Strained Si Film Research Articles

(Invited) Engineering Strain, Defects, and Electronic Properties of (110)-Oriented Strained Si

Strain engineering of group IV semiconductors has been extensively investigated with an aim to produce high mobility platform for high performance electronic devices. In particular, combination of strain and 3D structures requires elaborate engineering of strain to obtain optimum effect. Effects of utilization of non-(001)-oriented channel and anisotropic strain are major factors to be considered. The lattice structure of a strained material is generally anisotropic. Symmetry of the lattice structure is expected to have significant influence on electrical properties of crystals, which should be taken into consideration for advanced application of the lattice deformation. In addition, anisotropy in the lattice structure significantly affects the generation of defects during heterostructure formations, which can result in even larger asymmetry in the lattice structure. As a matter of fact, it is found that strain components and morphology of defects are highly asymmetric in the strained Si/SiGe heterostructures formed on (110) substrates. Recently, significant enhancement of the hole mobility was demonstrated in the topmost strained Si film. In this presentation, formation mechanism of the highly asymmetric strain in this heterostructure system and its influence on the properties of strained Si pMOSFET are discussed.Through a computational study of strain effect on the valence band structure, we found that the lowering of the hole effective mass due to strain is expected to be larger on (110) surface than on (001) surface. According to the calculation, the cyclotron effective mass of a hole on (110)-oriented strained Si can be lowered to ~ 0.2 m0 by strain. On the other hand, it was found that the minimum hole effective mass is ~ 0.3 m0 in the case of the (001)-oriented strained Si film. In addition, the result of the calculation shows that the hole effective mass on the (001)-oriented strained Si does not decrease with the increase of strain. These results motivated us to carry out experimental investigation on the growth and electrical properties of (110)-oriented strained Si. Strain-relaxed SiGe was employed as a buffer layer for strained Si layer. The growths were carried out using solid-source molecular beam epitaxy. On a Si(110) substrate, SiGe layer is subjected to biaxial compressive stress which induces glides of partial dislocations on (111) and (11-1) planes. Since the Burgers vectors of these partial dislocations are perpendicular to the in-plane axis [-110], the resultant strain relaxation is highly anisotropic. Accordingly, the surface morphology is also highly anisotropic. It was confirmed that strain in the topmost Si layer is also highly anisotropic in accordance with the lattice parameters of the SiGe layer. These anisotropic features play significant roles in electronic properties of the strained-Si MOSFET formed on this structure in the following manners. First, both the surface and the crystalline morphologies are almost homogeneous in the [-110] direction, for which the relaxation time is expected to be long for the motion in the [-110] direction. Secondly, the anisotropic strain shifts the valence band edges of the layers so that the holes are effectively confined to the high-mobility strained Si layer. A pMOSFET was fabricated on a (110)-oriented strained Si/SiGe structure and hole effective mobility was evaluated by IV and CV measurements. A significantly high hole effective mobility (480 cm2/Vs) was obtained in a pMOSFET with the [-110] channel. High hole mobility in the (110)-oriented strained Si with [-110] channel was also confirmed by gated Hall measurements.

Strain relaxation process and evolution of crystalline morphologies during the growths of SiGe on Si(110) by solid-source molecular beam epitaxy

In a previous study, we reported on a (110)-oriented strained Si pMOSFET with an effective hole mobility as high as 480 cm2/Vs. To impose the lattice strain on the Si channel layer, strain-relaxed SiGe buffer layer is utilized. Defect formation is requisite for strain relaxation in SiGe layer, and at the same time, has the adverse effects for electrical properties of the strained Si film. Therefore, it is necessary to control the formation of the crystalline defects. The aim of this study is to investigate the evolution of the crystalline morphology during the growth of SiGe layers by solid source molecular beam epitaxy, which is utilized to optimize the structural parameters of the strain-relaxed SiGe buffer layers. It is revealed in this study that the lattice strain in SiGe formed on Si(110) begin to relax at a lower film thickness than in the case of SiGe on Si(001). Orientation dependences of surface morphology and degree of strain relaxation are found to be dependent on Ge composition. The development process of the growth twin is also discussed. It is shown that the growth twin generation is effectively suppressed by the composition grading technique.

