Si Buffer Research Articles

Resistive Switching Behaviors of ReRAM Having W/CeO2/Si/TiN Structures

As one of emerging next-generation nonvolatile memories, Resistive RAM (ReRAM) still calls new material technology to improve its performance. By utilizing the special characteristics of cerium oxides, this paper proposes a new method to improve the performance of CeO2 based ReRAM devices by using Si buffer layer. It is confirmed that the device having W/CeO2/Si/TiN structure shows significant advantage over the device without Si layer for memory application in terms of lower forming voltage, smaller compliance current, larger window and better endurance characteristic. The effect of Si buffer layer is discussed in detail and a model based on filament switching mechanism was also proposed to explain the underlying reasons.

Control of semiconductor quantum dot nanostructures: Variants of SixGe1−x/Si quantum dot molecules

We examine variations in the basic structure of quantum dot molecules (fourfold quantum dot nanostructures forming around a central facetted pit) in the SixGe1−x/Si(100) system. Arrays of quantum dot molecules are seeded by Ga+ focused ion beam (FIB) prepatterning of the Si substrate prior to epitaxial Si buffer layer growth and GexSi1−x film deposition. Five main variants to the regular quantum dot molecule structure are observed. The populations of these variant structures depend on the initial FIB processing conditions; their frequencies generally increase with increasing prepatterned pit depth and with increasing incident ion energy. This work suggests both routes to improving uniformity of regular quantum dot molecule arrays as well as routes to enabling synthesis of a wider range of nanostructure geometries.

Low loss superconducting titanium nitride coplanar waveguide resonators

Thin films of TiN were sputter-deposited onto Si and sapphire wafers with and without SiN buffer layers. The films were fabricated into rf coplanar waveguide resonators, and internal quality factor measurements were taken at millikelvin temperatures in both the many photon and single photon limits, i.e., high and low electric field regimes, respectively. At high field, we found the highest internal quality factors (∼107) were measured for TiN with predominantly a (200)-TiN orientation. The (200)-TiN is favored for growth at high temperature on either bare Si or SiN buffer layers. However, growth on bare sapphire or Si(100) at low temperature resulted in primarily a (111)-TiN orientation. Ellipsometry and Auger measurements indicate that the (200)-TiN growth on the bare Si substrates is correlated with the formation of a thin, ≈2 nm, layer of SiN during the predeposition procedure. On these surfaces we found a significant increase of Qi for both high and low electric field regimes.

In situ Control of Si/Ge Growth on Stripe-Patterned Substrates Using Reflection High-Energy Electron Diffraction and Scanning Tunneling Microscopy

Si and Ge growth on the stripe-patterned Si (001) substrates is studied using in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). During Si buffer growth, the evolution of RHEED patterns reveals a rapid change of the stripe morphology from a multifaceted “U” to a single-faceted “V” geometry with {119} sidewall facets. This allows to control the pattern morphology and to stop Si buffer growth once a well-defined stripe geometry is formed. Subsequent Ge growth on “V”-shaped stripes was performed at two different temperatures of 520 and 600°C. At low temperature of 520°C, pronounced sidewall ripples are formed at a critical coverage of 4.1 monolayers as revealed by the appearance of splitted diffraction streaks in RHEED. At 600°C, the ripple onset is shifted toward higher coverages, and at 5.2 monolayers dome islands are formed at the bottom of the stripes. These observations are in excellent agreement with STM images recorded at different Ge coverages. Therefore, RHEED is an efficient tool for in situ control of the growth process on stripe-patterned substrate templates. The comparison of the results obtained at different temperature reveals the importance of kinetics on the island formation process on patterned substrates.

Growth of strained-Si material using low-temperature Si combined with ion implantation technology

In order to fabricate strained-Si MOSFETs, we present a method to prepare strained-Si material with high-quality surface and ultra-thin SiGe virtual substrate. By sandwiching a low-temperature Si (LT-Si) layer between a Si buffer and a pseudomorphic Si0.8Ge0.2 layer, the surface roughness root mean square (RMS) is 1.02 nm and the defect density is 106 cm−2 owing to the misfit dislocations restricted to the LT-Si layer and the threading dislocations suppressed from penetrating into the Si0.8Ge0.2 layer. By employing P+ implantation and rapid thermal annealing, the strain relaxation degree of the Si0.8Ge0.2 layer increases from 85.09% to 96.41% and relaxation is more uniform. Meanwhile, the RMS (1.1 nm) varies a little and the defect density varies little. According to the results, the method of combining an LT-Si layer with ion implantation can prepare high-quality strained-Si material with a high relaxation degree and ultra-thin SiGe virtual substrate to meet the requirements of device applications.

