Papers by Author: Wei Jie Lu

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Authors: Wei Jie Lu, D.T. Shi, T.R. Crenshaw, A. Burger, W.E. Collins
Authors: H.C. Lin, Zhe Chuan Feng, Ming Song Chen, Z.X. Shen, Wei Jie Lu, W.E. Collins
Abstract: The phonon anisotropy property of the GaN wurtzite crystal is studied using angular dependent Raman spectroscopy both theoretically and experimentally. The polarized Raman scattering spectra were recorded from cross-sections of c-axis oriented GaN films as a function of the angle between the incident laser polarization direction and the film normal direction in three different configurations. The Raman intensity of A1(TO) showed a sinusoidal dependence on the rotating angle, as also did the E1(TO) mode, while the E2 mode has a quite different behavior. The theoretical fit takes into account the susceptibility contribution and the phase differential of different vibrating elements.
Authors: Zhe Chuan Feng, C.W. Huang, W.Y. Chang, Jie Zhao, Chin Che Tin, Wei Jie Lu, W.E. Collins
Abstract: We have performed a combined investigation of experiment and theory on the infrared reflectance from cubic SiC grown on Si by chemical vapor deposition. A damping behavior of the interference fringes away from the reststrahlen band and a dip or notch within the “flat top” are observed from some samples while they does not occur in high quality 3C-SiC/Si samples. The former is interpreted due to an interfacial transition layer existed between SiC-Si and a rough surface, while the latter can be demonstrated by a three-component effective medium model.
Authors: Wei Jie Lu, J.A. Michel, C.M. Lukehart, W.E. Collins, W.C. Mitchel
Abstract: Ohmic contacts on SiC have been investigated extensively in the past decade. However, the mechanism for ohmic contact formation has been a troublesome issue. The interfacial structures at the atomic scale responsible for forming ohmic contacts have not been revealed. Our previous results have shown that carbon can form ohmic contacts on SiC after thermal annealing, and that an interfacial carbon layer between Ni and the SiC improves the contacts significantly. In this study, we have investigated the interactions between Ni and carbon, and ohmic contact formation on SiC using x-ray diffraction (XRD) and Raman spectroscopy. After annealing, ohmic behavior was observed and Ni graphite intercalated compounds (GICs) were found on Ni/C/SiC structures. Unlike conventional graphite intercalated compounds, the Ni atoms substitute for carbon atoms in the graphitic networks in these Ni-GICs. XRD peaks at 21.6° due to the Ni graphitic intercalation compound (Ni-GIC) and at 26.3° due to graphite have been observed. The distance between graphitic sheets is 0.403nm in the Ni graphite intercalated compounds, whereas it is ~20% larger in the graphite. The thickness of the interfacial carbon layer does not affect the formation of Ni-GIC.
Authors: J. Chen, S.C. Lien, Y.C. Shin, Zhe Chuan Feng, C.H. Kuan, J.H. Zhao, J.H. Zhao, Wei Jie Lu
Abstract: The polytype structural variations of a set of SiC bulk wafers with different Nitrogen (N) doping levels, prepared by Physical Vapore Deposition (PVD), are studied. The initial growth conditions were used to produce 6H-polytype SiC, which has been approved for the undoped and lightly doped materials. However, when extreme high N-dopants were applied, the obtained wafer was found with 4H- and 15R-polytype features. Our experimental results of HR-TEM and Raman scattering have revealed clearly the polytype transformation, indicating that the inducement of N in the reactor leads to the polytype transformation of the resulted SiC crystal.
