Papers by Author: Yue Ke

Abstract: This article discusses the electrochemical polishing of silicon-face p-type 4H SiC using diluted aqueous HF solution. Etchings on the silicon and carbon faces of SiC samples are performed and compared. The experimental results show that the RMS surface roughness of the electrochemically polished Si-face could be as low as about 2 nm. Carbon-face electrochemical polishing gives a rougher surface. Therefore, silicon-face 4H SiC is a better candidate for MEMS processing. The underlying mechanism is also discussed.

Abstract: Columnar porous Si-face 6H-SiC substrates were prepared by a photo-electrochemical etching method and applied as nanoimprint lithography (NIL) stamps. The diameter of the pores in the porous region was about 20 nm and the center-to-center separation between pores was about 60 nm. The columnar porous SiC substrates were subjected to a vapor phase silanization treatment whereby a monolayer of perfluorooctyltrichlorosilane (FOTS) was deposited in order to keep the stamps from sticking to the substrates during the imprint step. Subsequently, the porous SiC stamps were used to imprint polymethylmethacrylate (PMMA) at elevated temperatures and pressures. The imprinted PMMA could then be used to transfer the nanopattern on the columnar porous SiC to other substrates for various purposes; e.g. templates for GaN regrowth, catalysts for nanowire growth by vapor-liquid-solid type methods (VLS), etc. SiC is not typically used for NIL stamps since etch processing of SiC is less mature than that of Si. However, as demonstrated here, there is no reason why SiC cannot be used as a material for NIL stamps. The superior mechanical properties to Si make the use of SiC alluring as a master template for NIL processing.

Abstract: Brillouin light scattering spectroscopy was used to probe porous silicon carbide films formed from p-type 6H crystalline silicon carbide. The porosities of the films ranged from 30% to 58%. Surface and bulk acoustic wave velocities were measured and compared with those calculated from the Mori-Tanaka acoustic effective medium model. Qualitative agreement is obtained between the experimentally determined velocities and those predicted by Mori-Tanaka acoustic effective medium models with spherical pores and, in the case of surface acoustic waves, also with prolate spheroidal pores with shape factor equal to 0.2. The model demonstrates the importance of morphology in determining the behavior of acoustic waves in a porous material.

Abstract: We have fabricated columnar nano-porous SiC by photo-electrochemical etching on the C-face of n-type 6H SiC at constant voltage. SEM images reveal that the pores are long, straight and parallel with diameters of about 20 nm. We have produced such layers up to 250 μm thick. The pore morphologies for both Si and C-face SiC samples are compared and discussed as a part of the effort to understand the growth mechanism. It is found that the constant voltage etching condition on C-face SiC is crucial for this nano-columnar pore formation.

Abstract: A lightly doped n-type homo-epitaxial layer was grown by CVD onto a heavily doped n-type 4H-SiC substrate for which half of the surface had been made porous by photoelectrochemical etching. Raman spectra are obtained in the optic phonon region using three scattering geometries. An effective medium model for the porous layer is used to assist in the interpretation of the spectra. This work demonstrates that the contributions to the Raman spectra of the various layers in a sample with multiple 4H-SiC layers can be extracted.

Abstract: We present relative recovery data for six proteins diffusing through porous silicon carbide membranes having a hybrid columnar/dendritic morphology. These membranes are promising candidates for implantable biosensors. The results are interpreted using an effective medium model.

Abstract: Brillouin spectra have been recorded for a series of supported films of p-type porous 6H-SiC with a branched morphology and porosities in the range from 30% to 58%. Complex spectra comprising up to 7 identifiable components were observed in some cases. An effective medium model is being developed as an aid in interpreting the spectra, and preliminary results are presented.

Abstract: The effects of initial surface morphology on the early stages of porous SiC formation under highly biased photoelectrochemical etching conditions are discussed. We etched both Si-face and C-face polished n-type 6H SiC with different surface finishes prepared either by mechanical polishing or by chemical mechanical polishing at NOVASiC. For both Si-face and C-face porous SiC samples, a variety of surface and cross sectional porous morphologies, due to different surface finishes, are observed. The proposed explanation is based on the spatial distribution of holes at the interface of the SiC and electrolyte inside the semiconductor.

Abstract: A hybrid columnar and dendritic porous structure has been developed in n-type 6H SiC using photoelectrochemical etching with proper control of the applied voltage and current density. The diameter of the formed columnar pores is around 200-500 nm. A possible formation mechanism due to the spatial distribution of holes and the HF concentration gradient in the pores is proposed. A self-supporting film with this morphology is a promising candidate for protein dialysis.

Abstract: Hot-wall chemical vapor deposition has been used to epitaxially grow SiC layers on porous n-type 4H-SiC substrates. The growth was carried out at different speeds on porous layers of two different thicknesses. The quality of the SiC films was evaluated by X-ray diffraction and photoluminescence techniques. Based on the measurements, both the growth speed and the thickness of the porous layer buried underneath the epilayers do not appear to influence the structural integrity of the films. The intensity of the near bandedge low temperature photoluminescence appears stronger by a factor of two in films grown on porous layers.