α-SiC Single Crystals Research Articles

Thermodynamic design of a high temperature chemical vapor deposition process to synthesize α-SiC in Si-C-H and Si-C-H-Cl systems

In this study, we thermodynamically reviewed the suitable growth process conditions of α-SiC in the Si-C-H system using tetramethylsilane (TMS) and in the Si-C-H-Cl system using methyltrichlorosilane (MTS). In the Si-C-H-Cl system, pure solid SiC was obtained at high temperatures under a larger range of hydrogen dilution ratios than that tolerated in the Si-C-H system. X-ray diffraction and micro-Raman analysis of the products obtained at 1900, 2000, and 2100 °C showed that the α-SiC becomes more dominant with increasing temperature in the Si-C-H-Cl system. While TMS was unsuitable for high temperature processing due to its high C/Si ratio, MTS was found to be appropriate for growing α-SiC crystals at high temperatures under a range of conditions. These results indicate that a novel method to grow α-SiC single crystals through HTCVD using MTS as a precursor could be established.

Retention and thermal release behavior of deuterium and helium implanted in SiC

Retention and release of ion implanted deuterium (D) and helium (He) in silicon carbide (SiC) were studied with respect to damage accumulation and annealing behavior using ion beam analysis techniques. α-SiC single crystals were irradiated by 10keV D2+ and He+ at 300 and 770K, and depth profiles of retained atoms and lattice disorder were measured during the implantation and successive heat treatments. For the sample pre-irradiated with He at a fluence of 1.0×1021ions/m2, the thermal release of D atoms was completed at an annealing temperature of 1370K, while the retained D atoms still remained in the sample without pre-irradiation. In contrast to the facilitated thermal release of D by He pre-irradiation, the He release temperature increased in the sample followed by D ion implantation. No significant difference of He retention and damage accumulation behavior was observed between the samples implanted at 300 and 770K. The D retention and D-ion-induced disorder for 770K implantation, was reduced to approximately 1/2 comparing to that of the sample implanted at 300K. The D retention at 770K was not affected by the He pre-implantation induced damage.

The Role of Surface Chemistry on Spreading Kinetics of Molten Silicides on Silicon Carbide

The spreading time for millimeter-sized droplets of nonreactive molten silicides on silicon carbide in high vacuum is several orders of magnitude higher than typical spreading times observed in nonreactive metal/ceramic systems. To explain this paradox, two types of experiments were performed: (i) wetting experiments for various nonreactive CuSi alloys on α-SiC single crystals using the sessile drop and dispensed drop techniques, with emphasis on determining the initial contact angle; and (ii) characterization of surface chemistry of SiC after different heat treatments in high-vacuum furnaces. It is shown that spreading kinetics in these systems are controlled by the kinetics of removing of wetting barriers present or developed in situ on SiC surface.

Spreading of Cu–Si alloys on oxidized SiC in vacuum: experimental results and modelling

The wetting behaviour of a Cu–40 at.% Si alloy, non-reactive with SiC, on oxidized (0001) faces of α-SiC single crystals is studied at temperatures close to the melting point of copper by the “dispensed drop” technique under high vacuum. The experiments focused on wetting kinetics to determine the mechanisms controlling the rate of spreading under conditions allowing for “in situ” deoxidation of silicon carbide. It is shown that spreading occurs in three stages: (i) an initial stage consisting of a rapid spreading of the alloy on the oxidized SiC up to a contact angle which is close to that on vitreous silica, (ii) a second stage with a zero spreading rate during which the alloy goes through the oxide layer by dissolution near the triple line allowing an intimate contact between the alloy and the SiC surface to be established, and (iii) a final stage during which the alloy spreads with a constant rate up to a contact angle equal to that of clean SiC. These results are interpreted on the basis of a dissolution–diffusion–evaporation process occurring in the vicinity of the solid–liquid–vapour triple line.

Etching Kinetics of α-SiC Single Crystals by Molten KOH

The etching behaviour of {alpha}-SiC single crystals by molten KOH has been investigated. The etching rate is significantly affected by the etching ambience: dry air yields an etching rate eight times larger than nitrogen. This result suggests an essential role of dissolved oxygen in the melt for the molten KOH etching of SiC. The {l_brace}0001{r_brace} surface shows a surface polarity dependence: the etching rate of (000 anti 1)C is about four times larger than that of (0001)Si. The etching rate of (000 anti 1)C exhibits an Arrhenius type temperature dependence with an activation energy of 15-20 kcal/mol. The obtained activation energy and selectivity between (000 anti 1)C and (0001)Si are quite similar to those for thermal oxidation. The carrier concentration hardly influences the etching rate up to 3 x 10{sup 19}cm{sup -3}, while crystal with a larger hexagonality shows a larger etching rate. (orig.) 12 refs.

