Si Buffer Research Articles

Preferential nucleation of Ge islands at self-organized pits formed during the growth of thin Si buffer layers on Si(110)

The epitaxial growth of thin (∼20–40 nm) Si buffer layer on Si(110) leads to the formation of ∼100-nm-wide, uniformly sized faceted pits. The cause of these rhombohedral pits is revealed to be the overgrowth of a homoepitaxial layer over clusters of coherent contaminant particles, possibly SiC. Deposition of Ge on such “pitted” surfaces shows highly selective nucleation of pairs of coherent islands at the opposite corners of the pits along the 〈110〉 direction. Continued deposition leads to strain relaxation of one or both of the islands within the pit which then rapidly coarsen to form a single Ge island within the pit. Our observations offer insight into heterogeneous nucleation mechanisms important for producing controlled arrays of self-assembled quantum dots.

Quasi-medium energy ion scattering spectroscopy observation of surface segregation of Ge δ-doped layer during Si molecular beam epitaxy

We have observed the behavior of Ge δ-doped layers fabricated by molecular beam epitaxy (MBE) on an Si(001) substrate and the surface segregation of Ge atoms from δ-doped layers during a top Si buffer layer growth using quasi-medium energy ion scattering spectroscopy. We have found that the Ge atoms segregate to the surface even at a low substrate temperature of 300°C. This result is in contrast to the solid phase epitaxy case where Ge atoms diffuse around the interface. For MBE growth, at a substrate temperature of 600°C, the surface segregation was reduced. The distribution of Ge atoms is widely spread in the subsurface region. This result indicates that the Ge atoms are incorporated in the Si buffer layer with mixing by Si atoms.

Relaxed Si0.7Ge0.3 layers grown on low-temperature Si buffers with low threading dislocation density

Si 0.7 Ge 0.3 epilayers with low threading dislocation density have been grown on Si (001) substrates by introducing a low temperature Si buffer. Such a structure can be used as the buffer for the growth of device structures. In comparison with the conventional compositionally graded buffer system, it has the advantages of having lower threading dislocation density, smaller thickness for required degree of relaxation, and smoother surface. Experimental evidence suggests that an anomalous relaxation mechanism has been involved.

Quasi-medium energy ion scattering spectroscopy study of Ge δ-layer on Si(001)

We have investigated a Ge δ-layer on Si(001) formed by solid phase epitaxy (SPE) using recently developed quasi-medium energy ion scattering spectroscopy (Q-MEIS). We found that SPE of a top Si buffer layer results in a diffusion of Ge atoms from the δ-doped layer around the interface. This result is in contrast with the molecular beam epitaxy (MBE) case where Ge atoms are segregated to the grown surface. The incident angle dependence of Si and Ge intensities shows that the diffused Ge atoms occupy the Si lattice sites, indicating the GeSi alloying near the interface region. Our results demonstrates that Q-MEIS is suitable for in-situ observation of buried layers during thin film growth processes.

Formation of segregated cell structure for MBE growth of mismatched semiconductors

In this work the cellular structure is proposed as a buffer layer for defect-free heteroepitaxial growth of mismatched materials. The micro-cellular buffers were formed by 80 keV In + ion implantation into single-crystalline (001) silicon wafers (2 × 10 16 cm −2) followed by Q-switched laser annealing at an energy density of about 1.2–3.5 J/cm 2. The structure of the obtained surface was investigated by plan-view and cross-section transmission electron microscopy. It has been found that the In-implanted, and subsequently laser annealed, Si surface exhibits a structure consisting of a system of ‘seed’ pads (cells) separated by indium filled trenches (walls of the cell). The linear dimensions of the pads and trenches are measured to be 15–100 and 3–5 nm, respectively, which is close to the optimal values for defect-free epitaxial growth of mismatched materials. The obtained Si〈In〉 Si buffer layers were used for heteroepitaxial growth of GaAs films. The density of extended defects in GaAs grown on these buffers was found to be considerably lower as compared to conventional GaAs/Si structures grown at similar conditions.

Reduction of dislocation density in mismatched SiGe/Si using a low-temperature Si buffer layer

The reduction of the dislocation density in relaxed SiGe/Si heterostructures using a low-temperature Si(LT-Si) buffer has been investigated. We have shown that a 0.1 μm LT-Si buffer reduces the threading dislocation density in mismatched Si0.85Ge0.15/Si epitaxial layers as low as ∼104 cm−2. Samples were grown by both gas-source molecular beam epitaxy and ultrahigh vacuum chemical vapor deposition.

