NANOFABRICATION OF STRUCTURES FOR THE STUDY OF NANOWIRE DOPING

Open Access
Author:
Eichfeld, Chad M
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 17, 2009
Committee Members:
  • Suzanne E Mohney, Dissertation Advisor
  • Suzanne E Mohney, Committee Chair
  • Joan Marie Redwing, Committee Member
  • Elizabeth C Dickey, Committee Member
  • Theresa Stellwag Mayer, Committee Member
Keywords:
  • nanowire
  • silicon nanowire
  • selective plating
  • junction delenation
  • growth in place
  • nanofabrication
  • atom probe
  • local electrode atom probe
  • LEAP
Abstract:
A variety of techniques were developed for the characterization of nanowires and applied to the study of nanowire growth and dopant incorporation. A technique to selectively plate gold on the n-type regions of modulation-doped silicon nanowires for junction delineation was developed. The ability to electrolessly deposit metal on segments of nanowires could also facilitate electrical contact formation. More complicated structures such as controlled placement of forks along the nanowire could be made by placement of the gold catalyst at predetermined locations along a nanowire followed by a second growth. Additionally, a process was developed that focuses on using this plating ability to grow silicon nanowires horizontally from pre-determined locations. The processing was worked out and nanowires were grown horizontally from plated gold, but the selectivity of gold limited the ability to grow wires in only the desired locations. A silicon nanowire local electrode atom probe test structure is discussed from the initial design steps to successfully using the test structure to analyze silicon nanowires. Initial results using laser pulsed assisted local electrode atom probe indicated that the thermal properties of the nanowire prevent the tip from cooling fast enough and resulted in large thermal tails in the mass spectra. Thermal modeling was used to identify what nanowire diameters and metal coatings would allow the nanowire tip to cool sufficiently fast. A silver catalyzed silicon nanowire with a diameter large enough to allow for sufficient cooling was analyzed, and the concentration of silver in the silicon nanowire was below the detection limit of 10 ppm or 5 x 10^17 cm-3. The growth and characterization of Al catalyzed silicon nanowires is also discussed. Nanowires were grown at higher pressures and using H2 as a carrier gas resulting in much higher growth rates than previously observed for Al catalyzed nanowires in the literature. The nanowires were characterized by electron microscopy and found to be single crystal and to grow oriented to the growth substrate. 4-pt electrical resistivity measurements of the silicon nanowires were made. The resistivity measured ranged from of 0.005-0.017 Ω-cm, corresponding to an impurity concentration 7 x 10^18 to 2 x 10^19 cm-3 assuming bulk mobility. Calculations show that possible error from depletion at the surface has only a small effect on the calculated resistivity for a 100 nm diameter nanowire at this high of a doping concentration. The results of LEAP metrology analysis of the Al catalyzed nanowires show an estimated Al concentration of 0.4 atomic percent or 2 x 10^20 cm-3 about 200 nm from the nanowire tip, where the concentration appears to have leveled out. This value is higher than would be expected from the solubility published for Al.