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Crystal quality and electrical properties of p-type GaN thin film on Si(111) substrate by metal-organic chemical vapour deposition MOCVD

Wu G. M., Hsieh T. H.

Journal of Achievements in Materials and Manufacturing Engineering

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2007

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Vol. 24, nr 1

193-197

EN

Purpose: In this paper, p-GaN samples have been grown on silicon substrates under various processing conditions. The effects of growth temperature and thermal annealing on the crystal quality and strain were carefully investigated. The electrical properties such as hole concentration and mobility would be discussed. Design/methodology/approach: GaN-based III—V semiconductors have become promising materials for short-wavelength optoelectronic devices because of their large and direct band gap energies. In this paper, p-GaN has been grown by metal-organic chemical vapor deposition (MOCVD) at 900 , 950, 1000, and 1050 degrees centigrade with low temperature LT-deposited AlN/AlGaN buffer layer. Findings: The mobility was achieved at 150 square cm/Vs and the hole concentration was 8x10 to the 17 - cubic cm. SIMS and XRD were used to measure and explain the relationships between hole concentration and the growth temperature. When the growth temperature was increased to 1000 degrees centigrade, the hole concentration was increased by ten times. According to the experimental results, the optimal growth temperature was 1000 degrees centigrade. After the thermal annealing process at temperature 850 degrees centigrade for 2 minutes, the FWHM of p-GaN was lowered to 617 arcsec. The effects of growth temperature were explained in the two temperature regions. From 900 to 1000 degrees centigrade, the incorporation rate of Mg was slightly increased and the strain decreased with the growth temperature. Mg would provide holes and the lower strain would result in better crystal quality. The crystal quality and Mg concentration effects on hole concentration below 1000 degrees centigrade was thus beneficiary. On the other hand, when the growth temperature was further increased, the strain and FWHM increased while hole concentration decreased at 1050 degrees centigrade. At this high temperature, Si might become donor in GaN. Research limitations/implications: It was suggested that the hole concentration reduced at 1050 degrees centigrade due to the Si diffusion and the strain caused by Mg dopant. According to the experimental data, the optimal growth temperature was 1000 degrees centigrade. After the annealing process, the FWHM of p-GaN was lowered to 611 arcsec. Originality/value: Determination of crystal quality and electrical properties of p-type GaN thin film on Si(111) substrate by metal-organic chemical vapor deposition MOCVD.

2

Surface modification of PBO fiber by electrostatic discharge for composites

Wu G. M., Shyng Y. T.

Archives of Materials Science and Engineering

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2007

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Vol. 28, nr 12

722-728

EN

Purpose: PBO fibers provide great potential applications as reinforcement fibers for advanced composites due to the excellent thermal resistance and specific stiffness and strength. However, the interfacial adhesion between reinforcing fiber and polymer matrix in a composite system is a primary factor for the stress transfer from matrix to fiber. In this paper, the effects of surface treatment on the modification of PBO fiber and its composite materials have been investigated using electrostatic discharge under atmospheric pressure. The surface treatment process has been designed to improve fiber/matrix interfacial bonding quality while providing minimum alteration to the bulk characteristics of the reinforcement fiber. Design/methodology/approach: Both as-spun (AS) and high-modulus (HM) PBO fibers were surface treated and characterized in this study. The characterization techniques included scanning electron microscopy, MTS tensile tester, dynamic contact angle analysis system and microbond pull-out tests. Findings: The results showed that PBO fibers exhibited-10% reduction in tensile strength after the proposed treatment process. The AS fiber surface free energy could be increased from 49.90 mJ/m² to 65.42 mJ/m² (+31%) and the HM fiber surface free energy could be increased from 46.20 mJ/m² to 65.36 mJ/m² (+41%). The interfacial shear strength between PBO fiber and the epoxy matrix was improved to 41.6 MPa (+20%) for AS fiber system, and it improved to 40.1 MPa (+23%) for HM fiber system. The composite failure mode also shifted from fiber/matrix interface adhesive failure to partly cohesive failure. Research limitations/implications: The composite interfacial shear strength was improved through the increased surface free energy of PBO fiber. The more cohesive failure mode allowed more energy to be dissipated during failure. Originality/value: The proposed electrostatic discharge treatment process could improve the surface characteristics of PBO fiber and the applications in advanced composites.

3

Surface modification of PBO fiber by electrostatic discharge for composites

Wu G. M., Shyng Y. T.

