Structural characterization of 6H- and 4H-SiC polytypes by means of cathodoluminescence and x-ray topography

Abstract

The cathodoluminescence (CL) technique is used to analyse the radiative recombination properties of four distinct silicon carbide (SiC) samples: a 6H-SiC n+-type Lely wafer, two off-axis 4H-SiC epitaxial layers of n type and p type, and a ()-oriented 4H-SiC n+-type substrate. The CL spectra, recorded at various temperatures and at various excitation conditions, show strong differences between the polytypes, indicating a better homogeneous distribution of radiative centres inside the 6H polytype than in the 4H one, and also between the different orientations. For the ()-oriented 4H sample, luminescence features decrease when the excitation intensity increases, probably due to a more significant indirect transition band. The CL spectra also vary for the same sample, due to the impurity and the microscopic defect density variations. Comparisons between two local spectra taken in two distinct areas of the ()-oriented 4H sample, and with images obtained by x-ray topography in the same areas, allow us to establish that some structural defects are involved in luminescence centres. A deep centre involved in green luminescence (at 1.80 eV) is found to be associated with basal plane dislocations with the Burgers vector .

Source:IOPscience

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Preparation of SiC/Ge/graphene heterostructure on 4H-SiC(0001)

Highlights

•

SiC/Ge/graphene heterostructure on 4H-SiC was employed to realize VIS-NIR light operation of SiC devices.

•

Self-assembled submicron Ge islands have been prepared on 4H-SiC(0001) successfully.

•

Graphene on Cu foil was grown and wet transferred on Ge islands to form SiC/Ge/graphene heterostructure.

•

The structure of the SiC/Ge/graphene heterojunction is defined by XRD, TEM and Raman spectra characterizations.

Abstract

Due to the wide bandgap of SiC, SiC-based optoelectronic devices can not be operated by visible (VIS) and near-infrared (NIR) light sources. A promising way to solve this problem is to adopt a SiC/Ge/Graphene heterostructure. The Ge film can be used as VIS-NIR-absorbing layer. As a electrode, graphene also forms a Schottky junction with Ge on 4H-SiC. To form the SiC/Ge/graphene heterostructures, Ge film was prepared on 4H-SiC(0001) by chemical vapor deposition, and then graphene on Cu foil was wet transferred on Ge films. The Ge film on 4H-SiC(0001) has polycrystalline structure and consists of self-assembled submicron Ge islands. The monolayer graphene with a few cracks and wrinkles was transferred on the Ge rough surface. The SiC/Ge/graphene heterostructure with high light absorption in the 500–1200 nm range was prepared on 4H-SiC successfully.

Graphical abstract

Keywords

SiC;

Ge/graphene; heterostructure 

Raman

Microstructure;

Chemical vapour deposition;

Source:ScienceDirect

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Fundamental Summary of LED SiC Substrate

Silicon carbide is also called "emery powder" or "refractory sand." The usual manufacturing process of SiC is to combine silica sand, tar (or coke), woodchip (and salt when manufacturing green SiC) and other materials in an electric resistance furnace at a high temperature. Silicon carbide is usually divided into two categories, the black SiC and the green SiC, both having a hexagonal crystal structure, a density of 3.2 -3.25g/cm³ and microhardness of 2840-3320kg/mm2. The black SiC is manufactured with silica sand, tar and high quality silica as main materials in an electric resistance furnace at a high temperature. It is of hardness between corundum and diamond, mechanical robustness higher than corundum, crisp and sharp. The green SiC is manufactured with tar and high quality silica as main material, salt as additive, and in an electric resistance furnace at a high temperature. The hardness of green SiC is also between corundum and diamond, and mechanical robustness is higher than corundum.

