Papers by Author: Wolfgang J. Choyke

Abstract: In this paper we revisit sharp low temperature luminescence lines (LTPL) previously generated by high dose 1018 to 1020 cm-2 electron beams in an electron microscope and now produced by low dose 1015 cm-2 electron, 5x1010 cm-2 proton and helium ion irradiation. New no phonon lines E0, F0, θ0, Φ0, K0, G0, J0, M0 and phonon replicas are found. Phonon replicas up to the fifth harmonic are well accounted for by theory giving convincing new evidence that the di-carbon antisite is responsible for these deep defect lines.

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: Planar m-plane GaN was grown on (1100) m-plane 6H-SiC substrates using high-temperature AlN nucleation layers by metalorganic chemical vapor deposition. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images showed striated features on the sample surface aligned along the GaN [1120] direction, which are perpendicular to those associated with (1120) a-plane GaN. The epitaxial relationship between the m-GaN and 6H-SiC was analyzed using high-resolution x-ray diffraction (XRD). In order to reduce the defect density, epitaxial lateral overgrowth (ELO) was carried out on an m-GaN template with mask stripes along the GaN [1120] direction, which makes the lateral growth fronts advance along the GaN c-axis. On-axis XRD rocking curves show that the full width at half maximum (FWHM) values for the ELO samples were reduced by nearly half when compared to those of the m-plane template without ELO. Clear atomic steps were observed in the wing regions by AFM. The absence of the striated features that are associated with the template could be indicative of the reduction of basal stacking faults in the ELO wings. Low-temperature photoluminescence (PL) spectra showed an excitonic emission at 3.47eV, a basal stacking fault (BSF)-related emission at 3.41 eV, and other defect-related emissions at 3.29 eV and 3.34 eV.

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: The effect of the Schottky barrier height on the detection of the midgap defects EH6 and EH7 in 4H-SiC by deep level transient spectroscopy (DLTS) is systematically studied. The results show that the DLTS peak height - and as a consequence the observed defect concentration - increases with the increasing barrier height and saturates above 1.5 eV for EH6 and above 1.7 eV for EH7, while below 1.1 eV the DLTS peak height completely disappears. A model is applied, which determines the position of the quasi-Fermi level in the space charge region as a function of the barrier height and of the reverse bias applied and which explains the variation of the DLTS peak height.

Abstract: The atomic and electronic structure of 4H-SiC(1 1 02) surfaces were investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and photoemission (PES). Two well ordered phases existing on this surface, i.e. (2×1) and c(2×2) are discussed. The (2×1) phase consists of a Si adlayer which is topped by an array of ordered Si-nanowires with electronic states confined to one dimension. For the c(2×2) phase STM indicates the presence of adatoms and PES a surface composition close to bulk SiC stoichiometry. A detailed atomic model for this c(2×2) phase is proposed.

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: Ordered reconstruction phases on the 4H-SiC(1102) surface have been investigated using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). After initial hydrogen etching, the samples were prepared by Si deposition and annealing in ultra-high vacuum (UHV). Two distinct reconstruction phases develop upon annealing, first with a (2×1), and at higher temperatures with a c(2×2) LEED pattern. After further annealing the fractional order LEED spots vanish and a (1x1) pattern develops. For the (2×1) phase, STM micrographs show that adatom chains develop on large flat terraces, which in view of AES consist of additional Si. These highly linear and equidistant chains represent a self-assembled well-ordered pattern of nanowires developing due to the intrinsic structure of the 4H-SiC(1102) surface. For the c(2×2) phase AES indicates a surface composition close to the bulk stoichiometry. For the (1×1) phase a further Si depletion is observed.

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.