Diamond ultraviolet sensors are blind to visible light

17 Jun 2002

Diamond's bandgap is a close match to the energy of an ultraviolet photon. Thatmeans photodetectors based on thin-film diamond are blind to visible, infraredand solar radiation down to 220 nm, says Centronic of Croydon, UK.

Diamond's bandgap of 5.5 eV is too large to absorb visible photons. This isunlike solid-state silicon photodetectors with a band gap of 1.1 eV, narrowenough to absorb visible as well as ultraviolet light.

The insensitivity of the new ultraviolet photodetectors to wavelengths longerthan 220 nm means that they need neither filters nor screening to exclude visiblelight. That keeps their size down and reduces costs in applications such asphotolithography.

Centronic says that its new photodetectors are sensitive from 100 to 220 nm andblind to solar radiation to better than 1 part in 100,000 of their response at200 nm. They run at 5 to 30 V and show a dark current of less than 1 nA.

The photodetectors also show some of the general qualities of diamond, such aschemical and heat resistance. These particular devices are claimed to be stableup to 400 C and their general physical stability suits them to monitoring highpower ultraviolet lamps for curing adhesives or purifying water.

The company has been developing sensors based on chemical vapour deposition (CVD)diamond for many years. First as a part of a European Brite-Euram project,involving partners from several countries, into the feasibility of diamond andsilicon carbide sensors.

Later, as the price of synthetic diamond fell and its quality began to rise,Centronic began to support research at University College London (UCL). By 1996,that collaboration resulted in a prototype diamond-film photodetector.

The UCL group had to solve a number of practical problems. CVD diamond film ispolycrystalline, unlike pure diamond, so it had to minimize the effect of grainboundaries. It also had to learn how to grow the film with the minimum number ofdefects.

UCL's prototype sensor consisted of inter-digitated electrodes deposited on thesurface of a diamond thin film. Each digit is about 20 micrometres across andspaced by a similar amount. These dimensions are much smaller than the diamondgrains and so the electrode avoids the effect of boundaries draining awayelectrons generated by ultraviolet photons.

Centronic manufactures the diamond sensors in house. It can therefore provide anultraviolet sensor with a peak sensitivity tuned to a customer's specification orwith a special format.