Reinhard Zachai

Epitaxial diamond thin films on (001) silicon substrates

Epitaxial (001) diamond film were grown on mirror‐polished single‐crystalline (001) silicon substrates by microwave plasma chemical vapor deposition from a methane/hydrogen gas mixture. The films were characterized by means of scanning electron microscopy, Raman spectroscopy, and x‐ray analysis. The results show that the diamond crystallites are oriented to the silicon substrate with both the (001) planes and the [110] directions parallel to the silicon substrate.

Experimental investigation of thermal conduction normal to diamond‐silicon boundaries

Passive chemical‐vapor‐deposited diamond layers have the potential to improve thermal conduction in electronic microstructures because of their high thermal conductivities. The thermal resistances for conduction normal to the boundaries of diamond layers, which must be small in order to realize this potential, have not been measured. This research develops two independent experimental methods that measure the total thermal resistance for conduction normal to diamond layers thinner than 5 μm on silicon substrates, yielding an upper bound for the thermal resistance of the diamond‐silicon boundary. The data for layers as thin as 0.2 μm agree with predictions that account for phonon scattering on layer boundaries and couple the local scattering rate in the diamond to the grain size. The agreement lends support to the conclusion that the effective diamond‐silicon boundary resistance is dominated by a highly localized volume resistance in the diamond near the interface.

Silicon/germanium strained layer superlattices

High quality Si/Ge strained layer superlattices are achieved by low temperature molecular beam epitaxy on Si, SixGe1−x and Ge substrates. Various characterization techniques are used to obtain information on critical thickness, strain distribution, misfit dislocations, interface sharpness and superlattice periodicity. The band structure is strongly influenced by strain and zone folding effects. Two-dimensional electron systems can be realized in the wider gap Si layers due to the strain-induced lowering of the conduction band. New optical transitions in the infrared regime are observed with short period Si/Ge superlattices.

The effect of substrate bias voltage on the nucleation of diamond crystals in a microwave plasma assisted chemical vapor deposition process

Abstract Diamond films were deposited by microwave plasma chemical vapor deposition onto single-crystal (111), (110) and (100) silicon wafers. The deposition was preceded by an in situ bias pretreatment to enhance the nucleation. The effects of this substrate bias voltage on the nucleation density, orientation of the nuclei, and crystal quality of the diamond films were investigated. The influence of the hydrocarbon fraction of the gas mixture and the substrate temperature on the diamond nucleation under constant bias voltage was also studied.

Deposition and characterization of diamond epitaxial thin films on silicon substrates

Heteroepitaxial diamond growth has been attempted on mirror-polished monocrystalline (001), (111), and (110) silicon substrates by microwave plasma CVD. The surface morphology and the crystallographic properties of the films were characterized by means of Scanning Electron Microscopy (SEM), Raman spectroscopy, X-ray diffraction, and X-ray and Raman pole-figure analysis. The results demonstrate epitaxial growth of diamond on both (001) and (111) oriented silicon substrates. Preliminary results give strong evidence for substrate-induced orientation of the diamond crystallites also on (110) oriented silicon substrate. The heteroepitaxy can be assigned to the oriented covalent bonding across the interface between diamond and silicon.

Photoconductivity of undoped, nitrogen- and boron-doped CVD- and synthetic diamond☆

Abstract Nitrogen-doped CVD- and synthetic type IIa and Ib diamonds were investigated by the constant photocurrent method (CPM). Nominally undoped CVD-films containing nitrogen show broad absorption bands with threshold energies at 1, 2.3, 3 and 4.2 eV. The typical nitrogen donor absorption band with a threshold at 1.7 eV is partially masked by the 1 eV band in CVD-films. The absorption bands are too broad to be described by simple theories based on photoionization of single unbroadened impurity levels. Boron-doped CVD- and type IIb synthetic diamond was studied by photoconductivity and photothermal ionization in the near infra-red. The large electron-phonon coupling in diamond gives rise to oscillatory photoconductivity minima due to fast capture of holes by the excited states of boron acceptors. In CVD-films with boron concentrations around 1019 cm−3, the oscillation pattern inverts at low temperatures and sharp minima were found in the spectrum.

Influence of the microstructure on the thermal properties of thin polycrystalline diamond films

Highly oriented and columnar grained diamond layers only a few microns thick, deposited at different substrate temperatures (500, 550, and 800 °C) on silicon using microwave-plasma-assisted chemical vapor deposition, are investigated by special photothermal techniques and high-resolution transmission electron microscopy (HRTEM). Small effective diamond–silicon boundary resistances of <4×10−9 m2 K/W are determined for thermal conduction normal to the interface. Thermal conductivities normal to the interface, k⊥, are found to be about an order of magnitude greater than the conductivities parallel to the interface, k∥ (k⊥/k∥=9–18). The boundary resistances measured are in good agreement with limits estimated from the interface structure observed by HRTEM, which indicate a low near-interfacial disorder for the layers.

Thermal diffusivities of thin diamond films on silicon

Abstract Photothermal displacement spectroscopy at transient thermal gratings was used to characterize the thermal diffusivity of diamond films several microns thick grown by microwave plasma chemical vapour deposition on silicon substrates. With this very local lateral sensitive method a rather large variation in thermal diffusivities values was obtained, covering the range from 0.2 to 1.7 cm 2 s −1 . The thermal properties are related to the structural properties investigated by electron microscope imaging and Raman spectroscopy.

Lateral thermal diffusivity of epitaxial diamond films

Abstract Highly oriented heteroepitaxial diamond films grown on silicon (001) substrates were investigated by photothermal displacement spectroscopy at transient thermal gratings. Lateral thermal diffusivities were measured as a function of the height within the 90 μm thick films. A strong diffusivity variation was observed from less than 1 cm 2 s −1 near the substrate interface up to 10 cm 2 s −1 in the top growth region, which is about 80% of that for single-crystal type IIa diamond. The results are compared with those obtained from fibre-textured diamond films. The structural properties are characterized using micro-Raman spectroscopy.

Optical excitation of paramagnetic nitrogen in chemical vapor deposited diamond

Investigations of polycrystalline chemical vapor deposited diamond films by electron‐spin‐resonance (ESR), light‐induced (L)ESR, and the constant photoconductivity method have identified dispersed substitutional nitrogen (P1 center) as the main paramagnetic form of N incorporated in the CVD diamond. The density of N‐related paramagnetic states is strongly affected by illumination and heat treatment. It is found that the P1 center in CVD diamond gives rise to a deep donor state about 1.5 eV below the conduction band.