Browsing by Author "Nadia A. El-Masry, Committee Member"

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Growth of GaN from Elemental Gallium and Ammonia via a Modified Sandwich Growth Technique
(2005-01-07) Berkman, Elif; Robert M. Kolbas, Committee Member; Nadia A. El-Masry, Committee Member; Raoul Schlesser, Committee Member; Zlatko Sitar, Committee Chair
Gallium nitride (GaN) thin films were grown on (0001) sapphire substrates at 1050°C by controlled evaporation of gallium (Ga) metal and reaction with ammonia NH3. The feasibility of the growth process was demonstrated and discussed. One of the biggest challenges of working in the Ga–NH3 system was the instability of molten Ga under NH3 atmosphere at elevated temperatures, especially between 1100–1200°C. In the first part of the study, transport of Ga species from the source-to-substrate during the GaN growth process and the influence of ammonia—liquid Ga reaction on Ga transport were investigated. Experimental results under different conditions were studied and compared to theoretical predictions to quantify the mechanism of transport in the vapor growth technique. In presence of NH3, Ga transport far exceeded the predicted upper limit for the vapor phase transport. Visual observations confirmed that a significant amount of Ga left the source in a cluster rather than atomic form. A novel Ga source design was employed in an effort to obtain a stable and high vapor phase transport of Ga species at moderate temperatures. In this design, pure N2 was flowed directly above the molten Ga source. This flow prevented the direct contact and reaction between the molten Ga and NH3 and prevented Ga spattering and GaN crust formation on the source surface. At the same time, it significantly enhanced Ga evaporation rate and enabled control of Ga transport and V/III ratio in the system. Growth characteristics were described by a mass transport model based on process parameters and experimentally verified. The results showed that the process was mass transport limited and the maximum growth rate was controlled by transport of both Ga and reactive ammonia species to the substrate surface. A growth rate of 1.4 μm/h was obtained at 1050C, 800 Torr, 3 slm of ammonia flow rate, and 1250C Ga source temperature at a 24 mm source-to-substrate distance. It was found that the process required a more effective supply of active NH3 to the substrate in order to increase the crystal quality and growth rate. The surface morphology of the deposited layers was examined by optical and scanning electron microscopies. XRD analysis was used to determine the crystallinity of deposited films and revealed a full-width at half-maximum (FWHM) of 0.6 deg. for the (0002) GaN peak. EDX analysis was employed for the chemical characterization of the samples and showed that the deposited material contained only Ga an N elements. Room temperature PL spectrum demonstrated the optical quality of the grown samples.
INTERFACES IN NOVEL ELECTRONIC MATERIALS
(2008-01-10) Liu, Fude; Nadia A. El-Masry, Committee Member; Carl Osburn, Committee Member; Robert Nemanich, Committee Co-Chair; Gerd Duscher, Committee Chair
Reliable Local Strain Characterization in Si/SiGe Based Electronic Materials System
(2007-12-21) Zhao, Wenjun; Gerd Duscher, Committee Chair; George Rozgonyi, Committee Co-Chair; Robert J. Nemanich, Committee Member; Nadia A. El-Masry, Committee Member
In this research we first developed a procedure to determine the strain in a TEM sample. This procedure includes HOLZ line detection from a Convergent beam electron diffraction (CBED) pattern, kinematic calculation of high order Laue zone (HOLZ) line position and searching lattice parameters by χ2 minimization. With only CBED technique, strain measurement on the strained Si layer is not possible in a blanket strained Si⁄SiGe structure due to HOLZ line splitting (deformation). For sub-100nm short channel SiGe CMOS device structures strain could be determined in the center of the channel. We demonstrated the CBED strain measurement can be implemented in new generation short channel technology node with a nano meter spatial resolution and high accurate. For the first time, we developed a new approach combined with CBED and finite element (FE) modeling and quantitatively investigated the correlation of the strain in a thin TEM sample with that in the bulk. The new method successfully determined the strain in the strained Si layer on a blanket strained Si/SiGe wafer, in a good agreement with other measurements. The new results also gave some insight in strain relaxation in a TEM sample. We found the [-1,-1,0] strain component which is perpendicular to the TEM sample thinning direction stays the same in the TEM sample and in the bulk, while the [001]) strain component is relaxed because it is along the same direction as the TEM sample thinning direction. This relaxation causes the deformation of the TEM foil and HOLZ line splitting. Therefore a clear CBED pattern can not be obtained from a TEM sample with a biaxial stain state. Our findings from a recessed SiGe PMOS test structure with a uniaxial compressive strain showed a different strain redistribution behavior. The data showed that the εx [-1,1,0] strain is actually more than 20% higher in a TEM sample than in the bulk. The εy [-1,-1,0] strain which is parallel to the TEM sample thinning direction turns to tensile in the TEM sample due to the loss of constraints, while it is zero in the bulk. The new results can explain our experimental data and others' (which could not be explained before) and are consistent with UV Raman measurements.