Engineering topics

Trunek, Andrew J.

Publications and source records attributed to Trunek, Andrew J..

At least 19 records

NASA NTRS2018

Cryogenic Parametric Characterization of Gallium Nitride Switches

This report presents the parametric characterization results of four GaN field-effect transistor (FET) devices from three manufacturers, one of which is a cascode device, and compares those results to a Si power metal-oxide-semiconductor fieldeffect transistor (MOSFET) and a SiC power MOSFET. The devices were first characterized at ambient temperature, then at cryogenic temperatures down to -196 C (LN2 temperature), and finally at ambient temperature again in the event that the device parameters were permanently affected by the cryogenic temperatures. In general, the results indicate that the GaN devices show significant improvement overall at cryogenic temperatures in the parameters characterized, such as onresistance and leakage currents, compared to the Si and SiC devices. The results show that the SiC device tested should not be used at cryogenic temperatures due to the significant increase in on-resistance. The results also show that the GaN and Si parameters characterized were either not affected by the cryogenic temperatures or changed by no more than +/-20 percent post LN2 submersion. The device that exhibited the most parametric change post LN2 submersion was the SiC power MOSFET in its leakage currents.

Gonzalez, Marcelo C.↗

NASA NTRS2018

Extraction of Water from Martian Regolith Simulant via Open Reactor Concept

To demonstrate proof of concept water extraction from simulated Martian regolith, an open reactor design is presented along with experimental results. The open reactor concept avoids sealing surfaces and complex moving parts. In an abrasive environment like the Martian surface, those reactor elements would be difficult to maintain and present a high probability of failure. A general lunar geotechnical simulant was modified by adding borax decahydrate (Na2B4O7·10H2O) (BDH) to mimic the ~ 3 percent water content of hydrated salts in near surface soils on Mars. A rotating bucket wheel excavated the regolith from a source bin and deposited the material onto an inclined copper tray, which was fitted with heaters and a simple vibration system. The combination of vibration, tilt angle and heat was used to separate and expose as much regolith surface area as possible to liberate the water contained in the hydrated minerals, thereby increasing the efficiency of the system. The experiment was conducted in a vacuum system capable of maintaining a Martian like atmosphere. Evolved water vapor was directed to a condensing system using the ambient atmosphere as a sweep gas. The water vapor was condensed and measured. Processed simulant was captured in a collection bin and weighed in real time. The efficiency of the system was determined by comparing pre- and post-processing soil mass along with the volume of water captured.

Trunek, Andrew J.↗

NASA NTRS2014

Structural Characterization of Lateral-grown 6H-SiC am-plane Seed Crystals by Hot Wall CVD Epitaxy

The performance of commercially available silicon carbide (SiC) power devices is limited due to inherently high density of screw dislocations (SD), which are necessary for maintaining polytype during boule growth and commercially viable growth rates. The NASA Glenn Research Center (GRC) has recently proposed a new bulk growth process based on axial fiber growth (parallel to the c-axis) followed by lateral expansion (perpendicular to the c-axis) for producing multi-faceted m-plane SiC boules that can potentially produce wafers with as few as one SD per wafer. In order to implement this novel growth technique, the lateral homoepitaxial growth expansion of a SiC fiber without introducing a significant number of additional defects is critical. Lateral expansion is being investigated by hot wall chemical vapor deposition (HWCVD) growth of 6H-SiC am-plane seed crystals (0.8mm x 0.5mm x 15mm) designed to replicate axially grown SiC single crystal fibers. The post-growth crystals exhibit hexagonal morphology with approximately 1500 m (1.5 mm) of total lateral expansion. Preliminary analysis by synchrotron white beam x-ray topography (SWBXT) confirms that the growth was homoepitaxial, matching the polytype of the respective underlying region of the seed crystal. Axial and transverse sections from the as grown crystal samples were characterized in detail by a combination of SWBXT, transmission electron microscopy (TEM) and Raman spectroscopy to map defect types and distribution. X-ray diffraction analysis indicates the seed crystal contained stacking disorders and this appears to have been reproduced in the lateral growth sections. Analysis of the relative intensity for folded transverse acoustic (FTA) and optical (FTO) modes on the Raman spectra indicate the existence of stacking faults. Further, the density of stacking faults is higher in the seed than in the grown crystal. Bundles of dislocations are observed propagating from the seed in m-axis lateral directions. Contrast extinction analysis of these dislocation lines reveals they are edge type basal plane dislocations that track the growth direction. Polytype phase transition and stacking faults were observed by high-resolution TEM (HRTEM), in agreement with SWBXT and Raman scattering.

