Ultrapure Diamond Research Articles

Enhanced Hall mobility in graphene-on-electronic-grade diamond

The outstanding electronic properties of graphene make this material a candidate for many applications, for instance, ultra-fast transistors. However, self-heating and especially the detrimental influence of available supporting substrates have impeded progress in this field. In this study, we fabricate graphene-diamond heterostructures by transferring graphene to an ultra-pure single-crystalline diamond substrate. Hall-effect measurements were conducted at 80 to 300 K on graphene Hall bars to investigate the charge transport properties in these devices. Enhanced hole mobility of 2750 cm2 V−1 s−1 could be observed at room-temperature when using diamond with reduced nitrogen (Ns0) impurity concentration. In addition, by electrostatically varying the carrier concentration, an upper limit for mobility is determined in the devices. The results are promising for enabling carbon–carbon (C-C) devices for room-temperature applications.

Photoluminescence and First-principle Calculations of the Single Vacancy in Diamond

The isolated single vacancy is one of the most common defects in diamond, while it is of considerable interest and importance that significantly influences the optical properties of diamond. Although a number of literature publications about the electron structure and density states of single vacancy in diamond from the density functional theory (DFT) calculations, these results are very difficult to be identified from the perspective of experiments. In this paper, the low-temperature photoluminescence (PL) technology was employed to study the emission properties of single vacancy in diamond, and then the photoionization energy, migration energy and vibrionic structure obtained from PL experiments were compared with these obtained from the first-principle calculations. Results showed that the calculation results agreed well with the PL results for the neutral vacancy while differed for the negatively charged vacancy, indicating that for ultrapure diamond, the vacancy mainly existed in form of the neutral rather than the negatively charged.

Determination of the acoustic phonon deformation potentials in diamond

The interaction between acoustic phonons and electrons in diamond has been investigated by comparing state-of-the-art time-of-flight drift velocity measurements with Monte Carlo simulations. We use a multivariable anisotropic description of acoustic deformation potential scattering. The phonon-electron interaction is the limiting factor for the carrier mobility in ultrapure single crystal diamond. Hence, having a correct description is necessary for both device simulations and for predicting the maximum device performance. The experiments were performed at low temperature and using ultrapure diamond to minimize the influence of other scattering sources. The electronic valley polarization in diamond at low temperatures enables determination of both uniaxial and dilatation deformation potentials in the same experiment. The uniaxial and dilatation deformation potentials are found to be $18.5\ifmmode\pm\else\textpm\fi{}0.2$ and $\ensuremath{-}5.7\ifmmode\pm\else\textpm\fi{}0.3$ eV, respectively.

Intrinsic Mobility of Low-Density Electrons in Photoexcited Diamond

The cyclotron-resonance method reveals the drift mobility of carriers in semiconductors, which determines a device's (opto)electronic functionality. However, determining the intrinsic mobility value without interference from other carriers, dislocations, impurities, etc. remains challenging. By minimizing the density of photoexcited carriers in ultrapure diamond, the authors find an extraordinarily narrow cyclotron-resonance curve for electrons in diamond at 3 K. In this manner they obtain a corrected mobility value of 10${}^{8}$ cm${}^{2}$ V${}^{\ensuremath{-}1}$ s${}^{\ensuremath{-}1}$, a 16-fold increase compared to the previous record value for diamond.

A Valleytronic Diamond Transistor: Electrostatic Control of Valley Currents and Charge-State Manipulation of NV Centers.

The valley degree of freedom in many-valley semiconductors provides a new paradigm for storing and processing information in valleytronic and quantum-computing applications. Achieving practical devices requires all-electric control of long-lived valley-polarized states, without the use of strong external magnetic fields. Because of the extreme strength of the carbon–carbon bond, diamond possesses exceptionally stable valley states that provide a useful platform for valleytronic devices. Using ultrapure single-crystalline diamond, we demonstrate electrostatic control of valley currents in a dual-gate field-effect transistor, where the electrons are generated with a short ultraviolet pulse. The charge current and the valley current measured at the receiving electrodes are controlled separately by varying the gate voltages. We propose a model to interpret experimental data, based on drift-diffusion equations coupled through rate terms, with the rates computed by microscopic Monte Carlo simulations. As an application, we demonstrate valley-current charge-state modulation of nitrogen-vacancy centers.

