The effect of hydrogen in the mechanism of aluminum-induced crystallization of sputtered amorphous silicon using scanning Auger microanalysis

The metal-induced crystallization (MIC) of hydrogenated sputtered amorphous silicon (a-Si:H) using aluminum has been investigated using X-ray diffraction (XRD) and scanning Auger microanalysis (SAM). Hydrogenated, as well as non-hydrogenated, amorphous silicon (a-Si) films were sputtered on glass substrates, then capped with a thin layer of Al. Following the depositions, the samples were annealed in the temperature range 200 °C to 400 °C for varying periods of time. Crystallization of the samples was confirmed by XRD. Non-hydrogenated films started to crystallize at 350 °C. On the other hand, crystallization of the samples with the highest hydrogen (H₂) content initiated at 225 °C. Thus, the crystallization temperature is affected by the H₂ content of the a-Si. Material structure following annealing was confirmed by SAM. In this paper, a comprehensive model for MIC of a-Si is developed based on these experimental results.     

A study of the effects of annealing and outgassing on hydrogenated amorphous silicon

Annealing and outgassing of thin films of hydrogenated amorphous silicon (a-Si:H) are shown to produce major structural changes in the material. Outgassing can occur in three stages with thresholds at 350°C, 450°C and 575°C corresponding to conversion of SiH₃ groups to SiH₂ and SiH; conversion of SiH₂ groups to Si and SiH; and conversion of SiH to Si respectively. Heating to 575°C also appears to remove most of the hydrogen from vacancies and defects in the material. Such heat treatments could be useful for improving the stability of thin film a-Si:H devices.     

Hydrogen passivation and junction formation on APIVT-deposited thin-layer silicon by hot-wire CVD

The hot-wire chemical vapor deposition (HWCVD) technique was employed to deposit μc-Si emitters and a-SiN_x:H passivation/antireflection films, and to hydrogenate silicon thin layers grown by atmospheric-pressure iodine vapor transport (APIVT). Photovoltaic devices with HWCVD μc-Si emitters on APIVT epitaxial silicon exhibit greater than 8% efficiency, similar to those made with diffused junctions. On polycrystalline APIVT-Si layers, a HWCVD-deposited μc-Si emitter reduces open-circuit voltage loss caused by grain boundaries. Hot-wire hydrogenation improves Hall mobility by approximately 50%. HWCVD a-SiN_x:H films improve minority-carrier lifetime significantly after thermal annealing at temperatures up to 500 °C.     

Cat-CVD-prepared oxygen-rich μc-Si:H for wide-bandgap material

Microcrystalline phase-involved oxygen-rich a-Si:H (hydrogenated amorphous silicon) films have been obtained using catalytic chemical vapor deposition (Cat-CVD) process. Pure SiH₄ (silane), H₂ (hydrogen), and O₂ (oxygen) gases were introduced in the chamber by maintaining a pressure of 0.1 Torr. A tungsten catalyzer was fixed at temperatures of 1750 and 1950 °C for film deposition on glass and crystalline silicon substrates at 200 °C. As revealed from X-ray diffraction spectra, the microcrystalline phase appears for oxygen-rich a-Si:H samples deposited at a catalyzer temperature of 1950 °C. However, this microcrystalline phase tends to disappear for further oxygen incorporation. The oxygen content in the deposited films was corroborated by FTIR analysis revealing SiOSi bonds and typical SiH bonding structures. The optical bandgap of the sample increases from 2.0 to 2.7 eV with oxygen gas flow and oxygen incorporation to the deposited films. In the present thin film deposition conditions, no strong tungsten filament degradation was observed after a number of sample preparations.     

Deposition of SiC coatings by spraying a powder accelerated electrodynamically in a coaxial pulse plasma generator

This paper presents the results of examinations of silicon carbide coatings formed on a silicon single crystal by deposition of a SiC powder accelerated electrodynamically under conditions of pulse plasma. The coatings thus obtained appeared to be solid silicon carbide layers with a hexagonal 2H structure. The microstructure and the thermal conductivity of the coatings were examined. The measured effective thermal conductivity of thin (10 μm) polycrystalline silicon carbide coatings deposited on silicon was about 51-75 W/m K, which is one order of magnitude lower than that of monocrystalline silicon carbide (270 W/m K).     

fabrication of diamond membranes for MEMS using reactive ion etching of silicon

Polycrystalline diamond thin film has been grown on a silicon substrate using high pressure microwave plasma-assisted chemical vapor deposition from a gas mixture of methane and hydrogen at a substrate temperature of 950°C. A simple process flow has been developed to fabricate optically transparent polycrystalline synthetic diamond membranes/windows employing reactive ion etching (RIE) of a single crystal silicon substrate using an electron beam evaporated aluminum thin film mask pattern formed by photolithography. Scanning electron microscopy has been used to study the morphology of as-grown diamond thin films.     