Characterization of mismatched SiGe grown on low temperature Si buffer layers by molecular beam epitaxy

Several types of buffer layer structures, including superlattice and step-graded layers, have been employed to reduce the threading dislocation in SiGe epitaxial layers. A new technique, using a 0.1 μm thick Si buffer grown at 450°C by molecular beam epitaxy, provides the best results. For a 0.5 μm thick Si 0.85 Ge 0.15 layer, the dislocation density is ≤ 10 5 cm -2 . Hall measurements indicate an improvement in the hole mobility of a 1 μm thick Boron doped Si 0.7 Ge 0.33 layer. A SiGe/Si heterojunction bipolar transistor has been fabricated exploiting the low temperature Si buffer. Transmission electron microscopy of the structure does not indicate any evidence of threading dislocations.

Photoluminescence study of the optical properties of SiGe quantum wells on separation by implanted oxygen substrates

A detailed experimental study of the optical properties of separation by implanted oxygen (SIMOX) substrates is performed using photoluminescence techniques. The photoluminescence properties of SIMOX substrates are compared with those of Si substrates. The silicon-on-insulator (SOI) layer is found to be free of any strain. High-quality SiGe quantum wells are grown on SOI using gas source molecular beam epitaxy. It is found that the photoluminescence properties of these quantum wells on SOI differ significantly from those of the SiGe quantum wells grown on a Si substrate under identical growth conditions. Intense photoluminescence and higher operation temperature are obtained for quantum wells grown on SOI substrates. It is shown that growth of a few hundred Å Si buffer layer on SOI is essential in order to achieve a high quality SiGe epitaxy on SOI. Finally, a SiO2/Si/SiO2 optical cavity is proposed on SIMOX substrates, and it is employed to further enhance the photoluminescence intensity of SiGe quantum wells on SOI.

Correlation between oxygen distribution and luminescence efficiency in SiGe/Si layer structures measured by cathodoluminescence imaging and spectroscopy

SiGe layers and quantum wells grown on Si by low-pressure chemical vapour deposition (LPCVD) have been investigated by cathodoluminescence (CL) imaging and spectroscopy. Monochromatic imaging of the shallow bound exciton luminescence from the SiGe layers showed that in some cases it was not uniform. Dark regions were observed with dimensions mainly in the range 100 - 200 . As the density and size of these regions increased, the overall luminescence efficiency decreased. In this paper we report an investigation of the possibility that oxygen incorporation in the SiGe layer structures is responsible for the decrease in luminescence efficiency. Luminescence systems created in Si by radiation damage, known to be associated with carbon - oxygen and carbon - carbon complexes, were used to investigate the depth and spatial distribution of oxygen. A SiGe sample which contained a high density of dark regions was shown to have a high oxygen concentration close to the interface, either in the Si buffer layer or the substrate. Monochromatic CL images showed that the oxygen distribution was non-uniform, and that the areas where the oxygen concentration increased could be correlated with the regions where the shallow bound exciton luminescence efficiency decreased. This implies that oxygen incorporation in the SiGe layers during growth creates the non-radiative regions.

Process Improvements in the Selective Epitaxial Growth of Si1 − xGex / Si Strained Layers in a Conventional Hot‐Wall LPCVD System

Three process considerations were identified as essential for the selective epitaxial growth (SEG) of SiGe in hot‐wall tubular low pressure chemical vapor deposition reactors. A thermodynamic analysis of the system revealed that the hydrogen bake condition for SiGe SEG had to be modified from that for Si SEG to eliminate outgassing of residual Ge, which adversely affected the initial growth surface. A two‐step hydrogen bake was developed to eliminate the problem. A high temperature bake, carried out before wafer loading, removed residual Ge deposited on the reactor wall from the previous run. After wafer loading, removed residual Ge deposited on the reactor wall from the previous run. After wafer loading and purge, a lower temperature bake then removed the native oxide. Second, a selectively grown Si buffer layer improved the initial growth surface for later SiGe SEG. The Si buffer layer covered the remaining native oxide, which the previous hydrogen bake did not totally remove, and reduced the defects at the interface. Finally, a small flow of Si source gas during the temperature ramp down period helped to keep the buffer layer surface clean prior to SiGe SEG. All three steps proved to be essential in obtaining SiGe SEG of higher quality.

