Si Buffer Research Articles

Solid solutions GeSi grown by MBE on a low temperature Si (001) buffer layer: specific features of plastic relaxation

The role of a low temperature Si buffer layer (LT-Si) in the process of plastic relaxation of MBE grown GeSi/Si (001) is studied. Probable sources and mechanisms of a generation of misfit dislocations (MD) are discussed. Transmission electron microscopic and X-ray diffraction techniques are used for studying 100 nm Ge x Si 1− x films with LT-Si and those free of such a buffer layer. The MD density is found to be much lower in the former than in the latter and the level of the film plastic relaxation is not higher than 20% in both as-grown and annealed films with LT-Si. As the thickness of the solid solution layer reaches 400 nm, the plastic relaxation of the films increases to almost 100%. Therefore, the determining role of the MD multiplication is supposed. We assume the double role of the LT-Si buffer layer. Firstly, the diffusion flux of vacancies from the LT-Si layer to the GeSi/Si interface may cause erosion of the interface and as a result a decrease in the rate of MD generation at the early stages of epitaxy. Secondly, the generation of intrinsic defect clusters in the LT-Si, which are potential sources of MDs, occurs in the field of mechanical stresses of the growing pseudomorphic layer. This process is thought to be the key feature of plastic relaxation of GeSi/LT-Si/Si (100) films which promotes MD self-organization.

Uniformity control of 3 inch GaN/InGaN layers grown in Planetary Reactors®

We present results obtained from a 2400G3HT reactor with a maximum wafer capacity of 8×3 inch. To achieve uniformity of the growth, we increased the temperature uniformity on each satellite to 0.9°C and that from satellite to satellite to 0.8°C. The optimum reactor geometry has been found by extensive modeling of the reactor design. Thus, an optimization of uniformity and efficiency has been achieved. GaN, InGaN/GaN multi quantum wells, GaN:Si, GaN:Mg and AlGaN were obtained on 5×3 inch substrates by simple scaling of the corresponding process parameters of the 6×2 inch configuration. We demonstrate GaN:Si and GaN:Mg doping uniformity with a standard deviation of less than 5%, with thickness uniformities of less than 1.7% standard deviation without any edge exclusion. InGaN/GaN, quantum well emission at 480 nm shows a standard deviation of 1–2% without rim exclusion. We grew AlGaN with about 10% Al content and less than 2% standard deviation in Al composition across the 3 inch substrate. Simple electroluminescence test structures, consisting of a GaN:Si buffer, followed by a five-period InGaN/GaN quantum well and covered by a GaN:Mg cap with emission wavelengths of about 460 nm show wavelength variations across 3 inch wafers of less than 3 nm. All these results demonstrate that the AIXTRON Planetary Reactor® is a very flexible tool for mass production application, especially with respect to upgrading the system to larger wafer diameters, as is already well known from the standard GaAs and InP applications that are available up to 5×8 inch configurations.

Compliant effect of low-temperature Si buffer for SiGe growth

Relaxed SiGe attracted much interest due to the applications for strained Si/SiGe high electron mobility transistor, metal-oxide-semiconductor field-effect transistor, heterojunction bipolar transistor, and other devices. High-quality relaxed SiGe templates, especially those with a low threading dislocation density and smooth surface, are critical for device performance. In this work, SiGe films on low-temperature Si buffer layers were grown by solid-source molecular-beam epitaxy and characterized by atomic force microscope, double-axis x-ray diffraction, and photoluminescence spectroscopy. It was demonstrated that, with the proper growth temperature and Si buffer thickness, the low-temperature Si buffer became tensily strained and reduced the lattice mismatch between the SiGe and the Si buffer layer. This performance is similar to that of the compliant substrate: a thin substrate that shares the mismatch strain in heteroepitaxy. Due to the smaller mismatch, misfit dislocation and threading dislocation densities were lower.

Optical study of SiGe films grown with low temperature Si buffer

ABSTRACTIn this work, SiGe films on low temperature Si buffer layers were grown by solid-source molecular beam epitaxy and characterized by atomic force microscope, photoluminescence and Raman spectroscopy. Effects of the growth temperature and the thickness of the low temperature Si buffer were studied. It was demonstrated that using proper growth conditions of the low temperature Si buffer, the Si buffer became tensily strained and gave rise to the compliant effect. High-quality SiGe films with low threading dislocation density have been obtained.