Ge based nanostructures for electronic and photonic devices

Self-assembled Ge x Si 1− x islands were grown on Si(0 0 1) substrates by solid source molecular beam epitaxy. Two different morphological shapes with different sizes were evolved by tuning the growth time at a constant deposition temperature. Micro-Raman analysis was carried out to investigate the composition, intermixing and strain of resultant islands. The observed broad infra-red photoluminescence signal from grown samples was associated with radiative recombination of holes confined in the Ge islands and electrons localized in the Si buffer layer. The PL peak position and intensity were found to be influenced by the islands size and intermixing of Si and Ge. The electrical properties of the islands were studied through photoexcited I– V characteristics and current imaging using conducting mode atomic force microscopy.

MBE growth of Ge quantum dot structures in oxide windows

The implementation of Quantum Dots (QDs) in devices allows novel electronic and opto-electronic functions. Strain driven Stranski-Krastanov growth mode enables the formation of nanometric islands (on wetting layer) whose density and geometry depend on growth conditions (temperature, rate) and surface structure (cleaning). The island positions are random. However, they can be influenced by surface patterning. In this work, the MBE growth of self-organized Ge QD structures in oxide windows is investigated. The studied Ge QD structures are composed by either a single Ge layer directly grown on a Si substrate, or double layer formed by a Ge QD layer on top of a Si buffer layer. Different surface preparation (dry etching with and without anisotropic wet etching) and cleaning (HF dip or RCA cleaning) schemes have been used. It is found that the cleaning and the Si buffer layer growth have strong influence on island nucleation. Preferred nucleation at the window edge and/or nucleation at the window center is observed under certain conditions. Interestingly, negligible influence (this is needed for most device works) is found only if Ge is grown directly on the RCA cleaned window.

Boron-Induced Strain Relaxation in Hydrogen-Implanted SiGe/Si Heterostructures

The strain relaxation behavior of H-implanted heterostructures containing a B-doped Si buffer layer was investigated. The annealed H-implanted SiGe/Si samples with a B-doped Si buffer layer exhibit an additional relaxation compared to those with an undoped Si buffer layer. At an annealing temperature of 900°C, relaxation ratios of the H-implanted samples with and without a B-doped Si buffer layer were determined to be 79 and 53%, respectively. The increased relaxation can be attributed to the formation of the larger H-filled bubbles along the interface on both sides of the B-doped Si region. Such an annealed H-implanted SiGe/B-doped Si heterostructure was further demonstrated to have a threading dislocation density of with a root-mean-square roughness of only 0.48 nm. This work offers an effective approach to fabricate high quality strain-relaxed thin SiGe epilayers for high mobility device applications.

Shape transitions and island nucleation for Si/Ge molecular beam epitaxy on stripe-patterned Si (001) substrate

Si and Ge growth on the stripe patterned Si (001) substrates is studied using scanning tunneling microscopy. During Si buffer growth, the stripe morphology rapidly evolves from multifaceted ''U'' to ''V''-shaped forms. This involves successive transitions between different low energy (11n) side facets, where n continuously decreases from n=3 to 20. Ge growth on such stripes induces the formation of a pronounced side wall ripple structure when the Ge thickness exceeds three monolayers. This ripple structure consists of alternating (105) microfacets oriented perpendicularly to the stripes. Depending of the side wall geometry, Ge nanoislands subsequently nucleate either on the side walls or at the bottom of grooves. The latter only occurs for ''V''-shaped stripes, where the side wall ripples extend all the way from the top to the bottom of the grooves, allowing efficient downward mass transport. For multifaceted ''U'' stripes, the side wall ripples are interrupted by steeper side wall segments such that mounds and subsequently, pyramids and domes grow on the side walls instead of at the bottom of the grooves. The island shapes strongly depend on their position on the pattern topography, which also affects the critical coverage for island nucleation as well as for the transitionmore » from pyramids to domes. The mechanisms for nucleation at different positions are clarified by detailed analysis and the role of kinetic as well as energetic factors identified.« less

Crystallinity and Thermoelectric Properties of Si/GeB Multilayers Prepared with Si Buffer Layer and SiO2 Substrates

Si/GeB multilayers were prepared by ion-beam sputtering technique and the effect of the crystallinity on thermoelectric properties was investigated. The crystallinity of the samples was controlled using Si buffer and SiO2 layers. Samples prepared on the Si buffer layer show similar properties to samples prepared on a Si wafer without the buffer layer. Moreover, it is found that the resistivity of the samples prepared on the Si buffer layer is lower than that of the samples prepared on the Si wafer without the buffer layer because of the surface cleaning effect. On the other hand, samples prepared on SiO2 show poor thermoelectric properties. These results are caused by the change in the crystallinity and periodicity, which are related to the carrier confinement effect. After thermal annealing, it is found that the multilayer structure is degraded rapidly and that the sign of the thermoelectric power is changed. This is considered to be caused by crystalline defects in the interfaces due to the collapse of the multilayer structure.

