optical characterization of gan-based nanowires : from nanometric scale to light emitting devices...
TRANSCRIPT
400 500 600 700
0,0
0,2
0,4
0,6
0,8
1,0 PL - 7K
Inte
nsi
té N
orm
alis
ée
(u
.a.)
Longueur d'onde (nm)
3,5 3 2,5 2
400 500 600 700
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
CL - 300K CL - 9K
Inte
nsi
té N
orm
alis
ée
(u
.a.)
Longueur d'onde (nm)
3,5 3 2,5 2
Energie (eV)
350 400 450 500 550 6000
10000
20000
30000
40000
50000
60000
M
Longueur d'onde (nm)
50mA 45mA 40mA 35mA 30mA 25mA 20mA 15mA 10mA 5mA 2mA
400 600 8000
3000
6000
9000
1132_AprèsContactN_F.07_P1mm_50mA_4x_0.5mm_10s_400nm.dat
400 500 600 700
0,0
0,2
0,4
0,6
0,8
1,0
Inte
nsi
té (
u.a
.)
Longueur d'onde (nm)
3,5 3 2,5 2
Energie (eV)
Optical Characterization of GaN-based Nanowires : Optical Characterization of GaN-based Nanowires : From Nanometric Scale to Light Emitting DevicesFrom Nanometric Scale to Light Emitting Devices
A-L. Bavencove*, E. Pougeoise, J. Garcia, P. Gilet, F. Levy, B. AndréCEA, LETI, MINATEC, DOPT/SIONA/LTN, 38054 Grenoble, France
G. Tourbot, B. Gayral, B. Daudin CEA-INAC, SP2M/NPSC, CEA, 38054 Grenoble, France
Le Si Dang Institut Néel, CNRS et Université Joseph Fourier, 38042 Grenoble, France
CL measurements on Single Nanowire
Extraction of the nanowires
Electron Beam Scanning
(d~50nm)
Si Substrate (n+)
Overall CL Spectra (8K – 30 kV)Overall CL Spectra (8K – 30 kV)
Localized CL Spectra (8K – 30kV)Localized CL Spectra (8K – 30kV)
The different regions of the nanowire exhibit different optical responses under electron beam radiation :
p-type GaN : Structural Defects (Yellow Band Emission)
Undoped GaN : Excitonic Emission @ 357 nm
InGaN QWs : No optical signature
n-type GaN : DAP-Band (Interface defects?)
CL Imaging Mode (8K – 30kV)CL Imaging Mode (8K – 30kV)
400 500 600 700
0,0
0,2
0,4
0,6
0,8
1,0
Sp
ect
re T
ota
l (u
.a.)
Longueur d'onde (nm)
Spectre Total
3,5 3 2,5 2
Energie (eV)
GaN Band Edge
DAP
Band Edge 2nd order
Inte
nsity
(a.
u.)
Wavelength (nm)
Energy (eV)
383 nm
356 nm
CL and PL InGaN QWs Signals
500 nm
SEM Image
500 nm
CL Panchromatic Image
Aimed StructureSEM Image (Top View)
Si Substrate (n+)Undoped AlN Buffer
Undoped GaN
InGaN (3QWs)
100 nm
Conclusions et Perspectives
EL Measurements on Nanowire-based LEDs
Optical signature identification of InGaN Quantum Wells inserted in undoped GaN nanowires
Aimed Structure
400 500 600 700
0,0
0,2
0,4
0,6
0,8
1,0
Inte
nsi
té (
u.a
.)
Longueur d'onde (nm)
Spectre Spot 2
3,5 3 2,5 2
Energie (eV)
400 500 600 700
0,0
0,2
0,4
0,6
0,8
1,0
Inte
nsi
té (
u.a
.)
Longueur d'onde (nm)
Spectre Spot 3
3,5 3 2,5 2
Energie (eV)
400 500 600 700
0,0
0,2
0,4
0,6
0,8
1,0
Inte
nsité
(u.
a.)
Longueur d'onde (nm)
Spectre Spot 5
3,5 3 2,5 2
Energie (eV)
Wavelength (nm) Wavelength (nm)
Energy (eV)Energy (eV)
Inte
nsity
(a.
u.)
Inte
nsity
(a.
u.)
Inte
nsity
(a.
u.)
Inte
nsity
(a.
u.)
