Mass spectroscopic study of the cracking of trimethylgallium in a heated doser

Abstract

Trimethylgallium (TMGa) is a common source gas in GaAs film production by metalorganic chemical vapor deposition and atomic layer epitaxy. Recent studies have shown that TMGa is also a useful intermediate for chemical beam epitaxy (CBE). However, Suto et al. found that improved films are produced when the TMGa is ‘‘cracked’’ before it impinges on the substrate. The objective of this article is to examine the products of the cracking process. TMGa was passed through a heated tantalum or stainless steel tube at low pressure, and the products of the decomposition were measured with a mass spectrometer. It was found that TMGa is unreactive in a stainless steel and/or tantalum tube at 250 K. However, by 300 K there is significant attenuation of the parent TMGa peak in both tubes. The dimethylgallium (DMGa) disappears between 600 and 700 K, while the monomethylgallium (MMGa) contribution disappears between 650 and 800 K. The gallium signal is lost between 700 and 800 K. Calibration runs were performed to determine if the cracking pattern of TMGa changes with temperature. It was found that the cracking pattern of TMGa is essentially constant from 200 to 500 K. As a result, the changes in mass spectrometer signal observed in our work must be due to changes in the composition of the gas, and not due to changes in the cracking pattern of the various species in the mass spectrometer. Therefore, the conclusion of our study is that TMGa decomposes at room temperature on a stainless steel or tantalum tube, to yield DMGa. The DMGa decomposes between 500 and 700 K to yield MMGa. The MMGa decomposes between 700 and 800 K to yield gallium. The gallium signal is lost above 800 K, probably due to deposition of gallium inside the cracker. These results show that under UHV conditions, TMGa is much more reactive than previous investigators had supposed. In particular, the results indicate that special precautions must be taken to get TMGa into a vacuum system. If one puts TMGa through a standard doser at temperatures slightly above room temperature one will get significant DMGa even after days of passivation.