Single Crystal↗

NASA NTRS2011

Lateral Growth Expansion of 4H/6H-SiC m-plane Pseudo Fiber Crystals by Hot Wall CVD Epitaxy

Lateral expansion of small mixed polytype 4H/6H-SiC slivers were realized by hot wall chemical vapor deposition (HWCVD). Small slivers cut from m-oriented ..11..00.. SiC boule slices containing regions of 4H and 6H SiC were exposed to HWCVD conditions using standard silane/propane chemistry for a period of up to eight hours. The slivers exhibited approximately 1500 microns (1.5 mm) of total lateral expansion. Initial analysis by synchrotron white beam x-ray topography (SWBXT) confirms, that the lateral growth was homoepitaxial, matching the polytype of the respective underlying region of the seed sliver.

Trunek, Andrew J.↗

NASA NTRS2011

SiC growth by Solvent-Laser Heated Floating Zone

In an effort to grow single crystal SiC fibers for seed crystals the following two growth methods have been coupled in this work: traveling solvent and laser heated floating zone to create the solvent-laser heated floating zone (Solvent-LHFZ) crystal growth method. This paper discusses the results of these initial experiments, which includes: source material, laser heating, and analysis of the first ever Solvent-LHFZ SiC crystals (synchrotron white beam x-ray topography confirmed).

Woodworth, Andrew A.↗

NASA NTRS2009

Observations of Screw Dislocation Driven Growth and Faceting During CVD Homoepitaxy on 4H-SiC On-Axis Mesa Arrays

Previous studies of (0001) homoepitaxial growth carried out on arrays of small-area mesas etched into on-axis silicon-face 4H-SiC wafers have demonstrated that spiral growth emanating from at least one screw dislocation threading the mesa is necessary in order for a mesa to grow taller in the <0001> (c-axis vertical) direction while maintaining 4H stacking sequence [1]. However, even amongst mesas containing the screw dislocation step source necessary for vertical c-axis growth, we have observed striking differences in the height and faceting that evolve during prolonged homoepitaxial growths. This paper summarizes Atomic Force Microscopy (AFM), Electron Channeling Contrast Imaging (ECCI), Scanning Electron Microscopy (SEM), and optical microscopy observations of this phenomenon. These observations support our initially proposed model [2] that the observed large variation (for mesas where 3C-SiC nucleation has not occurred) is related to the lateral positioning of a screw dislocation step source within each etched mesa. When the screw dislocation step source is located close enough to the developing edge/sidewall facet of a mesa, the c-axis growth rate and facet angle are affected by the resulting interaction. In particular, the intersection (or near intersection) of the inward-sloping mesa sidewall facet with the screw dislocation appears to impede the rate at which the spiral provides new steps required for c-axis growth. Also, the inward slope of the sidewall facet during growth (relative to other sidewalls of the same mesa not near the screw dislocation) seems to be impeded by the screw dislocation. In contrast, mesas whose screw dislocations are centrally located grow vertically, but inward sloping sidewall facets shrink the area of the top (0001) growth surface almost to the point of vanishing.

Neudeck, Philip G.↗

NASA NTRS2008

Method for the growth of large low-defect single crystals

A method and the benefits resulting from the product thereof are disclosed for the growth of large, low-defect single-crystals of tetrahedrally-bonded crystal materials. The process utilizes a uniquely designed crystal shape whereby the direction of rapid growth is parallel to a preferred crystal direction. By establishing several regions of growth, a large single crystal that is largely defect-free can be grown at high growth rates. This process is particularly suitable for producing products for wide-bandgap semiconductors, such as SiC, GaN, AlN, and diamond. Large low-defect single crystals of these semiconductors enable greatly enhanced performance and reliability for applications involving high power, high voltage, and/or high temperature operating conditions.

Powell, J. Anthony↗

NASA NTRS2007

Hydrogen Gas Sensors Fabricated on Atomically Flat 4H-SiC Webbed Cantilevers

This paper reports on initial results from the first device tested of a "second generation" Pt-SiC Schottky diode hydrogen gas sensor that: 1) resides on the top of atomically flat 4H-SiC webbed cantilevers, 2) has integrated heater resistor, and 3) is bonded and packaged. With proper selection of heater resistor and sensor diode biases, rapid detection of H2 down to concentrations of 20 ppm was achieved. A stable sensor current gain of 125 +/- 11 standard deviation was demonstrated during 250 hours of cyclic test exposures to 0.5% H2 and N2/air.

