The optical instabilities and blinking
phenomena in the emission of InGaN
quantum wells
R. Micheletto, K. Oikawa, J. Komatsu
横浜市立大学
Yokohama City University

生命ナノシステム科学研究科
Graduate School of Nanobioscience12108Column 1Column 2Column 3

6420
Row 1

Row 2

Row 3

Row 4

Commercial InGaN device
This is a commercial InGaN device with high output.

There are many dislocation because ofmismatch in the lattice parameters for
substrate and crystal.

Many dislocation

Growth on SapphireNormalized Intensity (arb. units)

Spectral characteristics of InGaN device
GaN (undoped) 5nm
InxGa1-xN 3nm
GaN (undoped) 4μmSapphire

Samples central emission peak ：420nm,460nm,510nm,540nm

発光中心波長狙い：460nm：510nm：540nmPL Intensity (arb. units)

700

wavelength (nm)
600
500
400

Ex：365nm

22.5
3Photon energy (eV)

700

600

500

400

発光中心波長狙い：420nm：460nm：510nm：540nm
Ex：365nm

22.5
3Photon Energy (eV)Selective Excitation
3CCD
Camera①InGaN/GaN excitation
GaN InGaN GaNe

Microscope
MercurylampeesubstrateSample

surface

excitation

hhh

Moving Stage②InGaN selective excitationLowpass filter①400 nm ～②435 nm ～ Dichroic mirror
Interference filter
Reflection:① ～ 400nm①365 nm/10nm② ～ 430nm②405 nm/10nm
Transmission:①400nm ～② 430nm ～GaN InGaN GaN

eesubstratehhsurface
excitation

Photoluminescence temporal variation
A sample of 510nm central emission, Room temperature,Continuous excitation Hg lamp 365nm

After oneminuteExcitation start

After one minute ofcontinuous illumination

PL Spectrum

Intensity (arb.units)

wavelength (nm)800600700500(a)

22.5
33.5
photon energy (eV)Intensity (arb.units)

Intensity (arb.units)(a)20The yellow band peak
appears to be steady andunchanged

Main Peak

Yellow band10(b)There is a steep rise atthe beginning

1.50Yellow Band

40030Time (sec)40(b)

0102030
Time (sec)40

The first rise shape andtiming can be an artifact
related to the shutter
mechanism.
λ ＝３６５ nm

The behavior of themain peak
（ Center at 460nm
sample ）Intensity (arb.units)

PL Main peak rise behavior

λ ＝３６５ nm

1020Time (sec)Intensity (arb.units)

03040

PL Rise (expanded)

Steep rise and then slow
constant profile
00.2 0.4 0.6 0.8
Time (sec)1The rise characteristics

Intensity (arb.units)

Center wavelength of samples
15
λ ＝３６５ nm

Y0-A1exp(-t/τ1)-A2exp(-t/τ2)
Fitting Model

0102030

Time (sec)

40

●：τ１

○：τ２

1050400450500λ(nm)550Wavelength dependence
Ex:405nm
Intensity (arb.units)

Intensity (arb.units)

Ex:365nm
off:10min
Y0-A1exp(-t/τ1)-A2exp(-t/τ2)

0102030Time (sec)40

off:10min

Y0+A1exp(-t/τ1)

0102030
Time (sec)40

Selective excitation at λ ＝ 405nm
PL Intensity (a.u.)

460nm
540nm

10.90wavelength (nm)
600 550
500510 nm sample

0 s.
40 s.

eV-510nm
The behavior is complex
22.2
2.4
2.6-10It depends
on2030 Indium
40 concentration.
- Differences
Time (sec) are small in value, but curves have dissimilar profile

Intensity (a.u.)

Intensity (a.u.)

1.1

1
0.8
0.6
0.4
0.200

Ex:405nm
at 534.29nm
10 20 30 40
Time (sec)

10.8
0.6
0.4
0.2
00

Ex:405nm
at 504.36nm
10 20 30 40
Time (sec)Exposition “History” was found to be important

Design of ad hoc experiment

A routine of illumination and darkening Intervals

Sequence

40 secmeasure40 secIllumination

40 secmeasure40 secmeasureDark (light off)
1 hour5 min

10 min15 min

5

●：τ１
○：τ２

τ (sec)

43(a)

2
10

λ ＝３６５ nm

40sec up010

20 30 40 50
Dark-time (min)60・・・・・40 minSequence

40 sec40 sec measure

Illumination

40 secmeasure40 secmeasure・・・・
Dark (light- off)
Increase of dark-time provoke a longer time constant・
1 hour50 min40 min30 min- Graph (b) is the same as (a) in reverse time order- At corresponding times, in (b) the time constants are almost double.
- This demonstrate an evident memory of the process history.
8○：τ２τ (sec)●：τ１

640sec down(b)42

λ ＝３６５ nm

00

1020

30

40

Dark time (min)5060・・・・・min
20

Wavelength (nm)
470
460
450
64

Ex:365nm

202.6

2.65

2.7

PL (Imx/Io)

