Amorphous Si Layer Research Articles

Plasma-enhanced chemical vapour deposition growth of Si nanowires with low meltingpoint metal catalysts: an effective alternative to Au-mediated growth

Au nanoparticles are efficient catalysts for the vapour–solid–liquid (VLS) growth ofsemiconductor nanowires, but Au poses fundamental reliability concerns for applications inSi semiconductor technology. In this work we show that the choice of catalystsfor Si nanowire growth can be broadened when the need for catalytic precursordissociation is eliminated through the use of plasma enhancement. However, in thisregime the incubation time for the activation of VLS growth must be minimized toavoid burying the catalyst particles underneath an amorphous Si layer. We showthat the combined use of plasma enhancement and the use of a catalyst such asIn, already in a liquid form at the growth temperature, is a powerful methodfor obtaining Si nanowire growth with high yield. Si nanowires grown by thismethod are monocrystalline and generally oriented in the direction.

Structure and magnetic property of c-axis oriented L10-FePt nanoparticles on TiN/a-Si underlayers

L 1 0 -FePt is a promising material for high-density perpendicular magnetic recording media. The authors previously reported that c-axis oriented L10-FePt nanoparticle monolayers can be formed on (200)-oriented polycrystalline template TiN underlayers on SiO2 by using a conventional sputtering method. In this study, TiN nanostructures, such as the degree of (200) orientation, were improved by first depositing a buffer layer, such as amorphous Si onto SiO2, and the grain size could be controlled by adjusting either the deposition temperature or TiN thickness. When FePt nanoparticles were formed on a template TiN underlayer with a buffer layer of amorphous Si, both their degree of c-axis orientation and their magnetic properties were improved; FePt nanoparticles with nominal thickness of 1.4 nm had coercivity of 12.9 kOe in the out-of-plane direction at 300 K.

Silicon nanowire grid polarizer for very deep ultraviolet fabricated from a shear-aligned diblock copolymer template

A silicon (Si) nanowire grid ultraviolet (UV) transmission polarizer has been fabricated, and its performance was measured over the visible to deep UV range. A cylinder-forming polystyrene-b-poly(hexylmethacrylate) diblock copolymer was coated onto an amorphous Si layer supported on a fused silica substrate, then shear aligned and employed as a mask for reactive-ion etching, resulting in a Si grid of 33 nm period and multi-centimeter-squared area. Due to the high plasma frequency and UV reflectance of the deposited Si, this nanowire grid was able to polarize light down into the deep UV, including 193 nm.

Modulation of negative index metamaterials in the near-IR range

Optical modulation of the effective refractive properties of a “fishnet” metamaterial with a Ag∕Si∕Ag heterostructure is demonstrated in the near-IR range and the associated fast dynamics of negative refractive index is studied by pump-probe method. Photoexcitation of the amorphous Si layer at visible wavelength and corresponding modification of its optical parameters is found to be responsible for the observed modulation of negative refractive index in near-IR.

Investigation of Sb diffusion in amorphous silicon

Amorphous silicon materials and its alloys became extensively used in some technical applications involving large area of the microelectronic and optoelectronic devices. However, the amorphous-crystalline transition, segregation and diffusion processes still have numerous unanswered questions. In this work we study the Sb diffusion into an amorphous Si film by means of Secondary Neutral Mass Spectrometry. Amorphous Si/Si 1− x Sb x /Si tri-layer samples with 5 at% antimony concentration were prepared by direct current magnetron sputtering onto Si substrate at room temperature. Annealing of the samples was performed at different temperatures in vacuum ( p<10 −7 mbar) and 100 bar high purity (99.999%) Ar pressure. During annealing a rather slow mixing between the Sb-alloyed and the amorphous Si layers was observed. Supposing concentration independent of diffusion, the evaluated diffusion coefficients are in the range of ∼10 −21 m 2s −1 at 550 °C.

Spin-polarized transport of electrons from polycrystalline Fe3O4 to amorphous Si

Polycrystalline Fe3O4∕amorphous Si heterostructure was prepared by facing-target sputtering and its microstructure and electrical transport properties were studied. The polycrystalline Fe3O4 layer was grown in column structure. The electrical transport mechanism across the disordered interface between polycrystalline Fe3O4 and amorphous Si layers is tunneling above the Verwey temperature [Nature (London) 144, 327 (1939)] of 120K. Nonlinear I-V characteristics of the Schottky diode reveal thermionic emission∕diffusion mechanism below the Verwey temperature, and Schottky barrier height is 0.27eV, calculated by a standard theory of thermionic emission∕diffusion. Based on a simplified band structure, the spin polarization of the polycrystalline Fe3O4 layer was determined to be ∼45%.

