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’07 5/8 QTuL4 @ QELS Density tuning of one-dimensional electron gas in a doped T-shaped quantum wire ( studied by photoluminescence-excitation spectra ) Toshiyuki Ihara, Masahiro Yoshita, Hidefumi Akiyama Loren N. Pfeiffer, Ken W. West Institute for Solid State Physics, University of Tokyo, and CREST, JST, Japan Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, New Jersey 07974 Outline Ⅰ Ⅱ Ⅲ Ⅳ Introduction / Sample / Optical setup Results for high-density 1D electron gas Results for electron-density dependences Summary Introduction : Low-dimensional electron systems in semiconductor I Ek f kC 1 f kV k A Ek 1 f kC f kV k 1986 Asada et. al., IEEE J. Quantum Electron. Interests in Fundamental physics and Applications in Low-dimensional system Sharp density of states (DOS) Quantum wire Laser diode M. Okano et al., App. Phys. Lett. 90, 091108 (2007). Quantum (Fermi / Bose) statistics S. Liu et al., Jpn. J. Appl. Phys. 46, L330 (2007). Many-body effect (Exciton, Trion, FES, BGR) Early works of experiments on Low-dimentional electron systems 2D electron system ’87 M. S. Skolnick, PRL Fermi edge singularity (FES) ’93 K. Kheng, PRL Trions (Charged excitons) ’99 V. Huard, PRL evolution from trions to FES ’00 R. Kaur, PSS(b) [1] ’02 T. Ogawa, Nonlinear Opt. ’91 J. M. Calleja, SSC 1D FES effect ’01 D. Y. Oberli, Physica E ’02 H. Akiyama, SSC 1D BGR effect calculations (FES theory) [1] Electron density low FES 1D electron system high Excitons (X) Experimental Difficulties in sample growth optical measurement Trions (X-) Small number of experimental works Band-to-Band recombination Density-tuning of 2DEG by gate Absorption measurements Interesting physics < Targets of our investigation > High-quality single quantum wire Density-tuning of 1DEG by gate Absorption measurements T-wire fabricated by Cleaved edge overgrowth method Epitaxy was done by Dr. L. N. Pfeiffer in Lucent-Bell lab in U.S. ① ② ③ ④ [1] <wire size> 14 x 6nm x 4mm (single) <doping [2]> ①Si modulation doping ②gate electrode →tunable density [1] M. Yoshita, H. Akiyama, L. N. Pfeiffer, and K. W. West, Jpn. J. Appl. Phys. 40, L252 (2001). [2] H. Akiyama, L. N. Pfeiffer, A. Pinczuk, K. W. West, and M. Yoshita, Solid State Commun. 122, 169 (2002). Optical setup PL (photoluminescence) - emission PLE (PL-excitation) - absorption Point 1 keep excitation power stable Point 2 set excitation and detection perpendicular to each other Point 3 set the sample angle tilted and cut laser scattering by iris Point 4 set polarization of excitation and detection perpendicular to each other We succeeded PLE measurement on a ground state of a single T-wire Results for high-density 1D electron gas Exc. 5K Pex = 40mW Small hole density We observed PL peak at Band edge and PLE onset at Fermi edge Estimation of carrier temperature from PL/PLE ratio I exp A k BT Sawicki et al., Phys. Rev. A 54, 4837 (1996). Chatterjee et al., Phys. Rev. Lett. 92, 067402 (2004). We observed PL peak at Band edge and PLE onset at Fermi edge Estimated temperature : 9.8±0.5K (kBT ~ 1meV) Estimation of electron density by free-particle model calculation I Ek f kC 1 f kV k A Ek 1 f kC f kV k Asada et. al., IEEE J. Quantum Electron. (1986) me 0.067m0 mh 0.105m0 We observed PL peak at Band edge and PLE onset at Fermi edge Pronounced FES effect was NOT observed Estimated temperature : Estimated density : 9.8±0.5K 6x105 cm-1 (kBT ~ 1meV) (Ef ~ 5meV) Ef/kBT ~5 Temperature dependence of PL and PLE at 0.7V T=5K (Ef/kBT ~ 5) high T=50K temperature PLE onset at Fermi edge (FE) (Ef/kBT ~ 1) low sharp PLE peak at Band edge (BE) Good agreement with calculations. Characteristic of 1D systems. We observed Signature of 1D DOS singularity at Band-edge absorption peak Gate-voltage dependences at 5K Estimated density : 6x105 cm-1 (Ef ~ 5meV) Gate voltage high low Non-doped limit at Zero density (Ef ~ 0meV) Gate-voltage dependences at 5K (0.7 - 0.5 V) Red shifting PLE onset at Fermi edge Red shift of Fermi edge Decrease of Electron density Gate-voltage dependences at 5K (0.4 - 0.35 V) Characteristic double peak structure We observed Characteristic double peak PLE structure with Band edge and Fermi edge Gate-voltage dependences at 5K (0.3 – 0 V) Crossover to excitonic regime (Vg < 0.2V) from band-to-band recombination regime (Vg > 0.3V) Analogous to the results for 2D electron systems [’99 V. Huard, PRL, '00 R. Kaur, PSS(b), '02 T. Ogawa, Nonlinear Opt. ] Gate-voltage dependences at 5K (0.3 – 0 V) Crossover to excitonic regime (Vg < 0.2V) from band-to-band recombination regime (Vg > 0.3V) Analogous to the results for 2D electron systems [’99 V. Huard, PRL, '00 R. Kaur, PSS(b), '02 T. Ogawa, Nonlinear