Silicon carbide is of high hardness, good thermal and electrical conductivity, and is oxidation resistant under high temperature. It can be used as abrasive material or be made into abrasive tools such as abrasive wheel, sharpening stone, grinding unit, abrasive segment and so on. It can also be used as high temperature material and deoxidant in metallurgy. There are four major fields where silicon carbide is in general application: functional ceramics, high grade refractory, abrasive and metallurgy materials. And high-purity silicon carbide can further be used in semi-conductor and silicon carbide fibre production. Due to its unique physical and electrical properties, silicon carbide has become the best semi-conductor in some applications such as short wavelength photoelectric cell, high temperature, radiation resistant element and high frequency, high power component. Its major advantages are as following:

1.Wide energy level(eV)

 4H-SiC: 3.26 6H-Sic: 3.03 GaAs: 1.43 Si: 1.12

2.High thermal conduct efficiency(W/cm‧K@RT)

4H-SiC: 3.0-3.8 6H-SiC: 3.0-3.8 GaAs: 0.5 Si: 1.5

3.High disruptive field intensity

4H-SiC: 2.2x106 6H-SiC: 2.4x106 GaAs: 3x105 Si: 2.5x105

4.High saturated electron drift velocity(cm/sec @E 2x105V/cm)

4H-SiC: 2.0x107 6H-SiC: 2.0x107 GaAs: 1.0x10 Si: 1.0x107

With its wide energy level, the electronic components made of SiC are able to work under extremely high temperatures, can resist voltage and electric field 7 times larger than silicon and gallium arsenide, and is therefore especially good material in manufacturing high pressure and high power components, like high pressure diode. In addition, SiC is good thermal conductor with better conductivity than any other semi-conduct materials. These excellent characteristics have made SiC widely used in both industry and military fields.

Keywords:LED Substrate,

Source:LEDinside

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Combining graphene with silicon carbide: synthesis and properties – a review

Abstract

Being a true two-dimensional crystal, graphene possesses a lot of exotic properties that would enable unique applications. Integration of graphene with inorganic semiconductors, e.g. silicon carbide (SiC) promotes the birth of a class of hybrid materials which are highly promising for development of novel operations, since they combine the best properties of two counterparts in the frame of one hybrid platform. As a specific heterostructure, graphene on SiC performs strongly, dependent on the synthesis method and the growth modes. In this article, a comprehensive review of the most relevant studies of graphene growth methods and mechanisms on SiC substrates has been carried out. The aim is to elucidate the basic physical processes that are responsible for the formation of graphene on SiC. First, an introduction is made covering some intriguing and not so often discussed properties of graphene. Then, we focus on integration of graphene with SiC, which is facilitated by the nature of SiC to assume graphitization. Concerning the synthesis methods, we discuss thermal decomposition of SiC, chemical vapor deposition and molecular beam epitaxy, stressing that the first technique is the most common one when SiC substrates are used. In addition, we briefly appraise graphene synthesis via metal mediated carbon segregation. We address in detail the main aspects of the substrate effect, such as substrate face polarity, off-cut, kind of polytype and nonpolar surfaces on the growth of graphene layers. A comparison of graphene grown on the polar faces is made. In particular, growth of graphene on Si-face SiC is critically analyzed concerning growth kinetics and growth mechanisms taking into account the specific characteristics of SiC (0001) surfaces, such as the step-terrace structure and the unavoidable surface reconstruction upon heating. In all subtopics obstacles and solutions are featured. We complete the review with a short summary and concluding remarks.

Source:IOPscience

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Seeing X-rays in a new light: Soft X-ray detector could improve breast cancer imaging

Krishna Mandal examines a recently fabricated soft X-ray detector.

(Phys.org) -- A slice of light is about to come into focus for the first time, thanks to a new X-ray detector constructed at the University of South Carolina. And according to Krishna Mandal, the associate professor of electrical engineering who led the team that built it, the detector offers tremendous potential in breast cancer detection and treatment.

“There’s nothing available on the market that covers this range of X-rays,” Mandal said. “Nobody has explored this region, and there will be many innovations that will result from our being able to do so, particularly when it comes to medical imaging.”

X-rays are part of the electromagnetic spectrum, which ranges from low-energy radio waves to high-energy gamma rays. X-rays are on the high-energy end of the spectrum, just below gamma rays – they’re more energetic than ultraviolet light, which is more energetic than visible light.

As they just reported in Applied Physics Letters, the USC engineers have developed a laboratory-scale device that sensitively detects what are called “soft X-rays” – those on the lowest end of the X-ray energy scale.

At the other end of the X-ray spectrum are hard X-rays. The typical “X-ray” taken at a doctor’s or dentist’s office is a black-and-white photograph showing where hard X-rays were able to penetrate (the black area) or unable to penetrate (the white area) the object between the X-ray source and detector.

