1. Cathodoluminescence in semiconductor structures
under local tunneling electron injection
Petr Polovodov
M. Fabrice Charra CEA/Saclay, Gif sur Yvette Rapporteur
M. Philippe Dumas CINaM, Marseille Rapporteur
M. Razvigor Ossikovski LPICM, Ecole Polytechnique, Palaiseau Examinateur
M. Oleg Tereshchenko Rzhanov Institute of Semiconductor
Physics, Novosibirsk, Russie
Examinateur
M. Yves Lassailly LPMC, Ecole Polytechnique, Palaiseau Directeur de thèse
M. Jacques Peretti LPMC, Ecole Polytechnique, Palaiseau Co-Directeur de thèse
1
2. Context
2
e-
𝐼𝐼
A. Filipe et al., PRL 80 (1997) 2425
Polarized electron transport in ferromagnetic metal/semiconductorstructures.
,
X. Li et al., APL 105 (2014) 052402
3. 3
Transport and recombination in semiconductors
J. Iveland et al., PRL 110 (2013) 177406
V f
Electroemission spectroscopy
hn
Context
e-
𝐼𝐼
A. Filipe et al., PRL 80 (1997) 2425
,
X. Li et al., APL 105 (2014) 052402
Polarized electron transport in ferromagnetic metal/semiconductorstructures.
4. Objectives and approach
Studying transport and recombination phenomena at the
nanoscale in semiconductor structures incorporating
quantum wells
QW
Local injection
e-
Ballistic Electron Emission Microscopy configuration
Injection
layer semiconductor
e-
STM tip
It
IC
Advantages: overcome limits of electronic
measurements (Resistance of the junction, low
current detection, conductive wafer)
Injection
layer QW semiconductor
e-
STM tip
hn
Scanning Tunneling Luminescence (STL) configuration
It
Limits:
• Ra/RJ
• Analogic
• 2 terminals
• Spin-valve (2 layer)
4
5. BEEL: injection layer – ferromagnetic metal (spin filter)
STL: injection layer – semiconductor
tip
hnLEF
semiconductor
QW
Approach
Injection
layer QW semiconductor
e-
STM tip
hn
Scanning Tunneling Luminescence configuration
It
IL transmission
PL spin asymetery
5
6. Issues
Exploiting spin filtering effects for imaging magnetic domains:
• Domain imaging in buried single ferromagnetic layer
• Transport in “isolated” single nanostructure
• High resolution magnetic imaging in ferromagnetic metal/semiconductor structures
Understanding fundamental processes in large bandgap nitride semiconductors:
• Hot electron transport in heterostructures
• Recombination efficiency in optoelectronic devices
6
7. Outlook
1. Motivation: localization effects in InGaN/GaN structures
2. Experimental setup: optical detection
3. InGaN/GaN samples: structure and characterization
4. STL excitation spectroscopy
5. STL microscopy
6. Tip-induced surface oxidation
7. Conclusions and perspectives
7
8. 1. Motivation: localization effects in InGaN/GaN structures
Context : lightning = 20% of the electricity consumption
GaN LEDs : white light + efficiency > 80 %
50% energy saving is expected
Problem: efficiency droop at high injection current
0 1 2 3 4 5 6 7
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
Externalquantumefficiency Current (mA)
Low injection operation
Increased device number
Increased price 8
10. 0 1 2 3 4 5 6 7
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
Externalquantumefficiency
Current (mA)
1. Motivation: localization effects in InGaN/GaN structures
10
p-type GaN
n-type GaN
Ec
Ev
Vpol
A
hν
Efficiency droop at high current density
11. 0 1 2 3 4 5 6 7
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
Externalquantumefficiency
Current (mA)
eeh-processehh-process
hν hν
Ec
Ev
Auger processes limit light emission in low
dimensional semiconducting structures.
Auger processes depend on n3 / radiative
recombination depends on n2
Efficiency droop at high current density
Major droop mechanism:Auger recombination
1. Motivation: localization effects in InGaN/GaN structures
11
p-type GaN
n-type GaN
Ec
Ev
Vpol
A
hν
12. Auger processes limit light emission in low
dimensional semiconducting structures.
