Fahlman B.D. Materials Chemistry

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surface (Figure 4.87). When the dye absorbs a photon, the resultant excitation injects

electrons into the titanium, which transports them to the negative electrode, with the

positive electrode attached to the electrolyte (e.g., aqueous iodide or sodium sulﬁd e

solutions

[103]

) acting as the hole-scavenging medium.

[104]

To date, the maximum

efﬁciency of DSCs is only ca. 11%; however, it is likely that this value will

signiﬁcantly improve as alternate nanomaterials are utilized (e.g., TiO

-multiwalled

carbon nanotube nanocomposites,

[105]

quantum dot sensitized solar cells, QDSSCs,

discussed in Chapter 6). Even with a relative low overall efﬁciently, there are

already commercial products that contain DSC technology.

[106]

The TiO

used in DSCs is not present as a smooth ﬁlm, but as a porous

nanostructural sponge, with a very high surface area. This improv es the number of

active sites that may absorb light, analogous to the stacked structure of leaves that

convert light to energy using the light-harvesting molecule chlorophyll. The oxide

nanoparticles are carefully prepared to preferentially expose the (101) face of

anatase, which is the lowest energy surface – providing the lowest LUMO level to

accept electron density from the sensitizer. It should be noted that other wide

bandgap semiconductor oxides such as SnO

and ZnO have also been employed

within DSCs.

Figure 4.87. Schematic of a dye-sensitized photovoltaic cell. The TiO

-bound dye molecules act as the

light harvester. Electrons are injected into TiO

, ﬂow to the collector electrode and through the circuit to

the counter electrode. The dye is regenerated by electron donation from the I



/3I



couple (0.536 V).

Society.

338 4 Semiconductors

Not unlike their inorganic semiconductor counterparts, the light-harvesting mol-

ecule is perhaps the most important component of a DSC. The dyes employed for

this application are ruthenium complexes that feature highly conjugated ligands

similar to those employed for OLED applications. The absorption of UV–Visible

radiation causes p ! p

and metal-to-ligand charge transfer (MLCT) electronic

transitions. The latter transition is quite pronounced for ruthenium, since its v alence

d-electrons are heavily shielded from the nuclear charge and are more easily

promoted to ligand unoccupied orbitals. The donation of electron density from the

bound sensitizer is due to MLCT which results in the oxidation of the Ru(II) metal

center. (Figure 4.88a). The design of sensitizer ligands is not trivial; the p

LUMO of

Figure 4.88. (a) Detailed schematic of the forward/backward electron donation between the sensitizer

and TiO

surface. A carboxylate group is shown on the sensitizer ligand – important for Lewis acid/base

binding to the oxide surface. Shown in (b) is an example of a hole-trapping group that prevents back-

donation from the TiO

surface to the sensitizer (improving the photocurrent generated by the DSC).

4.4. Thermoelectric (TE) Materials 339

the ligand must be of appropriate energy to allow electron transport to ﬂow to the

TiO

surface from the metal ion. However, it is also possible for an electron to be

injected back to the sensitizer from the conduction band of the TiO

(Figure 4.88a).

By designing the sensitizer appropriately, the rate of electron ﬂow from the dye to

TiO

will be much faster than the reverse reaction. An intriguing way to accomplis h

this is thr ough the incorporation of a hole-trapping group attached to the ligands

(Figure 4.88b). After MLCT takes place and the electron is injected into the TiO

layer, the positive charge is transferred to the hole-trapping moiety. This increases

the distance of the hole from the surface, which slows the rate of TiO

-sensitizer

back-donation considerably.

References and Notes

http://ece-www.colorado.edu/~bart/book/book/chapter2/ch2_8.htm

For a thorough discussion of the complex threefold non-degeneracy of valence band edges, character-

ized by heavy hole and light hole bands degenerate at G, and a split-off band separated by the

magnitude of spin-orbit interaction, see: Kittel, C. Introduction to Solid State Physics, 8th ed.,

Wiley: New York, 2005. Another useful website resource is: http://mems.caltech.edu/courses/

EE40%20Web%20Files/Supplements/01_Effective_Mass.pdf

For instance, see: (a) Matsui, H.; Terashima, K.; Sato, T.; Takahashi, T.; Wang, S. -C.; Yang, H. -B.;

Ding, H.; Ueﬁji, T.; Yamada, K. Phys. Rev. Lett. 2005, 94, 047005. (b) Raj, S.; Hashimoto, D.; Matsui,

H.; Souma, S.; Sato, T.; Takahashi, T.; Sarma, D. D.; Mahadevan, P.; Oishi, S. Phys. Rev. Lett. 2006,

96, 147603.

Hemlock Semiconductor homepage: http://www.hscpoly.com

For example, see: (a) Sanjurjo, A. U.S. Patent 5006317. (b) Yoon, P.; Song, Y. U.S. Patent 4786477.