Microstructures of microcrystalline silicon thin films prepared by hot wire chemical vapor deposition

Undoped hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared at low temperature by hot wire chemical vapor deposition (HWCVD). Microstructures of the μc-Si:H films with different H₂/SiH₄ ratios and deposition pressures have been characterized by infrared spectroscopy X-ray diffraction (XRD), Raman scattering, Fourier transform (FTIR), cross-sectional transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). The crystallization of silicon thin film was enhanced by hydrogen dilution and deposition pressure. The TEM result shows the columnar growth of μc-Si:H thin films. An initial microcrystalline Si layer on the glass substrate, instead of the amorphous layer commonly observed in plasma enhanced chemical vapor deposition (PECVD), was observed from TEM and backside incident Raman spectra. The SAXS data indicate an enhancement of the mass density of μc-Si:H films by hydrogen dilution. Finally, combining the FTIR data with the SAXS experiment suggests that the Si---H bonds in μc-Si:H and in polycrystalline Si thin films are located at the grain boundaries.     

Polycrystalline silicon prepared by metal induced crystallization

Growth of large-grain polycrystalline silicon has been demonstrated using silicide-mediated crystallization of amorphous silicon (a-Si) by a pulsed rapid thermal annealing (RTA). The Ni atoms in concentration of 4.6×10¹²/cm² on the a-Si surface were heated at 700 °C in the RTA system for 10 s, ten times with 60 s intervals between the heat pulses. The Ni atoms on a-Si aggregate together, forming NiSi₂ precipitates. The crystallization proceeds from the NiSi₂ nuclei until the neighboring crystallites collide and forms distinct grain boundaries. It was found that 3.6×10⁷ Ni atoms form a seed for metal induced crystallization and the grain size was 40 μm when the Ni density was 4.6×10¹²/cm² on the a-Si. The grain size increases with decreasing metal density on a-Si.     

Depth profile measurements of aluminium film on phosphorus-doped hydrogenated amorphous silicon layers by Auger electron spectroscopy

Depth profile measurements are reported using Auger electron spectroscopy in aluminium films on phosphorus-doped hydrogenated amorphous silicon (a-Si:H) films prepared by r.f. glow discharge deposition. The data show that silicon is present in the aluminium film after heat treatment at a temperature as low as 200 °C. Thermal annealing before the aluminium deposition decreased the silicon signal strength in the aluminium film. The silicon signal was not detected in the aluminium film in contact with the a-Si:H through holes in an SiN film but the aluminium signal was detected in the a-Si:H film, probably at the uncovered margin of the SiN contact holes.     

Mechanical properties of amorphous and microcrystalline silicon films

Hydrogen-free amorphous silicon (a-Si) films with thickness of 4.5-6.5 μm were prepared by magnetron sputtering of pure silicon. Mechanical properties (hardness, intrinsic stress, elastic modulus), and film structure (Raman spectra, electron diffraction) were investigated in dependence on the substrate bias and temperature. The increasing negative substrate bias or Ar pressure results in simultaneous reducing compressive stress, the film hardness and elastic modulus. Vacuum annealing or deposition of a-Si films at temperatures up to 600 °C saving amorphous character of the films, results in reducing compressive stress and increasing the hardness and elastic modulus. The latter value was always lower than that for monocrystalline Si(111). The crystalline structure (c-Si) starts to be formed at deposition temperature of ∼ 700 °C. The hardness and elastic modulus of c-Si films were very close to monocrystalline Si(111). Phase transformations observed in the samples at indentation depend not only on the load and loading rate but also on the initial phase of silicon. However, the film hardness is not too sensitive to the presence of phase transformations.     

