Cree Research Research Articles

Development of a World Class Silicon Carbide Substrate Manufacturing Capability

Silicon carbide (SiC) semiconductor substrates provide the foundation for revolutionary improvements in the cost, size, weight and performance of a broad range of military and commercial radio frequency (RF) and power switching devices. Due to the lack of a viable, native gallium nitride (GaN) substrate, semi-insulating (SI) SiC substrates are the substrate of choice for high power AlGaN/GaN High Electron Mobility Transistors (HEMTs) due to their near lattice-match to GaN, superior thermal conductivity and commercial availability. GaN has emerged as the technology of choice for RF power because of its superior output power capability compared to other semiconductors. Similarly, semi-conducting (N+) SiC substrates are required for fabrication of high voltage Schottky Diodes and Metal Oxide Semiconductor Field Effect Transistor (MOSFET) power switching devices. Critical to this realization is the availability of affordable, high quality, large diameter SI and N+ SiC substrates for production of GaN and SiC power semiconductors. SiC is unique in that bulk single crystals cannot be grown via traditional melt-based manufacturing processes such as Czochralski. Rather, a high temperature sublimation process is required. In the late 1980’s, pioneering physical vapor transport research taking place at North Carolina State University ultimately led to the formation of Cree Research and subsequently the wide bandgap semiconductor industry. U.S. Department of Defense investment in wide bandgap semiconductor development has easily exceeded $1B spawning the creation of an entirely new industry. During the early 1990’s, SiC physical vapor transport growth development was fraught with perceived insurmountable technical challenges associated with micropipes, polytype conversion, diameter expansion and crystalline defects. Despite these monumental crystal growth technology hurdles, SiC substrates are now manufactured at a cost and quality never thought possible. This article highlights more than 20 years of Air Force Research Laboratory (AFRL) sponsored development with II-VI aimed at positioning itself as a merchant, world-class manufacturer of SiC substrates.

English semantic word-pair norms and a searchable Web portal for experimental stimulus creation

As researchers explore the complexity of memory and language hierarchies, the need to expand normed stimulus databases is growing. Therefore, we present 1,808 words, paired with their features and concept-concept information, that were collected using previously established norming methods (McRae, Cree, Seidenberg, & McNorgan Behavior Research Methods 37:547-559, 2005). This database supplements existing stimuli and complements the Semantic Priming Project (Hutchison, Balota, Cortese, Neely, Niemeyer, Bengson, & Cohen-Shikora 2010). The data set includes many types of words (including nouns, verbs, adjectives, etc.), expanding on previous collections of nouns and verbs (Vinson & Vigliocco Journal of Neurolinguistics 15:317-351, 2008). We describe the relation between our and other semantic norms, as well as giving a short review of word-pair norms. The stimuli are provided in conjunction with a searchable Web portal that allows researchers to create a set of experimental stimuli without prior programming knowledge. When researchers use this new database in tandem with previous norming efforts, precise stimuli sets can be created for future research endeavors.

P2-S3.12 Culture and research: how do you mix them?

BackgroundCommunity-based participatory research principles partner community and academics through all stages of the research process. Our purpose is to describe how we have been combining traditional Cree cultural practices (culture)...

Influence of Epitaxial Layer on SiC Schottky Diode Gas Sensors Operated under High-Temperature Conditions

Schottky diode gas sensors were fabricated on top of the epitaxial layer grown by three different methods, purchased from Cree Research Inc., by hot wall CVD, or by sublimation at a high growth rate. The epitaxial layers have different thickness and doping. The current-voltage characteristics of the gas sensors were compared in different gas ambient during operation in the high temperature region. The temperature dependence of the series resistance of the diodes revealed two types of carrier scattering mechanisms, impurity scattering for the sublimation epitaxial layer at 300-400degreesC and at 400-600degreesC, lattice scattering for all diodes. The ideality factor of the diode fabricated on the Cree substrate is higher than others. The higher ideality factor gives rise to a larger forward voltage change for a change in gas ambient. The amount of change in barrier height caused by a change in the ambient gas is almost the same for the three types of diodes. The value of the barrier height of the diode grown by the sublimation method is lower than for the others, which gives a higher reverse saturation current at temperatures above 400degreesC. The largest saturation current also shows the largest current change when switching between different gas atmospheres.

Role of the defects under porous silicon carbide formation

The resent study is concerned with the structure of porous silicon carbide (PSC) layers fabricated on 6H–SiC substrates produced by CREE Research and differ from each other by the defects presented (Type I and Type II). For Type I samples, the current density at the electrochemical etching was 20, 60, and 100 mA/cm 2, whereas for Type II samples a lower current density—5, 10, and 15 mA/cm 2—was used because of the presence of a large number of voids. It is shown that porous layer characteristics (pore size, porosity, layer thickness) depend on the structure of the initial substrate to a large extent.

