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Boron delta-doped profiles with peak concentrations above the full activation limit have been grown on (100)-oriented single crystal diamond substrates by microwave assisted CVD using a solid doping source technique. The growth process was optimized targeting electronic device applications. Up to now these profiles could only be analyzed by chemical/physical profiling and it had been difficult to relate these profiles to the electrical characteristics. For the first time, ERD (Electron Recoil Detection) profiles could be correlated with free carrier profiles extracted by electrochemical profiling based on electrochemical impedance analysis. The comparison shows, that it is possible to incorporate boron on acceptor site with high efficiency even for concentrations in the order of 1021 cm− 3 by the doping technique developed.

b ¨¨ Abstract The diffusion of carbon and five other elements in amorphous carbon (a-C) films was studied. One set of samples were sp 2 dominated a-C and the other set of samples were sp dominated tetrahedral amorphous carbon (ta-C). The films were deposited 3

Field effect transistors (FETs) in diamond should outperform FET structures on other wide bandgap materials like SiC and GaN in high power/high temperature applications due to the ideal diamond materials properties. However, the technology of these structures proved difficult leaving two device concepts to investigate: (1) the boron δ-doped p-channel FET and (2) the hydrogen induced p-type surface-channel-FET. The δ-channel-FET approach follows a traditional design path of power FET structures. Here, simulation results have enabled the extrapolation of a maximum RF output power to 27 W/mm, a value which is indeed higher than for any FET based on III-Nitrides or SiC. However, due to the narrow technological parameter window, fabricated δ-channel-FETs are still well behind expectations. In contrast, concerning the surface-channel-FET the physical/chemical nature of its channel remains still under discussion. Nevertheless, results obtained with this FET concept yielded a VDmax>200 V (LG=1 μm) and a IDmax>360 mA/mm a fT=11.5 GHz and fmaxU>40 GHz (LG=0.2 μm) and a recently obtained RF power measurement at 1 GHz. Furthermore, the 1 GHz power measurement result has been obtained on a diamond quasi-substrate grown on a Ir/SrTiO3 substrate. This result may therefore open up the perspective for wafer scale diamond electronics.

b ¨ Abstract The influence of H S on the CVD diamond growth, the sulfur incorporation and the electronic properties of sulfur containing 2 homoepitaxial diamond films were studied. Laser reflection interferometry (LRI) in combination with mass spectroscopy (MS) showed that H S modifies the gas phase chemistry by reducing the concentration of CH species. As a consequence thereof, at 2 x high deposition temperatures the growth rate decreased. At lower substrate temperatures, the observed increase in the growth rate after sulfur addition indicates that these gas phase effects are overcompensated by processes at the growing diamond surface. The incorporation coefficient of sulfur into the definitely boron free diamond films was very low (less than 10 ). Incorporation y6 seems to be enhanced by a reduction of the substrate temperature, by the presence of Si and, most effectively, by addition of CO . For 0.5% CO in the gas mixture a maximum S concentration of 480 ppm (9=10 ycm ) corresponding to an incorporation 19 3 22 coefficient of 6=10 was attained. Even for the highest H S concentrations (nearly 1%) the deposited diamond films preserve y4 2 their excellent quality as judged from m-Raman measurements. The electrical properties were not changed by the S incorporation. The electrical conductivity is thermally activated with typically 1.4-1.5 eV independent from the S concentration in the films. No values below 1.0 eV have been measured which argues against doping. 2002 Elsevier Science B.V. All rights reserved.

The oxygen incorporation at the interface between the silicon substrate and chemical vapour deposited (CVD) diamond films nucleated by the bias-enhanced nucleation (BEN) procedure has been studied by heavy-ion elastic recoil detection (ERD). Using standard process conditions for the realisation of heteroepitaxial films, oxygen with a concentration equivalent to about 1 nm SiO 2 has been found, which was mainly incorporated during textured growth with a certain CO 2 admixture to the process gas. By completely omitting CO 2 during nucleation and growth, the oxygen at the interface can be reduced by nearly one order of magnitude to 6.3×10 15 at cm −2 , corresponding to 0.14 nm SiO 2 . Intentional addition of highly enriched C 18 O 2 to the gas phase shows that the oxygen incorporation is strongly enhanced during BEN with hydrocarbon in the gas phase. The results indicate that roughening of the surface, the deposition of Si x O y C z phases and strong lateral inhomogeneities at the silicon interface may explain the coexistence of epitaxial crystallites and amorphous phases. It is suggested that a further reduction of the oxygen concentration at the interface may have consequences for an improved heteroepitaxy of diamond on silicon.

