Crystallization Of Amorphous Si Research Articles

Polycrystalline silicon thin film made by metal-induced crystallization

This paper investigates the crystallization of amorphous Si by using Al below eutectic temperature as a function of temperature on glass substrate. The crystalline structure from Raman spectroscopy represents amorphous phase of films perfectly changed to polycrystalline phase at the 450 °C. The ratio of crystalline volume fraction was observed to increase from 48.63% to 68.28% as the time increases from 1 to 40 min. Crystalline structure was preferentially grown along the 〈1 1 1〉 orientation with the increase in time duration above the temperature of 450 °C. Also, the sheet resistance decreases with the increase of crystallinity because many Al contents are expensed due to the crystallization process.

Combinatorial masked deposition: simple method to control deposition flux and its spatial distribution

Deposition flux is an important factor that determines the structures of vapor-deposited materials. However, controlling this flux over a wide range is difficult using only a single apparatus. In this work, we developed a simple method, called combinatorial masked deposition (CMD), that enables a series of deposition fluxes and their respective distribution to be realized on a single sample by just setting a mask with holes of different sizes above a substrate. The degree of reduction in deposition flux can be controlled by the hole size and distance between the given point and the hole. The characteristics and applicability of CMD were evaluated by two experiments. In the first experiment, Cu nanoparticles were formed by sputter–deposition on a-SiO 2 at different Cu deposition fluxes. The nanoparticles had a higher number density and smaller size when deposited at 0.80 nm/s for 2.5 s than when deposited at 0.014 nm/s for 140 s. In the second experiment, metal-induced crystallization of amorphous Si (a-Si) was done with spatially distributed Ni additives. The CMD method can realize a series of Ni flux distributions and was successfully used to form 100 different profiles of Ni concentration on a single sample, thus enabling efficient screening of concentration profiles to enhance grain size.

Formation of nickel disilicide using nickel implantation and rapid thermal annealing

Transmission electron microscopy (TEM), secondary ion mass spectroscopy (SIMS), and x-ray photoemission spectroscopy (XPS) have been used to investigate the nucleation, growth, and ripening behavior of nickel-disilicide precipitates formed by Ni implantation in an amorphous-Si layer on (100) Si and followed by a two-step annealing treatment. The TEM and XPS results show that amorphous-disilicide precipitates are formed in a depth of ∼21 nm in the amorphous-Si layer when pre-annealed at 380°C for 30 sec. It is also shown that the second-step annealing at temperatures in the range of 450–600°C causes the amorphous precipitates to transform to randomly oriented crystalline ones embedded in the amorphous-Si layer. Annealing above 550°C is shown to induce the crystallization of amorphous Si by solid-phase epitaxial growth (SPEG). It is further shown that, in a prolonged annealing at high temperatures, the disilicide has dissolved and reprecipitated on the Si surface. Based on the roles of the silicide-mediated crystallization (SMC), the dissolution and reprecipitation of silicides, and SPEG, possible mechanisms are given to explain how the surface-disilicide islands are formed during annealing at temperatures of 550–950°C.

Device Characteristics of Polysilicon Thin-Film Transistors Fabricated by Electroless Plating Ni-Induced Crystallization of Amorphous Si

Compared with conventional solid phase crystallized (SPC) thin-film transistors (TFTs), metal-induced laterally crystallized (MILC) TFTs exhibit significantly enhanced performance. Metal films are usually deposited by the physical vapor deposition (PVD) method, which is time-consuming and expensive in terms of equipment cost. In this work, a simpler electroless plating Ni was introduced to replace PVD Ni. It was found that the morphologies and the device characteristics of Ni-induced lateral crystallization TFT were as good as those of PVD Ni-induced lateral crystallization TFT.

Formation of crystalline Si nanodots in SiO2 films by electron irradiation

Amorphous SiO2 transforms into crystalline Si by 200 kV electron irradiation at ambient temperature. The transformation of amorphous SiO2 to crystalline Si takes place in two steps; the first step involves transformation of amorphous SiO2 into amorphous Si, while the second step is the crystallization of amorphous Si. Valence electron ionization is determined as the key factor for the transformation from SiO2 to amorphous Si; beam heating and knock-on displacement are responsible for the transformation from amorphous Si to crystalline Si. The energy threshold for the crystallization of amorphous Si is determined to be 150.2 kV.