Auger processes depend on n3 / radiative
recombination depends on n2
Carrier localization effect due to indium
concentration fluctuations
1. Motivation: localization effects in InGaN/GaN structures
T.-J. Yang et al., J. Appl. Phys. 116, 113104 (2014)
12
Efficiency droop at high current density
Major droop mechanism:Auger recombination
p-type GaN
n-type GaN
Ec
Ev
Vpol
A
hν
0 1 2 3 4 5 6 7
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
Externalquantumefficiency
Current (mA)
13. Outlook
1. Motivation: localization effects in InGaN/GaN structures
2. Experimental setup: optical detection
3. InGaN/GaN samples: structure and characterization
4. STL excitation spectroscopy
5. STL microscopy
6. Tip-induced surface oxidation
7. Conclusions and perspectives
13
17. 2. Experimental setup: alignment with LED
LED
Spectrometer
+
GaAs-Cs-O PMT
Multimode
optical fiber
p-contact
SiO2 SiO2
p+ GaN cap
100 or 200 nm p-GaN [Mg] = 2·1020
cm-3
In0.18Ga0.82N/GaN X 5, 25 nm in total
SiO2 n-contact SiO2 InGaN region 150 nm SiO2 n-contact SiO2
n-type GaN [Si]
Sapphire (0001)
17
18. 2. Experimental setup: alignment with LED
400 410 420 430 440 450 460 470 480 490 500
0.0
5.0x105
1.0x106
1.5x106
Intensity(counts/s)
Wavelength (nm)
0 5 10 15 20 25 30
0,0 2,5 5,0 7,5 10,0 12,5 15,0
0,00
0,05
0,10
0,15
0,20
0,25
0,30
Current density (A/cm2
)
EQE
Current (mA)
LED
18
19. 2. Experimental optical setup: alignment with LED
Summary
• We have built STM experiment allowing working with contacted devices
and coupled to an optical spectroscopysetup
• STM imaging allows achieving atomic resolution.
• The collection efficiency is 2·10-6 (configuration using mirrors) and 6·10-4
(configuration using optical fiber)
• The LED is used to align and to calibrate our optical system
19
20. Outlook
1. Motivation: localization effects in InGaN/GaN structures
2. Experimental setup: optical detection
3. InGaN/GaN samples: structure and characterization
4. STL excitation spectroscopy
5. STL microscopy
6. Tip-induced surface oxidation
7. Conclusions and perspectives
20
21. 3. InGaN/GaN samples: structure and characterization
QW in p-type GaN samples
p++-GaN cap - 10 nm
p-GaN [Mg]=2·1019 cm-3 – 10 or 90 nm
UID GaN - 10 nm
UID In0.18Ga0.82N QW - 3 nm
UID GaN - 100 nm
p-GaN [Mg]=2·1019 cm-3 - 1 µm
AlGaN [Mg]=2·1019
cm-3
- 15 nm
p-GaN [Mg]=2·1019
cm-3
- 10 nm
n-GaN [Si]=5·1018 cm-3 - 1-4 µm
(0001) Sapphire substrate
Injection energy is varied
by changing V
hνVth
p-type GaN
Unintentialy doped GaN
In0.18Ga0.78N
L = 30, 110 nm
Vgap
EF
L = 30, 110 nm
21
22. 3. InGaN/GaN samples: structure and characterization
SEM: hexagonal pits
Cross section
STM
etch pits
Atomic steps
Ha = 7 Å
STM
AFM
22
300 nm
100 nm
23. 400 420 440 460 480 500
110 nm p-type GaN
30 nm p-type GaN
Model
PLintensity(arb.units)
Wavelength (nm)
EF
hν-lum
hν-ex.
3. InGaN/GaN samples: structure and characterization
PL spectroscopyof InGaN/GaN samples
23
24. STL spectroscopyof InGaN/GaN samples
EF,m
EF,p hν
400 420 440 460 480 500
110 nm p-type GaN; Vg = 4V, It = 20nA
30 nm p-type GaN: Vg = 4V, It = 35 nA
30 nm p-type GaN: Vg = 4.V, It = 35 nAIntensity(counts/s)
wavelength (nm)
400 420 440 460 480 500
110 nm p-type GaN
30 nm p-type GaN
Model
PLintensity(arb.units)
Wavelength (nm)
EF
hν-lum
hν-ex.
