Low temperature carbon material deposition with photo-enhanced chemical vapor deposition

Abstract

Photo-enhanced chemical vapor deposition (CVD) technique is investigated here for low temperature deposition of carbon nanotubes (CNTs) and hexagonal diamond. Most current deposition methods require high substrate temperature. Photo-enhanced CVD utilizes light energy to dissociate carbon containing precursor molecules and hence has a potential for low temperature deposition. CCl4, having a high absorption coefficient compared to other commonly employed hydrocarbons in the UV emission spectrum from a Xe arc lamp, is selected as a carbon precursor in this work. Extensive experimentation conducted by varying Al/Ni/Al catalyst layer thicknesses on SiO2 coated Si substrates, substrate annealing temperature in the range 350 - 450 ¡ÆC for 25 min, and chamber pressure in the range 0.22 - 10 Torr in ammonia ambient, yielded suitable catalyst layers of thicknesses 3/2/3, 5/1/5 and 5/3/5 nm and annealing pressure of 10 Torr. For photo-enhanced CVD deposition, experiments are conducted with various Ar/CCl4 flow ratio in 1.5 - 19 range, total chamber pressure in 3 - 10 Torr range, and substrate temperatures in 350 - 450 ¡ÆC range. Optimal condition for CNT deposition in this work is found to be 30 min at 400 ¡ÆC at 5 Torr total pressure with Ar/CCl4 ratio of 9 with 5/1/5 nm thick catalyst annealed at 400 ¡ÆC. Raman spectroscopy indicates MWCNT growth and I-V measurements yield sheet resistivity of 22 k§Ù/sq. The densest hexagonal diamond deposition is obtained at 450 ¡ÆC, 3 hr deposition time, at 10 Torr with Ar/CCl4 ratio of 2.3 with 5/3/5 nm thick catalyst annealed at 450 ¡ÆC. Lesser dense hexagonal diamond platelets are obtained at 450 ¡ÆC, 3 hr deposition time, at 10 Torr with Ar/CCl4 ratio of 2.3 with 3/2/3 nm thick catalyst annealed at 450 ¡ÆC. Based on the physical structures observed at various stages of growth in SEM images, a model is proposed for nucleation and subsequent growth of hexagonal diamond platelets with graphene layer playing role both during nucleation and during platelet growth. Raman spectroscopy and XPS results confirm the deposition material to be hexagonal diamond. The grown material is characterized with UV-Vis spectroscopy for optical and with a nanoindenter for electrical and mechanical properties.