Japanese Team Measures Electric Fields Inside Working Semiconductor
Atomic-level sensors could help better characterise next generation wide band gap devices
Nitrogen–vacancy (NV) centre formed in a diamond device for electric field sensing. Left: Schematic of the structure of the NV centre. Middle: Confocal fluorescence image of a single NV centre in the device. Right: Schematic of the measurement configuration.
A Japanese research team has reported a method for sensing electric fields within operating wide band gap semiconductor devices, arguing that the ability to measure these internal fields at the nanoscale is essential for the development of next generation low loss electronics.
Takayuki Iwasaki, Mutsuko Hatano and colleagues 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) published their results in the journal ACS Nano.
Semiconductor devices function by controlling the electronic transitions across their band gap. While silicon and GaAs have a band gap of the order of 1eV, new types of wide band gap semiconductors, like SiC, GaN and diamond, have values as high as 3 to 5eV, which is why they can operate at temperatures over 300degC and sustain very high voltages and currents. In such semiconductors, the internal electric fields can become very strong - a property important for low loss electronic applications.
The technique developed by Iwasaki and co-workers 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 150V.
They applied their method to diamond, which can easily accommodates nitrogen–vacancy (NV) centres, a type of 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 means of ion implantation. A nearby electric field affects an NV centre’s energy state, which in turn can be probed by optically detected magnetic resonance (ODMR). In ODMR, the sample is irradiated with laser light, which excites the NV centre, after which the magnetic resonance spectrum can be recorded. An electric field makes the ODMR resonance split; the experimentally detected split width provides a measure for the electric field.
The researchers 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 localised 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.
From these spectra, values for 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.
Iwasaki and colleagues explain that the experimentally determined value for the electric field around a given NV centre is essentially the field’s component perpendicular to the direction of the NV centre — aligned along one of four possible directions in the diamond lattice. They reason that a regular matrix of implanted NV centres should enable reconstructing the electric field with a spatial resolution of about 10 nm by combining with super-resolution techniques, which is promising for studying more complex devices in further studies.
The researchers also point out that electric-field sensing is not only relevant for electronic devices, but also for electrochemical applications: the efficiency of electrochemical reactions taking place between a semiconductor and a solution depends on the former’s internal electric field. In addition, Iwasaki and co-workers note that their approach need not be restricted to NV centres in diamond: similar single-electron-spin structures exist in other semiconductors like e.g. silicon carbide.
This work was supported by a fund from Core Research Evolutionary Science and Technology (CREST) of the Japan Science and Technology Agency (JST).
‘Direct Nanoscale Sensing of the Internal Electric Field in Operating Semiconductor Devices Using Single Electron Spins’, by Takayuki Iwasaki et al; ACS Nano, DOI: 10.1021/acsnano.6b04460.