Note: homoepitaxial growth refers to growing a ﬁlm onto an atomically-ﬂat substrate, wherein both

ﬁlm and substrate are compositionally equivalent. In contrast, heteroepitaxial growth would refer to

growing a thin ﬁlm of different composition onto a substrate (e.g., GaAs thin ﬁlm on Si). Such ﬁlm

growth occurs usually by vapor-phase techniques, which facilitates exact lattice matching of the

crystal orientation and spacing of the growing thin ﬁlm with the underlying substrate.

Note: the solar market has been growing at a rate of 40%/year in recent years, as compared to 4–6%/

year for the semiconductor market. For more information on markets and production ﬁgures, see:

http://www.aiche.org/cep (Aug. 2008 issue).

The 65-nm Intel Core 2 Duo chip contained 290 million transistors, whereas its 45-nm successor

contains 410 million transistors. In comparison, various multi-core processors have been released in

2010 that feature over 1 billion transistors (http://en.wikipedia.org/wiki/Microprocessor_chronology):

IBM’s new 45-nm POWER7 processor (567 mm

) features 1.2 billion transistors; in contrast, the next

generation of Intel Itanium processors (“Tukwila” – 699 mm

) is the ﬁrst to feature over 2 billion

transistors for a microprocessor chip. The latest graphics processing unit (GPU – NVIDIA GF100)

contains over 3 billion transistors. It should be noted that ﬂash-memory chips have long featured over

1 billion transistors. Since 2005, 8 GB memory chips (146 mm

) have featured over 4 billion

transistors, and the latest 32 GB SD cards contain over 100 billion transistors! For an interesting

website that provides an updated timeline for PC developments, see: http://www.willus.com/archive/

timeline.shtml.

Imre, A.; Csaba, G.; Ji, L.; Orlov, A.; Bernstein, G. H.; Porod, W. Science 2006, 311, 205.

Note: though the current should, in theory, be zero for a reverse-bias diode, there will still be a very

small number of electrons/holes with enough energy to overcome the large junction potential, resulting

in a very small current.

340 4 Semiconductors

Note: whereas it is easy to conceptualize the uphill movement of electrons due to an applied voltage

that exceeds the junction potential, it is not as straight-forward to rationalize hole migration. That is,

they will move downhill from p–n regions during forward bias. A picture that may help visualize this is

to think of holes as helium-ﬁlled balloons that are adhered to a ceiling. Energy would be required in

order to pull them down; having a larger balloon with more helium would require even more energy

(analogous to reverse-bias), whereas a small balloon would be easier to pull down (foward bias).

(a) ftp://download.intel.com/research/silicon/Chau-paper-IEEE-0403.pdf (b) ftp://download.intel.

com/research/silicon/tri-gate-transistor-conference-paper-0603.pdf (c) Landgraf, E.; Rosner, W.; Sta-

dele, M.; Dreeskornfeld, L.; Hartwich, J.; Hofmann, F.; Kretz, J.; Lutz, T.; Luyken, R. J.; Schulz, T.;

Specht, M.; Risch, L. Solid-State Electron. 2006, 50, 38.

For a press release, see: http://www.rochester.edu/news/show.php?id¼2585

(a) http://www.soiconsortium.org/pdf/Consortium_9april09_ﬁnal.pdf (b) http://www.advancedsub-

stratenews.com/

For instance, see: (a) Jones, A. C. et al. J. Mater. Chem., 2004, 14, 3101 (b) Musgrave, C.; Gordon, R.

G. Future Fab International 2005, 18, 126. (c) Vincenzini, P.; Marletta, G. Adv. Sci. Technol. 2006,

51, 156.

http://www.thompson.ece.uﬂ.edu/Fall2008/intel%20TED-01347396.pdf

(a) Sadana, D. K.; Current, M. “Fabrication of Silicon-on-Insulator (SOI) Wafers Using Ion Implanta-

tion”, in Ion Implantation Science and Technology, Ziegler, J. F. ed., Edgewater: Ion Implantation

Technology Co., 2000. (b) Colinge, J. P. Silicon-on-Insulator Technology: Materials to VLSI, 2nd ed.,

Dordrecht: Kluwer Academic Publishers, 1997. (c) http://www.ibis.com – Ibis Technology Corpora-

tion, Danvers, MA, USA. (d) http://www.chips.ibm.com/bluelogic (e) Marshall, A.; Natarajan, S. SOI

Design: Analog, Memory, and Digital Techniques, Boston: Kluwer Academic Publishers, 2002.

An issue of the MRS Bulletin was recently devoted to scaling future CMOS logic devices with Ge and

III-V materials: MRS Bull. 2009, 34, 485.

(a) Nihei, R.; Usami, N.; Nakajima, K. Jpn. J. Appl. Phys. 2009, 48, 115507, and references therein. (b)

Jankovic, N. D.; O’Neill, A. Semicond. Sci. Technol. 2003, 18, 901.

