In order to optimize the opto-electronic properties of thin film materials for photovoltaic applications, mixed-phase films of hydrogenated amorphous silicon containing silicon nanocrystalline inclusions (a/nc-Si:H) have been synthesized in a novel dual chamber co-deposition system.

Particles generated in one chamber, at high silane pressures and pulsed RF powers, are injected into a second chamber in which the a-Si:H matrix is deposited at lower pressures and RF powers.

The flexibility of the dual chamber system enables the growth of materials that are difficult to synthesize in a single chamber. In addition to doped a/nc-Si:H, a-Si:H containing germanium nanocrystallites and silicon nitride with Si nanocrysts have been fabricated.

The electronic transport mechanism in n-type doped a/nc-Si:H is sensitive to Si nanocrystallite density Xc, and is found to transition from activated to variable range hopping to multiphonon assisted hopping as Xc increases.

Dual Plasma Co-Deposition of Mixed Phase Thin FilmsJames Kakalios, University of Minnesota-Twin Cities, DMR 0705675

Silicon nanocrystals are produced in the particle synthesis reactor at high pressure,and RF power and are then injected into the second chamber in which high quality a-Si:H is deposited. The concentration of 5 nm Si nanocrystals in the films depends on the substrate position relative to the particle synthesis reactor’s injection tube, as confirmed by Raman spectroscopy.

The dark conductivity for doped a/nc-Si:H films displays a transition from Arhennius behavior when the crystal fraction XC = 0, to variable range hopping for XC = 25%, to multiphonon hopping (indicated by the power-law temperature dependence) higher nanocrystal densities. Thermopower measurements are consistent with a change in transport mechanism as Xc increases in doped a-Si:H.

4.5 5.0 5.5 6.0-25

-20

-15

-10

-5

0

Power Law Conductivity

XC=0%

XC=25%

XC=30%

XC=35%

Ln(C

ondu

ctiv

ity)

Ln(Temperature) (K)

Dual Plasma Co-Deposition of Mixed Phase Thin Films James Kakalios, University of Minnesota-Twin Cities, DMR 0705675

From Oct. 2008 through Sept. 2010, Kakalios has given 24 public lectures on The Physics of Superheroes, from venues ranging from universities and high schools to international comic book conventions. He will be a featured speaker at the USA Science and Engineering Festival in Oct. 2010 on the National Mall in Washington DC.

Kakalios was the on-set science consultant for the 2009 Warner Bros. film Watchmen. In Feb. 2009, the University of Minnesota asked him to film a short video, describing the basic principles of quantum mechanics, as it related to the super-powered characters in Watchmen. The resulting video The Science of Watchmen received over 1.5 million views in its first few months of being posted on the University’s youtube.com page. The video won an Upper Midwest Regional Emmy Award in 2009 and was nominated for a national Webby award in 2010.

Kortshagen has been particularly involved in outreach to industry. His technology of synthesizing Si nanocrystals was licensed to Innovalight, Inc., in 2005, and recently relicensed to Dow Corning in 2010. As program leader of a group of faculty focused on “Renewable Energy Materials,” in 2010 he has organized University-industry workshops and seminars.

From The Science of Watchmen video

Giving a public lecture on the Physics of Superheroes to summer REU students