Single Crystal 3C-SiC Research Articles

Size- and surface-dependent electronic structures of crystalline SiC nanotubes

In this paper, the dependence of the stability and electronic structures of single-crystal 2H SiC and 3C SiC nanotubes on the sizes, surface facets, and surface saturation conditions are investigated with the help of density functional theory. The stabilities of each type of SiC nanotube increase as the wall-thicknesses increase no matter whether the surfaces are bare or saturated with hydrogen atoms. 2H SiC nanotubes are energetically more stable than 3C SiC nanotubes. The band gaps of each type of SiC nanotube decrease uniformly as the wall thicknesses increase when they are fully saturated or outer saturated with H atoms, while they increase uniformly when they are bare. The different trends of band gaps for bare and H-saturated SiC nanotubes reflect the competition between the quantum confinement effects and surface effects. Surface saturation can largely increase the band gaps especially for 2H SiC nanotubes. All the nanotubes possess direct band gaps when they are fully saturated with hydrogen atoms while most of them possess indirect ones when they are bare or half saturated. However, the bare or half-saturated single-crystal SiC nanotubes may also possess good optoelectronic properties because the top valance bands and bottom conduction bands are much flatter. Our results indicate that single-crystal SiC nanotubes, especially 2H SiC nanotubes, may have wider application prospects in optoelectronic devices.

Field emission performance of SiC nanowires directly grown on graphite substrate

Lawn-like SiC nanowire arrays were successfully synthesized on graphite substrates by thermal evaporation of silicon powders at high temperature. The morphology, microstructure and composition of the nanowires were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The product grown on graphite substrates was hexagonal prism-shaped single-crystal 3C-SiC nanowires with high aspect ratio. Planar defects, such as microtwins and stacking faults were observed in SiC nanowires. Field emission measurements of the SiC nanowires grown on graphite substrate showed a very low threshold field of 2.1 V μm −1, high brightness and stable field emission performance.

Amorphization of nanocrystalline 3C-SiC irradiated with Si+ ions

Irradiation-induced amorphization in nanocrystalline and single-crystal 3C-SiC has been studied using 1 MeV Si+ ions under identical irradiation conditions at room temperature and 400 K. The disordering behavior has been characterized using in situ ion channeling and ex situ x-ray diffraction methods. The results show that, compared with single-crystal 3C-SiC, full amorphization of small 3C-SiC grains (˜3.8 nm in size) at room temperature occurs at a slightly lower dose. Grain size decreases with increasing dose until a fully amorphized state is attained. The amorphization dose increases at 400 K relative to room temperature. However, at 400 K, the amorphization dose for 2.0 nm grains is about a factor of 4 and 8 smaller than for 3.0 nm grains and bulk single-crystal 3C-SiC, respectively. The behavior is attributed to the preferential amorphization at the interface.

Residual Stress Measurement and Simulation of 3C-SiC Single and Poly Crystal Cantilevers

The fabrication of SiC MEMS-based sensors requires new processes able to realize microstructures on either bulk material or on the SiC surface. The hetero-epitaxial growth of 3C-SiC on silicon substrates allows one to overcome the traditional limitations of SiC micro-fabrication. In this work a comparison between single crystal and poly crystal 3C-SiC micro-machined structures will be presented. The free-standing structures realized (cantilevers and membrane) are also a suitable method for residual field stress investigation in 3C-SiC films. Measurement of the Raman shift indicates that the mono and poly-crystal 3C-SiC structures release the stress in different ways. Finite element analysis was performed to determine the stress field inside the films and provided a good fit to the experimental data. A comprehensive experimental and theoretical study of 3C-SiC MEMS structures has been performed and is presented.

Study of the Spontaneous Nucleation of 3C-SiC Single Crystals Using CF-PVT Technique

The control of the nucleation step is a critical issue for a future development of 3C-SiC bulk growth. The possibility to get very high quality 3C-SiC single crystal through self-nucleation on graphite was already demonstrated but the large number of nuclei limits the growth of only one crystal. In this study, we have investigated different configurations that help improving the nucleation step. For that, the “necking” stage, well established in Bridgman or Czochralski growth processes, has been successfully applied to the growth of 3C-SiC with the CF-PVT technique. This has allowed getting only one 3C-SiC crystal. The enlarged parts, after having passed the neck, are of high structural quality.

High-quality 6 inch (111) 3C-SiC films grown on off-axis (111) Si substrates

Growth of single crystal 3C-SiC films on large area off-axis (111) Si substrate by chemical vapour deposition technique is here reported. The growth was conducted on off-axis Si substrates due to the ability of the misorientation to reduce anti-phase disorder in the 3C-SiC film. 3C-SiC films show an extremely flat surface and interface, stimulating further interest for electrical and mechanical device applications even if a very strong bow, due to the strain induced by the growth process, is observed. Film quality was proved to be high by several investigation techniques and a study of the crystalline defects is also presented. Optical profilometer measurements were also conducted to evaluate accurately the asymmetric curvature of the whole system.