460nm

PL (a.u.)τ (sec)8

Wavelength (nm)
470
460
450

2.75 2.8eV

2
102.6

460nm
Ex:365nm
2.652.7- For higher energies, we have higher time constants- Opposite behavior for intensities

2.75 2.8eV

Model concepts
EE- Distortion of the band can induce observable PLZintensity variationsZInGaNEGaN

GaN (undoped) 5nm
InxGa1-xN 3nm

λ ＝３６５ nm- Distortion of band due to trapped carriers

GaN (undoped) 4μm- Band overlap
GaN is induced, F^2 increase and PLSapphireincrease- We are observing the time constant of thisdeformation process
ZEEZ

E

λ＝40 ５ carriers
nm- Distortion of band due totrapped

GaN (undoped) 5nm
InxGa1-xN 3nm

GaN (undoped) 4μm- Band separation is induced, F^2 decrease and PL
decrease as wellSapphire- We are observing the time constant of thisdeformationZprocess
EEZZ

Commercial InGaN device
This is a commercial InGaN device with high output.

There are many dislocation because ofmismatch in the lattice parameters for
substrate and crystal.

Many dislocation

Growth on Sapphire

Instability Blinking Phenomena)

Two images taken from the CCD camera few seconds apartmicro Blinking in InGaN LEDs

micro Blinking in InGaN LEDs
A 540nmemitting InGaN
LED test
material
GaN (undoped) 5nm
InxGa1-xN 3nm
GaN (undoped) 4μm

Normalized Intensity (arb. units)

Sapphire
700

600

500400発光中心波長狙い：420nm：460nm：510nm：540nm
Ex：365nm

2

2.5
3Photon Energy (eV)

Optical microscope images
Centralemission

420nm

ext.
365nm
power
7.5W/cm2ext.
405nm
power
2.65W/cm2

460nm

510nm

540nm

Optical images Dependence on power
1.4W/cm2
exc.
365nm

exc.
405nm

0.23W/cm2

Intense domains
appears moredominant

No prominentchangesSome quantitative parameter
What parameter to measure?
This is a real system with a non ideal 3D potential

Quantitativeanalysis

Not clear explanationof slow behavior andmulti-level blinking

・ Observe and analyse with CCD・ Observe in near-field with a SNOM device
・ Measure the time-series characteristics.
. Avoid instrumental artifacts

CCD data analysis method
Deviation Mapping

The time-histogram of blinking
・ a ”Telegraphic” signal of on/off bits separated by few seconds
・ a ”multilevel” random-noise like signal

Bistable signal

Noise-like signalFast Fourier Transform(FFT)Frequency
characteristics

Peak in the range of 5~10Hz

Are blinking points ”interesting”?
Studying the blinking phenomena on LED device isimportant somehow?

We found blinking points on LED just bought in the local
shop (Akihabara)

If we understand the mechanism
involved we can imagine to improve
LEDs and make them more efficient
Hi-res Measurements Scaning Near-field Optical Microscope(SNOM)

３、 SNOM (Setup)
・ UV laser （３２５ nm ）（ collection mode ）・ pensil type probe・ room temperature

４、 sample structure (SampleData)

We used 540nm, 510nm
centered samples

５、 Results（１） We could see the typical opticalemission spatial distribution of InGaNdevices.

（２） Fixing the optical fiber in one single point,and recording the change in intensity, FFTanalysis shows similar peaks around 5-10 Hz

Signal (Time/Intensity) by SNOM

Signal by SNOM (Time/Intensity)

This is a blinking point before FFT analysis. Thereare 2250 samples in this plot. (12 ms rate)Results （２）Signal by SNOM (Time/Intensity)

FFT (Frequency/Intensity)Signal shapes

508060304020201000Imax

Intensity (arb.units)

Intensity (arb. units)

401020Time (sec)telegraphic

3000Imin
1020Time (sec)30multilevel

Maybe the quantum localization is degenerated in somedomain.

Temperature dependence
Sample 510nm, room temperatureand low temperature

Difference of behavior
at lower temperature
Intensity (arb.units)

120
100300K

604020001020Time (sec)30120
Intensity (arb.units)

冷却

8010080150K

604020001020Time (sec)Trap seems to loose effect a lower temperature

30

The process of recombination
The carrier is trapped in the QD

Quantum dote
Deep trap

hOur samples.Interfacestate.dislocation.migration･ Non-radiative recombined process withlong longevity
・ Auger nonradiative process
・ Quantum
confinement Stark effectby internal electric field

In-rich areas play a role, do they behave like aQD?
At the GaN/InGaN interface is there a trap orsemi-trap , does migration play a role.

