The development of CVD grown single-crystal Diamond-on-Iridium (DOD sensors for charged-particle detection in hadrons and nuclei physics research is reviewed. A variety of samples grown at the University of Augsburg has been investigated with alpha and beta sources in the laboratory, swift ions from the heavy-ion synchrotron SIS in Darmstadt, and relativistic protons from the COoler-SYnchrotron COSY in Julich. The results obtained by means of I-E(V) studies, transient-current techniques (TCT), alpha-spectroscopy, and heavy-ion time-of-flight (ToF) measurements are compared to those of commercially available \polycrystalline and homoepitaxial single crystal CVD diamond sensors of electronic grade quality. In many aspects, the performance of DOI sensors was found quite similar to that of homoepitaxial counters, and in any case far superior to that of polycrystalline detectors. Under single-carrier drift conditions, the CCE and energy resolution (delta E/E) for holes reached levels CCEh > 95% and delta E/E-h similar to 0.3%, respectively, which correspond to values of the Schubweg w(h,e) well above the detector thickness. In contrast, the CCEe for electrons was typically lower than similar to 40%, leading to appreciable reduction of the detection efficiency in the dual-carrier drift mode (CCE similar to 60%), which characterizes the experiments with swift heavy ions and high-energy particles. We measured transport parameters comparable to those of homoepitaxial devices: mu(h)(0) similar to 3080-1756 and mu(e)(0) similar to 2276-1150 cm(2)/Vs, v(sat)(h) similar to(1.7-1.4) *10(7) and v(sat)(e) similar to(1.5-1.0)* 10(7) cm/s, as well as intrinsic time resolutions sigma(i) similar to 15 ps. It is shown, that substantial improvements have been achieved in recent years, albeit reproducibility and the understanding of the reduced electron collection remain challenging issues. Prime novelty: Comprehensive electrical characterization of intrinsic single-crystal CVD Diamond-OnIridium sensors produced at the University of Augsburg and their classification into the range of commercial electronic grade polycrystalline and homoepitaxial diamond sensors supplied by Element Six.

Typical size of the chemical vapor deposition (CVD) homoepitaxially grown diamond material (also known as the single crystal SC) is limited to some 5 x 5 mm due to the availability of growth substrates made from the highpressure high-temperature (HPHT) diamond. Presently, the only material readily available for the production of diamond detectors with larger area (∼10 cm) is polycrystalline (PC) film grown on silicon wafers with electronic characteristics far inferior in comparison to SC material. In order to produce a large-surface high-quality material for diamond detectors different techniques for heteroepitaxial growing of diamond films are being investigated and developed at the University of Augsburg. By using yttriumstabilized zirconium oxide (YSZ) buffer layer to produce iridium terminated substrate on silicon wafers [1] one can grow diamond films (also know as the diamond on iridium DoI) that are far more homogeneous than PC, however, still burdened with defects. In last few years a remarkable improvement in lowering of the level of impurities and defects was achieved so that the presently produced samples while still not comparable to SC material are far superior to any PC material. Most significant structural defect arising in heteroepitaxial growth are dislocations.In a recent study [2] in which the density of threading dislocations was determined by few methods over a large sample thickness, an inverse growth depth behaviour was found. While this at least in principle confirms that films with very low density of dislocations can be grown, presently procedure would not be economically effective therefore different growing techniques e.g. epitaxial lateral overgrowth are being developed. In order to assess the quality i.e. electronic characteristics of new DoI samples a typical measurement of the charge collection efficiency (CCE) is performed by using the transient current technique (TCT). Alpha particles (Am) are used to test the sample with different polarizations and drift fields so that the properties for both types of charge carriers can be evaluated. In Fig. 1 the set of measurements with a recent DoI sample of 190 μm thickness at different drift fields is presented showing the saturation at values above 0.8 V/μm. While the overall triangular shape of wave forms indicate the presence of the charge recombination defects, additional flat-top slope is related to losses due to the charge carrier trapping within the sample. For this sample we have measured an average CCE of about 60% for holes, which is below the level of the best samples ∗This work was supported in part by HadronPhysics3 (grant agreement No 283286 under the EU FP7.) † m.kis@gsi.de Figure 1: Pulse-shapes (waveforms) obtained for the drift of holes across the sample. The CCE is determined by waveform integration. Each waveform is an average from 1000 recorded events.

