金属・量子井戸ヘテロ界面における動的共鳴現象 東京大学工学系研究科電気系工学専攻、松井裕章 光励起下で生成される量子井戸内の電子・正孔対（励起子）と、表面プラズモンとの動的相関科学は、高機能な 発光・受光素子及び光電変換材料の開発に向けての基礎科学になる。本研究の目的は、超短波パルス光及び近接 場光を用いた時空間分光を通じて、量子井戸上に微細加工した金属ナノ構造に誘起される局在光に関する諸特性 を解明する。本講演では、量子井戸内に生成される励起子エネルギーのヘテロ界面における金属層の帆表面自由 電子振動によるエネルギー緩和過程を考察する。これは、量子現象と電磁気現象の結合である。 Structure of 4.8 nm-thick SQW and PL quenching Critical temperature (Tc) for PL quenching effect Coupling of excitons and surface free electron oscillation 0 -4 Difference PL DI / I = 1000% -8 1.8 3 T = 300 K 100K 60K 2.2 2.6 20K 2.2 SQW emission peak energy (eV) PL quenching ratio (time) 0 -4 Tc = 125 K -8 -12 0 Tc = 90 K -4 -12 3.02 LW = 2.3 nm 3.005 10 100 Ag layer Photon-counting 2.93 LW = 3.2 nm 2.9 Ti: sapphire E(0) = 2.965 eV s = 20 meV 2.87 2.79 LW = 4.8 nm E(0) = 2.808 eV s = 8.5 meV 0 100 200 Lifetime Without Ag With Ag 10 K t1 = 0.266 ns t2 = 2.09 ns t1 = 0.242 ns t2 = 1.75 ns 75 K t1 = 0.148 ns t2 = 0.771 ns t1 = 0.136 ns t2 = 0.686 ns 300 Temperature (K) Temperature (K) SQW 6.2 nm 2.77 2.73 300 I (300K ) = 2.0% I (8K ) Lifetime (ns) Integrated PL intensity (a.u.) tNR t PL = I1t 1 + I 2t 2 I1 + I 2 ~ T1.5 1 40 80 1000 / T (K-1) 120 + t PL 1 tR + tET metal t2 t1 1 t ET -metal 1 hinteq = t NR T = 10 K tPL 0.1 10 = = 1 C.B. 100 1+ tR t NR I(t) = I1*exp(-t/t1) + I2*exp(-t/t2) Without Ag With Ag T = 10 K T = 75 K 0 1 2 Time (ns) tNR Energy Transfer was induced by longer lifetime (t2) than energy transfer lifetime (t ET metal). Ag metal nanodots by EB lithography for nano-optics 0.5 Effective lifetime:(tPL) Without Ag With Ag 0.3 0.1 10 T = 100 K × t1 V.B. Temperature (K) 1 × tNR InGaN laser: 403 nm 0 t PL ,metal tR 1 300 Long radiative lifetime to Ag metal layer 1 t PL hinteff = 100 200 Temperature (K) Energy transfer from quantum well to metal Radiative recombination process in SQW without Ag region DE = 10. 5 meV 0 Long lifetime (t2): localized states in the SQW Integrated PL intensity and effective PL lifetime 10 2.73 Selective excitation of the well (2.8 eV) 2.75 -8 3 Difference of lifetime between without and with Ag regions E(0) = 3.093 eV s = 29 meV 3.035 2.6 Photon energy (eV) s = 8.5 meV Ti: sapphire second harmonic laser (l = 408 nm) Laser pulse width: 2 ps Photo-counting system (resolution: 80 ps) 0 -12 2.75 Time-resolved PL response of 4.8 nm-thick SQW Excitonic localization and quenching temperature Tc = 140 K a 2T s2 T +b k BT Correlation of quenching temperature and localization of excitons Quantum well size and PL quenching effect -8 E (T ) = E ( 0 ) - 3 Photon energy (eV) -4 -8 2.77 30K 1.8 DI / I = 0% 1.8 Tc ~ 90 K -12 10K 0 -4 -8 -4 2.79 40K Lifetime (ns) SQW direction 2.6 200K 150K PL intensity (a.u.) Difference Exciton (e- -h+ pair) vibrating dipole 2.2 Photon energy (eV) 0 SQW energy (eV) With Ag layer PL intensity (a.u.) Electromagnetic field Space (t) 0 Without Ag layer Lifetime (ns) (SQW surface roughness : 1-1.5 nm) Ag (silver) metal 300K T = 10 K Difference PL intensity (a.u.) Difference ZnO Spacer CdZnO well ZnO Buffer HAADF-STEM Quantum confinement of excitons and the Quenching appearance PL quenching ratio PL intensity (a.u.) t = 4 nm ZnO spacer thickness 1 t ET metal ~ 1 ns 0.1 10 100 Temperature (K) Conclusion 1. PL quenching effect was observed at low temperatures and was related to a quantum confinement size. 2. PL quenching was ascribed with a decrease of lifetime, and energy transfer lifetime to the metal layer was about 1 ns. 3. Metal nanodots with 100 nm scale was successfully fabricated by EB lithography, which will open dynamic optical correlation between metal and SQW. 100 nm size; space: 500 nm 200 nm size; space: 500 nm 3
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