Stability of strain in Si layers formed on SiGe/Si(110) heterostructures

Strained Si/relaxed SiGe heterostructures grown on Si(110) substrates are attractive as a platform for high-hole-mobility Si-wafer-based electronic devices. The thickness of the strained Si has an impact on the performances of devices since the influence of the states at the Si/SiGe interface and the density of the diffused Ge atoms in the channel region can be suppressed by increasing the strained Si thickness. In this study, the critical layer thickness of a solid-source molecular beam epitaxy (MBE) grown strained Si film on a SiGe/Si(110) heterostructure with a Ge composition of about 30% was investigated by using polarized micro-Raman spectroscopy. Evolution of the surface morphology with the growth of the strained Si layer was also studied using atomic force microscopy. As a result, it is concluded that a nearly 70 nm thick smooth and anisotropically strained Si film can be formed on a Si0.7Ge0.3/Si(110) heterostructure by using the solid-source MBE method.

Gas-source MBE growth of strain-relaxed Si1−xCx on Si(100) substrates

The hole effective mass in a compressively strained Si formed on a (100) surface is expected to be low. The growth of a high quality strain-relaxed Si1−xCx increases the possibility of high performance electronic devices using compressively strained Si film. In this study, growth conditions and their influence on microstructural aspects of Si1−xCx grown by gas-source molecular beam epitaxy were studied. Disilane and trimethylsilane were used as source gases. It was found that the strain-relaxation process and defect formation were influenced not only by substrate temperature but also by flow rates of the source gases. Relationships between the morphological aspects and non-substitutional carbon concentration were studied.

Stress evaluation in thin strained-Si film by polarized Raman spectroscopy using localized surface plasmon resonance

We evaluated the stress in a thin strained-Si film on relaxed SiGe on a surface-oxidized Si substrate using surface enhanced Raman scattering (SERS). The strained-Si peak was enhanced by the SERS technique. However, the strained-Si peak shifted toward a higher wavenumber while the peaks from the Si substrate were unchanged. We performed Raman measurement under the optical geometry in LO and TO phonon active conditions. From these measurements, it was clarified that the peak shift was attributed to the TO phonon peak that appeared, which was caused by the excitation of the z polar component in the near-field light.

Atomically Controlled Plasma Processing for Quantum Heterointegration of Group IV Semiconductors

For the purpose of quantum heterointegration of group IV semiconductors, atomically controlled plasma processing by utilizing an electron-cyclotron-resonance (ECR) plasma enhanced chemical vapor deposition (CVD) at low temperature has been developed to achieve atomic-layer doping and heterostructure formation with nanometer-order thickness control as well as smooth and abrupt interfaces. In this paper, recent typical achievements in the plasma processing are reviewed: (1) By N and B atomic-layer formation and Si epitaxial growth on Si(100), heavy atomic-layer doping was demonstrated. Most of the incorporated N or B atoms can be confined in an about 2 nm-thick region. (2) highly-strained and relaxed Ge films on Si(100) with atomic-order flatness can be epitaxially grown. (3) Using a 84% relaxed Ge buffer layer formed on Si(100), formation of a highly strained Si film with nanometer-order thickness was achieved.

An Analytic Model for Electron Mobility in Strained Si/Si<sub>1-x</sub>Ge<sub>x</sub> nMOSFETs

In order to describe electron transport properties in inversion layer of strained Si/Si1-xGex nMOSFETs, a new analytic electron mobility model is proposed. The model not only takes into account the effect of germanium(Ge) content on phonon scattering-limited mobility and surface roughness-limited mobility, and but also includes the degradation effect of strained Si film thickness and temperature on the device mobility. For various Ge content and a wide range of normal electric field, temperature and strained Si film thickness, the model provides good agreement with the experimental data in references. In addition, the model can be expressed using the analytical expression and can be easily included in the device simulator.

Atomically Controlled Plasma Processing for Group IV Quantum Heterostructure Formation

By low-temperature epitaxial growth of group IV semiconductors utilizing electron-cyclotron-resonance (ECR) plasma enhanced chemical vapor deposition (CVD), atomically controlled plasma processing has been developed in order to achieve atomic-layer doping and heterostructure formation with nanometer-order thickness control as well as smooth and abrupt interfaces. In this paper, typical recent progress in plasma processing is reviewed as follows: (1) By N and B atomic-layer formation and subsequent Si epitaxial growth on Si(100) without substrate heating, heavy atomic-layer doping was demonstrated. Most of the incorporated N or B atoms can be confined in about a 2-nm-thick region of the atomic-layer doped Si film. (2) Using an 84 % relaxed Ge buffer layer formed on Si(100) by ECR plasma enhanced CVD, formation of a B-doped highly strained Si film with nanometer-order thickness was achieved and hole mobility enhancement as high as about 3 was observed in the highly strained Si film.