Opt. ] Splitting of X should be due to ML fluctuations of stem well PLE at 0V is consistent with that of non-doped quantum wire [ Itoh et al., APL 83, 2043 (2003). ] 0V corresponds to Limit of non dope (zero density) Comparison whole experimental results with calculations Vg = 0.7 - 0.4 V ne=6x105cm-1 ~ 3x105cm-1 Electron density high low Degenerated 1D electron gas (Ef/kBT > 2) Vg = 0.4 - 0.35 V ne=3x105cm-1 ~ 2.5x105cm-1 Non-degenerated 1D electron gas (Ef/kBT < 2) Double peak induced by 1D DOS Vg < 0.2 V ne < 1.5x105cm-1 Limit of non dope (zero density) Signature of 1D DOS singularity also appears in the characteristic double peak structure at 0.35-0.4V !! Differences between 2D and 1D systems 2D electron system 1D electron system ’87 M. S. Skolnick, PRL strong FES ’93 K. Kheng, PRL charged exciton (Trions) ’99 V. Huard, PRL evolution from trions to FES Characteristic double peak structure with band edge and Fermi edge ’00 R. Kaur, PSS(b) [1] ’01 G. Yusa, PRL Trions with Fractional QHE [1] Electron density low FES high Excitons (X) Trions (X-) Band-to-Band recombination Density-tuning of 2DEG by gate PL and PLE measurements Interesting physics Signature of 1D DOS singularity Unique feature of High-quality 1D electron systems Summary Variable-density 1D electron gas was realized in a T-shaped quantum wire with a gate structure. PLE measurements on a 1D ground states were achieved on an isolated single quantum wire. We observed a signature of 1D DOS represented by an absorption peak at the band edge, which indicates a high uniformity of our sample. The tunable density range covers from 0 to 6x105cm-1 Future investigation Remove ML fluctuations in the stem well (g ~0.2meV) Measurements at much lower temperature (T<5K) Measurements under magnetic field (B~10T) PLE measurement on ground states absorption peak The amount of laser scattering is smaller than that of PL We can set the PLE window for the ground state emission !! M. S. Skolnick et al., Phys. Rev. Lett. 58, 2130 (1987). PL of doped InGaAs quantum wells at various Temperature Observation of strong FES effects K. Kheng et al., Phys. Rev. Lett. 71, 1752 (1993). Absorption of dilute-doped CdTe quantum wells under magnetic field Observation of negatively charged excitons V. Huard et al., Phys. Rev. Lett. 84, 187 (1999). Absorption of doped CdTe wells at various density under magnetic field Observation of crossover from X- to FES R. Kaur et al., Phys. Status Solidi B 178, 465 (2000). PLE spectra on doped GaAs quantum wells with a gate Density-tuning of 2DEG by gate PLE measurements J. M. Calleja et al., Solid State Commun. 79, 911 (1991). PL and PLE spectra on doped GaAs quantum wires Investigation of 1D FES effects D. Y. Oberli et al., Physica E 11, 224 (2001). PL and PLE spectra on doped V-groove quantum wires FES effect can be strong due to extrinsic origin such as higher subband, hole localization. H. Akiyama et al., Solid State Commun. 122, 169 (2002). PL spectra on single doped T-wire at various density Investigation of 1D BGR effects M. Yoshita et al., Jpn. J. Appl. Phys. 40, L252 (2001). Succeeded to improve the uniformity of the second QW (Arm well) S. Chatterjee et al., Phys. Rev. Lett. 92, 067402 (2004). time-resolved PL spectra in conjunction with absorption spectra multi (20) quantum wells non-doped system pulse excitation non-resonant excitation (13.6meV higher energy) H. Itoh et al., App. Phys. Lett. 83, 2043 (2003). X 0V is consistent with the results for non-doped quantum wire Abstract figure 2 : results for the 1D wire PL and PLE spectra for doped T-wire at 0.7V and 0V Abstract figure 3 : results for the 2D Arm well PL and PLE spectra for Arm well at 0.8V and 0.2V Characterization ~ PL and PLE spectra overview A clear PLE peak of wire ground state stokes shift <0.2meV We assigned the peaks by PL and PLE measurement at various position Free particle model calculation (C-V expression) I Ek f 1 f C k k V k A Ek 1 f kC f kV k I 1D ' f C 1 f V B 'd ' 0 A 1D ' 1 f C f V B 'd ' 0 1D DOS, Effective mass approximation, k-conservation 1D 1 / 2 kC C 2me 2 C V 2 kV V 2 mh 2 kC kV Fermi distribution functions, Carrier densities fC C 1 exp C mC / kBT 1 me 0.067m0 mh 0.105m0 fV V 1 expV mV / kBT 1 ni 1D f i d 0 Broadening functions i C,V '2 B ' exp 2 g Free particle model calculation (e-h expression) I Ek f 1 f C k k V k A Ek 1 f kC f kV k I 1D ' f e f h B 'd ' 0 A 1D ' 1 f e 1 f h B 'd ' 0 1D DOS, Effective mass approximation, k-conservation 1D 1 / 2 2 ke e 2me e h 2 2 kh h 2 mh ke k h Fermi distribution functions, Carrier densities f e e 1 exp e me / kBT 1 me 0.067m0 mh 0.105m0 f h h 1 exp h mh / kBT 1 ni 1D f i d 0 Broadening functions i C,V '2 B ' exp 2 g Processing on T-shaped quantum wire