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

*ymchang@ntu.edu.tw

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