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Introduction to Cathodo-luminescence (CL)
陰極螢光分析系統簡介
Department of Electrophysics,
National Chiao Tung University (NCTU)
電子物理系周武清 Wu-ching Chou
CL
Scanning electron
microscopy (SEM)
Luminescence
analysis system
References:
1. Physical Principles of Electron Microscopy
By Ray F. Egerton, Springer
2. Electron Microscopy
By J.J. Bozola and L.D. Russell,
2015/05/28
Jones and Bartlett Publishers
Cathodo-luminescence (CL)
Principle：the samples are excited by high energy (30 keV)
and nano-meter scale e-beam to produce luminescence signal
and second electron for the investigation of nano-meter scale
composition distribution, morphology and photonic
properties.
Features of system:
broad spectrum range：IR(2000nm) to UV (180nm)
Broad temperature range：4K to room temperature
highest spatial resolution (1.5 nm at 15KV)
Principle of electron microscope
Electron sources: tungsten
filament, LaB6, Schottky
emitter, or tungsten field
emission tip
30 KV < 100 KV of TEM
Beam size < 10 nm
Much smaller than TEM 1μm
Spatial resolution > beam size
Principle of electron microscope
Principle of electron microscope
Principle of electron microscope
SEM
TEM use a stationary incident beam
x, y scan
(a)Line scan waveform (x), (b) frame scan waveform (y),
(c) Digital equivalent, and (d) a single-frame raster scan
(x + y)
Detector’s signal (secondary or backscattered electrons)
to drive the brightness
Penetration of electrons into a solid
Backscattered electron BSE
Secondary electron (SE)
Inelastic scattering
elastic scattering ≈ Z2
Back scattered primary
electron ≈ 30 keV
(elastic scattering)
SE < 100 eV
Monte Carlo simulation
25 primary electrons of 10 nm
(a)30 keV, 6.4 μm, (b) 10 keV, 0.8 μm, and (c) 3 keV,
0.12 μm, for Al (Z=13), (d) 30 keV, 1.2 μm for Au (Z=79)
Secondary electron (SE) image
Cross section diagram of a specimen
Sample tilt angle
SE yield 0.1 to 10
(average escaping SE/primary)
SE signal, detector is located to
the right of the specimen
SE < 100 eV, escape depth < 2nm, topographical image
Back scattered primary electron ≈ 30 keV
(elastic scattering), penetration depth ≈ μm
(a) SE image of a small crystal; using a side-mounted
Everhart-Thornley detector.
(b) BSE image recorded by the same side-mounted
detector.
Everhart-Thronley detector to improve the SEM signal
Compound eye of an insect, coated with gold to make
the specimen conducting. (a) SE image recorded by a
side-mounted detector (located toward the top of the
page) and showing a strong directional effect, including
dark shadows visible below each dust particle. (b) SE
image recorded by an in-lens detector, showing
topographical contrast but very little directional or
shadow effect.
Backscattered electron (BSE) image
Robinson detector: an annular (ring-shape) scintillator
Feature of JSM-7000F
 Electron gun: In-lens Shottky Thermal FEG (0.5~30KV)
 Resolution:HR condition 1.5nm(15KV), 3.0nm(1KV)
Analysis 3.0nm(5nA,15kV,WD 10mm)
 Probe current:a few pA to 200nA(continuous)
 OBJ lens: super conical lens
 Specimen exchange:one action mechanism (150mm,)
 Specimen stage:X=70mm,Y=50mm, WD=3 to 41mm,
tilt=-5 to 70°,R=360°
 Option stage: X=110mm,Y=80mm, tilt=-5 to 60° (8 inch wafer)
 Motor control:X,Y rotation (standard),WD,Tilt(option)
 Digital Image display:1280 x 1024 pixels(for live and stored image)
 Operation system:Window XP
 Take off angle: 35° for EDS and WDS
 Option attachment: EDS, CL,
Emission spectra
Band information
e-beam
Morphology:
Primary electron,
secondary electron
Holes in VB
Issue of spatial resolution and spectrum resolution.
PMT, CCD, length of spectrometer
Comparison of PL and CL
Photoluminescence (PL)
Cathodoluminescence (CL)
Closed Refrigerator
computer
15K
PL Intensity (a.u.)
ZnTe
L1
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
Photon Energy (eV)
L2
Argon - ion Laser
L3
PMT
SPEX 1403
Double-Grating
Spectrometer
Issue of spatial resolution and spectrum resolution.