3C-SiC Films on Si for MEMS Applications: Mechanical Properties

Single crystal 3C-SiC films were grown on (100) and (111) Si substrate orientations in order to study the resulting mechanical properties of this material. In addition, poly-crystalline 3C-SiC was also grown on (100)Si so that a comparison with monocrystaline 3C-SiC, also grown on (100)Si, could be made. The mechanical properties of single crystal and polycrystalline 3C-SiC films grown on Si substrates were measured by means of nanoindentation using a Berkovich diamond tip. These results indicate that polycrystalline SiC thin films are attractive for MEMS applications when compared with the single crystal 3C-SiC, which is promising since growing single crystal 3C-SiC films is more challenging. MEMS cantilevers and membranes fabricated from a 2 µm thick single crystal 3C-SiC grown on (100)Si under similar conditions resulted in a small degree of bow with only 9 µm of deflection for a cantilever of 700 µm length with an estimated tensile film stress of 300 MPa. Single crystal 3C-SiC films on (111)Si substrates have the highest elastic and plastic properties, although due to high residual stress they tend to crack and delaminate.

Single Crystal and Polycrystalline 3C-SiC for MEMS Applications

Cantilever resonators have been fabricated from two types of materials, single crystal and polycrystalline 3C-SiC films. The films have been grown in a hot-wall chemical vapor deposition reactor on 100 mm diameter p-type boron-doped (100) Si wafer without rotation of the wafer. The crystal structure of the films have been accessed with X-ray diffraction. The cantilever devices have been fabricated using a one-step etch and release process; the beam length has been varied between 50 and 200 µm. Resonant frequencies in the range 110 KHz – 1.5 MHz and 50 – 750 KHz have been obtained for single crystal and polycrystalline SiC devices, respectively. Furthermore, the experimental resonance frequencies have been used to calculate the Young’s Modulus E for the two different types of SiC. The single crystal SiC, possessing a very high Young’s Modulus (446 GPa), should be an optimal material for RF-MEMS applications.

Towards Large Area (111)3C-SiC Films Grown on Off-Oriented (111)Si

The choice of off-axis (111) Si substrates is poorly reported in literature despite of the ability of such an oriented Si substrate in the reduction of stacking faults generation and propagation. The introduction of off-axis surface would be relevant for the suppression of incoherent boundaries. We grew 3C-SiC films on (111) Si substrates with a miscut angle from 3° to 6° along <110> and <11 >. The film quality was proved to be high by X-Ray diffraction (XRD) characterization. Transmission electron microscopy was performed to give an evaluation of the stacking fault density while pole figures were conducted to detect microtwins. Good quality single crystal 3C-SiC films were finally grown on 6 inch off-axis (111)Si substrate. The generated stress on both 2 and 6 inch 3C-SiC wafers has been analyzed and discussed.

Disorder accumulation and recovery in gold-ion irradiated 3C-SiC

A single-crystal 3C-SiC film on a Si/SiO2/Si (separation by implantation of oxygen ) substrate was irradiated in different areas at 156 K with Au2+ ions to low fluences. The disorder profiles as a function of dose on both the Si and C sublattices have been determined in situ using a combination of 0.94 MeV D+ Rutherford backscattering spectrometry and nuclear reaction analysis in channeling geometry along the ⟨100⟩, ⟨110⟩, and ⟨111⟩ axes. The results indicate that for the same damage state, the level of disorder on the Si sublattice in 3C-SiC follows a decreasing order along the ⟨111⟩, ⟨100⟩, and ⟨110⟩ axes, while that on the C sublattice shows comparable values. Similar levels of Si and C disorder are observed along the ⟨111⟩ axis over the applied dose range. However, the level of C disorder is higher than that of Si disorder along all axes at low doses. The amount of disorder recovery during thermal annealing depends on the sublattice (Si or C) and crystallographic orientation. Room-temperature recovery occurs for both sublattices in 3C-SiC irradiated to a dose of 0.047 dpa or lower. Significant recovery is observed along all directions during thermal annealing at 600 K. The results are discussed and compared to those for 6H-SiC and 4H-SiC under similar irradiation conditions.

