We have found that an oxygen plasma exposure of a-Si:H films can cause thesefilms to crystallize with a much lower thermal-budget than that required forthe same a-Si:H films without this plasma exposure. Based on this uniquefinding, a selective area crystallization process has been developed tosuccessfully form patterned polycrystalline Si films. In this study, 1500 ÅPECVD a-Si:H films were first covered by 500 Å sputtered SiO2which was then patterned by lithography to form various islands covered bythe SiO2. These patterned films were exposed to an oxygen plasmaand were then thermally annealed in a furnace at 600 °C for 6 hours. Afterthis annealing it was found that the islands covered by SiO2during the oxygen plasma treatment remained a-Si while the oxygen plasmaexposed regions were crystallized completely. Both XRD and TEM were used toestablish the existance of these controlled regions of a-Si and poly-Si inthese films.

Application of excimer laser crystallization of Amorphous silicon (a-Si) hasintroduced a new, interesting potential technology for the fabrication ofpolycrystalline (poly-Si) thin film transistors. We are currently studyingpolycrystalline Si1−xGex thin films in order todetermine whether this material can lead to improved electrical propertiesor to better processing requirements when compared with polycrystalline Sifilms. In this work we analyze by RBS, TEM, Raman spectroscopy and surfacereflectance, the structure of thin Amorphous Si1−xGexfilms after irradiation with a XeCl excimer laser. The Amorphous SiGe filmswere prepared by evaporation of Si and Ge onto oxidized Si substrates usingan electron gun in vaccum. The effects of laser energy fluence duringirradiation are investigated. The Amorphous to crystalline transition isfollowed by in-situ measurement of time-resolved reflectivity.

Hydrogenated Amorphous silicon films deposited on molybdenum sheet metalsubstrates have been crystallized by thermal annealing at 850°C for 4 hoursin a nitrogen atmosphere. X-ray diffraction and scanning electron microscopyresults indicated that the average grain size in the crystallized films wasapproximately 200A. Palladium contacts were fabricated and the resultingPd/Si/Mo structures were electrically characterized.Current-voltage-temperature measurements for phosphorus doped and undopedannealed samples resulted in a J “V characteristic consistent withspace-charge-limited current. Using this data, Mobility as a function oftemperature from 100K-300K was obtained. In phosphorus doped samples, themobility appeared to be limited by energy barriers at the grain boundaries.In undoped samples, a σ “T’ exp (-b/T’) temperature dependence consistentwith variable-range hopping conduction was observed.

Thin film hydrogenated Amorphous silicon (a-Si:H) was deposited onMolybdenum (Mo) substrates by d.c. glow discharge. We investigated thea-Si:H crystallization using four anneal techniques; nitrogen atmospherefurnace, vacuum, rapid thermal anneal (RTA), and excimer laser anneal.Anneal temperature ranged from 100 to 1200 °C. Excimer laser energy perpulse 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 determinethe degree of crystallization. The a-Si:H started to crystallize attemperatures over 600 °C. An 850 °C anneal reduced film resistivity to 10s (ω-cm) for intrinsic and 1 (ω-cm) for n-type. Coplanartype thin film transistors (TFT) with gate channel length of 25 μm and widthof 220 μm were fabricated with various insulating layers; if sputtered SiO2, Si3N4, BaTiO3, MgO, andevaporated SiO. The first two exhibited the least leakage current. Theas-grown intrinsic a-Si:H field effect mobility was around 0.03 (cmVV.s) anddelay time was 5×10−7 s. The solid phase crystallized siliconfilm exhibited high leakage current. The delay time of an excimer laseranneal treated TFT was reduced to 2.5×10−7 s. Crystallized Sifilm mobility was improved to 15 (cm2 /V.s).

Silicon thin films were deposited by Electron Cyclotron Resonance PECVDusing silane as a source gas at room temperature. Deposited films werecrystallized either by conventional furnace annealing(FA) or by rapidthermal annealing (RTA) process. The films deposited on SiO2/Siwafer substrates were Amorphous or microcrystalline depending on themicrowave power. Deposited films were annealed at 600TC in a furnace. Asexpected, higher crystallinity was obtained in the case of the Amorphousfilms than the microcrystalline films after 7.5 hours annealing. It took 15hours at 600δC for the Amorphous films to reach their maximum crystallinityin case of FA, but it only took 1 second at 900 δC for RTA. In addition, itwas shown that RTA can be applied to the rapid crystallization of Amorphoussilicon thin films deposited on a fused quartz substrate utilizing a newfilm structure.

