Y. Shishkin

Porous Silicon Carbide as a Membrane for Implantable Biosensors

This work demonstrates the creation of free-standing, nanoporous SiC membranes, and their ability to pass molecules of biologic interest. We have shown that proteins up to 29000 Daltons in molecular weight can pass through porous SiC. Also, we have shown that porous SiC resists protein adsorption comparable to the best commercially available polymer membrane that has been specifically developed to avoid protein adsorption.

Fabrication and morphology of porous p-type SiC

Porous silicon carbide fabricated from p-type 4H and 6H SiC wafers by electrochemical etching in hydrofluoric electrolyte is studied. An investigation of the dependence on wafer polarity reveals that pore formation is favored on the C face while complete dissolution occurs on the Si face. When the etching is done on the C face, the pore wall thickness decreases with increasing current density. The morphology of the front surface of the sample depends on the prior treatment of the workpiece surface. The porosity is estimated based on the analysis of scanning electron microscope images, charge-transfer calculations, and gravimetric analysis.

Growth of 3C-SiC on Si molds for MEMS applications

A hetero-epitaxial 3C-SiC growth process in a low-pressure hot-wall CVD reactor has been developed on planar Si (100) substrates. The growth rate achieved for this process was about 10 μm/h. The process consists of silane/propane/hydrogen chemistry with HCl used as a growth additive to increase the growth rate. 3C-SiC has also been grown on 22, 52 and 123 +m deep etched MEMS structures formed by DRIE of (100) Si at a rate of about 8 +m/h. Secondary electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) were used to analyze the quality of the 3C-SiC films.

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.

High Epitaxial Growth Rate of 4H-SiC Using Horizontal Hot-Wall CVD

A 4H-SiC epitaxial growth process has been developed in a horizontal hot-wall CVD reactor using a standard chemistry of silane-propane-hydrogen, producing repeatable growth rates up to 32 μm/h. The growth rate was studied as a function of pressure, silane flow rate, and growth time. The structural quality of the films was determined by X-ray diffraction. A 65 μm thick epitaxial layer was grown at the 32 μm/h rate, resulting in a smooth, specular film morphology with occasional carrot-like and triangular defects. The film proved to be of high structural quality with an X-ray rocking curve FWHM value of the (0004) peak of 11 arcseconds.

Interface Defects in n-Type 3C-SiC/SiO2: An EPR Study of Oxidized Porous Silicon Carbide Single Crystals

The defects at the 3C-SiC/SiO2 interface have been studied by X-band EPR spectroscopy in oxidized porous 3C-SiC. One interface defect is detected; its spin Hamiltonian parameters, spin S=1/2, C3V symmetry, g//=2.00238 and g⊥=2.00317, central hyperfine interaction (CHF) with one carbon atom and AB//[001]=48G and superhyperfine (SHF) interaction with three equivalent Si neighbour atoms and TB//[001]=12.4G, allow us to attribute the center to a sp3 coordinated carbon dangling bond center, PbC.

Microscopic Structure and Electrical Activity of 4H-SiC/SiO2 Interface Defects : an EPR Study of Oxidized Porous SiC

