Today 2PM CISX Auditorium - Deterministic Germanium Nanowire Growth - Ph.D. Defense

Jacob Woodruff jacobw at stanford.edu
Mon Jun 4 10:44:34 PDT 2007


Ph.D. Dissertation Defense 



DETERMINISTIC GERMANIUM NANOWIRE GROWTH:

CONTROLLING THE POSITION, DIAMETER, AND ORIENTATION OF

GERMANIUM NANOWIRES

 

Jacob Huffman Woodruff

Chemistry Department

Stanford University

 

Advisor: Christopher E.D. Chidsey

 

2PM Today, June 4th, 2007

Allen Center for Integrated Systems, CISX 101 Auditorium



ABSTRACT



Semiconductor nanowires grown by chemical vapor deposition (CVD) are promising materials for active device elements in 3D nanoelectronics.  In particular, gold-catalyzed germanium nanowires (GeNWs) are interesting due to their low growth temperature. The ability to control the nanowire's position, diameter, and orientation is crucial to enable the use of these materials for 3D electronics.  This dissertation reports studies to obtain this type of deterministic growth of GeNWs on silicon substrates.

            A lamp-heated, cold-wall CVD reactor was used to grow GeNWs from germane in hydrogen.  Growth conditions investigated included temperatures of 260-400 °C and partial pressures of germane of 0.012-1.8 Torr.  The growth rate was linearly dependent on germane partial pressure and weakly dependent on temperature, whereas the sidewall deposition rate was strongly dependent on temperature, resulting in tapered nanowires at high temperatures.  To obtain non-tapered nanowires, a two-step temperature process was used which initiated nanowire nucleation at a high temperature to improve yield, followed by growth at a lower temperature.

            GeNW epitaxy on silicon substrates was investigated as a means to control orientation during growth.  Maintaining an oxide-free silicon surface was necessary to obtain epitaxy.  GeNWs grew predominantly along the <111> crystallographic directions for larger diameter GeNWs and along the <110> directions for small diameter GeNWs, irrespective of the underlying substrate orientation.  The use of gold colloids as catalysts provided a model system to study epitaxy because of their tight size distribution.  Gold colloids were deposited onto silicon by acidifying the colloid solution with HF or HCl.  Using this system, conditions were found that resulted in predominantly vertical GeNW growth on Si(111).  While diameter and orientation control was achieved, control of the nanowire position was still needed. 

            Therefore, several techniques were investigated to control the catalyst placement, and hence the nanowire position, including site-selective gold vapor deposition, electrochemical gold deposition on patterned silicon nanoelectrodes, and lift-off using a nanoimprint-lithography-patterned mask.  Electrochemical deposition and lift-off have the advantage over site-selective vapor deposition of being room temperature methods. Simultaneous control of the nanowire position, diameter, and orientation was achieved by GeNW epitaxy from nanoimprint-lithography-patterned gold catalysts on silicon.

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