3. InGaN/GaN samples: structure and characterization
PL spectroscopyof InGaN/GaN samples
24
25. 25
3. InGaN/GaN samples: structure and characterization
Summary
• STM images show atomic steps on the GaN surface and etch pits related
to emerging dislocations
• AFM images confirm these features. Etch pits with hexagonal structure
are typical for c-plane GaN are resolved in SEM
• STL spectroscopyperformed for electron injection well above the
minimum of the GaN conduction band shows that luminescence signal
all comes from carrier recombination in QW’s (no signal at the GaN
bandgap energy is observed) which is confirmed by PL measurement.
26. Outlook
1. Motivation: localization effects in InGaN/GaN structures
2. Experimental setup: optical detection
3. InGaN/GaN samples: structure and characterization
4. STL excitation spectroscopy
5. STL microscopy
6. Tip-induced surface oxidation
7. Conclusions and perspectives
26
30. 4. STL excitation spectroscopy
1 eV
Vgap
We observe a saturation at 1 eV above the threshold.
Vth
EF
EF,p
hν detection
1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0
0
200
400
600
800
1000
0
2x10-2
4x10-2
6x10-2
8x10-2
1x10-1
Externalquantumefficiency
Luminescence(counts/s)
-Vgap
Vth
30
31. 4. STL excitation spectroscopy
1 eV
EF
G-valley
L-valley
Vgap
We observe a saturation at 1 eV above the threshold. Injection in side-valley
Y. C. Yeo et al., J. Appl. Phys. 83 (3) (1998)
2 eV
(ab initio calculations)
1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0
0
200
400
600
800
1000
0
2x10-2
4x10-2
6x10-2
8x10-2
1x10-1
Externalquantumefficiency
Luminescence(counts/s)
-Vgap
Vth
31
CB
VB
32. 4. STL excitation spectroscopy
1 eV
EF
G-valley
L-valley
Vgap
We observe a saturation at 1 eV above the threshold. Injection in side-valley
M. Piccardo et al., Phys. Rev. B 89, 235124 (2014)
1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0
0
200
400
600
800
1000
0
2x10-2
4x10-2
6x10-2
8x10-2
1x10-1
Externalquantumefficiency
Luminescence(counts/s)
-Vgap
Vth
1 eV
(Experiment)
E
k
32
33. 4. STL excitation spectroscopy
1 eV
EF
G-valley
L-valley
Vgap
Similar saturation is observed in another system: AlGaAs/GaAs
Interpretation: dip in DOS ???
1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0
0
200
400
600
800
1000
0
2x10-2
4x10-2
6x10-2
8x10-2
1x10-1
Externalquantumefficiency
Luminescence(counts/s)
-Vgap
Vth
T. Tsuruoka et al., Appl. Phys. Lett., Vol. 73 ,No. 11 (1998) 33
34. 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0
0
200
400
600
800
1000
0
2x10-2
4x10-2
6x10-2
8x10-2
1x10-1
Externalquantumefficiency
Luminescence(counts/s)
-Vgap
Vth
4. STL excitation spectroscopy
STL threshold: eVth = Ec,bulk-EF
STL saturation: injection in bulk side-valley.
Dependence on the QW position
2 3 4 5 6
0
10
20
30
40
50
60
70
Luminescence(counts/s)
-Vgap
110 nm GaN/InGaN
30 nm GaN/InGaN
400 420 440 460 480 500
110 nm p-type GaN90 nm p-type GaN
30 nm p-type GaN10 nm p-type GaN
Model
PLintensity(arb.units)
Wavelength (nm)
EF
G-valley
L-valley
It = 20 nA
It = 5 nA
Vgap
34
35. 4. STL excitation spectroscopy
Summary
• The Vth value of 3.2 eV corresponds to the electron injection in the bulk
GaN conductionband
• The STL saturation at injection energy of 1 eV above G coincides with
electron injection in the bulk first side-valley of GaN and confirms the G-L
separation measured by photoemission.
