Nagarajan Sridhar

Effect of deposition temperature on the structural and electrical properties of laser‐crystallized hydrogenated amorphous silicon films

The deposition temperature of hydrogenated amorphous silicon films deposited by dc glow discharge was found to affect the crystallinity, hydrogen content, and silicon–hydrogen bonding after laser crystallization of the film. This in turn affected the electrical properties of the crystallized film. The crystallinity of the film after laser annealing was always higher than that of the corresponding furnace‐crystallized films, for the same deposition temperature, and it increased with decreasing deposition temperature, similar to that observed in furnace crystallized films (650 °C, 30 h). However, the dark and photoconductivity, photoresponse (defined as the ratio of photo to dark conductivity), and the carrier diffusion length increased with increasing deposition temperature (150–350 °C). This was due to both an increase in hydrogen content and the SiH and SiH2 bonding, as shown by evolved gas analysis and infrared spectroscopy. Carrier transport measurements indicated that the dominant transport mechanism ...

Crystallization of Hydrogenated Amorphous Silicon Thick Films on Molybdenum Substrates

Crystallization of hydrogenated amorphous silicon thick films deposited by dc glow discharge on molybdenum substrates was studied by Raman scattering and x-ray diffraction. Investigation was made as a function of amorphous silicon film deposition temperature. On heating the films at a rate of 5 °C/min to 650 °C for various times, it was observed that the film deposited at 300 °C started crystallization faster than the film deposited at 150 °C. The degree of cirystallinity increased with increasing annealing time for all the films. However, at all annealing times, the degree of crystallinity for the annealed film deposited at 150 °C was higher than that of the annealed film deposited at 300 °C, indicating that the crystallization growth rate was higher for the film deposited at a lower temperature. These results were consistent with the dark conductivity measurements. The film deposited at 150 °C showed a photoresponse which increased with increasing annealing time whereas no photoresponse was observed for the film deposited at 300 °C. This was probably due to the degree of crystallinity and grain size being much larger for the film deposited at 150 °C than the film deposited at 300 °C.

POLYSILICON FILMS OF HIGH PHOTORESPONSE, OBTAINED BY VACUUM ANNEALING OF ALUMINUM CAPPED HYDROGENATED AMORPHOUS SILICON

Nickel aluminide Ni3AI in the single phase form, with grain size ~ 10 gm, porosity ~ 5%, tensile strength 425 MPa, modulus 92 GPa and ductility 9.5% at room temperature, was fabricated by reactive infiltration at 800 ~ of liquid aluminium into a porous preform containing 78 vol % nickel and made by sintering 3-7 lam size nickel particles. Without sintering, the preform contained 58 vol % nickel and reactive infiltration resulted in an aluminium-matrix NiAla particle ( ~ 50 ~m size) composite and extensive growth of Ni-AI needles from the preform to the excess liquid aluminium around the preform.Hydrogenated amorphous silicon (a‐Si:H) films having a thickness of 7 μm were deposited on molybdenum by dc glow discharge and then capped with a 0.1‐μm‐thick aluminum (Al) layer by thermal evaporation. Subsequent vacuum annealing at 500–725 °C resulted in the formation of crystalline Si, as observed by Raman scattering and x‐ray diffraction. This was in contrast to the uncapped a‐Si:H films which were still amorphous at the same annealing temperatures, except at ≳700 °C. That the Al capped films were crystalline caused a ten‐fold increase in the dark conductivity in comparison to the uncapped film annealed at the same temperature. The capped films annealed at 500 °C showed a photoresponse (the ratio of the photoconductivity to dark conductivity) of 30, a photoconductivity of 2×10−4 (Ω cm)−1, and a carrier diffusion length of 5.3 μm—values much higher than those of the uncapped films (heated or not). This was due to a large grain size combined with the retention of hydrogen, which passivated the grain bou...

Kinetics Of Hydrogen Evolution And Crystallization In Hydrogenated Amorphous Silicon Films Studied By Thermal Analysis And Raman Scattering

We observed the processes of hydrogen evolution and crystallization in hydrogenated Amorphous silicon 0.5–7 μm thick films (deposited by dc glow discharge on Molybdenum) by differential scanning calorimetry (DSC), Raman scattering and thermogravimetric analysis (TGA). Investigation was made as a function of doping, deposition temperature and film thickness. For all the films, an endothermic DSC peak was observed at 694 °C (onset). That this peak was at least partly due to hydrogen evolution was shown by TGA, which showed weight loss beginning at 694 °C, and by evolved gas analysis, which showed hydrogen evolution at 694 °C. This temperature (658–704 °C) increased with increasing heating rate (5–30 °C/min). Doping reduced this temperature from 694 to 625 °C for boron doping and to 675 °C for phosphorous doping. Hydrogen evolution kinetics and FTIR results suggest that the silicon-hydrogen bonding in the intrinsic film was a mixture of SiH and S1H2, and was predominantly SiH in the phosphorous doped films and SiH 2 in the boron doped films. Crystallization was independent of silicon-hydrogen bonding in the as-deposited Amorphous silicon film. It was bulk (not interface) induced. No exothermic DSC peak accompanied the crystallization. The film deposition temperature had little effect on the DSC result, but crystallization was enhanced by a higher deposition temperature.

