Si Cones Research Articles

Varactor diodes of hydrogenated nanocrystalline Si with hydrogenated amorphous SiC prepared by PECVD

Hetero-structural barrier varactor diodes (HBVDs) can be widely used as variable capacitors, frequency multipliers, parametric amplifiers, etc. in electronic systems. Herein, n-type hydrogenated nano-crystalline Si (n-nc-Si:H) films with narrow band-gap and un-intentional doped hydrogenated amorphous SiC (a-SiC:H) ones with wide band-gap are adopted to innovatively design the n-nc-Si:H/a-SiC:H/n-nc-Si:H-based HBVDs. Two periodic hetero-structures with different doped nc-Si:H modulation layers and different thickness a-SiC:H barriers are severally fabricated by plasma enhanced chemical vapor deposition (PECVD), the optical band-gaps of n-nc-Si:H and a-SiC:H are derived from the ultra violet-visible (UV–vis) absorption spectra, the prepared devices are demonstrated as the HBVDs by measuring the relations of current density–voltage and capacitance–voltage, respectively. The conduction mechanisms of the fabricated devices in different biased voltage ranges are probed, the variation of capacitance modulation ratio with changing the doped-level in modulation layers and thickness of barriers are analyzed. This paper will be helpful to extend the material combination and preparation method for HBVDs.

Diamond synthesis on Si by plasma chemical vapor deposition using microwave sheath-voltage combination plasma

Microwave sheath-voltage combination plasma (MVP) represents a new method to generate plasma and coat diamond-like carbon films onto the three-dimensional metal surfaces . The present work investigates the synthesis of diamonds using MVP deposition, focusing in particular on how pressure and bias voltage affect the diamond structure. Using MVP, silicon wafers were coated with diamond in gas mixtures of methane and hydrogen with the substrate bias ranging from 0 to −200 V and pressure ranging from 800 to 1000 Pa. Although diamond particles are created with the substrate biased between 0 and −200 V at 1% methane concentration, at −200 V the particles are affixed atop Si or SiC cones that form on the Si substrate due to plasma etching . X-ray diffraction analysis reveals that the diamond structure is created up to a bias voltage of −100 V at 3% methane concentration. Nanodiamond is produced at methane concentration of 3%. These experiments suggest that MVP, a new diamond coating method, makes depositing the diamond on 3D shape substrates possible. • Diamond coatings are deposited by microwave sheath-voltage combination plasma. • Higher substrate temperature with nonzero bias voltage than with zero bias voltage • Higher negative bias produces cone-like structures on the substrate.

Single-Crystal Integrated Photoanodes Based on 4H-SiC Nanohole Arrays for Boosting Photoelectrochemical Water Splitting Activity.

Photoelectrochemical (PEC) splitting of water into H2 and O2 by direct use of sunlight is an ideal strategy for the production of clean and renewable energy, which fundamentally relies on the exploration of advanced photoanodes with high performance. In the present work, we report that single-crystal integrated photoanodes, that is, 4H-SiC nanohole arrays (active materials) and SiC wafer substrate (current collector), are established into a totally single-crystal configuration without interfaces, which was based on a two-step electrochemical etching process. The as-fabricated SiC photoanode showed a rather low onset potential of -0.016 V vs reversible hydrogen electrode (RHE) and a high photocurrent density of 3.20 mA/cm2 vs RHE 1.23 V, which were both superior to those of all reported SiC ones. Furthermore, such a rationally designed photoanode exhibited a fast photoresponse, wide photoresponse wavelength range, and long-term stability, representing its overall excellent PEC performance.

Review of Packaging Schemes for Power Module

SiC devices are promising for outperforming Si counterparts in high-frequency applications due to its superior material properties. Conventional wirebonded packaging scheme has been one of the most preferred package structures for power modules. However, the technique limits the performance of a SiC power module due to parasitic inductance and heat dissipation issues that are inherent with aluminum wires. In this article, low parasitic inductance and high-efficient cooling interconnection techniques for Si power modules, which are the foundation of packaging methods of SiC ones, are reviewed first. Then, attempts on developing packaging techniques for SiC power modules are thoroughly overviewed. Finally, scientific challenges in the packaging of SiC power module are summarized.

