Here - PMOD/WRC

NEWRAD 2005
Fractional self-calibration of silicon photodiodes
J. Gran
JV, Kjeller, Norway
T. E. Hanssen
AME, Horten, Norway
A. Hallén
KTH, Kista, Sweden
J. C. Petersen
DFM, Lyngby, Denmark
Abstract. We describe the first results aiming at using
specially designed silicon photodiodes as primary
standards in optical radiometry. Currently the most
accurate measurements of optical power are obtained using
a cryogenic radiometer (CR), where the principle of
operation is by electrical substitution. The CR is a rather
complex and expensive device, and therefore simpler more
straightforward methods are desirable. We propose to use
specially designed silicon photodiodes as a primary
standard, where one half of the surface will be used for
calibration purposes only whereas the other half is used for
measurements. By biasing the detector we eliminate the
internal losses in the detector and by measuring the
reflectance of the surface of the detector we estimate the
two loss mechanisms separately and thus expresses the
responsivity of the detector in terms of fundamental
constants. The first design and initial measurements of the
detectors is reported.
Introduction
The principles for the establishment of a new primary
standard based on specially designed silicon photodiodes
is presented. The method is a modified method of the
self-calibration method established by Geist and Zalewski
in 1980. The basic idea is that the responsivity, R, of an
ideal photodiode can be described by fundamental
constants and the wavelength of the radiation while
deviations from ideal performance is described by the two
loss mechanisms, reflectance from the surface and
non-perfect internal quantum efficiency (IQE). The
reflectance and the IQE are estimated separately in
different relative experiments. Unfortunately, the initial
methods turned out to change the IQE resulting in a
decreased interest for using the method. The method
proposed here is expected not to have the disadvantages
experienced by the original self-calibration method. The
initial measurements and characterizations of the detectors
are presented.
Theory
The responsivity of a semiconductor photodiode can be
expressed as
( 1−
ρ(
λ)
)( 1−
δ ( ))
R ( λ)
= eλ
hc
λ , (1)
where e is the elementary charge, h is Planck’s constant, c
is the speed of light in vacuum, and λ is the vacuum
wavelength of the radiation. These quantities form the
ideal term of a quantum detector. The reflectance is given
by ρ(λ), and the internal quantum deficiency (IQD) by δ(λ).
From (1) it is seen that by reducing the reflectance and the
quantum deficiency to negligible levels the silicon
photodiode would become a primary standard detector,
because the spectral response R(λ) is then expressed in
terms of fundamental constants and the wavelength.
In a photodiode, one electron hole pair is created per
incident photon and transported through the diode by its
built in potential at the pn junction of the diode. The finite
lifetime for electrons and holes makes it unlikely that all of
them reach the connectors of the diode and an IQE less
than one is expected and observed. In the proposed method
a potential is created throughout the whole diode in order
to reduce transport time of electrons and holes and thus
practically eliminate the recombination probability. Since
it is impossible to create fully transparent electrodes over a
wide spectral range we avoided the problem by depositing
a semitransparent electrode on a fraction (half) of the
surface and thereby eliminated the IQD on this part. We
thereby find the IQD on the uncoated surface. A picture of
the detector is shown in fig.1.
Figure 1. Picture of the Si detector. A semitransparent layer
of gold is deposited on the left half of the surface. The
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 63