Boron-doped Homoepitaxial Diamond Research Articles

Multi-microscopy characterization of threading dislocations in CVD-grown diamond films

We present site-coincident imaging of a high dislocation density boron-doped chemical vapor deposition-grown homoepitaxial (001) diamond film using electron channeling contrast imaging (ECCI), cathodoluminescence, transmission electron microscopy (TEM), and scanning secondary ion mass spectroscopy (SIMS). With growing interest in large area heteroepitaxial substrates, we show that ECCI is a promising technique to accurately evaluate dislocations in diamond films. We find the electron backscatter yield is sufficient to distinguish individual threading dislocations in ECCI despite the low atomic number of carbon, and we have generally good agreement between dislocations observed in ECCI and TEM of the same region. Importantly, relying on luminescence in the 430 nm dislocation-related A-band alone results in an underestimation of the defect density as we find only 20%–40% of the threading dislocations luminesce. We further show that dislocations do not perturb the spatial uniformity of boron doping measured by SIMS, even when the dislocations are clustered with high density, and we can tentatively rule out strong segregation effects at the dislocations. Our results establish the complementary use of microscopy and microanalysis methods to rapidly characterize and understand the impact of dislocations in diamond thin films.

Pseudo-vertical Mo/Au Schottky diodes on {113} oriented boron doped homoepitaxial diamond layers

Pseudo-vertical diodes on {113} oriented homoepitaxial boron-doped diamond were successfully fabricated using molybdenum as a metal for both the Schottky and ohmic contacts. After metal deposition, diodes were stabilized by annealing for 20 min at 300 °C and their I-V and C-V characteristics were measured at temperatures from 30 to 180 °C. Results show that Mo forms a very good and stable Schottky contact on {113} oriented homoepitaxial boron-doped diamond. At 180 °C, forward current densities higher than 1kA/cm2 are achieved, while the reverse current density stays unchanged at 10−8 A/cm2. The ideality factor of the diode I-V characteristic drops to 1.23 and the barrier height of the Schottky contact increases to 1.71 eV. This effect is attributed to inhomogeneity in the Schottky barrier. These characteristics are fully comparable with those of the best Schottky diodes fabricated on {100} oriented homoepitaxial boron-doped diamond. Results confirm that the use of {113} oriented homoepitaxial boron-doped diamond for the fabrication of high-temperature power devices is advantageous, as it enables high quality Schottky and ohmic contacts.

Effect of substrate crystalline orientation on boron-doped homoepitaxial diamond growth

Boron-doped diamond layers have been grown by MW PECVD on substrates with a misorientation over the range of 0 to 90° relative to the (100) crystalline plane while keeping all other growth conditions constant. Deposited layers were characterized by scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. The deposition rate, surface roughness, and boron incorporation vary according to the substrate misorientation. These results show the essential need for rigorous control of the substrate crystalline orientation in the synthesis of doped diamond for electronic applications. Thick, smooth, and free of surface defects boron-doped epitaxial diamond layers were obtained over a broad range of crystalline orientations at high growth rates and high boron incorporation efficiency. Novelty statementThis work reports on the effect of the substrate crystalline orientation on the morphology, growth rate, and boron incorporation in epitaxial diamond layers deposited by plasma-enhanced chemical vapor deposition.

Misfit dislocations between boron-doped homoepitaxial films and diamond substrates studied by X-ray diffraction topography

Boron-doped diamond epilayers grown over diamond substrates have a different lattice parameter from the undoped diamond substrate, which introduces a lattice mismatch between substrates and epilayers. This can generate misfit dislocations at the interface when the epilayer reaches a certain critical thickness. For a boron concentration of about 1 × 1020 atoms cm−3, the calculated lattice mismatch is about 1.3 × 10−4 and the critical thickness is of the order of 0.2 µm. In the epilayers studied, grown over high-pressure high-temperature 1b (001) substrates, the lattice mismatch and the epilayer thickness are 1.3 × 10−4, 30 µm and 6.5 × 10−4, 4 µm. The epitaxial strain has been relaxed by the generation of two orthogonal misfit dislocation systems. These are edge dislocations parallel to the [100] and [010] directions with a Burgers vector making an angle of 45° with the (001) interface. Their lengths are 40–60 µm and their lineal densities 200–240 cm−1. They are heterogeneously nucleated, propagated in the form of half-loops along the slip planes (011) and (101), respectively, and related mainly to 〈111〉 threading dislocations emerging from octahedral growth sectors. Another kind of half-loop originates from the substrate growth sector boundaries. Limited X-ray topography has been demonstrated to be a very useful tool to discriminate between substrate and epilayer defects when their lattice mismatch is not sufficient to separate such defects in conventional Lang topography. X-ray section topography has confirmed the presence of [001] dislocations in the epilayers and the misfit half-loops related to threading dislocations propagating from the interface.

