Purity and Uv Absorption Cross Sections of TMA, TMG, and TMAs
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PURITY AND UV ABSORPTION CROSS SECTIONS OF TMA, TMG, and TMAs HIDEO OKABE AND M.K. EMADI-BABAKI Department of Chemistry and Materials Science Research Center of Excellence, Howard University, Washington, DC, 20059 Vapor pressures of commercial electronic grade TMA,.TMG and TMAs samples have been measured at various temperatures Only TMA and compared with those of known pure samples. The UV absorption spectrum showed the presence of impurities. of impure TMA shows toluene to be an impurity. The UV absorption cross sections of TMA, TMG and TMAs have been measured and tabulated at several wavelengths. INTRODUCTION The purity of source materials for metalorganic chemical vapor deposition(MOCVD) is an important factor for the fabrication of group III-V electronic materials with efficient light emitting and photo voltaic qualities, high electron mobilities and high temperature operating qualities[l]. While gas chromatography in combination with mass spectrometry(gc/ms) gives more thorough analysis of A much simpler impurities, it is not readily available. method of finding impurities is to measure the vapor pressure at various temperatures. If the observed pressure at a given temperature is higher than that of the known pressure of a pure compound, it indicates the presence of impurities. The pressure deviates more for impurities with higher vapor pressure than that of the pure compound. Vacuum distillation is an excellent way to purify small quantities of sample without decomposition by heat[2]. An impure sample can easily be purified by vacuum distillation at a temperature corresponding to about 1 torr of the pure compound. The purity of the sample can be monitored by measuring the vapor pressure after pumping for several Vapor pressures of three organometallic compounds, minutes. TMG(trimethylgallium), and TMAs TMA(trimethylaluminum), Only TMA showed the (trimethylarsenic), were measured. presence of an impurity. The UV absorption cross sections of three purified The cross sections provide samples were measured. important information on the extent of light absorption at a laser wavelength being used. The measured absorption cross section also can be used to measure amounts of organometallic compound useý in a reactor. The spectrum of impure TMA sample taken at -2 0C was used to identify the presence of toluene as an impurity. EXPERIMENTAL Commercial bubblers containing electronic grade organometallic compounds with a stated purity of 99.9999%, were obtained from Alfa(Morton Thiokol, Inc). They were attached to a glass vacuum system with a capacitance manometer(MKS Baratron Model 220BA, up to 10 mtorr), mechanical pressure gage(wallace & Tiernan, up to 800 torr). Mat. Res. Soc. Symp. Proc. Vol. 145. 01989 Materials Research Society
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Pressures of a sample was measured at various temperatures in All compounds are pyrophoric and the range from -20 to 30°C. TMAs was pumped off easily, but react violently with air. TMA, and TMG to a lesser extent, adsorbed strongly on walls. Consequently, the cell had to be evacuated
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