A study of plasma-film interactions in He/H2, Ar/H2, and Xe/H2 radio frequency sputtered a-Si:H

Abstract

A study of plasma-film interactions present during the deposition process of a-Si:H prepared by radio frequency (rf) sputtering in 10 mTorr of either He, Ar, or Xe and 0.5 mTorr H2, at rf power levels of 0.27–3.3 W/cm2 (50–600 W), on nominally unheated substrates, is presented. Measurement techniques included scanning electron microscopy (SEM), infrared (IR) and optical absorption, and electron-spin resonance (ESR). The SEM images indicated that the films prepared in He/H2 at low power are morphologically porous and/or columnar; the evidence suggests that as the rf power is increased, the islands coalesce to form larger ones and the images appear glassy and cracked. IR absorption spectra indicated that the porous films undergo post-depositional oxidation. A comparison between the observed deposition rates and the rates expected from previously published sputtering yields indicated that resputtering of the growing film is a dominant feature in films deposited in Ar/H2 and Xe/H2 but a weaker one in those grown in He/H2. The dependence of the total H concentration, as determined from the 640 cm−1 IR wagging mode absorption, on the deposition rate, is in reasonable agreement with the kinetic model of Moustakas et al.; the comparison with that model indicates that the product of the sticking coefficient and capture cross section for hydrogen is much lower when depositing in He/H2 than in Ar/H2 or Xe/H2. The analysis of the concentration of hydrogen in dihydride and trihydride configurations, as determined from the 850–890 cm−1 IR bending mode absorption, indicated that this quantity is determined by the number of sites available for these configurations and a random statistical process. The number of these available sites is generally much higher in He/H2 sputtered films than in Ar/H2 or Xe/H2 ones, and in the former sputtering medium also sharply increases with decreasing rf power. These observations are in excellent agreement with the picture of dihydride and trihydride configurations residing at internal surfaces, the area of which increases as the rf power is decreased as the film resputtering process weakens and the morphology becomes increasingly porous and/or columnar. Analysis of the ESR measurements indicates that the density of unpaired bonds sharply increases as the dihydride and trihydride, and apparently the internal surface area, also increase. It is also concluded that the density of weak bonds, essentially constructed by the pairing of dangling bonds on the internal surfaces of the film and invisible by ESR, is much larger in the films prepared with He than with Ar or Xe. The optical absorption measurements in the Tauc region indicate that the He/H2 films are considerably more disordered than the Ar/H2 ones; the subgap Urbach slopes of the former are also much larger (as high as 490 meV) than the former and indicate that band tail states originate, to a considerable extent, from weak SiSi bonds created as mentioned above. Finally, it should be noted that the films deposited in Ar/H2 at high power approach device quality, in spite of the relatively low effective substrate temperature, which is around 100–150 °C. The analysis may thus indicate that the source of the special care and attention to the various deposition conditions required in the synthesis of high photovoltaic quality rf-sputtered films may be due to the double role of plasma-film processes, which are both responsible for destroying incipient porous and/or columnar morphology yet introduce other defects into the growing film.