Influence of overlayers of photodetectors on their optical properties

Terubumi Saito(a) and Lanny R. Hughey

National Institute of Standards and Technology (NIST)
B119 Radiation Physics, Gaithersburg, MD 20899  USA
(a)  on leave from Electrotechnical Laboratory (ETL), 
1-1-4, Umezono, Tsukuba-shi, Ibaraki 305, Japan

        Most photodetectors are known to have layered structures on their
photosensitive surfaces: for example, SiO2 on Si for a silicon photodiode,
Au on GaAsP for a GaAsP Schottky photodiode, Al2O3 on Al for an aluminum
vacuum phototube. Response of such photodetectors are closely related to
the optical properties of their surface, because detector response in the
case of a photodetector using internal photoelectric effect, for example,
is proportional to the transmittance of overlayers into the sensitive
region. The rest of the photons are lost due to reflection at the surface
and absorption in a dead layer. Therefore, even a single overlayer
structure on a detector surface may play a significant role in its
photoresponse.
        In the present study, we have characterized some photodiodes such
as silicon photodiodes in terms of reflectance and absorption loss and
investigated  their polarization dependence experimentally and
theoretically in the spectral range from VUV to visible. The study for a
silicon photodiode reveals that its reflectance and polarization dependence
change significantly in the wavelength range approximately from 120 to 300
nm depending on its oxide thickness due to interference effects. The
experimental results for silicon photodiodes on the responsivity and its
polarization dependence are explained very well by an optical model. This
good agreement confirms that there is no dead layer except for an oxide
layer in certain photodiodes[1]. By overcoating a layer or some layers to
be investigated directly onto the surface of such silicon photodiode, we
can obtain the transmittance of the film. Since detector responsivity
polarization dependence is predictable, this technique will be useful for
film characterization by ellipsometric techniques using both reflectance
and transmittance. 
        
Reference
  [1]  L. R. Canfield, Jonathan Kerner, and Raj Korde, Appl. Opt. 28, 3940
(1989).