Assessment of candidate metallization systems deposited on diamond using nano-indentation and nano-scratching tests

Mechanical suitability of ohmic contacts among the select metallization systems, deposited on a p-type heavily boron-doped homoepitaxial diamond layer, was evaluated via mechanical tests on the nanoscale. Two candidate metallization systems were considered: Si/Al and Ti/Pt/Au. Metallizations were performed using two different techniques: plasma-enhanced chemical vapour deposition and “lift-off”. Effectiveness of the techniques was assessed via mechanical tests on the microscale and the nanoscale. Nano-indentation experiments were performed to determine the mechanical properties of the layers. Nano-scratching experiments were used to evaluate the mechanical adhesion on the diamond substrate. Scanning electron microscopy was applied for observation of the morphology of the surface and the indent and for detecting defects.

Electrochemical Property of Boron-Doped Heteroepitaxial Diamond Treated By Different Oxidation Method

Boron-doped diamond electrode is expected to realize high sensitive electrochemical sensors because of its unique properties such as wide potential window and low background current. Many studies of diamond electrochemistry have been already reported but most of these works were carried out using polycrystalline diamond. Because several crystal faces and many grain boundary exists, electrochemical reactions at the surface of polycrystalline diamond becomes complex. On the other hand, epitaxial diamond (nearly single crystal) is structurally uniform, we believe that boron-doped epitaxial diamond will be more ideal material in order to realize highly-sensitive and highly-reliable sensor. Furthermore, it is useful to clarify basic electrochemical reactions at the surface of diamond. Several studies have been already reported using boron-doped homoepitaxial diamond. In the case of homoepitaxial growth, HPHT (high pressure high temperature) diamond is used for substrate but available size of HPHT diamond is small and also costs much. On the other hand, heteroepitaxial growth is much easier to fabricate diamond in large size at low cost. From the viewpoint of practical use, studies on boron-doped heteroepitaxial diamond will be important. In this study, fundamental electrochemical property of hydrogen-terminated and oxygen-terminated boron-doped heteroepitaxial diamond was evaluated. There are several methods to change the surface termination of diamond from hydrogen to oxygen such as heat mixed acid, oxygen plasma, UV irradiation, anodic oxidation treatment. The treatment conditions were optimized at each method and the oxidation peak shift of potassium ferrocyanide was compared. First, non-doped epitaxial diamond(100) was grown on Ir(100)/MgO(100) substrate in thickness of 100 μm by direct current plasma enhanced chemical vapor deposition (PE-CVD) method. After the growth, Ir/MgO substrate was removed by acid treatment in order to fabricate free-standing diamond platelet. Finally, boron-doped epitaxial diamond was grown on non-doped epitaxial diamond at boron concentration of 5000 ppm. Oxidation peak potential of potassium ferrocyanide (5mM in 0.1 M sodium sulfate) was measured by cyclic voltammetry after oxidation treatment under defined condition. To optimize oxidation treatment condition, this operation was repeated until the peak potential becomes constant. For heat mixed acid treatment, sample was dipped in mixed acid (60% nitric acid:95% sulfuric acid = 3:1) at 523 K for 15 min. For anodic oxidation treatment, 0.5 M sulfuric acid was used for electrolyte and the potential was kept at 3.0 V for 5 s. From the X-ray diffraction measurement, single sharp peak at around 120 degrees which is due to diamond(004) was detected from 2theta-omega spectrum and four-fold symmetry peaks were detected from diamond{111} pole figure which indicates that the diamond was epitaxially grown on Ir/MgO substrate. When the sample was terminated by hydrogen, sharp oxidation and reduction peaks were observed from the cyclic voltammogram and its peak separation was around 70 mV which reflects a quasi-reversible reaction (very close to reversible reaction). Shift of oxidation peak was observed after both heat mixed acid and anodic oxidation treatment. By repeating heat mixed acid treatment for four times (total 60 min treatment), oxidation peak became constant and its potential was 0.42 V vs Ag/AgCl. On the other hand, repeating anodic oxidation treatment for six times (total 30 s treatment), the peak became constant at potential of 1.52 V. Heat mixed acid treatment is a well-known method to remove surface conductive layer of diamond but the peak potential at optimized treatment condition was extremely different between two methods which indicates that termination structure differs. Termination structures analyzed by X-ray photoelectron spectroscopy will be discussed. Furthermore, the results of other methods such as oxygen plasma and UV irradiation treatment will be also discussed.

