Low temperature carbon nanotube and hexagonal diamond deposition with photo-enhanced chemical vapor deposition

Deposition of carbon nanotube and hexagonal diamond thin films at low substrate temperature with photo-enhanced chemical vapor deposition is described here. Extensive experimentation is conducted to optimize the catalyst layer utilized for deposition by varying Al/Ni/Al metal layer thicknesses on SiO₂ coated Si substrates. The coated substrates are annealed to transform the thin metal layers into nanoparticles. Suitable catalyst layer thicknesses obtained are 3/2/3, 5/1/5 and 5/3/5 nm for Al/Ni/Al sandwich metal layers. Suitable annealing conditions are in the range of 350–450 °C for substrate temperature and from 0.22 to 10 Torr for chamber pressure in ammonia ambient for 25 min. Carbon tetrachloride (CCl₄) is used as a carbon precursor in this work. Argon to CCl₄ flow ratio is varied in 1.5–19 range, chamber pressure is varied in 3–10 Torr range, and the substrate temperature is varied in 350–450 °C range. Carbon nanotubes (CNT) growth is observed at lower chamber pressure, lower partial pressure of CCl₄, lower substrate temperature and for thin Ni catalyst layer. The optimal CNT deposition condition observed is 5 Torr total chamber pressure, 9:1 partial pressure ratio of Ar to CCl₄, 400 °C substrate temperature and 5/1/5 nm thick Al/Ni/Al catalyst layers. The hexagonal diamond deposition is observed at a higher chamber pressure, higher partial pressure of CCl₄, higher substrate temperature and for a thicker Ni catalyst layer. The optimal condition for hexagonal diamond deposition observed is 10 Torr total chamber pressure, 7:3 partial pressure ratio of Ar to CCl₄, 450 °C substrate temperature and 5/3/5 nm thick Al/Ni/Al catalyst sandwich layers.