Mechanisms of laser interaction with metal carbonyls adsorbed on Si(111)7×7: Thermal vs photoelectronic effects

Abstract

Ultrahigh vacuum studies of the interaction of 514 nm radiation from a cw Ar ion laser and its second harmonic at 257 nm with mono- and multilayer coverages of Mo(CO)6, W(CO)6, and Fe(CO)5 adsorbed on Si(111)7×7 at 90 K using thermal desorption spectroscopy (TDS), laser induced desorption spectroscopy, high resolution electron energy loss spectroscopy (HREELS), and Auger electron spectroscopy were performed. A model for the temperature rise of the sample due to cw laser heating is developed. By directly measuring the substrate temperature, these experiments were able to distinguish between photoelectronic and thermal effects active in the decomposition and desorption mechanisms of the adsorbed carbonyls. Results from TDS and HREELS show that Mo(CO)6 and W(CO)6 are molecularly adsorbed, while Fe(CO)5 partially dissociates upon adsorption. The decomposition of adsorbed Mo(CO)6 is caused by electronic excitation due to direct absorption of the 257 nm radiation. Irradiation with 514 nm radiation results in no photochemistry. The same mechanism is dominant for adsorbed W(CO)6 and Fe(CO)5; however, new excitation mechanisms are available to these molecules that lead to bonding changes in W(CO)6 and Fe(CO)5 with 514 nm irradiation. The photodecomposition products of the adsorbed carbonyls are found to be different from the gas-phase decomposition products. The surface stabilizes the adsorbed carbonyls, preventing complete removal of all the CO ligands. Desorption of CO due to photoelectronic excitation is found to occur via sequential single photon absorption and extraction of CO ligands. Evidence of clustering of carbonyl fragments was observed after 257 nm irradiation.

Formation of iron nano-dot arrays by electron beam-induced deposition using an ultrahigh vacuum transmission electron microscope

“…The main reaction took place here is the dissociation of the adsorbed carbonyl molecules or those in gas phase traveling near the target on the substrate. Once the first CO ligand is removed, the subcarbonyls (Fe(CO) x , x ¼ 124) rather interact with the substrate due to CO bridging between the Fe atoms, and resulted in the fast loss of the remaining ligands [12]. The process then can be simply written as FeðCOÞ 5ðg; adsÞ !…”

“…The vapour of iron (Fe) pentacarbonyl is utilized as a precursor. The initial stage of Fe chemical vapour deposition (CVD) in the UHV environment has been investigated vigorously [11][12][13][14]. The mechanism of pyrolysis, photolysis, and electron beam desorption has been clarified systematically and important physical factors have been determined.…”

Mechanistic studies of the thermal decomposition of metal carbonyls on Ni(100) surfaces in connection with chemical vapor deposition processes

“…Both electron and photon excitation have been used to induce metal film deposition from metal carbonyls in those cases. 8,10,12,[17][18][19][20][21][22][23][24]40 Once a nucleation step is triggered, however, steady state deposition of additional material on the nucleation sites is expected to be easy in all systems. Figures 11 and 13 show that metal films can be grown in a steady state manner as long as the substrate temperature is kept above 300 K. Unfortunately, the thermal deposition of metals by using metal carbonyls will still not be practical as long as the codeposition of carbon and oxygen in the growing films remains a problem.…”

Infrared and ultraviolet laser‐assisted deposition from iron pentacarbonyl

“…Using in situ Auger electron spectroscopy, the species occurring during pyrolytic and UV photolytic decomposition of Fe͑CO͒ 5 on Si ͑100͒ were identified. 23 The laser interaction with monoand multilayer coverage of carbonyls, adsorbed on Si ͑111͒ 7ϫ7 at 90 K, were studied by thermal and laser desorption spectroscopy; 22 the results show that Fe͑CO͒ 5 partially dissociates on absorption and that the surface stabilizes the fragments. The decomposition of Fe͑CO͒ 5 adsorbed at 120 K on Ag ͑110͒ and single-crystal Al 2 O 3 has been monitored by mass-spectrometric detection 21 of released CO. Unstable Fe͑CO͒ 4 intermediate species appear in the photolysis of Fe͑CO͒ 5 adsorbed on a Pt surface, while on alumina insulating surfaces mainly Fe 2 ͑CO͒ 5 is produced.…”

“…17,18 The UV spectrum of gas-phase iron pentacarbonyl exhibits a charge-transfer band ͑maximum near 200 nm͒, a secondary maximum at 280 nm ͑assigned to a metal-ligand charge-transfer band͒, and a weaker absorption feature 21 Different techniques have used other radiation wavelengths to photolyse Fe͑CO͒ 5 , such as 514-and 257-nm ͑second harmonic͒ emission from the Ar ion laser, 22 337-nm emission from the N 2 laser, 21 the broadband UV radiation of mercury lamps, 23 and UV synchrotron orbital radiation. 24 A field of recent interest is the CO 2 -laser pyrolysis of Fe͑CO͒ 5 for the production of ultrafine ␥-iron particles, 25-27 by a method using the sensitized decomposition of the carbonyl molecules.…”