The oxidation related defects in porous n-type 4H-SiC have been studied by electron paramagnetic resonance spectroscopy. Two main centers are observed after a 1000°C oxidation in dry oxygen. The first one is isotropic with a g-factor of 2.0028 ; it is attributed to a carbon related center in the oxide. The second center is anisotropic with C3v or C1h symmetries depending on its orientation relative to the c-axis. Its g-factors are g//c=2.0024 and g⊥c=2.00315 and gxx=2.0031, gyy=2.0028, gzz=2.0023. From its central and ligand hyperfine interactions it is attributed to a Pb like carbon dangling bond center at the SiC side of the interface:(Pbc). The Pbc is electrically active and introduces a deep level. The concentrations of the two defects are estimated to [C]=10 18 cm -3 and [PbC]=10 12 cm -2 . Introduction The oxide and interface structure of 4H-SiC/SiO2 and 6H-SiC/SiO2 have been the object of numerous experimental and theoretical studies[1]. As in the case of Si a thermal oxide of SiO2 composition can be obtained by high temperature oxidation of SiC. The oxidation kinetics are different from the Si case and depend on the carbon or silicon face for the most studied (0001) surface[2]. The interface structure is also different from the Si case and it is a still open question whether an atomically abrupt interface can be formed for 4H-SiC or whether a transition layer of SiOxCy can not be avoided[3]. These issues are important for the feasibility of acceptable MOSFET structures[4]. We have investigated the nature and concentration of the defects at the interface and in the oxide by EPR spectroscopy. This technique has been shown previously to be extremely useful for interface defect studies in Si/SiO2[5]. For (111)Si interfaces it had allowed the identification and characterization of the so called Pb center, which is the dominant identified defect in this structure. The use of porous silicon, which is a monocristalline form of Si with a high internal surface area has given additional information due to the well resolved hyperfine structures of the Pb center in spite of their low intensity[6]. In previous EPR studies on bulk SiC/SiO2 samples [7-10] oxidation and postoxidation anneal related defects have been investigated. One defect characterized by an isotropic single line EPR spectrum with g=2.0024 (4H-SiC) was observed; based on the g-value it had been tentatively attributed to a C dangling bond defect in the amorphous SiO2 layer. No defect at the cristalline part of the interface had been observed. We present here the results of an EPR study of oxidized porous 4H-SiC. First results have been published already recently [11,12]. Experimental The porous layers were prepared by photo assisted electrochemical dissolution of bulk n-type doped 4H-SiC substrates. For details see ref.[13]. Layers with a thickness of several tens of μm were obtained. Depending on the dissolution conditions samples with different pores structures can be obtained. Figure 1 shows a cross section of a sample with triangular pore structure. At higher Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 1457-1462 doi:10.4028/www.scientific.net/MSF.457-460.1457

Modification of the Oxide/Semiconductor Interface by High Temperature NO Treatments: A Combined EPR, NRA and XPS Study on Oxidized Porous and Bulk n-Type 4H-SiC

The effect of thermal treatments in nitric oxide (NO) on the paramagnetic defects at the 4H-SiC/SiO2 interface are analyzed by EPR in oxidized porous samples. The results on ultrathin thermal oxides show that the NO treatment at 1000°C is insufficient for an efficient reduction of the two dominant paramagnetic interface defects: PbC centers and carbon clusters. From the NRA and XPS analysis of bulk samples treated under the same conditions we attribute the weak effect to the low nitrogen concentration of only 1% at the interface.

Triangular Pore Formation in Highly Doped n-Type 4H SiC

Our study of photoassisted electrochemical etching of highly doped n-type 4H SiC shows that the anodization proceeds anisotropically. As a result, a triangular-channel porous structure is formed independent of the direction of the external electric field applied to the sample. It is proposed that the observed pore morphology is due to differences in oxidation rates of the crystallographic planes terminated with silicon and carbon atoms.

Brillouin Scattering Studies of Surface Acoustic Waves in SiC

Brillouin spectra due to thermally excited surface ripples (Rayleigh waves) have been observed on both porous and non-porous 6H SiC after deposition of aluminium coatings varying in thickness from 20 nm to 70 nm. The spectra were obtained at room temperature in a back-scattering geometry using a tandem Fabry-Perot spectrometer with a probe wavelength of 532 nm. The incident beam power was normally less than 50 mW in a focal-spot area of approximately 50 μm. Data were typically obtained for a series of incident angles varying from 20 ° to 80 ° . The surface wave velocity for the ) 10 2 1 ( face of the non-porous material was 6000±200 m/s. The corresponding velocities for the (0001) face of the porous samples were in the range from 2400±100 m/s to 2850±50 m/s.