I/526 I/5758
I/9325I/39193
Introduction Efficient integration of GaN-based nanowires into light emitting devices (LEDs) requires deep investigation of their optical properties. GaN-based nanowires, with and without InGaN multiple quantum wells were grown on n-doped silicon (111) substrate by molecular beam epitaxy (MBE). Si and Mg can be used during the growth process to obtain nanodiode structures (p-i-n junctions) which are characterized through 3 different techniques: Cathodoluminescence (CL), Photoluminescence (PL) and Electroluminescent (EL) spectroscopy. CL measurements have been conducted on both single nanowires and vertically self-assembled nanowires. A technological process has been developed in order to fabricate and characterize LEDs. For 350x350 m2 devices, approximately 106 nanowires are electrically connected and operating in parallel. Thanks to this multiple-scale characterization approach, the light emission characteristics of GaN-based nanowires can be accurately investigated.
Cold Plate (He Flow) (300 4K)
Tungsten Filament
Light Collection
Optical System
Monochromator
Diffraction Grating
Electron Beam
Sample
Detector (CCD Camera or PM)
Cathodoluminescence Setup
GaN-based Nanowires Integration
Direct visualization of the luminescent regions can be
achieved through CL imaging mode experiments
Autoplanarized Process : 350x350 m2 and 1x1mm2 (~3x106 and 3x107 nanowires electrically connected) LEDs have been processed by directly depositing semitransparent electrodes and thick metal pads on the cone-shaped p-type region of the nanowires. The n-type contact is taken on the back-side of the wafer.
GaN-based nanowires have been efficiently characterized : - Structural defects have been precisely localized - Active InGaN-based quantum wells have been identifiedThese results have led to the drastic improvement of nanowire-based LEDs performances (EBL).
Further electrical characterizations are in process: - n-type doping level evaluation (FET measurements) - Single vertical nanowires I-V characteristics by AFM (conductive mode)
Aimed Structure
n-type GaN (Si)
p-type GaN (Mg)
Undoped GaN
5 x InGaN QWs
Si Substrate (n+)n-type AlN Buffer (Si)
Schematic Cross Section of a GaN Nanowire-based LED
350 m
350 m
350 m
p-type Transparent Contact
Thick p-type Contact
Schematic Top View of a GaN Nanowire-based LED
p-type Contacts
n-type Contact
Si Substrate (n+)
n-type AlN Buffer (Si)
2 excitation sources are used on vertically self-assembled nanowires
:
CL : Electron Beam - d~50nm – E = 30keV
PL : Ar+ Laser Beam - d~50m – = 244nm
Inte
nsity
(a.
u.)
Cathodoluminescence Spectra (300 et 9 K)
Wavelength (nm)
Energy (eV)
350 meV
Photoluminescence Spectrum (7K)
Inte
nsity
(a.
u.)
Wavelength (nm)
Energy (eV)
270 meV
Inte
nsity
(a.
u.)
Wavelength (nm)
Inte
ns
ity
(a.u
.)
Wavelength (nm)
With EBLWithout
EBL
(@50 mA)
Without EBL (@ 50mA)
With EBL (@ 50mA)
Room temperature EL characteristics of 1x1 mm2 GaN nanowire-based LED. (A) EL spectra at various applied DC currents of nanowire-based LEDs with and without (INSET) an EBL. (B,C) Associated CCD Images. (D) Room Temperature Current-Voltage characteristics of GaN nanowire-based LEDs.
GaN-based nanowires containing a p-type AlGaN Electron Blocking Layer (EBL) have been epitaxied and processed into LEDs according to the integration process described above. The AlGaN layer is inserted at the active region/p-type part interface and is aimed at localizing the recombinations of e-/h pairs in the InGaN-based QWs.The performances of devices with and without EBL are compared :
n-type GaN (Si)
p-type GaN (Mg)
Undoped GaN
3 x InGaN QWs
p-type AlGaN (Mg)
Band Edge : RED SHIFT
1m
m
InGaN Signal : BLUE SHIFT
(A)(B)
(C)
-5 0 5 10 15
0
10
20
30
Cou
ran
t (m
A)
Tension (V)
With EBL Without EBL
Voltage (V)
Cur
rent
(m
A)
(D)
Exposition / 300
SEM image of a nanowire
CL Panchromatic Image
CL Monochromatic Image @ 356 nm
CL Monochromatic Image @ 383 nm