Processing of hybrid laminates integrating ZrB<sub>2</sub>/SiC and SiC layers

Tape casting technique was used to develop hybrid laminates constituting by SiC and ZrB2–SiC layers; the main aim is obtaining a structure which integrate the unique properties of these materials and potentially extent their application temperature range. Multilayer with ZrB2–SiC layers stacked in between SiC ones were successfully processed. Thin cracks propagated in the composite layers without affecting SiC ones; their formation was due to residual stresses developed in the two materials because of the differences in their shrinkage and coefficients of thermal expansion. However, these cracks did not significantly affect the material properties: relative density, elastic modulus and flexural strength of hybrid laminates was indeed only slightly lower than those of laminates made up of layers with the same composition.

Effect of copper over-pad metallization on reliability of aluminum wire bonds

Recently, there have been moves to replace Si power devices with SiC ones, which will require new materials with higher thermal conductivity for power module assemblies owing to their higher operating temperatures. Al wire bond interconnections suffer from a mismatch in the coefficient of thermal expansion (CTE) between the SiC chip and the wire material, leading to a degradation of the power module reliability owing to the higher junction temperature (Tj) operation caused by the current density increase. Various studies have been conducted to improve the reliability of such interconnections. In this study, Al wires were bonded on a thick Cu over-pad metallization (OPM) layer that was formed on an Al pad of a SiC Schottky barrier diode, and active power cycling tests were performed while changing Tjmax to investigate the contribution of such a thick Cu-OPM layer on the reliability of Al wire bonds. Compared with the case of Al wire bonds on an Al pad, in active power cycling tests, the lifetime of Al wire bonds on a 25 μm-thick Cu-OPM layer was 1.3-times longer at ΔTj = 75 °C, and 1.5-times longer at ΔTj = 100 °C. The results of Joule heating analysis and elastic thermal-stress analysis showed that the lifetime prolongation effect of Al wire bonds on Cu-OPM was not derived from the heat dissipation effect of the thick Cu-OPM layer, but from a stress-reduction effect obtained by keeping the Al wire bonds away from the SiC diode chip, with which the CTE mismatch is large.

Truncated titanium/semiconductor cones for wide-band solar absorbers

A truncated Ti and Si cones metasurface has been proposed for wide-band solar absorber (WSA), which produced a high average absorption of 94.7% in the spectral region from 500 to 4000 nm. A maximal enhancement factor of 166.0% was achieved by the WSA in comparison with the absorption of Ti/Si cylinder resonators based absorber. Under the standard solar radiance, a high full-spectrum solar absorption efficiency of 96.1% was obtained for the WSA in the energy range from 0.28 to 4.0 eV. The spectral bandwidth with absorption above 90% is up to 3.402 μm, which shows an enhancement factor of 165.0% than that of the WSA intercalated by the SiO2. Other semiconductors such as Ge, GaAs have been utilized to form the WSA, which also maintained the near-unity absorption in the wide-band spectrum. The plasmonic resonant response of the Ti material and the strong electromagnetic coupling capability of the Si resonator, and the plasmonic near-field coupling by the adjacent truncated cones were the main contributions for the impressive absorption behaviors. These findings pave a new way for achieving full-spectrum solar absorber via combining the Ti material and semiconductors, which could open potential approaches for active optoelectronic devices such as photo-detectors, hot-electron related modulators, and solar cells, etc.

Effect of particle size on the thermal and mechanical properties of aluminum composites reinforced with SiC and diamond

The effect of particle size on thermal conductivity (TC) and bending strength was investigated in the SiC/Al and diamond/Al (Dia/Al) composites for thermal management applications. The results showed that with the particle size of SiC increasing from 7 to 250μm, the TC of the 40vol.% SiC/Al composite was improved by 50%, from 182 to 272W/mK, while the bending strength decreased by 100% from 352 to 175MPa. Comparatively, with diamond particles increasing from 30 to 200μm, the TC of the 40vol.% Dia/Al composite increased by around 52% from 313 to 475W/mK, and the bending strength declined by 32% from 327 to 223MPa. Characterization of fracture surface indicated that large SiC particles featured by an irregular shape tended to fracture prior to the Al matrix during loading, while large diamond particles with a regular shape did not have such a tendency. Moreover, diamond particles with a high intrinsic TC contributed to global TC much more than SiC ones with a relatively low TC. Thus, in order to balance thermal and mechanical properties of particulate reinforced aluminum composites, particle size of reinforcements should be well compromised according to their intrinsic properties.

