Mapping the three-dimensional stress distribution of GaN-based light emittingdiode with confocal Raman and photoluminescence spectromicroscope
Hui-Yu Cheng1, Wei‐Liang Chen1, Yi-Hsin Huang1, Tien‐Chang Lu2, and Yu‐Ming Chang1*1 Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
2 Department of Photonics, National Chiao Tung University, Hsinchu, Taiwan
Recently patterned sapphire substrate (PSS) has become widely used for growing GaN-based light emitting diode (LED) nanostructures,
such as InGaN/GaN superlattices (SLs) and multi‐quantum wells (MQWs). The LED active layer greatly increases the emission efficiency
while allowing the tunability of emission wavelength. However, it was suspected that epi-growth related defects (v-pits) in the MQWs could
be associated with the droop of the LED emission efficiency. To investigate this issue, we performed Raman and PL mapping on a typical
GaN-based LED nanostructure grown on PSS with our home-built laser scanning confocal spectromicroscope. It was found that both the
spatial inhomogeneity in the PL and Raman mapping of the MQWs layer can be directly correlated with the hexagonal structure of PSS and
the position of v-pits. The variation in the PL peak and Raman line shift clearly reveals the stress distribution in the MQWs layer. Our results
clarify the propagation and relaxation of stress originated from the PSS toward the LED surface.
Abstract
AcknowledgementThis work was supported by the National Science Council under
the grant No. NSC102-2119-M-002-015-MY3.
References[1] S. Nagarajan et al., Appl Phys Lett 104, 151906 (2014).
[2] W.-L. Chen et al., Rev Sci Instrum 84, 113108 (2013)
Axial Mapping of E2(high) Mode
These figures show E2(high) intensity mapping at various sample
depths, where 0 mm corresponds to the sample surface and
increases towards the substrate. The measured E2(high)
bandwidth was set to 20 cm-1.
60 pairs InGaN/GaN SLS insertion layer
(1.1 nm InGaN wells, 7nm GaN barrier)
Substrate
9 pairs InGaN/GaN MQWs (In ~22%)
6 pairs shallow InGaN/GaN MQWs (In ~15%)
12 nm GaN barrier , 2.5 ~3 nm InGaN wells
The GaN LED sample was grown using metal organic chemical
vapor deposition system (MOCVD) on a pattern sapphire substrate.
The layer structure is shown above. The active region consists of
15 pairs of InGaN/GaN MQWs. No p-GaN capping layer was
grown in order to explore the effects of the v-pits.
Sample Description
Pattern Sapphire Subtrate: Hexagonal Pattern
Surface: V-pit on MQW
1.1 nm InGaN well
7nm GaN barrier
InGaN/GaN SLS
AFM image SEM image
D=2.5 um
H=1.5 um
r
Conclusion1. PL mapping of MQW layer shows that the MQW can be divided
into v-pits, bright, and dark areas. The dark and bright areas form
pattern that can be directly correlated to PSS structure.
2. E2(high) peak position mapping shows stress variation which can
be also correlated with PSS pattern.
3. 3D construction of the axial E2(high) mapping reveals that the v-
pits in MQW can be traced to the protruded tips of PSS.
Experimental Setup
All the experiments were performed on a home-built confocal
spectromicroscope using a 100x NA 0.9 objective, providing ~0.3 μm
spatial resolution and ~1 μm axial resolution. PL and Raman
mapping were obtained using 375 nm and 532 nm laser excitation
respectively. Raman spectra were acquired with JY-FHR640
spectrometer with liquid nitrogen cooled CCD, and PL spectra were
acquired with a home-built spectrometer. For E2(high) Raman
mapping, the signal was detected with a PMT in combination with a
monochrometer.
3D Construction of Raman Mapping
The 3D reconstruction of
axial E2(high) Raman
mapping. The two lower
dark areas (red arrows)
correspond to the tip of
two protruded patterns in
PSS. From this three-
dimensional image plot,
the origin of the two v-
pits (yellow arrows) in
MQW can be traced to
the two tips in PSS.
PL and Reflectivity Mapping
(a)-(c): Reflectivity mapping of PSS, and (d)-(f): PL mapping of
MQW in different scales. (g),(h): Fast Fourier transform (FFT) of
images (a) and (d) reveal that PL patterns in MQW is directly
correlated with the structural pattern in PSS. The PL mapping of
MQW layer can be divided into three areas: v-pit, bright area, and
dark area. Normalized spectra correspond to these three areas as
shown in (i).
(g) (h)
(i)
(a) (b) (c)
(d) (e) (f)
PSS
MQW
PSS MQW
(a), (b): The mapping of E2(high) phonon peak intensity of MQW
and PSS. (c), (d): The mapping of E2(high) phonon peak position
of MQW and PSS, where the peak position is determined by
Gaussian peak fitting. (e) shows a representative Raman
spectrum inside the sample. (f) Plot of peak position variation
along the white lines shown in (c) and (d).
Raman Mapping
n-GaN layer～ 3 μm
u-GaN layer ～ 5 μm
Pattern sapphire
SLS ~ 108 nm
MQW～225 nm
350 400 450 500
0.0
0.5
1.0
a
b
c
Wavelength (nm)
350 400 450 500
0
200
400
600
800
1000
1200
1400 a
b
c
Co
un
ts
Wavelength (nm)
b
c
a
0 1 2 3 4 5 6 7 8 9 10
569.0
569.1
569.2
569.3
569.4
569.5
569.6
569.7
569.8
569.9
570.0
570.1
570.2
570.3
570.4
570.5
E2(h
igh
) P
eak S
hif
t (c
m-1)
Position (mm)
(c)
(d)
400 450 500 550 600 650 700 750
1000
1500
2000
2500
Ram
an In
ten
sity
(a.
u.)
Raman shift (cm-1)
(e)
0 μm 1.85 μm
5.55 μm3.7 μm 9.25 μm7.4 μm 11.1 μm
-1.85 μm-3.7 μm-5.55 μm
Surface
PSS