Defect-free strain relaxation in locally MBE-grown SiGe heterostructures

Defect-free SiGe layers and virtual substrates are of great interest for SiGe heterostructures and device processing. To overcome the problems arising from strain-induced defects, local growth of Si and SiGe on a micrometer scale with ultra high vacuum compatible shadow masks can be used. The geometrical shape of locally grown SiGe mesa structures was investigated by scanning electron microscopy. This reveals a facet formation like in silicon, which can be explained by a self-organized growth mechanism. The defect density can be observed by transmission electron microscopy analysis. A decreasing defect density for decreasing mesa width is found. Growth exceeding the critical layer thickness for uniformly deposited layers is possible for locally grown mesa structures. This effect can be explained by suppressed nucleation and suppressed multiplication of defects. MicroRaman spectroscopy measurements of these local structures can be used to investigate the strain relaxation. For locally grown SiGe on a (001) Si substrate the formation of dislocations is suppressed resulting in increasing strain. In contrast to this locally grown SiGe on local Si buffer layers is defect free, but reveal strain relaxation for micron dimensions. This effect of defect-free elastic relaxation offers a new method to fabricate virtual substrates, with buffer thicknesses on a nanometer scale. © 1997 Elsevier Science S.A. All rights reserved.

Behavior of Ge .DELTA.-doped Layer in Si(001) Observed by TOF-LEIS.

We have investigated a Ge δ-doped layer in Si(001) formed by solid phase epitaxy (SPE) using time-of-flight low-ener-gy ion scattering spectroscopy (TOF-LEIS). The Ge δ-doped layer was formed by annealing a Si buffer layer deposited on a Ge(1ML)/Si(001) surface. We found that Ge atoms diffuse from the δ-doped layer around the interface as a result of SPE growth of the Si buffer layer. Incident angle dependence of Ge and Si intensities at various annealing temperatures showed that the Ge atoms began to occupy Si lattice sites above 300°C; indicating Ge-Si alloying near the interface region, and that the diffusion of Ge atoms was promoted above 500°C accompanied by the crystallization process of the Si buffer layer. The thickness of the Si1-xGex layer at 600°C was estimated to be about 30 Å, where the Si buffer layer was nearly perfectly crystallized.

Strain relaxation and self-organizing MBE-growth of local SiGe-structures

SiGe wires and dots of μm size were fabricated by a self-organizing MBE growth mode. Layer thickness and Ge content on top of a Si mesa buffer are varied. It is shown that defect free growth exceeding the critical thickness is possible for locally grown mesa structures. Raman spectroscopy exhibits reduced strain relaxation for decreasing lateral dimensions. The SiGe-growth on the top of a locally grown Si buffer layer results in the nucleation of dislocations mainly in the Si buffer. This effect of defect accumulation in the locally grown buffer offers a method for the fabrication of relaxed, defect free virtual substrates.

Improvement of the crystallinity of a GaAs epitaxial film grown on a Si substrate using a Si/SiGe/Ge buffer layer

Double-crystal X-ray diffraction (DCXD), photoluminescence (PL), and Raman spectroscopy measurements on GaAs/Si heterostructures grown with various kinds of buffer layers by molecular beam epitaxy were performed in order to characterize the crystallinity of the samples. The strain and dislocation density of the GaAs epitaxial layer determined from the DCXD curve for the GaAs/Si/SiGe/Si heterostructure are 3.90×10 −3 and 6.99×10 6 cm] −3, respectively, and the full width at half maximum (FWHM) of the GaAs epitaxial layer for the heterostructures is 405 arcsec. The results of the PL spectrum for GaAs/Si with a Si/SiGe buffer showed that the FWHM of the band-to-band luminescence is only 5.4 meV. The ratio of the peak intensity of the longitudinal optical phonon to that of the transverse optical phonon for GaAs/Si with a Si/SiGe/Ge buffer increased dramatically in comparison with the corresponding rates for the GaAs/Si with Ge and SiGe/Ge buffers. These results indicate that the crystallinity of the GaAs epilayer was remarkably improved using a Si/SiGe/Ge buffer, and a strong binding energy of the SiSi bond in the top Si buffer layer reduced the formation of the dislocation in the GaAs active layer.

Low-temperature buffer layer for growth of a low-dislocation-density SiGe layer on Si by molecular-beam epitaxy

A method using a low-temperature Si (LT-Si) buffer layer is developed to grow a SiGe epilayer with low density of dislocations on a Si substrate by molecular-beam epitaxy. In this method, a LT-Si layer is used to release the stress of the SiGe layer. The samples have been investigated by x-ray double-crystal diffraction and transmission electron microscopy. The results indicate that the LT-Si is effective to release the stress and suppress threading dislocations.