• Radiative recombination efficiency is lower in the structure with QW
close to the surface, but threshold and saturation are identical
35
36. Outlook
1. Motivation: localization effects in InGaN/GaN structures
2. Experimental setup: optical detection
3. InGaN/GaN samples: structure and characterization
4. STL excitation spectroscopy
5. STL microscopy
6. Tip-induced surface oxidation
7. Conclusions and perspectives
36
37. 5. STL microscopy in InGaN/GaN structures: localization effects
110 nm GaN/InGaN
Vth
EF,m
EF,p
hν
detection
Topography Luminescence
Vg = -6V, It = 20 nA
Luminescence 3D
37
We observe localized luminescence features.
Light fluctuations are much larger than the statistical fluctuations (by about one order of magnitude)
38. 5. STL microscopy in InGaN/GaN structures: localization effects
110 nm GaN/InGaN
Vth
EF,m
EF,p
hν
detection
EF,nEF,p
EF,m
S. K. Manson-Smith et al., Phys. Stat. Sol. (b) 228, No. 2, 445–448 (2001)
LED
38
Vg = -3.3 V, It = 0,5 nA
Topography Luminescence
Topography Luminescence
Vg = -6V, It = 20 nA
39. 5. STL microscopy in InGaN/GaN structures: localization effects
110 nm GaN/InGaN
Vth
EF,m
EF,p
hν
detection
S. K. Manson-Smith et al., Phys. Stat. Sol. (b) 228, No. 2, 445–448 (2001)
39
Electron Injection in LED
Vg = -3.3 V, It = 0,5 nA
Topography Luminescence
Topography Luminescence
Vg = -6V, It = 20 nA
40. 5. STL microscopy in InGaN/GaN structures: localization effects
Scan 1
Scan 4 Scan 5 Scan 6
40Significant variations in the topograpy and luminescence images vs time
Scan 2 Scan 3
42. 5. STL microscopy in InGaN/GaN structures: localization effects
Summary
• Light emission localization is observed in the range of 10 to 100 nm
• The tunneling current does not change during the experiment
• There is no correlation with etch pits related to emerging dislocations
• But there are significant changes in the topography and STL images during experiment
42
Possible interpretation of observed localized luminescence:
• Preferential transport path
• Fluctuations of indium composition
T.-J. Yang et al., J. Appl. Phys. 116, 113104 (2014)
43. Outlook
1. Motivation: localization effects in InGaN/GaN structures
2. Experimental setup: optical detection
3. InGaN/GaN samples: structure and characterization
4. STL excitation spectroscopy
5. STL microscopy
6. Tip-induced surface oxidation
7. Conclusions and perspectives
43
48. • STL experiments on wide band gap semiconductors require high injection conditions:
large tip-to-sample bias (between 4 to 6 V)
large tunneling current (a few 10 nA)
• Surface is strongly modified during spectroscopyand microscopy experiment
• The tip-induced surface modification is found to be due to GaN oxidation
Summary
6. Tip-induced surface oxidation
48
49. 7. Conclusions and perspectives
• STL spectroscopy:light is emitted from QW
• STL excitation spectroscopy:
threshold corresponds to injection in G bulk
saturation corresponds to injection in L bulk
• STL microscopy: radiative recombination and localization are
observed
• Tip-induced surface oxidation is observed in high injection
conditions,it influences the STL
Conclusions
49
400 420 440 460 480 500
110 nm p-type GaN; Vg = 4V, It = 20nA
30 nm p-type GaN: Vg = 4V, It = 35 nA
30 nm p-type GaN: Vg = 4.V, It = 35 nA
Intensity(counts/s)
wavelength (nm)
1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0
0
200
400
600
800
1000
0
2x10-2
4x10-2
6x10-2
8x10-2
1x10-1
Externalquantumefficiency
Luminescence(counts/s)
-Vgap
Vth
50. • Limit oxidation process:
Working in vacuum condition
Covering the GaN surface with a metal layer
• Working at low temperature (freezing out the non-recombination processes )
• Modelling transport in the band bending region
• STL experiment in biased LED
7. Conclusion and perspectives
Perspectives
50
400 410 420 430 440 450 460 470 480 490 500 510 520
0
5
10
15
20
25
signal with electron injection
No electron injection
Intensity(counts/s)
Wavelength (nm)
Topography Tunneling current
