Si Buffer Layer Research Articles

Low-resistance titanium/n-type silicon (1 0 0) contacts by monolayer selenium passivation

Low-resistance Ohmic contacts fabricated by selenium passivation between titanium and n-type silicon (1 0 0) substrates have been characterized by the four-point probe and circular transmission line method. Selenium passivation terminates dangling bonds on the Si(1 0 0) surface and reduces the Schottky barrier height between Ti and n-type Si(1 0 0) substrates. Sheet resistance of the Ti–Si contacts on Se-passivated 1019 cm−3 doped Si substrates shows a ∼30% reduction as compared with control samples. Accordingly, the extracted contact resistance is reduced by about an order of magnitude for samples with different Ti thicknesses and different annealing temperatures. A 29-times reduction in contact resistivity is achieved by Se passivation on highly-doped Si-on-insulator substrates with a 500 Å un-doped Si buffer layer.

Sb as surfactant at plastic relaxation of GeSi/Si(0 0 1) films grown by molecular-beam epitaxy: Reduction of surface roughness value

Plastically relaxed GeSi films with a fraction of Ge ranging from 0.19 to 0.29 are grown on Si(0 0 1) substrates with the use of a low-temperature (350 °C) buffer layer of Si. Their root mean square (RMS) surface roughness is in the 1.7–2.7 nm range. Addition of Sb as a surfactant smoothing the surface of the stressed film during its growth is shown to reduce the surface roughness of the plastically relaxed heterostructure. The RMS roughness lower than 1 nm is reached for a film with a 0.29 fraction of Ge and a threading dislocation density close to 10 6 cm −2.

Ge Selective Epitaxial Growth with Ultra Thin SiGe Buffer Layer

In this study, we introduced another very thin SiGe layer that was selectively grown on the Si buffer layer prior to the ultra- thin Si0.5Ge0.5 buffer growth. As a result, selective epitaxial growth of Ge film with good crystalline quality was realized without increasing intermediate layer thickness or thermal budget significantly. This technique is expected to be applicable for the Ge channel formation and other application that require selective Ge film formation with low thermal budget. The mechanism of selectivity enhancement by the first SiGe layer as well as its influence on crystalline quality of the Si0.5Ge0.5 buffer layer and Ge epitaxial layer above will be discussed in the presentation.

Initial stage of the two-dimensional to three-dimensional transition of a strained SiGe layer on a pit-patterned Si(001) template

We investigate the initial stage of the 2D-3D transition of strained Ge layers deposited on pit-patterned Si(001) templates. Within the pits, which assume the shape of inverted, truncated pyramids after optimized growth of a Si buffer layer, the Ge wetting layer develops a complex morphology consisting exclusively of {105} and (001) facets. These results are attributed to a strain-driven step-meandering instability on the facetted side-walls of the pits, and a step-bunching instability at the sharp concave intersections of these facets. Although both instabilities are strain-driven, their coexistence becomes mainly possible by the geometrical restrictions in the pits. It is shown that the morphological transformation of the pit surface into low-energy facets has strong influence on the preferential nucleation of Ge islands at the flat bottom of the pits.

GaN full-vertical p-i-n rectifiers employing AlGaN:Si conducting buffer layers on n-SiC substrates

The development of a full-vertical GaN p-i-n rectifier on a 6H n-type SiC substrate by employing a conducting AlxGa1−xN:Si (x=∼0.1) buffer layer scheme is reported. In this vertical configuration, the n contact is made on the backside of the SiC substrate using a Ni∕Au metallization scheme. Epitaxial layers are grown by low-pressure metal organic chemical vapor deposition. The AlxGa1−xN:Si nucleation layer is proven to provide excellent electrical properties while also acting as a good buffer layer for subsequent GaN growth. The reverse breakdown voltage for a relatively thin 2.5μm thick i region was found to be over −330V. The devices also show a low on resistance of Ron of 7.5×10−3Ωcm2. This full-vertical configuration provides the advantage of the reduction of sidewall damage from plasma etching and lower forward resistance due to the reduction of current crowding in the bottom n-type layer.

