate the influence of the growth temperature and grading rate on surface morphology, crystalline quality and the extent of the lattice tilts in the constant composition layer.

Transport and absorption in strain-compensated Si/Si 1−xGe x multiple quantum well and cascade structures deposited on Si 0.5Ge 0.5 pseudosubstrates

Various strain-compensated Si/SiGe quantum well structures with high Ge concentrations in the SiGe wells (70–85% Ge) have been successfully deposited by very low-temperature molecular beam epitaxy on thick strain-relaxed Si 0.5Ge 0.5 buffer layers. The samples have been subject to intensive and careful characterization of their structural, electronic and optical properties. X-ray diffraction and reflectivity measurements using synchrotron radiation allowed accurate determination of layer thicknesses and composition of the quantum well structures. The r.m.s. roughness at Si/SiGe interfaces is less than 0.4 nm I– V characteristics of resonant tunneling devices reveal strong negative differential resistance peaks for heavy hole tunneling but no trace for tunneling via light hole states. Two-dimensional hole gases with a mobility of up to 0.49 cm 2/Vs in 7 nm wide, modulation-doped Si 0.8Ge 0.2 quantum wells with Si barriers and relaxed Si 0.5Ge 0.5 spacer layers underline the high structural quality. The dependence of hole mobility on well width can be explained by an interface roughness of 0.4 nm, in accordance with the X-ray reflectivity measurements, and the associated fluctuation of the strain at the interfaces. Using the structural parameters determined by X-ray analysis as input parameters, the intersubband absorption spectra of strain-compensated quantum well structures have been calculated by the 6 band k· p method. The theoretical predicted spectra are in excellent agreement with those obtained by experiments. Our comprehensive study results in a set of very congruent data enabling to custom design complex Si/SiGe structures such as p-type quantum cascade emitters.