Plummer, J. D.; Deal, M. D.; Grifﬁn, P. B. Silicon VLSI Technology: Fundamentals, Practice, and

Modeling, Prentice Hall: New Jersey, 2000.

A detailed video of the Si(111) 7  7 reconstruction may be found at: http://www.vimeo.com/

1086112

For more details, see: (a) http://www.chem.qmul.ac.uk/surfaces/scc/ (b) http://www.cem.msu.edu/%

7Ecem924sg/Topic05.pdf

Note: even though single-crystal Si is used as the substrate, the lattice spacings of SiO

are much

greater than Si (5.431 A

Hess, D. W. Proc. Electrochem. Soc. 96(1), 143.

Kriegler, R. J. Semiconductor Silicon 1973, The Electrochemical Society: Princeton, NJ, 1973.

Note: the majority of ICs utilize a Si(100) substrate, largely due to the desirable electrical properties of

the Si(100)/SiO

interface.

A comprehensive users guide to photolithography may be found online at: http://www.ee.washington.

edu/research/microtech/cam/PROCESSES/PDF%20FILES/Photolithography.pdf

Note: for GaAs substrates, primers such as xylene or trichlorobenzene are used, as GaAs is already a

polar surface.

Note: masks are either comprised of soda-lime glass (coated with either a photographic emulsion,

, or Cr ﬁlms), or quartz (with a Cr ﬁlm). Due to the absorption of UV light by glass, the latter is

required for deep UV (DUV) photolithography. Masks may be classiﬁed as either “light-ﬁeld” or

“dark-ﬁeld”; whereas the former is mostly clear with opaque patterns, the latter is an opaque mask,

with transparent features.

For example: (a) Nonogaki, S. Polymer J. 1987, 19, 99. (b) Liu, H. -H.; Chen, W. -T.; Wu, F. -C.

J. Polym. Res. 2002, 9, 251.

The use of double- and triple-patterning has extended the 193-nm photolithography timeline beyond

that originally anticipated. For more details on this technology, see: Wu, B.; Singh, A. K. Extreme

References 341

Ultraviolet Lithography, McGraw-Hill: New York, 2009 (page 5 of the Introduction has a lithography

roadmap for the extension of 193-nm photolithography, and EUV not likely being instituted until the

22 or 16 nm node (ca. 2015þ).

For more details regarding EUV, see: Hutcheson, G. D. et al. Scientiﬁc American 2004, 290, 76.

For information regarding EUV mask design may be found online at: http://www.sematech.org/

meetings/archives/litho/euvl/20030930/presentations/2C%20Shoki%20EUV%20Symp.pdf

Hiroshi, I. Adv. Polym. Sci. 2005, 172, 37, and references therein.

(a) http://people.ccmr.cornell.edu/~cober/MiniPresentations/PAG_RBSPC.pdf (b) Kim, K. -M.;

Ayothi, R.; Ober, C. K. Polym. Bull. 2005, 55, 333.

For instance, see: (a) Dai, J.; Ober, C. K.; Wang, L.; Cerrina, F.; Nealey, P. F. Proc. SPIE – Int. Soc.

Opt. Eng. 2002, 4690, 1193. (b) Kessel, C. R.; Boardman, L. D.; Rhyner, S. J.; Cobb, J. L.; Henderson,

C. C.; Rao, V.; Okoroanyanwu, U. Proc. SPIE – Int. Soc. Opt. Eng. 1999, 3678, 214. (c) Bratton, D.;

Yang, D.; Dai, J.; Ober, C. K. Polym. Adv. Technol. 2006, 17, 94, and references therein.

Note: the depth of focus (DOF ¼ l/(NA)

) is also paramount toward resolution, as wafers are not

atomically ﬂat. Though it may be possible to adjust the wavelength and NA to achieve better

resolution, the depth of ﬁeld will decrease, making it difﬁcult to deﬁne features simultaneously at

the top and bottom surfaces. Consequently, chemical mechanical polishing (CMP) is used to planarize

the wafer prior to high-resolution photolithography, and as thin a layer as possible of photoresist is

applied to the wafer.

Article may be accessed online at: http://turroserver.chem.columbia.edu/PDF_db/publications_801_850/

NJT849.pdf

There are two methods used to remove the patterned material. Etching is where the photoresist is

developed on top of the deposited layer. The underlying material is then removed by etching through

openings in the mask. In contrast, lift-off is used when the material is deposited on top of the developed

photoresist. The material is then lifted off when the resist is removed. For a nice summary of wet/dry

etching, as well as etching vs. lift-off, see: http://www.mrsec.harvard.edu/education/ap298r2004/Erli

%20chen%20Fabrication%20III%20-%20Etching.pdf

Note: a plasma is considered the fourth class of matter, in addition to solids, liquids, and gases. A

plasma contains a mixture of ground-state and excited-state atoms, as well as ions.