Efficiency enhancement for SiN-based light emitting device through introduction of Si nanocones in emitting layer

Silicon nitride-based light-emitting devices were fabricated with a SiNx emitting layer grown on annealed Si film of dense nano-crystalline cones. Comparative studies revealed that the patterned SiNx emitting layer, with embedding nanocrystalline Si cones and a rough surface morphology of its own, manifests a much enhanced, even doubled at sufficiently large injected current density, electroluminescence efficiency. Both the increased light-extraction capability and the effective hole-blocking by the presence of Si nanocones, the latter is favorable for the balance of carrier injection in emitting layer, are responsible for this remarkable efficiency enhancement. The current work established an alternative approach toward the fabrication of more efficient SiN-based light-emitting devices.

Effect of reinforcing particle type on morphology and age-hardening behavior of Al–4.5 wt.% Cu based nanocomposites synthesized through mechanical milling

Abstract The effects of reinforcing particle type (SiC and TiC) on morphology and precipitation hardening behavior of Al–4.5%Cu based nanocomposites synthesized via mechanical milling were investigated in the current work. In order to study the microstructure and morphology of mechanically milled powder, X-ray diffraction technique, scanning electron microscopy and high resolution transmission electron microscopy were utilized. Results revealed that at the early stages of mechanical milling, when reinforcing particles are polycrystal, the alloying process is enhanced more in the case of using the TiC particles as reinforcement. But, at the final stages of mechanical milling, when reinforcing particles are single crystal, the alloying process is enhanced more in the case of using the SiC ones. Transmission electron microscopy results demonstrated that Al–4.5 wt.%Cu based nanocomposite powders were synthesized and confirmed that the mutual diffusion of aluminum and copper occurs through the interfacial plane of (200). The hardness results showed that not only does introducing 4 vol.% of reinforcing particles (SiC or TiC) considerably decrease the porosity of the bulk composite samples, but also it approximately doubles the hardness of Al–4.5 wt.%Cu alloy (53.4 HB). Finally, apart from TEM and scanning electron microscopy observation which are localized, a decline in hardness in the TiC and SiC contained samples, respectively, after 1.5 and 2 h aging time at 473 K proves the fact that the size of SiC particles is smaller than the size of the TiC ones.

Arrays of Si cones prepared by ion beams: growth mechanisms

AbstractArrays of uniformly distributed Si cones with controlled orientations are fabricated on Si substrates by an ion beam technique. The axes of produced Si cones are well aligned with the direction of ion beams. Each cone essentially consists of a metal containing tip and a single crystalline Si base with the crystallographic orientation identical to the Si substrate. TEM studies show tapered pit interface between the tip and the base, visualizing the ion impinging process. The tip comprises tetragonal MoSi2, domains of Mo‐modulated Si ordered superstructures and a few Mo nanoparticles. Experimental evidence indicates that the cone formation follows the left‐standing model. Since the cones progressively are sharpened with the elapsed time of their formation, they might develop into whiskers if the ion bombardment and supply of sputtered metal clusters are prolonged.

Ultraviolet femtosecond, picosecond and nanosecond laser microstructuring of silicon: structural and optical properties

The effect of laser pulse duration on the morphology, composition, crystallinity and optical properties of self-organized Si microcones fabricated using 248 nm laser pulses (500 fs, 5 ps and 15 ns) in an SF(6) atmosphere, is presented in this paper. Despite distinct differences in the morphology, the Si cones show similar structure and composition independently of the laser pulse duration used: a core of single-crystalline Si, covered by a few hundred nanometer thick, sulfur-doped nanocrystalline Si layer, where no amorphous Si is present. The obtained features exhibit strong below-bandgap absorptance, making them excellent candidates for Si based photodetectors with improved spectral response.

