Eric Carlson

GaN thin films deposited via organometallic vapor phase epitaxy on α(6H)–SiC(0001) using high‐temperature monocrystalline AlN buffer layers

Monocrystalline GaN(0001) thin films, void of oriented domain structures and associated low‐angle grain boundaries, have been grown via organometallic vapor phase epitaxy (OMVPE) on high‐temperature monocrystalline AlN(0001) buffer layers predeposited on vicinal α(6H)–SiC(0001) wafers using TEG, TEA, and ammonia in a cold wall, vertical, pancake‐style reactor. The surface morphology was smooth, and the PL spectrum showed strong near‐band‐edge emission with a full width at half‐maximum (FWHM) value of 4 meV. The dislocation density within the first 0.5 μm was ≊1×109 cm−2; it decreased substantially with increasing film thickness. Controlled n‐type Si doping of GaN has been achieved for net carrier concentrations ranging from ∼1×1017 to 1×1020 cm−3. Double‐crystal XRC measurements indicated a FWHM value of 66 arcsec for the GaN(0004) reflection.

Pendeoepitaxy of gallium nitride thin films

Pendeoepitaxy, a form of selective lateral growth of GaN thin films has been developed using GaN/AlN/6H–SiC(0001) substrates and produced by organometallic vapor phase epitaxy. Selective lateral growth is forced to initiate from the (1120) GaN sidewalls of etched GaN seed forms by incorporating a silicon nitride seed mask and employing the SiC substrate as a pseudomask. Coalescence over and between the seed forms was achieved. Transmission electron microscopy revealed that all vertically threading defects stemming from the GaN/AlN and AlN/SiC interfaces are contained within the seed forms and a substantial reduction in the dislocation density of the laterally grown GaN. Atomic force microscopy analysis of the (1120) face of discrete pendeoepitaxial structures revealed a root mean square roughness of 0.98 A. The pendeoepitaxial layer photoluminescence band edge emission peak was observed to be 3.454 eV and is blueshifted by 12 meV as compared to the GaN seed layer.

Undoped and doped GaN thin films deposited on high-temperature monocrystalline AlN buffer layers on vicinal and on-axis α (6H)–SiC(0001) substrates via organometallic vapor phase epitaxy

Monocrystalline GaN(0001) thin films have been grown at 950 °C on high-temperature, ≈ 100 nm thick, monocrystalline AlN(0001) buffer layers predeposited at 1100 °C on α (6H)−SiC(0001) Si substrates via OMVPE in a cold-wall, vertical, pancake-style reactor. These films were free of low-angle grain boundaries and the associated oriented domain microstructure. The PL spectra of the GaN films deposited on both vicinal and on-axis substrates revealed strong bound excitonic emission with a FWHM value of 4 meV. The near band-edge emission from films on the vicinal substrates was shifted slightly to a lower energy, indicative of films containing residual tensile stresses. A peak attributed to free excitonic emission was also clearly observed in the on-axis spectrum. Undoped films were too resistive for accurate Hall-effect measurements. Controlled n -type, Si-doping in GaN was achieved for net carrier concentrations ranging from approximately 1 × 10 17 cm −3 to 1 × 10 20 cm −3 . Mg-doped, p -type GaN was achieved with n A −n D ≈ 3 × 10 17 cm −3 , ρ ≈ 7 Ω · cm, and μ ≈ 3 cm 2 /V · s. Double-crystal x-ray rocking curve measurements for simultaneously deposited 1.4 μ m GaN films revealed FWHM values of 58 and 151 arcsec for deposition on on-axis and off-axis 6H−SiC(0001) Si substrates, respectively. The corresponding FWHM values for the AlN buffer layers were approximately 200 and 400 arcsec, respectively.

Ion implantation of epitaxial GaN films: damage, doping and activation

Single-crystal GaN films grown on AlN buffer layers previously deposited on 6H-SiC(0001) were studied for radiation damage and its recovery using Rutherford backscattering/channeling, photoluminescence, and cross-sectional TEM. The highest fluence of (1e15 cm{sup -2}) 110 keV Mg and 160 keV Si produced little damage at implantation temperature 550 C. RT damage was higher for same fluences compared to 550 C implantation. The damage was partially annealed by RTA at 1000 C, however, this was not enough to recover the PL signal even for the lowest fluence (1e14 cm{sup -2}). XTEM of as-implanted samples revealed small clusters of defects extended beyond the projected ion range. To recover damage completely, perhaps one needs to go either much higher RTA temperature and/or implant samples in a smaller fluence increment and anneal in between implants to recover the damage.

