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Journal of ELECTRICAL ENGINEERING, VOL. 58, NO. 6, 2007, 301–306
ANALYSIS OF NANOSTRUCTURES BY MEANS
OF AUGER ELECTRON SPECTROSCOPY
∗
∗
Gernot Ecke — Volker Cimalla — Katja Tonisch
∗
∗
— Vadim Lebedev — Henry Romanus
∗∗
∗∗∗
— Oliver Ambacher — Jozef Liday
∗
In modern nanotechnology analysis such methods are needed which are able to investigate extremely small volumes, thus
surface sensitive techniques with a high spatial and depth resolution. Concerning the capability of high lateral and depth
resolution, Auger electron spectroscopy (AES) is one of the outstanding analytical methods for nanotechnology. By field
electron guns probe diameters below 10 nm are reached. Depth resolution of Auger electron spectroscopy, depending on
the kinetic energy of the Auger electrons, is approximately 0.5 to 4 nm. Whereas large area AES has a detection limit of
0.1 at% it impairs for laterally highly resolved measurements. The article will give some examples for the application of Auger
electron spectroscopy to nanostructures mostly in group III-nitride semiconductor technologies: (i) nanowires, consisting of
Si and AlN with diameters of about 20 to 200 nm; these nanowires and nanorods have been grown by different technologies
and some of them are contacted on both ends by FIB grown Pt contacts, (ii) nanoflowers,ie specially shaped up to 5 µm
sized networks of AlN nanowires of about 20 nm in diameter, (iii) segregation structures of Si of 200 nm width, grown during
PIMBE AlN epitaxy on Si substrate.
On the basis of these measurements the benefits and limits of Auger electron spectroscopy on nanostructures as well as
some special effects which are characteristic especially for nanostructures, for instance resputtering and background signal
contribution, are discussed.
K e y w o r d s: nanotechnology, Auger electron spectroscopy, nanowires, nanoflowers, segregation
1 INTRODUCTION
There are a lot of analytical methods using focused
electron, ion or photon beams as well as mechanical tips
as a probe for the excitation of a sample. Electrons, ions,
neutrals or photons coming back from the sample as well
as interaction forces between tip and sample surfaces can
be measured. Each of these analytical methods is denoted by its special characteristics, possibilities, excellences, disadvantages and fields of application. This huge
family of analysis methods can be classified into several
groups: methods for chemical composition measurements
and methods for the investigation of the structural, morphological and optical and electronic characteristics of the
sample. Key characteristics of those methods are sensitivity, detection limit, elemental range, quantification and,
as one of the most important features, the spatial and the
depth resolution.
In modern nanotechnology analysis such methods are
needed which are able to investigate extremely small volumes, thus surface sensitive techniques with a high spatial and depth resolution. Concerning the capability of
high lateral and depth resolution Auger electron spectroscopy (AES) is one of the outstanding analytical methods for nanotechnology. In modern Auger spectrometers,
equipped with a field electron gun, probe diameters below
10 nm are reached. Electron beams can be focused very
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∗∗
well, whereas the focusing of ion beams and x-rays or visible light beams is much more complicated or impossible
on the nanoscale. The depth resolution of Auger electron
spectroscopy depends on the energy of the Auger electrons. But within the usual range from 50 eV to 2000 eV
the attenuation length is approximately 0.5 to 4 nm. As
a consequence most of the Auger signal comes from a
thin sample surface layer with a thickness of some atomic
layers. Whereas large area AES has a detection limit of
0.1 at% it impairs for laterally highly resolved measurements. With a detection limit of approximately 1 %, a
spatial resolution of 10 nm and a depth resolution of 1 nm
50 atoms of one kind can be detected. No other method
is able to reach the same detection limit, except for analytical cross-sectional transmission electron microscopy
(XTEM) with very high preparation efforts and other
constrictions.
With these properties AES can play an outstanding role for the analysis of nanostructures among other
analysis methods like scanning transmission electron microscopy (STEM), photo emission electron microscopy
(PEEM), small spot secondary ion mass spectroscopy
(SIMS) and the scanning probe microscopy methods.
According to [1], problems are expected due to the
complexity of nanostructures — due to the higher importance of surface phenomena compared to conventional
microanalysis.