Structural defects in α-SiC single crystals grown by the modified-Lely method

Structural characterization of single crystalline α-SiC has been conducted by X-ray topography. Crystals were grown in the [0001¯] and the [11¯00] directions by the modified-Lely method, and wafers perpendicular and parallel to the growth directions sliced from the grown crystals were examined by transmission topographs of the Lang method. The crystals grown in the [0001¯] and the [11¯00] directions showed a significant difference in both types and densities of crystal defects. Each growth direction exhibited a peculiar defect formation, and the topography revealed that most of the defects were formed at the very initial stage of growth.

Wetting kinetics and bonding of Al and Al alloys on α-SiC

The wetting behaviour of aluminium on basal planes of α-SiC single crystals was investigated between 1000 and 1200 K by the sessile drop technique in high vacuum. The experiments were focused on wetting kinetics to determine the mechanisms controlling the rate of spreading and to identify all the characteristic contact angles formed during an isothermal experiment. Pure Al and Al alloys with silicon, copper and tin were studied. In some experiments SiC substrates covered by a silica layer 10–50 nm thick were used.

High energy Ni ion implantation and thermal annealing for α-SiC single crystal

The annealing behavior of α-SiC(0001) single crystals is studied using high energy ion implantations and thermal annealings. 1.0 MeV Ni+ ion implantations at RT were carried out on samples with fluence of 1 × 1015 and 1 × 1017cm2. Post-implantation annealings were performed in 1 atm Ar gas flow at 500, 1000 and 1500°C for 2 h. Samples were analyzed by Rutherford backscattering spectroscopy with channeling and scanning electron microscopy.During the 500°C annealing, the thickness of the introduced amorphous layer, TA, did not change for both samples. During the 1000°C annealing, TA decreased for the 1 × 1015 Ni+cm2 sample, but not for the 1 × 1017 Ni+cm2. The initiation temperature of the recrystallization increases with the fluence. During the 1500°C annealing, an explosive recrystallization occurred for both samples. Moreover, in the recrystallized region of the 1 × 1017 Ni+cm2 sample, a layer with a high defect concentration was found in the depth range from 0.5 μm to 0.7 μm, but not for the 1 × 1015 Ni+cm2. The recrystallization at 1500°C was accompanied by an extensive Ni diffusion. Then, the Ni profile changed from a Gaussian-like distribution to a trapezoid which extends from the surface to a depth 0.5 μm. The new Ni distribution is faced with the layer with a high defect concentration at a depth of 0.5 μm.

Transmission electron microscopy and high-resolution electron microscopy studies of structural defects induced in 6H α-SiC single crystals irradiated by swift Xe ions

Abstract Single crystals of 6Hα-SiC, p type, were irradiated with 5·5GeV Xe ions supplied by GANIL close to the 〈0001〉 crystal axis with fluences up to 1015 Xe cm−2. After irradiation the colour of the crystals changed in the irradiated zone. Transmission electron microscopy observations of samples which had been irradiated and then annealed at 1373 K revealed the presence of loops lying in basal and prismatic planes. Basal faults have an interstitial character with a Burgers vector of the type 1/6〈0001〉. Contrast analyses as well as high-resolution electron microscopy observations allowed us to propose a geometrical model for the new defects observed in prismatic planes of the 6H polytype. These prismatic loops are the consequence of interstitial precipitation in {1012} pyramidal planes; the fault plane has a ‘zigzag’ configuration and a Burgers vector of the type 1/18〈8081〉.

Characterization by atomic-force microscopy of nanometre-scale plastic damage in Sic single crystals after being sintered by hot isostatic pressing

Abstract The plastic deformations introduced in silicon carbide (SiC) single crystals after they had been sintered by hot isostatic pressing (HIP) have been investigated by atomic-force microscopy (AFM). The starting α-SiC single crystals, mainly consisting of 6H and 4H polytypes, were platelet shaped with an average diameter and thickness of 24.6 and 3.2 μm respectively. After HIP sintering for 1 h in a silicon nitride (Si3N4) matrix carried out at 2050°C and 180 MPa to produce dense composite bodies, the SiC platelets were freed from the composite microstructure by a chemical treatment using a dilute NaOH aqueous solution. They were then imaged by AFM. Nanometre resolution was obtained by a contamination tip grown in the scanning electron microscope. Besides the presence of macroplastic deformation observed in some cases up to 16%, the SiC platelets typically showed two kinds of plastic damage: impressions by Si3N4 grains (average depth, 45.5 nm) left on the top surfaces and shear slip along the basal p...