“If you take mammography as an example, hard X-rays pose difficulties,” Mandal said. “First, they have very high energy, and so we have to minimize exposure to them.” Soft X-ray devices are potentially less harmful to patients than those based on hard X-rays, he said.

“And more importantly, the soft X-rays interact with calcifications in the tissue,” he added. “Hard X-rays do not – they just pass through calcium deposits.”

Calcification is the deposition of calcium minerals in body tissue; in the breast it can be an indicator of pathology. Not as opaque as bone to X-rays, calcium deposits represent an very promising target for detailed soft X-ray mapping, Mandal said. He envisions the new soft X-ray detectors being at the forefront of a new way of imaging breast tissue, so that physicians can follow progression of calcification over time.

“It’s common for women even under 40 years of age to have calcifications,” Mandal said. “It’s critical to know whether it exists in the tissue and especially whether it is spreading.”

“But to see that, we need very high resolution detection systems, which is what we’ve made. These detectors are instantaneous, real-time and will be able to operate at room temperature with high resolution.”

Mandal’s team constructed the detector through epitaxial growth of silicon carbide on wafers of 4H-SiC. They were tested for response to soft X-rays at both the Los Alamos National Laboratory and Brookhaven National Laboratory.

The resulting detectors exhibited high sensitivity for soft X-rays (50 to 10,000 electron volts). There are no commercially available soft X-ray detectors covering this range, Mandal said, and comparison with an off-the-shelf ultraviolet detector showed a much more robust response for soft X-rays with the new device.

Source:PHYS

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Electrical properties and microstructural characterization of Ni/Ta contacts to n-type 6H–SiC*

Abstract

A Ni/Ta bilayer is deposited on n-type 6H–SiC and then annealed at different temperatures to form an ohmic contact. The electrical properties are characterized by I–V curve measurement and the specific contact resistance is extracted by the transmission line method. The phase formation and microstructure of the Ni/Ta bilayer are studied after thermal annealing. The crystalline and microstructure properties are analyzed by using glance incident x-ray diffraction (GIXRD), Raman spectroscopy, and transmission electron microscopy. It is found that the transformation from the Schottky to the Ohmic occurs at 1050 °C and the GIXRD results show a distinct phase change from Ta2C to TaC at this temperature. A specific contact resistance of 6.5× 10−5 Ωcm2 is obtained for sample Ni(80 nm)/Ta(20 nm)/6H–SiC after being annealed at 1050 °C. The formation of the TaC phase is regarded as the main reason for the excellent Ohmic properties of the Ni/Ta contacts to 6H–SiC. Raman and TEM data reveal that the graphite carbon is drastically consumed by the Ta element, which can improve the contact thermal stability. A schematic diagram is proposed to illustrate the microstructural changes of Ni/Ta/6H–SiC when annealed at different temperatures.

Source:IOPscience

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DFT study of stepped 4H-SiC{0001} surfaces

Highlights

•

DFT simulations of atomic steps on 4H-SiC{0001} in [11–20] and [10–10] directions.

•

[11–20] step with C atom on the edge at Si-terminated is the most favourable.C-terminated: structures with CC bonds and Si as the edge atom are preferable.

•

C-terminated: atomic and electronic structure is modified only at the very edge.

•

Si-terminated: surface buckling and electronic transfer among Si atoms on terraces.

Abstract

The growth of hexagonal crystals of the 4H-SiC and 6H-SiC polytypes occurs in the 〈0001〉 direction. The exposed crystal surface is composed of terraces of the {0001} oriented crystal planes separated by atomic steps of half-unit-cell or single-unit-cell height. We present results of density functional theory (DFT) study of the atomic structure and morphology of the atomic steps formed in the $[10\overline{1}0]$ and $[11\overline{2}0]$ directions on the Si-terminated 4H-SiC{0001} and the C-terminated 4H-SiC$\{000\overline{1}\}$ surfaces. On the latter surface atomic and electronic structure is modified only in the nearest neighbourhood of the step whereas the structure of the terrace is almost unchanged compared to that of the smooth stoichiometric surface. In contrast, on the Si terminated surface for both types of the applied model steps and on the widest terraces considered a substantial surface buckling is observed in the regions between steps. We calculated ledge energy and found that $[11\overline{2}0]$ step with C atom on the edge at Si-terminated surface is the most favourable energetically of all examined cases.