Stress Effects on Nanocrystal Formation by Ni-Induced Crystallization of Amorphous Si

AbstractMetal-induced crystallization (MIC) of amorphous Si is gaining increased interest because of its potential use for low-temperature fabrication of integrated circuits. In this work, the MIC technique was used to make Si nanocrystals and the effects of stress on the crystallization were studied. Amorphous Si films were deposited onto the Si substrate with thermal oxides on top by low-pressure chemical vapor deposition (LPCVD) and then patterned into nanoscale pillars by electron beam lithography and reactive ion etching. A conformal low-temperature oxide (LTO) layer was deposited to cover the pillars, followed by an anisotropic etch back to form a spacer, leaving only the top surface of the pillars exposed to the 5 nm Ni sputtering deposition afterwards. An HF dip was used to partially remove the LTO spacers on the pillars, leading to different LTO thicknesses on different samples. These samples were then annealed to crystallize the amorphous Si pillars, forming Si nanocrystals. Transmission electron microscope (TEM) observations after anneal found a clear dependence of the crystallization rate on the pillar size as well as the LTO thickness. The crystallization rate was lower for pillars with thicker LTO spacers, while for the same LTO thickness the crystallization rate was lower for pillars with narrower width. A model based on the stress in the pillars is proposed to explain this dependence. This model suggests some methods to control the nickel-induced crystallization process and achieve higher quality Si nanocrystals.

Single-crystalline Si on insulator in confined structures fabricated by two-step metal-induced crystallization of amorphous Si

We present a technology and its mechanism to obtain single-crystalline Si pillars on SiO2 using a two-step Ni-induced crystallization process on amorphous Si pillars with confined sizes. The amorphous Si pillars with a Ni cap were first annealed at 400 °C for 15 h so that a single-crystalline NiSi2 template was formed on top of each pillar. In the second step, they were annealed at 550 °C for 2 h, during which single-crystalline Si pillars were formed by NiSi2-mediated solid-phase epitaxy. These single-crystalline Si pillars can be used for advanced vertical metal–oxide–semiconductor transistors and surround-gate structures, especially where low-temperature processing is required.

Metal-Free Germanium-Induced Crystallization of Amorphous-Si on Glass

AbstractA metal-free germanium-induced crystallization technique is reported to realize poly-silicon films on glass at temperatures as low as 480°C. The reduction in crystallization temperature is achieved by using ultra-violet illumination during thermal treatment of the amorphous silicon layer. The annealed samples have been studied using SEM and XRD analyses. Also the electrical conductivity of samples with various annealing conditions has been examined. Conductivity of the samples annealed in the presence of ultra-violet exposure shows more than three orders of magnitude improvement compared to that of amorphous films. Samples treated without UV exposure do not show much enhancement. Size of poly-silicon grains is 0.3μm, as revealed using SEM study.

Microwave-induced low-temperature crystallization of amorphous Si thin films

Microwave heating was utilized for low-temperature crystallization of amorphous Si (a-Si) films. Microwave heating lowered the annealing temperature and reduced the annealing time. By microwave heating the hydrogen in the amorphous films was diffused out long before the nucleation of polycrystalline Si (poly-Si). The combination of NiCl2 coating on a-Si and microwave heating greatly reduced crystallization temperature. The combination of metal-induced crystallization and microwave-induced crystallization might be a useful technique to develop high-quality poly-Si films at low temperature.

Reply to “Comment on ‘Molecular-dynamics simulations of solid-phase epitaxy of Si: Growth mechanisms’ ”

We have investigated atomic diffusion properties near the amorphous/crystalline $(a/c)$ Si(001) interface during solid-phase epitaxy (SPE) of Si based on molecular-dynamics simulations using the Tersoff potential. It has been found that the mean-square displacement of the Si atoms in an amorphous layer with a thickness of 10 \AA{} at the $a/c$ interface is larger than that in bulk amorphous Si. This can be attributed to local heating due to crystallization of amorphous Si during SPE growth.

Ion-beam stimulated solid-phase crystallization of amorphous Si on SiO2

Influences of ion-beam irradiation on solid-phase-crystallization of a-Si on SiO2 were studied in the temperature range between 200 and 700°C. Significant enhancement of crystal nucleation was observed under ion irradiation (25 keV, 1×1016 Ar+ cm−2). As a result, nucleation at a temperature lower than that of the softening of soda-lime glass (450°C) becomes possible. In addition, nuclei growth along the [111] and [110] directions was detected using X-ray diffraction methods. These are a big advantage for the fabrication of high-quality and low-cost thin-film transistors on glass substrates.