Optical Characterization Of Self-Assembled Ge Dots On Silicon

AbstractSelf assembled quantum dots of Ge were obtained by magnetron sputter epitaxy of seven monolayers of Ge on a 33nm thick undoped Si buffer grown on top of a p-doped (100) Si substrate. The samples obtained in this manner were then capped with an increasing number of silicon layers in order to study the effect of Si deposition on the strain and the morphology of the dots. They were characterized “ex situ” by spectroscopic ellipsometry and Raman spectroscopy. The optical experiments revealed well defined differences between the capped and uncapped samples and among samples with different cap thicknesses.By monitoring the energy and the splitting of the E0', E1 and E2 interband optical transitions of Ge and the Ge-Si vibrational mode, the optical measurements evidence strain effects as well as the formation of SiGe alloy, in agreement with the ``in situ'' STM measurements.

Ｓｉ下地層上のＣｏ／Ｃｕ多層膜の結晶性とＧＭＲ特性

Ｓｉ下地層上のＣｏ／Ｃｕ多層膜の結晶性とＧＭＲ特性

Optical Characterization of Self-Assembled Ge Dots on Silicon

ABSTRACTSelf assembled quantum dots of Ge were obtained by magnetron sputter epitaxy of seven monolayers of Ge on a 33nm thick undoped Si buffer grown on top of a p-doped (100) Si substrate. The samples obtained in this manner were then capped with an increasing number of silicon layers in order to study the effect of Si deposition on the strain and the morphology of the dots. They were characterized “ex situ” by spectroscopic ellipsometry and Raman spectroscopy. The optical experiments revealed well defined differences between the capped and uncapped samples and among samples with different cap thicknesses.By monitoring the energy and the splitting of the E0', E1 and E2 interband optical transitions of Ge and the Ge-Si vibrational mode, the optical measurements evidence strain effects as well as the formation of SiGe alloy, in agreement with the “in situ” STM measurements.

Giant magnetoresistance and structural properties in Co/Cu/Co sandwiches with Si and Cr buffer layers

Cobalt 5.5 nm/Cu 3 nm/Co 5.5 nm sandwiches with Si and Cr buffer layers were prepared by ultra-high vacuum electron beam evaporation. A large in-plane anisotropy of the giant magnetoresistance (GMR) effect was found in Si buffered sandwiches when the buffer layer thickness was equal to or larger than 0.9 nm. In the easy axis, the GMR effect reached a value of 5.5% with a high field sensitivity of approximately 0.7%/Oe, while in Cr-buffered Co/Cu/Co sandwiches, the GMR effect showed only in-plane isotropic properties with a maximum GMR value of 6%. The XRD spectrum and HRTEM image revealed that there exists a Co 2Si compound between the Si buffer and the lower Co magnetic layer. All these results indicate that the anisotropic GMR effect in Si-buffered sandwiches results from the cobalt silicide between the Si buffer and the lower Co magnetic layer.

The effect of N +-implanted Si(1 1 1) substrate and buffer layer on GaN films

We present the results of GaN characteristics affected by the strain field existing in the Si(1 1 1) substrate, which was implanted with 60 and 100 keV nitrogen ions (N +) to a dose ranging from 2×10 14 to 2×10 16 cm −2 prior to the film growth. GaN epitaxial films were grown on GaN/AlN-buffered and AlN-buffered implanted-substrates, respectively, by the metalorganic chemical vapor deposition technique. They were investigated by Raman scattering and photoluminescence. In the case of GaN film with the single-buffered layer, the tensile stress in the film decreased as the stress in the substrate increased. In contrast, the properties of the GaN films with the double-buffered layers made little difference regardless of the magnitude of stress in the substrate. It can be concluded that the implanted layer in substrate contributes to relax the stress that is generated in film and then to make better optical property of GaN film.