Electron-Beam Lithography Patterning of Ge2Sb2Te5 Nanostructures Using Hydrogen Silsesquioxane and Amorphous Si Intermediate Layer

We report the successful electron-beam patterning of nanostructures, in the form of both lines and dots, using hydrogen silsesquioxane (HSQ) resist. Although HSQ has proven to be a good resolution electron-beam resist, the adhesion between and the HSQ resist is problematic when trying to form fine patterns. To promote their adhesion, we introduced an amorphous Si (a-Si) layer between the and HSQ layers, and the layers were then sequentially removed using reactive ion etching and Ar etching with good selectivity of each layer. The selectivities for HSQ:a-Si etched by reactive gas and etched by Ar gas were 1:2.7 and 1:17, respectively. On the basis of this multilayer structure and a two-step dry etching process, various nanostructures as small as were successfully fabricated.

Effect of laser annealing on crystallinity of the Si layers in Si/SiO 2 multiple quantum wells

We report on continuous-wave laser induced crystallisation processes occurring in Si/SiO 2 multiple quantum wells (MQW), prepared by remote plasma enhanced chemical vapour deposition of amorphous Si and SiO 2 layers on quartz substrates. The size and the volume fraction of the Si nanocrystals in the layers were estimated employing micro-Raman spectroscopy. It was found that several processes occur in the Si/SiO 2 MQW system upon laser treatment, i.e. amorphous to nanocrystalline conversion, Si oxidation and dissolution of the nanocrystals. The speed of these processes depends on laser power density and the wavelength, as well as on the thickness of Si-rich layers. At optimal laser annealing conditions, it was possible to achieve ∼100% crystallinity for 3, 5 and 10 nm thickness of deposited amorphous Si layers. Crystallization induced variation of the light absorption in the layers can explain the complicated process of Si nanocrystals formation during the laser treatment.

Dopant-enhanced solid-phase epitaxy in buried amorphous silicon layers

The kinetics of intrinsic and dopant-enhanced solid-phase epitaxy (SPE) is studied in buried amorphous Si layers in which SPE is not retarded by H. As, P, B, and Al profiles were formed by multiple energy ion implantation over a concentration range of $(1--30)\ifmmode\times\else\texttimes\fi{}{10}^{19}∕{\mathrm{cm}}^{3}$. Samples were annealed in air at temperatures in the range $460--660\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and the rate of interface motion was monitored using time-resolved reflectivity. The dopant-enhanced SPE rates were modeled with the generalized Fermi level shifting model using degenerate semiconductor statistics. The effect of band bending between the crystalline and amorphous sides of the interface is also considered.

Molecular dynamics simulation of the recrystallization of amorphous Si layers: Comprehensive study of the dependence of the recrystallization velocity on the interatomic potential

The molecular dynamics method is applied to simulate the recrystallization of an amorphous/crystalline silicon interface. The atomic structure of the amorphous material is constructed with the method of Wooten, Winer, and Weaire. The amorphous on crystalline stack is annealed afterward on a wide range of temperature and time using five different interatomic potentials: Stillinger-Weber, Tersoff, EDIP, SW115, and Lenosky. The simulations are exploited to systematically extract the recrystallization velocity. A strong dependency of the results on the interatomic potential is evidenced and explained by the capability of some potentials (Tersoff and SW115) to correctly handle the amorphous structure, while other potentials (Stillinger-Weber, EDIP, and Lenosky) lead to the melting of the amorphous. Consequently, the interatomic potentials are classified according to their ability to simulate the solid or the liquid phase epitaxy.

