Analysis of the Crystallization Kinetics and Microstructure of Polycrystalline Sige Films by Optical Techniques
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Polycrystalline Si1 _xGex films are of interest for the fabrication of thin film transistors (TFT) for applications in flat panel displays [1, 2]. TFT using solid-phase crystallized SiGe with x=0.16 have been successfully fabricated [2]. However, only a few works on the SPC of amorphous SiGe alloys deposited by MBE, sputtering or LPCVD have been published in the past years [3-6]. The characteristics of the grain nucleation and growth processes determine the final structure of the films (grain size, preferred grain orientations, crystallographic defects, surface roughness...), which has a critical influence on the possible device applications of the films. In this work, optical techniques have been used to obtain, in a fast way, information on the crystallization kinetics and structural properties of SiGe films. EXPERIMENT 100 nm thick Sil.xGe, layers have been deposited using a hot wall LPCVD reactor. Pure disilane (Si 2H6) and germane (GeH 4) were used as precursor gases. The GeH 4 to Si2H 6 gas flow ratio was varied between 0 and 1.4, while keeping the total flow constant at 10 sccm. No carrier gas was used. The values of x obtained were in the 0-0.38 range. During the deposition, both temperature and pressure were kept constant at 450 'C and 300 mTorr, respectively. Thermally oxidized silicon wafers were used as substrates. Further details on the deposition process are described elsewhere [7]. The thickness and the composition of the amorphous layers were 221 Mat. Res. Soc. Symp. Proc. Vol. 588 © 2000 Materials Research Society
determined by Rutherford backscattering spectrometry (RBS). The composition of the films was found to be constant throughout their whole thickness. After the deposition, the films were annealed under vacuum (below 60 mTorr) at temperatures ranging from 525 to 600 'C to study their crystallization behavior. Raman spectra were acquired using the 514.5 nm line of an Ar+ laser. The excitation and scattered light collection were done by means of a metallographic microscope attached to the DILOR XY Raman spectrometer. The detection was done with a liquid nitrogen cooled CCD multichannel detector [8]. UV reflectance spectra in the 200 to 450 nm wavelength range were obtained using a Perkin-Elmer Lambda 2 spectrometer equipped with a retro-mirror attachment for specular reflection measurements. X-ray diffractometry (XRD) measurements were also taken in the 0-20 configuration for comparison [6]. The minimum measurement time necessary to achieve reliable results using the different techniques was around 5 min for Raman spectroscopy, 10 min for UV reflectance and 60 min for XRD. The optical techniques used in this work are useful to carry out a fast, even in-situ, characterization of the film crystallization. The crystalline fraction of the films, X(t), was determined as a function of the annealing time using the integrated intensity of a) the Si-Si Raman band of the crystallized phase, b) the peak located around 280 nm in the UV reflectance spectra, corresponding to optical transitions at the X p
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