Keywords

Surfaces

Semiconductors

DFT

Steps

Atomic structure

Electronic structure

Source:ScienceDirect

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Growth and characterization of 3C–SiC and 2H–AlN/GaN films and devices produced on step-free 4H–SiC mesa substrates

Abstract

While previously published experimental results have shown that the step-free (0 0 0 1) 4H–SiC mesa growth surface uniquely enables radical improvement of 3C–SiC and 2H–AlN/GaN heteroepitaxial film quality (>100-fold reduction in extended defect densities), important aspects of the step-free mesa heterofilm growth processes and resulting electronic device benefits remain to be more fully elucidated. This paper reviews and updates recent ongoing studies of 3C–SiC and 2H–AlN/GaN heteroepilayers grown on top of 4H–SiC mesas. For both 3C–SiC and AlN/GaN films nucleated on 4H–SiC mesas rendered completely free of atomic-scale surface steps, TEM studies reveal that relaxation of heterofilm strain arising from in-plane film/substrate lattice constant mismatch occurs in a remarkably benign manner that avoids formation of threading dislocations in the heteroepilayer. In particular, relaxation appears to occur via nucleation and inward lateral glide of near-interfacial dislocation half-loops from the mesa sidewalls. Preliminary studies of homojunction diodes implemented in 3C-SiC and AlN/GaN heterolayers demonstrate improved electrical performance compared with much more defective heterofilms grown on neighbouring stepped 4H–SiC mesas. Recombination-enhanced dislocation motion known to degrade forward-biased 4H–SiC bipolar diodes has been completely absent from our initial studies of 3C–SiC diodes, including diodes implemented on defective 3C–SiC heterolayers grown on stepped 4H–SiC mesas.

Source:IOPscience

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Cree Offers 4H Silicon Carbide Epitaxial Wafers Featuring Very Low Basal Plane Dislocation

Cree,Inc,announces its latest silicon carbide (SiC) offering with low basal plane dislocation (LBPD) 100-mm 4H SiC epitaxial wafers.
 This LBPD material exhibits a total BPD density of < 1 cm-2 in the epitaxial drift layer, with BPDs capable of causing Vf drift as low as < 0.1 cm-2.
 
This low BPD material further demonstrates Cree’s long-standing commitment to continuous improvement and investment in SiC materials technology.

“Bipolar devices in SiC have long been held back by forward voltage degradation caused by the presence of BPDs,” said John Palmour, CTO, Cree Power & RF. “This Low BPD material enables very high voltage bipolar devices such as IGBTs (insulated-gate bipolar transistors) and GTOs (gate turn-off thyristor) to have improved stability over time. This recent development helps remove roadblocks to commercialization of these extremely high power devices.

SiC is a high-performance semiconductor material used in the production of a broad range of lighting, power and communication components, including light-emitting diodes(LEDs), power switching devices and RF power transistors for wireless communications. 

Keywords:SiC Epitaxial Wafer,North America,Cree,

Source:IOPscience

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Size effect on high temperature variable range hopping in Al+ implanted 4H-SiC

Abstract

The hole transport properties of heavily doped 4H-SiC (Al) layers with Al implanted concentrations of 3  ×  1020 and 5  ×  1020 cm−3 and annealed in the temperature range 1950–2100 °C, have been analyzed to determine the main transport mechanisms. This study shows that the temperature dependence of the resistivity (conductivity) may be accounted for by a variable range hopping (VRH) transport into an impurity band. Depending on the concentration of the implanted impurities and the post-implantation annealing treatment, this VRH mechanism persists over different temperature ranges that may extend up to room temperature. In this framework, two different transport regimes are identified, having the characteristic of an isotropic 3D VRH and an anisotropic nearly 2D VRH. The latter conduction mechanism appears to take place in a rather thick layer (about 400 nm) that is too large to induce a confinement effect of the carrier hops. The possibility that an anisotropic transport may be induced by a structural modification of the implanted layer because of a high density of basal plane stacking faults (SF) in the implanted layers is considered. The interpretation of the conduction in the heaviest doped samples in terms of nearly 2D VRH is supported by the results of the transmission electron microscopy (TEM) investigation on one of the 5  ×  1020 cm−3 Al implanted samples of this study. In this context, the average separation between basal plane SFs, measured along the c-axis, which is orthogonal to the carrier transport during electrical characterization, appears to be in keeping with the estimated value of the optimal hopping length of the VRH theory. Conversely, no SFs are detected by TEM in a sample with an Al concentration of 1  ×  1019 cm−3 where a 3D nearest neighbor hopping (NNH) transport is observed.