Journal of Achievements in Materials and Manufacturing Engineering

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2006

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Vol. 17, nr 1-2

169--172

EN

Purpose: PBO fibers provide great potential applications as reinforcement fibers for advanced composites due to the excellent thermal resistance and specific stiffness and strength. However, the interfacial adhesion between reinforcing fiber and polymer matrix in a composite system is a primary factor for the stress transfer from matrix to fiber. In this paper, the effects of surface treatment on the modification of PBO fiber and its composite materials have been investigated using electrostatic discharge under atmospheric pressure. The surface treatment process has been designed to improve fiber/matrix interfacial bonding quality while providing minimum alteration to the bulk characteristics of the reinforcement fiber. Design/methodology/approach: Both as-spun (AS) and high-modulus (HM) PBO fibers were surface treated and characterized in this study. The characterization techniques included scanning electron microscopy, MTS tensile tester, dynamic contact angle analysis system and microbond pull-out tests. Findings: The results showed that PBO fibers exhibited -10% reduction in tensile strength after the proposed treatment process. The AS fiber surface free energy could be increased from 49.90 mJ/m2 to 65.42 mJ/m2(+31%) and the HM fiber surface free energy could be increased from 46.20 mJ/m2 to 65.36 mJ/m2 (+41%). The interfacial shear strength between PBO fiber and the epoxy matrix was improved to 41.6 MPa (+20%) for AS fiber system, and it improved to 40.1 MPa (+23%) for HM fiber system. The composite failure mode also shifted from fiber/matrix interface adhesive failure to partly cohesive failure. Research limitations/implications: The composite interfacial shear strength was improved through the increased surface free energy of PBO fiber. The more cohesive failure mode allowed more energy to be dissipated during failure. Originality/value: The proposed electrostatic discharge treatment process could improve the surface characteristics of PBO fiber and the applications in advanced composites.

4

Mechanical properties and thermogravimetric analysis of PBO thin films

Wu G. M., Hung C. H.

Journal of Achievements in Materials and Manufacturing Engineering

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2006

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Vol. 17, nr 1-2

27--32

EN

Purpose: In the continued quest for high performance, high temperature, light weight materials, the research and development of poly(1,4-phenylene-cis-benzobisoxazole) or PBO polymer was a significant step. The thermal stability, stiffness, and tensile strength surpass those of many other engineering polymers. In this report, the superior mechanical properties of PBO thin film materials and the thermal characteristics of the isotropic thin films were investigated. Thermogravimetric analysis (TGA) and the Ozawa method were employed to analyze the thermal degradation kinetic parameters. Design/methodology/approach: The quasi-isotropic thin films were processed from 14 wt-% PBO dope with the molecular weight about 130,000. Mini-tensile bar specimens of dog-bone-shape were prepared for MTS testing machine. The ASTM D1708 microtensile testing was carried out at room temperature. The TGA spectra were obtained by employing Thermal Analysis (TA) TA-2050 thermogravimetric analyzer under various heating rates in nitrogen. The Ozawa method has been used for the kinetic data analysis. Findings: The results showed that PBO thin film exhibited outstanding specific tensile properties and thermal resistance. The onset of the decomposition temperature in nitrogen atmosphere was about 670°C. The temperature that corresponded to the maximum decomposition rate was around 750°C. The residual weight was still as high as 73% when heated up to 850°C. The reaction followed a first order mechanism. In addition, the activation energy (Ea ) for PBO thin film material has been estimated to be 445 kJ/mol and the frequency factor (logA) to be 25.2 min-1. Practical implications: The better understanding in PBO thin film processing and characteristics could help to advance the structure design and unique applications. Originality/value: The outstanding specific mechanical properties and excellent thermal resistance should provide great potential applications for this new class of high performance rigid rod polymers.

5

Defect structures in InGaN/GaN multiple quantum wells on Si(111) substrates

Wu G. M., Kao Y. L.

Journal of Achievements in Materials and Manufacturing Engineering

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2006

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Vol. 18, nr 1-2

195--198

EN

Purpose: Nitrides are compound semiconductor nanomaterials that are suitable for use in light-emitting diodes. It has been desired to grow high quality gallium nitride crystal thin film on silicon substrates because silicon substrates have the advantages of low cost, large wafer size, and good electrical and thermal conductivity. However, the higher defect density can limit the industrial applications due to lower quantum efficiency. The purpose of this study has been to investigate the crystal defect structure within InGaN/GaN multiple quantum wells on Si(111) substrates. In addition, the variation in quantum well thickness was also explained by the selective area growth model. Design/methodology/approach: InGaN/GaN nano-structures were prepared by metal-organic chemical vapor phase epitaxy (MOVPE) using composite buffer layers. The crystal defect structures in the buried multiple quantum wells on both (0001) and {10-11} sidewalls were carefully studied by transmission electron microscopy. Previous studies on sapphire substrates have been compared and discussed. Findings: The V defect structures have been found in InGaN/GaN multiple quantum wells on Si(111) substrates. A simplified structural model with increasing barrier thickness has been reported. The barrier thickness increased on both (0001) and {10-11} facets along thin film growth. A decreased fill factor based on the selective area growth model was proposed. In addition, the average thin film growth rate was found to be four times higher along (0001) than that along {10-11} facet. As the number of multiple quantum wells increased, the barier thickness increasing was also intensified. Research limitations/implications: The understanding in defect structure could help to modify the processing and design parameters. Originality/value: The V-defect structure and model were reported for the first time using silicon substrates. The different growth rates along defect structures were quantified. High quality gallium nitride crystal could be manufactured along with better substrate design.