Authors: Wei Jie Lu, G.R. Landis, W.E. Collins, W.C. Mitchel
Abstract: Al based alloys, such as Ti/Al, are commonly used for ohmic contacts on p-type SiC. The interfacial structures of a metal alloy film on SiC are very complicated after annealing. Al is considered as the key element responsible for forming ohmic contacts on p-type SiC, and reacts with C from SiC and forms Al4C3 and Si during annealing. In this study, we have investigated ohmic contact formation of a single component Al4C3 film on p-type SiC. Based on the stoichiometric formation of Al4C3 between Al and C at high temperatures, several samples with various Al/C mole ratios have been examined for ohmic contact formation after different annealing temperatures. Carbon rich and stoichiometric Al4C3 films form ohmic contacts on p-type 4H-SiC (~2.8 x1018 cm-3 ) after annealing at 800 and 900°C. X-ray diffraction (XRD) data have shown that a single component Al4C3 is formed when an ohmic contact on p-type SiC is activated. Al/SiC, as the control sample, does not form ohmic contacts under the same conditions. This study reveals that Al4C3 can be responsible for forming ohmic contacts on p-type SiC. However, its chemical instability requires that the secondary metal is necessary to form stable ohmic contacts when Albased films are used.
Authors: Zhe Chuan Feng, J.W. Yu, J.B. Wang, R. Varatharajan, B. Nemeth, J. Nause, Ian T. Ferguson, Wei Jie Lu, W.E. Collins
Abstract: ZnO bulk crystal wafers, undoped and doped with various impurities of Ga, Er, Co, Ho, Fe, Mn, and co-doped Mg-Li, have been prepared by a modified melt growth method, and characterized by optical techniques of Raman scattering, photoluminescence and UV-visible transmission. Their wurtzite structures were confirmed, with a small degree of crystalline imperfection. It is shown that with some dopants, such as, Co and Fe, the electronic energy gap is affected much less than the optical absorption gap. Computer analysis has helped greatly in obtaining useful information of the optical properties of the ZnO bulk materials.
Authors: Wei Jie Lu, John Boeckl, W.C. Mitchel, J. Rigueur, W.E. Collins
Abstract: Carbon nanotubes (CNTs) grown on SiC are metal-free, well-aligned, and with low structural defects. In this study, CNT formation on SiC is examined in high vacuum (10-5torr) and ultra-high vacuum (10-8torr). Multi-wall carbon nanotubes and graphitic structures are the main products on the SiC surface at 1400-1800°C in 10-5torr. Under ultra-high vacuum, the decomposition rate of SiC is much lower than in high vacuum, indicating that SiC is decomposed by oxidation reaction. Using X-ray photoelectron spectroscopy (XPS), the intensity of the O1s peak at 530.3 eV decreases with increasing take-off angle, indicating that this oxygen species exists on the walls of CNTs. The results show that oxygen with a low pressure not only oxidizes SiC, but also forms a highly thermally stable carbon-oxygen compound, and interacts with the CNTs at high temperatures.
Authors: John Boeckl, W.C. Mitchel, Wei Jie Lu, J. Rigueur
Abstract: Aligned carbon nanotubes (CNT’s) are formed on the surface of silicon carbide (SiC) wafers during high temperature anneals. The exposed 4H SiC surface transforms into CNT’s for temperatures in the range of 1400-1700°C and under moderate vacuum conditions (10-2 – 10-5 torr). The rate of formation on the C-face (0001,‾) is about three times the rate on the Si-face (0001), but both rates increase with anneal temperature. SEM, TEM and Raman scattering measurements have confirmed the presence of both single-wall and multi-wall CNT’s. The carbon source is believed to be residual carbon from the SiC left on the surface after preferential evaporation of Si. CNT formation is believed to be catalyzed by low concentrations of residual oxygen in the chamber. Subsequent I-V measurements provide insight into the electrical characteristics of the CNT’s and the SiC/CNT interface.
Authors: John Boeckl, W.C. Mitchel, Edwina Clarke, Roland L. Barbosa, Wei Jie Lu
Abstract: Graphene growth on SiC in atmospheric pressure argon exhibits large terrace sizes and coverage over the entire substrate surface. Graphene growth and the resulting morphology are correlated with the characteristics of the growth chamber and the surface quality of the starting SiC substrate. Without in-situ surface preparation prior to growth, we observe “wrinkles” in the graphene surface. Graphitic-like disordered structures are formed at 1500°C while atomically flat graphene terraces are formed above 1600°C.
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