Source:IOPscience

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Mysterious molecules in space

Over the vast, empty reaches of interstellar space, countless small molecules tumble quietly though the cold vacuum. Forged in the fusion furnaces of ancient stars and ejected into space when those stars exploded, these lonely molecules account for a significant amount of all the carbon, hydrogen, silicon and other atoms in the universe. In fact, some 20 percent of all the carbon in the universe is thought to exist as some form of interstellar molecule.
This graph shows absorption wavelength as a function of the number of carbon atoms in the silicon-terminated carbon chains SiC_(2n+1)H, for the extremely strong pi-pi electronic transitions. When the chain contains 13 or more carbon atoms - not significantly longer than carbon chains already known to exist in space - these strong transitions overlap with the spectral region occupied by the elusive diffuse interstellar bands. Credit: D. Kokkin, ASU

Many astronomers hypothesize that these interstellar molecules are also responsible for an observed phenomenon on Earth known as the "diffuse interstellar bands," spectrographic proof that something out there in the universe is absorbing certain distinct colors of light from stars before it reaches the Earth. But since we don't know the exact chemical composition and atomic arrangements of these mysterious molecules, it remains unproven whether they are, in fact, responsible for the diffuse interstellar bands.

Now in a paper appearing this week in The Journal of Chemical Physics, from AIP Publishing, a group of scientists led by researchers at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. has offered a tantalizing new possibility: these mysterious molecules may be silicon-capped hydrocarbons like SiC3H, SiC4H and SiC5H, and they present data and theoretical arguments to back that hypothesis.

At the same time, the group cautions that history has shown that while many possibilities have been proposed as the source of diffuse interstellar bands, none has been proven definitively.

"There have been a number of explanations over the years, and they cover the gamut," said Michael McCarthy a senior physicist at the Harvard-Smithsonian Center for Astrophysics who led the study.

Molecules in Space and How We Know They're There

Astronomers have long known that interstellar molecules containing carbon atoms exist and that by their nature they will absorb light shining on them from stars and other luminous bodies. Because of this, a number of scientists have previously proposed that some type of interstellar molecules are the source of diffuse interstellar bands—the hundreds of dark absorption lines seen in color spectrograms taken from Earth.

In showing nothing, these dark bands reveal everything. The missing colors correspond to photons of given wavelengths that were absorbed as they travelled through the vast reaches of space before reaching us. More than that, if these photons were filtered by falling on space-based molecules, the wavelengths reveal the exact energies it took to excite the electronic structures of those absorbing molecules in a defined way.

Armed with that information, scientists here on Earth should be able to use spectroscopy to identify those interstellar molecules—by demonstrating which molecules in the laboratory have the same absorptive "fingerprints." But despite decades of effort, the identity of the molecules that account for the diffuse interstellar bands remains a mystery. Nobody has been able to reproduce the exact same absorption spectra in laboratories here on Earth.

"Not a single one has been definitively assigned to a specific molecule," said Neil Reilly, a former postdoctoral fellow at Harvard-Smithsonian Center for Astrophysics and a co-author of the new paper.

Now Reilly, McCarthy and their colleagues are pointing to an unusual set of molecules—silicon-terminated carbon chain radicals—as a possible source of these mysterious bands.

As they report in their new paper, the team first created silicon-containing carbon chains SiC3H, SiC4H and SiC5H in the laboratory using a jet-cooled silane-acetylene discharge. They then analyzed their spectra and carried out theoretical calculations to predict that longer chains in this family might account for some portion of the diffuse interstellar bands.