High-resolution electron microscopy study of electron-irradiation-induced crystalline-to-amorphous transition in α-SiC single crystals

Abstract An electron-irradiation-induced crystalline-to-amorphous (CA) transition in α-SiC has been studied by high-resolution electron microscopy (HREM). The irradiation-produced damage structure was examined as a function of dose of electrons by taking high-resolution maps extending from the unirradiated crystalline region to the completely amorphized region. In the intermediate region between those two regions, that is in the CA transition region, the damage structure was essentially a mixture of crystalline and amorphous phases. The volume fraction of the amorphous phase was found to increase with increasing dose of electrons and no discrete crystalline-amorphous interface was observed in CA transition region. These facts indicate the heterogeneous and gradual nature of the CA transition. In the transition region close to the unirradiated crystalline region, a sort of fragmentation of the crystal lattice was observed to occur; crystallites with slightly different orientations with respect to the paren...

The hardness and elastic modulus of chromium-implanted silicon carbide

The hardness and elastic modulus of α-SiC single crystals were determined using an ultra-low load micro-indentation hardness tester for implantation conditions that produced both crystalline and amorphous as-implanted surfaces. The effect of fluence for 260 keV chromium implantation was noted as was the effect of substrate temperature. For implantation at room temperature, the relative hardness (implanted/unimplanted) rises to a maximum of 1.10 at a fluence of 1.5 × 10 14 Cr/cm 2 (260 keV) and then decreases with increasing fluence as amorphization begins. The fully amorphous silicon carbide has a hardness value that is about 40% of the unimplanted value. The elastic modulus of the implanted material is lower for all implanted conditions.

Electron-irradiation-induced crystalline to amorphous transition in α-Sic single crystals

Abstract An electron-irradiation induced crystalline-to-amorphous (C-A) transition in α-Sic single crystals of the 6H polytype has been studied as a function of irradiation temperature, incident electron energy and orientation of the incident beam, by means of ultra-high voltage electron microscopy. The C-A transition can be induced at temperatures below 290 K. The minimum energy of incident electrons to cause the C-A transition is 725 ± 25 keV. The electron dosage required for the C-A transition is essentially constant at temperatures below 220 K, while at temperatures above 220 K, the dosage increases quickly with temperature until no amorphization can be induced any more at the critical temperature of 290 K. At fixed temperatures below the critical temperature, the required dosage decreases with increasing incident electron energy. The temperature and incident energy dependence of the required dosage indicate that a knock-on mechanism rather than an ionization mechanism is responsible for the C-A transition. The amorphization dosage of the irradiation along the ⟨1120⟩ and ⟨1100⟩ directions (i.e. parallel to the basal plane) is smaller than that of the irradiation along the ⟨0001⟩ direction (i.e. perpendicular to the basal plane). The crystallization temperature of the irradiation-produced amorphous Sic is 1148 ± 25 K.

Defects in β-SiC thin films grown on α-SiC substrates

Beta-SiC is an ideal candidate material for use in semiconductor device applications. Currently, monocrystalline β-SiC thin films are epitaxially grown on {100} Si substrates by chemical vapor deposition (CVD). These films, however, contain a high density of defects such as stacking faults, microtwins, and antiphase boundaries (APBs) as a result of the 20% lattice mismatch across the growth interface and an 8% difference in thermal expansion coefficients between Si and SiC. An ideal substrate material for the growth of β-SiC is α-SiC. Unfortunately, high purity, bulk α-SiC single crystals are very difficult to grow. The major source of SiC suitable for use as a substrate material is the random growth of {0001} 6H α-SiC crystals in an Acheson furnace used to make SiC grit for abrasive applications. To prepare clean, atomically smooth surfaces, the substrates are oxidized at 1473 K in flowing 02 for 1.5 h which removes ∽50 nm of the as-grown surface. The natural {0001} surface can terminate as either a Si (0001) layer or as a C (0001) layer.

High-temperature depletion-mode metal-oxide-semiconductor field-effect transistors in beta-SiC thin films

Depletion-mode n-channel metal-oxide-semiconductor field-effect transistors were fabricated on n-type β-SiC (111) thin films epitaxially grown by chemical vapor deposition on the Si (0001) face of 6H α-SiC single crystals. The gate oxide was thermally grown on the SiC; the source and drain were doped n+ by N+ ion implantation at 823 K. Stable saturation and low subthreshold current were achieved at drain voltages exceeding 25 V. Transconductances as high as 11.9 mS/mm were achieved. Stable transistor action was observed at temperatures as high as 923 K, the highest temperature reported to date for a transistor in any material.

