Nathaniel R. Quick

Laser synthesis of carbon-rich SiC nanoribbons

A nanosecond pulsed laser direct-write and doping (LDWD) technique is used for the fabrication of carbon-rich silicon carbide nanoribbons heterostructure in a single crystal 4H–SiC wafer. Characterization by high-resolution transmission electron microscope and selected area electron diffraction pattern revealed the presence of nanosize crystalline ribbons with hexagonal graphite structure in the heat-affected zone below the decomposition temperature isotherm in the SiC epilayer. The nanoribbons exist in three layers each being approximately 50–60 nm thick, containing 15–17 individual sheets. The layers are self-aligned on the (0001) plane of the SiC epilayer with their c axis at 87° to the incident laser beam. The LDWD technique permits synthesis of heterostructured nanoribbons in a single step without additional material or catalyst, and effectively eliminates the need for nanostructure handling and transferring processes.

Laser conversion of electrical properties for silicon carbide device applications

A direct conversion technique has been demonstrated to produce highly conductive tracks on silicon carbide by irradiating it with a laser beam. It is found that laser irradiation of insulating silicon carbide substrates decreases its resistivity from 1011 to 10−4 Ω cm. Scanning electron microscopy of laser-irradiated α-silicon carbide substrate reveals dispersed globules on the irradiated track. The atomic force microscopic images of the tracks indicate the conversion of larger structures into smaller, more round-shaped structures suggesting the formation of globules. However, laser irradiation of silicon carbide conductors in the presence of pure oxygen transforms the conducting track into an insulator. The effect of annealing on the electrical properties of the laser-generated conducting tracks is also examined. This technique provides a means of directly writing conducting and insulating tracks on silicon carbide to produce electronic devices for high temperature applications.

SiC-based optical interferometry at high pressures and temperatures for pressure and chemical sensing

Crystalline silicon carbide is a chemically inert wide band gap semiconductor with good mechanical strength and oxidation-resistant properties at elevated temperatures, which make it a good sensor material for harsh environments such as combustion chambers and turbine systems. For such cases, optical sensors are generally superior to electrical sensors in many aspects such as responsivity, detectivity, and sensitivity. This paper presents a wireless technique for pressure and chemical sensing based on the pressure-and temperature-dependent refractive indices of silicon carbide. A helium-neon laser with a wavelength of 632.8nm was used as a probe laser to obtain the complementary Airy pattern of the laser power reflected off a silicon carbide wafer segment at high temperatures (up to 300°C) and pressures (up to 400psi). The interference patterns revealed unique characteristics for nitrogen and argon test gases. This pattern is different at the same pressure and temperature for the two gases, indicating the...

Temperature-dependent optical properties of silicon carbide for wireless temperature sensors

The temperature-dependent optical properties of materials, such as refractive index and reflectivity, can be used for remote sensing of temperatures using a laser beam. Wireless sensing mechanism eliminates the need for external electrical contacts to the sensor, typically required in electrical property-based sensors. Such contacts tend to melt at elevated temperatures. This paper investigates the optical response of silicon carbide to temperature changes, indicating the potential for realizing silicon carbide-based high temperature wireless sensors by using a helium?neon laser of wavelength 632.8?nm. The laser power reflected by single crystal 6H?silicon carbide substrates has been measured at different temperatures up to 750??C. The reflected powers exhibited oscillatory patterns caused by multiple beam interference for which the substrate itself acts as a Fabry?P?rot etalon. These results were used to calculate the refractive index, reflectivity and thermo-optic coefficients of silicon carbide as a function of temperature. Two samples with different dopant profiles were investigated to examine the effects of dopants on the optical response of silicon carbide substrates. With proper modification of the microstructure in the substrate, the temperature dependence of reflectivity can be used directly to measure temperature remotely.

Modelling of microvia drilling with a Nd : YAG laser

Drilling of microvias with Nd : YAG laser of wavelength 1.06 µm is studied for polymer materials used in high density electronic packaging applications. Generally thermal ablation is the dominant mechanism for such polymer materials in the nanosecond and microsecond laser pulse regimes. A transient thermal model is developed to describe the drilling process. Since a fraction of the laser energy is absorbed inside the polymer layer, volumetric heating occurs and an overheated small metastable region can form inside the layer. Thus the drilling mechanism involves thermal phase change, chemical decomposition and possibly explosion due to rapid thermal expansion and vaporization inside the polymer material. The volumetric heating can cause large thermal stresses inside the material, resulting in convex forms at the free surface of the substrate. Experimental results show the formation of such convex surfaces during laser irradiation. These surfaces eventually rupture and the material is removed explosively due to high internal pressures.

