Silicon Surface Layer Research Articles

Extension of the lifetime of tantalum filaments in the hot-wire (Cat) Chemical Vapor Deposition process

One of the prime components of a hot-wire (Cat) Chemical Vapor Deposition system is the filament used to pyro-catalytically crack the gases like silane. Burnt out tantalum filaments were studied to determine the possible improvement of lifetime for these filaments. The structure and chemical distribution in the filament were investigated, using electron microscopy and elemental analysis. It was found that at the high temperature centre section of a filament a thin surface silicide layer develops around a central metal core. At the filament ends a surface layer of metal silicide develops, that eventually consume the metal completely, causing breakages. Changes in the pre- and post-treatments of gas type and pressure were tested to improve the filament lifetime, arriving at a very simple procedure that increases the lifetime of the filament to several months. By annealing the hot filament both before and after a deposition run in a low pressure hydrogen ambient, a surface layer of pure silicon developed at the lower temperature ends, essentially protecting the metal from further silicidation.

Bioactive properties of nanostructured porous silicon for enhancing electrode to neuron interfaces

Many different types of microelectrodes have been developed for use as a direct brain–machine interface (BMI) to chronically recording single-neuron action potentials from ensembles of neurons. Unfortunately, the recordings from these microelectrode devices are not consistent and often last for only on the order of months. For most microelectrode types, the loss of these recordings is not due to failure of the electrodes, but most likely due to damage to surrounding tissue that results in the formation of non-conductive glial scar. Since the extracellular matrix consists of nanostructured fibrous protein assemblies, we have postulated that neurons may prefer a more complex surface structure than the smooth surface typical of thin-film microelectrodes. This porous structure could then act as a drug-delivery reservoir to deliver bioactive agents to aid in the repair or survival of cells around the microelectrode, further reducing the glial scar. We, therefore, investigated the suitability of a nano-porous silicon surface layer to increase the biocompatibility of our thin film ceramic-insulated multisite electrodes. In vitro testing demonstrated increased extension of neurites from PC12 pheochromocytoma cells on porous silicon surfaces compared to smooth silicon sufaces. Moreover, the size of the pores and the pore coverage did not interfere with this bioactive surface property, suggesting that large highly porous nanostructured surfaces can be used for drug delivery. The most porous nano-porous surfaces were then tested in vivo and found to be more biocompatible than smooth surface, producing less glial activation and allowing more neurons to remain close to the device. Collectively, these results support our hypothesis that nano-porous silicon may be an ideal material to improve biocompatibility of chronically implanted microelectrodes. The next step in this work will be to apply these surfaces to active microelectrodes, use them to deliver bioactive agents, and test that they do improve neural recordings.

A numerical model to simulate precipitate growth and ripening in oxygen-implanted silicon-on-insulator materials

A finite element model was developed to simulate the production of silicon-on-insulator substrates through the technique known as SIMOX. The simulation initiates from an as-implanted distribution of SiO2 precipitates and calculates the time evolution during annealing of the number, size and shape of precipitates, until the eventual formation of the buried oxide layer under a surface-silicon layer. During the evolution, the precipitates can grow, dissolve, merge or split and they can adopt arbitrary shapes under the dynamic interaction of the oxygen concentration annealing and capillary forces. The simulations show that the model reproduces several phenomena observed during the SIMOX process, like Ostwald ripening and the formation of silicon islands.

Monolithic 3-D silicon photonics

A monolithic CMOS compatible process has been developed to realize vertically integrated devices in silicon. The method involves the implantation of an oxygen into a patterned silicon substrate to form buried guiding structures. These buried devices are separated from a surface silicon layer by an intervening layer of silicon dioxide formed through the implantation process. Photolithography and etching is used to define devices on the surface silicon layer. The method has been utilized to realize the vertically coupled microdisk resonators and a variety of microresonator-based integrated optical elements. A new method for extraction of the unloaded Q of a cavity from its measured spectrum is also described.

Influence of silicon processing in atomic hydrogen on the formation of local molten regions as a result of pulsed irradiation with optical photons

The effect of intense atomic hydrogen flux on the defect density in the surface layer of single-crystal silicon is studied. It is shown that the formation of local molten regions by pulsed-light heating of Si samples and further analysis of the local melting pattern can be an efficient tool for determining the number of defects introduced by the processing in atomic hydrogen. It was found that the processing conditions in atomic hydrogen with an exposure dose lower than 2.7 × 1017 cm−2 do not change the number of defects in Si; in contrast, conditions with an exposure dose above 3.6 × 1018 cm−2 significantly increase the defect density. The increase in the number of defects can be caused by the interaction of atomic hydrogen with the Si surface.