We observed the processes of hydrogen evolution and crystallization inhydrogenated Amorphous silicon 0.5–7 μm thick films (deposited by dc glowdischarge on Molybdenum) by differential scanning calorimetry (DSC), Ramanscattering and thermogravimetric analysis (TGA). Investigation was made as afunction of doping, deposition temperature and film thickness. For all thefilms, an endothermic DSC peak was observed at 694 °C (onset). That thispeak was at least partly due to hydrogen evolution was shown by TGA, whichshowed weight loss beginning at 694 °C, and by evolved gas analysis, whichshowed hydrogen evolution at 694 °C. This temperature (658–704 °C) increasedwith increasing heating rate (5–30 °C/min). Doping reduced this temperaturefrom 694 to 625 °C for boron doping and to 675 °C for phosphorous doping.Hydrogen evolution kinetics and FTIR results suggest that thesilicon-hydrogen bonding in the intrinsic film was a mixture of SiH andS1H2, and was predominantly SiH in the phosphorous doped films and SiH2 in the boron doped films. Crystallization was independentof silicon-hydrogen bonding in the as-deposited Amorphous silicon film. Itwas bulk (not interface) induced. No exothermic DSC peak accompanied thecrystallization. The film deposition temperature had little effect on theDSC result, but crystallization was enhanced by a higher depositiontemperature.

Hydrogenated amorphous silicon thick films deposited by dc glow discharge onmolybdenum substrates were annealed by a pulsed Nd:glass laser. Massspectrometry showed hydrogen remaining in all the laser annealed films. Theamount of hydrogen remaining decreased with decreasing scan rate. Thehydrogen evolved upon heating at 365 °C and mainly at 658 °C before laserannealing, 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 Ramanscattering. The photo and dark conductivity of the film increased by one andthree orders of magnitude respectively with increasing laser energy densityup to 12 J/cm2 at a fixed scan rate. This Means that thephotoresponse was decreased with laser annealing, in spite of the associatedincrease in crystallinity. This photoresponse decrease is attributed to thehydrogen evolution.

To better understand the involved phase transformation Mechanism, we arestudying the excimer laser-induced amorphization (ELA) of ultra-thin Sifilms on oxidized Si substrates. In this paper, we show that the onset ofamorphization of hydrogen-free Si films on SiO2 substrates uponincreases in the energy density is associated with the onset of completemelting of the film. Once complete melting occurs, further increases in theincident energy density and/or increases in the substrate temperature canlead to incomplete amorphization of the film. Planar view TEM analysis ofnearly-amorphized Si films reveals a heterogeneous microstructure, whichconsists of a mixture of densely dispersed amorphous-like annular regions(∼20 to 40 μm−2), embedded within and typically separated by aregion containing finegrained small crystals. Such a cellular microstructurestrongly suggests that amorphization occurred not via a homogeneous but viaa heterogeneous transformation. In particular, the microstructure paints ascenario in which amorphization proceeded via nucleation of solids, which isthen followed by interfacial amorphization. The experimental resultsunambiguously reveal (1) that the previously proposed criteria of the meltduration and the vertical temperature gradient are irrelevant in determiningamorphization of supercooled liquid Si films and (2) that the quenchingrate, not surprisingly, is the important parameter.

Properties of μc-Si:H films grown by rf sputtering and by glowdischarge-chemical vapor deposition (GD-CVD) using diluted-hydrogen andhydrogen-atom-treatment method were compared employing TEM, X-raydiffraction, Raman scattering and FT-IR. The films deposited by both methodsall exhibited comparable grain sizes in the range of 10–18 nm. and showedthe same tendency in almost all the Measurements.

Microcrystalline silicon films were deposited by diluted-hydrogen method andhydrogen-atom-treatment method at 250°C in a plasma enhanced chemical vapordeposition system and they were characterized by nuclear magnetic resonance,Raman spectroscopy, and optical bandgap Measurements. One-Mask a-Si:H thinfilm transistors (TFT's) were fabricated with those microcrystallinematerials as the channel layer. The highest electron mobilities of the TFT'sfabricated by diluted-hydrogen method and hydrogen-atom-treatment methodwere 1.23 and 1.04 cm2/V•s, respectively without any thermaltreatment steps.

The heating rate dependence of the phase transition temperature wasformulated based on the temperature dependence of nucleation of a new phase.The glass transition temperature of a-Si was explained in terms of van derWaals fluid of a-Si pseudo-Molecules which are produced by the fragmentationof continuous random networks of Si atoms. Transient phases and theirtransition temperatures as a function of the heating rate are summarized inthe phase diagram.