Neudeck, Philip G.↗

NASA NTRS2006

CFD Growth of 3C-SiC on 4H/6H Mesas

This article describes growth and characterization of the highest quality reproducible 3C-SiC heteroepitaxial films ever reported. By properly nucleating 3C-SiC growth on top of perfectly on-axis (0001) 4H-SiC mesa surfaces completely free of atomic scale steps and extended defects, growth of 3C-SiC mesa heterofilms completely free of extended crystal defects can be achieved. In contrast, nucleation and growth of 3C-SiC mesa heterofilms on top of 4H-SiC mesas with atomic-scale steps always results in numerous observable dislocations threading through the 3C-SiC epilayer. High-resolution X-ray diffraction and transmission electron microscopy measurements indicate non-trivial in-plane lattice mismatch between the 3C and 4H layers. This mismatch is somewhat relieved in the step-free mesa case via misfit dislocations confined to the 3C/4H interfacial region without dislocations threading into the overlying 3C-SiC layer. These results indicate that the presence or absence of steps at the 3C/4H heteroepitaxial interface critically impacts the quality, defect structure, and relaxation mechanisms of single-crystal heteroepitaxial 3C-SiC films.

Neudeck, Philip G.↗

NASA NTRS2006

Recent Results from Epitaxial Growth on Step Free 4H-SiC Mesas

This paper updates recent progress made in growth, characterization, and understanding of high quality homoepitaxial and heteroepitaxial films grown on step-free 4H-SiC mesas. First, we report initial achievement of step-free 4H-SiC surfaces with carbon-face surface polarity. Next, we will describe further observations of how step-free 4H-SiC thin lateral cantilever evolution is significantly impacted by crystal faceting behavior that imposes non-uniform film thickness on cantilever undersides. Finally, recent investigations of in-plane lattice constant mismatch strain relief mechanisms observed for heteroepitaxial growth of 3C-SiC as well as 2H-AlN/GaN heterofilms on step-free 4H-SiC mesas will be reviewed. In both cases, the complete elimination of atomic heterointerface steps on the mesa structure enables uniquely well-ordered misfit dislocation arrays to form near the heterointerfaces with remarkable lack of dislocations threading vertically into the heteroepilayers. In the case of 3C-SiC heterofilms, it has been proposed that dislocation half-loops nucleate at mesa edges and glide laterally along the step-free 3C/4H interfaces. In contrast, 3C-SiC and 2H-AlN/GaN heterofilms grown on 4H-SiC mesas with steps exhibit highly disordered interface misfit dislocation structure coupled with 100X greater density of dislocations threading through the thickness of the heteroepilayers. These results indicate that the presence of steps at the heteroepitaxial interface (i.e., on the initial heteroepitaxial nucleation surface) plays a highly important role in the defect structure, quality, and relaxation mechanisms of single-crystal heteroepitaxial films.

Neudeck, Philip G.↗

NASA NTRS2005

Measurements of Breakdown Field and Forward Current Stability in 3C-SiC P-N Junction Diodes Grown on Step-Free 4H-SiC

This paper reports on initial fabrication and electrical characterization of 3C-SiC p-n junction diodes grown on step-free 4H-SiC mesas. Diodes with n-blocking-layer doping ranging from approx. 2 x 10(exp 16)/cu cm to approx.. 5 x 10(exp 17)/cu cm were fabricated and tested. No optimization of junction edge termination or ohmic contacts was employed. Room temperature reverse characteristics of the best devices show excellent low-leakage behavior, below previous 3C-SiC devices produced by other growth techniques, until the onset of a sharp breakdown knee. The resulting estimated breakdown field of 3C-SiC is at least twice the breakdown field of silicon, but is only around half the breakdown field of <0001> 4H-SiC for the doping range studied. Initial high current stressing of 3C diodes at 100 A/sq cm for more than 20 hours resulted in less than 50 mV change in approx. 3 V forward voltage. 3C-SiC, pn junction, p+n diode, rectifier, reverse breakdown, breakdown field,heteroepitaxy, epitaxial growth, electroluminescence, mesa, bipolar diode

Neudeck, Philip G.↗

NASA NTRS2004

Step Structures Produced by Hydrogen Etching of Initially Step-Free (0001) 4H-SiC Mesas

This paper presents new observations resulting from in-situ high temperature hydrogen etching of 4H-SiC mesas that were step-free prior to initiation of etching. In particular, it was found that well-ordered pyramidal-shaped step train structures could be produced on mesa top surfaces via stepflow etching proceeding inward from the sides of mesas. In many cases, the height of steps etched inward from {112 0} mesa sides is 0.5 nm (2 Si-C bilayers), while the height of steps etched inward from {11 00} sides of the same mesa is 1.0 nm (4 Si-C bilayers, the repeat distance of the 4H-SiC polytype). We propose that stepflow etching starting from the mesa sidewall and involving step-to-step repulsive forces produces the observed step train structures.

