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Two-dimensional carrier profiling of InP-based structures using scanning
spreading resistance microscopy
P. De Wolfa)
IMEC, Kapeldreef 75, B3001 Leuven, Belgium
M. Geva and C. L. Reynolds
Lucent Technologies, Breinigsville, Pennsylvania 18031
T. Hantschel and W. Vandervorstb)
IMEC, Kapeldreef 75, B3001 Leuven, Belgium
R. B. Bylsma
Lucent Technologies, Breinigsville, Pennsylvania 18031
~Received 5 October 1998; accepted 22 February 1999!
Scanning spreading resistance microscopy ~SSRM!is a powerful tool originally developed for
measuring two-dimensional ~2D!carrier distributions in Si device structures with nm spatial
resolution. Whereas its application has been explored in detail on Si structures, very little work was
made on III–V materials. In this article, we report on the application of SSRM for the analysis of
III–V semiconductor structures, and in particular metalorganic chemical vapor deposition-grown
InP-based structures. We found that the application of SSRM to InP-based structures is much
simpler than to Si. Minimal surface preparation is required, much lower tip forces are needed, and
metal tips instead of diamond tips can be used. When imaging complex multilayer epitaxial
structures ~containing p,n, and semi-insulating layers!, close agreement between the SSRM profile
and secondary ion mass spectroscopy profiles can be obtained. More importantly is the capability of
SSRM to image and determine 2D structures in actual devices, such as mesas and trenches, common
in semiconductor laser devices. SSRM also proved very valuable in characterizing with high spatial
resolution 2D dopant and implant distributions. In this article, we present SSRM analysis of
dedicated InP test structures with multiple p-nlayers, varying in concentration level and in
thickness. © 1999 American Vacuum Society. @S0734-2101~99!10904-7#
I. INTRODUCTION
Recently, several methods have been explored to measure
the two-dimensional ~2D!carrier distribution in InP-based
device structures. Hull et al. have reported a strong transmis-
sion electron microscopy ~TEM!contrast between p-, i- and
n-doped layers in InP laser diodes,1,2 although the precise
mechanism of the observed doping contrast is not yet fully
understood. Also, reproducibility and quantification are poor
and remain to be studied. Inspection by scanning electron
microscopy ~SEM!after stain etching the sample gives only
the gross features and cannot delineate the pn junctions, and
is insensitive to small variations in doping. However, it al-
lows the doped layers to be detected with a typical resolution
of the order of tens to hundreds of nm, which is defined by a
combination of the inherent instrumental resolution and in-
terface broadening due to the etch. Scanning capacitance mi-
croscopy ~SCM!has also been used for pn-junction delinea-
tion and qualitative carrier profiling of InP-based
structures.3–6 At present, none of these methods allows one
to measure the quantitative 2D carrier profile with high spa-
tial resolution (,20nm) in combination with a large dy-
namic range of carrier concentrations (1015–1020
atoms/cm3). In this work, the limitations and possibilities of
the scanning spreading resistance microscopy ~SSRM!
method for 2D carrier profiling of InP-based structures are
studied.
SSRM was originally developed for 2D carrier profiling
of Si devices with nanometer resolution.7It is in essence
based on an atomic force microscope ~AFM!equipped with a
conductive tip that is biased relative to the sample. The
spreading resistance value derived from the measured elec-
trical current is a function of the local carrier concentration at
the surface region surrounding the probe’s tip. The spreading
resistance value is determined primarily by the resistivity of
the material in the small semihemispherical volume at the
surface region where the tip contacts it. The resistivity is
closely related to the local carrier concentration ~and mobil-
ity!in this surface region. The spatial resolution thus mainly
depends on the tip radius and pressure. Since SSRM mea-
sures resistance, a material property that depends upon car-
rier concentration, rather than detecting the carriers directly,
it is capable of producing images of both high spatial reso-
lution and wide dynamic range of carrier concentration. We
have demonstrated spatial resolution values as small as 20
nm, and a dynamic range of 1015–1020 atoms/cm3in analy-
ses of Si devices.8Previously, it has been demonstrated that
SSRM can also be applied for the analysis of InP mesa-like
structures.9Moreover, it has been found that the application
a!Present address: Digital Instruments, 112 Robin Hill Road, Santa Barbara,
CA 93117.