Nucleation and Pattern Formation of Coherent Islands During Epitaxial Growth of Ge on Si(110) Surfaces

AbstractMolecular beam epitaxial growth of Ge on Si(110) surfaces reveals interesting aspects of the heterogeneous nucleation of coherent Ge islands. Cleaning of the Si substrate by desorption of a passivating oxide layer at high temperature creates surface pits. Two sets of experiments, including deposition of Ge on as-cleaned substrates, and surfaces with a thin Si buffer layer are compared to illustrate the nucleation behavior of Ge. Typical Ge deposition temperatures range from 600°C to 725°C.For Ge deposition on as-cleaned surfaces, the faceted edges of pits serve as preferential sites for the heterogeneous nucleation of coherent Ge islands. Experiments were also performed on surfaces with thin (˜20nm) Si buffer layers grown on the as-cleaned surface. Though the faceted pits have not been completely covered by the Si buffer layer, they have decreased in lateral size. In addition, the Si(110) surface shows ledges that are formed along specific crystallographic directions. Ge deposited on the Si buffer nucleates first at the corners of the pits, in an interesting dipole orientation, as well as along the ledges on the surface.

Nitrogen in-situ doped poly buffer LOCOS: simple and scalable isolation technology for deep-submicron silicon devices

This paper describes a novel LOCOS (LOCal Oxidation of Silicon) technology that uses nitrogen in-situ doped amorphous-Si as a buffer layer instead of the undoped poly-Si used in conventional Poly Buffered LOCOS (PBL). This technology makes it possible to use a thin 6-nm pad oxide by preventing the formation of voids in Si buffer layer and improves edge morphology and effective dimension loss. Therefore, the technology will be used in advanced LSI fabrication with KrF lithography, notwithstanding that the number of processing steps is the same as conventional PBL. This new LOCOS technology is the most promising isolation technology for the deep-submicron era due to its simplicity and scalability.

Time-resolved photoluminescence of pseudomorphic SiGe quantum wells.

We report low-temperature time-resolved photoluminescence experiments on a pseudomorphic SiGe quantum-well structure. Under the condition of optical absorption in the Si buffer layers, the decay time of the SiGe quantum-well luminescence is controlled by the capture of excitons and electron-hole droplets. From the onset of the SiGe luminescence, the exciton lifetime in the investigated 59-\AA{}-wide ${\mathrm{Si}}_{0.72}$${\mathrm{Ge}}_{0.28}$ quantum wells is found to be about 100 ns.

Crucial role of Si buffer layer quality in the photoluminescence efficiency of strained Si 1− xGe x/Si quantum wells

The crucial role of Si buffer layer quality in strained Si 1− x Ge x /Si quantum well structures is investigated in terms of quantum efficiency of luminescence. The close correlation between etch-pit density (EPD) increase and luminescence quenching is presented for the first time. It is shown that strained Si 1− x Ge x /Si quantum wells with a high degree of luminescence efficiency can be obtained only on a high-quality Si buffer layer grown at temperatures higher than 700°C, which gives an EPD lower than 10 6 cm −2.

The effect of in situ boron doping on the strain relaxation of Si 0.8Ge 0.2 : B/Si heterostructure grown by molecular beam epitaxy

The effect of in situ elemental boron doping (boron concentration N B = 1 × 10 19 to 3 × 10 20 cm −3 on the strain relaxation of a Si 0.8Ge 0.2 : B/Si(001) heterostructure grown at 680°C is investigated. Although boron gives rise to lattice contraction in bulk Si, it does not compensate the lattice mismatch between the Si 0.8Ge 0.2 layer and Si substrate. On the contrary, it stimulates the strain relaxation. The strain relaxation of the Si 1- xGe x : B/Si(001) heterostructure mainly gives way to dislocation half-loops generated not in the Si 0.8Ge 0.2 layer, but in the Si buffer layer just below it. This means that heterointerfacial quality may be one of the major control factors of strain relaxation. The strain relaxation induced by boron doping is observed even at N B ∞ 10 19 cm −3, which is in the doping range typically used in Si 1- xGe x / Si heterojunction bipolar transistors (HBTs). Therefore, as well as Ge mole fraction and growth temperature, N B should also be considered in determining critical thickness of a Si 1- xGe x layer grown on a Si substrate.