Batch ALD: Characteristics, comparison with single wafer ALD, and examples

A comparison is made between single-wafer and (vertical) batch systems for Atomic Layer Deposition (ALD). Various characteristic times that play a role in such systems are compared. Many of these characteristic times are substantially shorter in single wafer systems. In case wafers with a high density of deep trenches are processed in a batch system, the required length of the precursor pulse could be as long as 10–60 s, depending on the specific chemistry of the process. This time is largely determined by the time it takes to saturate the very large surface of the trenched substrates. Despite that, batch systems with load sizes of 50–100 wafers are superior in terms of overall throughput. Two classes of ALD processes are distinguished, ‘normal’ ALD, in which the active precursors are separated by inert gas purges, and partial ALD in which one of the ALD steps is replaced by a pulsed CVD step. In the first category two examples are discussed, Al 2O 3 and HfO 2 ALD, characterized by short and long surface saturation times, respectively. In the second category (partial) ALD of TiN and Si 3N 4 are discussed. First a thin TiN layer is deposited through a short CVD pulse; in the second, true ALD pulse NH 3 reacts with the TiN surface to remove any remaining Cl. It is demonstrated that this procedure combines a high overall growth rate with a low sheet resistance. In the Si 3N 4 process the first pulse is a CVD process in which a thin layer of amorphous Si is deposited. In the second (ALD) pulse N and/or NH2 radicals generated by a remote plasma are used to convert the Si into Si 3N 4. The paper concludes with a short discussion on new types of carbide and nitride films that have potential to be deposited in batch type ALD systems.

Dopant Diffusion during Amorphous Silicon Crystallization

We have investigated the redistribution of B during the crystallization of an amorphous Si layer homogeneously doped with P. The redistribution of B only occurs for concentrations lower than 2 × 1020 at cm−3. Crystallization leads to a non “Fickian” redistribution, allowing an abrupt interface between the regions doped and undoped with B. Once the crystallization is ended, B diffuses through the layer in the type B regime with a coefficient which is in agreement with the literature data for diffusion in polycrystalline Si. Although the P distribution is homogeneous in the entire layer, for a temperature as high as 755 °C, P diffuses towards the region the most concentrated in B. The B and P interactions are interpreted as chemical interactions.

Surface chemistry and preferential crystal orientation on the H and Cl terminated silicon surface

Surface chemistry and determining factors of the preferential crystal orientation are discussed through the deposition studies on hydrogenated chlorinated crystalline silicon films by rf plasma-enhanced chemical vapor deposition of a dichlorosilane, SiH2Cl2, and H2 mixture. The growth of randomly oriented crystal Si films occurred from the initial growth stage. On the other hand, the incubation layer of amorphous Si was formed in the initial stage, and subsequently, the growth of (220) preferred crystal orientation proceeded. They are determined by the thermal abstraction of H from the growing surface at substrate temperature above 350°C. Higher degree of Cl termination was effective in suppressing the oxygen incorporation into the Si network, although it did not contribute directly to the preferred crystal orientation. The insertion of atomic hydrogen to the Si–Si back bond in the subsurface region promoted the SiHClx complex formation, which was the most possible nucleation site for promoting the (220) preferential crystal orientation.

Ultrathin amorphous Si layer formation by femtosecond laser pulse irradiation

Formation of ultrathin amorphized Si layer by femtosecond laser irradiation is reported in this letter. Below the fluence of ablation threshold, femtosecond laser irradiation induced an amorphization of crystalline Si. The authors confirmed the thickness of amorphous Si layer by transmission electron microscope. The thickness of the amorphized layer was found to be quite uniform and did not depend on the number of irradiated laser pulses and fluence, which was related to the effective light penetration depth.

Study of Si wafer surfaces irradiated by gas cluster ion beams

The surface structures of Si (100) wafers subjected to gas cluster ion beam (GCIB) irradiation have been analyzed by cross-sectional transmission electron microscopy (XTEM) and atomic force microscopy (AFM). GCIB irradiation is a promising technique for both precise surface etching and planarization of Si wafers. However, it is very important to understand the crystalline structure of Si wafers after GCIB irradiation. An Ar-GCIB used for the physically sputtering of Si atoms and a SF6-GCIB used for the chemical etching of the Si surface are also analyzed. The GCIB irradiation increases the surface roughness of the wafers, and amorphous Si layers are formed on the wafer surface. However, when the Si wafers are annealed in hydrogen at a high temperature after the GCIB irradiation, the surface roughness decreases to the same level as that before the irradiation. Moreover, the amorphous Si layers disappear completely.