TU Ilmenau Center of Micro- and Nanotechnologies PO Box 100565 D-98684 Ilmenau, Germany, gernot.ecke@tu-ilmenau.de;
∗∗∗
Slovak
Fraunhofer Institut für Angewandte Festkörperphysik, Tullastr 72, Munich, Germany, oliver.ambacher@iaf.fraunhofer.de;
University of Technology, Department of Microelectronics, Ilkovičova 3, 812 19 Bratislava, Slovakia
ISSN 1335-3632 c 2007 FEI STU
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Gernot Ecke et al : ANALYSIS OF NANOSTRUCTURES BY MEANS OF AUGER ELECTRON SPECTROSCOPY
Fig. 1. Secondary electron microscopy (SEM) images of a Si segregation structure on AlN in different magnification
A hemispherical energy analyzer of 0.03% energy resolution serves as an electron energy spectrometer. The
energy resolution can be increased by retarding the electrons before passing through the analyzer. The measurements have been carried out in the direct mode E N (E)
with a constant retard ratio (CRR).
The nanostructure samples which are discussed in the
paper have been grown by plasma induced molecular bam
epitaxy (PI-MBE) in a BALZERS UMS500 and metallorganic chemical vapour deposition (MOCVD) in an
AIX200 by Aixtron. Si and sapphire wafers have been
used as substrates.
Fig. 2. AES spectra of Si, N and Al: (a) on the segregation structure
and (b) besides the structure of the Al and Si images after sputter
cleaning
2 EXPERIMENTAL
The following examples have been carried out by an
Auger Microlab 350 by THERMO, equipped with a small
spot electron gun with a field electron tip, an electron current of 0.1–100 nA. A spatial resolution of 7 nm for SEM
images and 10 nm for Auger images can be achieved.
Sputtering was carried out by a conventional Ar sputtering ion gun under 43◦ – 74◦ with respect to the surface
normal in the energy range of 0.5–5 keV.
Fig. 3. AES spectra of Si, N and Al: (a) on the segregation structure
and (b) besides the structure after sputter cleaning by Ar + ion
bombardment
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Journal of ELECTRICAL ENGINEERING VOL. 58, NO. 6, 2007
the AlN surface (see Fig. 4), we can conclude that there is
only a thin Si rich layer on the AlN, even after sputtering.
This could be a result of sputter induced redeposition. To
answer the question of the real concentration of Si within
the AlN layer additional analysis is necessary which has
been made by cross sectional TEM an electron energy loss
spectroscopy, which is summarized in [3]. These measurements confirmed clearly the existence of Si segregation at
dislocations within the AlN layer. With the good lateral
resolution of the Auger spectrometer there is the possibility of acquiring scanning Auger maps (SAM) of these
structures. Figure 5 shows the SEM images of the structure and Auger maps of Si and Al. The contrast can be
enhanced after removing the surface adsorbates by sputter cleaning.
Fig. 4. AES spectra of Al and Si besides the structure after sputter
cleaning, (a) LVV Auger peaks and (b) KLL Auger peaks
3 EXAMPLES
3.1 Example 1: Si segregating on epitaxial AlN
After the epitaxial growth of 50-200 nm thick Si doped
AlN layers on Si substrates by plasma induced molecular beam epitaxy (PIMBE) segregation structures on the
surface of the AlN layer could be detected [2]. These segregation structures consist of stripes with 0.1– 1 µm width
and up to some 10 µm length. Figure 1 shows the secondary electron microscopic image of the sample surface
with one of the segregation structures in different magnifications.
It was assumed that silicon passes the AlN layer along
grain boundaries from the substrate and segregates on
the surface. AES spectra on and besides the segregation
structure are shown in Fig. 2. One can see that in fact the
segregation structure consists of silicon on the AlN layer.
But traces of Al can be found on the Si structure and Si
can be found on the AlN layer more surface more than the
doping concentration. In order to decide, whether this is
a surface effect or these are bulk concentrations, the sample has been sputter cleaned. The spectra of the sputter
cleaned structure and layer are shown in Fig. 3. One can
see that there is still Al on the segregation structure and
Si on the AlN layer surface. If we measure the low energy
LVV peaks and the high energy KLL peaks of Si and Al at
Fig. 5. SAM investigation of Si segregation: (a) and (b) SEM
images in different magnification, (c) and (d) Al and Si SAM images
before sputter cleaning and (e) and (f) with enhanced contrast of
the Al and Si images after sputter cleaning.
