Dependence of the Decomposition of Trimethylaluminum on Oxygen Concentration

Abstract

Trimethylaluminum ͑TMA͒ is often used as a source gas for composite semiconductor or gate insulator films containing aluminum. However, TMA readily reacts with O 2 and this reaction causes film performance to degrade. Film formation is affected by the decomposition properties of the source gas, so it is important to investigate the influence of O 2 on the decomposition behavior of TMA. The starting decomposition temperature of TMA was 332°C in an Al 2 O 3 tube filled with Ar gas, and the decomposition rate increased rapidly above 380°C. However, when TMA was heated in an atmosphere containing more than 1 ppm O 2 , the temperature at which TMA began to decompose increased. It is assumed that this phenomenon resulted from the formation of methoxy groups through reaction between TMA and O 2 . Thus, the presence of O 2 in TMA not only caused the films to be contaminated with oxygen atoms but also altered the decomposition behavior of TMA. This means that fluctuations in the deposition rate and film performance are caused by the presence of O 2 in TMA when deposition conditions are otherwise kept constant. As a result, it is desirable that the O 2 concentration present in TMA is maintained below 0.1 ppm during deposition.Films that contain aluminum, such as the composite semiconductors AlGaAs, AlGaInP, and Al 2 O 3 , which act as a gate insulator for the composite semiconductors including GaAs and GaN, are often formed using metal-organic ͑MO͒ chemical vapor deposition or atomic layer deposition ͑ALD͒. Trimethylaluminum ͑TMA͒ is often used as an aluminum precursor in the formation of such films. Al 2 O 3 thin films and films containing Al 2 O 3 have been studied as a potential alternative gate dielectrics, and a plasma-enhanced ALD method using TMA and O 2 is one of the processes employed to fabricate these films. 1-3 This method is more effective than the ALD process using TMA and water because it proceeds rapidly at a low temperature and results in a little contamination of the film with impurities. However, TMA exhibits high reactivity with oxygen, and this is influenced by the purity of the TMA that is used in the formation of the composite semiconductor films such as AlGaAs. 4 Furthermore, there are some reports that the oxygen contamination in the AlGaAs films was increased with the tendency of flow rate of TMA at the low range of Al mole fraction, and the oxygen contamination in the films was increased rapidly at the high range of Al mole fraction by the reaction of TMA with O 2 or H 2 O in a process chamber. 5,6 So, high reactivity between TMA and O 2 is the problem to the contamination of oxygen in the composite semiconductor films.It has been reported that Al͑CH 3 ͒ 2 ͑OCH 3 ͒ generated by the reaction between O 2 and TMA is transported into the growth zone of the AlGaAs films and that this organic compound negatively affects the film properties by incorporating oxygen into the AlGaAs films. 7 Furthermore, there are some reports that decreasing the concentration of oxygen or Al͑CH 3 ͒ 2 ͑OCH 3 ͒ in TMA reduc...

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