C. J. Mogab, A. C. Adams, and D. L. Flamm, J. Appl. Phys. 1978, 49(7), 3796.

As its name applies, chemical mechanical polishing/planarization utilizes a hybrid of chemical and

mechanical forces to yield a ﬂat surface. It should be noted that using mechanical force alone (e.g.,

grinding) would successfully planarize a surface; however, this would cause too much surface degrada-

tion. For more information about this process, see: http://maltiel-consulting.com/CMP-Chemical-

mechanical_planarization_maltiel_semiconductor.pdf

In contrast to crysta lline Si, polysilicon i s formed at much l ower temperatures (150–350



C) and is

comprised of individual grains of dimensions between ca. 10 nm and 1 mm. The electrical conductiv-

ity of a polysilicon gate may be altered by doping or increased by depositing a surface coating of

a metal (e.g., W) or silicide (e.g., WSi

). It should be noted that the 32-nm Intel chi ps now feature a

metallic gate that, in combination with a high-k dielectric gate insulator (HfO

), leads to a

20% +increase in transistor switchi ng speeds and a 100-fold decrease in the gate oxide leakage

current(http://www.intel.com/technology/silicon/high-k.htm). For more details regarding the future

scaling of CMOS transistors past 32 nm, se e: http://download.intel.com/pressroom/pdf/kkuhn/

Kuhn_Advanced_Semicond uctor_Manufacturing_Conference_k eynot e_July_13 _2010_text.pdf

It should be noted that for memory chips and removable ﬂash drives, a gate stack structure known as

SONOS (polysilicon/SiO

/Si

/SiO

/Si substrate) is typically used. The device is programmed by

applying a voltage to the gate to inject charge into the conduction band of the Si

layer, resulting in a

change in the threshold voltage. When the voltage is removed, the charge remains trapped within the

layer. High-k metal oxides such as ZrO

and HfO

are also capable of trapping injected

electrons; if these replace Si

, the memory device is known as SOMOS (where M refers to metal

oxide). The charge-trapping ability is most pronounced for materials that exhibit a high dielectric

constant (i.e., are nonconducting and polarizable), and have sufﬁcient density to prevent tunneling to

342 4 Semiconductors

surrounding layers. To erase the device, a reverse bias voltage is applied that removes the trapped

charge from the Si

region. Typically, the SiO

adjacent to the polysilicon gate is thicker than the

SiO

/Si interface, resulting in charge injection occurring through the bottom SiO

layer.

This is typically performed through use of templates that are sacriﬁcially removed following ﬁlm

deposition. For example, see: (a) Fuertes, M. C.; Soler-Illia, G. J. A. A. Chem. Mater. 2006, 18, 2109.

(b) Xiao, L.; Zhang, H.; Scanlon, E.; Ramanathan, L. S.; Choe, E.-W.; Rogers, D.; Apple, T.;

Benicewicz, B. C. Chem. Mater. 2005, 17, 5328. (c) Kanungo, M.; Deepa, P. N.; Collinson, M. M.

Chem. Mater. 2004, 16, 5535. (d) Li, X. S.; Fryxell, G. E.; Birnbaum, J. C.; Wang, C. Langmuir 2004,

20, 9095.

For an animated website to illustrate the DC-diode and magnetron sputtering processes, see: http://

www.ajaint.com/whatis.htm

For example, Sigel, G. H.; Homa, D. S. U.S. Patent 7181116.

(a) Nasibulin, A. G.; Shurygina, L. I.; Kauppinen, E. I. Colloid J. 2005, 67, 1, and references therein.

(b) Suzuki, K.; Kijima, K. Jpn. J. Appl. Phys. 2005, 44, 2081. (c) Chen, R. S.; Huang, Y. S.; Liang, Y.

M.; Tsai, D. S.; Tiong, K. K. J. Alloys Compd. 2004, 383, 273. (d) Wang, Y. Q.; Chen, J. H.; Yoo, W.

J.; Yeo, Y. -C. Mat. Res. Soc. Symp. Proc. 2005, 830, 269.

Barron, A. R. in CVD of Nonmetals, Rees, W. S. ed., Wiley: New York, 1996.

For a thorough description of kinetic and mass-transport mechanisms involved in CVD, as well as

dependent variables, see: Pierson, H. O. Handbook of Chemical Vapor Deposition, 2nd ed., William

Andrew: Norwich, NY, 1999.

(a) Schropp, R. E. Mater. Res. Soc. Symp. Proc. 2003, 762, 479. (b) Schropp, R. E. Thin Solid Films

2004, 451, 455. (c) Lau, K. K. S.; Murthy, S. K.; Lewis, H. G.; Pryce, C.; Jeffrey, A.; Gleason, K. K. J.