Diamond junction field effect transistors (FETs) utilizing δ-boron-doped diamond films were fabricated and analyzed. In order to allow full charge modulation by the gate, the total channel sheet charge must not exceed the order of 1013 cm−2. However, boron doping shows full activation only for concentrations above ca 1020 cm−3 [1] . This yields a thickness for a fully activated channel in the range of ca 1 nm. To approach such narrow doping spikes any parasitic boron doping tails need to be eliminated. One possible way of achieving this is to compensate boron doping with nitrogen doping, an extremely deep donor. This results in the formation of a pn-junction, where the nitrogen doped part is not activated at room temperature and which therefore represents a semi-insulating (lossy) dielectric at low temperature and high frequency. At elevated temperature and low frequency the nitrogen doped layer becomes conducting acting as a series resistor to the interfacial pn-junction. Using this concept of a lossy dielectric pn-junction in the δ-doped channel FET, two gate diode configurations were investigated. In the first the nitrogen doped (Ib) synthetic diamond substrate served as a large area back gate, while in the second the nitrogen doped gate layer was grown on top of the δ-channel. The devices show high drain currents of up to 100 mA mm−1 and full channel modulation even at moderate operation temperatures of 200–250 °C. By extrapolation a current density of 1 A mm−1 is expected for a 0.25 μm gate length device.

To investigate the stability of the cubic (c-) and the hexagonal (h-) phase of boron nitride (BN) against radiation damage induced predominantly by electronic energy loss processes of high energy density (typical value 25 keV nm−1), thin films prepared by ion-beam-assisted sputter deposition (IBAD) were irradiated at room temperature with 170-MeV iodine ions. Though the average sample stoichiometry, as continuously monitored during the irradiation by elastic recoil detection (ERD), remains unchanged in both cases, Fourier transform infra-red (FTIR) spectra as well as reflection electron energy loss spectra (REELS) taken before and after each irradiation step clearly demonstrate that c-BN is converted to h-BN, even by small ion fluences, while h-BN remains stable. Additionally, interferometric measurements indicate a dramatic stress release for both, h-BN and c-BN starting structures. In all cases, a pronounced volume expansion of the samples perpendicular to the beam direction is observed maintaining a constant mass area density as again monitored by ERD. Atomic force microscopy (AFM) on bombarded samples reveals characteristic surface features on different length scales, which can be interpreted as being due to stress release and ion track formation. These results are consistent with h-BN being the stable high-temperature phase under relaxed pressure conditions.

The similar crystalline structure of diamond and cubic boron nitride suggests the synthesis of superhard thin films containing boron, carbon and nitrogen. BCN thin films have been prepared by reactive r.f.-magnetron sputtering of a hexagonal boron nitride target in an argon/acetylene atmosphere of a constant pressure of 2 × 10−2 mbar. A deposition temperature of 600 °C leads to nearly hydrogen-free BCN films. At a flow ratio of 0.05% C2H2/Ar, the composition of the deposited BCN films is close to B5CN3 as a result of energy dispersive X-ray (EDX) and elastic recoil detection (ERD) measurements. By applying a r.f.-substrate bias, the ion energy has been varied at a constant ratio of ions to film-forming particles of 4. The BCN films show a maximum in stress at an ion energy of 110 eV due to knock-on subplantation of argon ions and a phase separation into carbon, boron and cubic boron nitride regions as a result of X-ray diffraction investigations. Auger electron- and infra-red spectroscopy.

Highly oriented diamond films can play an important role in replacing single crystal diamond for their use as substrates in active electronic devices. However, for practical applications, large, homogenous films with low defect densities are required. The focus of our investigations is the nucleation of highly oriented diamond on (001) silicon via Bias Enhanced Nucleation (BEN) over large areas. A modified BEN process using repetitive pulse bias ‘RP-BEN’ was developed, resulting in an area of oriented nucleation of up to 30 cm2. The density of azimuthally oriented diamond seeds was measured by scanning electron microscopy (SEM) images and found to be 8 × 108 cm−2 with only 30% variation over the whole deposition area. After the nucleation, a microwave plasma assisted chemical vapor deposition (MPACVD) resulted in highly oriented and 〈100〉 textured diamond films. X-ray diffraction (XRD) measurements of the {111} diamond peak, for a 60-μm-thick film, showed azimuthally misorientations (FWHM) of 2.8° for the tilt and 5.5° for the rotation. Raman spectroscopy was used to evaluate the radial distribution of the phase purity within the films. Introducing a final growth step with a low nitrogen concentration in the gas phase yielded a significant decrease in nitrogen incorporation in the films while maintaining the morphology. Elastic recoil detection (ERD) revealed impurity concentrations in the surface layer of 1.4 ppm for the N/C ratio and 210 ppm for the H/C ratio.

