Demonstration of near-ﬁeld scanning photoreﬂectance spectroscopy
Charles Paulson and A. B. Ellis
Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
Department of Electrical and Computer Engineering, University of Wisconsin, Madison, Wisconsin 53706
Brian Hawkins, Jingxi Sun, and T. F. Kuech
Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706
͑Received 10 April 2000; accepted for publication 26 July 2000͒
A near-ﬁeld scanning optical microscope ͑NSOM͒ was developed to perform photoreﬂectance ͑PR͒
spectroscopy experiments at high spatial resolution ͑ϳ1
m͒. Representative PR spectra are shown,
along with an image illustrating the capability of observing contrast in images due to the strength of
a PR feature. It was found that sufﬁciently high intensity light from the NSOM tip can produce
photovoltages large enough to limit the spatial resolution of the electric ﬁeld determination by PR.
The photovoltage effect is measured as a function of light intensity, and the results are discussed in
terms of a simple photovoltage expression. © 2000 American Institute of Physics.
The photoreﬂectance ͑PR͒ spectroscopy technique is a
form of electromodulation spectroscopy used to determine
the electronic properties at the surface of direct band gap
semiconductors, e.g., surface electric ﬁeld in the depletion
region, the surface Fermi level E
, doping density, the band
gap energy E
, and other critical point energies.
conductor is modulated at a frequency
, with an above-band
gap light source, the pump. A tunable light source serves as
the probe and is reﬂected from the modulated area of the
sample’s surface. The reﬂected probe light contains both a dc
and an ac component, with the ac component having the
frequency of the pump modulation. The modulated reﬂectiv-
ity, normalized by the dc reﬂectivity, is the PR signal. The
magnitude of ⌬R/R is typically 10
and depends on
the wavelength of the probe light. As the probe wavelength
is scanned, some samples will exhibit Franz–Keldysh oscil-
lations ͑FKOs͒ in the ⌬R/R spectra due to modulation of the
internal electric ﬁeld.
The extrema in the FKOs are given by
and are related to the magnitude of the electric ﬁeld F by
is the electrooptic energy, n is the index of the
nth extreme of the oscillations, E
is the energy of the probe
is the gap energy,
is an arbitrary phase factor,
is the reduced mass of an electron in the sample. F can
be determined from a plot of (4/3
dex number n, and should be a straight line with slope equal
, which is proportional to F.
We have developed and demonstrated a near-ﬁeld scan-
ning PR measurement technique based on near-ﬁeld scan-
ning optical microscopy ͑NSOM͒.
The tapered, aluminum-
coated tip used here had an aperture diameter of ϳ100 nm,
and was held at a constant distance of ϳ10–40 nm from the
surface, permitting the simultaneous measurement of surface
electronic properties and surface topography when the tip is
scanned. The optical signals can be gathered with a spatial
resolution that is nominally limited by the aperture size.
Typically, only 1–10 mW of light is launched down the ﬁ-
ber, since higher power levels are apt to damage the aperture
by heating and thus degrade the resolution. A drawback to
NSOM-based measurements is that only 10
incident light passes through the small aperture, requiring
that the optical detection system be sensitive to signal levels
in the 1–1000 nW ͑and smaller͒ range for reﬂection mea-
surements made with unity collection efﬁciency.
The photoreﬂectance NSOM ͑PRNSOM͒ system uses a
chopped HeNe laser ͑632.8 nm͒ for the pump and a cw tita-
nium sapphire ͑TiS͒ laser ͑that is tuned from 770 to 905 nm͒
for the probe in this experiment. The two laser beams are
combined using a beam splitter and coupled into a single-
mode ﬁber optic, ensuring that the pump and probe beams
have strong spatial overlap at the NSOM tip.
The light was reﬂected at near-normal angles to the
sample surface, and a high collection efﬁciency was realized
using a large ͑7.5 cm diam͒, fast (f/No.ϭ1) lens placed on
axis with the ﬁber, one focal length away from the tip–
sample interface. A long-pass ﬁlter was placed in front of the
detector to remove the HeNe light from the reﬂected signal.
Detection was accomplished with a large ͑1cm
silicon photodiode and a precision current ampliﬁer followed
by a lock-in ampliﬁer. The probe laser intensity was con-
trolled to produce a constant power output which aids in the
normalization of the PR signal.
Both the photoluminescence
͑PL͒ and the PR signals are passed by the ﬁlter and detected.
The PL signal generated by the pump laser and detected by
the lock-in ampliﬁer was independently measured by block-
ing the probe beam. The magnitude of the PR signal was
then corrected by subtracting the pump-induced PL back-
Author to whom correspondence should be addressed; electronic mail:
APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 13 25 SEPTEMBER 2000
19430003-6951/2000/77(13)/1943/3/$17.00 © 2000 American Institute of Physics