Metalorganic Chemical Vapor Deposition and Investigation of AlGaInN Microstructure
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The group III-nitride family of semiconductor materials grown by metalorganic chemical vapor deposition (MOCVD) has had a dramatic impact on optoelectronics and high-frequency, high-power devices in recent years. The nitrides possess wide, direct transition band gaps ranging from 6.2 eV for AlN to 3.4 eV for GaN to 0.9 eV for InN which has allowed the development of LEDs and laser diodes in the blue and UV spectrum. Furthermore, the band gap, combined with a high electron saturation velocity and the piezoelectric properties of the (AlGaIn)N/GaN interface allow for the formation of a two-dimensional electron gas which can be utilized to produce high electron mobility transistors (HEMTs). To date most research has focused on AlGaN/GaN heterostructures, which have proven extremely useful for HEMTs but less than ideal for UV LEDs. The quaternary AlGaInN has emerged as an alternative to AlGaN that promises to further enhance HEMT performance and produce better quality UV LEDs. However, the addition of In to the alloy can be problematic because it is not completely soluble and tends to form clusters through spinodal decomposition. Studies of phase separation in AlGaInN have been made, but have focused on films composed mostly of Ga. Recently, HEMTs made using a high Al content AlGaInN layer have shown comparable performance to devices made using AlGaN, but no studies of phase separation have been done on these high Al content films. In this thesis, the installation of a MOCVD reactor at Texas State University and the modifications necessary to make it operational will be presented. Furthermore, the progress made in developing of processes to deposit A1GaN/GaN heterostructures will also be discussed. Finally, evidence of phase separation by spinodal decomposition in a sample of high aluminum content A1GaInN, provided by IQE, from a study by scanning transmission electron microscopy in collaboration with the Yacaman group at the University of Texas at San Antonio will be presented, and the results discussed within the context of the regular solution model.