APPLICATION NOTE

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Correlative Raman Imaging of
Semiconducting Materials

Confocal Raman microscopy is a
non-destructive analysis method thatis uniquely capable of characterizing
semiconducting materials. It canreveal the chemical composition ofa sample, identify possible contaminants and even visualize strain
fields in 3D volumes. In combination
with techniques such as AFM or SEM,
semiconducting materials can beinvestigated more comprehensively.

The Raman principle

No need for compromises

The Raman effect is based on the inelastic
scattering of light by the molecules ofgaseous, liquid or solid materials. Theinteraction of a molecule with photons
causes vibrations of its chemical bonds,
leading to specific energy shifts in the
scattered light. Thus, any given chemicalcompound produces a particular Raman
spectrum when excited and can be easily
identified by this individual “fingerprint.”
Raman spectroscopy is a well-established,label-free and non-destructive method foranalyzing the molecular composition of asample.

The Raman effect is extremely weak, so
every Raman photon is important for
imaging. Therefore WITec Raman imaging
systems combine an exceptionallysensitive confocal microscope with anultra-high throughput spectrometer(UHTS). Precise adjustment of all opticaland mechanical elements guarantees thehighest resolution, outstanding speed andextraordinary sensitivity - simultaneously!
This optimization allows the detectionof Raman signals of even weak Raman
scatterers and extremely low material
concentrations or volumes with thelowest excitation energy levels. This isan unrivaled advantage of WITec systems.

Raman imaging
In Raman imaging, a confocal microscopeis combined with a spectrometer anda Raman spectrum is recorded at every
image pixel. The resulting Raman image
visualizes the distribution of the sample’s
compounds. Due to the high confocality of
WITec Raman systems, volume scans and3D images can also be generated.

Resolution
Lateral resolution is physicallylimited to ~200 nm, dependingon the wavelength of theincident light.

Speed
The more sensitive a system is, the shorter
the acquisition time for a single spectrum.
WITec’s Ultrafast Raman Imaging reduces
acquisition times for single Raman spectra
down to well below 1 ms.

Sensitivity
A high confocality increases the signal-to-noise ratio by reducing
the background. With the UHTS Series, WITec developed lens-based,
wavelength-optimized spectrometers with a spectral resolution
down to 0.1 cm-1 relative wavenumbers.

Correlative Raman Imaging of Semiconducting Materials
Semiconducting materials are the basis for many electronic and optoelectronic devices such as light-emitting diodes (LED), laser diodes,
photodiodes, solar cells and integrated circuits. Prominent examples of semiconducting materials include silicon (Si), silicon carbide(SiC), gallium arsenide (GaAs) and gallium nitride (GaN). In addition, 2D materials, such as molybdenum disulfide (MoS2), are the focus
of extensive research due to their potential in the field of semiconductors. During research, fabrication and quality control, the structure of the crystals as well as defects and stress fields in the crystals must be detected efficiently and without damaging the sample inorder to produce high-quality devices. Raman microscopy is ideally suited to this task.
This application note presents several
examples of studies on semiconducting
materials using 2D and 3D Raman imaging in combination with various othertechniques such as atomic force microscopy (AFM), scanning electron microscopy(SEM) or second harmonic generation(SHG) microscopy. Raman imaging candistinguish between the semiconducting
material and possible contaminants andidentify them chemically. It can reveal
stress fields in the crystal, even in 3D insome samples. At the same time, AFM andSEM yield information on the sample‘s
topography. Thus, the combination ofthese techniques enables detailed sample
characterization.

Silicon wafer inspection with a WITec alpha300 confocal Raman microscope

Correlative topography and Raman measurements
When investigating semiconducting materials, the sample’s surface structure is often of interest. WITec TrueSurface technology obtains topographic and chemical information simultaneously by combining a Raman microscope with an optical profilometer. Additionally, the profilometry data is used for focus stabilization, so that even rough, irregularly-shaped and inclined surfaces are kept in focus
during the Raman measurement [1]. The application example in Figure 1 shows the topographic Raman image of multi-crystalline silicon. The height difference between the areas was up to 9 µm and the measurements were performed with a 100x objective featuring adepth resolution of about 900 nm. Nevertheless, the surface was kept in focus by TrueSurface over the entire measured region.

Figure 1: Topography and chemical compositionof multi-crystalline silicon. The surface structure
information and Raman image were recorded
simultaneously using TrueSurface technology
and overlaid. The Raman signal from silicon iscolor coded in turquoise and the yellow spots
represent areas of increased fluorescence signal.