X-Ray Reciprocal Space Mapping and Synchrotron Radiation Topography of Strained Si/Si1-xGex on Bonded SOI

In this work, X-ray triple-axis diffractometry (TAD) and synchrotron radiation double-crystal topography (SRDT) have been utilized to investigate strained-Si/Si1-xGex on bonded SOI. The structural characteristics of the Si capping layer and the underlying Si layers (the Si buffer and SOI top Si layer) have been described using high resolution reciprocal lattice mapping (HRRLM). The mosaicity in the SiGe layer leads to (004) diffraction peak broadening asymmetrically. The crosshatched contrast spatially correlated with the misfit dislocations (MDs) has been observed both in the SiGe layer and the underlying Si layers. The granular pattern due to the mosaic blocks is also shown in the topograph of the SiGe layer. It is argued that the compressive strain in the SiGe layer has been primarily released at the interface between the SiGe layer and the underlying Si layers.

Low-resistance Ti/n-type Si(100) Contacts by Monolayer Se Passivation

Low-resistance contacts fabricated by selenium passivation between Ti and n-type Si(100) substrates have been characterized by low-temperature I-V, four-point probe and circular transmission line methods. The Ti-Si contacts on Se- passivated samples demonstrate a significant reduction in Schottky barrier height over control samples in low- temperature I-V. Sheet resistance of the contacts on Se- passivated 1019 cm-3 doped n-type Si(100) substrates shows a 30% reduction as compared with control samples. Accordingly, the extracted contact resistance decreases by about one order of magnitude for samples with different Ti thicknesses and different annealing temperatures. A 125%-2900% reduction in contact resistivity is achieved by Se passivation on highly- doped n-type SOI substrates with 500 Aå un-doped Si buffer layer. The reduction in contact resistance is attributed to the minimization of interface states between Ti and Si(100) surface.

Photoluminescence of Ge(Si) self-assembled islands embedded in a tensile-strained Si layer

We report photoluminescence (PL) studies of Ge(Si) self-assembled islands embedded into a tensile-strained Si layer grown on smooth relaxed Si0.75Ge0.25∕Si(001) buffer layers subjected to chemical-mechanical polishing. The intense PL from Ge(Si) islands embedded into a strained Si layer compared to the PL from islands grown on unstrained Si(001) is associated with efficient confinement of electrons in a strained Si layer on the heterojunction with islands. The observed dependence of the island PL peak position on thickness of strained Si layer confirms the validity of the model for real-space indirect optical transition between electrons confined in the strained Si layer, and holes localized in islands.

Misfit Dislocations in SiGe/Si Heterostructures: Nucleation - Propagation - Multiplication

We present experimental data on the effect of low-temperature buffer layers on the dislocation structure formation in SiGe/Si strained-layer heterostructures under thermal annealing. Specific subjects include mechanisms of misfit dislocation nucleation, propagation and multiplication as well as the role of intrinsic point defects in these processes. Samples with lowtemperature Si (400°C) and SiGe (250°C) buffer layers were grown by MBE. In general, the processes of MD generation occur similarly in the heterostructures studied independently of the alloy composition (Ge content: 0.15, 0.30) and kind of buffer layer. Intrinsic point defects related to the low-temperature epitaxial growth influence mainly the rate of misfit dislocation nucleation.

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.

Thermally stimulated relaxation of misfit strains in Si1−x Gex/Si(100) heterostructures with different buffer layers

The regularities of the defect formation in Si1−xGex/Si heterostructures (x = 0.15 and 0.30), consisting of a low-temperature Si buffer layer and a SiGe solid solution, during their growth and subsequent annealings at temperatures 550–650°C are investigated by the methods of optical and transmission electron microscopy and X-ray diffraction. It is shown that the misfit-strain relaxation by plastic deformation under the conditions studied occurs most intensively in heterostructures with low-temperature SiGe buffer layers. The maximum degree of misfit-strain relaxation (no higher than 45%) is observed in the heterostructures with x = 0.30 after annealing at 650°C. The results obtained are explained by the effect of the nature and concentration of dislocation-nucleation centers, existing in low-temperature buffer layers, on the characteristics of the formation of a dislocation structure in the heterostructures under consideration.