Al plasmon-enhanced diamond solar-blind UV photodetector by coupling of plasmon and excitons

Localised surface plasmon (LSP) enhanced metal-semiconductor-metal interdigital solar-blind UV photodetector was fabricated by assembling of Al crescent-shaped arrays on boron-doped homoepitaxial diamond thin films. The spectral responsivity at 225 nm is about 28 m A/W biased at 5 V, which is over 10-fold more than that of bare diamond detector. The photodetector exhibits a rejection ratio up to 103 between 225 and 280 nm light. These observations can be attributed to near-field optical coupling between Al LSPs and diamond excitons through the improved optical absorption in the semiconductor layer. The complementary optical properties of metal Al nanostructures and diamond semiconductor make them attractive components for solar-blind UV photodetectors.

Fundamentals and Applications of Diamond

Dear colleagues, We are pleased to introduce you to this Special Issue of physica status solidi (a), focussed on fundamental and applied science and technology of diamond and nanodiamond materials. This special issue aims at providing a brief but hopefully broad overview of those topics that are today at the centre of the diamond community's current interests, such as diamond growth and doping, colour centres, nanophotonics, devices and electronics, bioapplications of diamond and nanodiamonds, surface properties, etc. The Feature Article by Dean Ho and Han Man 1 reviews the use of diamond materials in various nanomedicine applications, with a particular focus on therapeutics and drug delivery. They demonstrate that, in addition to their well-known properties such as biocompatibility and outstanding imaging capability, diamond nanoparticles are promising vehicles for drug delivery resulting in enhanced therapeutic efficacy and reduced premature drug release. This exciting field of applications is expected to grow rapidly with the full understating and control over the surface functionality of diamond nanomaterials. A relatively recent field of excitement for the diamond community is nanophotonics, an area of research that has rapidly evolved from the study of colour centres in bulk diamond to a plethora of phenomena and applications enabled by a precise nanostructuring of diamond single crystals. In their Feature Article, Birgit Hausmann and colleagues 2 from Harvard University review some of their recent contributions, such as the use of diamond nanowires to increase single photo collection from nitrogen-vacancy (NV) centres embedded in these nanostructures, or the coupling of single NV centres to planar resonators on-chip. This article highlights the outstanding challenges that still need to be addressed both in terms of fabrication nanotechnology, implantation of colour centres, and device design. The third Feature Article in this special issue, by Markus Dankerl and colleagues 3 of the Walter Schottky Institute, reviews solution-gated field effect transistors (SGFETs) based on surface-conductive single crystalline diamond with a focus on bioelectronic applications, in particular for the development of cell-transistor electronic interfaces. The article covers technology aspects, fundamental issues of the diamond/electrolyte interface, and finally reports on the successful recording of cell's action potentials using arrays of diamond SGFETs. In a special issue related to diamond, the most demanding challenge for the community, i.e. the growth of high quality diamond films and their doping, cannot be missing. In the Editor's Choice article, André Sartori and co-workers 4 provide a detailed study of in-situ boron doping during heteroepitaxial growth of diamond, a promising path towards the preparation of doped, high-quality, large area diamond thin films, which is a step that seems mandatory for the success of any of the potential applications of diamond in electronics. In another contribution, Jocelyn Achard et al. 5 explore the suitability of boron-doped freestanding diamond single crystals for vertical power electronic devices. In contrast to the most standard co-planar structures, the vertical configuration offers several advantages, at the expense of requiring high quality, heavily doped, thick films. The electrical characterization of diamond films as well as of diamond devices is still a very active topic in the diamond community, and is well represented in this special issue of physica status solidi (a). For instance, the contribution by Yuto Hoshino and colleagues 6 reports on the electrical properties of a novel lateral p–n junction diode fabricated by selective growth of phosphorous n+ diamond, aiming at avoiding some of the drawbacks of vertical p–n junctions. Two studies report on fundamental aspects of the electronic properties of charge carriers in diamond films. J. Barjon et al. 7 characterize homoepitaxial boron-doped diamond, grown with a very residual donor impurity concentration which results in low compensation and thus films exhibiting high conductivities. In their contribution, P. Ščajev et al. 8, provide an extensive study on the effect of the crystalline structure of diamond films on the carrier recombination and diffusivity. With this special issue of physica status solidi (a) we do not only aim at providing you with a selection of the currently most active fields of research related to the science, technology, and applications of diamond. In addition, we also hope that the contributions included in this issue can help to point out the challenges ahead while at the same time provide inspiration for your future work. Garching & Hasselt, September 10, 2012 Jose A. Garrido Miloš Nesládek Ken Haenen