Enhanced nucleation in solid-phase crystallization of amorphous Si by imprint technology

A method to enhance crystal nucleation at controlled sites in solid-phase crystallization of amorphous Si is demonstrated. The method uses imprint with Ni-coated Si tips prior to conventional furnace annealing of amorphous Si films deposited on SiO2 substrates. The incubation time for crystallization is found to be greatly reduced at sites imprinted with the tips. This enhanced nucleation can be used to form large crystal grains up to about 7 μm in diameter at controlled sites. Results obtained from imprint with SiO2-covered Si tips suggest that the enhanced nucleation results not from physical effects of indentation but from a chemical effect of metal transfered from the tip to the film surface.

Study of Vacancy and Impurity Complexes in Si Solid-Phase Epitaxial Crystallization with Positron Annihilation Spectroscopy

AbstractWe investigate the residual vacancy defect species after crystallization of amorphous Si (a-Si) by solid phase epitaxy (SPE). To this end, we correlate the total and electronically-active doping concentrations measured by secondary mass spectrometry and spreading resistance analysis, and data from positron annihilation spectroscopy (PAS), which is sensitive to openvolume defects. Float-zone silicon substrates were implanted with boron, phosphorus and both phosphorus and boron ions to create nonuniform doping profiles at degenerate doping levels, after an amorphization step by 29Si+ ions. Samples were vacuum annealed at 600°C to induce SPE, and the SPE rate was measured by time-resolved reflectivity. PAS was used for identification of the impurity-defect complexes. Momentum-resolved PAS measurements enable the detection of phosphorus-vacancy (P-V) and oxygen-vacancy (O-V) complexes.

Crystallization of amorphous Si 0.5 Ge 0.5 films studied by means of in-situ X-ray diffraction and in-situ transmission electron microscopy

Crystallization of amorphous Si 0.5 Ge 0.5 films studied by means of in-situ X-ray diffraction and in-situ transmission electron microscopy

Raman scattering on crystallites growing at the interface of Au and amorphous Si during annealing

Au-induced crystallization of amorphous Si (a-Si) during annealing at different temperatures (450–620 K) was observed in situ by Raman scattering. From spectral analysis, we obtain information on the crystallite size, shape and orientation. We show that the crystallite correlation length is 5–8 nm and that the best fit of the line shape can be achieved with the assumption of cylindrical shapes in the thin Au film case. We also performed an angle-dependent Raman scattering experiment, which confirms that the Si crystallites are randomly oriented.

The Role of Vacancies and Dopants in Si Solid-Phase Epitaxial Crystallization

AbstractThe role and interaction of vacancies and dopants in the crystallization of amorphous Si (a-Si) by solid-phase epitaxy (SPE) was investigated. To this end, we studied: (i) the solid-phase epitaxy rate measured by time-resolved reflectivity (TRR), (ii) the dopant and carrier concentrations measured by secondary ion mass spectrometry (SIMS) and spreading resistance (SR) analysis, and (iii) the vacancy concentration measured by positron annihilation spectroscopy (PAS). Phosphorus was implanted into a-Si on Si (001), which was previously amorphized by 29Si+ implantation, to create a nonuniform P doping profile. Phosphorus doped samples compensated with a similar boron profile were also studied. Samples were vacuum annealed for various times so that the amorphous-crystal interface was stopped at various depths providing frozen frames of the SPE process. These samples were then studied with PAS to investigate the vacancy population and to identify the impurity-defect complexes. Using this method, we have observed a population of phosphorus-vacancy complexes in the epitaxial layer.

