InSbN photovoltaic infrared detectors offer a promising alternative to the HgCdTe incumbent by combining superior material quality with lower Auger recombination and a range of fabrication techniques.
A team of researchers in Singapore claims to have built the first InSbN-based photodiodes for mid- and long-wavelength infrared detection.
These photovoltaic devices incorporate tiny amounts of nitrogen and produce their strongest photocurrent peak at 5.3 microns, which originates from the binary InSb. Subsidiary peaks in three different samples occurred at 6.30, 6.33 and 6.47 microns. According to team member Dao Hua Zhang from Nanyang Technology University, one application for these InSbN devices is night vision. In addition, they could be used to detect gases such as sulphur dioxide, ammonia and chlorofluorocarbon refrigerant compounds. If InSbN detectors are to kick-on and enjoy commercial success, then they needs to take market share from HgCdTe devices that
provide detection across the 1 to 25 micron spectral range.
The incumbent technology has many strengths: a tunable bandgap governed by alloy composition; a high optical absorption coefficient; high electron mobility; and readily available doping techniques. These benefits have to be weighed against several disadvantages, including lattice, surface and interface instabilities of HgCdTe, which can lead to large variations in stoichiometry and transport properties.
"Mass production [of HgCdTe detectors] has a very low uniformity and low yield, and HgCdTe is a highly toxic material," adds Zhang. In comparison, InSbN features better material quality and uniformity, thanks to the very small amounts of nitrogen needed to red-shift the bandgap. There are also many options for fabrication, because this ternary can be fabricated by MOCVD, MBE or multi-step ion implantation.
"More importantly, the Auger recombination rate of the InSbN alloy is only one-third of HgCdTe with an equivalent bandgap, which makes InSbN the best candidate for making mid- and long-wavelength infrared photodetectors," says Wang.
Fabrication of InSbN devices began by depositing a 100 nm-thick, SiN film onto n-type InSb substrates by PECVD. InSbN layers were formed by nitrogen ion implantation, using energies of 90, 180 and 530 keV to ensure a uniform nitrogen profile. The top p-region was then created by magnesium ion implantation.
Annealing the wafers at 550K for 4 hours removed damage caused by implantation, and activated the incorporated nitrogen. Standard photolithography then created mesa-like structures with a 250-micron diameter. Detectors produced a range of photocurrent spectra, and the longest wavelength device had a photocurrent peak at 6.47 microns and a cut-off wavelength of 9.4 microns. This device has a nitrogen composition of 0.43 percent, according to theoretical band structure calculations with a 10-band k.p model. Zhang and his colleagues are now planning to build devices spanning the mid and longwavelength infrared.
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