A study of contacts to p- and n-type silicon nanowires synthesized via vapor-liquid-solid growth process

Open Access
Dey, Soham
Graduate Program:
Materials Science and Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
March 27, 2007
Committee Members:
  • Suzanne E Mohney, Committee Chair/Co-Chair
  • Joan Marie Redwing, Committee Member
  • Theresa Stellwag Mayer, Committee Member
  • S Ashok, Committee Member
  • aluminide
  • silicide
  • nanowire
  • silicon
  • ohmic
  • contact
  • vapor-liquid-solid growth
Silicon nanowires have been the subject of intense research in recent years for their potential application in future nanoscale electronic systems and for studying physical phenomena in restricted geometries. In order for nanoscale electronic devices to realize their full potential in certain applications, ohmic contacts are required. The present study focuses on the fabrication and characterization of electrical contacts to p- and n-type Si nanowires grown by the vapor-liquid-solid method with the objective of designing ohmic contacts with a low contact resistance. The actual contacts to these nanowires could exist in two possible geometries: firstly, as an integral part of the nanowires fabricated during growth of the nanowires, also referred to as nanowire heterojunction contacts, and secondly, in the form of lithographically defined metal pads overlapping the nanowires. Cobalt silicide/Si and Au/Si heterojunction nanowires grown in anodized alumina templates with an average pore diameter of 200 nm were studied in the former geometry. An optimum amount of Au catalyst was deemed necessary for obtaining cobalt silicide/Si nanowires with good morphology. Investigations using transmission electron microscopy helped determine this amount to be approximately two-thirds of the average pore diameter of the templates in which the nanowires were grown. Subsequently, detailed scanning transmission electron microscopy revealed that even some of the nanowires with good morphology contained Au nanoparticles, more so near the interface of the Si segment with the silicide in the case of cobalt silicide/Si nanowires and with the Au in the case of Au/Si nanowires. Such Au nanoparticles were shown to cause discontinuities in some nanowires upon annealing at temperatures above the Au-Si eutectic point. Nanowires grown out the top of the membranes contained very few Au nanoparticles away from the Au/Si interface and were used for studying contacts in the metal-pad geometry. Contacts in a two-point probe configuration based on Pd, Ti/Au and Co/Al metallizations were studied for heavily doped p-type Si nanowires of average diameter 80 nm that were grown out the top of membranes, and a Ti/Au metallization was studied for heavily doped n-type Si nanowires of similar diameter. Such studies revealed that for p-type nanowires, the Co/Al contacts were the least resistive under similar fabrication and annealing conditions. Annealing from 200 – 300 °C in a N2 ambient was shown to be optimal for obtaining ohmic contacts and avoiding thermal degradation. For n-type nanowires, the Ti/Au contacts were also shown to form ohmic contacts readily upon annealing between 250 – 300 °C under similar conditions. Field emission scanning electron microscopy was used to study some of the defective p- and n-type Si nanowire devices including contact electrodes. A multi-contact test structure was next fabricated, which made it possible to extract various contact parameters based on a nanowire transmission line model. Contacts based on Al, Co and Co/Al metallizations were fabricated on heavily doped p-type nanowires of average diameter 80 nm that were grown out the top of membranes and from Au thin films, and an Er/Al metallization was used for heavily doped n-type nanowires of similar diameter grown by similar methods. The specific contact resistance of as-deposited Al contacts was found to be within 1 x 10^-5 – 4 x 10^-4 Ù-cm^2 when extracted at the origin of the device current-voltage (I-V) characteristics for p-type Si nanowires of resistivity 0.53 (±0.48) Ù-cm. Cobalt contacts, upon annealing in N2 within 250 – 300 °C in the multicontact fabrication, showed lower resistance and more linear current-voltage characteristics than the Al contacts but were more difficult to lift off at the required metallization thickness. A contact based on the Co/Al metallization was later fabricated that combined the ease of lift-off of the Al multicontacts with the low resistance of the annealed Co contacts. The specific contact resistance of ohmic contacts based on such a metallization was extracted at the origin of the device I-V characteristics and found to be within 6 x 10^-6 – 6 x 10^-4 Ù-cm^2 in the as-deposited condition, and within 2 x 10^-6 – 2 x 10^-4 Ù-cm^2 upon annealing in N2 at 200 – 250 °C for p-type Si nanowires of resistivity 0.27 (±0.18) Ù-cm. Ohmic contact formation for such devices was attributed to high doping of the Si nanowires. The formation of a cobalt silicide phase combined with an increase in the doping of the nanowires adjacent to the metallization upon annealing is proposed to have led to a decrease in the specific contact resistance of the contacts after annealing.