Photoelectrochemical characterization of p- and n-doped single crystalline silicon carbide and photoinduced reductive dehalogenation of organic pollutants at p-doped silicon carbide

In this work, the photoelectrochemical properties of single crystalline p- and n-SiC, now available from Cree Research, Inc., will be presented. 4H- and 6H-SiC were investigated by electrochemical impedance spectroscopy and cyclic voltammetry. Flatband potentials of p-SiC determined by application of the Mott–Schottky-relation varied from 1.8 to 2.5 V versus NHE depending on the C or Si face and the doping concentration of the single crystalline samples. The flatband potential of n-SiC was found to be −1.3 V vs. NHE. A nearly Nernstian dependence on the pH was found for both the n-SiC and the p-SiC-types. The onset of the cathodic photocurrent at p-SiC occurred with a large overpotential in aqueous solution. P-SiC was used as photocathode for the reductive dehalogenation of halogenated acetic acids due to its chemical stability, its highly energetic conduction band and the overvoltage for the hydrogen evolution. It was especially suitable for the reduction of less reducible chlorinated acetic acids in the aqueous phase. The dehalogenation of brominated acetic acids resulted in the complete release of bromine in form of bromide. The bromide yields were about 98%. In case of chlorinated acetic acids, the dehalogenation stopped at the level of monochloroacetic acid, since this halide was very slowly reduced.

Plastic behavior of 4H-SiC single crystals deformed at low strain rates

Department of MaterialsScience and Engineering, Case Western Reserve University, Cleveland, OH 44106-7204, USA(Received April 18, 2000)(Accepted in revised form May 24, 2000)Keywords: Semiconductors; Mechanical properties; Dislocations; Brittle-to-ductile transitionIntroductionPlasticity of elemental and III-V compound semiconducting materials has been extensively studied ina wide range of stress and temperature; for a review, see, e.g., Refs. [1,2]. However, data on the yieldstress and other plastic properties of IV-IV compounds, e.g. different SiC polytypes, is lacking mainlybecause it was not possible to grow single crystals of these materials in large dimensions until recently.The first experiments on plasticity of 6H-SiC single crystals were performed by Fujitaet al. [3] on smallsamples cut from industrial Acheson-grown crystals. In the last few years, reasonably high quality bulksingle crystals of 4H-SiC and 6H-SiC are being grown by the modified Lely technique [4] and waferscut from these bulk ingots are now commercially available (Cree Research, Inc., Durham, NC, USA).Recently, Samant [5–7] performed compression experiments on the new high-quality 6H-SiC crystalsat moderate and high strain rates in order to measure the yield stress, t

Microwave Simulation on the Performance of High Power GaN/AlGaN Heterostructure Field Effect Transistors

We present the results of the simulation of dc and microwave performance of a GaN/AlGaN heterostructure field effect transistors and compare the results with recent record-breaking experimental data from Cree Research, Inc. Our simulation results are in good agreement with the data on the small signal transistor performance. The calculated maximum stable power gain decreases with frequency with a 3 dB per octave slope in agreement with experimental results. The simulated values of the saturation power are somewhat less than experimentally measured values. This difference might be caused by self-heating, which is smaller at microwave frequencies than at dc.

Progress in SiC: from material growth to commercial device development

Silicon carbide technology has made tremendous strides in the last several years, with a variety of encouraging device and circuit demonstrations in addition to volume production of nitride-based blue LEDs being fabricated on SiC substrates. The commercial availability of relatively large, high quality wafers of the 6H and 4H polytypes of SiC for device development has facilitated these exciting breakthroughs in laboratories throughout the world. These have occurred in numerous application areas, including high power devices, short wavelength optoelectronic devices, and high power/high frequency devices. This paper will describe progress made in increasing the quality and size of SiC wafers, advances in SiC epitaxy and some of the resulting device demonstrations and commercialization by Cree Research.

High quality β-SiC films obtained by low-temperature heteroepitaxy combined with a fast carbonization step

The present paper describes a new low-temperature process for the deposition of crystalline cubic silicon carbide on [100] silicon using methylsilane as precursor. A substantial improvement of crystal quality was achieved by applying a special fast carbonization step based on ethylene diluted in argon. The influence of the buffer-layer growth conditions, the SiC film thickness, and the deposition temperature on crystal quality and surface morphology was studied using SEM, PDS, and X-ray diffraction. In comparison to SiC layers grown without a carbonization step the FWHM of the [200] reflex was reduced from 0.54° to 0.21°, the surface roughness from 350nm to 38nm and the defect density by a factor of 9. The state-of-the-art quality was proved by the comparison to commercial films from Cree Research Inc., presumably grown at considerably higher temperatures.

