The control of the charge state of nitrogen-vacancy (NV) centers in diamond is of primary importance for the stabilization of their quantum-optical properties, in applications ranging from quantum sensing to quantum computing. To this purpose, in this work current-injecting micro-electrodes were fabricated in bulk diamond for NV charge state control. Buried (i.e. 3 {mu}m in depth) graphitic micro-electrodes with spacing of 9 {mu}m were created in single-crystal diamond substrates by means of a 6 MeV C scanning micro-beam. The high breakdown field of diamond was exploited to electrically control the variation in the relative population of the negative (NV-) and neutral (NV0) charge states of sub-superficial NV centers located in the inter- electrode gap regions, without incurring into current discharges. Photoluminescence spectra acquired from the biased electrodes exhibited an electrically induced increase up to 40% in the NV- population at the expense of the NV0 charge state. The variation in the relative charge state populations showed a linear dependence from the injected current at applied biases smaller than 250 V, and was interpreted as the result of electron trapping at NV sites, consistently with the Space Charge Limited Current interpretation of the abrupt current increase observed at 300 V bias voltage. In correspondence of such trap-filling-induced transition to a high-current regime, a strong electroluminescent emission from the NV0 centers was observed. In the high-current-injection regime, a decrease in the NV- population was observed, in contrast with the results obtained at lower bias voltages. These results disclose new possibilities in the electrical control of the charge state of NV centers located in the diamond bulk, which are characterized by longer spin coherence times.
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