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 Pfeiffer A ， Ken West A Institute for solid state physics, University of Tokyo,and CREST, JST Bell Laboratories, Lucent Technologies, USA. A 14pYc JPS fall

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

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. Why (110) GaAs surface is so important for us ?

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, L (2001). [2] M. Yoshita, H. Akiyama, L. N. Pfeiffer, K. W. West, Appl. Phys. Lett. 81, (2002) Introduction: Novel fabrication of atomically flat GaAs (110) surface using growth-interrupt annealing and cleaved-edge overgrowth 600 ℃ 10 min 5X5  m Annealed surface

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 10 3 carrier/cm 2 ) → exciton-excition scattering ↓

[ 3] J. W. Oh, M. Yoshita, H. Akiyama, L. N. Pfeiffer, K. W. West, Appl. Phys. Lett. 82, ,2003. Quantum well structure with atomically smooth interface and excitation positioning by Micro-PL 4] J. W. Oh, M. Yoshita, H. Akiyama, L. N. Pfeiffer, K. W. West, J. Appl. Phys. To be published. Sample structure Excitation positioning by micro PL imaging Cross-sectional image of quantum well Position Selected atomically smooth Interface region by micro-PL image

6.8  m Micro-PL spectroscopy with point excitation Experimental parameter: Excitation source: He-Ne: 630 nm =1.967 (eV) Ti:Sa: 730 nm =1.698 (eV) Carrier density Ti:Sa : 6e+02 ~ 1.8e+12 ( excitons/cm 2 ) He-Ne : 6e+02 ~ 4e+10 (estimated) Exposure Time: ~ 3600 sec (1hr) 4k Liquid He Atomically flat region in a quantum well 6 nm (30ML) PL Objective lens: x 40, N.A. 0.4

Ti:Sa He-Ne Multi-peaks under at various intensity ( Excitons / cm 2 ) Single peak PL intensity (Arb. Units) Peak 1 Peak ( = 1.7 ×10 5 W / cm 2 ) ( = 5.3× 10 –3 W / cm 2 ) ( 1.6 x 10 3 W/cm 2 ) = (7.8 X10 -5 W/cm 2)= 1.8e e e e e e e e e e e e e e e e e e+03 3e+03 6e+02

PL spectra at low excitation intensity and  L ( line-width of peaks) by curve fitting Photon Energy (eV) Ti:Sa PL intensity (Arb. units) Ti:Sa Peak 1 Ti:Sa Peak 2 He-Ne Peak 1 He-Ne Peak 2 Fitting curve of peak 1 Fitting curve of peak 2  L =  G (Gaussian full widths at half maximum ( FWHM) ) PL intensity (Arb. units) He-Ne Photon Energy (eV)

Pos. eV = Peak positions in eV excitons / cm 2 meV excitons / cm 2  L = Linewidth of peak 1 &2  L of Peak meV * 1.6 meV * * :at single peaks He-Ne Peak 1 He-Ne Peak 2 Ti:Sa Peak 1 Ti:Sa Peak 2 LL Peak positions and  L ( line-width of peaks) Pos. eV

[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) Comparison of experimental results with previous works Calculation for Width fluctuation 0.03 nm (1.06 ML) 60 This study Exciton linewidth Experimental condition Notes: This study 6 nm (110) GaAs QW ~1.6 meV Ti:Sa W/cm2 4K Al Ga As barrier ~1.5 meV He-Ne W/cm2 4K Other works 6 nm (001) GaAs QW 1.6 meV Ti:Sa (0.2 W /cm 2 ) 8K Al 0. 3 Ga As barrier [ 5] meV Ti:Sa or Ar + ( W /cm 2 ) 6K AlAs barrier [6]

Considering the reason for broad linewidth at (110) GaAs QW Broadening mechanism 1. Lifetime broadening 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 × × × △ △ × △ Possibilities △

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):  L ~ 1.5~1.6 meV Future works ・ elucidating the origin for broad linewidth ・ fabricating quantum wells with narrower linewidth

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 10 ｎｍでキャップ層が 10 ｎｍ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 0.2  m 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４０ nm 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。

Ti:Sa PL curve fitting by Gaussian Model

atomically smooth (110) interface evolved in b-5b at the ends of sample 350  m: PL spectra4450  m 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

Excitation Density ( W/  m 2 ) (Arb. units) Exciton density ( num./ cm 2 )

backgrounds noises defending exposure time. Comparison with previous results 20 min (0.5 nW) 10 min (0.92 nW) 5 min (3.2 nW) 3 min (6.7 nW) 2min ( 20.3 nW) 78  W 24  W 10.8  W 3.1  W 960 nW 280 nW 90 nW 22 nW 6.32 nW 2 nW 0.72 nW 0.2 nW 0.05 nW Reference series : 300 grating at 250  m