Fluorine Chem. 2003, 122, 93. (d) Mahan, A. H. Solar Energy Mater. Solar Cells 2003, 78, 299. (e)

Stannowski, B.; Rath, J. K.; Schropp, R. E. Thin Solid Films 2003, 430, 220. (f) Schroeder, B. Thin

Solid Films 2003, 430, 1. (g) Duan, H. L.; Zaharias, G. A.; Bent, S. E. Curr. Opin. Solid State Mater.

Sci. 2002, 6, 471. (h) Mahan, A. H. Solar Energy 2004, 77, 931. (i) Matsumura, H.; Umemoto, H.;

Masuda, A. J. Non-Cryst. Solids 2004, 338, 19.

(a) Fahlman, B. D.; Barron, A. R. Adv. Mater. Opt. Electron. 2000, 10, 135. (b) Richards, V. N.; Vohs,

J. K.; Williams, G. L.; Fahlman, B. D. J. Am. Ceram. Soc. 2005, 88, 1973.

Zhang, W. J.; Bello, I.; Lifshitz, Y.; Chan, K. M.; Meng, X. M.; Wu, Y.; Chan, C. Y.; Lee, S. T. Adv.

Mater. 2004, 16, 1405.

For instance, see: (a) Xiong, G.; Elam, J. W.; Feng, H.; Han, C. Y.; Wang, H.-H.; Iton, L. E.; Curtiss, L.

A.; Pellin, M. J.; Kung, M.; Kung, H.; Stair, P. C. J. Phys. Chem. B. 2005, 109, 14059. (b) Niinisto, J.;

Rahtu, A.; Putkonen, M.; Ritala, M.; Leskela, M.; Niinisto, L. Langmuir 2005, 21, 7321. (c) Sechrist,

Z. A.; Fabreguette, F. H.; Heintz, O.; Phung, T. M.; Johnson, D. C.; George, S. M. Chem. Mater. 2005,

17, 3475. (d) Reijnen, L.; Meester, B.; de Lange, F.; Schoonman, J.; Goossens, A. Chem. Mater. 2005,

17, 2724. (e) Matero, R.; Rahtu, A.; Ritala, M. Langmuir 2005, 21, 3498. (f) Min, Y.-S.; Cho, Y. J.;

Hwang, C. S. Chem. Mater. 2005, 17, 626. (g) Gu, W.; Tripp, C. P. Langmuir 2005, 21, 211.

Fahlman, B. D.; Barron, A. R. Adv. Mater. Opt. Electron. 2000, 10(3–5), 135.

(a) Hansen, B. N.; Hybertson, B. M.; Barkley, R. M.; Sievers, R. E. Chem. Mater. 1992, 4, 749. (b)

Lagalante, A. F.; Hansen, B. N.; Bruno, T. J.; Sievers, R. E. Inorg. Chem. 1995, 34, 5781. (c)

Fernandes, N. E.; Fisher, S. M.; Poshusta, J. C.; Vlachos, D. G.; Tsapatsis, M.; Watkins, J. J. Chem.

Mater. 2001, 13, 2023. (d) Cabanas, A.; Long, D. P.; Watkins, J. J. Chem. Mater. 2004, 16, 2028. (e)

Blackburn, J. M.; Long, D. P.; Watkins, J. J. Chem. Mater. 2000, 12, 2625. (f) Blackburn, J. M.; Long,

D. P.; Cabanas, A.; Watkins, J. J. Science 2001, 294, 141. (g) Cabanas, A.; Blackburn, J. M.; Watkins,

J. J. Microelectron. Eng. 2002, 64, 53. (h) Ohde, H.; Kramer, S.; Moore, S.; Wai, C. M. Chem. Mater.

2004, 16, 4028.

For a thorough review of CVD/ALD precursors, see: Fahlman, B. D. Curr. Org. Chem. 2006, 10, 1021.

Gardiner, R. A.; Gordon, D. C.; Stauf, G. T.; Vaarstra, B. A.; Ostrander, R. L.; Rheingold, L. Chem.

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For an example of ﬂuorine-free polyether ligands used to successfully prevent oligomerization of

barium complexes (particularly problematic for heavy Group II complexes due to the large ionic radius

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Chem. 2000, 39, 3148.

For example, see: Gillan, E. G.; Bott, S. G.; Barron, A. R. Chem. Mater. 1997, 9, 796.

Hansen, B. N.; Brooks, M. H.; Barkley, R. M.; Sievers, R. E. Chem. Mater. 1992, 4, 749.

(a) Banger, K. K.; Jin, M. H.-C.; Harris, J. D.; Fanwick, P. E.; Hepp, A. F. Inorg. Chem. 2003, 42,

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15, 3142.

http://nextbigfuture.com/2009/06/euv-lithography-could-be-commercial.html – a recent press release

indicates that EUV lithography will likely be implemented for high volume production by 2012–2014,

with feature sizes well below 32 nm.

