Reminder: MSE PhD Oral Examination: Yi Wei Chen (Wednesday, June 8th @ 3:15 PM in Packard 101)

Vincent Yi Wei Chen chenyw at stanford.edu
Tue Jun 7 13:56:28 PDT 2011


University PhD Dissertation Defense

*Atomic Layer Deposited Metal Oxides for Semiconductors Used in Aqueous 
Solutions*

(Vincent) Yi Wei Chen

Department of Materials Science and Engineering

Advisor:Prof.Paul C. McIntyre

When:*Wednesday, June 8th2011, 03:15 pm*(Refreshments at3:00pm)
Where:*Packard 101*
http://campus-map.stanford.edu/index.cfm?ID=04-030

In recent years, atomic layer deposition (ALD) has become a popular 
technique to deposit ultra-thin films with superior conformality and 
thickness control. Because of its unique surface adsorption-limited 
mechanism and the resulting capability of deposition at low temperatures 
and moderate pressures, ALD has found industrial applications in field 
effect transistor fabrication and coating of multilayer interconnection 
metallization.In this work, I have explored the potential of ALD-grown 
metal oxide layers in applications beyond typical electronics 
technologies. In particular, this research has focused on using 
ALD-grown metal oxides to enhance the performance and stability in 
aqueous solutions of biomolecular sensors and semiconducting anodes for 
photoelectrochemical fuel synthesis.

In the biosensing application, we have replaced the SiO_2 gate 
dielectric material typically used in high sensitivity 
bio-field-effect-transistors (bioFET) with high dielectric constant 
HfO_2 . The SiO_2 bioFET gate dielectric suffers from poor stability and 
non-ideal dielectric response at the very small physical thicknesses 
required to achieve high sensitivity. ALD-grown HfO_2 , on the other 
hand, is capable of providing high capacitance density with a physically 
thicker dielectric layer, thanks to its large dielectric constant. With 
the ALD-HfO_2 gate dielectric, biosensor switching behavior was 
demonstrated using capacitance-voltage measurements in water, while at 
the same time maintaining the desired high capacitance. In addition, we 
have verified bio-functionalization of the HfO_2 film surface with 
biotin molecules via photoelectron spectroscopy, and detected 
streptavidin and avidin binding with capacitance-voltage analysis and 
molecular AFM imaging methods respectively.

For the solar fuel synthesis, we have studied the behavior of ALD-TiO_2 
tunnel oxides that can protect heretofore unstable semiconductors, such 
as Si, used as photoanodes in water splitting. For several decades, 
intense research effort has been devoted to identifying an efficient 
photoelectrochemical cell for oxidizing water under solar 
illumination.The resulting hydrogen and oxygen can be used to store 
energy from the intermittent terrestrial solar resource renewably, using 
water as a feedstock. However, photoanode materials choices have always 
been limited because the water oxidation half reaction at the anode 
surface is highly corrosive and requires large overpotentials. As a 
result, only oxidation-stable wide bandgap semiconductors such as TiO_2 
and Fe_2 O_3 have been used as the photoanode.These photoanodes exhibit 
poor efficiency, however, because of their large bandgaps. Lower bandgap 
semiconductors, such as Si, are capable of absorbing solar light much 
more efficiently, but are easily corroded during water oxidation. In 
this work, a silicon photoanode was passivated by a thin and 
pinhole-free layer of ALD-TiO_2 such that efficient light absorption in 
the Si and the chemical stability of the TiO_2 can be exploited at the 
same time. This ALD-grown nanocomposite photoanode has been demonstrated 
to perform water oxidation with low overpotentials, while at the same 
time maintaining good stability with hours of continuous operation. The 
tunneling of electronic carriers through the thin ALD-TiO_2 , required 
to sustain high oxidation rates, has also been investigated by varying 
the TiO_2 thickness.

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