Pendeo-Epitaxy - A New Approach for Lateral Growth of Gallium Nitride Structures

A new process route for lateral growth of nearly defect free GaN structures via Pendeoepitaxy is discussed. Lateral growth of GaN films suspended from {11 2 0} side walls of [0001] oriented GaN columns into and over adjacent etched wells has been achieved via MOVPE technique without the use of, or contact with, a supporting mask or substrate. Pendeo-epitaxy is proposed as the descriptive term for this growth technique. Selective growth was achieved using process parameters that promote lateral growth of the { 11 2 0) planes of GaN and disallow nucleation of this phase on the exposed SiC substrate. Thus, the selectivity is provided by tailoring the shape of the underlying GaN layer itself consisting of a sequence of alternating trenches and columns, instead of selective growth through openings in SiO 2 or SiN x mask, as in the conventional lateral epitaxial overgrowth (LEO). Two modes of initiation of the pendeo-epitaxial GaN growth via MOVPE were observed: Mode A - promoting the lateral growth of the {11 2 0} side facets into the wells faster than the vertical growth of the (0001) top facets; and Mode B - enabling the top (0001) faces to grow initially faster followed by the pendeo-epitaxial growth over the wells from the newly formed {11 2 0} side facets. Four-to-five order decrease in the dislocation density was observed via transmission electron microscopy (TEM) in the pendeo-epitaxial GaN relative to that in the GaN columns. TEM observations revealed that in pendeo-epitaxial GaN films the dislocations do not propagate laterally from the GaN columns when the structure grows laterally from the sidewalls into and over the trenches. Scanning electron microscopy (SEM) studies revealed that the coalesced regions are either defect-free or sometimes exhibit voids. Above these voids the PEGaN layer is usually defect free.

A simulation-based decision support system for a specialty chemicals production plant

The subject of this paper is the development of an interactive simulation-based decision support system for a batch processing facility. Thk+ system is a decision-making tool that can be used by both the production scheduler and the production staff to help them in both long and short-term scheduling of production and equipment and in capacity planning. Simulation has been combined with artificial intelligence methodology to produce an interactive, simulation-based decision support system that can be used in an on-line or off-line mode. Off line, the system can be used for long-term capacity and debottlenecking studies. Operated on line, the model can be used for short-term planning, scheduling, and decision-makhtg. The different modules of the system interface with the user in a menu-driven format, collect work-in-process information from the plant’s distributed control system, run simulations, and provide interpretations of the results in an easy-to-understand format.

Simulation and queueing network modeling of single-product production campaigns☆

Abstract A two-stage approach to modeling single-product campaigns in specialty chemical production facilities has been developed. In the first stage a simple analytical queueing network model is formulated. In some instances this approximate model is accurate enough for its intended applications (e.g. debottlenecking, capacity planning or scheduling). If it is not, the results indicate which portions of the process must be subjected to more detailed modeling, and a general-purpose simulation language is used for that purpose. In an illustrative application of the approach, a queueing network model was developed for a single-product campaign in an existing multiproduct pharmaceutical plant, and a SLAM II simulation model was subsequently used to model finite intermediate storage and non-Markovian arrival and service times. The final model was validated and used to explore alternative production procedures to reduce the campaign duration and yield additional production capacity.

Diluent Gas Effects on Properties of Ain and GaN Thin Films Grown by Metalorganic Vapor Phase Epitaxy on α(6H)-SiC Substrates

Thin films of AIN and GaN were deposited on α(6H)-SiC(0001) wafers using metalorganic vapor phase epitaxy (MOVPE) and H 2 and N 2 diluents. A computational fluid dynamic model of the deposition process was used to analyze the film growth conditions for both diluents. Low temperature (12 K) photoluminescence of the GaN films grown in N 2 had peak intensities and full widths at half maximum of ∼7 meV which were equal to or better than those films grown in H 2 . Cross-sectional and plan-view transmission electron microscopy of films grown in both diluents showed similar microstructures with a typical dislocation density of 10 9 /cm 2 . Hall measurements of n-type (Si doped) GaN grown in N 2 revealed Hall mobilities equivalent to those films grown in H 2 . Acceptor-type behavior of Mg-doped GaN grown in N 2 was repeatably obtained without post-growth annealing. Secondary ion mass spectrometry revealed equivalent levels of H in Mg-doped GaN films grown in both diluents.