Defect characterization and numerical modelling of single-crystal ultra-pure intrinsic diamond

In this paper, the native defects in the bulk of single-crystal semiconducting diamond have been characterized by thermally stimulated current spectroscopy (TSC), its variant (i.e. Zero-bias TSC) and Photoluminescence (PL) measurements. The Zero-bias TSC measurements reveal the presence of a hole trap and an electron trap located at Ev+0.75 eV and Ec−0.95 eV respectively, with trap density in ~1013cm−3 range whereas the PL measurements confirm the presence of the Nitrogen-Vacancy centers in the diamond bulk. Based on the identified native defects and their extracted trap signatures, a three-level bulk trapping model has been developed for bulk diamond photoconductor-like structure. The trapping parameters are incorporated into the commercial TCAD software and the simulated characteristics are found to be in excellent agreement with the experimental results thus validating the developed model.

Window into NV center kinetics via repeated annealing and spatial tracking of thousands of individual NV centers

Knowledge of the nitrogen-vacancy center formation kinetics in diamond is critical to engineering sensors and quantum information devices based on this defect. Here we utilize the longitudinal tracking of single NV centers to elucidate NV defect kinetics during high-temperature annealing from 800-1100 $^\circ$C in high-purity chemical-vapor-deposition diamond. We observe three phenomena which can coexist: NV formation, NV quenching, and NV orientation changes. Of relevance to NV-based applications, a 6 to 24-fold enhancement in the NV density, in the absence of sample irradiation, is observed by annealing at 980 $^\circ$C, and NV orientation changes are observed at 1050 $^\circ$C. With respect to the fundamental understanding of defect kinetics in ultra-pure diamond, our results indicate a significant vacancy source can be activated for NV creation between 950-980 $^\circ$C and suggests that native hydrogen from NVH$_y$ complexes plays a dominant role in NV quenching, in agreement with recent {\it ab initio} calculations. Finally, the direct observation of orientation changes allows us to estimate an NV diffusion barrier of 5.1~eV.

Temperature dependence of optical centres in ultrapure diamond after 200 keV electron irradiation

In this work, we conduct a temperature-dependent study of optical centres in ultrapure diamond after 200 keV irradiation by photoluminescence spectra, which are performed in a temperature range of 80–200 K. The zero phonon lines (ZPLs) include sizes of 550.3 nm, 593 nm, and 741 nm (GR1 centre). With an increase in measurement temperature, the ZPLs shift to the lower energy side, together with intensity quenching and an increase in the full width at half maximum. These results are noted in models of lattice contraction and electron–phonon coupling, and their inhomogeneous and homogeneous broadening mechanisms are also carefully distinguished by the Voigt function. The 550.3 nm and 593 nm emissions present harder bonds than the GR1 centre, close thermal quenching energies to the interstitial-related defect, a lower thermal softness than that of the ideal diamond, weak phonon bands at each lower energy side, and different broadening mechanisms with the GR1 centre, indicating that the 550.3 nm and 593 nm lines are possibly interstitial-related.

ENHANCED ENRICHMENT OF SMALL CD31+ BLAST-LIKE HUMAN CORD BLOOD CELLS

We previously reported the enrichment and sorting of small CD31+ cells from human umbilical cord blood by magnetic bead depletion of lineage+ and CD34+ cells. We also reported different lineage/CD34+ depletion strategies [combining Lineage depletion and CD34+ UltraPure kits vs Diamond CD34+ kit] results in different input populations for sorting; CD31+ cells could be sorted from either. Here we report adding the missing component from the alternate lineage-depletion cocktail [anti-CD61 microbeads to Lineage depletion kit/anti-CD123 microbeads to Diamond kit] results in identical input populations, with the CD31+ target cells comprising >20% of input population, permitting rapid, high-purity sorting. We have performed sorts with Influx [B-D] and Tyto [Miltenyi] sorters, yielding 125,000 to 750,000 cells [92-98% pure] in 15 minutes. We show purified small CD31+ blast-like cells express the ESC marker alkaline phosphatase, as well as CD4, CD90 and CxCR4. We have been able to successfully isolate our CD31+ cell population from frozen cord blood samples stored in DMSO/FBS after red cell depletion. Frozen storage results in decreased MNC recovery, but CD31+ cell yields appear unaffected, further improving the enrichment of the CD31+ cell population. This is similar to what we have seen with frozen samples for CD34+ HSC isolation. The ability to use frozen samples is greatly improved by the using of Miltenyi's Tyto Running Buffer, a cGMP-compliant wash and transport solution which contains Tytonase to prevent cell clumping. Tyto Running Buffer can be used in the magnetic bead depletion steps without affecting overall yields. These findings permit rapid sorting of highly purified CD31+ cells, which can be used for gene expression profiling and cell culture.