Ge integration on Si via rare earth oxide buffers: From MBE to CVD (Invited Paper)

Single crystalline rare earth oxide heterostructures are flexible buffer systems to achieve the monolithic integration of Ge thin film structures on Si. The development of engineered oxide systems suitable for mass-production compatible CVD processes is hereby of special importance. In this paper, the interaction of Ge with PrO 2(1 1 1)/Si(1 1 1) heterostructures is studied in detail to achieve this goal. MBE based in situ growth studies unveil the chemical reduction of the PrO 2 buffer during the initial Ge deposition, the occurrence of a Volmer Weber growth mode of Ge on the resulting Pr 2O 3 heterostructure and the final formation of single crystalline, atomically smooth and c(2 × 8) reconstructed Ge(1 1 1) film structures. Comparative CVD Ge heteroepitaxy studies on MBE grown PrO 2(1 1 1)/Si(1 1 1) and Pr 2O 3(1 1 1)/Si(1 1 1) buffer systems indicate that the highly reactive lattice oxygen of PrO 2 plays an active role to avoid during initial exposure to the reducing ambient of the GeH 4 precursor chemistry the decomposition of the oxide buffer system.

X-ray investigation of buried SiGe islands for devices with strain-enhanced mobility

In this work self-organized SiGe islands are used as stressors for Si capping layers, which later act as carrier channels in field effect transistors. To be able to address individual islands and to obtain a sufficiently narrow distribution of their properties, the SiGe islands are grown by molecular beam epitaxy on prepatterned Si substrates, with a regular two-dimensional array of pits. This combination of lithographic patterning and self-assembled island growth combines the advantages of both approaches and leads to very homogeneous island shape, size, and chemical composition. For processing, 4in. wafers are used, and fields with pit periods between 600 and 1000nm are defined by optical lithography. After growth of a Si buffer layer several monolayers of Ge are deposited, leading to island formation (dome or barn shaped) in the pits. Subsequent Si capping is performed at a low substrate temperature of 300°C to avoid intermixing and shape changes of the buried islands. The Ge distribution in the buried islands and the strain distribution in the islands and the surrounding Si matrix are assessed by x-ray diffraction experiments, combined with three-dimensional model simulations using finite elements. Tensile strain values in the Si cap up to 8×10−3 can be achieved using this approach, which is difficult to achieve using other methods without introduction of dislocations.

X-ray diffraction study of the composition and strain fields in buried SiGe islands

We report on studies of strain and composition of two-dimensionally ordered SiGe islands grown by molecular beam epitaxy using high resolution x-ray diffraction. To ensure a small size distribution of the islands, pit-patterned \(4''\) (001) Si wafers were used as substrates. The Si wafers were patterned by optical lithography and reactive ion etching. The pits for island growth are ordered in regular 2D arrays with periods ranging from 500 to 1000 nm along two orthogonal 〈110〉 directions. After the growth of a Si buffer layer, 5 to 9 monolayers of Ge are deposited, leading to the formation of islands with either dome- or barn shape, depending on the number of monolayers deposited. The Si capping of the islands is performed at low temperatures (300○C) to avoid intermixing and thus strain relaxation. Information on the surface morphology obtained by atomic force microscopy (AFM) was used to set up models for three-dimensional Finite Element Method (FEM) simulations of the islands including the patterned Si substrate. In the model, special attention was given to the non uniform distribution of the Ge content within the islands. The FEM results served as an input for calculating the diffracted x-ray intensities using kinematical scattering theory. Reciprocal space maps around the vicinity of symmetric (004) and asymmetric (113) and (224) Bragg peaks were recorded in coplanar geometry. Simulating different germanium gradients leads to altered scattered intensity distribution and consequently information on this quantity is obtained for both dome- and barn-shaped islands as well as on the strain fields.

Molecular beam epitaxy of Si/Ge nanoislands on stripe-patterned Si (0 0 1) substrates with different stripe orientations

Molecular beam epitaxy of Si and Ge on the stripe-patterned Si (0 0 1) substrates is studied by scanning tunneling microscopy. The stripe patterns consist of one-dimensional etch grooves oriented along different azimuth directions within the Si (0 0 1) surface. During Si buffer growth, the stripe morphology rapidly changes and the side walls of the grooves are transformed into differently inclined low-energy Si facets, depending on the stripe orientation and growth conditions. Subsequent Ge growth drastically changes the surface topography to form energetically favorable {1 0 5} sidewall facets. Depending on the sidewall geometry, Ge nanoislands formed at a coverage of around 5 monolayers nucleate either in the middle of the sidewalls or in the center of the grooves as a result of material accumulation induced by capillary forces and strain relaxation.

