INTERFACES IN NOVEL ELECTRONIC MATERIALS

by Liu, Fude

LIU, FUDE. Interfaces in Novel Electronic Materials. (Under the direction of Dr. Gerd Duscher.) Si-based devices have been scaling down over a forty year time span, following Moore?s law. However, materials are now an important constraint. One of the most serious problems is now the field effect transistor (FET) gate dielectric. The thickness of the gate SiO2 layer presently is becoming so thin that gate current leakage becomes a problem due to the tunneling effect. So it is desirable to find high-K dielectrics to replace SiO2 so that a physically thicker gate dielectric can be used and then the tunneling effect can be reduced or avoided. Very recently, more interest has been shown in the La-based material system. In this study, a main part of effort was put on amorphous LaScO3 and La2O3/SiO2 alloys grown on Si (001) substrate, using a lot of TEM techniques. We have demonstrated that amorphous LaScO3 (?3.5 nm) can be deposited directly on Si substrate as a gate dielectric for CMOS devices. However, oxygen diffusion through the thin LaScO3±x film and reaction with Si substrate increased the EOT and the interface roughness at 1000 oC. So possible oxygen sources at 1000 oC need to be eliminated. As to the high-K La-silicate layer on Si, we also studied the interface structure and the chemical composition in detail. It was shown that the high-K La-silicate capped with UHV-W is a good choice for the next generation gate dielectric. Excess oxygen caused the TaN grain growth and then the rough interface between the high-K layer and the TaN layer, and gave rise to the SiOx layer between the high-K layer and the Si substrate. The chemical changes across the high-K layer were well determined. All these results give us a deep understanding about the promising La-based oxides as the next generation gate dielectric. Compared to the mature Si industry, III-nitrides had been long regarded as a scientific curiosity. They have now earned a most respected place in modern electronic and optoelectronic devices. Unlike silicon and other traditional materials such as GaAs, IIInitrides are particularly suitable for high-frequency, high-power and high-temperature applications. Unfortunately, the choice of appropriate substrate materials for III-nitrides is still one of the biggest issues to be solved. Sapphire offers a compromise as the most widely used substrate material to date. And much remains to be known about the interface structures and GaN inversion domain boundaries (IDBs). In this study, we tried to determine these interface structures and GaN-IDBs. To the GaN films on c-sapphire with a low temperature AlN (LT-AlN) nucleation layer, the 7.5:8.5 theory about misfit edge dislocations of LT-AlN on c-sapphire was confirmed. The theoretical threading dislocation density in AlN was derived to be 8.48 x 1013/cm2. The direct polarity determination of GaN was successfully realized for the first time. The interface atomic structure of LT-AlN on c-sapphire was determined. For the first time, we experimentally explained why LT-AlN layers can invert the polarity of GaN. The exact 3-D geometry of AlN pits was determined for the first time. The GaN IDBs were also studied in detail. A transition region with mixed polarities was found with the convergent beam electron diffraction (CBED) method. The transition region was related to the not well defined edge of the LT-AlN. As to the interface of GaN on csapphire, the 7.2:6.2 theory was confirmed. The dislocation loops were observed experimentally. The chemical reaction between GaN and sapphire at high temperature was identified. The interface structure of GaN on sapphire was determined for the first time. Also for the first time, we experimentally explained the N-face polarity of GaN directly grown on sapphire. Finally, a more accurate GaN IDB was proposed.