b!Also at: KULeuven, INSYS, Kard. Mercierlaan 92, B-3001 Leuven, Bel-
gium; electronic mail: vdvorst@imec.be
1285 1285J. Vac. Sci. Technol. A 17„4…, Jul/Aug 1999 0734-2101/99/17„4…/1285/4/$15.00 ©1999 American Vacuum Society
of SSRM to InP-based structures is more simple than to Si-
based structures. Minimal surface preparation is required in
the cross-sectioning process, much lower tip forces become
possible, and metal tips instead of doped diamond tips can be
used. In the present work, the use of SSRM for InP-based
material is further explored by characterizing specially pre-
pared InP test structures. These test structures allow one to
determine the spatial resolution and the sensitivity to small
changes in dopant concentration for both n- and p-type InP
materials.
II. RESULTS AND DISCUSSION
A. General
The SSRM analysis was done on a Digital Instruments’
AFM model Nanoscope D-3000, equipped with a resistance
measurement unit, which is based on a 6-decade logarithmic
current amplifier.7Pyramidal metal tips,8,10 witha20nm
thick Cr outer layer, integrated into a silicon cantilever, with
a spring constant of 15 N/m were used. Note that these
probes cannot be used for SSRM analyses of Si structures,
where a high force, of 15–20
m
N is typically required.
Therefore, for SSRM imaging of Si structures, one must use
chemical vapor deposition ~CVD!diamond-coated silicon
tips,11 where the diamond is made conductive by the incor-
poration of a high B concentration during deposition. The
sample preparation of InP-based structures is simple: in most
cases, it involves cleaving the structure along a crystallo-
graphic plane, exposing an atomically flat cross section
through the region of interest. No additional cleaning is re-
quired. The cleaved samples were mounted on a metal plate
using silver paint, thus connecting the exposed samples’ sur-
faces ~other than the cleaved one!to the other electrode. The
cleaving was done about half an hour prior to analysis, and
both the preparation and measurement were performed at
ambient conditions. The measurement time for an image
with SSRM is very similar ~minutes!to a standard ~contact!
topographical image acquisition.
While the bias voltage applied for SSRM imaging of Si-
based structures is usually below 100 mV, an order of mag-
nitude higher bias voltage has to be applied for InP-based
structures. At lower bias voltages, the measured current was
FIG. 1. Test sample A: ~a!SIMS profiles showing the alternating n- and p-type layers with equal thickness and increasing dopant concentration. ~b!SSRM
resistance image showing the differently doped regions ~scan size: 535
m
m2). ~c!Topographic image measured simultaneously with the resistance image. ~d!
Section through the SSRM image taken at the line AA8showing the different carrier concentration steps in agreement with the SIMS profile.
1286 De Wolf
et al.
: Two-dimensional carrier profiling of InP-based structures 1286
J. Vac. Sci. Technol. A, Vol. 17, No. 4, Jul/Aug 1999
too low for acceptable signal-to-noise ratio. This might be
related to the fact that a metal point contact generally has a
high barrier resistance with III–V semiconductor materials.
Finally, it is interesting to note that the quality of repeated
images taken at the same area on the sample, decreases. The
best images, i.e., with the highest signal-to-noise ratio, were
the ones taken in the first scan of a cross-sectional area.
B. Specially prepared test samples
The SSRM method has been applied to characterize dedi-
cated InP test structures. The test structures consist of several
n- and p-type layers and have a mesa-like structure. The first
test structure ~sample A!has alternating n- and p-type layers
with different dopant concentrations but equal thickness. The
secondary ion mass spectroscopy ~SIMS!profiles for this
sample are shown in Fig. 1~a!. The dopant concentration
varies from 1017 to 2.331018 atoms/cm3for n-type ~Si-
doped!and 1.731017 to 1.231018 atoms/cm3for p-type ~Zn-
doped!InP. Figures 1~b!and 1~c!respectively show the
SSRM resistance image and the topography which are mea-
sured simultaneously in a 535
m
m2area located at the edge
of the mesa. No information regarding the differently doped
regions is found in the topographic image. The resistance
image, on the other hand, clearly shows the differently doped
FIG. 2. SSRM calibration curve for InP obtained from the data presented in
Figs. 1~a!and 1~d!.