Note: in order to reduce the adhesion between a polymeric mold and a silicon/quartz master, the master

surface is typically modiﬁed with a ﬂuorosilane (e.g., CF

(CF

)

(CH

)

SiCl

3(g)

). In addition, the ﬁnal

removal of the mold may also be carried out in the presence of a liquid with a low viscosity such as

methanol (solvent-assisted micromolding (SAMIM)).

For a nice survey of the beneﬁts for (nano)imprint lithography relative to photolithography, see: http://

www.molecularimprints.com/NewsEvents/tech_articles/new_articles/SPIE_07_MMS.pdf

A recent thorough review of nanofabrication using both hard and soft molds, as well as other forms of

soft lithography, see: Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M.

Chem. Rev. 2005, 105, 1171; for instance, CDs are made by imprinting patterns from Ni masters in

polycarbonate (a) J. S. Winslow, IEEE Trans. Consumer Electron. 1976 (Nov.), 318; holograms are

made by imprinting patterns from a fused quartz master in SURPHEX photopolymer (F. P. Shvarts-

man in Diffractive and Miniaturized Optics (Ed. : S.-H. Lee), SPIE Optical Engineering Press,

Bellingham, WA, 1993, 165)

For example, see: Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272, 85.

For example, see: Jackman, R. J.; Wilbur, J. L.; Whitesides, G. M. Science 1995, 269, 664.

Im, J.; Kang, J.; Lee, M.; Kim, B.; Hong, S. J. Phys. Chem. B 2006, 110, 12839.

Myung, S.; Lee, M.; Kim, G. T.; Ha, J. S.; Hong, S. Adv. Mater. 2005, 17, 2361.

(a) Odom, T. W.; Thalladi, V. R.; Love, J. C.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 12112.

(b) Odom, T. W.; Love, J. C.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Langmuir 2002, 18, 5314.

Li, H.-W.; Muir, B. V. O.; Fichet, G.; Huck, W. T. S. Langmuir 2003, 19, 1963.

Steward, A.; Toca-Herrera, J. L.; Clarke, J. Protein Sci. 2002, 11, 2179.

Note: PDMS is known to swell in organic solvents and leaves a silicone residue behind during its

release from the substrate; these limitations are overcome for PFPE molds; for example, see: (a) Lee, J.

N.; Park, C.; Whitesides, G. M. Anal. Chem. 2003, 75, 6544. (b) Rolland, J. P.; Hagberg, E. C.;

Denison, G. M.; Carter, K. R.; DeSimone, J. M. Angew. Chem., Int. Ed. 2004, 43, 5796. (c) Rolland, J.

P.; Van Dam, R. M.; Schorzman, D. A.; Quake, S. R.; DeSimone, J. M. J. Am. Chem. Soc. 2004, 126,

2322.

Maynor, B. W.; Larue, I.; Hu, Z.; Rolland, J. P.; Pandya, A.; Fu, Q.; Liu, J.; Spontak, R. J.; Sheiko, S.

S.; Samulski, R. J.; Samulski, E. T.; DeSimone, J. M. Small 2007, 3, 845.

Kelly, J. Y.; DeSimone, J. M. J. Am. Chem. Soc. 2008, 130, 5438.

Gratton, S. E. A.; Williams, S. S.; Napier, M. E.; Pohlhaus, P. D.; Zhou, Z.; Wiles, K. B.; Maynor, B.

W.; Shen, C.; Olafsen, T.; Samulski, E. T.; Desimone, J. M. Acc. Chem. Res. 2008, 41, 1685.

Gratton, S. E. A.; Pohlhaus, P. D.; Lee, J.; Guo, J.; Cho, M. J.; DeSimone, J. M. J. Controlled Release

2007, 121, 10.

Gates, B. D.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 14986.

Note: we will discuss the operating principle of atomic force microscopy (AFM) and other scanning

force microscopies in more detail in Chap. 7. At this point, simply think of this technique as analogous

to an antiquated record player, in which the needle gently touches the surface of the record to produce

music. Similarly, the AFM tip either gently taps, or hovers immediately above, the surface of a planar

substrate to reveal its surface topography.

(a) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661. (b) Hong, S.; Zhu, J.;

Mirkin, C. A. Science 1999, 286, 523. (c) Hong, S.; Mirkin, C. A. Science 2000, 288, 1808.

344 4 Semiconductors

A very nice review of the various DPN methodologies is provided by: Ozin, G. A.; Arsenault, A. C.

Nanochemistry: A Chemical Approach to Nanomaterials,2

ed. RSC: Cambridge, UK, 2009, pp.

173–204.

Bowers, M. J.; McBride, J. R.; Rosenthal, S. J. J. Am. Chem. Soc. 2005, 127, 15378.

For a review of transparent conductive ﬁlms (fabrication and applications), see: Gordon, R. G. MRS

Bull. 2000, 8, 52.

For a review of the band structure of doped tin oxide, see: Batzill, M.; Diebold, U. Prog. Surf. Sci.

2005, 79, 47.

