Evidence of a Crossover from Trions to an Electron-Hole Gas


Masumi Yamaguchi, Shintaro Nomura*, Hiroyuki Tamura, and Tatsushi Akazaki
Physical Science Laboratory, *University of Tsukuba

   A trion, the bound state of two electrons and a hole, cross over to the unbound state of the two-dimensional electron gas plus a hole state (2DEG-h) as the excess electron density is increased (see Fig. 1). This is because the Coulomb attraction between the electron and hole is screened by the degenerate 2D electron gas. In studying this crossover, a difficulty lies in the unavoidable spatial disorder of the electrostatic potential existing in samples. In the low- electron-density regime, the electrons are localized at the potential valleys. The electron-density regime where the crossover occurs often overlaps the electron-density regime where the electron localized-delocalized transition occurs.
   In this work, we studied the crossover by analyzing the evolution of the PL spectrum using an undoped GaAs QW [1]. Unlike in modulation-doped QWs, which are commonly used for studying 2D electron system, the electron localization occurs at much lower density than the crossover regime since the typical length scale of the disorder potential for our sample is long. Therefore, the electron localization and the crossover are clearly separated in our sample. By measuring the photoluminescence linewidth under perpendicular electric fields, we have traced the variation of the effective radius a* of the trion as a function of electron density (Fig. 2). The a* increases sharply above ns = 2 ~ 1014 m-2, at which the screening length drops. Here, the screening length is predicted by nonlinear-screening theory. This is a clear evidence of the trion-2DEG-hole crossover related to the dissociation of the trion state screened by the 2D electron gas.
   This work was supported by KAKENHI.

[1] M. Yamaguchi et al., Phys, Rev. B 87 (2013) 081310(R).

Fig. 1. Tron state and two-dimensional electron gas plus hole state.
Fig. 2. Electron density dependence of the trion radius a*. Inset: Screening length.