3.2 Example 2: Pt contacted Si nanowires
On the SiO2 surface of a Si/SiO2 substrate Ti/Au
metallisation pads have been created by means of magnetron sputtering. Afterwards, Si nanowires of approximately 150 nm diameter have been grown by low pressure
chemical vapor deposition (LPCVD) on glass substrate
and transferred onto the SiO2 surface with the Ti/Au
pad matrix. Focused ion beam technique (FIB) was applied to deposit Pt locally to connect both ends of the
Si nanowires to the metallisation pads, in order to allow
measuring the I − U characteristics of these nanowires
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Gernot Ecke et al : ANALYSIS OF NANOSTRUCTURES BY MEANS OF AUGER ELECTRON SPECTROSCOPY
Fig. 6. SEM image of a contacted Si nanowire on SiO2 surfaces:
(a) nanowire and (b) (c) both contacts, in (c) the position of the
AES measurements are indicated
position of the FIB deposited Pt connects. AES results
on two positions, indicated in Fig. 6 (c) show that the
nanowire is consisting of Si which contains a high amount
of nitrogen or SiNx and that the metal connect which is
made intentionally of Pt by FIB consists of Pt with a high
C content (Fig. 7). After sputtering there is still a lot of
N on the Si nanowire, indicating that the SiNx mantle is
relatively thick. After sputtering there is still the impurity of carbon in the Pt nanowire. Additionally we found
that there is a heavy C contamination layer produced simultaneously during the Pt deposition by FIB all around
the grown structure. The C amount is much higher and
thicker than a common C contamination created by air.
3.3 Example 3: AlN nanoflowers
Fig. 7. AES spectra of (a) the Si nanowire according to position 1
in Fig. 6 (c) and (b) of the Pt metal connect according to position 2
in Fig. 6(c)
[4]. Figure 6 shows the SEM images of one nanowire and
the contacts to the metallisation pads on both ends.
AES measurements were applied in order to measure
the elemental composition of the nanowires and the com-
Under special conditions for the AlN growth on sapphire by metal organic chemical vapor deposition
(MOCVD) special structures are developing, the so called
’nanoflowers’ [5]. They consist of nanowires with diameters of about 20 nm. Figure 8 shows the SEM images
of typical nanoflowers grown on a sapphire substrate in
different magnifications. In Fig. 8 (c) one can see the
nanowires which are the basis of these flowers.
With the high spatial resolution of AES it is possible
to measure the composition of the nanowires either on
one nanowire or on a conglomerate of nanowires.
Auger measurements shown in Fig. 9 reveal that the
nanowires consist of AlNx , but the surface of these wires
is highly covered by C and O (not shown) which are adsorbates from the technological process or from the contamination by air before the Auger measurement. In such
a case quantification of the measurement and conclusions
on the stoichiometry of the AlNx cannot be executed
because the surface is contaminated and because of complete other backscattering conditions in nanowires compared to bulk or layer samples which must be used as
standards.
3.4 Summary of examples
The three examples show the power of high spatially
resolved Auger analysis on its application to nanostruc-
Fig. 8. SEM images of a ’nanoflower’ in different magnifications
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Experiments have been carried out in order to explore
this effect.
An InN particle of a size of approximately 1 µm on Si
substrate has been measured with a relatively broad electron beam under excitation energies of 25 keV, 10 keV
and 3 keV.
In and N signal comes from the particle itself, Si signal
comes from the substrate due to electron backscattering
and primary electron bombardment besides the particle.
Auger spectra for the three primary energies are shown
in Fig. 10.
Fig. 9. Auger spectra of N KLL and Al KLL on a nanowire of a
nanoflower
tures. But in all mentioned cases one can see that some
artifacts could disturb the quality of the measurements,
like sputtering induced redeposition, problems of quantification due to the backscattering behavior and the signal coming from the surrounding of a nanostructure by
electron backscattering within the substrate.
If we define the useful signal as a ratio of the signal
of the particle (in our case In or N) to the signal of the
substrate (Si), the useful signal can be evaluated of the
measurement presented in Fig. 10. The useful signal In/Si
and its dependence on the primary excitation energy is
shown in Fig. 11. The insets show the corresponding SEM
images and give an impression on the sharpness of the primary electron beam used for these measurements. It can
be seen that for this measurement there is a maximum of
the useful signal, ie an optimum of the primary electron
excitation energy for acquiring the best measurement result. The energetic position of this maximum depends on
(i) the relation of the structure size and electron beam
diameter,
(ii) the material and its backscattering behaviour and
4 GENERAL CONSIDERATIONS
In Auger analysis it is commonly known that
(i) the lower the primary energy of the electron gun, the
worse is the spatial resolution,
(ii) the higher the beam current, the worse the spatial
resolution,
(iii) but, the lower the primary excitation energy, the
smaller is the area emitting Auger electrons due to
backscattering.