For a review of exciton formation and OLEDs, see: Yersin, H. Top. Curr. Chem. 2004, 241,1.

Note: spin-orbit coupling refers to the interaction of the spin magnetic moment of an electron with the

magnetic moment arising from the orbital motion of the electron.

For a nice summary of triplet emitters for OLED applications, see: Yersin, H.; Finkenzeller, W. J. in

Highly Efﬁcient OLEDs with Phosphorescent Materials, Yersin, H. ed., Wiley-VCH: Weinheim,

2008.

A nice brief overview of thermoelectricity may be found online at: http://www.iue.tuwien.ac.at/phd/

mwagner/node48.html

Note: for details regarding all aspects of thermoelectric materials, refer to the March 31, 2006 issue of

the MRS Bulletin – devoted entirely to this topic.

For example, see: Mangersnes, K.; Lovvik, O. M.; Prytz, O. New J. Phys. 2008, 10,1.

For example, see: Kleinke, H. Chem. Mater. 2009, ASAP.

For example, see: Hebert, S.; Lambert, S.; Pelloquin, D.; Maignan, A. Phys. Rev. B 2001, 64, 172101,

and references therein.

For example, see: Wilson-Short, G. B.; Singh, D. J.; Fornari, M.; Suewattana, M. Phys. Rev. B 2007,

75, 035121, and references therein.

For instance, see: Ham, J.; Shim, W.; Kim, D. H.; Lee, S.; Roh, J.; Sohn, S. W.; Oh, K. H.; Voorhees,

P. W.; Lee, W. Nano Lett. 2009, 9, 2867, and references therein.

For example, see: Liang, W.; Hochbaum, A. I.; Fardy, M.; Rabin, O.; Zhang, M.; Yang, P. Nano Lett.

2009, 9, 1689, and references therein.

Lee, J. -H.; Galli, G. A.; Grossman, J. C. Nano Lett. 2008, 8, 3750.

Mingo, N.; Hauser, D.; Kobayashi, N. P.; Plissonnier, M.; Shakouri, A. Nano Lett. 2009, 9, 711.

More details regarding the beneﬁts of nanostructures for thermoelectric applications may be found at:

http://www.cs.duke.edu/~reif/NSF.NanoEnergy/Report/

100

A nice summary of the development of materials with high ZT may be found at: http://www.ms.ornl.

gov/correlated/pdf/talks/2004/Boston04.pdf

101

http://www.nrel.gov. It should be noted that solar cells are limited by a number of intrinsic and

extrinsic losses. Extrinsic sources include reﬂection, series resistance, absorption within interlayers,

nonradiative recombination, and many others. Intrinsic sources include inefﬁcient collection of solar

photon energies by each layer, and radiative recombination. For details on these limitations, see:

Henry, C. H. J. Appl. Phys. 1980, 51, 4494.

102

For a recent review of dye-sensitized solar cells, see: Peter, L. Acc. Chem. Res. 2009, ASAP.

103

There have been recent efforts toward designing solid-state DSSCs, which represent a more commer-

cially-viable device. Previous designs featured liquid electrolytes, which have issues with leakage

and volatilization of the liquid. Common designs have featured solid electrolytes poly (N-alkyl-4-

vinyl-pyridine) iodide/N-methyl pyridine iodide and 2,2

,7,7

-tetrakis(N,N-di-p-methoxyphenyla-

mine)-9,9

-spiro-biﬂuorene (Spiro-OMeTAD, Figure 4.85b). For example, see: (a) Wu, J.; Hao, S.;

Lan, Z.; Lin, J.; Huang, M.; Huang, Y.; Li, P.; Yin, S.; Sato, T. J. Am. Chem. Soc. 2008, 130, 11568.

(b) Moon, S. -J.; Yum, Y -H.; Humphry-Baker, R.; Karlsson, K. M.; Hagberg, D. P.; Marinado, T.;

Hagfeldt, A.; Sun, L.; Gratzel, M.; Nazeeruddin, M. K. J. Phys. Chem. C 2009, 113, 16816. (c)

Cappel, U. B.; Karlsson, M. H.; Pschirer, N. G.; Eickemeyer, F.; Schoneboom, J.; Erk, P.; Boschloo,

G.; Gagfeldt, A. J. Phys. Chem. C 2009, 113, 14595.

104

For a thorough review of DSCs, see: Gratzel, M. Inorg. Chem. 2005, 44, 6841.

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Muduli, S.; Lee, W.; Dhas, V.; Mujawar, S.; Dubey, M.; Vijayamohanan, K.; Han, S. -H.; Ogale, S.

ACS Appl. Mater. Interfaces 2009, 1, 2030.

106

Note: solar panels using DSC have been produced by Sustainable Technologies, International (www.

sta.com.au); other applications are being actively pursued.