Powell, J. Anthony↗

NASA NTRS2004

Web Growth Used to Confine Screw Dislocations to Predetermined Lateral Positions in 4H-SiC Epilayers

Silicon-carbide- (SiC-) based power devices could enable substantial aerospace electronics benefits over today's silicon-based electronics. However, present-day SiC wafers contain electrically harmful dislocations (including micropipes) that are unpredictably distributed in high densities across all commercial 4H- and 6H-SiC wafers. The NASA Glenn Research Center recently demonstrated a crystal growth process that moves SiC wafer dislocations to predetermined lateral positions in epitaxial layers so that they can be reproducibly avoided during subsequent SiC electronic device fabrication. The process starts by reactive ion etching mesa patterns with enclosed trench regions into commercial on-axis (0001) 4H- or 6H-SiC substrates. An example of a pregrowth mesa geometry with six enclosed triangular-shaped trench regions is shown. After the etch mask is stripped, homoepitaxial growth is carried out in pure stepflow conditions that enable thin cantilevers to grow laterally from the tops of mesas whose pregrowth top surfaces are not threaded by substrate screw dislocations. The image in the bottom figure shows the postgrowth structure that forms after the lateral cantilevers expand to coalesce and completely roof over each of the six triangular trench regions. Atomic force microscope (AFM) measurements of the roof revealed that three elementary screw dislocation growth spirals, each shown in the AFM insets of the bottom image on the previous page, formed in the film roof at three respective points of cantilever film coalescence. The image above shows the structure following an etch in molten potassium hydroxide (KOH) that produced surface etch pits at the dislocation defects. The larger KOH etch pits--S1, S2, and S3--shown in this image correspond to screw dislocations relocated to the final points of cantilever coalescence. The smaller KOH etch pits are consistent with epilayer threading edge dislocations from the pregrowth substrate mesa (P1, P3, and P4) and a final cantilever coalescence point (P2). No defects (i.e., no etch pits) are observed in other cantilevered portions of the film surface. On the basis of the principle of dislocation Burgers vector conservation, we hypothesize that all vertically propagating substrate dislocations in an enclosed trench region become combined into a single dislocation in the webbed film roof at the point of final roof coalescence. The point of final roof coalescence, and therefore the lateral location of a webbed roof dislocation, can be designed into the pregrowth mesa pattern. Screw dislocations with predetermined lateral positions can then be used to provide the new growth steps necessary for growing a 4H/6H-SiC epilayer with a lower dislocation density than the substrate. Devices fabricated on top of such films can be positioned to avoid the preplaced dislocations.

Powell, J. Anthony↗

NASA NTRS2004

Atomic Force Microscope Observation of Growth and Defects on As-Grown (111) 3C-SiC Mesa Surfaces

This paper presents experimental atomic force microscope (AFM) observations of the surface morphology of as-grown (111) silicon-face 3C-SiC mesa heterofilms. Wide variations in 3C surface step structure are observed as a function of film growth conditions and film defect content. The vast majority of as-grown 3C-SiC surfaces consisted of trains of single bilayer height (0.25 nm) steps. Macrostep formation (i.e., step-bunching) was rarely observed, and then only on mesa heterofilms with extended crystal defects. As supersaturation is lowered by decreasing precursor concentration, terrace nucleation on the top (111) surface becomes suppressed, sometimes enabling the formation of thin 3C-SiC film surfaces completely free of steps. For thicker films, propagation of steps inward from mesa edges is sometimes observed, suggesting that enlarging 3C mesa sidewall facets begin to play an increasingly important role in film growth. The AFM observation of stacking faults (SF's) and 0.25 nm Burgers vector screw component growth spirals on the as-grown surface of defective 3C films is reported.