Bias-enhanced nucleation of oriented diamonds on cone-like Si

Uniform distribution of bias-enhanced nucleation of diamond has been improved on Si substrate of an area of 1 × 1 cm 2 by using a dome-shaped Mo counter electrode in a microwave plasma chemical vapor deposition reactor. A nucleation density of 10 9 cm 2 can be reached within a few minutes when the bias voltage of − 100 V is applied on the substrates. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that a single-crystalline diamond in a few nanometered size can be deposited on a volcano-shaped cubic SiC which is epitaxially formed on a Si cone. Examination reveals a large fraction of diamond nuclei are oriented along with one side of SiC on each Si cone. The Si cone formed on the Si substrate is due to plasma etching. The diamond nuclei have a shape close to rhombus in TEM. With further growth, secondary nucleation of diamond occurs on top of diamond nuclei and SiC which grows with Si cones. As a result, polycrystalline diamonds are deposited on each Si cone.

Fabricating needle arrays with a gray-scale x-ray mask

Microscopic needles long enough to penetrate skin but short enough to avoid touching nerves provide a method for painless injections. Research into a process for mass-producing microneedle arrays for drug delivery is leaping forward as the needles become a commodity. A painless needle can be less than 150μm long for injecting cosmetics, but for delivering medicine it must be 500μm–1mm, so that it can reach the subcutaneous tissue or even the underlying muscle. Short needles are easily created by silicon (Si) dry etching. Fabricating longer ones, however, is quite difficult. To solve this problem, we used x-ray lithography, which can handle a resist thicker than 1mm, combined with a gray-scale mask. A microstructure fabrication method called LIGA (from the German: ‘Lithographie,’ ‘Galvanik,’ and ‘Abformung’)1 already incorporates binary x-ray lithography. In a typical LIGA process, a resist master is produced by x-ray lithography, and a metallic mold is replicated from the master by electroforming. Then the pattern can be mass-produced using a molding technique. Because of its laserlike directional property, a synchrotron source is used to produce x-rays. LIGA has been successfully employed to make pillar structures with vertical sidewalls. However, because x-rays cannot be easily focused or reflected, it has not been successful in building 3D structures with sloped sidewalls. We proposed that the slopes needed to make pointed needles could be achieved by incorporating a gray-scale x-ray mask into the LIGA process. A conventional binary x-ray mask consists of a membrane with a pattern made of an x-ray-absorbing heavy element such as gold. The x-rays penetrate the membrane but are blocked completely by the absorber. Fabrication of the mask involves photolithography and electroplating to create 4μm-high vertical absorber sidewalls. We developed a gray-scale mask with inclined-sidewall absorber sections so that we could create a reFigure 1. (a) Photograph of x-ray gray-scale mask with silicon (Si) absorbers on an x-ray-transparent membrane. (b) Scanning electron microscope (SEM) image of Si cones before backside etching of the substrate.

Emission properties of DLC films on Si substrates

DLC films were deposited on polished both n-type and p-type silicon substrates. The silicon resistivity was ∼0.02 Θ cm. Some of the DLC films 20 nm thick were deposited on the n-type Si surface with the submicron cones. SEM and Raman spectroscopy were used for structural investigations. Field electron emission occurs after dielectric breakdown, except for the samples with Si cones for which the emission seems to originate from SiC formed during the first stage of electron emission. It seems that too much sp2 graphite phase may give rise to the observed increase in the turn-on field from 50 V/μm up to 150 V/μm.

Measurements of Size, Morphology, and Spatial Distribution of Oxygen Precipitates in Si Wafers Using RIE

A new technique for detecting oxygen precipitates in Si wafers, which utilizes a reactive ion etching (RIE) process with a Si/ etching rate ratio of about 200, has been applied to measurements of the size, morphology, and distribution of oxygen precipitates. The detectable precipitate size limit of the technique, which detects oxygen precipitates as needle-shaped Si cones formed under the precipitates, was estimated to be 8 nm. When the RIE depth was shallow, the size and morphology of oxygen precipitates were revealed by TEM observation of the top of Si cones. Also, the plane and depth distributions of oxygen precipitates were successfully obtained by measuring the position and height of Si cones regardless of the depth position of the precipitates and the RIE depth. Furthermore, effect of oxygen precipitates on gate oxide integrity (GOI) yield was examined using the technique, and it was shown that small-sized (15-40 nm) oxygen precipitates strongly degrade GOI yield.