Epitaxial recrystallization of amorphous Si layers by swift heavy ions

Epitaxial recrystallization of 200nm amorphous Si layers by swift heavy ions (50 and 100MeV Au8+) and the role of electronic energy loss was investigated by Rutherford backscattering spectrometry and Micro-Raman spectroscopy. We observed good epitaxial crystallization in the range of 473–673K, which is a much lower temperature regime as compared to the one needed for conventional solid phase epitaxial growth. Planar recrystallization has been observed as a function of temperature giving rise to an activation energy of 0.26±0.02eV. A considerable amount of stress still remains even after full epitaxy at 673K implantation. A possible mechanism of recrystallization is discussed on the basis of the production of vacancies along the track of the swift heavy ion and their migration at elevated temperatures.

Flux Penetration in a Ferromagnetic/Superconducting Bilayer

An array of miniature Hall sensors is used to study the magnetic flux penetration in a ferromagnetic/superconducting bilayer consisting of Nb as a superconducting layer and Co/Pt multilayer with perpendicular magnetic anisotropy as a ferromagnetic layer, separated by an amorphous Si layer to avoid the proximity effect. It is found that the magnetic domains in the ferromagnetic layer create a large edge barrier in the superconducting layer which delays flux penetration. The smooth flux profiles observed in the absence of magnetic pinning change into terraced profiles in the presence of domains.

Effect of hydrogen implantation on semiconductor–metal transition and high-pressure thermopower in Si

In the present work Czochralski-grown silicon single crystals were investigated implanted with different doses of H + ions. The Si wafers were characterized by the Raman scattering technique. Thermoelectric power was studied in a pressure range of 0–20 GPa of p-type Si single crystal wafers containing a thin hydrogenated layer consisting of amorphous Si, nanocrystalline Si and H-rich Si layers. In the region of the pressure-induced phase transition from the initial semiconductor diamond-like into the metal β-Sn lattice, a lowering of thermopower values was noticed in comparison with ones of p-Si.

Effect of amorphous Si layer on the reaction of carbon and silicon in the C/Si multilayer by high vacuum annealing

Crystalline silicon carbide can be formed on the crystalline silicon (c-Si) substrates by reaction of amorphous silicon (a-Si) and carbon (C) layers when annealed at high temperature. The a-Si layer position under or sandwiching the C layer will affect reaction temperature between the a-Si and C layers at constant annealing time. Vacuum annealing at 500–900 °C was performed to study the effect of a-Si layer on the reaction between C and Si in the three kinds of structures, i.e. C, C/a-Si and a-Si/C/a-Si deposited on c-Si substrates. There was no interdiffusion between the single-layer C and c-Si substrate when annealed up to 900 °C for 1.5 h. But some interdiffusion occurred between C and a-Si of the two-layer C/a-Si structure when annealed at 900 °C for 1.5 h. Simultaneously, transformation of a-Si to polycrystalline Si was detected. The interdiffusion between C and Si in the three-layer a-Si/C/a-Si was more pronounced than in the two-layer C/a-Si. In the a-Si/C/a-Si structure, the transformation of a-Si to polycrystalline Si was observed at 700 °C and significant interdiffusion occurred at 900 °C as well as crystalline SiC formation in the multilayer. The SiC formation temperature is lower than conventional crystalline SiC which is formed at 1200 °C in chemical vapor deposition process. The two a-Si layers sandwiching the C layer in the three-layer a-Si/C/a-Si structure lead to the formation of the nanocomposite structure of SiC, C and Si phases at 900 °C. A possible interdiffusion and reaction mechanism between C and Si in the C/Si multilayers through the grain boundaries of polycrystalline Si is proposed.

Enhanced room temperature ferromagnetism in Co- and Mn-ion-implanted silicon

The authors report on ferromagnetism at room temperature in cluster-free, cobalt- and manganese-ion-implanted crystalline silicon. Through magnetic and structural analysis it is shown that the ion-implanted Si consists of two layers of Co- and Mn-containing silicon: (1) an amorphous Si layer on the surface and (2) single crystalline Si beneath. The amorphous layer shows very little magnetism by itself but seems to be responsible for partially canceling out or masking the ferromagnetism in the crystalline Si. The authors also observe that etching of the amorphous Si layer dramatically enhances the measured magnetism by as much as 400%.