Deposition of Amorphous Hydrogenated Silicon Films by VUV Laser CVD: Influence of Substrate Temperature
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H.KARSTENS AND P.HESS Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany
ABSTRACT Amorphous hydrogenated silicon (a-Si:H) films were deposited from disilane at substrate temperatures between 180 and 390 °C using a F2-1aser (157 nm) in a parallel configuration. Material properties such as hydrogen content, Sill and Sill2 group concentration, photoand dark conductivity, band-gap energy and the Urbach parameter were determined as a function of the deposition temperature. The material with the best optical and electronical properties was found for a substrate temperature of 260 °C. Using argon as the buffer gas instead of helium results in films of poor quality.
INTRODUCTION The PECVD process has become the most important method for the deposition of amorphous hydrogenated silicon films, and up to now the highest quality material has been prepared by this technique. Although material properties such as defect density, Urbach energy and drift mobility have been improved in recent years, there has been very little real improvement in reducing the light-induced degradation of the PECVD a-Si:H films. Although the fundamental reasons for this problem are not well understood, the solution lies possibly in a better understanding and control of the a-Si:H deposition process. The plasma CVD process, however, is based on very complicated chemical processes that are quite difficult to understand. This is due to the great variety of different radicals, ions and stable molecules that are generated by the source gas dissociation in the electrical discharge. Although it is possible to separate the discharge region from the growing film, there is only limited information on the composition and concentration of the flux of molecules reaching the substrate surface. Fluorine laser CVD in a parallel configuration is a suitable technique for the exploration of gas phase and surface processes, since - the growing film is not affected by irradiation or particle bombardement -- the excitation energy is lower than the ionization threshold of the process gas, so only nonionic photolysis products of a certain energy are obtained - the composition of the photolysis products is independent of the absorbed laser energy - the number of dissociated source gas molecules can be calculated from the laser light absorption - the high absorption coefficient of disilane1 at A = 157 nm allows one to work at low source gas partial pressures; this reduces the probability of secondary reactions in the gas phase - the important species such as monoradicals and diradicals are generated in the gas phase at this wavelength - due to the very high reactivity of the diradicals, the monoradical/diradical ratio at the surface will increase with increasing distance of the laser beam from the surface2. One of the basic requirements for an understanding of film formation processes is the knowledge of the dependence of the growth rate and the material properties on the deposition 93 Mat. Res. Soc. Symp. Prec. Vo
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