Ge self-assembled islands grown on SiGe/Si(0 0 1) relaxed buffer layers

In this work, the results obtained in growth of Ge(Si) self-assembled islands on relaxed Si 1− x Ge x /Si(0 0 1) buffer layers ( x ∼ 25%) and their photoluminescence study are presented. It is found out that growth of Ge(Si)/Si 1− x Ge x islands proceeds with an abrupt change in the surface morphology that is similar to the earlier observed transition (from dome to hut islands with a decreasing Ge growth temperature) in the case of island growth on Si(0 0 1) substrates. It is revealed that in growth of Ge(Si)/Si 1− x Ge x islands, in contrast to the Ge(Si)/Si(0 0 1) islands case, the interval of growth temperatures, in which there is a change in the islands morphology (dome–hut transition) shifts towards higher temperatures. For the first time a photoluminescence signal from Ge(Si) self-assembled islands embedded in a strained Si layer is observed.

Fracture behavior of diamond-like carbon films on stainless steel under a micro-tensile test condition

We investigated the stability of the DLC film coated on 304 stainless steel substrate by r.f. PACVD method. Fracture and spallation behaviors of the coating were observed during micro-tensile test of the film/substrate composite. As the tensile deformation proceeded, the cracks of the film appeared in the perpendicular direction to the tensile axis. Further deformation resulting in the local necking with shear band of 55° inclined to the tensile axis, induced the spallation of the film, which was initiated at the cracks of the film, and was aligned along the slip directions. We found that both the cracking and the spallation behaviors are strongly dependent on the pretreatment condition, such as Ar plasma pretreatment or Si buffer layer deposition. The spallation of the film was significantly suppressed in an optimized condition of the substrate cleaning by Ar glow discharge. These results show that the spallation behavior during the tensile test can be used to estimate the interfacial strength of the coating with relatively poor adhesion.

Strain relaxation of GeSi/Si(001) heterostructures grown by low-temperature molecular-beam epitaxy

Plastic relaxation in GexSi1−x∕Si(001) heterostructures with x=0.18–0.62, grown at temperatures of 300–600 °C with the use of a low-temperature (350 °C) Si buffer layer, is considered. It is shown that the use of low-temperature Si and low temperature of growth of GeSi films decreases the density of threading dislocations to the value of 105–106cm−2 in heterostructures with a germanium content x<¯0.3, whereas the density of the threading dislocations in heterostructures with a higher content of Ge remains at the level of ∼108cm−2 and higher. By means of transmission electron microscopy, it is shown that the origination of dislocation half-loops from the surface in the case of a high content of germanium in the film is the main reason for the high density of threading dislocations. Growing of GeSi films with a two-step change in composition is considered. The fact that the density of the threading dislocations in the first step of the film is significantly higher than that in the substrate is noted. Because of their presence, the real thickness of insertion of misfit dislocations into the second step of the film is in ten times less than for the first layer. With an allowance for this effect, almost complete plastic relaxation of the second and further heterostructure steps can be reached at low temperatures and at a smaller thickness of GeSi films. It is concluded that the main factors of low-temperature epitaxy of GeSi, which reduce the density of the threading dislocations in heterostructures are (i) a decrease in the initial threading dislocation density and (ii) an increase in the rate of expansion of dislocation loops, which facilitates plastic relaxation with a smaller number of threading dislocations.

Dielectric properties of highly (100) oriented (Pb0.5,Sr0.5)TiO3 thin films grown on Si with MgO buffer layer

Pb 0.5 , Sr 0.5 Ti O 3 (PST) thin films were deposited on Si with MgO (100) buffer layer by the alkoxide-based sol-gel method. Structural and dielectric properties of PST thin films for the tunable microwave device applications were investigated. For the MgO∕Si buffer layer, the PST thin films exhibited highly (100) orientation. The MgO buffer layer affects the stress state of the (100)-oriented PST thin films. The dielectric constant, tunability, and figure of merit of the highly (100)-oriented PST thin film increased with increasing annealing temperature due to the decrease in lattice distortion. The differences in dielectric properties may be attributed to the change in the film stress. The dielectric constants, dielectric loss and tunability of the PST thin films deposited on the MgO∕Si substrates measured at 10kHz were 822, 0.025, and 80.1%, respectively.