Influence of freely diffusing excitons on the photoluminescence spectrum of Si thick films with depth distribution of strain

We present an interpretation of photoluminescence (PL) for relatively thick Si films (200–300 nm) with depth distribution of strain, in which freely diffusing excitons influence the PL signal that originates from layers with different strain conditions. Micro-PL was excited by a 325 nm laser at 8.5 K for the Si films. The PL spectra clearly depended on the thickness of the strained part (ts) in the strained Si film. Under the condition of ts greater than the penetration depth (dp) of 325 nm line for Si, only the strained-part-related PL (PLs) could be observed but not the unstrained-part-related PL (PLus). With a decrease in ts, PLus gradually appeared and became strong, and simultaneously PLs became weak. For positions with very small ts, PLs could never be observed. The strain completion for the Si films was also investigated based on exciton behaviors in strained semiconductor. These characteristics of PL are useful references for understanding exciton behavior in semiconductor with depth distribution of band gap.

Threshold voltage model of strained Si channel nMOSFET

The model of nMOSFET threshold voltage was established based on study of voltage distribution in strained Si film,which was grown on relaxed SiGe virtual substrate.The model was analyzed with reasonable parameters, and the dependence of threshold voltage on Ge fraction and channel doping was revealed. The dependence of threshold voltage shift on Ge fraction was also obtained. The relationship between threshold voltage and strained Si layer thickness and doping was studied. The results indicates, the threshold voltage increases with increasing doping concentrations of relaxed SiGe layer, decreases with increasing Ge fraction of relaxed SiGe layer, and increases with increasing strained Si layer thickness. This threshold voltage model provides valuable reference to the strained-Si device design.

Highly sensitive detection of near-field Raman scattered light from strained Si∕SiGe heterostructures by scanning near-field optical Raman microscope using ultraviolet resonant Raman scattering

We have developed a scanning near-field Raman microscope (SNORM) with a hollow pyramidal probe that uses ultraviolet resonant Raman scattering and measured changes in the near-field and far-field Raman intensities of strained Si∕SiGe heterostructures as a function of a probe-sample distance R. We observed that the near-field Raman intensity of a strained Si film dramatically decreased with an increase in the probe-sample distance. The decay curve of the near-field Raman intensity was approximately expressed by the R−3 plot calculated from a simple model based on dipole-dipole interaction. This confirms that our SNORM detects the near-field light from the sample.

Measurement of incomplete strain relaxation in a silicon heteroepitaxial film by geometrical phase analysis in the transmission electron microscope

Relaxation of strain by a partial dislocation and stacking fault in a strained Si film was characterized using geometric phase analysis of high-resolution transmission electron microscope (HRTEM) images. Movement of a 60° glide dislocation from the free surface to the film-substrate interface created a complex state of strain in the film. HRTEM image analysis was used to produce a quantitative measure of the atomic displacement fields that could be used as input to finite-element simulations of stress distributions and resulting affects on band structures.

Effects of high-temperature anneals and Co60 gamma-ray irradiation on strained silicon on insulator

Strained silicon on insulator was exposed to high-temperature annealing and high-dose Co60 gamma (γ)-ray irradiation to study the tenacity of the bond between the strained Si film and the underlying buried oxide. During the high-temperature anneals, the samples were ramped at a rate of 150°C/s to 850°C then ramped to 1200, 1250, and 1300°C at a rate of approximately 5×105°C∕s for millisecond duration anneals. For the irradiation experiments, the samples were irradiated with Co60 γ rays to a dose of 51.5kGy. All samples were characterized by ultraviolet (UV) Raman, pseudo metal-oxide-semiconductor field-effect transistor (Ψ-MOSFET) current voltage, Hall mobility, and photoluminescence (PL) to verify changes in strain. UV Raman, PL, and Ψ-MOSFET measurements show no strain relaxation for the high-temperature annealed samples and only very slight relaxation for the γ-ray irradiated samples.

Deposition of polycrystalline SiGe by surface wave excited plasma

With application to underlayer of strained Si film in mind, polycrystalline SiGe films were deposited by plasma chemical vapor deposition (PCVD) using a high-density surface wave-excited plasma in SiH 4/GeH 4/H 2 gas. The atomic ratio of Si/Ge in the film was controlled by adjusting the gas flow rate ratio of SiH 4/GeH 4. The lattice spacing of the film was also controlled by the gas flow rate ratio. Polycrystalline SiGe film with large grain size of ∼ 200 nm and high crystallinity was successfully deposited by surface wave-excited plasma.