Characterization of 3C-SiC micro-pillars on Si(100) substrate grown by vapor-liquid–solid process

Cubic silicon carbide (3C-SiC) micro-pillars were fabricated on Si (100) substrate by vapor–liquid–solid (VLS) process. The microstructure and residual stress of the micro-pillars were investigated by electron microscopy and micro-Raman spectroscopy, respectively. The selected area diffraction pattern of the SiC micro-pillar indicated that the micro-pillar was 3C-SiC single crystal. The residual stress of about 0.3 GPa in the micro-pillar calculated from Raman spectrum indicated that the VLS grown 3C-SiC micro-pillars had good crystalline quality.

Electrical Characterization of Microelectromechanical Silicon Carbide Resonators.

This manuscript describes the findings of a study to investigate the performance of SiC MEMS resonators with respect to resonant frequency and quality factor under a variety of testing conditions, including various ambient pressures, AC drive voltages, bias potentials and temperatures. The sample set included both single-crystal and polycrystalline 3C-SiC lateral resonators. The experimental results show that operation at reduced pressures increases the resonant frequency as damping due to the gas-rarefaction effect becomes significant. Both DC bias and AC drive voltages result in nonlinearities, but the AC drive voltage is more sensitive to noise. The AC voltage has a voltage coefficient of 1∼4ppm/V at a DC bias of 40V. The coefficient of DC bias is about -11ppm/V to - 21ppm/V for poly-SiC, which is more than a factor of two better than a similarly designed polysilicon resonator (-54 ppm/V). The effective stiffness of the resonator decreases (softens) as the bias potential is increased, but increases (hardens) as drive voltage increase when scan is from low to high frequency. The resonant frequency decreases slightly with increasing temperature, exhibiting a temperature coefficient of -22 ppm/°C, between 22°C and 60°C. The thermal expansion mismatch between the SiC device and the Si substrate could be a reason that thermal coefficient for these SiC resonators is about twofold higher than similar polysilicon resonators. However, the Qs appear to exhibit no temperature dependence in this range.

Increased Growth Rates of 3C-SiC on Si (100) Substrates via HCl Growth Additive

Growth rates from 10 to 38 μm/h of single crystal 3C-SiC on planar Si (001) substrates have been obtained in a low-pressure horizontal hot-wall CVD reactor. The propane-silanehydrogen gas chemistry system with HCl added as a growth additive, which allows an increased amount of silane to be introduced into the reactor during growth, was used. The 3C-SiC film growth rate versus silane mole fraction was found to be a linear function in the range from 0.43x10-3 to 1.50x10-3. Nomarski optical microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, atomic force microscopy and X-ray diffraction were used to characterize the deposited layers. The X-ray rocking curve taken on the (002) diffraction plane of a 12 μm thick 3CSiC (001) layer displayed a FWHM of 360 arcsec, which indicates the films are mono-crystalline.

Reduction of defects propagating into 3C-SiC homoepilayers by reactive ion etching of 3C-SiC heteroepilayer substrates

The crystallinity and morphology of single-crystal 3C-SiC homoepilayers grown on heteroepilayer (0 0 1) substrates by low-pressure chemical vapor deposition were investigated. The crystalline qualities of homoepilayers were critically dependent upon the defects of heteroepilayer substrates because planar defects, predominantly stacking faults and twins, and protrusions existing on the heteroepilayer surface propagated into the homoepilayers. A surface etching process using reactive ion etching (RIE) of the backside of free-standing heteroepilayers, the interface with Si substrates, was proposed to minimize the defect densities on the heteroepilayer surface. Analyses of high resolution X-ray diffraction (HRXRD) and atomic force microscope (AFM) reveal that simultaneous reductions of both surface roughness and defect densities on the heteroepilayer surface are achieved by an etching depth of 4 μm. Cross-sectional scanning electron microscopy (SEM) observation clearly shows that protrusions on the RIE-treated heteroepilayer surface are readily buried by growing homoepilayers. Furthermore, the results of cross-sectional transmission electron microscopy (XTEM) indicate that (i) significant amounts of planar defects are removed by the RIE process of the backside of heteroepilayers and (ii) most of planar defects propagating into the homoepilayers are terminated by coalescences between one another during early homoepitaxial growth stages.

Mechanical Properties of 3C-SiC Films for MEMS Applications

AbstractThere is a technological need for hard thin films with high elastic modulus and fracture toughness. Silicon carbide (SiC) fulfills such requirements for a variety of applications at high temperatures and for high-wear MEMS. A detailed study of the mechanical properties of single crystal and polycrystalline 3C-SiC films grown on Si substrates was performed by means of nanoindentation using a Berkovich diamond tip. The thickness of both the single and polycrystalline SiC films was around 1-2 μm. Under indentation loads below 500 μN both films exhibit Hertzian elastic contact without plastic deformation. The polycrystalline SiC films have an elastic modulus of 457 GPa and hardness of 33.5 GPa, while the single crystalline SiC films elastic modulus and hardness were measured to be 433 GPa and 31.2 GPa, respectively. These results indicate that polycrystalline SiC thin films are more attractive for MEMS applications when compared with the single crystal 3C-SiC, which is promising since growing single crystal 3C-SiC films is more challenging.