Spectrum resolution:(1)specrtometer length, (2) # of gratings,
and (3) # of groves/mm
E=1239.8 nm eV/ λ Available lasers:
He-Ne: 632.8 nm (1.96)
He-Cd: 325 nm (3.81eV)
Ar : 488 nm (2.54 eV), 514.5 nm (2.41 eV)
Kr, CO2, YAG …..
大自然分光儀
Diffraction Grating
Photomultiplier tube
Quantum efficiency
Thermionic emission
Dark current
Wavelength dependence of
quantum efficiency
M=δn
Intrinsic semiconductor
N-type semiconductor
P-type semiconductor
excitation
emission
可能發生那些光學躍遷?
si
si
si
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si
si
si
si
si
si
CB
D
Eg
VB
donor
si
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si
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si
N+
si
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si
N-type semiconductor
(1) Free exciton X 自由激子
(2) Neutral donor (acceptor) bound
exciton 施子(授子)束縛激子
(3) Donor-acceptor (D-A) recombination
D-A 1LO, DA 2LO
(3) Phonon assisted recombination
DA-1LO, DA-2LO, DX-1LO ……
可能發生那些光學躍遷?
D(n=3)
D(n=2)
D(n=1)
CB
D
VB
donor
si
si
si
si
si
si
N+
si
si
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si
si
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Donor-valence band (D-VB) recombination N-type semiconductor
Ionization energy of donor level Ei=ED(n= ∞)-ED(n=1)
Neutral donor bound exciton
Se Zn
Zn
Cl+ Zn
Se Zn
Zn
Se Zn
Se
Se Zn
Se Zn
Se
Emission spectra
Band information
e-beam
Morphology:
Primary electron,
secondary electron
Holes in VB
Fixed wavelength + e-beam scanning
Fixed e-beam position and spectrum analysis
Deep Ultraviolet Light–Emitting Hexagonal Boron Nitride
Synthesized at Atmospheric Pressure
Science 317, 932 (2007)
Application: information storage, medical treatment, ….
Fig. 1. Optical micrographs of recrystallized hBN obtained with
a Ni-Mo solvent. (A) Typical hBN crystal on the solidified solvent
(as grown). (B) A fragment of aggregate hBN crystals after acid
treatment (the inset is an optical micrograph of a recovered
sample). The shiny white regions are reflected light.
There is no excitation laser for PL.
CL
X
BX
E=1239.8 nm eV/ λ
Fig. 3. Cathodoluminescence spectra of recrystallized hBN at
room temperature. (A) hBN obtained with a Ni solvent. (B) hBN
obtained with a Ni-Mo solvent. (C) Direct quality comparison of
the emission characteristics. Solid line, hBN obtained with a NiMo solvent at atmospheric pressure (in this study); dotted line,
hBN obtained with a Ba-BN solvent at high pressure and high
temperature (HP-HT).
Fig. 4. Images of hBN crystal grown on the a-plane sapphire
substrate (obtained with a Ni-Mo solvent prepared at 1400°C).
(A) Differential interference microscopic image. (B)
Cathodoluminescence image for 215-nm band. We did not find
any intensity change of the measured spectra between the
grain boundary and the plane surface area when measuring the
point-to-point mode of the cathodoluminescence system, where
the electron beam remained stationary and the measured
luminescence was confined to the exposed spot area.
Ultraviolet Emission from a Diamond pn Junction
Science 292, 1899 (2001)
N-type
p-type
Fig. 1. Impurity depth profile of pn junction measured by SIMS,
where solid circles represent phosphorus (31P) and open
squares represent boron (11B).
Fig. 2. Representative I-V characteristics of diamond pn junction.
In the linear plot (A), the voltage shows the applied voltage to ptype diamond. In the semilogarithmic plot (B), the forward
direction corresponds to the case when the negative voltage was
applied to n-type diamond.
Fig. 3. Optical emission spectra
of the pn junction operated with
forward current of (A) 1, (B) 5,
and (C) 10 mA. A representative
CL spectrum of P-doped
diamond thin film taken at room
temperature is shown in (D). The
inset is a representative optical
image of the diamond LED with
light emission. The circularshaped electrodes (diameter, 150
μ m) are formed by the
separating each other by 150 μ
m. Light emission can be seen
around the electrode located at
the center of the image.
Microstructural evolution
in m-plane GaN growth on
m-plane SiC
APL 92, 051112 (2008)
FIG. 3. Color online a–c Plan-view SEM images, d perspective
view SEM image of sample C near-edge area, and e cross
sectional view SEM image of sample C fully coalesced center
area. The blue rectangular and red oval dotted lines in d mark out
the regions with partial and full coalescence along the c-axis
between adjacent mesas, respectively.