3D Raman imaging of stress fields
Gallium nitride (GaN) is a direct band
gap semiconductor commonly used, forexample, in light-emitting diodes, solar
cells and transistors. In order to obtainhigh-quality GaN crystals with a low
density of dislocations, the material canbe grown on structured substrates. To
evaluate the growth methods and the
produced materials, non-destructive
techniques are needed for detecting
stress in the crystals.
In the example presented here, a 20 µmthick layer of GaN was grown on a patterned sapphire substrate in a two-stepprocess, using metal organic vapor phase
epitaxy and hydride vapor phase epitaxy.
The substrate featured evenly-spaced
hexagonal pits. The sample was provided

A

C

D

courtesy of Dr. Eberhard Richter from theMaterials Technology Department of the
Ferdinand Braun Institute, Berlin, Germany,
where fabrication of the pattern and GaN
growth were performed. Stress fields inthe crystal were then investigated by 3D
confocal Raman imaging using a WITecalpha300 Raman microscope equippedwith an ultra-high throughput spectrometer (UHTS).
First, an overview white-light image was
acquired using a 100x air objective with
NA 0.9 (Figure 2). As GaN is transparent forvisible light, the honeycomb-like structureof the substrate is visible beneath the GaN
layer. The sample was then investigatedin detail by 3D Raman imaging using an
excitation wavelength of 532 nm.

B

Ehigh
2

Figure 2: White-light image of a GaN crystal
grown on a patterned sapphire substrate. The
hexagonal structure of the substrate is visible
through the 20 µm-thick GaN layer grown on
top. A Raman depth scan was performed along
the red line and a z-stack of 20 x-y scans wasrecorded in the green area (see Figure 3).

A1(LO)Elow
2

Figure 3: 3D Raman analysis of a GaN crystal grown on a patterned
sapphire substrate.
A: Depth scan (240 x 80 pixels on an area of 60 x 20 µm2) along the
red line in Figure 2. The Raman image is color coded according to thespectra in B.
B: Corresponding Raman spectra. In the inset, the green spectrum isomitted for clarity.
C, D: 3D Raman image of the sample volume. Scans of the green areain Figure 2 were performed at different focal planes (180 x 45 x 20
pixels in a volume of 60 x 15 x 20 µm3) and the images were color
coded according to the spectra in B. C: Example 2D Raman image
extracted from the stack. D: 3D reconstruction of the sample volume.
At the front right corner, a part of the structure is removed.
Sample courtesy of Dr. Eberhard Richter (Materials Technology
Department of the Ferdinand Braun Institute, Berlin, Germany).

A 2D depth scan was performed along the
red line in Figure 2 and a Raman spectrum
was recorded at each pixel. The resulting
Raman image (Figure 3A) was color coded
according to the measured spectra (Figure
3B), revealing three distinct areas in thesample. The red spectrum shows the
typical features of GaN [2] and is dominantat the surface of the GaN layer, showingthat the crystal grown there was highin quality. The green spectrum showsenhanced fluorescence at low wavenumbers and is found only at the wallsof the pits in the substrate. Most likely, it
is a remnant of the etching process. The
blue spectrum measured above the pits
is distinct from the red one, mainly at
the A1(LO) peak near 740 cm-1 (Figure 3B,
inset). This peak is upshifted and strongly
broadened compared to the red spectrum,indicating a lower quality GaN crystal in
the areas above the pits (blue) comparedto the regions at the surface (red). Next,
a stack of 2D scans was performed at
different focal planes of the area indicatedin Figure 2. Figure 3C shows one layerfrom the stack, in which the discussed
spectral features are again visible. A 3D
representation of the sample volume was
generated from the image stack (Figure
3D). The fluorescence signal (green) forms
a ring at the walls of the pits, while thespace on top of the pits is dominated by
the distorted GaN spectrum (blue). The
topmost layers are characterized by theundistorted GaN spectrum (red).
Strain in crystalline samples is often associated with Raman peak shifts. In orderto reveal stress fields in the GaN sample,
a peak shift analysis was performed forthe entire z-stack. The position of the E2highpeak near 570 cm-1 was quantified for each
spectrum by fitting a Lorentzian function.
Figure 4 shows a 3D reconstruction of thesame sample volume as in Figure 3D, but
color coded according to the determinedpeak shift. The positions of the pits in thesubstrate are indicated by the picture in
the bottom plane. The stress fields propagate from the interface to the surface