Low-temperature growth of in situ phosphorus-doped silicon films: two-step growth utilizing amorphous silicon buffers

Two-step growth has been developed to achieve in situ phosphorous-doped epitaxial and polycrystalline Si films, and the characteristic transitions in microstructures have been investigated for different phosphorus concentrations and annealing conditions of Si buffers. Our experimental results showed microstructures of Si films were controlled significantly by phosphorus doping and annealing of Si buffers grown at low temperature. While undoped Si buffers grown at 500°C were deposited as amorphous, Si buffers with phosphorus doping above ∼1020 cm−3 converts the growth modes from amorphous to single crystalline. Moreover, we thought that heavily doped phosphorus could dissociate native oxide at Si buffers/substrate interface by kinetically activated process during annealing. Using heavily phosphorus doped buffers, we can get high quality epi Si films. In addition, uniform polycrystalline Si films grew on undoped Si buffers.

Growth of germanium crystal films on amorphous silicon by sputter deposition

Epitaxial Ge crystal and amorphous films grown on the amorphous Si buffer layers are reported. After optimizing growth condition, we obtained Ge crystal films by magnetron sputter deposition. The film crystallinity was characterized by Raman scattering and X-ray diffraction. We discuss the possible roles of the growth temperature and the amorphous Si buffer layer in promoting epitaxial growth of Ge films.

Influence of Si buffer layer on the giant magnetoresistance effect in Co/Cu/Co sandwiches

The Co/Cu/Co sandwiches with a semiconductor Si buffer layer were prepared by high vacuum electron-beam evaporation. The influence of the Si buffer layer with different thickness on the giant magnetoresistance (GMR) effect in the Co/Cu/Co sandwiches was investigated. It was found that the GMR showed an obvious anisotropy when the thickness of Si buffer layer was larger than or equal to 0.9 nm, and that the GMR was basically isotropic with an Si buffer layer thinner than, 0.9 nm. The anisotropic behavior of GMR can be ascribed to the in-plane magnetic anisotropy in the sandwiches. Due to the interdiffusion at the Si buffer/Co interface, a Co2Si interface layer with a good (301) texture formed and induced the in-plane magnetic anisotropy in the sandwiches. The dependence of the crystalline texture of the sandwiches on the thickness of Si buffer layer was also studied.

A-Si buffer layer for improvement of crystallographic and magnetoresistance characteristics of Co/Cu multilayers

Co/Cu multilayers deposited on an amorphous Si (a-Si) buffer layer revealed good crystallinity and lower resistivity than ones deposited on other buffer layers. The periodicity and the sharpness at the interface between each layer were not as good as those in the multilayer with a Ni–Fe buffer. Therefore, the multilayer with a Si buffer exhibited smaller resistivity ρ but slightly smaller change of resistivity Δρ. The annealing process improved the periodicity and the sharpness at the interface in the Co/Cu multilayer with a Si buffer, and the value of the magnetoresistance ratio in the multilayer with a Si buffer (17.1%) became larger than that with Ni–Fe (15.5%).

Normal-incidence SiGe/Si photodetectors with different buffer layers

Normal-incidence SiGe/Si p–i–n photodetectors with relaxed GeSi alloy layers grown on Si buffer layers at low temperature and fraction-graded Si1−xGex buffers were fabricated by rapid thermal process/very-low-pressure chemical-vapor deposition. The response wavelength of these detectors ranges from 0.7 to 1.55 μm. The peak wavelengths are 0.98 and 1.06 μm, at which the responsivities are 2.7 and 1.8 A/W (−2 V), respectively. The responsivities at 1.3 μm are 0.15 and 0.07 A/W (−5 V), and the dark current densities are 0.05 and 0.03 μA/mm2 (−2 V), respectively. The influences of the Ge fraction, epilayer thickness, and bias voltage on the detectors are discussed.

Electrical properties of ZrO2 gate dielectric on SiGe

We report the electrical properties of a high dielectric constant (high-k) material, ZrO2, deposited directly on SiGe, without the use of a Si buffer layer or a passivation barrier. ZrO2 thin films of equivalent oxide thickness (EOT) down to 16.5 Å were deposited on strained SiGe layers by reactive sputtering. Results indicate that ZrO2 films on SiGe have good interfacial properties and low leakage currents. Sintering in forming gas at 350 °C for 1 h could further improve the film quality. Although threshold voltage stability and dielectric dispersion become a concern for thick ZrO2 films, thin ZrO2 films of EOT less than 20 Å exhibit excellent electrical properties making them a good candidate for SiGe applications.

