Structure of AlN on Si (111) Deposited with Metal Organic Vapor Phase Epitaxy
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th on Si (111) was carried out using metal organic vapor phase epitaxy (MOVPE). In MOVPE growth, the metal organic precursor, trimethyl aluminum, and ammonia are carried to the growth surface in a hydrogen carrier gas. The precursors undergo a series of chemical reactions, which can take place both in the gas stream and on the substrate. These reactions result in a growth surface possessing a distribution of organic compounds and deposited film species. On the other hand, the UHV-based techniques only deliver the specific inorganic film species. MOVPE films could therefore have different material properties due to the modified surface kinetics. A possible limitation in the MOVPE process is the purity of the gas sources. Impurities may be present in the reactor from a variety of sources. Contaminants in the source materials, reaction by-products and real as well as ‘virtual’ leaks can all lead to unwanted chemical impurities in the growing film. Virtual leaks are the slow release of impurities, such as water, absorbed on internal reactor and gas line surfaces. The virtual leaks are largely due to air exposure during installation or sample exchange. These impurity sources contain notably different contaminants. Most of the air is quickly removed, but the surface adsorbed water tends to persist in the system for extended periods. The gas source-based impurities are metal alkoxides and reaction byproducts from the decomposition of the metalorganic specie. These contaminants are complex metalorganic molecules that may decompose and yield C impurities in the growing film. During MOVPE growth of most III-V materials, the requirements on the oxygen or water content in the reactor are relaxed because of the volatility of III-V oxides within a H2 ambient at high temperatures. Alternatively, Si readily forms surface oxides in the presence of trace amounts of oxygen or water, a process that was studied by Ghidini and Smith [6]. Higher temperatures were found to promote oxide desorption. At the high temperature of 1000°C, the partial pressure of water within the reactor must be less than 10-4 Torr to have an oxide-free Si surface. This constraint is a significant practical limitation to growth upon Si substrates. AlN growth was studied here at growth temperatures between 825 and 1175°C. Under these conditions, the maximum allowable water vapor pressure in the reactor tolerated before surface silicon oxides will form range from 5x10-7 to 5x10-2 Torr. The Si substrates initially received a brief dilute HF dip prior to sample loading. A 2.5 minute anneal at 1175°C in hydrogen was performed to initiate each sample with hopefully identical oxide-free Si surfaces. Growth was carried out for 30 min. with a V/III ratio of 10000 at a reactor pressure of 76 Torr, which resulted in films 200 to 300 nm in thickness. To probe the influence of reactor chemistry, an additional sample was grown at 1125°C with the ammonia flow rate doubled. The samples were analyzed for chemical, structural, and surface defects, using ex situ reflection high ener
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