Boundary effects on the plastic flow of amorphous layers during high-energy heavy-ion irradiation

Swift heavy-ion irradiation of amorphous materials causes nonsaturating plastic deformation. Thin samples deform anisotropically as if they were hammered. Thick samples or thin layers on top of thick crystalline substrates flow plastically during nonperpendicular irradiations. In this paper, we demonstrate that the constitutive equation of ion hammering not only describes the homogeneous flow of uniformly irradiated matter but also the deformation effects at boundaries between irradiated and unirradiated matter. An analytic expression is derived which describes the flow of thin surface layers in the vicinity of an abrupt boundary. This solution is complemented by finite-element calculations, which reveal the influence of finite transition widths. The constitutive equation is also solved numerically to unveil the boundary effects for thick samples, where the ion-induced deformation depends on the distance from the specimen surface. The calculations are compared with experiments carried out with thin amorphous silicon surface layers and thick crown glasses. The excellent agreement confirms the validity of the constitutive equation and supports its microscopic basis, namely the idea of an efficient relaxation of thermally induced shear stresses along the ion path during the thermal spike period.

Electrical characterization of 5 keV phosphorous implants in silicon

Si(100) wafers were implanted with 5-keV phosphorous (fluences of 1×1014–1×1016cm−2) and then annealed at 600–1000 °C, for 30 s or for 5 mins, to study shallow junction formation. Carrier densities were obtained by Van de Pauw Hall measurements, and carrier concentrations were depth profiled via a native-oxide stripping, differential Hall technique. For implanted fluences ⩾5×1014cm−2, an amorphous silicon surface layer is created. This results in solid-phase epitaxial growth at low annealing temperatures (600 °C), with reverse annealing upon dissolution of end-of-range interstitial clusters at higher temperatures (>700°C). Sheet resistances as low as 20Ω∕sq with electrical activation approaching 100% were achieved after annealing at 1000 °C.

직접 접합에 의한 Al2O3SOI 구조 제작

The SOI structure with buried alumina was fabricated by ALD followed by bonding and etchback process. The interface of alumina and silicon was analyzed by CV measurements and cross section was investigated by SEM analysis. The density of interface state of alumina and silicon was 2.5E11/-eV after high temperature annealing for wafer bonding. It was confirmed that the surface silicon layer was completely isolated from substrate by cross section SEM and AES depth profile. The device on this alumina SOI structure would have better thermal properties than that on conventional SOI due to higher thermal conductivity of alumina than that of silicon dioxide.

Characterization of epitaxial germanium grown on (LaxY1−x)2O3∕Si(111) using different surfactants

The uniformity and quality of epitaxial, ultrathin germanium-on-insulator (GOI) layers are studied as a function of Ge regrowth under different surfactant conditions. It is shown that using antimony as a surfactant during the solid phase epitaxial regrowth of the germanium layers provides a higher crystal quality and much flatter surfaces than samples grown without antimony. However, the diffusion of even a small percentage of a monolayer of antimony into the GOI layer may cause a thin n+ “delta-doped” layer to remain on the surface of the wafer, making device fabrication difficult. It is shown that using a surface layer of silicon as a surfactant that is not expected to interact electronically with the Ge (unlike Sb), acceptable surface smoothness may be achieved. Physical mechanisms behind this are discussed.

Structure and Magneto-Optical Properties of “Porous Silicon–Cobalt” Granular Nanocomposites

The structural and magneto-optical properties of “porous silicon-cobalt” nanocomposites prepared through electrochemical deposition on silicon substrates of different porosities are investigated. It is revealed that, under galvanostatic conditions, cobalt micrograins are formed only in a surface layer of porous silicon. The greater the pores in silicon, the larger the mean size of the ferromagnetic micrograins. The nanocomposites thus fabricated possess ferromagnetic properties and, at specified compositions, are characterized by anomalously large magnitudes of the equatorial or transverse magneto-optical Kerr effect (TMOKE). The magneto-optical properties of the nanocomposites are simulated in the Bruggeman effective-medium approximation. It is shown that the anomalous negative transverse magneto-optical Kerr effect is associated with the oxidized state of porous silicon in the vicinity of the ferromagnetic metal micrograins.

