Kosuke Tahara

Germanium-Vacancy Single Color Centers in Diamond

Atomic-sized fluorescent defects in diamond are widely recognized as a promising solid state platform for quantum cryptography and quantum information processing. For these applications, single photon sources with a high intensity and reproducible fabrication methods are required. In this study, we report a novel color center in diamond, composed of a germanium (Ge) and a vacancy (V) and named the GeV center, which has a sharp and strong photoluminescence band with a zero-phonon line at 602 nm at room temperature. We demonstrate this new color center works as a single photon source. Both ion implantation and chemical vapor deposition techniques enabled fabrication of GeV centers in diamond. A first-principles calculation revealed the atomic crystal structure and energy levels of the GeV center.

Effect of radical fluorination on mono- and bi-layer graphene in Ar/F2 plasma

Fluorinated graphene has the possibility to achieve unique properties and functions in graphene. We propose a highly controlled fluorination method utilizing fluorine radicals in Ar/F2 plasma. To suppress ion bombardments and improve the reaction with fluorine radicals on graphene, the substrate was placed “face down” in the plasma chamber. Although monolayer graphene was more reactive than bilayer, fluorination of bilayer reached the level of ID/IG ∼ 0.5 in Raman D peak intensity at 532 nm excitation. Annealing fluorinated samples proved reversibility of radical fluorination for both mono- and bi-layer graphenes. X-ray photoelectron spectroscopy showed the existence of carbon-fluorine bonding.

Quantifying selective alignment of ensemble nitrogen-vacancy centers in (111) diamond

Selective alignment of nitrogen-vacancy (NV) centers in diamond is an important technique towards its applications. Quantification of the alignment ratio is necessary to design the optimized diamond samples. However, this is not a straightforward problem for dense ensemble of the NV centers. We estimate the alignment ratio of ensemble NV centers along the [111] direction in (111) diamond by optically detected magnetic resonance measurements. Diamond films deposited by N2 doped chemical vapor deposition have NV center densities over 1 × 1015 cm−3 and alignment ratios over 75%. Although spin coherence time (T2) is limited to a few μs by electron spins of nitrogen impurities, the combination of the selective alignment and the high density can be a possible way to optimize NV-containing diamond samples for the sensing applications.

Asymmetric transport property of fluorinated graphene

Carrier transport properties of fluorinated graphene with various fluorination rates are presented. Onset of transition from insulating to metallic conduction is observed in dilute fluorinated graphene. Highly fluorinated graphene shows electron-hole asymmetry in transport properties and local resistivity maximum at the hole conduction region, which are presumably caused by the existence of resonant fluorine impurities. Drastic change of the asymmetric feature occurs after removing fluorine atoms and creating structural defects by thermal annealing. These results suggest that the type of impurities or defects in graphene is detectable by examining asymmetry in transport properties.

Direct Nanoscale Sensing of the Internal Electric Field in Operating Semiconductor Devices Using Single Electron Spins

The electric field inside semiconductor devices is a key physical parameter that determines the properties of the devices. However, techniques based on scanning probe microscopy are limited to sensing at the surface only. Here, we demonstrate the direct sensing of the internal electric field in diamond power devices using single nitrogen-vacancy (NV) centers. The NV center embedded inside the device acts as a nanoscale electric field sensor. We fabricated vertical diamond p-i-n diodes containing the single NV centers. By performing optically detected magnetic resonance measurements under reverse-biased conditions with an applied voltage of up to 150 V, we found a large splitting in the magnetic resonance frequencies. This indicated that the NV center senses the transverse electric field in the space-charge region formed in the i-layer. The experimentally obtained electric field values are in good agreement with those calculated by a device simulator. Furthermore, we demonstrate the sensing of the electric field in different directions by utilizing NV centers with different N-V axes. This direct and quantitative sensing method using an electron spin in a wide-band-gap material provides a way to monitor the electric field in operating semiconductor devices.

Formation of perfectly aligned nitrogen-vacancy-center ensembles in chemical-vapor-deposition-grown diamond (111)

Selectively aligning a nitrogen-vacancy (NV) ensemble in diamond is an important technique for obtaining a high-sensitivity magnetic sensor. Nitrogen-doped diamonds were grown on (111) substrates by microwave plasma chemical vapor deposition to perform the selective alignment of high-density NV ensembles, yielding perfectly aligned NV ensembles along the [111] direction with a density greater than 1016 cm−3 and a spin relaxation time of 2 µs. Such alignment results in a high signal contrast with an optical magnetic resonance close to the typical value reported with an isolated NV center. These results indicate the possibility of achieving a high sensitivity through the selective alignment of NV ensembles.

Improvement of fluorescence intensity of nitrogen vacancy centers in self-formed diamond microstructures

We present umbrella-shaped diamond microstructures with metal mirrors at the bottom in order to improve the amount of collected photons from nitrogen vacancy centers. The metal mirrors at the bottom are self-aligned to the umbrella-shaped diamond microstructures which are selectively grown through holes created on a metal mask. By the finite-difference time-domain simulations, we found that the umbrella-shaped microstructures, which have an effect similar to solid immersion lens, could collect photons more efficiently than bulk or pillar-shaped microstructures. Improvement of the fluorescence intensity by factors of from 3 to 5 is shown experimentally.

Fluorinated Graphene FETs Controlled by Ionic Liquid Gate

Fluorinated graphene field effect transistors (FETs) with an ionic liquid (IL) top-gate are demonstrated. Fluorinated graphene functionalized with different fluorine concentrations were prepared. Highly fluorinated graphene FETs controlled by ionic liquid gating (ILG) exhibited higher on/off ratios than pristine (non-fluorinated) graphene FET, while conventional back gating (BG) configuration resulted in lower on/off ratios. ILG can induce high charge density in fluorinated graphene with localized states because of extremely high electric field effect of an electric double layer between IL and fluorinated graphene. Thus, a higher on/off ratio was obtained by the combination of graphene fluorination and ILG.

Charge-state control of ensemble of nitrogen vacancy centers by n–i–n diamond junctions

We fabricated n-type–intrinsic–n-type (n–i–n) diamond junctions to stabilize negatively charged nitrogen vacancy (NV−) centers. An ensemble of NV centers was generated in a high-purity intrinsic diamond in order to maintain the spin coherence time. The Fermi-energy of the i-layers was controlled by band bending at the two n–i junctions. We confirmed that the NV centers can be modulated toward NV− centers by decreasing the width of the i-layers between adjacent n-layers. The spin coherence time for the NV− centers was maintained even for narrow i-layers.