Transcript Diapositiva 1

Andrés Cantarero
University of Valencia
Spain
OPTICAL PROPERTIES OF III-N
NANOSTRUCTURES
Valence band
Composing one of the most important facets of Valencia's music scene, both
past and present, are its internationally renowned "bandes." Found in every city
and village of the Valencian community, these "bandes" are performing brass
bands that play an integral part in festivals; in fact, they even have a music
festival of their own: the Certamen Internacional de Bandas de Música
(International Band Competition). Taking place annually since 1886, thousands of
musicians descend upon the city as parts of regional, national, international,
civilian and military brass bands.
Banda de Chiva
Valencia and its University
Valencia, 138 bce
810,000 p;1,500,000 mr
We have 45,000 students
The University was founded
in 1499
Outline
 Interest in nitride semiconductors: applications
 Generalities on III-N semiconductors
 GaN/AlN self-assembled quantum dots
 Growth of GaN/AlN/SiC self-assembled QDs
 Optical properties of polar and non polar QDs
 Q1D semiconductor nanostructures
 InN nanowires
 Growth of InN nanowires
 Optical properties of InN nanowires
 Conclusions
Optical storage devices
Installed (1000000)
1000
World database on installed optical memories
CD
DVD
HD/BD VD
NG-VD
800
600
400
200
0
1985
1990
1995
Year
2000
2005
2010
Sony launches its Blueray recorder (Sept 20th,
2006)
54 Gb capacity
400 nm laser
(Nichia Corp)
Prof. Nakamura (UCSB) fabricated a 443.9 nm laser
based on NP InGaN/GaN
White LEDs
1st prize street
(ecological) illumination
Fallas 2009
Kittilä (Finland) , 2.2.2009
Nitride semiconductors
wurtzite
zincblende
Lattice constant (wurtzite c 0, zincblende a 0) (10-10 m)
Photon wavelength hc/ Eg (nm)
Band gap energy Eg (eV)
Wurtzite lattice constant (10 -10 m)
Crystal structure
GaN crystallizes in the wurtzite structure under
normal conditions
Difference of packing between wurtzite and ZB
Origin of PSP in Ga-face GaN
Structural parameters and PSP
But, it is grown on
Al2O3, SiC or Si(111)
There is, additionally, PPZ
1010 disl/cm2
F. Bernardini, V. Fiorentini, D. Vanderbilt, PRB 56, R10024 (1997).
Growth of GaN/AlN QDs
Modified Stranski-Krastanow mode
Ga Ga
+ N
AlN
6H-SiC
AlN
GaNdot
[0001]
[2-1-10]
AlN
2nm
Elastic relaxation
C. Adelmann et al APL 81, 3064 (2002); N. Gogneau et al JAP 94, 2254 (2003)
PL and electric field
Photoluminescence of different samples
growth with different number of periods
800
FWHM (meV)
1.0
0.8
600
400
dQW (ML)
1
10
2
100
4
6
8
10
12
GaN Periods
0.4
5.0
3 periods
Absorption
Emission
10 periods
4.5
50 periods
0.2
200 periods
0.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Energy (eV)
The built in electric field manifests in the
optical properties of GaN/AlN heterostructures
through the Stark effect
Is there a way to reduce dislocations
and Stark effect?
PL Emission Energy (eV)
Normalized PL
200
0.6
(0001) GaN/AlN QWs
4.0
3.5
GaN Bulk
F=10 MV/cm
3.0
Ref a
Ref b
Ref c
Ref d
Ref e
2.5
2.0
0.5
1.0
Stokes
shift
1.5
2.0
2.5
3.0
dQW (nm)
(a): J. M. Llorens, PhD (2006), Univ. Valencia; (b) Salviati et al., J. Phys. Cond. Matt. 16, S115 (2004); (c) Miyamura et
al., APL 80 3937 (2002); (d) Kako et al., APL 83, 984 (2003); (e) Widmann et al., APL 83, 7619 (1998).
