Atomic layer processing such as atomic layer deposition (ALD) and thermal atomic layer etching (ALE) is usually described in terms of sequential, self-limiting surface reactions. This picture for ALD and thermal ALE leaves out the possibility that the metal precursor in ALD and thermal ALE can also convert the surface material to another new material. This perspective introduces the previous evidence for conversion reactions in atomic layer processing based on a variety of studies, including Al₂O₃ ALD on ZnO, growth of Zn(O,S) alloys, “self-cleaning” of III-V semiconductor surfaces, and thermal ALE of ZnO and SiO₂. The paper then focuses on the reaction of Al(CH₃)₃ [trimethylaluminum (TMA)] on ZnO as a model conversion system. A variety of techniques are utilized to monitor ZnO conversion to Al₂O₃ using TMA at 150 °C. These techniques include FTIR spectroscopy, quadrupole mass spectrometry (QMS), x-ray reflectivity (XRR), gravimetric analysis, x-ray photoelectron spectroscopy (XPS), and quartz crystal microbalance (QCM) measurements. The various studies focus on ZnO conversion to Al₂O₃ for both hydroxyl-terminated and ethyl-terminated ZnO substrates. FTIR studies observed the conversion of ZnO to Al₂O₃ and provided evidence that the conversion is self-limiting at higher TMA exposures. QMS studies identified the volatile reaction products during the TMA reaction with ZnO as CH₄, C₂H₄, C₂H₆, and Zn(CH₃)₂. The CH₄ reaction product preceded the appearance of the Zn(CH₃)₂ reaction product. XRR investigations determined that the thickness of the Al₂O₃ conversion layer on ZnO limits at ∼1.0 nm at 150 °C after larger TMA exposures. A gravimetric analysis of the conversion reaction on ZnO nanoparticles with a diameter of 10 nm displayed a percent mass loss of ∼49%. This mass loss is consistent with an Al₂O₃ shell of ∼1 nm on a ZnO core with a diameter of ∼6 nm. XPS studies revealed that ZnO ALD films with a thickness of 2 nm were almost completely converted to Al₂O₃ by large TMA exposures at 150 °C. QCM investigations then measured the mass changes for lower TMA exposures on hydroxyl-terminated and ethyl-terminated ZnO films. More mass loss was observed on ethyl-terminated ZnO films compared with hydroxyl-terminated films, because TMA does not have the possibility of reacting with hydroxyl groups on ethyl-terminated ZnO films. The mass losses also increased progressively with temperatures ranging from 100 to 225 °C on both hydroxyl-terminated and ethyl-terminated ZnO films. The perspective concludes with a discussion of the generality of conversion reactions in atomic layer processing.

中文翻译：

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原子层加工中的转化反应，重点是三甲基铝将ZnO转化为Al2O3

原子层处理（例如原子层沉积（ALD）和热原子层蚀刻（ALE））通常是按照顺序的，自限性的表面反应来描述的。这张有关ALD和热ALE的图片排除了ALD和热ALE中的金属前体也可以将表面材料转换为另一种新材料的可能性。该观点介绍了基于各种研究的原子层加工中转化反应的先前证据，这些研究包括ZnO上的Al ₂ O ₃ ALD，Zn（O，S）合金的生长，III-V半导体表面的“自清洁”以及ZnO和SiO _2的热ALE 。然后，论文重点讨论了Al（CH ₃）₃的反应。ZnO上的[三甲基铝（TMA）]作为模型转换系统。使用各种技术在150°C下使用TMA监测ZnO向Al ₂ O _3的转化。这些技术包括FTIR光谱，四极质谱（QMS），X射线反射率（XRR），重量分析，X射线光电子能谱（XPS）和石英晶体微量天平（QCM）测量。各种研究集中于羟基封端的和乙基封端的ZnO基板的ZnO转化为Al ₂ O ₃。FTIR研究观察到ZnO向Al ₂ O ₃的转化并提供证据表明，在更高的TMA暴露量下，这种转化是自我限制的。QMS研究确定了TMA与ZnO分别为CH ₄，C ₂ H ₄，C ₂ H ₆和Zn（CH ₃）_2时的挥发性反应产物。CH ₄反应产物先于Zn（CH ₃）₂反应产物出现。XRR研究确定了Al ₂ O ₃的厚度较大的TMA暴露后，在150°C下ZnO上的ZnO转换层限制在约1.0 nm。对直径为10 nm的ZnO纳米颗粒的转化反应的重量分析显示质量损失百分比为〜49％。这种质量损失与直径约6 nm的ZnO芯上约1 nm的Al ₂ O ₃壳层一致。XPS研究表明，厚度为2 nm的ZnO ALD膜几乎完全转化为Al ₂ O _3。在150°C下大量TMA暴露。然后，QCM研究人员测量了羟基封端和乙基封端的ZnO膜上较低TMA暴露的质量变化。与羟基封端的薄膜相比，在乙基封端的ZnO薄膜上观察到更多的质量损失，因为TMA不可能与乙基封端的ZnO薄膜上的羟基发生反应。在羟基封端和乙基封端的ZnO薄膜上，质量损失也随着温度从100到225°C逐渐增加。该观点最后讨论了原子层处理中转化反应的一般性。