CVD growth of ultrapure diamond, generation of NV centers by ion implantation, and their spectroscopic characterization for quantum technological applications

Abstract Applications of nitrogen-vacancy (NV) centers in diamond in quantum technology have attracted considerable attention in recent years. Deterministic generation of ensembles of NV centers can advance the research on quantum sensing, many-body quantum systems, multipartite entanglement and so on. Here we report the complete process of controlled generation of NV centers in diamond as well as their characterisation: growing diamond films through chemical vapor deposition (CVD), ion implantation and spectroscopic characterization of the defect centers using a confocal microscope. A microwave-assisted CVD set-up is presented which we constructed for the preparation of single-crystalline homoepitaxial diamond films. The films were prepared with minimized nitrogen concentration, which is confirmed through photoluminescence measurements. We demonstrate an in situ ultra high vacuum (UHV) implantation and heating process for creation of NV centers using a novel experimental set-up. For the first time hot implantation has been shown which prevents surface charging effects. We do not observe graphitization due to UHV heating. By optimizing the implantation parameters it has been possible to implant NV centers in a precise way. We present large area mapping of the samples to determine the distribution of the centers and describe the characterization of the centers by spectroscopic techniques. Reducing the decoherence caused by environmental noise is of primary importance for many applications in quantum technology. We demonstrate improvement on coherence time T_{2} of the NV spins by suppression of their interaction with the surrounding spin-bath using robust dynamical decoupling sequences.

Optical properties of ultrapure nano-polycrystalline diamond

We synthesized an ultrapure nano-polycrystalline diamond (NPD) containing very few chemical impurities (<1 ppm). The 13C concentration of the carbon source was reduced to less than 0.01% by using 12C-enriched high-purity carbon. The ultrapure NPD was synthesized by direct conversion from graphite under high-pressure and high-temperature (HPHT) conditions. We measured the optical properties of the ultrapure NPD, which appeared yellowish, attributed to the structural features of the specimen. Also, the one-phonon absorption peak at 1220 cm−1 is attributed to the broken symmetry of the diamond lattice. Moreover, a defect-related PL peak was found at 730 nm.

Magnetotransport study of valley-polarized electrons in synthetic diamond

We demonstrate that the highly stable valley-polarized electron states in ultrapure single-crystalline diamond allow for investigation of charge transport, magnetoresistivity, and determination of ...

Magnetic properties of point defects in proton irradiated diamond

We investigate the magnetic properties of ultra-pure type-IIa diamond following irradiation with proton beams of ≈1–2MeV energy. SQUID magnetometry indicate the formation of Curie type paramagnetism according to the Curie law. Raman and Photoluminescence spectroscopy measurements show that the primary structural features created by proton irradiation are the centers: GR1, ND1, TR12 and 3H. The Stopping and Range of Ions in Matter (SRIM) Monte Carlo simulations together with SQUID observations show a strong correlation between vacancy production, proton fluence and the paramagnetic factor. At an average surface vacancy spacing of ≈1–1.6nm and bulk (peak) vacancy spacing of ≈0.3-0.5nm Curie paramagnetism is induced by formation of ND1 centres with an effective magnetic moment μeff~(0.1–0.2)μB. No evidence of long range magnetic ordering is observed in the temperature range 4.2-300K.

Sensitive detection of Majorana fermions based on a hybrid spin–microcantilever via enhanced spin resonance spectrum

Motivated by recent experimental progress towards the detection and manipulation of Majorana fermions in ferromagnetic atomic chains on a superconductor, we present a novel proposal based on a single-crystal diamond (SCD) microcantilever with a single nitrogen-vacancy (NV) center spin embedded in ultrapure diamond substrate to probe Majorana fermions in an all-optical domain. With this scheme, a possible distinct Majorana signature is investigated via the electron spin resonance spectrum. In the proposal, the SCD microcantilever behaves as a phonon cavity and is robust for detecting of Majorana fermions, while the NV center spin can be considered as a sensitive probe. Further, the vibration of the microcantilever will enhance the coupling effect, which makes the Majorana fermions more sensitive to detection and the well-established optical NV spin readout technology will certainly promote the detection. This proposed method may provide a potential supplement for the detection of Majorana fermions.

Annealing of electron radiation damage in a wide range of Ib and IIa diamond samples

The diffusion of vacancies in diamond is of considerable practical and fundamental interest. This work undertakes a new investigation of this property, based on photoluminescence of electron irradiated and annealed samples, exploiting the recent availability of ultra-pure diamond where the very low nitrogen level dictates the necessity for long-distance migration of vacancies on annealing if nitrogen-vacancy complexes are to be formed. The results reveal that an annealing temperature of 850°C is required for long-range vacancy diffusion rather higher than the generally accepted 700°C, but that in Ib samples with high nitrogen concentrations marked reductions of vacancy concentrations can occur at temperatures as low as 500°C by short range diffusion to nearby nitrogen atoms. As a result of this study, two new optical centres have been discovered and evidence is provided for the hypotheses that they are the divacancy and the positively charged nitrogen-vacancy complex.Interstitials created by the electron irradiation are well known to produce several optically active defects, but in particular, the centre known in the literature as 3H exhibits properties that have so far eluded convincing explanations. As a by-product of this investigation, a number of the properties of this centre were encountered that may assist a final determination of the atomic structure of this complex but very common defect. It is deduced that, contrary to conclusions in the literature, the centre is negatively charged.