Surface micro-grooves for near-wall exergy and flow control: application to aircraft intake de-icing

This paper develops a new surface micro-profiling technique for reducing exergy losses and controlling near-wall flow processes, particularly for anti-icing of a helicopter surface. Fabrication of embedded surface microchannels entails surface etching with KOH and XeF2 gas, so that the interspersed microchannels can be assembled into a surface layer of silicon. Testing of the micro-profiled surfaces is performed with particle image velocimetry in a water tunnel. Experimental data indicate that converging open microchannels lead to certain differences of flow patterns on the downstream side of an engine cooling bay. Furthermore, exergy losses for external flow past the parallel embedded microchannels are shown to be lower than previous benchmark results without microchannels. Analytical results are presented for these losses of available energy and exergy destruction. Reduced drag of slip-flow conditions within each microchannel offsets the added friction irreversibility of larger surface area. By altering the near-wall flow patterns, it is anticipated that embedded surface microchannels can provide a useful new approach for dealing with flow-related problems of aircraft icing.

Hydrogen Evolution Efficiency at Illuminated Silicon

The change of kinetic characteristics of photoelectrochemical hydrogen evolution at p-type silicon in acid aqueous electrolyte solutions under prolonged continuous illumination is studied. It is shown that, during the transfer of full charge Qt 350 C/cm2, in breaking-down the direct relation between the current and the charge transfer time, despite the electrode's retaining high photosensitivity. The effect is probably caused by significant energy and structure distortions in the surface layer of silicon.

Formation of ultra-shallow p +/n junctions in silicon-on-insulator (SOI) substrate using laser annealing

Laser annealing (LA), in which the laser melts the surface layer of silicon and causes the dopants to be distributed uniformly within the melted region, produces abrupt, highly activated and ultrashallow junctions. The degree of melting is determined by the extent of laser absorption and rate of heat dissipation, which are dependent on the substrate properties. When applying LA on substrates such as silicon-on-insulator (SOI), the heating and cooling characteristics are expected to be different from that of a typical Si substrate. This work compares the redistribution of boron atoms in silicon (1 0 0) and SOI substrates after laser annealing. SIMS analysis shows that laser induced melting is significantly deeper for the SOI than the silicon substrates using the same laser fluence. The enhancement of melting is attributed to the heat insulating effect of the buried oxide (BOX) layer. With multiple-pulse LA, the junction depth in the SOI substrate increases with subsequent laser pulses, a feature that is absent in silicon substrate. In the SOI substrate, the sheet resistance remains relatively constant regardless of deeper junction formed with multiple pulse conditions, implying the maximum dopant activation at a given laser fluence is reached. Boron profiles annealed in the non-melt regime with 20 laser pulses or less overlap with the as-implanted profiles, suggesting that no melting has occurred. However, significant melting is observed at 50-pulse annealing. The corresponding sheet resistance shows a sharp decrease with the initial pulses and consequently decreases slightly with increasing pulses.

Step-debunching in Sn/Si(001) surfaces

Step-debunching, or the transformation of step structure from double atomic height to single atomic height, has been observed using scanning tunneling microscopy (STM) on the vicinal Si(001) surface upon adsorption of 0.8 ML Sn. The surface shows a two-domain c(8×4)-Sn structure containing defective features such as antiphase boundaries and so on. One of the causes of step-debunching is the release of misfit-induced strain between the substrate and adsorbates. The other is related to kinetics, where Sn replaces surface silicon layers during the formation of c(8×4) and the removed Si atoms are incorporated into the step to extend the terrace. The kinetics on the small terrace width is responsible for the observed defective surface structure.

Recombination of an electron-hole plasma in silicon under the action of femtosecond laser pulses

Experimental data are obtained on the dynamics of conduction-electron relaxation at the stage preceding the melting of a silicon surface layer. The energy of a quantum of probe radiation is smaller than the band gap, making it possible to obtain information about the electron-phonon relaxation processes for an electron concentration of ∼ 1021 cm−3 in the conduction band.

Concentration Dependence of Boron-Interstitial Cluster (BIC) Formation in Silicon-on-Insulator (SOI)

ABSTRACTThe effect of boron concentration on boron-interstitial cluster (BIC) dissolution was studied in separation by implantation of oxygen (SIMOX) and SOITEC silicon-on- insulator (SOI) materials. SOI substrates having surface silicon thickness of 750 Å and 1450 Å were ion implanted with 11B+ at a energy of 15 keV. The dose of the implant was varied from 3×1014 cm−2 to 1×1015 cm−2 to provide different boron peak concentrations. Anneals were performed at 825 °C in a nitrogen ambient using RTA for short times and furnace anneals for longer times. The retained dose of boron in the surface silicon layer was estimated using simulations from UT-Marlowe 5.0. Hall Effect was used to measure the sheet resistance, mobility, sheet number, and fraction of active boron. At lower concentrations, SOI exhibited significantly lower sheet numbers. As the boron concentration was increased SOI began to approach bulk Si. However, degraded mobility and sheet resistance was observed at all concentrations. Truncating the boron profile over the surface Si/buried oxide interface enhanced the fraction of active boron in the 750 Å SOI. This also lead to a higher fractional activation in 750 Å SOI than bulk Si as the concentration increased due to a loss of interstitials, effectively reducing the BIC population.