The topographic surface features that develop during chemical vapour deposition of heteroepitaxial diamond on off‐axis Ir/YSZ/Si(001) and their implications on the incorporation and modification of 0D and 1D defects have been studied. After growth with nitrogen in the gas phase, the surface has split into alternating stripes of opposite inclination. The terrace regions are tilted towards the crystallographic [001] axis, while the risers are tilted in the opposite direction. AFM measurements on the macroscopic terraces reveal a microscopic substructure consisting of terraces and risers on a smaller length scale. Cross‐section SEM images display inclined dark and bright striations. In photoluminescence maps, the stripes ending at risers show a higher NV and SiV emission intensity thus indicating a stronger defect incorporation of colour centres. Relative growth rates on terrace and riser areas have been evaluated. In high resolution Raman measurements, the cross‐section maps display bands of high line width embedded in a background of lower line width. Conversion of these data into dislocation density maps reveals local variations by one order of magnitude. The tilt angle of the threading dislocation bundles is attributed to an interaction induced by the lateral step flow on the off‐axis growth surface.Scheme of the investigated diamond samples.

The development of dislocation density and micro-strain in heteroepitaxial diamond films on iridium was measured over more than two decades of thickness up to d ≈ 1 mm. Simple mathematical scaling laws were derived for the decrease of dislocation density with increasing film thickness and for its correlation with micro-strain. The Raman line width as a measure of micro-strain showed a huge decrease to 1.86 cm−1, close to the value of perfect single crystals. The charge collection properties of particle detectors built from this material yield efficiencies higher than 90% in the hole-drift mode, approaching the performance of homoepitaxial films.

Diamond has several desirable features when used as a material for radiation detection. With the invention of synthetic growth techniques, it has become feasible to look at developing diamond radiation detectors with reasonable surface areas. Polycrystalline diamond has been grown using a chemical vapour deposition (CVD) technique by the University of Augsburg and detector structures fabricated at the James Watt Nanofabrication Centre (JWNC) in the University of Glasgow in order to produce pixelated detector arrays. The anode and cathode contacts are realised by depositing gold to produce ohmic contacts. Measurements of I-V characteristics were performed to study the material uniformity. The bias voltage is stepped from -1000 V to 1000V to investigate the variation of leakage current from pixel to pixel. Bulk leakage current is measured to be less than 1nA.

The beams extracted from thermal neutron sources such as nuclear reactors are monochromatised by Bragg diffraction using imperfect single crystals with an angular mosaic spread of typically 0.2-0.8 degrees. For neutron wavelengths below 1.5 angstrom, the highest reflectivity of all crystalline materials is expected for diamond. Nowadays diamond single crystals with an appropriate mosaic spread exceeding a thickness of 1 mm can be grown by heteroepitaxy on an Ir/yttria-stabilised zirconia bilayer deposited on a Si(001) single crystal.To explain the observed neutron reflectivity being below the theoretically expected value, we have studied the spatial distribution of the mosaic structure of two crystals by high resolution X-ray diffraction using a laboratory X-ray source and synchrotron radiation. The first sample (A) showed a uniform mosaic spread of 0.18 degrees +/- 0.02 degrees across the 1 cm wide sample. The peak shift of the X-ray rocking curves of 0.08 degrees indicated a weak curvature of the crystal lattice. The measured absolute neutron peak reflectivity of 34% corresponded to 90% of the value predicted by theory. The peak width of the neutron rocking curve for the second sample (B) was twice as big, but here the peak reflectivity of 13% corresponded to only half of the theoretical value. This unfavourable behaviour could be assigned to a substantial spatial variation of the mosaic spread deduced from the synchrotron X-ray studies. X-ray diffraction with high spatial resolution indicated a mosaic block size below 50 pm for sample A. This was consistent with chemical etching experiments on the surface of a comparable sample which showed both randomly distributed dislocations and others that are arranged in boundaries of several 10 mu m large domains. (C) 2013 Elsevier B.V. All rights reserved.