Nickel aluminide (Ni3Al) fabricated by reactive infiltration

Nickel aluminide Ni3AI in the single phase form, with grain size ~ 10 gm, porosity ~ 5%, tensile strength 425 MPa, modulus 92 GPa and ductility 9.5% at room temperature, was fabricated by reactive infiltration at 800 ~ of liquid aluminium into a porous preform containing 78 vol % nickel and made by sintering 3-7 lam size nickel particles. Without sintering, the preform contained 58 vol % nickel and reactive infiltration resulted in an aluminium-matrix NiAla particle ( ~ 50 ~m size) composite and extensive growth of Ni-AI needles from the preform to the excess liquid aluminium around the preform.Hydrogenated amorphous silicon (a‐Si:H) films having a thickness of 7 μm were deposited on molybdenum by dc glow discharge and then capped with a 0.1‐μm‐thick aluminum (Al) layer by thermal evaporation. Subsequent vacuum annealing at 500–725 °C resulted in the formation of crystalline Si, as observed by Raman scattering and x‐ray diffraction. This was in contrast to the uncapped a‐Si:H films which were still amorphous at the same annealing temperatures, except at ≳700 °C. That the Al capped films were crystalline caused a ten‐fold increase in the dark conductivity in comparison to the uncapped film annealed at the same temperature. The capped films annealed at 500 °C showed a photoresponse (the ratio of the photoconductivity to dark conductivity) of 30, a photoconductivity of 2×10−4 (Ω cm)−1, and a carrier diffusion length of 5.3 μm—values much higher than those of the uncapped films (heated or not). This was due to a large grain size combined with the retention of hydrogen, which passivated the grain bou...

Thermodynamics and Kinetics of Hydrogen Evolution in Hydrogenated Amorphous Silicon Films

The enthalpy (endothermic) of hydrogen evolution from p-type (boron doped) amorphous silicon with 17 at. % H was 4.8, 10.3, 15.8 and 17.3 kJ/g, the evolution temperature was 585, 606, 625 and 644 °C and the entropy of evolution was 5.6, 11.7, 17.5 and 18.9 J/g.K at heating rates of 5, 10, 20 and 30 °C/min respectively. That the enthalpy and entropy increased with heating rate means that the evolution involves not only Si-H bond breaking, but also Si-Si bond breaking and other defect formation. The Si-Si bond breaking and defect formation were enhanced at high heating rates, which caused high rates of hydrogen evolution. For n-type (phosphorous-doped) and intrinsic amorphous silicon with 25 and 23 at. % H respectively, the enthalpy and entropy of hydrogen evolution were higher than the p-type case, due to severe defect formation resulting from the higher hydrogen content. The activation energy of hydrogen evolution was 1.38, 2.5 and 4 kJ/g for the p-type, intrinsic and n-type materials respectively. Crystallization which occurred at temperatures higher than hydrogen evolution, was delayed for the amorphous silicon film in a higher disordered state after hydrogen evolution, suggesting that hydrogen evolution influenced the crystallization process.

Amorphous Silicon Crystallization For Tft Applications

Thin film hydrogenated Amorphous silicon (a-Si:H) was deposited on Molybdenum (Mo) substrates by d.c. glow discharge. We investigated the a-Si:H crystallization using four anneal techniques; nitrogen atmosphere furnace, vacuum, rapid thermal anneal (RTA), and excimer laser anneal. Anneal temperature ranged from 100 to 1200 °C. Excimer laser energy per pulse ranged from 90 to 340 M.J. Transmission electron Microscopy (TEM) revealed microstructure of crystallized Si film with grain size over 0.5 μm. X-ray diffraction (XRD) and Raman spectroscopy were employed to determine the degree of crystallization. The a-Si:H started to crystallize at temperatures over 600 °C. An 850 °C anneal reduced film resistivity to 10 s (ω-cm) for intrinsic and 1 (ω-cm) for n-type. Coplanar type thin film transistors (TFT) with gate channel length of 25 μm and width of 220 μm were fabricated with various insulating layers; if sputtered SiO 2 , Si 3 N 4 , BaTiO 3 , MgO, and evaporated SiO. The first two exhibited the least leakage current. The as-grown intrinsic a-Si:H field effect mobility was around 0.03 (cmVV.s) and delay time was 5×10 −7 s. The solid phase crystallized silicon film exhibited high leakage current. The delay time of an excimer laser anneal treated TFT was reduced to 2.5×10 −7 s. Crystallized Si film mobility was improved to 15 (cm 2 /V.s).

Laser Annealing of Hydrogenated Amorphous Silicon Thick Films

Hydrogenated amorphous silicon thick films deposited by dc glow discharge on molybdenum substrates were annealed by a pulsed Nd:glass laser. Mass spectrometry showed hydrogen remaining in all the laser annealed films. The amount of hydrogen remaining decreased with decreasing scan rate. The hydrogen evolved upon heating at 365 °C and mainly at 658 °C before laser annealing, but at 365, 575 (Mainly) and 645 °C after laser annealing, indicating weakening of the silicon-hydrogen bonding after laser annealing. The presence of hydrogen inhibited crystallization, as indicated by Raman scattering. The photo and dark conductivity of the film increased by one and three orders of magnitude respectively with increasing laser energy density up to 12 J/cm 2 at a fixed scan rate. This Means that the photoresponse was decreased with laser annealing, in spite of the associated increase in crystallinity. This photoresponse decrease is attributed to the hydrogen evolution.