Laser doping of chromium in 6H-SiC for white light emitting diodes

Laser doping has been utilized for fabricating white light emitting diodes with 6H-SiC wafers. The emission of different colors to ultimately generate white light is tailored on the basis of donor acceptor pair (DAP) recombination mechanism for luminescence. Chromium (Cr), which is an unconventional dopant that produces multiple acceptor sites per atom, was incorporated into SiC and conventional dopants such as aluminum (Al) and nitrogen (N) were also laser-doped to produce acceptor and donors states, respectively. A p−n junction was fabricated with these dopants and an electroluminescent broad spectrum (400–850 nm) corresponding to white light was observed. This white light is a result of the combination of red, green, and blue lights formed due to DAP recombination between Al–N (blue, 460–498 nm), Cr–N (green, 521–575 nm) and additional other impurity state transitions (red, 698–738 nm).

Laser direct writing and doping of diamond-like carbon, polycrystalline diamond, and single crystal silicon carbide

Nanosecond pulsed Nd:yttrium–aluminum–garnet laser treatment is applied to alter the electric properties of free standing polycrystalline diamond substrates, diamond-like carbon (DLC), and 4H–SiC single crystal wafers. In the case of DLC samples, the laser irradiation, nitrogen doping, as well as the incorporation of cobalt into the DLC layer reduces its electric resistance. Laser fabrication of Schottky barrier diode at the DLC–cobalt contact is demonstrated, and its nonlinear 1/C2 vs V curve indicates a nonuniform dopant distribution. The nitrogen dopant profile in the laser-doped SiC wafer is obtained by secondary ion mass spectroscopy and the corresponding nitrogen diffusion coefficient under laser processing parameters is calculated. Laser doping enhances the nitrogen diffusivity in SiC and allows in situ fabrication of metal contacts with no additional materials. Scanning electron microscopy, wavelength dispersive spectroscopy, and x-ray photoelectron spectroscopy are used to study the surface compo...

Structural evolution and drawability in laser dieless drawing of fine nickel wires

Abstract Drawability of Nickel 200 wires in laser dieless drawing was investigated. Influencing factors under consideration include the laser power, the heat-treatment state (as-drawn or annealed), and the initial wire diameter. Microstructural evolutions in laser dieless drawing were studied by scanning electron microscopy (SEM). The wires exhibit optimal drawability at an intermediate laser power range corresponding to the wire temperature in the range of 1000–1300 K. The as-drawn precursor wire has better drawability than that of the annealed wire. The drawability decreases as the precursor wire diameter deceases. Microcrystalline structures were found in nickel 200 wires after being laser-drawn from as-drawn precursor wires. These experimental observations are explained using the concepts of dynamic recovery and recrystallization.

Laser microprocessing of wide-bandgap materials

Laser direct-write and doping technique (LDWD) is used to introduce variations in electric properties of wide band gap materials such as SiC and diamond. Conductive, p-type doped, n-type doped and insulative tracks are created on different diamond and SiC substrates using this method. The effects of various processing parameters such as laser-matter interaction time, number of repeated exposures, and type of irradiation environment are investigated. SEM, SIMS, XPS and Raman spectroscopy are used to study the effect of laser irradiation on the microstructure, chemical binding and to obtain dopant depth profile in the substrates, respectively. LDWD technique proved to enhance the dopant (nitrogen) diffusivity into SiC resulted in a diffusion coefficient (available in paper)that is four orders of magnitudes faster than the reported value (5 x 10-12 cm2s-1). Process modeling is conducted to study the atomistic of laser-doping process and to utilize laser irradiation to increase both dopant penetration and concentration. Laser doping of nitrogen alters the Raman spectrum of the 4H-SiC suggesting that Raman spectroscopy can be used as a non-contact method to characterize the laser-doped SiC.

One-dimensional transient analysis of volumetric heating for laser drilling

Generally laser energy is considered to interact only with the substrate surface, as in metals, where the laser beam does not propagate into the substrate beyond a very small absorption depth. There are, however, many instances, particularly for ceramics and polymers, where the laser beam can penetrate into the substrate to substantial depths depending on the laser wavelength and laser-material interaction characteristics. Specifically there are polymeric dielectrics used as multilayer electronic substrates in which a laser beam of wavelength 9.3μm can penetrate into the substrate. The laser energy interacts at the substrate surface as well as inside the substrate. This particular aspect of laser-material interactions is important in laser drilling of blind microvias in polymeric multilayer electronic substrates. A one-dimensional transient heat conduction model including vaporization parameters is constructed to analyze this behavior. The absorption coefficient of the dielectric is also considered in thi...