Enhanced crystallization of amorphous Si containing hydrogen without oxygen during ion-beam irradiation at 310 °C and during furnace annealing below 450 °C

The epitaxial crystallization rates of amorphous Si layers on crystalline Si substrates containing a considerable number of hydrogen atoms are markedly increased in the absence of oxygen atoms. This enhanced crystallization occurs both during 1-MeV Xe-ion-beam irradiation at 310 °C and during furnace annealing in vacuum at temperatures below 450 °C. Implantation-amorphized crystal Si layers epitaxially grown on the (100)-crystalline Si substrates by ultrahigh vacuum chemical vapor deposition (UHV-CVD) are epitaxially crystallized by furnace annealing in vacuum at temperatures below 450 °C. Implantation-amorphized bulk-crystal Si substrates, however, are not entirely crystallized by the same low-temperature annealing. Nanometer-scale microcrystallites, remaining at the near-surface region in the amorphous Si layer after 80-keV phosphorus implantation into the UHV-CVD epitaxial Si layer, grow three-dimensional during 1-MeV Xe-ion-beam irradiation at 310 °C, but not during furnace annealing at 600 °C in dry Ar ambient. This three-dimensional crystal growth does not occur in implantation-amorphized bulk-crystal Si substrates, even during 1-MeV Xe-ion-beam irradiation. Amorphous Si layers directly deposited by low-pressure CVD (LP-CVD) are crystallized epitaxially by 1-MeV Xe-ion-beam irradiation at 310 °C. The crystallization rate of the LP-CVD sample heated in the deposition furnace after evacuation is twice as high as that of the sample heated before evacuation. All these results are explained consistently by the presence of a considerable amount of hydrogen in polyhydride states in the amorphous Si layers in the absence of oxygen. It is suggested that the mechanism of low-temperature crystallization of amorphous Si containing hydrogen atoms in polyhydride states is closely related, in terms of vacant spaces and dangling-bonds, to the mechanism of ion-beam-induced epitaxial crystallization.

Thermal stability and crystallization of amorphous Si 1−xB x, Si 1−xP x and Si 1−xSb x alloy thin films

The thermal stability and crystallization of the amorphous Si 1−xP x, Si 1−xB x and Si 1−xSb x alloy systems have been studied by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). It was found that the amorphous Si-P and Si-B alloys have a high thermal stability and crystallize at temperatures as high as 1150°C, whereas amorphous Si-Sb alloys are unstable and crystallize already at 350°C. The thermal stability is explained by strong Si-P and Si-B bonds, whilst the instability is explained by the weakening of Si-Si bonds by the presence of Sb atoms (< 17 at.% Sb) and by weak Si-Sb and Sb-Sb bonds (> 17 at.% Sb). The thermal stability and instability of the amorphous alloys were correctly predicted from calculated Gibbs free energy diagrams (GFED). The calculated GFED were also used together with the observed crystallization temperatures to successfully predict the first crystalline phase to form in the Si-P and Si-B systems. The first crystalline phases that formed were Si (< 40 at.% B and < 30 at.% P), a newly reported phosphide, Si 12P 5 (30 at.% P), and SiP (> 30 at.% P).

Enhanced Solid-Phase Crystallization of Amorphous Si by Plasma Treatment Using Reactive Ion Etching

Solid-phase crystallization of amorphous Si (a-Si) films was found to be enhanced by oxygen- or nitrogen-plasma treatment prior to annealing for crystallization using a reactive ion etching system. Amorphous Si films were deposited by vacuum evaporation under ultrahigh-vacuum conditions. Plasma treatments were carried out using a conventional parallel-plate reactive ion etching apparatus. The incubation time for crystallization of a-Si was found to be greatly reduced by plasma treatment. Nitrogen-plasma treatment resulted in more enhanced crystallization than oxygen plasma. The incubation time was found to decrease with the increase of the self-bias voltage of the reactive ion etching system during plasma treatment. Impurity analysis suggests that metal atoms adsorbed during treatment at the surface of a-Si is one cause of the enhanced crystallization.

Segregation and trapping of erbium at a moving crystal-amorphous Si interface

Segregation and trapping of Er during solid-phase crystallization of amorphous Si on crystalline Si is studied in a concentration range of 1016–5×1020 Er/cm3. Amorphous surface layers are prepared on Si(100) by 250 keV Er ion implantation, recrystallized at 600 °C, and then analyzed using high-resolution Rutherford backscattering spectrometry using 2 MeV He+ or 100 keV H+. The segregation coefficient k depends strongly on Er concentration. At Er interface areal densities below 6×1013 Er/cm2 nearly full segregation to the surface is observed, with k=0.01. At higher Er densities, segregation and trapping in the crystal are observed, with k=0.20. The results are consistent with a model in which it is assumed that defects in the a-Si near the interface act as traps for the Er.