Non polar QDs
a- and m-plane contain the same amount of
Ga and N per layer
5 nm
Intensity (arb. units)
c-plane QDs
HRTEM
[1-100]
a-plane QDs
RT
-2
1 Wcm
GaN
2.0
2.2
2.4
2.6
2.8
3.0 3.4
Laser
3.6
3.8
4.0
Energy (eV)
RT
[1120]
[0001]
Plano a
[0001]
a-plane QDs
Intensity (arb. units)
AFM
c-plane QDs
100 Wcm
10 Wcm
-2
-2
-2
1 Wcm
S. Founta et al, APL 86, 171901 (2005)
N. Garro et al., APL 87, 011101 (2005)
2,0
2,2
2,4
2,6
2,8
3,0 3,4
Energy (eV)
3,6
3,8
4,0
4.2
1,0
Z
Potential (V)
0,5
C-plane QD
piezo
4
spont
2
tot
0
0,0
-2
-0,5
Etot
-4
-6
-1,0
X
-6
-4
-2
0
2
4
6
8
Electric Field (MV/cm)
V and electric field
-8
Z (nm)
0,4
Potential (V)
X
A-plane QD
0,5
0,2
0,0
0,0
-0,2
-0,4
Z
-20
-0,5
-15
-10
-5
0
Z (nm)
5
10
15
20
Electric Field (MV/cm)
1,0
Growing interest in SNWs
Published papers on semiconductor
NWs (Web of Sciences)
Comparison of the number of
citations on QDs and NWs
8000
140000
7000
120000
6000
100000
5000
80000
4000
3000
60000
2000
40000
1000
20000
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
0
0
•Quasi-one dimensional symmetry
•Large surface/volume ratio
•New possible heterostructures
•High quality materials (strain free)
•High quality heterointerfaces (larger LM)
From QWs to NWs
GaN grown on AlN
Fixed N flux
Ga bilayer conditions
GaN
Ingredients:
-lattice mismatch
-(2.5 % Da/a GaN on AlN)
- surface energy (Ga bilayer)
Thanks to Bruno Daudin, CEA Grenoble
AlN
N-rich conditions
Self-organized growth of InN
NWs
Growth details
InN nanocolumns growth:
• Grown by plasma-assisted MBE.
• p-Si (111) substrate.
• Growth time of 300 minutes.
• N2 rich conditions.
Sample
Ts
(ºC)
In-BEP
(10-8 mbar)
N2 Flux
(sccm)
G053
400
3.0
2.0
G071
475
3.0
2.0
G047
500
3.0
2.0
G044
500
3.0
1.5
G041
500
1.5
1.5
Ts: Substrate temperature.
In-BEP: Base equivalent pressure of In.
J. Segura et al, ICNS7 (Las Vegas), 2007.
Two sets of samples:
Set A: Different substrate
temperature.
Set B: Different In-BEP
and N2 flux conditions.
Raman modes of the wurtzite
structure
G=2A1+2B1+2E1+2E
A1
Wurtzite : hexagonal
structure with 4 atoms
in the unit cell.
E1
B1h
B1l
E2l
E2h
447 (TO)
585.4 (LO)
476 (TO)
593 (LO)
InN
87
490.1 cm-1
546 (TO)
732 (LO)
555 (TO)
741 (LO)
GaN
137
592 cm-1
Raman scattering results
h
E2
Intensity (arb. units)
E1(LO)
0.8
PLP
A1(LO)
-
(x2)
E1(TO)
A1(TO)
0.4
E2h*
FWHM
(E2h)
E1(LO)
FWHM
(E1(LO))
G041
489,46
3,62
592,83
7,29
G071
489,24
3,45
592,53
7,79
G047
489,24
3,51
592,73
9,14
G044
488,92
4,45
591,42
7,55
G053
489,36
4,20
592,00
9,22
Very narrow E2h non polar mode
peak, an indication of the high
crystalline quality
1.2
G041
G053
Sample
Forbidden modes
Lower plasmon-LO coupled
(PLP-) mode is observed
0.0
420
440
460
480
500
580
600
-1
Raman shift (cm )
*X. Wang et al. Appl. Phys. Lett. 89, 171907 (2006).
Raman results
E1(LO)
G041
G053
Intensity (arb. units)
0,8
NCs homogeneous (G041) or with
tapering effect (G071).
A1(LO)
0,4
0,0
570
580
590
600
-1
Raman shift (cm )
610
NCs with Baseball bate shape
(G047) and coalescence (G053).
Forbidden modes can be observed in the Raman spectra because the laser
light enters and scatters mainly from the lateral surface of the NCs.
Higher intensity of E1(LO) peak is observed in samples with morphologies
which allow an easier access to the NCs lateral surface.
J. Segura et al, ICNS7 (Las Vegas), 2007; J. Segura et al, Phys. Rev. B 79, 115305 (2009).
Conclusions
 GaN QDs
 GaN QDs grown along the c axis emit in the green
region of the spectrum due to the Stark effect
 GaN QDs grown on non polar directions show
quantum confinement and emit in the UV
 InN NWs
 NCs have a high crystalline quality and are strain-free
 The appearance of forbidden modes has been
correlated to the sample morphology
 Raman scattering shows the existence of two emitting
regions, a surface region giving rise to PLP modes and
an inner region