Spectroscopic ellipsometry studies on hydrogenated amorphous silicon thin films deposited using DC saddle field plasma enhanced chemical vapor deposition system

Hydrogenated amorphous silicon (a-Si H) films deposited on crystalline silicon substrates using the DC saddle field (DCSF) plasma enhanced chemical vapor deposition (PECVD) system have been investigated. We have determined the complex dielectric function, ε(E) = ε₁(E) + iε₂(E) for hydrogenated amorphous silicon (a-Si:H) thin films by spectroscopic ellipsometry (SE) in the 1.5-4.5 eV energy range at room temperature. The results indicate that there is a change in the structure of the a-Si:H films as the thickness is increased above 4 nm. This is attributed to either an increase in the bonded hydrogen content and, or a decrease of voids during the growth of a-Si:H films. The film thickness and deposition temperature are two important parameters that lead to both hydrogen content variation and silicon bonding change as well as significant variations in the optical band gap. The influence of substrate temperature during deposition on film and interface properties is also included.     

High deposition rate of amorphous silicon thick layers using a gas mixture of 10% silane in hydrogen

The deposition rate of amorphous silicon of the order of 0.9 μm/h, has been obtained using a gas mixture of 10% silane (SiH₄) in hydrogen (H₂), with a RF source of 13.56 MHz. Best films were deposited at a total flow rate of 100–200 sccm, 300°C substrate temperature, 66.7 Pa, and RF power density of 150 mW/cm². The geometrical configuration of the reaction chamber included a gas injector that was specially designed for this purpose. Films were characterized by Fourier transform infrared (FTIR), secondary ion mass spectrometry (SIMS), and profilometer. In addition, thick p-i-n diodes were prepared and characterized, obtaining reverse current densities lower than 5×10⁻⁶ A/cm² at full depletion.     

Crystallization of amorphous silicon thin films: The effect of rapid thermal processing pretreatment

In the present work, the effect of low temperature short-time rapid thermal processing (RTP) pretreatment on the average grain size and the crystallinity of the polycrystalline silicon thin films, fabricated by subsequent solid phase crystallization (SPC) of amorphous silicon (a-Si) thin films grown by radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) at high temperature has been studied. The average grain size and the crystallinity results were estimated using X-ray diffraction (XRD) and Raman spectroscopy, respectively. It was found that RTP at 800 °C for 60 s resulted in slightly larger average grain size and higher crystallinity than those without the RTP pretreatment after SPC at 800 °C for 5, 10 and 22 h. The results suggest that the low-temperature short-time RTP pretreatment can promote the crystallization process of the as-deposited a-Si thin films during the following SPC and then improve their crystallinity. Finally, the mechanism is also discussed in detail in the paper.     

Study on the mechanism of rapid solid-phase recrystallization of hydrogenated amorphous silicon film by rapid thermal processing

High-quality hydrogenated amorphous silicon films (a-Si:H) were deposited on quartz glass substrates by radio-frequency plasma-enhanced chemical vapor deposition method. The films were then annealed at 800 °C for 3 min by rapid thermal processing (RTP). As confirmed by X-ray diffractometry and Raman spectrometry, hydrogenated microcrystalline silicon films were obtained after the annealing procedure. The mechanism of the rapid solid-phase recrystallization of a-Si:H film by RTP was theoretically mainly attributed to the interaction between short-wavelength photons and ground-state precursor radicals (silicon, SiH₂ and SiH₃).     

Low-temperature crystallization of amorphous silicon films in contact with palladium by hydrogen plasma heating

A new annealing process using hydrogen plasma heating was suggested for the fabrication of poly-Si (polycrystalline silicon) films. This fabrication process had the advantages of low processing temperature approximately 450 °C and a short processing time of 1 h. The a-Si (amorphous silicon) films and a-Si/Pd (palladium) bilayers were deposited by r.f. sputtering and subsequently annealed by conventional furnace heating and hydrogen plasma heating. It was found that the Pd layer, introduced to the surface of the glass substrate prior to deposition of the a-Si layer, enhanced the nucleation reaction of c-Si (crystalline silicon) during the annealing, and that hydrogen plasma heating enhanced the grain growth reaction effectively. These resulted in lowered processing temperature and reduced processing time, while the grain size in the poly-Si films annealed by hydrogen plasma heating was much larger than that in the films by conventional furnace heating. The grain size of the poly-Si films annealed by hydrogen plasma heating was approximately 0.3 μm.     