D(A) steps and 2D islands of double layer height in the SiGe(001) system

The surfaces of step graded, partially relaxed Si(1-x)Ge(x)/Si(001) buffers were studied by scanning tunneling microscopy. The surface slips along <110> forming the crosshatch pattern, consisting of bunches of D(A) steps of double layer height. The D(A) steps are present in regions of large surface gradients close to the slips, as well as in planar regions between the slips. These regions are also characterized by the appearance of 2D islands of double layer height. The observations can be explained by assuming the strain due to the misfit dislocations to be locally anisotropic. Anisotropic misfit strain and efficient strain relaxation by the ( 2x8) Ge reconstruction were identified as the main factors causing the unusual step structure.

Orientation control in PZT/Pt/TiN multilayers with various Si and SiO2 underlayers for high performance ferroelectric memories

To suppress fluctuation in capacitor characteristics, highly oriented lead zirconate titanate (or PbZrxTi1−xO3 (PZT) films are required in ultralarge scale ferroelectric memories whose capacitor size is comparable to PZT grain size. The (111) orientation, which is closely related to spontaneous polarization, is achieved when PZT film is grown on highly (111) oriented platinum film. The degree of Pt(111) orientation is dependent on the crystallinity of the underlying TiN barrier metal. Furthermore, the crystallinity of the TiN is strongly dependent on the type of underlying material. The crystallinity of the TiN film grown on amorphous Si is much better than the crystallinity achieved on thermally oxidized Si. The surface energy difference between the TiN and underlayer may affect the growth mode of the TiN films. The crystallinity of the TiN film on rougher Si surfaces, such as in situ crystallized poly-Si, is inferior to the crystallinity observed on amorphous Si. Therefore, a smooth Si buffer layer is required underneath the capacitor to improve performance of the PZT/Pt/TiN capacitor. The modified planarized stacked structure presented in this article uses a postannealed crystalline Si as a buffer layer underneath the capacitor.

Role of Si1−xGex buffer layer on mobility enhancement in a strained-Si n-channel metal–oxide–semiconductor field-effect transistor

Strained-Si n-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) were fabricated on molecular-beam epitaxially grown strained Si with various Si1−xGex buffer layers. Effective electron mobility in n-MOSFETs with a Si1−xGex (x=0.2, 0.3) graded buffer layer was 60% higher than that in an unstrained-Si n-MOSFET. However, mobility of samples with a Si1−xGex buffer layer on a low-temperature grown Si buffer layer was not increased as much as that of samples on a graded buffer layer. Atomic-force microscopic observation suggests that the power spectrum of surface roughness of the strained-Si layer varies according to the buffer layer, and this variation may affect the enhancement of mobility.

Characteristics of ultrathin [4–7 nm] gate oxides for SiGe quantum well MOS structures

SiGe quantum well metal-oxide-semiconductor (MOS) structures were realized by the preparation of the undoped Si buffer layer, the undoped SiGe quantum well layer, and the undoped Si cap layer by molecular-beam epitaxy or low pressure chemical vapor deposition, and low thermal budget rapid thermal oxidation of the Si cap layer in dry oxygen at 1100 °C for 30–65 s. Comprehensive electrical characterization by steady state admittance spectroscopy was carried out. The relaxation of the strained SiGe layer was monitored by x-ray diffraction. The effects of the composition of the SiGe layer [Ge content, and minute C content] on the trap density and the device characteristics were investigated. Also examined were the effects of the oxidation temperature, the post-oxidation annealing [temperature and time] in nitrogen, and the post-metallization annealing [temperature and time] in forming gas, on the trap density and the device admittance characteristics.

In-situ observation of Ge δ-layer in Si(001) using quasi medium energy ion scattering spectroscopy

We have performed in-situ observation of Ge δ-layers in Si(001) fabricated by different methods, using quasi medium energy ion scattering spectroscopy. The following has been revealed: (1) the Ge atoms segregate to the surface even at a low substrate temperature of 200°C during MBE growth of Si buffer layer. (2) In the δ-layers fabricated by hydrogen mediated epitaxy, the surface segregation of Ge atoms was reduced compared with that by conventional MBE. The Ge atoms, however, were widely spread in the growing film. Hydrogen atoms are segregated to the topmost layer of the growth surface. (3) In a solid phase epitaxy (SPE) case, Ge atoms are confined and diffuse around the interface. For realizing δ-layer structure of doping atoms such as Ge, which are strongly segregated on the growth front, SPE is found to be an effective method.