Hetero-Epitaxial Growth of 3C-SiC on Silicon Substrates by Plasma Assisted CVD

3C-SiC films grown on carbonized Si (100) by plasma-assisted CVD have been investigated with systematic changes in flow rate of monosilane (SiH4) and propane (C3H8) as source gases. The deposition rate of the films increased monotonously and the microstructures of the films changed from 3C-SiC single crystal to 3C-SiC polycrystal with increasing flow rate of SiH4. Increasing C3H8 keeps single crystalline structure but results in contamination of α-W2C, which is a serious problem for the epitaxial growth. To obtain high quality 3C-SiC films, the effects of C3H8 on the microstructures of the films have been investigated by reducing the concentration of C3H8. Good quality 3C-SiC single crystal on Si (100) is grown at low net flow rate of C3H8 and SiH4, while 3C-SiC single crystal on Si (111) is grown at low net flow rate of C3H8 and high net flow rate of SiH4. It is expected that 3C-SiC epitaxial growth on Si (111) will take placed at a higher deposition rate and lower substrate temperature than that on Si (100).

Characterization of Bulk <111> 3C-SiC Single Crystals Grown on 4H-SiC by the CF-PVT Method

Because of the formation of DPB (Double Positioning Boundary) when starting from a hexagonal <0001> seed, DPB-free 3C-SiC single crystals have never been reported up to now. In a recent work we showed that, using adapted nucleation conditions, one could grow thick 3C-SiC single crystal almost free of DPB [1]. In this work we present the results of a multi-scale investigation of such crystals. Using birefringence microscopy, EBSD and HR-TEM, we find evidence of a continuous improvement of the crystal quality with increasing thickness in the most defected area, at the sample periphery. On the contrary, in the large DPB-free area, the SF density remains rather constant from the interface to the surface. The LTPL spectra collected at 5K on the upper part of samples present a nice resolution of multiple bound exciton features (up to m=5) which clearly shows the high (electronic) quality of our 3C-SiC material.

Ultraviolet photoluminescence from 3C-SiC nanorods

An intensive sharp photoluminescence at 3.3eV is observed from single-crystal 3C-SiC nanorods. Structural characterization reveals that the nanorods contain a fairly large amount of threefold stacking faults. We tentatively attribute the emission to these stalking faults, which structurally resemble 6H-SiC nano-layers of 1.5nm embedded in a 3C-SiC matrix. The emission mechanism is discussed in terms of spontaneous polarization at the stacking faults.

Dependence of stacking fault and twin densities on deposition conditions during 3C-SiC heteroepitaxial growth on on-axis Si(0 0 1) substrates

Single-crystal 3C-SiC epilayers were grown on on-axis Si(0 0 1) substrates by low-pressure chemical vapor deposition. The dependence of the densities of stacking faults and twins on epilayer thicknesses and growth conditions—including the reactor pressure, the substrate temperature, and the inlet gaseous composition—were investigated by a series of experiments and simulations. Simple indexes were developed to predict the planar defect densities in terms of the flux ratio of adatoms on the deposition surface. The planar defect densities were significantly reduced with increasing the epilayer thickness until continuous surfaces with {1 0 0} planes were formed at 0.7 μm. The stacking fault density was a function of the surface flux ratio of carbon adatom to atomic hydrogen, while the twin density was a function of that of silicon adatom to atomic hydrogen. Those densities were decreased almost linearly with increases in the surface flux of atomic hydrogen at fixed flux values of carbon and silicon adatoms. The surface flux of atomic hydrogen was increased as either the reactor pressure was decreased or the substrate temperature was increased.

SiC cantilever resonators with electrothermal actuation

Cubic SiC cantilever resonators designed for electrothermal actuation are presented. Metal electrodes with both open circuit and short circuit designs have been deposited and patterned on top of the 3C–SiC cantilevers. Pt electrodes on single crystal 3C–SiC cantilevers and NiCr electrodes on poly-crystalline 3C–SiC cantilevers have both been fabricated and tested in order to investigate the material property effect on the performance of the devices. An analytical model has been developed to understand the electrical power distribution in the cantilevers for the different material systems as well as the different metal terminations. Electrothermal actuation of resonance has been successfully achieved in all the fabricated cantilevers. The dynamic performance of the cantilever resonators has been systematically studied including resonance frequencies, amplitude response with voltage and actuation efficiencies. During the discussion of these results, the mechanism of the electrothermal actuation in these devices has been identified which allows actuation frequencies up to 100 MHz to be possible.