FIG. 4. Color online a Plan-view SEM image and b micro-CL
monochromatic mapping collected at 380 nm of a near-edge area
of sample C. The blue rectangular I and red circular II dotted lines
mark out mesas with a trapezoidal and an irregular shape,
respectively.
APPLIED PHYSICS LETTERS 91, 251911 2007
Impacts of dislocation bending and impurity
incorporation on the local cathodoluminescence
spectra of GaN grown by ammonothermal method
Spatially resolved cathodoluminescence CL spectra of GaN films
grown on freestanding GaN seeds via fluid transport by the
ammonothermal method were correlated with the microstructure
and growth polarity.
FIG. 1. Color online (a) and (c)
Plan-view and cross-sectional
SEM images of Ga-polar and (b)
and (d) N-polar ammonothermal
GaN films grown on a 400-m-thick
freestanding HVPE GaN seed
wafer. (e) Widearea CL spectra at
100 K and PL spectra at 293 K of
the same GaN films. For
comparison, PL spectrum of the
HVPE GaN seed is shown in (f).
FIG. 2. a Wide-area and spotexcitation CL spectra at 100 K,
b representative local SEM
image, and c–f corresponding
monochromatic CL intensity
mapping images recorded for
various photon energies at 100
K of the 5-μm-thick Ga-polar
ammonothermal GaN film. The
spectra numbered 1–6 in a were
taken at the numbered spots in
b–f. In c–f, white areas
correspond to those emitting
bright lights.
FIG. 4. a Wide-area and spot-excitation CL spectra at 100 K, b
representative local SEM image, and c monochromatic CL
intensity mapping images recorded at 3.50 eV at 100 K of the 4m-thick N-polar ammonothermal GaN film. The spectra numbered
1–4 in a were taken at the numbered spots in b. In c, white areas
correspond to those emitting bright lights.
Spatial fluctuations of optical emission from
single ZnO/MgZnO nanowire quantum wells
Nanotechnology 19 (2008) 115202
MgZnO/ZnO quantum wells on top of ZnO nanowires were
grown by pulsed laser deposition. Ensembles of spatially
fluctuating and narrow cathodoluminescence peaks with single
widths down to 1 meV were found at the spectral position of the
quantum well emission at 4 K. In addition, the number of these
narrow QW peaks increases with increasing excitation power in
micro-photoluminescence, thus pointing to quantum-dot-like
emission centers. Indeed, laterally strained areas of about 5 nm
diameter were identified at the quantum well positions on top of
the nanowires by high-resolution transmission electron
microscopy.
(30nm)
(2nm)
O partial pressure
Ar partial pressure
Figure 1. Idealized growth scheme of the MgZnO–ZnO–
MgZnO heterostructures on top of ZnO nanowire arrays (only
one wire shown here) with the indicated number of laser
pulses for each PLD growth step. In addition to the shown
axial growth, also some radial growth took place.
Figure 2. Typical SEM images of the ZnO nanowire arrays on aplane sapphire with ZnO–MgZnO QW structure on top of the wires.
Three different samples with wire diameters of 150 nm (top, ex
situ process) and 1 and 2.5 μm (bottom, both by in situ process)
are shown. The ex situ growth route typically yields structures of
smaller diameter than the in situ process.
Figure 4. Cathodoluminescence spectra of a single
nanowire taken at RT. Emission bands from ZnO nanowire,
ZnO QW and MgZnO barrier are observable, as indicated
in the image. The QW emission is dominant at RT.
Figure 6. Strong dependence of low temperature CL spectra on electron beam
spot position on top of a single selected nanowire with QW structure, grown (a)
by an in situ process within one chamber and (b) by a two-step ex situ process
with ZnO nanowire and MgZnO–ZnO–QW structure from different PLD
chambers. The wire diameters are 2 μm (a) and 150 nm (b). The spectra were
taken at positions a few tens of nanometers apart. The Mg content of the
ZnMgO barriers is 28% (a) and 21% (b), respectively.
Figure 7. Temperature
dependence of the CL
signal measured
approximately at the same
lateral position of a wire.
The additional features on
the QW band are
observable up to 110 K.
Figure 8. Comparison between micro-PL spectra of a
nanowire under different excitation conditions. The number of
lines is increasing with increasing excitation power at 5 K.
Here, P is the excitation power and equals 10 μW.
