Transcript Ti:Sa

14pYc-7
2004 JPS fall
超弱励起による原子平坦面をもつ
量子井戸の発光スペクトル
Ji-Won Oh，Masahiro Yoshita，Hidefumi Akiyama, Loren PfeifferA，
Ken WestA
Institute for solid state physics, University of Tokyo,and CREST, JST
Bell Laboratories, Lucent Technologies, USA.A
Outline
• Introduction: Novel fabrication method for
atomically smooth (110) surface: why (110) surface?
• Experimental method using Micro-PL spectroscopy
• PL profiles of ideal 2-D quantum well system under
very weak excitation power and liquid He temp.:
Linewidth of photoluminescence by curve fitting
Why (110) GaAs surface is so important for us ?
Schematics of the cleaved-edge overgrowth method with molecular beam epitaxy
Schematic of a T-shaped
quantum-wire structure which consists
of a 14-nm-thick (001) QW (stem
well) and a 6-nm-thick (110) QW (arm
well). Percentages show Al-contents
(x) in AlxGa1.xAs layers.
Introduction: Novel fabrication of atomically flat GaAs (110) surface
using growth-interrupt annealing and cleaved-edge overgrowth
Annealed surface
600 ℃
10 min
Novel
growth-interrupt annealing technique with a cleaved-edge
overgrowth (CEO) method in MBE growth:Atomically flat (110) GaAs
quantum well
[1] M. Yoshita , H. Akiyama, L. N. Pfeiffer, K. W. West Jpn. J. Appl. Phys. 40,
L252-254 (2001).
[2] M. Yoshita, H. Akiyama, L. N. Pfeiffer, K. W. West, Appl. Phys. Lett. 81,
49-51 (2002)
5X5 mm
Purpose of experiment
PL spectroscopy of quantum well with atomically flat interface ( ideal 2-D
electron-hole system) under very weak point excitation power and 4 K.
. Ideal 2-D electron-hole system: minimizing the following factors !
Atomically smooth interface→ exciton scattering by surface fluctuation↓
Liquid-He temperature (4K) → phonon scattering ↓
Very low excitation power (1 x 103 carrier/cm2) → exciton-excition scattering
↓
Quantum well structure with atomically smooth interface
and excitation positioning by Micro-PL
Selected atomically smooth
Interface region by micro-PL image
Position
Sample structure
Excitation
positioning
by micro PL
imaging
Cross-sectional image
of quantum well
[3] J. W. Oh, M. Yoshita , H. Akiyama, L. N. Pfeiffer,
K. W. West, Appl. Phys. Lett. 82, 1709-1711,2003.
4] J. W. Oh, M. Yoshita , H. Akiyama, L. N. Pfeiffer,
K. W. West, J. Appl. Phys. To be published.
Micro-PL spectroscopy with point excitation
PL
4k Liquid He
Objective lens:
x 40, N.A. 0.4
Experimental parameter:
Excitation source:
He-Ne: 630 nm =1.967 (eV)
Ti:Sa: 730 nm =1.698 (eV)
Atomically flat region
in a quantum well
6 nm
(30ML)
Carrier density
6.8 mm
Ti:Sa : 6e+02 ~ 1.8e+12 ( excitons/cm2)
He-Ne : 6e+02 ~ 4e+10 (estimated)
Exposure Time: 10-2 ~ 3600 sec (1hr)
Multi-peaks under at
various intensity
He-Ne
Ti:Sa
Peak 1
Peak 2
1.8e+12
( = 1.7 ×10 5 W / cm2 )
5.8e+11
1.8e+11
5.8e+10
( 1.6 x 10 3 W/cm2) =
1.8e+10
5.8e+09
1.8e+09
5.8e+08
PL intensity (Arb. Units)
1.8e+08
Single peak
5.8e+07
1.8e+07
5.8e+06
1.8e+06
5.8e+05
1.8e+05
5.8e+04
( = 5.3× 10 –3 W / cm 2 )
2.9e+04
5.8e+03
Single peak
3e+03
(7.8X10-5 W/cm2)=
6e+02
1.57 1.58 1.59 1.60 1.57 1.58 1.59 1.60 ( Excitons / cm2 )
PL spectra at low excitation intensity
and GL ( line-width of peaks) by curve fitting
Ti:Sa Peak 1
Ti:Sa Peak 2
Ti:Sa
He-Ne Peak 1
He-Ne Peak 2
He-Ne
1100
120
1000
PL intensity (Arb. units)
PL intensity (Arb. units)
1200
900
800
700
600
500
400
300
200
100
80
60
40
20
100
0
1.575
1.580
1.585
1.590
1.595
Photon Energy (eV)
Fitting curve of peak 1
Fitting curve of peak 2
0
1.575
1.580
1.585
1.590
1.595
Photon Energy (eV)
GL = GG (Gaussian full widths
at half maximum (FWHM) )
Peak positions and GL ( line-width of peaks)
GL = Linewidth of peak 1 &2
Pos.eV = Peak positions in eV
Ti:Sa Peak 1
Ti:Sa Peak 2
Pos.eV