Neudeck, Philip G.↗

NASA NTRS2003

Surface Characterization of 3C-SiC Mesa Heterofilms: Evidence for Growth by Edge/Corner Nucleation Mechanism

This paper presents the first detailed observations of unique surface morphologies of 3C-Sic films grown on 4W6H-SIC mesas by the step-free surface heteroepitaxy technique. The top surfaces of 3C-Sic films were extensively studied by optical microscopy and atomic force microscopy (AFM) as both film thickness (i.e,, growth time) and growth temperature (i.e., terrace nucleation rate) were varied following complete coverage of each 4W6H mesa by an initial 3C-Sic film. Almost all surface steps observed by AFM were 0.25 nm, the height of a single Si-C bilayer. However, strikingly different step patterns were observed, suggesting that radically different processes dominate the nucleation of new 3C-Sic bilayers on top of existing 3C-Sic film surfaces.

Trunek, Andrew J.↗

NASA NTRS2003

Step Free Surface Heteroepitaxy of 3C-SiC Layers on Patterned 4H/6H-SiC Mesas and Cantilevers

Most SiC devices are implemented in homoepitaxial films grown on 4H/6H-SiC wafers with surfaces 3 degrees to 8 degrees off-axis from the (0001) basal plane. This approach has not prevented many substrate crystal defects from propagating into SiC epilayers, and does not permit the realization of SiC heteropolytype devices. This presentation describes recent advances in SiC epitaxial growth that begun to overcome the above shortcomings for arrays of mesas patterned into on-axis 4H/6H-SiC wafers. First, we demonstrated that atomic-scale surface steps can be completely eliminated from 4H/6H-SiC mesas via on-axis homoepitaxial step-flow growth, forming (0001) basal plane surfaces (up to 0.4 mm x 0.4 mm) for larger than previously thought possible. Step-free surface areas were then extended by growth fo thin lateral cantilevers from the mesa tops. These lateral cantilevers enabled substrate defects to be reduced and relocated in homoepitaxial films in a manner not possible with off-axis SiC growth. Finally, growth of vastly improved 3C-SiC heterofilms was achieved on 4H/6H-SiC mesas using the recently develop step-free surface heteroepitaxy process. These epitaxial growth developments should enable improved homojunction and heterojunction silicon carbide prototype devices.

Neudeck, Philip G.↗

NASA NTRS2002

Confinement of Screw Dislocations to Predetermined Lateral Positions in (0001) 4H-SiC Epilayers Using Homoepitaxial Web Growth

This paper reports initial demonstration of a cantilevered homoepitaxial growth process that places screw dislocations at predetermined lateral positions in on-axis 4H-SiC mesa epilayers. Thin cantilevers were grown extending toward the interior of hollow pre-growth mesa shapes etched into an on-axis 4H-SiC wafer, eventually completely coalescing to form roofed cavities. Each completely coalesced cavity exhibited either: 1) a screw dislocation growth spiral located exactly where final cantilever coalescence occurred, or 2) no growth spiral. The fact that growth spirals are not observed at any other position except the central coalescence point suggests that substrate screw dislocations, initially surrounded by the hollow portion of the pre-growth mesa shape, are relocated to the final coalescence point of the webbed epilayer roof. Molten potassium hydroxide etch studies revealed that properly grown webbed cantilevers exhibited no etch pits, confirming the superior crystal quality of the cantilevers.

Neudeck, Philip G.↗

NASA NTRS2002

Enlargement of Step-Free SiC Surfaces by Homoepitaxial Web-Growth of Thin SiC Cantilevers

Lateral homoepitaxial growth of thin cantilevers emanating from mesa patterns that were reactive ion etched into on-axis commercial SiC substrates prior to growth is reported. The thin cantilevers form after pure stepflow growth removes almost all atomic steps from the top surface of a mesa, after which additional adatoms collected by the large step-free surface migrate to the mesa sidewall where they rapidly incorporate into the crystal near the top of the mesa sidewall. The lateral propagation of the step-free cantilevered surface is significantly affected by pregrowth mesa shape and orientation, with the highest lateral expansion rates observed at the inside concave comers of V-shaped pregrowth mesas with arms lengthwise oriented along the {1100} direction. Complete spanning of the interiors of V's and other mesa shapes with concave comers by webbed cantilevers was accomplished. Optical microscopy, synchrotron white beam x-ray topography and atomic force microscopy analysis of webbed regions formed over a micropipe and closed-core screw dislocations show that c-axis propagation of these defects is terminated by the webbing. Despite the nonoptimized process employed in this initial study, webbed surfaces as large as 1.4 x 10(exp -3) square centimeters, more than four times the pregrowth mesa area, were grown. However, the largest webbed surfaces were not completely free of bilayer steps, due to unintentional growth of 3C-SiC that occurred in the nonoptimized process. Further process optimization should enable larger step-free webs to be realized.

Neudeck, Philip G.↗