Characterization of macrodefects in pure silcon carbide films using X-ray topography and Raman scattering

Raman and X-ray topographic measurements were carried out on epitaxial films of silicon carbide grown at Cree Research Inc. by vapor-phase epitaxy on bulk 6H-SiC substrates. The objective was to identify ranges of the Raman spectrum of 6H-SiC that were particularly sensitive to macrostructural defects (dislocations, inclusions, etc.) in these films, and to determine what conclusions could be drawn about the properties of the corresponding portions of the films.

Microwave power MESFET on 4H-SiC

We present static and microwave characterization of MESFETs processed on n+ 4H-SiC wafers supplied by Cree Research. An eight-level technology including airbridge interconnections has been performed. Transistors with total gate width up to 6 mm and 1.2 μm gate length optically designed are obtained. High breakdown voltage (up to 140 V) and high drain current (up to 0.7 A) show the high power capability of the transistors. RF characterization was performed. Because of the use of a conductive substrate, parasitic capacitances account for the relatively low values of fT and fmax (4 and 9 GHz, respectively). Power measurements were carried out at 1 and 2 GHz in cw. Output powers up to 1.7 W mm−1 were measured with a power added efficiency of 37% at 2 GHz.

Comparative investigation of ECR-RIE patterns on Si and C faces of 6H-SiC using a gas mixture

We report on the relative results of dry etching 〈0001〉 Si-like or 〈0001〉 C-like oriented faces of bulk natural 6H-SiC Lely crystals. For comparison, a 6H epilayer from Cree Research Inc. was also investigated. In all three cases, the initial and resulting surface morphology has been studied by means of scanning electron and atomic force microscopy.

SiC Seeded Crystal Growth

The availability of relatively large (30 mm) SiC wafers has been a primary reason for the renewed high level of interest in SiC semiconductor technology. Projections that 75 mm SiC wafers will be available in 2 to 3 years have further peaked this interest. Now both 4H and 6H polytypes are available, however, the micropipe defects that occur to a varying extent in all wafers produced to date are seen by many as preventing the commercialization of many types of SiC devices, especially high current power devices. Most views on micropipe formation are based around Frank's theory of a micropipe being the hollow core of a screw dislocation with a huge Burgers vector (several times the unit cell) and with the diameter of the core having a direct relationship with the magnitude of the Burgers vector. Our results show that there are several mechanisms or combinations of these mechanisms which cause micropipes in SiC boules grown by the seeded sublimation method. Additional considerations such as polytype variations, dislocations and both impurity and diameter control add to the complexity of producing high quality wafers. Recent results at Cree Research, Inc., including wafers with micropipe densities of less than 1 cm -2 (with 1 cm 2 areas void of micropipes), indicate that micropipes will be reduced to a level that makes high current devices viable and that they may be totally eliminated in the next few years. Additionally, efforts towards larger diameter high quality substrates have led to production of 50 mm diameter 4H and 6H wafers for fabrication of LEDs and the demonstration of 75 mm wafers. Low resistivity and semi-insulating electrical properties have also been attained through improved process and impurity control. Although challenges remain, the industry continues to make significant progress towards large volume SiC-based semiconductor fabrication.

6H-Silicon Carbide Light Emitting Diodes and UV Photodiodes

Silicon carbide has been used to fabricate a variety of short wavelength optoelectronic devices including blue LEDs, green LEDs and UV photodiodes. As a light emitter, 6H-SiC junctions can be tailored to emit light across the visible spectrum. The most widely commercialized device is the blue LED. Over the past years, the quantum efficiency of the Cree Research blue LED has increased significantly. The devices emit light with a peak wavelength of 470 nm with a spectral halfwidth of ≈︂70 nm. The optical power output is typically between 25 and 35 μW at a forward current of 20 mA and 3.2 V. This represents an external quantum efficiency of ≈︂0.05 to 0.07%. Green LEDs have been demonstrated which emit with a peak wavelength of 530 nm. As opposed to the epitaxial junction used in the blue LED, the green devices use ion implanted junctions. The typical output power is similar to that of the blue LED. However, with respect to photometric units, the die luminous intensity is a factor of two higher than the blue LED, 1.2 mcd (millicandela) for a radiant flux output of 33 μW. In addition to short wavelength light emission, the energy bandgap of ≈︂3.0 eV allows for inherently low dark currents and high quantum efficiencies for ultraviolet photodiode detectors made in 6H-SiC, even at high temperatures. These devices typically exhibit a quantum efficiency of 80 to 100% and peak response of ≈︂250 to 280 nm. These characteristics are maintained to at least 350 °C. The dark current density at —1.0 V and 473 K is ≈︂10—11 A/cm2. This corresponds to an extrapolated room temperature current density of ≈︂2 × 10—17 A/cm2 at —1.0 V.