mainly in tube-like structures, which become narrower towards the surface, again
indicating that the GaN crystal quality
is higher at the surface than close to theinterface. However, the overall differencesin the peak positions were quite small(< 1 cm-1) and thus, the overall differencesin the strain of the GaN crystal were also
small. Nevertheless, the peak shift sensitivity of the UHTS300 spectrometer was
sufficient to measure and visualize them.
Using a similar approach, Joonas Holmi(Aalto University, Finland) and co-workers
recently published a method for characterizing threading dislocations (TDs) inammonothermally-grown α-GaN crystals using 3D Raman microscopy [3]. Byquantifying the Raman peak shift of theE2H peak at each position, they were able
to locate the distortions, estimate the TD
density in their sample and visualize thepropagation of the stress fields into the
crystal volume in 3D. Their measurementseven enabled them to distinguish betweenedge a-type and mixed a+c-type TDs [3].

Figure 4: 3D representation of stress fields in the GaN crystal (the same sample volume as in Figure 3D). The position of the E2high Raman peak near
570 cm-1 is color coded, revealing stress fields in the crystal. The image in the bottom plane indicates the positions of the pits.
Sample courtesy of Dr. Eberhard Richter (Materials Technology Department of the Ferdinand Braun Institute, Berlin, Germany).

Stress in silicon – correlative Raman-AFM imaging
When characterizing materials, the effectthat deforming forces have on themis often of interest. In a Vickers test, asquare-based diamond pyramid is pushed
into the sample material with a controlled
force. From the size of the resulting indent,
a value for the material’s hardness canbe obtained. Such microindents can be
efficiently characterized using a combination of atomic force microscopy (AFM)
and Raman imaging [4]. In this example,
the indentation was made with a force of
50 mN in a silicon (111) substrate using a
Fischer Indentation System. Raman and
AFM images were acquired at precisely thesame sample position using an alpha300
RA combined Raman-AFM microscope.

Indentation resulted in a pyramidal indentof 2.6 µm diagonal length with protrusions at the edges (Figure 5A). The indent
was 160 nm deep and the protrusions
were about 7 nm high (Figure 5B). The
observed deformations caused stress inthe silicon around the indent, which was
visualized by Raman imaging. For every
recorded Raman spectrum, the relative
shift of the silicon peak at 520 cm-1 wasquantified and color coded in the Raman
image (Figure 5C). As stress leads to peakshifts in the Raman spectra, the symmetricstress fields around the indent are visible
in this image [4, 5]. Tensile strain occurredat the corners of the pyramidal indent(shift to lower wavenumbers, blue/green
areas in Figure 5C) and compressive strain

A

B

D

Cross Section 1

at the protruding sides (shift to higher wavenumbers, yellow/red areas in Figure 5C).
Stress profiles along three different axesof the indent are displayed in Figure 5D.
In a statistical analysis of the stress-freeareas, the standard deviation of the Raman
peak position was about 0.02 cm-1 (seealso Cross Section 1 in Figure 5D). In silicon,
a peak shift of 1 cm-1 corresponds to a
stress of 435 MPa [4]. Thus, the precisionof 0.02 cm-1 corresponds to a stresssensitivity of less than 9 MPa. This highdegree of precision was achieved with an
integration time of 70 ms per spectrum.
The example illustrates the usefulness ofcorrelative Raman-AFM measurements forcharacterizing topography and strain insemiconducting materials.

C

Cross Section 2

Figure 5: Characterization of topography and stress in silicon around a Vickers indent.
A: AFM image of the topography around an indent, 10 x 10 µm2.
B: Depth profile along the blue line in the AFM image (A).
C: Raman stress image of the same sample area as in A. The shift of the silicon Raman peak at 520 cm-1 is color coded.
D: Stress profile cross sections along three different axes, named 1, 2 and 3.
Sample courtesy of Helmut Fischer GmbH, Sindelfingen, Germany.

