Photoluminescence Excitation Spectra of a Doped Single Quantum Wire
H. Akiyama
We studied photoluminescence (PL) and PL excitation (PLE) spectra of one-dimensional electron system in an n-type doped single quantum wire with a gate to tune the electron density (ne). The sample structure and experimental setup are shown in Fig.1.
The measured PL and PLE spectra at various gate voltages (Vg) are shown in Fig.2 (a) with calculation of free-electron approximation (b). The evolution of these spectra shows the metal-insulator crossover from excitonic discrete peaks (0-0.2V) to band-to-band broad structure with large Burstein-moss shift (0.5-0.7V). At low ne (0-0.2V), we observed complete oscillator strength (OS) transfer from excitons (X) to trions (X-) with increasing ne, which is a nature of 1D system where the concentration of OS into the lowest bound state is expected. At high ne (0.5-0.7V), we observed broad absorption onset showing blue-shift with increasing ne, which corresponds to Fermi edge. At the crossover region from trion to band-to-band transition (0.3-0.4V), we found a double peak structure corresponding to Fermi edge and square root divergence of 1D DOS.
These features are very different from those of 2D system [1]. We believe, so far, this is the first experiment for the direct observation of 1D DOS singularity. Even in high quality nondoped quantum wire, the continuum absorption
Fig.1: Schematic structure of n-type doped quantum wire. The cross-sectional aria of 14nm quantum well (stem well) and 6nm quantum well (arm well) works as a 1D wire. The electron density of stem well is increased by Si modulation doping. We can tune the electron density of wire and arm well by applying gate voltage (Vg). The direction of excitation and the detection are perpendicular, and their polarizations are orthogonal to each other to eliminate intense laser scattering.
decreases and the 1D DOS singularity disappears due to the formation of exciton peak, in other words, Sommerfeld factor is less than 1 in 1D system [2].
Although PLE spectra at high ne are vary similar to the calculation (b), the data is not fully explained. To understand the evolution of the sharp sharp peaks due to bound states at low ne (0-0.2V), we need to consider the three-body calculation [3]. Furthermore, PL spectra at high ne (0.4-0.7V) show red-shifts and broadening with increasing ne, which is inconsistent of calculation. We guess this difference is due to the many-body effect such as Band Gap Renormalization.
Fig.2: Experimental (a) and theoretical (b) spectra of photoluminescence (PL:red line) and PL excitation (PLE:blue line). Each lines are normalized and the electron density increases with gate voltage. The evolution of PLE spectra shows the metal-insulator crossover where trion (X-) evolves to band-to-band structure. The theoretical lines are obtained by free electron with effective mass approximation.
where fefh is Fermi distribution, L is Lorenz function with the broadening of, and D1D is inverse-square-root joint density of state.
[1] Huard et al. Phys. Rev. Lett. 84, 187 (1999)
[2] Ogawa et al. Phys. Rev. B 43, 14325 (1991)
[3] Esser et al. Phys. Status Solidi B 227, 317 (2001)