Experimental equipment - Accelerator

It is a Cockcroft-Walton type electron beam accelerator, and the acceleration energy is variable in the range of 0.5 to 2 MeV. By scanning 120cm wide, it is possible to irradiate the electron beam uniformly. The beam current is often selected in the range of about 0.5 to 10mA, and if the current is lowered, the irradiation amount per beam will be 1x10¹¹ electrons/cm². If the current is increased, the irradiation amount will be about 1x10¹⁷ electrons/cm² per one day. For an electron beam of 2 MeV, the electron beam will penetrate a diamond. The irradiated electrons eject the carbon atoms that make up the diamond, forming atomic vacancies. When a diamond containing nitrogen (P1 center) is irradiated with an electron beam, nitrogen and atomic vacancies are created in the crystal. When heat treated after irradiation, nitrogen (N) and atomic vacancies (V) combine to form NV centers.

Fig. Diagrams and photos of electron beam irradiation equipment

This picture shows a device for implanting ions into extremely shallow areas (10 nm or less) on the sample surface. The ion beam from the ECR ion source can be extracted at a low acceleration voltage of 1 to 10 kV, and the ion beam can be irradiated after mass spectrometry. XY beam scanner enables a uniform beam. The irradiation area is approximately 1.5cm x 1.5cm. ¹⁴N⁺ and ¹⁵N⁺ ions can be implanted. Irradiation is possible with a wide range of fluences from 10⁸ ions/cm² to 10¹⁴ ions/cm². We can perform injection experiments with up to 4 conditions in one day.

Fig.  Test bench

This shows a device for implanting ions into a shallow region (10 to several 100 nm) of the sample surface. It is possible to heat the sample to a high temperature (~1000˚C), allowing implantation with less radiation damage than normal room temperature ion implantation. We can obtain uniform beam irradiation by using the XY beam scanner. For diamond samples, ¹⁵N⁺ ions, ¹⁵N²⁺ ions (molecular ions), ²⁸Si⁺ ions, etc. are implanted at room temperature. Additionally, ³¹P⁺ ions, ²⁷Al⁺ ions, etc. are implanted into silicon carbide samples. In 2019, we successfully demonstrated triple NV center fabrication as the three-qubits system (M. Haruyama, S. Onoda, et al., Nature Communications, DOI: 10.1038/s41467-019-10529-x ). This is based on a technology that uses an ion implanter to implant an ion beam of organic compound ions (C₅N₄H_n) containing multiple nitrogen atoms.

Fig. Vacuum chamber of ion implanter

Fig. Sample holder heated to 800℃

We can perform particle beam writing using a focused ion beam of light elements (H, He). After determining the irradiation position using an optical microscope equipped with a long focal length lens, we can control the irradiation with a positional accuracy of 12 nm (however, the ion beam diameter (FWHM: half width) was around 1 µm) within the irradiation area = 800 x 800 µm². Ion irradiation dose can be controlled from 10¹⁰ to 10¹⁷ ions/cm².

Fig. Confocal microscope image of silicon vacancies formed by particle beam lithography
                  using an H (proton) beam on a silicon carbide substrate
(T. Ohshima, et al., Nano Lett. 2017).

This device is capable of implanting ions to a depth of several micrometers from the sample surface. We can successfully focus the ion beam to a few micrometers, which is called a micro beam line. By appropriately selecting the chopper pulse, it is possible to perform stochastic single ion hit irradiation. Within the irradiation range of 300 x 300 µm², it is possible to irradiate the targeted position with ions while controlling the number of ions. The figure shows an example of drawing Roman numerals and grids on CR-39 (plastic track detector) using a microbeam. (As of May 2023: Injection experiments are suspended due to overhaul.)

Fig. The microbeam irradiation device

Fig. The drawing of heavy ion microbeam on CR-39

This allows ions to be implanted deep into the sample (several tens of micrometers). It is possible to irradiate ions with various ion types and linear energies, such as ¹⁵N³⁺ 56 MeV, ²⁰Ne⁴⁺ 75 MeV, ⁴⁰Ar⁸⁺ 150 MeV, ⁸⁴Kr¹⁷⁺ 322 MeV, ¹²⁹Xe²⁵⁺ 454 MeV, and ¹⁹²Os³⁰⁺ 490 MeV.

Fig. Vacuum chamber