Compact Analytical Threshold-Voltage Model of Nanoscale Fully Depleted Strained-Si on Silicon–Germanium-on-Insu lator (SGOI) MOSFETs

In this paper, a physically based analytical threshold-voltage model is developed for nanoscale strained-Si on silicon-germanium-on-insulator MOSFETs for the first time, taking into account short-channel effects. The model is derived by solving the 2-D Poisson equation in strained-Si and SiGe layers. The effects of various important device parameters like strain (Ge mole fraction in the SiGe layer), body doping, gate workfunction, strained-Si thin film and SiGe layer thickness, etc., has been considered. We have demonstrated that increasing strain in order to enhance device performance can lead to undesirable threshold-voltage rolloff. The model is found to agree well with the 2-D simulation results

A new method of fabricating strained Silicon materials

Strain-relaxed SiGe virtual substrates are of great importance for fabricating strained Si materials. Instead of using graded buffer method to obtain fully relaxed SiGe film, in this study a new method to obtain relaxed SiGe film and strained Si film with much thinner SiGe film was proposed. Almost fully relaxed thin SiGe buffer layer was obtained by Si/SiGe/Si multi-structure oxidation and the SiO 2 layer removing before SiGe regrowth. Raman spectroscopy analysis indicates that the regrown SiGe film has a strain relaxation ratio of about 93% while the Si cap layer has a strain of 0.63%. AFM shows good surface roughness. This new method is proved to be a useful approach to fabricate thin relaxed epilayers and strain Si films.

Suppression of Structural Imperfection in Strained Si Thin Film by Utilizing SiGe Bulk Substrate

We demonstrate suppression of structural imperfection in strained Si by utilizing homemade SiGe bulk crystal as a substrate. X-ray reciprocal space mapping clarified that the growth of a Si thin film on a SiGe bulk substrate leads to reduction in the orientation fluctuation compared with that on a SiGe virtual substrate. Furthermore, analysis of Raman spectra revealed dramatic decrease of the strain fluctuation in the strained Si film on the SiGe bulk substrate. These results suggest that the SiGe bulk crystal can be utilized as a substrate for various strain-controlled heterostructures for fundamental studies as well as improvement of device performance.

Imaging defects in strained-silicon thin films by glancing-incidence x-ray topography

X-ray topographical images from thin (⩽50nm) strained-Si films grown on relaxed, planarized crystalline SiGe-on-Si (001) virtual substrates have been imaged by glancing-incidence monochromatic x-ray topography. This extremely asymmetric diffraction geometry, utilizing (311) diffraction planes, can limit penetration into the sample to as little as 6nm and allows separate images from the thin strained-Si film, the SiGe layer, and the base Si wafer to be recorded at different angles above the critical angle. Strain fields from the misfit dislocations in the SiGe layer penetrate the Si wafer and act as a template for the defect structure of the strained-Si films, even after an ex situ planarization step was inserted during the growth of the SiGe layer. This defect structure remains in the strained-Si film throughout the fabrication of strained-Si-on-insulator substrates.

Suppression of structural imperfection in strained Si by utilizing SiGe bulk substrate

We attempted to utilize homemade SiGe bulk crystal as a substrate for epitaxy of strain-controlled heterostructures. X-ray reciprocal space mapping clarified that the growth of a Si thin film on a SiGe bulk substrate leads to reduction in the orientation fluctuation compared with that on a SiGe virtual substrate. Furthermore, analysis of Raman spectra revealed a dramatic decrease of the strain fluctuation in the strained Si film on the SiGe bulk substrate. These results suggest that the SiGe bulk crystal can be utilized as a substrate for various strain-controlled heterostructures for fundamental studies as well as improvement of device performance.

Strained silicon thin-film transistors fabricated on glass

Strained-Si thin-film transistors were fabricated on glass substrate by direct transfer of a 35nm strained Si film onto glass. The strained Si films were originally grown on a relaxed SiGe layer on Si substrate. The tensile strain for the strained Si on glass (SSOG) was found to be 0.80%±0.02%. The effective electron mobility of the fabricated NMOS TFTs is 820cm2∕Vs. These devices show low interface charge densities at the bonding interface and at the gate oxide interface, as confirmed by the low subthreshold swing of 77mV∕dec for the 0.5μm SSOG device.