Topics for Further Discus sion

1. If a circular silicon wafer, with a diameter of 300 mm and thickness of 0.5 mm, is 99.9999999% pure,

how many impurity atoms are present in the wafer?

2. Describe the steps involved in transforming sand to UHP silicon. What techniques are used to

fabricate silicon wafers from the larger pieces of puriﬁed silicon sold by a company such as Hemlock

Semiconductor?

3. What is meant by the current ‘45 nm’ technology node (i.e., what distance does 45 nm refer to)? What

is the next IC technology node, and when is this scheduled to be initiated?

4. The cleanrooms that house photolithographic equipment must have yellow lighting. Why?

5. Can one over-expose a photoresist? If so, what will happen?

6. Provide examples (with literature references) for surface patterning techniques that do not use organic

photoresists. Be sure to also include techniques other than “soft lithography”.

7. Do any commercial processes currently utilize soft lithography techniques? If so, describe these

applications.

8. Explain the beneﬁts and disadvantages of using EUV as a development source for photolithography.

9. Describe the inﬂuence of the incident exposure wavelength on the resulting line resolution in

photolithography. Would you expect the resolution to be better by using X-rays (l ¼ ca. 1.5 A

)

instead of EUV? Explain.

10. What is meant by “density of states,” and how does this inﬂuence the electrical conductivity of a

semiconductor?

11. Your supervisor has asked you to deposit the following thin ﬁlms; what transition metal/main group

complexes would you employ that would impart a high volatility of the overall complex, and low C

incorporation in the growing ﬁlm? Also, think of what coreactant gas(es) would be most appropriate

for CVD. (a) ZrO

(b) Zr

1x

(d) PZT (e) Mg (f) Alq

(i.e., Figure 4.46a).

12. Explain how LEDs/OLEDs operate, and think of some intriguing applications for these types of

materials.

13. Why is TiO

used in dye-sensitized solar cells (DSCs)? What other oxides would possibly be useful?

14. Explain how triplet harvesting improves the efﬁciency of OLEDs.

15. As discussed in this Chapter, next-generation CMOS ICs will employ high-k dielectric ﬁlms as the

gate insulator. A potential roadblock to this replacement is the possible incompatibility with current

gate stacks – explain this problem, with possible solutions.

16. Explain the differences between ALD, CVD, and PVD. Which one would be the method of choice to

deposit a silica (SiO

) ﬁlm of 1 nm ( 0.1 nm) onto a carbon ﬁber? Rationalize your choice.

17. How would you design a CVD system (reactor type/material; nature of the precursor, co-reactant, and

carrier gas; precursor decomposition methodology; etc.) to deposit Si

onto the surface of fumed

alumina powder (particulate diameters of ca. 70 nm)? Use diagrams with full explanation of your

design choices.

18. Explain the relative effect of growth rate vs. temperature for surface-reaction and mass-transport

kinetically controlled thin ﬁlm MOCVD processes. Why does the growth rate decrease for deposi-

tions taking place at very high temperatures?

19. In this Chapter, there was a lot of discussion related to silicon-based devices and applications. As we

attempt to continue Moore’s Law into the future, how likely is it that Si will be replaced entirely? If

this paradigm shift was proposed, what would be some negative consequences associated with such a

move?

20. Why are chlorinated coreactants used during the initial oxidation step of CMOS fabrication, and what

are the mechanisms involved with its reactive participation?

346 4 Semiconductors

21. How are the bandgap (E

) and dielectric constant (k) of a thin ﬁlm determined experimentally?

22. Heusler alloys are intermetallics with the general formula M

NiSn (M ¼ Zr, Ti), with the unit cell

being a NaCl lattice of two metals, with the third metal occupying tetrahedral interstitial sites. In

contrast, “half-Heusler alloys” have the structure MNiSn, with the third metal occupying only one-

half of the interstitial sites. With this in mind, would “Heusler” or “half-Heusler” alloys be a more

efﬁcient thermoelectric material? Explain your choice.

23. Your stellar academic performance at CMU has resulted in a job interview for a very lucrative

materials scientist position. As a test, they have given you the following LEDs: GaP, Al

0.5

As,

GaAs, GaN, GaSb. Arrange these in order of observed color, from red – blue. Explain your rationale.

24. How are the bandgap and dielectric constant of a thin ﬁlm determined experimentally?

25. Whereas the positive effects of tensile strain of a MOSFET channel is easy to visualize (increases the

lattice parameter & the free mean path of electrons), the effects of channel compression strain seems

counter-intuitive. Explain how compression strain increases the conductivity of the channel region.

Why is this type of strain not preferred for n-type Si?

26. The military has designed a futuristic soldier protective suit that would provide various functionalities

such as: artiﬁcial muscles, vision enhancement, chemical/biological warfare agent protection, and

temperature modulation. What strategy(ies) would you use to provide long-term electrical power to

this suit, other than battery power? Note that the ﬁrst missions to use this suit will be in the caves of

Afghanistan.