Spectrosocopy and imaging are important to provide insight into analysis, and characterization of a chemical physical processes. \cite{Dai_2014}Furthermore, optical data links, such as vertical cavity surface emitting lasers, rely on photodetectors on the receiving end to be sensitive enough to detect different frequencies of light.\cite{Seyedi_2014} Therefore improvements of photodetectors and other sensors are always desired.
Peng and coworkers developed single GaAs NWs as microscopic coherent THz sensors.\cite{Peng_2015} These single GaAs NW detectors were fabricated using a direct laser lithographic technique and were even used as a spectrometer to measure the transmission spectrum of a 290 GHz low pass filter.
Dai and coworkers have developed a NW photodetector using a GaAs core and AlGaAs shell NWs.\cite{Dai_2014} In typical III-V NW the large density of surface states pin the surface fermi energy, limiting carrier mobility. These surface states were “passivated" by placing a shell on top. The GaAsAl shell and GaAs are found to be significantly more sensitive as a photodetector than the GaAs NW alone. Low dark current and high photoresponsivity was found in the range of 300-890 nm.
Seyedi and coworkers also demonstrated the power of GaAs core and AlGaAs shell NW. They explain how NW based photodetectors have detection limits based on active area diameter and GaAs NWs have large active areas. A bandwidth sensitivity of several GHz was found for the NW.\cite{Seyedi_2014} Additionally Joyce and coworkers found that AlGaAs shell thickness significantly improved both the electron mobility and the photoconductivity lifetime. Electron mobility increased to 3000 \(\frac{cm^{2}}{V s}\) with 16 and 34 nm thick AlGaAs shells.\cite{Joyce_2014}
Luo and coworkers use passivation to improve the sensistivity of their bilayer graphene/gallium arsenide Schottky junction based near infrared photodetector (NIRPD). An aluminum oxide layer allows for high sensitivity 850 nm illumination at zero bias voltage. Passivation also causes dark current to be reduced and the responsivity of the detector to increase. It was also found to work in a wide range of switching frequencies with fast response rates.