FIG. 3. Test sample B: ~a!SIMS profiles showing the alternating n- and p-type layers with equal dopant concentration and different width. ~b!SSRM
resistance image of the ‘‘field’’ region showing the different n- and p-type layers ~scan size: 535
m
m2). ~c!SSRM resistance image taken at the edge of the
mesa-like structure ~scan size: 737
m
m2). ~d!Section through SSRM image taken at the line AA8.
1287 De Wolf
et al.
: Two-dimensional carrier profiling of InP-based structures 1287
JVST A - Vacuum, Surfaces, and Films
regions of the test sample. In the lower part of Fig. 1~b!, the
different n- and p-type doped layers are resolved and found
to be parallel. Upon approaching the mesa-like structure,
some of these layers are bending and converge, while the
others remain straight. No signal is measured in the upper
right corner of the image, as the probe is no longer in contact
with the sample’s surface @as can be seen in the topographic
image in Fig. 1~c!#. The resistance image of Fig. 1~b!can be
used to make sections in a particular direction, for example,
along the line AA8. This resistance scan is displayed in Fig.
1~d!. Based on this line scan and the SIMS profile of Fig.
1~a!one can quantify the SSRM signal. The measured resis-
tance versus dopant concentration is presented in Fig. 2 for
both n-type and p-type materials. It is estimated from Fig. 2
that both n- and p-type materials have a sensitivity of about
15 nA per 1017 cm23, and, thus, the SSRM method has suf-
ficient sensitivity to measure small variations in dopant con-
centration. One can use this type of plot to calibrate the
SSRM resistance image of an arbitrary InP sample or device
with an unknown dopant profile. However, one must remem-
ber that this plot is only valid for the particular probe being
used at the force and bias voltage which were applied. Other
measurement conditions result in different calibration curves.
The second test sample ~sample B!has alternating n- and
p-type layers with different thickness values. The p-type lay-
ers are all with a constant Zn concentration of about 3
31017 cm23, while the n-type layers are at a constant Si
concentration level of about 731017 cm23. The SIMS pro-
files are given in Fig. 3~a!. Figures 3~b!and 3~c!show the
2D SSRM resistance image measured respectively in the
‘‘field’’ region and at the edge of the mesa-like structure of
this sample. The different dopant regions are clearly ob-
served in both images. The streaks which are observed in
Fig. 3~c!are due to the tip picking up debris at the edge of
the sample’s cross section. From this, it is clear that one has
to avoid scanning the tip across the edge of the sample. A
section made through the image of Fig. 3~c!is displayed in
Fig. 3~d!and shows the expected carrier profile. From this
profile, it is clear that the dopant layers which are thinner
than 50 nm could not be observed ~i.e., the region at a depth
smaller than 0.5
m
m!. This is probably related to the fact that
the radius of the probe contact is on the order of 20 nm,
resulting in a limited spatial resolution. Also, in SSRM, the
carrier profile is measured rather than the dopant profile as
measured with SIMS. The carrier profile might be different
from the dopant profile due to carrier diffusion which is even
more enhanced at pn junctions.
III. CONCLUSIONS
In conclusion, SSRM high spatial resolution imaging of
carrier distributions was demonstrated on InP-based
multilayer test structures, demonstrating its capability in ana-
lyzing multiple p-nlayers of varying thickness or concen-
tration. The experimental conditions with InP material are
much less stringent than those with Si structures, allowing
for high quality images to be obtained within a short period
of time. Sample preparation is minimal; usually just cleaving
the sample to reveal the desired cross section. Metal tips
prove to be sufficiently rigid for stable and reproducible
SSRM measurements. Very low probe pressures are re-
quired. For good signal-to-noise ratio, relatively high bias
voltage is required.
ACKNOWLEDGMENTS
The authors would like to thank Andy Erickson ~Digital
Instruments!for providing the AFM instrumentation, Ph.
Niedermann and W. Ha
¨nni ~CSEM!for providing the dia-
mond coated probes. T. Hantschel is indebted to the Belgian
IWT for a research grant. They also acknowledge J. H.
Eisenberg of Lucent Technologies for her support of this
project.
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