Computer simulation of the vacancy defects interaction with shuffle dislocation in silicon

The interactions between the 60 ∘ shuffle dislocation and two different types of vacancy defects in silicon are separately studied via the molecular dynamics simulation method. The Stillinger–Weber potential is used to describe the atomic interactions. The results show that the dislocation slip velocity will decrease due to the interaction with the vacancy cluster (V 6). The simulation also reveals that the divacancy will be absorbed by the dislocation. Meanwhile, a climbing of the dislocation occurs during their interactions. However, the divacancy has little effect on the dislocation slip velocity. Based on the above results, the decrease in threading dislocation density in SiGe/Si heterostructures with the use of low-temperature Si buffer layer may be explained.

Mechanical stability of the diamond-like carbon film on nitinol vascular stents under cyclic loading

The mechanical stability of diamond-like carbon (DLC) films coated on nitinol vascular stents was investigated under cyclic loading condition by employing a stent crimping system. DLC films were coated on the vascular stent of a three dimensional structure by using a hybrid ion beam system with rotating jig. The cracking or delamination of the DLC coating occurred dominantly near the hinge connecting the V-shaped segments of the stent where the maximum strain was induced by a cyclic loading of contraction and extension. However the failures were significantly suppressed as the amorphous Si (a-Si) buffer layer thickness increased. Interfacial adhesion strength was estimated from the spalled crack size in the DLC coating for various values of the a-Si buffer layer thickness.

Si (001) surface preparation for the antiphase domain free heteroepitaxial growth of GaP on Si substrate

The present paper summarizes results of thermal annealing steps on epitaxially grown Si buffer morphology in dependence of Si offcut angle as well as miscut direction. A Si buffer layer was grown on an undulated Si substrate by vapour phase epitaxy, which provides due to its roughness deliberate off-orientations. An optimized annealing procedure under a hydrogen pressure of 950 mbar was used after growth. Atomic force investigation shows that there is a clear tendency to form bi-atomic steps along both <110> directions as long as the surface miscut is low in angle. For the off-orientations in between these two directions the surface steps delimit mono-layer high terraces. An exact (001) Si substrate with a low miscut of 0.12° towards [11̄0] was treated by the same annealing condition. The Si surface is strongly dominated by one kind of sublattice but not completely double-stepped. Using this Si wafer as a substrate for GaP heteroepitaxy by metal organic vapour phase epitaxy, an antiphase domain free GaP layer is achieved after 50 nm of overgrowth, proven by transmission electron microscopy investigation.

Observation of in-plane strain fluctuation in relaxed SiGe virtual substrate

We investigate the morphology and strain distribution of the surface crosshatch undulation on a strain-relaxed SiGe layer grown on a low-temperature Si buffer layer by atomic force microscopy and spatially-resolved UV-Raman spectroscopy. Surface crosshatch undulation resulted from misfit dislocations at the SiGe/Si interface is associated with the strain relaxation in the SiGe layer. By means of thermal annealing, strain relaxes and the inhomogeneous in-plane strain fluctuation can be eliminated.

Monolithic integration of InP based heterostructures on silicon using crystalline Gd2O3 buffers

An original approach of monolithic integration of InP based heterostructures on silicon is proposed based on the peculiar properties of the heterointerface between InP and crystalline Gd2O3. When grown on a crystalline Gd2O3∕Si(111) buffer, InP takes its bulk lattice parameter as soon as the growth begins, and the lattice mismatch (7.9%) is fully accommodated by the formation of a misfit dislocation network at the InP∕Gd2O3 heterointerface. This plastic compliant effect allows the monolithic growth of good quality InAsP∕InP heterostructures on Si, as attested by room-temperature photoluminescence experiments.

Impact of Local Strain From Selective Epitaxial Germanium With Thin Si/SiGe Buffer on High-Performance p-i-n Photodetectors With a Low Thermal Budget

This letter reports on the impact of selective epitaxial germanium, specifically its local strain effects, on high-performance p-i-n photodetectors for near-infrared applications. By combining a thin compliant Si epitaxial layer (~6 nm) with SiGe buffer (10-15 nm), we demonstrated a high-quality Ge film (~150 nm) prepared by two-step growth. Without using high-temperature cyclic anneal, Ge films with smooth surface (root mean square = ~ 0.67 nm) and low dislocation density (4 x 106 cm-2) have been produced. The Si buffer locally enhances the tensile strain (epsiv = 0.63%) in Ge while slightly suppressing the dark current by half to 0.12 muA (with circular ring area = 1230 mum2 and spacing = 2 mum). A lateral p-i-n Ge photodetector has been demonstrated with enhanced photoresponse of ~190 mA/W at 1520 nm and a 3-dB bandwidth of 5.2 GHz at 1 V.