Raman scattering in boron-doped single-crystal diamond used to fabricate Schottky diode detectors

Thanks to its exceptional physical and electronic properties, diamond is an attractive material for electronic devices working at high temperature and in harsh chemical environment. Its use as a semiconducting material for electronics is related to the possibility of doping it in order to control its conductivity. Semiconducting p-type diamond films can be grown when boron is introduced into the film. In this work, boron-doped (B-doped) homoepitaxial diamond films are grown by Microwave Plasma Enhanced Chemical Vapor Deposition. Raman and electrical characterizations are carried out on the films as a function of boron doping level. As the boron content increases, we observe systematic modifications in the Raman spectra of single-crystal diamonds. A significant change in the lineshape of the first-order Raman peak, as well as a wide and structured signal at lower wavenumbers, appears simultaneously in samples grown with higher boron content. A single crystal diamond Schottky diode based on a metal/intrinsic/p-type diamond junction is analysed.

RAMAN SCATTERING IN HEAVILY BORON-DOPED SINGLE-CRYSTAL DIAMOND

A series of boron-doped homoepitaxial diamond films grown by Microwave Plasma Enhanced Chemical Vapor Deposition at the University of Rome Tor Vergata have been investigated with Raman spectroscopy. As the boron content increases, we observed systematic modifications in the Raman spectra of single-crystal diamonds. A significant change in the lineshape of the first-order Raman peak as well as a wide and structured signal at lower wavenumbers appeared simultaneously in samples grown at higher boron content.

Hall hole mobility in boron-doped homoepitaxial diamond

Hall hole mobility of boron-doped homoepitaxial (100) diamond samples has been investigated in the temperature range of 100--900 K, both experimentally and theoretically. The temperature dependence of the mobility measured in high-quality and low boron-doped materials was compared with theoretical calculations to determine the phonon-hole coupling constants (deformation potential for acoustic phonons and coupling constant for optical phonons). The maximum hole mobility is found to be close to $2000\text{ }{\text{cm}}^{2}/\text{Vs}$ at room temperature. For boron-doped material, the hole scattering by neutral boron atoms is shown to be important in diamond due to the high ionization energy of the boron acceptor. The doping dependence of the Hall hole mobility is established for boron-doping levels ranging between ${10}^{14}$ and ${10}^{20}\text{ }{\text{cm}}^{\ensuremath{-}3}$ at 300 and 500 K. The physical reasons which make diamond a semiconductor with a higher mobility than other semiconductors of column IV are discussed.

Development of ionizing radiation detectors based on synthetic diamond material for the nuclear power industry

Results of the experimental study of boron-doped single-crystal synthetic diamond substrates and thin homoepitaxial diamond films grown on them with the predetermined properties are discussed. The homogeneity of the substrates and films is examined. Diamond detectors based on the grown diamond structures have been produced. The sensitivity of these detectors to UV photons, X and γ rays, α particles, and fast neutrons under stationary and pulsed irradiation has been determined.

Hydrogen passivation of boron acceptors in as-grown boron-doped CVD diamond epilayers

A homoepitaxial boron-doped diamond single layer is investigated by means of Fourier transformed infrared spectroscopy (FTIR) and cathodoluminescence (CL). Both techniques are shown to be complementary. µ-FTIR mapping allows to determine the location of active boron while CL allows discernability between passivation and compensation. Hydrogen incorporation during chemical vapour deposition (CVD) growth is revealed to passivate boron acceptors. The obtained results highlight that plasma etching can induce a dissociation of B–H centres.

Front Cover (Phys. Status Solidi A 9/2009)

Homoepitaxial diamond films are more and more frequently proposed as the basic component of electronic devices, but like in other semiconductors, defects and unwanted impurities may hamper optimal operation. The article by Pierre Muret et al. (p. 2016) deals with these aspects in boron-doped homoepitaxial diamond films. A new understanding of the role and structure of impurities comes both from ab-initio calculations of the density of states and total energy of possible defect configurations, and from the electrical response of elementary devices, mainly Schottky diodes, whose transient regimes of current, charge or capacitance are influenced by local defects. The Fourier transforms of capacitance transients in a borondoped diamond Schottky diode, shown on the front cover, are representative of the experimental turning back to equilibrium of the hole trap population in the frequency domain at several temperatures. The small image displays the local structure of the most stable BH2 defect. The results obtained in this work demonstrate that defects incorporating hydrogen atoms are the most probable, but that their concentration profiles decrease rapidly inward the diamond film.