Surface preparation of [formula omitted]-SiC for the epitaxial growth of 3C–SiC

In this paper, we present a study of the surface preparation of on-axis, Si face 6H–SiC(0 0 0 1) and 4H–SiC(0 0 0 1) substrates for the growth of 3C–SiC heteroepitaxial layers. The treatments were performed in a vertical cold-wall reactor, at temperatures ranging from 1350 to 1650 °C for 10–30 min, under H 2 or H 2+C 3H 8 atmospheres. By atomic force microscopy investigation, the 4H–SiC surfaces were always found to be rougher than the 6H–SiC ones so that we mainly studied the use of 6H–SiC. All the samples displayed a more or less regular surface covered with small steps (up to 1 nm height). Si droplet formation was observed in some specific conditions, which, hence, are to be avoided. The most regular array of steps (∼0.7 nm height) was obtained on 6H–SiC seeds under H 2 treatment at 1450 °C for 10 min. On such surfaces, 3C–SiC growth was performed using a mixture of SiH 4+C 3H 8 at 1450 °C with a C/Si ratio of 3 in the gas phase.

Synthesis of conical Si array on Si(100) for a field electron emitter by plasma-enhanced chemical vapor deposition

We synthesized a highly aligned conical Si array on Si(100) for a field electron emitter by plasma-enhanced chemical vapor deposition (PECVD). The number density of these submicron Si cones was controlled in the range 10 6–10 9 cm −2. The 10 6 cm −2 conical Si array exhibited a low threshold voltage for its field electron emission, while for an array with a high number density, the emission property was degraded due to the electric field screening effect.

Laser microstructuring of Si surfaces for low-threshold field-electron emission

Dense arrays of high aspect ratio Si micro-pyramids have been formed by cumulative high intensity laser irradiation of doped Si wafers in an SF6 environment. A comparative study using nanosecond (XeCl, 308 nm) and femtosecond (Ti: Sapphire, 800 nm and KrF, 248 nm) laser pulses has been performed in this work. The influence of pulse duration and ambient gas pressure (SF6) is also presented. Scanning electron microscopy has shown that upon laser irradiation conical features appear on the Si surface in a rather homogenous distribution and with a spontaneous self alignment into arrays. Their lowest tip diameter is 800 nm; while their height reaches up to 90 μm. Secondary tip decoration appears on the surface of the formed spikes. Areas of 2×2 mm2 covered with Si cones have been tested as cold cathode field emitters. After several conditioning cycles, the field emission threshold for the studied Si tips is as low as 2 V/μm, with an emission current of 10−3 A/cm2 at 4 V/μm. Even though these structures have smaller aspect ratios than good quality carbon nanotubes, their field emission properties are similar. The simple and direct formation of field emission Si arrays over small pre-selected areas by laser irradiation could lead to a novel approach for the development of electron sources.

Field-emission properties of multihead silicon cone arrays coated with cesium

Field emission from multihead silicon (Si) cones was substantially improved by cesium (Cs) coating. Increasing the Cs coating lowered the emission turn-on field (for 10 μA/cm2) from 25 V/μm to a saturated value of 13 V/μm, while the threshold field (for 10 mA/cm2) decreased by 30%, dropping from 27 V/μm for Si cones coated with 1.8 monolayers (ML) of Cs to a saturated value of 19 V/μm with 4.1 ML of Cs. The Cs-treated Si cones could give an emission current density that was three to ten times that delivered by bare Si cones. The work function reduced by a factor of 1.43 for Si cones coated with 4.9 ML of Cs with reference to the untreated Si cones. From the slope of Fowler–Nordheim plot, the field enhancement factor β was found to increase by a factor of 2.02 for Si cones coated with 2.5 ML of Cs and then reduce to 1.57 after the 4.9 ML of Cs deposition. Reduction of the factor β might occur because of a thick Cs layer, which could flatten the sharp cone features. Stability test showed that no current decay was observed at a current density of 0.8 mA/cm2 under a constant applied field of 16 V/μm during the 10 h investigation.