Self-diffusion in alpha and beta silicon carbide

Abstract The self-diffusion coefficients of 14C and 30Si have been measured for lattice transport in high purity and N-doped α-SiC single crystals and in high purity polycrystalline CVD β-SiC in the temperature range of 2123–2573 K. Grain boundary diffusion of 14C has also been determined in the β-SiC material. The results of these studies reveal a vacancy mechanism wherein 14C diffuses considerably faster than 30Si in both materials. Furthermore, Dc★ in the N-doped single crystals is smaller than for the undoped materials, while the opposite is true for the 30Si transport in these crystals. Changes in the concentration of the charged C and Si vacancies are reasoned to be the underlying cause of this last phenomena. A discussion of the effect of Si evaporation and its effect upon the value of the diffusion coefficient is also presented.

Effect of crystal orientation, structure and dimension on vickers microhardness anisotropy of β-, α-Si 3N 4, α-SiO 2 and α-SiC single crystals

Variation of Vickers microhardness on planes parallel to [VMH(∥C)] and perpendicular to the C-axis [VMH(⊥C)] of hexagonal β-Si 3N 4, α-Si 3N 4, α-SiO 2 (α-quartz) and α-SiC(6H) single crystals has been studied. The VMH (∥ C) VMH (⊥ C) ratios have been correlated with a c ratios, a and c being unit cell parameters of the single crystals. VMH(⊥C) was found to be independent of orientation of Vickers indenter on basal plane (0001) of α-SiC, while VMH(∥C) was found to vary with Vickers indenter direction on prismatic plane (10-10) of α-SiC. VMH(∥C) of β-Si 3N 4 single crystals was found to depend on the aspect ratio of β-Si 3N 4, while VMH(⊥C) was found to be independent of the dimension of basal plane. VMH(⊥C) of α-SiC single crystals was also found to be independent of the dimension of basal plane of α-SiC.

Self-diffusion of silicon-30 in ?-SiC single crystals

The self-diffusion of30Si in high purity and N-doped α-SiC single crystals has been measured in the temperature range 2273 to 2573 K. The diffusion (DSi*) in N-doped crystals exceeds that in the pure crystals because of the increase in the concentration of the charged acceptor-type Si vacancies in the presence of the N species. A comparison ofDC* andDSi* shows that the former exceeds the latter by approximately 102, primarily because of the greater entropy of migration of C. Possible crystallographic paths of transport for both species are also discussed.

Self-diffusion in alpha and beta silicon carbide

The self-diffusion coefficients of 14C and 30Si have been measured for lattice transport in high purity and N-doped α-SiC single crystals and in high purity polycrystalline CVD β-SiC in the temperature range of 2123–2573 K. Grain boundary diffusion of 14C has also been determined in the β-SiC material. The results of these studies reveal a vacancy mechanism wherein 14C diffuses considerably faster than 30Si in both materials. Furthermore, D c ★ in the N-doped single crystals is smaller than for the undoped materials, while the opposite is true for the 30Si transport in these crystals. Changes in the concentration of the charged C and Si vacancies are reasoned to be the underlying cause of this last phenomena. A discussion of the effect of Si evaporation and its effect upon the value of the diffusion coefficient is also presented.

Growth of SiC whiskers in the system SiO 2-C-H 2nucleated by iron

A study has been made of the nucleation, growth and morphology of α- and β-SiC whiskers obtained from silica and carbon in hydrogen between 1200 and 1300°C by a VLS process with iron as agent. On polycrystalline substrates - the solid starting materials or an inert third material - β-whiskers are obtained mainly grown in the [111] direction; on basal {0001} and prismatic {10 1 0} faces of α-SiC single crystals in addition α-[0001] and α-[1 1 0] whiskers respectively grow epitaxially. The growth rate is in the order of 1 mm/hr, irrespective of polytype and growth direction. The rate depends on the relative position of C, SiO 2 and substrate; in a given arrangement the rate increases with increasing temperature with an activation energy of about 85 kcal/mole. As to be expected for a VLS process in which the supply of vapour is rate-limiting, the rate is higher for whiskers grown from relatively larger droplets. The β-[111] whiskers often show a fluctuation in diameter, whereas the α-whiskers are always smooth. For smooth whiskers the ratio of the diameter of the Fe-Si-C alloy droplet at the tip and the diameter of the whiskers is about 2, and for knotted whiskers generally larger. These ratios are associated with minima in the Gibbs free surface energy at the vapour-liquid-solid interface. It is discussed how instabilities in the liquid can give rise to knottiness of β-[111] whiskers in a system where solid SiC grows from a gas phase with Si/C ⪡ 1 via a liquid phase with Si/C ⪢ 1.