The mechanism of laser-assisted CVD of germanium

Abstract

Previous workers have shown that germane does not decompose below 700 K on silica substrates. However, if the germane is irradiated with an ArF or KrF laser, germanium films will grow at temperatures down to 610 K. In the work reported here, the role of the laser in the growth process was explored. First, the species impinging on the substrate during ArF laser-assisted chemical vapor deposition (LCVD) were analyzed with a mass spectrometer. The analysis showed that when germane is irradiated with an ArF laser under growth conditions, digermane is formed in the gas phase. The digermane then impinges on the substrate and pyrolyses. Many other species have been reported previously to be formed transiently during the laser pulse. However, we were not able to detect any other species with a high enough flux onto the substrate to condense at an appreciable rate. Next, temperature programmed decomposition was used to see if photochemical decomposition of chemisorbed germane played an important role in film growth. It was found that a laser dose comparable to that needed for film growth did not produce an appreciable change in the decomposition spectra. Lastly, film growth studies were used to examine the role of the digermane formed by the laser in the film growth process. It was found that a mixture of germane and digermane will decompose on a silica substrate down to 580 K to produce a germanium film. The growth characteristics of the germane/digermane mixtures were virtually the same as those seen in LCVD. Thus, it appears that under the conditions used previously for LCVD, the main role of the ArF laser is to convert some of the germane source gas to digermane. It also has been found that under other conditions, one can grow germanium films down to at least 300 K via digermane photolysis.