Residual stress in silicon films deposited by LPCVD from disilane

Measurements of the thermomechanical stress in amorphous silicon films deposited by low pressure chemical vapour deposition (LPCVD) from disilane Si₂H₆ are reported as a function of the deposition parameters (temperature, gas pressure and wafer spacing). Major influences of the deposition temperature and the deposition rate are put into evidence and related to the films ordering and hydrogenation. The effects of a 600°C anneal are also investigated and a transition from highly compressive to highly tensile stress is characterised whatever the deposition. Such behaviour has been explained thanks to hydrogen atoms out-diffusion and crystallisation effects.     

Si-rich a-SiC:H thin films: Structural and optical transformations during thermal annealing

Amorphous hydrogenated silicon-rich silicon carbide (a-Si_0.8C_0.2:H) thin films were prepared by plasma enhanced chemical vapour deposition and were thermally annealed in a conventional resistance heated furnace at annealing temperatures up to 1100 °C. The annealing temperatures were varied and the samples were characterised with Auger electron spectroscopy, glancing incidence X-ray diffraction, Raman spectroscopy, Fourier transformed infrared spectroscopy, transmission electron microscopy and photoluminescence (PL) spectroscopy. As-deposited a-Si_0.8C_0.2:H thin films contain a large amount of hydrogen and are amorphous. When annealing the films, the onset of Si crystallisation appears at 700 °C. For higher annealing temperatures, we observed SiC crystallites in addition to the Si nanocrystals (NCs). The crystallisation of SiC correlates with the occurrence of a strong PL band, which is strongly reduced after hydrogen passivation. Thus PL signal originates from the SiC matrix. Si NCs exhibit no PL yield due to their inhomogeneous size distribution.     

Thickness-dependent properties of sprayed iridium oxide thin films

Iridium oxide thin films with variable thickness were deposited by spray pyrolysis technique (SPT), onto the amorphous glass substrates kept at 350 °C. The volume of iridium chloride solution was varied to obtain iridium oxide thin films with thickness ranging from 700 to 2250 Å. The effect of film thickness on structural and electrical properties was studied. The X-ray diffraction (XRD) studies revealed that the as-deposited samples were amorphous and those annealed at 600 °C for 3 h in milieu of air were polycrystalline IrO₂. The crystallinity of Ir-oxide films ameliorate with film thickness thereby preferred orientation along (1 1 0) remains unchanged. The infrared spectroscopic results show Ir–O and Ir–O₂ bands. The room temperature electrical resistivity (ρ_RT) of these films decreases with increase in film thickness. The p-type semiconductor to metallic transition was observed at 600 °C.     

铝诱导晶化P型非晶硅薄膜实验研究

利用PECVD设备在普通玻璃基片上沉积硼掺杂P型非晶硅薄膜,采用铝诱导晶化法（AIC）在氮气气氛保护下进行退火处理制备出P型多晶硅薄膜,研究了不同厚度的金属铝膜和热处理温度对非晶硅薄膜的微观结构、表面形貌的影响。实验结果表明：铝膜相对厚度越厚,对a—Si的晶化诱导效果则越好,在一定温度条件下,相对较厚的铝膜可以缩短a—Si晶化为polv-Si的时间,并且能使a—Si的晶化更加完整,产生尺寸较大的硅晶颗粒。在铝膜厚度相同,退火温度相同的条件下,热处理的时间越长,则晶化发生的程度越深,晶化越为彻底。     

A simple tool for quality evaluation of the microcrystalline silicon prepared at high growth rate

Silicon thin films were grown by plasma enhanced chemical vapor deposition at high-pressure (700 Pa), high-power (4– W/cm²) depletion regime using multi-hole cathode. Series of samples were deposited by varying hydrogen/silane ratio or plasma power to study evolution of film structure and transport properties near a-Si:H/μc-Si:H transition. We suggest a simple “μc-Si:H layer quality factor” based on the ratio of subgap optical absorption (1.4 eV)/ (1 eV) measured by constant photocurrent method. This ratio correlates well with the values of ambipolar diffusion lengths measured by surface photovoltage method perpendicularly to the substrate, i.e., in the direction of the collection of the photogenerated carriers in solar cells.