Materials Science: UD Chemist Proposes New Description Of Reactions For "Growing" Computer Chips

April 14, 1997

SAN FRANCISCO, CA--University of Delaware research might someday help computer companies "grow" next-generation semiconductors faster while also achieving greater control over material properties, chemist Douglas J. Doren reported April 14 during the American Chemical Society meeting.

Doren, an associate professor of chemistry and biochemistry at UD, notes that different chemical additives known as "dopants" alter the growth rate of silicon and silicon-germanium alloys, new materials promising faster computing speeds than traditional silicon wafers.

By understanding exactly how dopants alter the material's growth rate, he says, it may be possible to better control the electronic properties of films grown from chemical vapors at low temperatures.

Inside a chemical vapor deposition (CVD) chamber, hydrogen atoms in silicon-hydride precursor materials tend to slow the rate at which silicon is deposited on a surface. When hydrogen sticks to the surface of a substrate, Doren explains, it blocks silicon atoms from forming bonds, thereby slowing the deposition rate.

Semiconductor companies typically use heat to break hydrogen's hold on the substrate. Immediately after those bonds are broken, Doren says, electrons on the surface of the substrate are left in an "excited energy state." The energy required to reach this energy state depends on the dopants. If the material was doped with phosphorous, for instance, more energy is needed to excite these electrons, which slows the deposition rate. Doping with boron, however, will result in a lower energy state, which speeds deposition.

In summary, "We have developed circumstantial evidence to suggest that breaking these hydrogen bonds excites the motion of electrons, more than atoms," Doren says. "We therefore believe that a key barrier to the deposition reaction or growth rate for these alloys correlates with the final energy state. This explains why manipulating the phosphorous or boron content of the material controls the reaction rate."

Doren developed his theory after completing a review of existing experimental data in an effort to reconcile inconsistencies he describes as "apparent paradoxes." He then performed calculations on excited electronic states of silicon surfaces, to identify a mechanism for hydrogen desorption involving two different electronic states. Doren's resesarch has received support from the National Science Foundation.

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CONTACT: Ginger Pinholster / Beth Thomas
Office of Public Relations, University of Delaware
(302) 831-6408 (direct) or (302) 831-2791
(302) 435-6733 - Media Beeper
* During the ACS meeting, contact Ginger at (415) 394-1111 or send e-mail to

ACS presentation information - Douglas J. Doren:
Sheraton Palace, Mendocino Room, 2nd Floor
Monday, April 14, 11:20 a.m. (Pacific Time)

University of Delaware

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