In the last year, we continued with the development of large-area diamond sensors grown at the University of Augsburg by chemical vapour deposition (CVD) on wafer scale Ir/YSZ/Si(001) substrates. Early results of diamondon-iridium (DoI) sensors [1] were indicating significant defect densities within the detector bulk, which led to reduced charge collection efficiencies CCE = QColl/QGen= 40% (with QGen the particle induced charge and QColl the charge measured by the sensor) and broad spectra of energy resolution δE/E ≈ 30%. These values were obtained with traversing Am-α-particles for the favourable case of a sensor of thickness d = 12μm = α-range in diamond. A remarkable improvement of the crystal-quality has been achieved in 2010 by using ultra-pure H2/CH4 gas mixtures and optimized growth techniques [2]. Two DoI samples D1 and D2 of thicknesses dD1 = 293 μm and dD2 = 320 μm, respectively, were investigated. Ti/Pt/Au quadrant electrodes with common mass potential were applied in order to prove the spatial homogeneity of the detector performance. Almost symmetrical IED characteristics with respect to the bias polarity were obtained for each sector, while the dark current at electric fields ED ≥ ± 2 V/μm were of the order of 10 A, well comparable to homoepitaxial single-crystal (SC) diamond sensors.

For future particle accelerator experiments, e.g. at FAIR, we are developing novel, advanced diamond sensors grown by chemical vapour deposition (CVD), capable on one hand of replacing the commonly used silicon tracking devices and being on the other hand an advantageous alternative to polycrystalline or single-crystal diamond sensors used so far in beam diagnostics and timing applications. By using heteroepitaxial diamond films grown on Ir/YSZ/Si(001) we want to bridge the gap between polycrystalline (which exhibits inhomogeneous incomplete charge collection) and single-crystal CVD diamond (i.e. of small areas), enabling the fabrication of large-area diamond sensors of good homogeneity, high drift velocity of the charge carriers, and of almost complete collection of the particle induced charge.

The thermal diffusivity of heteroepitaxial CVD diamond films grown on iridium buffer layers has been measured using a combined laser flash and converging thermal wave setup. Absolute values and anisotropy for a fiber-textured reference sample were in the range of former reports in the literature. The in-plane thermal conductivity for three heteroepitaxial samples grown on Ir/YSZ/Si(001) as deduced from the diffusivity measurements was around 20W/cmK, similar to high purity large grain polycrystalline films. Laser flash measurements of the perpendicular diffusivity suggest that the defect rich first microns of the heteroepitaxial films represent a thermal series resistance which limits the perpendicular heat transport especially for thin films. For the parallel component of the diffusivity the contribution of this shunt resistance is negligible. The absolute values for the parallel component in the heteroepitaxial films with in-plane angular spread of the crystal lattice below 0.5° were discussed in the framework of the model proposed by Klemens for phonon scattering by grain boundaries. The present data indicate that the remaining defects in heteroepitaxial diamond films with low mosaic spread are significantly less detrimental for the heat transport than large angle grain boundaries. In addition we speculate that the exclusive deposition on the {100} growth sector may also reduce the influence of nitrogen in the gas phase on the heat transport properties.

Within the ADAMAS collaboration, the role of the diamond group at the University of Augsburg is to develop and manufacture large-area high-quality single crystal diamond suitable for the construction of detectors for heavy ions and minimum ionising particles. In contrast to current samples grown by homoepitaxy on substrates with maximum lateral dimensions of ~10 mm, the heteroepitaxial deposition is done on the Ir/YSZ/Si(001) multilayer system that is available in 100 mm wafer size. As a consequence, these samples that are often known as “Diamond on Iridium” (DoI) offer a realistic chance for a scaling to large areas as required for tracking applications. Major challenges, however, are the occurrence of polarization effects and an incomplete charge collection with charge collection efficiencies (CCE) varying from sample to sample. These effects are mainly attributed to properties of the bulk, namely lattice defects like dislocations and point defects, generating both deep and shallow charge carrier traps. Being able to control and further reduce the density of these defects is compulsory for future application of DoI material not only in particle physics.