The quality of CVD diamond films degrades severely with decreasing substrate temperatures. In this report, the impurity and defect incorporation in diamond films deposited from a carbon-hydrogen-oxygen gas system at substrate temperatures between 560 and 345 C has been investigated using elastic recoil detection (ERD), FTIR and micro-Raman spectroscopy. In approaching the low temperature limit which coincides with the formation of cauliflower morphologies, the hydrogen incorporation rises steeply. Hydrogen contents beyond 1 at.% have been measured, roughly 20 times higher than in the upper temperature range. By contrast, there was a much smaller rate of rise in the concentration of nitrogen and oxygen, despite a marked change in the microstructure of the deposited films. At the lowest substrate temperatures, the absolute hydrogen content measured by ERD increases more steeply than those measured by FTIR spectroscopy, which refers to C-H stretch vibrations only. There is evidence that hydrogen is incorporated also in the bulk rather than being concentrated at grain boundaries as at higher temperatures. This conclusion is supported by micro-Raman spectroscopy exhibiting significant peak broadening in the low temperature region.

The hydrogen content of BN films deposited by the ICP plasma method from trimethyl borazine and N2 has been investigated integrally and spatially resolved by elastic recoil detection. Hydrogen contents between 15% and 25% have been found for conditions allowing the formation of c-BN compared with 4%–10% for h-BN conditions (low bias voltages). However, under c-BN conditions hydrogen is concentrated within the sp2 parts of the film, particularly within the nucleation layer. Addition of hydrogen to the gas phase prevents the formation of c-BN and leads to h-BN films with high hydrogen contents. FTIR examination shows that the films do not contain BH and NH bonds; traces of OH and CHx do not add up to the total hydrogen content. Therefore most of the hydrogen must be free, for example incorporated on interstitials.

CVD diamond samples were investigated for their content of light elements and their influence on the physical properties by high resolution elastic recoil detection (ERD) analysis. ERD allows quantitative measurements of depth profiles for all light elements with a depth resolution better than 1 nm (near the surface) using 58Ni or 127I ions with a specific energy of about 1 MeV per nucleus and a high resolution magnetic spectrograph. The measurements were focused on the content of hydrogen in (100)-oriented CVD diamond 90 μm thick grown on (100) silicon. The hydrogen content varied from 0.07 at.% in the bulk to several atomic per cent for fine crystalline diamond near the interface to its silicon substrate. The hydrogen coverage of (100)-oriented diamond could also be determined to be near the value which is expected if the (100) surface has a (2 × 1) reconstruction. In addition to hydrogen, other light elements were simultaneously measured. Silicon and oxygen impurities were also analysed in order to get information about their physical relevance. A correlation between the Si content and the luminescence of silicon centres was attempted.

Some experiments on the growth of cubic boron nitride (c-BN) show that the content of c-BN in BN films depends on the film thickness. Non-cubic phases, such as amorphous and hexagonal BN, can be identified, particularly near the substrate. To understand these phase variations, elemental depth profiles of thin BN films were measured by high resolution elastic recoil detection (ERD). BN films have been prepared by r.f. glow discharge (r.f = 13.56 MHz) from a mixture of B2H6 (10% in argon) and N2. Layers 48 nm and 90 nm thick have been deposited on crystalline silicon with integrated portions of the cubic phase of 55% and 67% respectively. ERD measurements showed that the integrated boron-to-nitrogen ratio is close to unity for the thicker film, but significant enhancements of boron are found near the interface with the silicon substrate and at the surface. These enhancements are coupled with a decrease in the hydrogen content, which is about 9% on average. The elemental fluctuations will be correlated with the occurrence of different BN phases and improvements for c-BN deposition will be suggested.

The electron stimulated desorption of negatively charged hydrogen ions from diamond surfaces is studied with respect to an application as a bright ion source. Desorption of macroscopic currents of negative ions from a diamond surface by stimulated desorption has been demonstrated recently. To determine the dependence of the ionization cross-section on electron affinity, an oxygen covered diamond was used as a model system for positive electron affinity diamond. On this surface, different amounts of deuterium have been adsorbed. The D − ionization cross-section has been proven to vary by one order of magnitude between 3.5×10 −18 cm 2 and 5×10 −19 cm 2 with hydrogen coverage of the surface. The energy distribution of desorbed negative ions has been measured by an electrostatic analyser. For the measured broad energy distribution, an attempt of an explanation is made.

A series of CVD diamond films was analysed for their nitrogen content by high resolution elastic recoil detection (ERD) in order to investigate the incorporation of nitrogen from the gas phase in a CVD reactor into the growing diamond film. CVD diamond films were deposited from a mixture of 1.5% CH4 in H2 and an admixture of nitrogen gas varying from 0.33% up to 67% [N] [C] . The measurements revealed that the probability of nitrogen incorporation is only about 0.4% with respect to carbon. In addition, the morphology and texture of the polycrystalline films were investigated by scanning electron microscopy and X-ray texture analysis and showed a significant dependence on the nitrogen admixture.