Cross Section 3

Correlative Raman-SEM imaging of strained nanopillars
Semiconducting nanostructures areinvestigated for their application in
optoelectronic devices, for example,
efficient solar cells. RISE (Raman Imaging
and Scanning Electron) microscopy
was applied here to study the physicalproperties of silicon nanopillars that were
etched out of Si(111) and Si(100) substrates[6, 7]. A strain gradient, modifying theelectronic band structure of the pillars,
was introduced via wet thermal oxidation.
High-resolution SEM images revealed thegeometric dimensions of the nanopillars,with a height of 2 - 3 µm and a diameterof 500 nm or 700 nm, depending on thesubstrate. The symmetry of the nanopillars followed a densely-packed pattern(hexagonal symmetry) on both substrates(Figure 6).
Raman images were acquired from thesame sample area by transferring thesample inside the vacuum chamber tothe Raman measuring position [6]. From
Raman spectra, it is possible to evaluate
the crystallinity and stress state of theanalyzed Si-sample [4, 5]. A shift of thesilicon peak at 520 cm-1 indicates stress inthe sample, while low crystallinity leads toa broadening of the band. Raman spectra
acquired from the nanopillars are shown
color coded in Figure 6. The Raman imagesof the two samples revealed different patterns of strain and crystallinity (Figure 6).
On the (111) substrate, all pillars showed
concentric regions of different strain levels(Figure 6A), while some pillars on the (100)substrate showed a more complex pattern(Figure 6B). The combination of Raman
and SEM thus made it possible to correlate
the high-resolution structural informationof SEM with the Raman information on
strain and crystallinity.

A

B

Figure 6: RISE imaging of strained silicon pillars
etched out of differently-oriented Si substrates.
A section of the respective SEM image is overlaid
with the Raman image of this region. The Raman image is color coded according to the spectra shown on the right. The respective maximum
is indicated next to the peaks. Note the different
scale bars and color codes in panels A and B.
A: Nanopillars etched out of a (111) silicon
substrate.
B: Nanopillars etched out of a (100) silicon
substrate.
Sample courtesy of Prof. Alois Lugstein (Institutefor Solid State Electronics, Vienna University ofTechnology, Vienna, Austria).

Correlative characterization of 2D semiconducting materials
Two-dimensional semiconducting
materials, such as transition metal dichalcogenides (TMDs), are receiving increasing
attention due to their unique physical,
chemical and electronic properties. An
important example is molybdenum
disulfide (MoS2). Single-layer MoS2 is adirect band gap semiconductor, in contrast
to bulk MoS2, which has an indirect bandgap. 2D MoS2 is thus investigated for its

A

Optical image

E

B

potential applications in optoelectronic
devices. As grain boundaries can affectthe properties of 2D materials, sensitiveand fast methods for their visualization
are needed [8]. Here, we present correlative confocal Raman and nonlinear
multi-photon imaging measurements thatcomprehensively characterize the crystal
structure and local strain/doping effects atthe crystal boundary of 2D materials.

SHG imageC

A1g

THG imageThe following experiments were performed using an alpha300 RA microscope
equipped with a 488 nm laser for Raman
measurements and a 1560 nm fs-pulsed
laser for second and third harmonic
generation (SHG and THG), at theLaboratory of Advanced Nanomaterials,
Wuhan University, China. The sample was
single-layer MoS2, synthesized by chemical
vapor deposition (CVD).

D

Raman image

Si

E2g

Figure 7: Analysis of grain boundaries and strain on monolayer CVD MoS2.
A: Optical image.
B: SHG intensity image. Grain boundaries are indicated by white arrows.
C: THG intensity image.
D: Raman image displaying the position of the A1g mode (see panel E).
E: Raman spectra averaged over the dark blue areas in panel D (A1g peak positions between 395.0 cm-1 and 395.5 cm-1 relative wavenumbers). The peak at
520 cm-1 originates from the silicon substrate.
Scan range for Raman, SHG, THG: 150 x 150 pixels in 30 x 30 µm2.
WITec is grateful to Prof. Lei Fu (Laboratory of Advanced Nanomaterials, Wuhan University, China) for granting access to his Raman microscope.

In the optical image, a crack in the crystal
is faintly visible, but no other features canbe discerned (Figure 7A). For more information, SHG and THG measurements were
performed simultaneously (Figure 7B, C).
SHG is a nonlinear optical processthat radiates a photon with twice thefrequency of the excitation photon. The
SHG theory requires non-centric symmetry
and is sensitive to changes in crystal
symmetry, layer thickness, stacking orderand crystal orientation [9]. SHG imaging isthus very useful for characterizing crystal
structures of 2D materials. The SHG image
revealed several grain boundaries in the
MoS2 flake (Figure 7B) that were not visible
in the optical or THG images.
Further lattice features, such as the strain/
doping change at the boundary, could

be revealed by high-resolution Raman
imaging (Figure 7D, E). The exact positionsof the in-plane E2g and out-of-plane A1g
peaks (Figure 7E) were quantified for every
image pixel by fitting a double Lorentzianfunction. In the resulting Raman image,the frequency of the A1g mode was color
coded (Figure 7D). In all recorded Raman
spectra, both the E2g and A1g Raman modes were obviously red-shifted comparedto published data on monolayer MoS2 [8,10]. A calibration artifact can be ruled out
because the Rayleigh peak and the silicon
Raman peak were at 0 cm-1 and 520 cm-1
relative wavenumbers, respectively, as
expected. The red-shift of the E2g and A1g
peaks implies that the investigated flake
was under strong tensile strain over its
entire area. However, the high-resolution