Impurity band structure of boron-doped homoepitaxial diamond

Impurity band structure of boron-doped homoepitaxial diamond

Deposition of thick boron-doped homoepitaxial single crystal diamond by microwave plasma chemical vapor deposition

The deposition of high quality single crystal boron-doped diamond is studied. The experimental conditions for the synthesis of 1–2 mm thick boron-doped diamond are investigated using a high power density microwave plasma-assisted chemical vapor deposition reactor. The boron-doped diamond is deposited at a rate of 8–11.5 μm/h using 1 ppm diborane in the feed gas as the boron source, and the capability to overgrow defects is demonstrated. The experimental study also investigates the deposition of diamond with both 10 ppm diborane and 2.5–500 ppm of nitrogen added to the feedgas. Synthesized material properties are measured including the electrical conductivity using a four-point probe and the substitutional boron content using infrared absorption.

Recovery of negative electron affinity by annealing on (111) oxidized diamond surfaces

We discovered that oxidised (111) p-type boron-doped homoepitaxial diamond film surfaces grown by chemical vapor deposition (CVD) recover negative electron affinity (NEA) after annealing the samples in vacuum or under an Ar atmosphere. On the other hand, (001) p-type boron-doped homoepitaxial CVD diamond surfaces do not show NEA recovery. The oxidised (111) surface possesses positive electron affinity of + 0.6 eV, whereas the subsequently annealed surface has a negative electron affinity of − 0.8 eV, indicating a partial hydrogen termination condition that is supported by XPS results. These distinct behaviours should be taken into consideration for investigating the surface electronic properties of diamond.

Substrate influence on MPCVD boron‐doped homoepitaxial diamond

AbstractWe report a study of boron‐doped diamond epilayers grown on (100) HPHT diamond substrates by microwave plasma chemical vapor deposition (MPCVD) in simultaneous or successive runs. The structural and crystalline quality of the samples have been investigated by several techniques. The profiles performed by secondary ion mass spectrometry (SIMS) show the homogeneity in depth of the boron concentration in all layers. Cathodoluminescence (CL) spectroscopy indicates state‐of‐the‐art epilayers. The substrate misorientation angles have been investigated by X‐ray diffraction (XRD). Confocal micro‐Raman mapping of the full width at half‐maximum of the 1332 cm–1 diamond line was also preformed on all substrates. It evidences crystalline inhomogeneities that are known to be growth sectors. We show a relation between the epilayer surface morphology, observed by Nomarski optical microscopy, and the substrate crystallinity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Persistent positive and transient absolute negative photoconductivity observed in diamond photodetectors

Persistent photoconductivity and optical quenching phenomena in boron-doped homoepitaxial diamond thin films grown on type Ib (100) substrates were investigated. Boron doping induced positive persistent photoconductivity (PPPC) and photocurrent gain upon the illumination of deep ultraviolet (DUV) light. A deep defect with a thermal energy of around 1.37 eV was deduced from the results of thermally stimulated current measurements. An optical quenching experiment revealed a threshold photon energy of around 2 eV. The difference between the thermal and optical energies of the deep defect suggests a large lattice relaxation. An absolute transient negative photocurrent (TNPC) was observed for zero or weak electric fields with DUV light illumination after optically quenching the PPPC. The absolute TNPC was due to transient electron filling of the charged nitrogen in the substrate. The combination of the nitrogen in the substrate and the boron-related defects in the epilayer was apparently responsible for the PPPC and TNPC effects. Therefore, the overall photoresponse properties of a thin epilayer deposited on a type Ib diamond substrate can be tailored by adjusting the concentration of nitrogen in the substrate and of boron in the epilayer.

Pressure effect of superconducting transition temperature for boron-doped diamond films

The superconducting transition temperature of homoepitaxial boron-doped diamond thin films fabricated by microwave plasma-assisted chemical vapor deposition technique depend on substrate orientation. In addition, heavily doped diamond thin films indicate anisotropic lattice expansion. From these points of view, pressure effect will give us knowledge of the superconducting mechanism of boron-doped diamond. We report measurements of the electrical resistivity of heavily boron-doped diamond thin film under pressure up to P = 1.45 GPa and 1.27 GPa for (1 1 1) and (1 0 0) homoepitaxial thin films, respectively. The superconducting transition temperature decreases linearly with increasing pressure by a rate of δ T c/δ P = −1.17 × 10 −1 K/GPa and −1.51 × 10 −2 K/GPa for (1 1 1) and (1 0 0) thin film, respectively.