He-Ne Peak 1
He-Ne Peak 2
1.5880
2.4
1.5875
2.2
1.5870
2.0
1.5865
1.8
1.5860
1.6
1.5855
1.4
1.5850
6 8
10
3
2
4
6 8
10
4
2
4
6 8
10
1.2
5
meV
GL
6 8
10
excitons / cm2
GL of Peak 1
GL of Peak 1
3
2
4
6 8
10
4
excitons / cm2
1.5 meV *
1.6 meV *
2
4
6 8
10
5
* :at single
peaks
This study
6 nm
(110)
GaAs QW
Other works
6 nm (001)
GaAs QW
Exciton
linewidth
Experimental
condition
~1.6 meV
Ti:Sa
W/cm2
4K
Notes:
Comparison of
experimental results
with previous works
Al0.33Ga0.67As
barrier
~1.5 meV
He-Ne
W/cm2
4K
1.6 meV
Ti:Sa
(0.2 W /cm2)
8K
Al0.3Ga0.67As
barrier [5]
1.6 - 2.0
meV
Ti:Sa or Ar+
(1-104 W /cm2)
6K
AlAs
barrier [6]
This study
[5] V. Srinivas, et al. Phys. Rev. B. 46, 16 (1992)
[6] D. Katzer et al. J.Vac.Sci.Tech.B 10(2), 800
(1992)
Calculation for
Width fluctuation
0.03 nm (1.06 ML)
60
Considering the reason for broad linewidth at (110)
GaAs QW
Broadening mechanism
Possibilities
1. Lifetime broadening
 r ( lifetime )  300 ps
E  
r
 14 m eV
2. Phonon scattering ・・・・small at 4K
×
×
3. Interface roughness:
① monolayer fluctuation
② intermixing / segregation during MBE growth
4. Alloy scattering ・・・・small
GaAs well / AlGaAs barrier
5. Barrier concentration fluctuation
6. Impurities scattering:
7. Surface charge
① surface bulit-in potential・・・active layer to surface
: only 20 nm
② inhomogeneous field distribution・・・charged impurities
on sample surface by exposing it to the air
×
△
△
×
△
△
Description of experiment
・PL on atomically flat (110) interface in
GaAs/GaAlAs QW under various excitation
powers via microscopic PL
・ Exciton peak (Ti:Sa & He-Ne):
GL ~ 1.5~1.6 meV
Future works
・elucidating the origin for broad linewidth
・fabricating quantum wells with narrower
linewidth
9
10
10
11
10
10
6
10
9
5
10
8
10
7
4
10
6
10
5
10
4
10
3
10
2
8
10
Exp_Time_HeNe_warutime
Exposure_Time_counts_warutime
Exciton_cm2_TiSa
7
10
Photon Counts / time
10
10
10
3
2
12
Excitons / cm
10
10
2
10
1
10
0
10
0
10
10
1
10
2
10
3
10
4
10
5
10
6
7
10
8
10
Photon Incident / cm
9
10
10
10
10
11
10
12
10
13
2
Estimating carrier density by comparing integrated PL
Q&A
•
•
•
•
•
•
•
•
•
野村晋太郎： He-Ne とTi:Sa両方で励起した理由は？
Answer: バリアー層での吸収がある励起波長を用いたかったが、同じ励起強度下で
は発光量の差はほとんど無かった。 He-Neのバリアー吸収はわずかであると考えら
れる。
松田一成： 活性層から表面まで距離は？表面との距離をどの位離せば
表面効果によるbroadeningは無くなると思うのか？
Answer: バリアー層が10ｎｍでキャップ層が10ｎｍでトータル２０ｎｍ程度である。どの
くらい離せばいいのかはわからないが、ピークシフトがないサンプルは0.2mmほど
AlGaAsバリアーを積んで、離している。
松田さんの経験からすると４０nm以上じゃないと表面効果が出る。
中山正昭: 表面効果が均一に利いてくるとピークがシフトするだけである。効果に斑が
ある場合のみ、広がりに対して利くのでは？低エネルギー側のピーク２の正体は何
か？
Ａｎｓｗｅｒ：Charged excitonもしくはexciton分子だと考えられますが、正体は
はっきりしない、時間分解測定をすれば一発でわかると思う。ちなみに励起強度に対
するピークの積分面積はべき乗の関係であった。
1000
100
10
1
1.575
1.580
1.585
1.590
Ti:Sa PL curve fitting by Gaussian Model
1.595
atomically smooth (110) interface evolved in b-5b at
the ends of sample
Two atomically smooth region
with very weak excitation power
Measuring PL spectra upon varying
Excitation power using CCD with
1200 grating by point excitation
with the long exposure times
350 mm: PL spectra
4450 mm
PL inten/ time
1000
(Arb. units)
100
10
10
9
10
7
10
5
10
3
10
1
10
(Arb. units)
1
1.575
10
6
10
5
10
4
10
3
10
2
10
1
10
0
10
1.580
1.585
1.590
10
9
10
7
10
5
10
3
10
1
10
3
10
5
10
7
10
9
Excitation Density ( W/ mm2)
10
10
1.595
-1
10
-1
11
-1
10
3
10
5
10
7
10
9
10
11
Exciton density ( num./ cm2)
5
8
11
2
10
10
Photons / cm
10
14
Reference
series : 300 grating at 250 mm
Comparison with previous results
backgrounds noises defending exposure time.
205
20 min (0.5 nW)
10 min (0.92 nW)
5 min (3.2 nW)
3 min (6.7 nW)
2min ( 20.3 nW)
200
78 mW
195
24 mW
190
10.8 mW
3.1 mW
960 nW
185
280 nW
180
90 nW
22 nW
175
170
6.32 nW
2 nW
165
1.555
0.72 nW
0.2 nW
0.05 nW
1.560
1.565
1.570
1.575
1.580
1.585
1.590
1.595
1.600
1.605