Reducedopticalemission

Quantum Jump model[5]R. J. Cook and H. J. Kimble, Phys. Rev. Lett., 54 1023 (1985)

Two types of transitionEx) |0 ＞ ： QD minimal state| 1 ＞： QD excited state|2 ＞ ： trapped state・ normally transition from |0>
to |1>(from exited state to groundU1→∞)｜ 1 ＞： Excited｜ 2 ＞： Trapped P2

P1

・ rare jumps to state 2
A(B) ： Einstein A(B) coefficients
P ： probability
U ： Energy potential

Exc light

A1

B1

B2U(hν)｜ 0 ＞： groundstate P0

Rate equations

A2

dP1=-A1P1+B1U1(P0-P1)
dt
dP2=-A2P2+B2U2(P0-P2)
dt

Quantum Jump model[5]R. J. Cook and H. J. Kimble, Phys. Rev. Lett., 54 1023 (1985)

onoff

Energy dependence[5]R. J. Cook and H. J. Kimble, Phys. Rev. Lett., 54 1023 (1985)- Our phenomena does not fit standard QD models- We have no clear explanation- We are working on ”fancy” models that are on theway of publication

Systematic analysis of single pixel time profile

yVideo data
(3D file)t(sec)

yx①Image is decomposed in 6
variables: time, x, y, R, G, B

Singleframestx

Intensity

Algorithmt② A single point (x,y) on sample
is isolate and its time profile
recordedSystematic analysis of single pixel time profile

yVideo data(3D file)t(sec)

yx①Image is decomposed in 6
variables: time, x, y, R, G, B

Singleframestx

Intensity

Algorithmt② A single point (x,y) on sample
is isolate and its time profile
recordedMap of Intensity captured from movie file
Every pixelon this map
is theintensity ofthe light at
t=0. The
region is
the one in
the grayeddashed box,
150x150
pixels,about 60x60
μm large.
Each pixel
is about
400x400 nm.

Map of standard deviation

Intensity

Intensity

This map is the70219y50199.txt Profile of (x143y134)
video18
standard
240
220
deviation of each200(c)pixel time profile.
180
For each pixel we160(c)140
have 450 data
120(frames) for one
050
100
150
200
250300350400450(frames)- SD analysis timeshows
spatial structures
minute of- SD video18
analysiscan be use to put foward crystal spatialrecording. The
70219y50199.txt Profile of (x8y22)
region is same as
200
characteristics
notvisibleoptically(b)180
above, 150x150
160
pixels. In simple
140
words: blue
120
100
points do not80
blink, brightpoints blinks andvideo1870219y50199.txt Profile of (x24y3)x
intensity gives a200180
measure of the of(a)160
blinking vigor in140
120
standard
100
deviation units.
80 050100 150 200 250 300 350 400 450
Intensity

x(b)
(a)

time (frames)

Time-Autocorrelation of Photoluminescence

R=0.0099
R=0.993000
1R=0.24

R=0.63
2.5

4

0.8

2

3

0.6

1.5

2

0.4
1000

10.2

0.500

0-0.2

-0.5

-0.4

-1

-0.6
-2000

-1.5

-3-0.8

-2-4

2000

-1000

-3000
-1005050

100 150150 200
200 250250 300
300 350350 400
400 450450
100-2.510-1-2050

100 150 200 250 300 350 400 450-5050

100 150 200 250 300 350 400 450

Map of Autocorrelation(a)video1870219y50199.txt Profile of (x144y134) and (x81y39)

260

(a)

240
220
200

- AC shows new spatial domain within the crystal

180
160

- These are not(b)visible optically140- AC will be studied for Sample Quality Control

120050

100

150

200
250
time (frames)

300350(b)400450In this map
each pixel is theautocorrelation
coefficient ofthe pixel time
profile. Also thismap show clear
domains andstructurescompatible with
the PL map. (a)and (b) points
are shown as
example ofhighlyautocorrelated
pixels.

Time-correlation between different
Photoluminescence domains

R=1.003
2.5
2

R=0.02
R=0.62

R=0.85
55

4

4

3

3

2

2

1

1

00

1.5
1
0.5
0
-0.5
-1-1050

100 150 200 250 300 350 400 450

-1050

100

150 200

250 300 350

400

450

050

100

150200

250

300

350

400

450

Map of Correlation

video1870219y50199.txt Profile of (x144y134) and (x78y42)

21.5

Intensity(a)10.5(b)

0050

100

150

200time (frames)

250

300

350

400450In this map eachpixel is thecorrelation
coefficient of thepixel time profile
with the time profileof point (144,134).
For each pixel wehave 450 data(frames) for one
minute of recording.
The region is sameas above, 150x150
pixels. Distant
blinking domainsthat showcorrelation better
than 60% are noted.
The map show
structurescompatible with thePL map.

- We detected intermittency in PL fluorescence in GaN/InGaN
sample- We found dependence with Indium concentration, Excitation
wavelength, excitation power, temperature- Blinking is not a random process, it is not pure chaos.
- We found several domains that show autocorrelation.
- There is correlation between distant blinking domain.
- Correlation depends on time lag, suggesting space drift- We found new spatial structures in the crystal that manifest
themselves only in the blinking phenomenon.