Transparent ElectronicsResearchers at Oregon State

University (OSU) have recently de-veloped a new class of materials.They have used these materials in alaboratory setting to create trans-parent th in- f i lm transistors (TTFTs).These new materials might one dayallow you to transform the wind-shield of your car into a GPS-acti-vated map at the touch of a button,or turn a picture window in yourhouse into a high-definition videoscreen. How far into the future youwill have to wait for these and otherconsumer products is hard to say,but HP has already taken the firststep by recently licensing OSU's newtechnology.

The first transparent electroniccircuit was created using a thin f i lmzinc oxide substrate at Oregon StateUniversity in 2003. Since then, re-searchers have made major ad-vances and discovered 28 differentnew material compounds that canall render transparent electroniccircuits.

Looking at Photo 1, you will seethe corner of a one-dollar bill covered witha small square glassplate. Since you cansee through the glassplate, you will prob-ably assume that theglass is empty. In fact,though, a transparentelectronic circuit thatcontains 56 patternedtransistors and 24resistance test struc-tures covers the glass."You might be amazedto learn that the thinfilm that you can seethrough was createdusing a zinc oxide thinfilm, the same zinc oxide that goesinto the cream that you put on yournose when you go to the beach toprotect your skin from a sunburn,"

Alan Piercepierceaj@optonline.net

says John Wager, a professor ofelectrical engineering at OSU.

Since seeing is believing, Photo 2shows the same glass plate en-

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Wager worked in collaborationwith Doug Keszler, a professor ofchemistry and Janet Tate, a profes-sor of physics. The developmentteam also included Chris Tasker, thelaboratory manager and graduateresearch assistants working in thethree departments. The successwith zinc oxide led to combining ofthe zinc oxide with tin oxide. Thiscombination improved conductivityfar beyond what the researchers hadachieved with zinc oxide alone.

Photo 3 shows Wager lookingthrough the window of a sputteringmachine, which deposits the thinfilms used to make the circuits andother devices. The zinc oxide and

Photo 1—Spin-coated ZnO TTFTs. 56patterned ZnO TTFTs and 24 contact

resistance test structures arepresent inside the red box.

hanced to allow you to see whatwould normally be invisible. Wagerexplains that "you see through theelectronic circuit in [the first photo]because the material that it is madeof has a very wide band gap. Youcan't see through a silicon circuit or

This sputtering machine is used to deposit the thin films tomake the circuits and other devices.

Photo 2-Sputtered ZnO TTFTs.An enhanced-contrast, magnified

image of a bottom-gate transparenttransistor test structure.

zinc-tin oxide circuits contain 10-75nrn thick channel layers for electron

flow. Wager explains that"a 10 nm thick layerwould only be about 40atoms thick!"

Up to this point, crys-talline materials havebeen used in most elec-tronic circuits becausethey effectively moveelectrons through theirorganized lattice of at-oms. The new materialsthat these engineers andscientists have created bydepositing zinc and zinctin oxides randomlyshould not have pro-duced a good conducting

other opaque materials becausethey have a narrower band gap thatdoesn't allow visible light to passthrough."

Alan Pierce, Ed.D., CSIT, is a tech-nology education consultant. VisitWWW. technology today, us for pastcolumns and teacher resources.

www.techdirections.com TECHNOLOGY TODAY

circuit. The researchers were sur-prised Jo find that electrons movedthrough their amorphous material (amaterial with randomly distributedatoms) efficiently. Wager and I dis-cussed the team's breakthrough in aphone interview. He also e-mailedme a PowerPoint presentation tohelp me understand the complexi-ties of what he and his colleagueshad achieved.

It was easy to understand howelectrons move through a crystallinematerial but hard to understandhow they could efficiently movethrough an amorphous thin-filmzinc-tin oxide. I asked i f [ could usethe analogy that the atoms movedthrough the zinc-tin oxide amor-phous material in much the sameway that a figure skater createsintersecting circles to move aroundan ice rink. Wager thought that rnyanalogy worked if we viewed theskater as using his or her circularroute to transfer from pond to pond(atom to atom).

Wager and his team have nowidentified 28 inorganic oxide mate-

rial combinations that might be justas effective as or even more effec-tive than the zinc-tin oxide that theyhad first discovered. Each of the 28material combinations would havethe capability to produce transpar-ent, inexpensive, chemically stable,diff icul t to damage and even envi-ronmentally friendly materials thatcan he turned into electronic cir-cuits. Since the percentage in themix can vary, Wager says that thenew material possibilities are actu-ally inf ini te in number.

By building electronic circuitsusing these amorphous noncrystal-line materials, you can greatly re-duce production cost because youcould print the new materials on aplastic substrate at very low tem-peratures. These very inexpensiveelectronic circuits could lead to awhole new category of throw-awayelectronic devices.

The circuits could eventually beprinted using gravure, offset, inkjetand/or screen process printing.Since the surface of these amor-phous materials is very smooth, it is

possible to stack electronic circuitson top of each other by registeringa second, third or more printedlayer.

Soon, you may very well findyourself purchasing a nonbreakable,bendable computer screen that youcan roll or fold up to fit in yourpocket! To further explore this topic,Google "transparent electronics,""TTFT," "John Wager" or "amor-phous heavy-metal cation multicom-ponent oxides."

Recalling the Facts1. What rfo you see as the great-

est advantage of transparent elec-tronics?

2. Can you envision any negativeeffects or possible misuses of thistechnology?

3. What is it about this new mate-rial that makes i t possible for peopleto create invisible electronic cir-cuits?

4. The new class of materialsdescribed in this column share atleast five significant characteristics.Name them. <£>

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techdlrections * MAY 2006