Passive charge state control of nitrogen‐vacancy centres in diamond using phosphorous and boron doping

The control and stabilisation of the charge state of nitrogen‐vacancy centres in diamond is an important issue for the achievement of reliable processing of spin‐based quantum information. The effect of phosphorous and boron doping of diamond on the charge state of nitrogen‐vacancy (NV) centres is shown here. Ensembles of NV centres are produced at a depth of 60 nm in ultrapure diamond by implantation of nitrogen ions. Overlapping with the NV ensembles, donor and acceptor doped regions of different doping levels are prepared by ion implantation of phosphorus and boron followed by annealing in vacuum at 1500 °C. We show how the charge state of NV centres is controlled by the presence of phosphorous or boron atoms in their neighbourhood. For the lowest doping level, spectral measurements on the ensemble of NV centres reveal a higher amount of NV0 in the case of boron and a higher amount of NV− in the case of phosphorus, as compared with undoped regions. This behaviour is strengthened when the doping level is increased. Interestingly, the charge state control of native silicon‐vacancy centres is also evidenced. Finally, we discuss the role of the surface termination of diamond on the average charge state of the NV ensemble (still dominant even at a depth of 60 nm) and confirm that the surface 2D‐hole‐gas (H‐termination) can be compensated by nitrogen itself.

Isotopic identification of engineered nitrogen-vacancy spin qubits in ultrapure diamond

Nitrogen impurities help to stabilize the negatively-charged-state of NV$^-$ in diamond, whereas magnetic fluctuations from nitrogen spins lead to decoherence of NV$^-$ qubits. It is not known what donor concentration optimizes these conflicting requirements. Here we used 10-MeV $^{15}$N$^{3+}$ ion implantation to create NV$^-$ in ultrapure diamond. Optically detected magnetic resonance of single centers revealed a high creation yield of $40\pm3$% from $^{15}$N$^{3+}$ ions and an additional yield of $56\pm3$% from $^{14}$N impurities. High-temperature anneal was used to reduce residual defects, and charge stable NV$^-$, even in a dilute $^{14}$N impurity concentration of 0.06 ppb were created with long coherence times.

Mass-anisotropy splitting of indirect excitons in diamond

We have investigated fine structure splitting of indirect excitons in ultrapure diamond by high-resolution spectroscopy. The role of the mass-anisotropy splitting has been revealed by comparison with group-theoretical calculations of the reduced matrix elements for phonon scattering of electrons and holes followed by the radiative recombination. The observed energy levels and coupling strengths of the fine structure states are successfully reproduced by inclusion of the mass-anisotropy splitting in addition to the exchange and spin-orbit interactions.

High carrier mobility in ultrapure diamond measured by time-resolved cyclotron resonance

We have performed time-resolved cyclotron resonance measurements in ultrapure diamond crystals for the temperature range of T=7.3–40 K and obtained the temperature-dependent momentum relaxation times based on the cyclotron resonance widths for optically generated electrons and holes. The relaxation time follows a T−3/2 law down to 12 K, which is expected for acoustic-phonon scattering without impurity effect because of the high purity of our samples. The deviation from the law at lower temperatures is explained by the impurity scattering and the breakdown of the high-temperature approximation for the phonon scattering. We extract the carrier drift mobility by using the directly measured effective masses and the relaxation times. The mobility at 10 K for 600 ns delay time after optical injection is found to be μe=1.5×106 cm2/V s for the electrons, and μlh=2.3×106 cm2/V s and μhh=2.4×105 cm2/V s for the light and heavy holes, respectively. These high values are achieved by our high-sensitivity detection for low-density carriers (at &amp;lt;1011 cm−3) free from the carrier-carrier scattering as well as by the suppression of the impurity scattering in the high-purity samples.

Nanosecond cyclotron resonance in ultrapure diamond

We have investigated temporal properties of photo-excited carriers in ultrapure diamond by the nanosecond cyclotron resonance (CR) method. The ramp time and the amplitude of the CR signals are found to be dependent on the incident laser pulse energy as well as on the photon energy for excitation. The dependences are discussed based on the photoabsorption process due to the indirect excitons in diamond, which is assisted by absorption of the transverse-acoustic phonons. Similarities to the free-carrier generation via two-body excitonic collisions in a dipole-forbidden direct gap semiconductor, Cu2O, are pointed out.