Incorporation of Ga in ZnO/GaN epitaxial films
APPLIED PHYSICS LETTERS 92, 131905 2008
Growth of zinc oxide ZnO layers on gallium nitride GaN substrates
benefits from the small lattice mismatch of these two materials.
We report on spatially resolved cathodoluminescence studies of
ZnO layers grown by a modified chemical vapor deposition
process on GaN templates deposited on sapphire substrates. Line
scans across the ZnO/GaN interface reveal the incorporation of
gallium from the template into the ZnO layer. Transmission
electron microscopy and micro-Raman measurements both
indicate that strain relaxation occurs within a distance of a few
nanometers from the ZnO/GaN interface. The diffusion coefficient
of gallium in ZnO is determined.
FIG. 1. CL spectrum obtained by exciting a 55 m2 area
of the surface of the sample.
FIG. 2. SEM micrograph, pseudocolor representation of the
SEM-CL line scan, and the corresponding spectra recorded
from the cross section of the sample.
Optical studies of dislocationfree GaAs nano-wires grown on
trenched Si (001) substrate by
cathode-luminescence
Dr. Ling Lee & Prof. Wu-Ching Chou(周武清)
Department of Electrophysics,
National Chiao Tung University, Hsinchu,
Taiwan, Republic of China
Lee et al. Nanotechnology 21, 465701 (2010)
Strengths and challenges of GaAs nano-wires on Si
Higher carrier mobility of GaAs
benefit HEMT in nano-scale
3/16
Dislocations and antiphase
domains degrade the device
performance
Reduced dislocation in GaAs by shallow trenches
Threading dislocations extend along
<110> at GaAs (111).
Is it possible to block threading dislocation
by trench?
[001]
[110]
[110]
Threading dislocation
free region
Threading
dislocation
4/16
Reduced dislocation in GaAs by shallow trenches
For nano-wire width <150nm, the real
emission volume is reduced which results a
weaker intensity.
Emission efficiency could be calculated for
evaluating structural quality in terms of
defect density near surface
5/16
L. Lee, W. C. Chou et al.
Nanotechnology 2010
Lee et al. Nanotechnology 21, 465701 (2010)
(a) Plan-view SEM image for GaAs nano-wires grown
on nano-scale shallow-trench-patterned (001) Si. Planview TEM images along [110] with traench width of (b)
1000, (c) 80, and (d) 40 nm.
Lee et al. Nanotechnology 21, 465701 (2010)
Reduced dislocation in GaAs by shallow trenches
1000nm SEM
1000nm CL
80nm SEM
80nm CL
6/16
L. Lee, W. C. Chou et al.
Nanotechnology 2010
Low-temperature cathode-luminescence
GaAs grown on Si wafer exhibit a
reduced eA emission than that grown
on GaAs substrate
A red-shift and splitting of near band
edge emission are observed for GaAs
grown on planar Si wafer
As trench width is reduced, red shift
and splitting of near band edge
emission is reduced
9/16
Biaxial strain and band structure splitting
heavy hole E g
light hole E g
3 3
C C
C  2C12

 E g 0  a i  b a  E g 0  [2a 11 12  b 11
]   lateral
2 2
C11
C11
3 1
C C
C  2C12

 E g 0  a i  b a  E g 0  [2a 11 12  b 11
]   lateral
2 2
C11
C11
 i  2 lateral   vertical ,  a   lateral   vertical
11/16
S. C. Jain et al.
Semicond Sci Tech 1996
Temperature-dependent CL analysis
Homoepitaxial GaAs epilayer shows exciton emission obeys the
Varshini’s prediction
A decreased splitting is observed for GaAs grown on planar Si
wafer, indicating a thermal induced strain
10/16
Biaxial strain and band structure splitting
GaAs grown on planar Si wafer exhibit
coherent strain at growth temperature,
and a biaxial tensile strain during
cooling down
12/16
Non-biaxial strain for GaAs nano-wire in trenches
For GaAs grown in trenches with width
>500 nm, a biaxial strain remains
For GaAs grown in trenches with width
<300nm, splitting is reduced
13/16
Uniaxial stress induced by SiO2 masks
A (001) direction tensile stress is
induced near SiO2 mask because of
less thermal expansion coefficient of
SiO2 than GaAs
14/16
Lee et al. CrystEngComm (to be published)
Lee et al. CrystEngComm (to be published)
Lee et al. CrystEngComm (to be published)
Conclusion:
The cathodoluminescence system consists of SEM, low
temperature stage, and spectra analysis system.
It is a powerful tool to study both the emission properties
and morphology of the nano-structured material.