Deposition of NbTe x Thin Films Using Laser Ablation: Crystallographic Structure And Spatial Composition of Deposits
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500 A.
139 Mat. Res. Soc. Symp. Proc. Vol. 397 01996 Materials Research Society
The roughness of films was studied using an atomic force microscope ( DIGITAL NANOSCOPE III ) and a scanning electron microscope ( JEOL JSM 35C ). The deposit composition have been determined using X-ray fluorescence technique ( spot area = 100 4tm 2 ). All the values of the Te/Nb ratio mentioned hereafter will characterize the central area of the films excepting the Part 3 of this paper where we studied a spatial composition. X-ray diffraction technique ( Cu Kcxl ) was used for structural characterization. The conducting behaviour of the crystallized films was determined using the four probe measurement technique. Ablation plume was characterized by two set-up. An Optical Multichannel Analyzer ( OMA II EG&G ) and a spectrograph ( 1 m focal length, Jobin Yvon THR 1000 MSL ) allow to obtain the spectral lines emitted by the ablation plume and to identify the different species present in the plume. A Mac Pherson monochromator ( 50 cm focal length ) and a fast photomultiplier RCA 4837 perform emission Time of Flight ( ToF ) spectroscopy. RESULTS During our study, we have principally investigated the role of four parameters : the target composition, the fluence (F), the substrate temperature (Ts) and the background pressure. Before using sintered targets, we tried to utilize unsintered one ( compacted mixtures of tellurium and niobium powders ). The films were amorphous and composed of small solidified niobium droplets [ 6 ]. Consequently, NbTex sintered targets appeared to be much more convenient for our purpose. Different crystalline films ( Nb5Te4-type structure or NbTe2-type one ) were produced by using different sintered target compositions ( NbTe 2 or NbTe4 ). All films mentionned hereafter have been deposited on Si (100) and at 10-6 mbar. 1.NbTe2 Targe The films deposited at substrate temperature Ts < 250'C, are amorphous independly of fluence. Increasing the substrate temperature leads to the evaporation of Te atoms ( Tb = 150'C at 10-6 mbar); consequently the Te/Nb ratio decreases with respect to Ts. In this case, for Ts > 250'C and a fluence above 2.5 J/cm 2 , ablation of a NbTe2 target results in crystalline films with rather low composition in Te ( 0.7 < Te/Nb < 1.1 ). The few peaks observed on the corresponding diffractograms were assigned to harmonics of the d 110 interreticular distance of the Nb 5 Te4 structure. Indeed, if the c axis of the tetragonal cell of this one-dimensional structure grows parallel to the substrate, only dl10 could be reached. When increasing the substrate temperature to 400'C, the Te/Nb ratio and the d1 10 value of the film decreased towards Nb 5Te4 bulk values, 0.8 and 7.21 A, respectively. This variation of the d1 10 is shown on the Fig 1. e 7.8
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