Scientists from the University of Minnesota in Minneapolis and from Vanderbilt University in Nashville and Oak Ridge National Laboratory, both in Tennessee, have achieved the room-temperature photodesorption of hydrogen from a Si(111) surface using 4.8-μm radiation from Vanderbilt’s free-electron laser. Although the mechanism underlying the excitation and breaking of the Si-H vibrational stretch mode has not yet been identified, they suggest that the phenomenon may enable the development of new materials and material structures.Philip I. Cohen, a professor of electrical and computer engineering at the University of Minnesota, explained that the typical photodesorption process is nonselective, involving the absorption and redistribution of the radiation and the unintended dissociation of various bonds. In pursuit of a means of performing low-temperature chemical vapor deposition of silicon, however, he and his colleagues had calculated that it should be possible to target only the Si-H bond on Si(111) using a sufficiently intense laser tuned to the proper wavelength in the infrared. Doing so, they proposed, might enable them to selectively prevent hydrogen passivation of the silicon substrate and to promote particular crystal growth modes in a process they call selectively enhanced adatom diffusion.The infrared source at Vanderbilt’s W.M. Keck Free-Electron Laser Center offered a means to confirm their predictions. Free-electron lasers subject a beam of electrons to a periodic magnetic field, causing the particles to emit radiation as they oscillate. The wavelength of the radiation is a function of the period of the field and the energy of the electrons in the beam, so the operating wavelength of the laser can be tuned by injecting electrons with the proper energy. Because its 25- to 45-MeV electrons offer 8-μs-long “macropulses” of 2.1- to 9.8-μm radiation with pulse energies of 100 mJ, the university’s free-electron laser offered the requisite high photon flux at the proper wavelength, said Leonard C. Feldman, a professor in the department of physics and astronomy at Vanderbilt.In their demonstration, the scientists monitored the temperature of hydrogen-passivated silicon under irradiation and the H2 desorption yield under various wavelengths. They detected no direct laser-induced heating of the silicon, which is transparent in the mid-IR. The H2 yield periodically peaked with the 30-Hz repetition rate of the free-electron laser, and it was found to be at a maximum under 4.8-μm irradiation, corresponding to the 0.26-eV Si-H vibrational stretch mode and in accordance with the researchers’ calculations.They also compared the yields from silicon coadsorbed with hydrogen and deuterium that they subjected to 4.8-μm irradiation and to high temperatures. The hydrogen/silicon and deuterium/silicon bond energies are different, Cohen explained, so the fact that photodesorption produced almost entirely H2 while thermal desorption produced mostly deuterium gas, some hydrogen deuteride and less H2 indicates that the mechanism underlying the photodesorption is not local heating.Although it is not yet clear how the irradiation works to break the Si-H bond, Cohen noted that the low-temperature growth it may enable would have many potential applications, including the deposition of semiconductor on flexible substrates or on integrated circuits that have delicate, pre-existing device structures. Any application would require a move to a less exotic source than a free-electron laser, and the team is consulting with the photonics industry to determine whether frequency-doubled CO2 lasers might offer the necessary performance.Multiphoton processesHe said that the investigators are working to rule out various multiphoton phenomena as they seek to understand the photodesorption process. They also hope to investigate whether silicon can be grown preferentially under laser irradiation using silane as the source gas.Science, May 19, 2006, pp. 1024-1026.