Phase transformations initiated in thin layers of amorphous silicon by nanosecond excimer laser pulses

The reflection of a probing beam with a wavelength λ=0.63 µm from a silicon surface layer (a-Si) amorphized by ion implantation was detected during its melting and solidification initiated by excimer ArF laser radiation. When the irradiation energy is below the epitaxial threshold, a single event of a-Si layer melting leads to the formation of single nanocrystals separated from one another in the amorphous matrix rather than to the appearance of polycrystalline material. The presence of nanocrystals makes possible the formation of polycrystalline Si from a melt under exposure to a second laser pulse and allows the intermediate crystallization of Si in a laser-induced sequence of phase transitions. The data obtained are compared to the results of studying the phase transitions initiated in similar experimental conditions in thin layers of hydrogenated a-Si layers on glass substrates.

In situ laser recrystallization of Si layers during low-pressure chemical vapor deposition: Recrystallization dynamics and influence of the seed layer

The growth of polycrystalline silicon on glass by low-pressure chemical vapor deposition and in situ laser induced recrystallization was investigated with the aim to study the influence of the seed layer and the mechanism of the recrystallization dynamics on the structural and morphological properties of the grown film. A seed layer was used to trigger the solidification process of many additional in situ laser-crystallized overlayers. One-dimensional calculations of the thermal flow produced by laser irradiation were used to clarify the complex interaction between the substrate and the molten silicon surface layer during nucleation and growth. The experiments show the relevant role played by the seed layer and the peculiar shaping of the film surface due to the preferential aggregation of molten silicon. Compact polysilicon films with thicknesses up to 4 μm with almost monocrystalline grains of 1–2-μm size were obtained.

Ultrafast and direct imprint of nanostructures in silicon.

The fabrication of micrometre- and nanometre-scale devices in silicon typically involves lithography and etching. These processes are costly and tend to be either limited in their resolution or slow in their throughput. Recent work has demonstrated the possibility of patterning substrates on the nanometre scale by 'imprinting' or directed self-assembly, although an etching step is still required to generate the final structures. We have devised and here demonstrate a rapid technique for patterning nanostructures in silicon that does not require etching. In our technique which -- we call 'laser-assisted direct imprint' (LADI) -- a single excimer laser pulse melts a thin surface layer of silicon, and a mould is embossed into the resulting liquid layer. A variety of structures with resolution better than 10 nm have been imprinted into silicon using LADI, and the embossing time is less than 250 ns. The high resolution and speed of LADI, which we attribute to molten silicon's low viscosity (one-third that of water), could open up a variety of applications and be extended to other materials and processing techniques.

The effect of dense compression plasma flow on silicon surface morphology

The dense compression plasma flow action on silicon surface layers is investigated. Samples of monocrystalline silicon of (111) and (100) orientations were subjected to plasma flow processings. Values of energy absorbed by the silicon surface range from 5 to 25 J per pulse, the latter corresponding to the flow power density range 0.5·10 5–3·10 5 W/cm 2. Microreliefs of sample surfaces and slices were recorded by means of high-resolution scanning microscopy. The action of the compression plasma flow on the sample results in the melting and subsequent modification of silicon material down to the depth of 6 μm. Micrographs of surface layers clearly show regular cylindrical structures that are the first of their kind ever observed. Cylindrical fragments measure 50–100 μm in length and 0.7–1.5 μm in diameter. These fragments are located on the surface of the sample at intervals of 1–2 μm with a surface density of (2–6)·10 6 cm −2. Possible mechanisms of cylindrical periodic structure formation are discussed. The roles of Rayleigh–Taylor instability, Kelvin–Helmholtz instability and Benard instability in transformations of molten surface layers are considered. It may be inferred that structural-phase changes in the state of silicon surface are related to the fast crystallization of molten layer accompanied by the development of various instabilities in the presence of an induced magnetic field.