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Photoluminescence excitation spectra of one-dimensional electron systems in an n-type doped quantum wire Toshiyuki Ihara, Y. Hayamizu, M.Yoshita, H. Akiyama, L. N. Pfeiffer and K.W. West Institute for Solid State Physics, University of Tokyo and CREST, JST, Chiba, 277-8581, Japan Bell Laboratories, Lucent Technologies, Murray Hill, NJ, USA Abstract. We measured photoluminescence (PL) and PL excitation (PLE) spectra of one-dimensional (1D) excitons, charged excitons, and an electron plasma in a high-quality n-type-modulation-doped single GaAs quantum wire with a gate, where density of 1D electron gas (1DEG) is tuned by gate voltage. We demonstrate that the absorption line shape of the charged excitons varies gradually from symmetric to asymmetric with electron density in the wire, which then changes to a shoulder corresponding to an absorption onset of an electron-plasma system. The high-energy tail of the asymmetric absorption line of the charged excitons is analogous to a power-law anomaly appearing in Fermi-edge singularity (FES). INTRODUCTION Interband optical transition spectra in the presence of Fermi Sea are expected to exhibit a power-law singularity, which reflects the final state response of the Fermi Sea electrons to the attractive potential of the valence-band hole. This singularity, which is well known as the Fermi edge singularity (FES), has been studied in detail in 2D quantum wells (QWs) [1]. In 1D electron system, we previously reported this effect in photoluminescence (PL) spectra of a high-quality ntype-modulation-doped quantum wire (QWR) with 1D electron densities tuned by gate voltage [2]. In this paper, we report our first study on PL excitation (PLE) spectra of the QWR, which reveals many-body absorption features of the 1D system. increased by Si delta-doping (4 x 1011cm-2). By applying DC gate voltage (Vg) to the n+ Al0.1Ga0.9As layer relative to modulation-doped 2D electron gas (2DEG) in the stem-QW, we accumulated or depleted additional electrons in the QWR and arm-QW. MicroPL and PLE measurements on the single QWR were performed with a cw titanium-sapphire laser with a 1m spot size. The excitation power was 20 W which is the weakest power we achieved to measure the PLE spectrum. The direction of PL detection was perpendicular to the laser excitation and their polarization are orthogonal each other to improve signal-to-noise ratio. PL resolution was 0.15meV and PLE resolution was 0.04meV. SAMPLES AND EXPERIMENTS The sample was grown by the cleaved-edge overgrowth method with molecular beam epitaxy and growth interrupt annealing [3]. As schematically shown in Fig.1, the T wire consisted of a 14-nm-thick Al0.07Ga0.93As quantum well (stem-QW) grown on a (001) substrate, and an intersecting 6-nm-thick GaAs QW (arm-QW) overgrown on a cleaved (110) edge of the stem-QW. The electron density in stem-QW is FIG. 1 The schematic structure of T-shaped QWR, stemQW, arm-QW, and a gate. RESULTS We measured PL and PLE spectra of QWR with various gate voltages from 0 to 0.7V at 5K. Fig.2 (a) shows the normalized PL (dotted lines) and PLE (solid lines) spectra, where PL excitation energy is adjusted between 1.569 eV and 1.575eV. At low electron density (0V), the PLE spectrum is dominated by the neutral exciton peak (X) that split into double peak due to monolayer fluctuations of QWR. As electron density become higher, the PLE peak of the neutral excitons buleshifts and becomes weak. The peak of the negatively charged excitons (X-) appears and becomes strong at about 2 meV below the exciton peak. At Vg = 0.2V, PLE shape of the X- becomes asymmetric (a fast rise at low energies and a slow fall at high energies). The high-energy tail corresponds to the power-law singularity in FES for 1DEG. When the gate voltage is above 0.35V, the PLE peak of the charged excitons changes to a shoulder. For comparison, we measured PL and PLE spectra of arm-QW. Fig.2 (b) shows the normalized PL (dotted lines) and PLE (solid lines) spectra of arm-QW with various gate voltages from 0.2 to 0.8V. We observed absorption peak of neutral excitons (X), charged exciton peak (X-), and electron plasma. Comparing these results of 1D QWR and 2D armQW, we find the following: (a) X peak blueshifts and X- peak doesn't shift with electron density in both 1D-QWR and 2D-QW which means that the energy gap between X- and X increases with electron density. This effect has been reported in 2D-QW by other group and studied theoretically in 2D system. (b) FES of X- absorption peak is observed strongly in 1D-QWR, while other group has reported strong FES of X peak in 2D-QW. By comparing our result of 1D-QWR and 2D-QW, we wish to mention that this difference is because X peak becomes weak with fermi energy more quickly in 1D-QWR than in 2D-QW. (c) The PLE line shape varies dramatically from Xpeak to 1D electron plasma absorption (0.3-0.4V) in 1D-QWR. In 2D-QW, X- peak changes to 2D electron plasma absorption (0.4-0.6V) continuously. Electron plasma absorption peak blueshifts with electron density both 1D-QWR and 2D-QW. But we observed large energy gap between PL shoulder and PLE peak in 1D-QWR. FIG. 2 Normalized PL (dotted lines) and PLE (solid lines) spectra of QWR (a) and arm-QW (b) at various gate voltage. CONCLUSION We measured the electron density dependence of PL and PLE spectra in an n-type-modulation-doped QWR. We observed the energy gap between exciton and charged exciton increases with electron density in both 1D-QWR and 2D-QW. FES of charged exciton is observed strongly in 1D-QWR. We observed dramatic change of the PLE line shape from charged exciton peak to 1D electron plasma absorption that differs from 2D-QW result. REFERENCES 1. M. Takagiwa and T ogawa, J. Phys. Chem. Solids. 63, 1587 (2002). 2. H. Akiyama, L. N. Pfeiffer, A. Pinzuk, K. W. West, and M. Yoshita, Solid. State. Commun. 122 169 (2002) 3. M. Yoshita, H. Akiyama, L. N. Pfeiffer and K. W. West, Jpn. J. Appl. Phys. 40, L252 (2001). 4. G. Yusa, H. Shtrikman and I. Bar-Joseph, Phys. Rev. B 62 , 15390 (2000)