The researchers at the Tokyo Institute of Technology, the Japan Science and Technology Agency (JST) and Toshiharu Makino at the National Institute of Advanced Industrial Science and Technology (AIST) used a diamond device, with nitrogen–vacancy (NV) centres acting as local electric-field probes. The technique can also be applied to silicon carbide power devices.
The technique exploits the response of an artificially introduced single electron spin to variations in its surrounding electric field, and enabled the researchers to study a semiconductor diode subject to bias voltages of up to 150 V.
Diamond has the advantage that it easily accommodates NV centres, a point defect that arises when two neighbouring carbon atoms are removed from the diamond lattice and one of them is replaced by a nitrogen atom. NV centres can be routinely created in diamond by ion implantation and can be probed by optically detected magnetic resonance (ODMR). Similar single-electron-spin structures exist in other wideband semiconductors such as silicon carbide.
Takayuki Iwasaki, Mutsuko Hatano and colleagues first fabricated a diamond p–i–n diode (an intrinsic diamond layer sandwiched between an electron- and a hole-doped layer) embedded with NV centres. They then localized an NV centre in the bulk of the i-layer, several hundreds of nanometers away from the interface, and recorded its ODMR spectrum for increasing bias voltages. ODMR is a technique where irradiating the sample with laser light excites the NV centre and the resulting magnetic resonance spectrum can be captured. An electric field makes the ODMR resonance split and the experimentally detected split width provides a measure for the electric field. From this data the electric field could be obtained using theoretical formulas.
The experimental values were then compared with numerical results obtained with a device simulator and found to be in good agreement — confirming the potential of NV centres as local electric-field sensors. A regular matrix of implanted NV centres should enable reconstructing the electric field with a spatial resolution of about 10 nm to study the operation of more complex devices.
The researchers also point out that electric-field sensing is not only relevant for electronic devices, but also for electrochemical applications such as batteries: the efficiency of electrochemical reactions taking place between a semiconductor and a solution depends on the internal electric field.