Silicon Carbide Electronic Materials and Devices

The development of SiC for electronic applications has been a subject of intense research for nearly 40 years. Much of this research is motivated by the extraordinary combination of physical properties possessed by SiC, especially in the development of SiC-based devices for specific high-temperature, high-power, or high-frequency applications that are not suitable for Si- or GaAs-based devices. During the early years of SiC research and development, a significant amount of good, fundamental research was performed, but the development of commercially available SiC-based devices was retarded by low-quality bulk materials and inadequate epitaxial processes. In the late 1980s, research at academic institutions, such as North Carolina State University, and industrial laboratories, such as Westinghouse (now Northrup-Grumman), Advanced Technology Materials, Inc. (ATMI), and Cree Research, Inc., coupled with the commercial offering of highquality SiC wafers from Cree, created an opportunity for further advancement. Improvements in epitaxial processes and device processing strategies were also realized during this time. Together these factors have enabled the fabrication of high-quality device structures and have generated increased research and funding activities in SiC electronic devices.

SiC-Seeded Crystal Growth

To take advantage of the superior properties of SiC-based devices, the industry must be able to produce high-quality SiC homoepitaxial films on single-crystal SiC wafers. Silicon- and GaAs-based semiconductor devices have taken advantage of the industry's ability to produce largediameter, high-quality substrates to develop large-volume, low-cost devices. For the same reasons, the development of low-cost, high-quality single-crystal SiC substrates has been pursued since the first Lely (nonseeded growth process) platelets were used to make devices. Currently several groups (ATMI, USA; Northrop Grumman Research Center, USA; Okmetic Ltd., Finland; Nippon Steel Corporation, Japan; and our company, Cree Research, Inc., USA) are working toward SiC substrate fabrication. University researchers also play an important role in the search for solutions to fundamental outstanding issues. These efforts are important because the future success of the SiC industry is closely related to the successful development of the SiC bulk growth process.

MBE growth and properties of GaN on GaN/SiC substrates

Growth of III–V nitrides by molecular beam epitaxy (MBE) is being studied at NCSU using an r.f. nitrogen plasma source. GaN SiC substrates consisting of ∼ 3 μm thick GaN buffer layers grown on 6HSiC wafers by MOVPE at Cree Research Inc. are being used as substrates in the MBE film growth experiments. The MBE-grown GaN films exhibit excellent structural and optical properties—comparable to the best GaN films grown by MOVPE—as determined from photoluminescence, X-ray diffraction, and vertical-cross-section TEM micrographs. Mg and Si have been used as dopants for p-type and n-type layers, respectively. Al x Ga 1 − x N films ( x ∼ 0.06-0.08) and Al x Ga 1 − x N GaN multi-quantum-well structures have been grown which display good optical properties. Light-emitting diodes based on double-heterostructures of Al x Ga 1 − x N GaN which emit violet light at ∼400 nm have also been demonstrated. Growth of GaN on LiGaO 2 substrates is also reported for comparison.

The First Nitride Laser Diode on Silicon Carbide

AbstractThis paper reports the events at NCSU leading up to and including those of June 5, 1997 which produced the first demonstration of a nitride laser diode on silicon carbide – and the very first nitride laser demonstration outside of Japan. All of the laser diode samples tested at NCSU were designed, grown, and fabricated into cleaved cavity test structures at Cree Research. Laser testing at NCSU consisted of spectral emission versus current measurements, light output power versus current (L-I) measurement, and light output polarization measurements versus current. The first successful laser on silicon carbide emitted at 402.6 nm. Subsequently, lasers displaying outputs ranging from 402.6 to 430.2 have been successfully tested at NCSU.

An infrared study of Ge + implanted SiC

The damage produced by 200 keV Ge + ion implantation in 6H-SiC (from Cree Research) has been studied by fast Fourier transform infrared (IR) reflectance spectroscopy. A new, recently developed at the spectroscopy laboratory of AUTh, non-destructive optical method, based on the normalized atomic vibrational damping of the ‘reststrahlen band’, has been applied to determine damage depth profiles of SiC for damage over three orders of magnitude, up to amorphisation. For the amorphized state IR results show an up to 30% reduction of heteronuclear SiC bonds. No new broad SiSi or CC vibrational bands were observed.