(iii) the acquired Auger peak energies, because low energy Auger electrons are generated farther away from
the structure by backscattering
(iv) the ionization cross-section of the core electron level
of atom for primary electrons excitation energy.
It should be a fundamental aim for measuring nanostructures by AES to know about the optimal measuring
conditions and to know about the possible artifacts. Additional measurements are necessary to clear this in a wide
range of measuring parameters and materials.
Fig. 10. Auger spectra of a InN particle on Si substrate with (a) 3 keV; (b) 10 keV and (c) 25 keV primary excitation energy
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5 CONCLUSIONS
Three examples of Auger measurements on nanostructures are presented to document the capability of Auger
electron spectroscopy as a surface sensitive technique
with a brilliant spatial resolution. These properties enable this method to be one of the most promising surface
analysis methods in nanotechnology research and development.
It has been shown that some artifacts can occur and
it is important to know about them and regard them.
It is of fundamental importance to know about optimal
primary exciting electron energy and their dependence on
the measured material and measuring conditions.
Fig. 11. Useful Auger signal In/Si of a InN nanoparticle on Si
substrate and its dependence on the primary exciting energy, insets
show the corresponding SEM images.
Acknowledgement
The authors would like to thank the Thuringian ministry
of education (project B609-02004), the Deutsche Forschungsgemeinschaft (project AM105/1-1 and CI 148/2)
and Office of Naval Research (project Nicop) and the
Ministry of Education of Slovak Republic (Nem/Slov/1/
DAAD, VEGA 1/4079).
References
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Received 15 February 2007
Gernot Ecke (Dr, Ing) was born in Calbe, Germany in
1958. He graduated in electronic devices from TU Ilmenau in
1983, and gained the PhD degree in 1986. Since that time
he has been Assistant Professor. His teaching and research
activities include surface science and materials analysis.
Volker Cimalla was born in 1967. He received the MSc
and PhD degrees in engineering from the Technical University
Ilmenau, Ilmenau, Germany, in 1993 and 1998, respectively.
Since 2002, he has been a Research Assistant with the Technical University Ilmenau and has been involved in the synthesis,
characterization, and application of wide – bandgap semiconductors in nanostructures and sensors. He is also working on
the development of micro- and nanoelectromechanical device
sensing applications.
Katja Tonisch was born in 1980. She is currently a PhD
student at the Technical University of Ilmenau, Germany. She
received her Diploma (MS) in Electrical Engineering in 2005.
Her research interests include the growth of group III-nitrides
with MOCVD and nitride-based MEMS.
Vadim Lebedev was born in 1967. He received the degree
in physics from the St. Petersburg’s State Technical University, Russia, and the PhD degree from Jena University, Germany, in 2001. Since 2002, he has held a research position
at the Center of Micro- and Nanotechnologies, Technical University of Ilmenau, Germany, working in the field of growth
of wide bandgap semiconductors. His current research interests include applied and background research in the area of
nanosensoric and optoelectronic applications based on groupIII nitrides and oxides. He has authored or coauthored more
than 60 publications.
Henry Romanus studied electrical engineering and materials science in specialization at the Technical University Ilmenau and received the PhD degree in 2004. At present he is
scientific staff member of the Center for Micro- and Nanotechnologies (ZMN) at Technical University Ilmenau. Main fields
of activities are the research of materials in field of microand nano-technologies with electron microscopy (SEM, TEM,
FIB) and X-ray diffraction, thin film technology of contacts
and metallizations (PVD, RTP) and the processing of different
specimens by focus ion beam technology.
Oliver Ambacher (Prof, Dr) was born 1963 in Welmerskirchen, Germany. He graduated in physics at the LMU Munich in 1989 and received his PhD in physics from the TU
Munich in 1993. Since 2002 he has been professor of the Department of Nanotechnology at the TU Ilmenau. His teaching
and research activities cover nanotechnology, nanoanalytics
and nanodiagnostics as well as technology of devices based on
Group III nitrides.
Jozef Liday (Doc, Ing, CSc) graduated in solid state
physics in 1968 and received his PhD in electronics and vacuum technology, both from the Slovak University of Technology, in 1985. His teaching and research activities include materials analysis, thin films and surface science.