Raman image revealed local differences,such as areas with reduced strain (yellow
in Figure 7D). A similar study recently
used multi-photon, Raman and photoluminescence microscopy for visualizing
grain boundaries in monolayer MoS2 [8].
Correlative SHG and Raman imaging could
not only make the boundaries visible
but also demonstrate the strain change,
providing a comprehensive understandingof crystal symmetry and lattice. These
examples illustrate the utility of correlative
non-linear imaging (SHG/THG) and Raman
imaging for characterizing 2D materials
and optoelectronic, ferroelectric or ferromagnetic devices. Changes of structureand strain at the grain boundaries could
easily be visualized at the same sample
location.

References[1] B. Kann, M. Windbergs, Chemical imaging of drug delivery systems with structured surfaces – a combined analytical approach of confocal Raman
microscopy and optical profilometry. The AAPS Journal, 2013, 15: p. 505-510. DOI: 10.1208/s12248-013-9457-7.
[2] M. Kuball, Raman spectroscopy of GaN, AlGaN and AlN for process and growth monitoring/control. Surface and Interface Analysis, 2001, 31: p. 987999. DOI: 10.1002/sia.1134.

[3] J.T. Holmi, B.H. Bairamov, S. Suihkonen, H. Lipsanen, Identifying threading dislocation types in ammonothermally grown bulk α-GaN by confocal
Raman 3-D imaging of volumetric stress distribution. Journal of Crystal Growth, 2018, 499: p. 47-54. DOI: 10.1016/j.jcrysgro.2018.07.024.

[4] U. Schmidt, W. Ibach, J. Müller, K. Weishaupt, O. Hollricher, Raman spectral imaging — A nondestructive, high resolution analysis technique for local
stress measurements in silicon. Vibrational Spectroscopy, 2006, 42: p. 93-97. DOI: 10.1016/j.vibspec.2006.01.005.

[5] I. De Wolf, Micro-Raman spectroscopy to study local mechanical stress in silicon integrated circuits. Semiconductor Science and Technology, 1996, 11:
p. 139-154. DOI: 10.1088/0268-1242/11/2/001.
[6] J. Jiruše, M. Haničinec, M. Havelka, O. Hollricher, W. Ibach, P. Spizig, Integrating focused ion beam–scanning electron microscope with confocal Raman microscope into a single instrument. Journal of
Vacuum Science & Technology B, 2014, 32: 06FC03. DOI: 10.1116/1.4897502.
[7] U. Schmidt, K. Hollricher, P. Ayasse, O. Hollricher, Correlative RISE microscopy: Raman imaging
meets scanning electron probe microscopy. Microscopy and Analysis, 2015, 29: p. 24-27.

[8] L. Karvonen, A. Säynätjoki, M.J. Huttunen, A. Autere, B. Amirsolaimani, S. Li, R.A. Norwood,
N. Peyghambarian, H. Lipsanen, G. Eda, K. Kieu, Z. Sun, Rapid visualization of grain boundaries inmonolayer MoS2 by multiphoton microscopy. Nature Communications, 2017, 8: 15714.
DOI: 10.1038/ncomms15714.

[9] X. Yin, Z. Ye, D.A. Chenet, Y. Ye, K. O‘Brien, J.C. Hone, X. Zhang, Edge nonlinear optics on a MoS2
atomic monolayer. Science, 2014, 344: p. 488-490. DOI: 10.1126/science.1250564.

[10] H. Li, Q. Zhang, C.C.R. Yap, B.K. Tay, T.H.T. Edwin, A. Olivier, D. Baillargeat, From Bulk to Monolayer
MoS2: Evolution of Raman Scattering. Advanced Functional Materials, 2012, 22: p. 1385-1390.
DOI: 10.1002/adfm.201102111.

[11] J. Toporski, T. Dieing, O. Hollricher, eds. Confocal Raman Microscopy. 2nd ed. 2018, Springer
International Publishing AG. DOI: 10.1007/978-3-319-75380-5.

For more information on confocal Raman
imaging and its applications, see the book:
Confocal Raman Microscopy [11]WITec Microscopesalpha300 R

alpha300 Raman-AFM-SNOM
apyronautomated Raman-AFM
alpha300 Raman-AFM
alpha300 access

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