Misfit dislocation nucleation and multiplication in fully strained SiGe/Si heterostructures under thermal annealing

An evolution of dislocation structure formed in fully strained Si 1− x Ge x /Si(0 0 1) heterostructures during thermal annealing was studied. Heterostructures with Ge content x = 0.15 and 0.30 were grown by MBE on low-temperature Si(400 °C) and SiGe(250 °C) buffer layers. The main attention was devoted to the initial stages of strain relaxation and to the role of intrinsic point defects in misfit dislocation nucleation. A mechanism is proposed for the misfit dislocation nucleation at heterogeneous sources placed within SiGe epitaxial layer.

High-current-density thin-film silicon diodes grown at low temperature

High-performance thin-film silicon n–i–p diodes are fabricated at temperatures below 160°C using hot-wire chemical vapor deposition. The 0.01mm2 diodes have a forward current-density of near 1000A∕cm2 and a rectification ratio over 107 at ±2V. Use of microcrystalline silicon i and n layers results in higher current-density diodes than with amorphous silicon, primarily by lowering a barrier to carrier injection. A 30nm intrinsic Si buffer layer between the i and p layers is needed to reduce the reverse leakage current. Minimizing diode area increases forward current density by reducing the voltage drop across the external series resistances.

EVOLUTION AND ORDERING OF MULTILAYER Ge QUANTUM DOTS ON Si(001)

Evolution and ordering of multilayer Ge islands (up to 14 layers) on Si (001), prepared by physical vapor deposition at about 550°C; were studied using Scanning Tunneling Microscope (STM). In the growth process, it was observed that long huts split during the deposition of Si buffer layer. It can be explained as a consequence of strain due to lattice mismatch and intermixing of Si and Ge to form solid solution. Such a mechanism of splitting could lead the dots to order and uniformity in the subsequent layers. Apart from splitting of huts, the surface corrugation of Si spacer layer influences the nucleation of Ge islands in the next layer.

Growth and optical properties of Ge/Si quantum dots formed on patterned [formula omitted] substrates

Single and stacked layers of Ge/Si quantum dots were grown in SiO 2 windows patterned by electron-beam lithography on oxidized Si (0 0 1) substrates. The growth of a silicon buffer layer prior to Ge deposition is found to be an additional parameter for adjusting the Ge-dot nucleation process. We show that the silicon buffer layer evolves towards [1 1 3] -faceted pyramids, which reduces the area of the topmost (0 0 1) surface available for Ge nucleation. By controlling the top facet area of the Si buffer layers, only one dot per circular window and a high cooperative arrangement of dots on a striped window can be achieved. In stacked layers, the dot homogeneity can be improved through the adjustment of the Ge deposited amount in the upper layers. The optical properties of these structures measured by photoluminescence spectroscopy are also reported. In comparison with self-assembled quantum dots, we observed, both in single and stacked layers, the absence of the wetting-layer component and an energy blue shift, confirming therefore the dot formation by selective growth.

Growth of Ge islands on prepatterned Si (0 0 1) substrates

We report on the growth and properties of Ge islands grown on (0 0 1) Si substrates with lithographically defined two-dimensionally periodic pits. After thermal desorption and a subsequent Si buffer layer growth these pits have an inverted truncated pyramid shape. We observe that on such prepatterned substrates lens-like Ge-rich islands grow at the pit bottoms with less Ge deposition than necessary for island formation on flat substrates. This is attributed to the aggregation of Ge at the bottom of the pits, due to Ge migration from the pit sidewalls. At the later stages of growth, dome-like islands with dominant {1,1,3} or {15,3,23}, or other high-index facets [i.e. {15,3,20} facets] are formed on the patterned substrates as shown by surface orientation maps using atomic force microscopy. Furthermore, larger coherent islands can be grown on patterned substrates as compared to Ge deposition on flat ones.