However, McCarthy cautioned that the work has not yet revealed the smoking gun source of the diffuse interstellar bands. In order to prove that these larger silicon capped hydrocarbon molecules are such a source, more work needs to be done in the laboratory to define the exact types of transitions these molecules undergo, and these would have to be directly related to astronomical observations. But the study provides a tantalizing possibility for finding the elusive source of some of the mystery absorption bands—and it reveals more of the rich molecular diversity of space.

"The interstellar medium is a fascinating environment," McCarthy said. "Many of the things that are quite abundant there are really unknown on Earth."

Explore further: Mysterious stellar absorption lines could illuminate 90-year puzzle

More information: The Journal of Chemical Physics, July 29, 2014. DOI: 10.1063/1.4883521 

Journal reference: Journal of Chemical Physics  

Provided by: American Institute of Physics

Source: PHYS

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Investigation of spatial charge distribution and electrical dipole in atomic layer deposited Al2O3 on 4H-SiC

Abstract

Charge distribution and electrical dipole in an Al2O3/4H-SiC structure are investigated by capacitance–voltage measurement and x-ray photoelectron spectroscopy (XPS). The charge densities in Al2O3 and at the Al2O3/4H-SiC interface are negligible and  −6.89  ×  1011 cm−2, respectively. Thus the small charge amount indicates the suitability of Al2O3 as a gate dielectric. The dipole at the Al2O3/4H-SiC interface is  −0.3 to  −0.91 V. The XPS manifests electron transfer from Al2O3 to 4H-SiC. The dipole formation is explained by a gap state model and the higher charge neutrality level of Al2O3 than the Fermi level of 4H-SiC, which confirms the feasibility of the gap state model on investigating band lineup at heterojunctions. The electrical dipole at the Al2O3/4H-SiC interface is critical for threshold voltage tuning. These results are helpful in engineering the SiC based gate stacks.

Keywords:Al2O3/4H-SiC,

Source:  iopscience

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Evaluation of nitrogen incorporation into bulk 4H-SiC grown on seeds of different orientation from optical absorption spectra

Abstract

The effectiveness of n-type nitrogen doping of bulk 4H-SiC grown on seeds of different orientation is studied by optical absorption measurements. The 4H-SiC ingots have been grown by physical vapour transport (PVT), with nitrogen doping from the SiC source. The nitrogen concentration was determined at room temperature from the absorption peak intensity at 464 nm, with account for the degree of donor ionization. It has been shown that 4H-SiC ingots grown on Si (11-22) faces are significantly less doped by nitrogen than the ones grown on C (11-2-2).

Keywords:4H-SiC,

Source:  iopscience

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α-SiC nanoscale transit-time diodes: performance of the photo-irradiated terahertz sources at elevated temperature

Abstract

The effects of elevated junction temperature on the terahertz (THz) frequency characteristics of α-(hexagonal, 4H and 6H) silicon carbide (SiC) based double-drift region (DDR, p++ p n n++ type) impact ionization avalanche transit-time (IMPATT) devices are studied and compared for the first time through simulation experiments. This study reveals that at 300 K < T < 600 K, a 4H-SiC IMPATT diode may yield 3.5 W of output power (efficiency (η) ~ 8.6%) at 1.3 THz, while its 6H-SiC counterpart can deliver 3 W of output power (η = 6.3%) at 1.2 THz. It is interesting to observe that at elevated temperature, the performance of a 6H-SiC IMPATT diode degrades more in comparison with its 4H-SiC counterpart. These comparative analyses reveal the superiority of 4H-SiC diodes over their 6H-SiC counterparts, and thus establish the potential of the former as a high-power THz IMPATT oscillator even in harsh environments. Mobile space charge effects and the effect of positive series resistance on the high-temperature performance of the THz devices are also simulated, and it is found that series resistance reduces the output power level of the diodes by at least 15.0%. Moreover, the effects of increased junction temperature on the photo-sensitivity of top mounted (TM) and flip chip (FC) α-SiC IMPATTs are also investigated using a modified simulation technique. The device operating frequencies, under TM illumination configuration, shifts upward by at least 40.0 GHz, whereas the operating frequency shifts upward by at least 100.0 GHz under FC illumination configuration. The simulation results and the proposed experimental methodology presented here may be used for realizing optically controlled α-SiC transit-time devices for application in THz communication.

Keywords:SiC,

Source:  iopscience

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