A high-quality low-density two-dimensional electron system (2DES) in semiconductors can be formed on gated undoped GaAs quantum wells (QWs) grown by molecular beam epitaxy. Some interesting phenomena, such as Wigner crystallization, are predicted to appear in low-density 2DES theoretically because the large Coulomb interaction between electrons plays a role. However, the unavoidable potential randomness in real samples disturbs the ideal 2D condition. Investigating the screening of the potential randomness by the induced electrons would therefore help us to better understand the low-density 2DES properties.
In this study, we performed photoluminescence (PL) measurements on an undoped GaAs QW for different electron densities with a gate bias. We found a two-stage transition reflecting the change of the potential-screening properties when the electrons were induced [1]. By PL measurement, we can seamlessly follow the transition from the insulating to the metallic regimes through the PL spectrum change from the exciton (X0) and charged excion (X-) to the 2DES-hole recombination. The typical PL spectra are shown in Fig. 1 from the bottom to top with increasing gate bias voltage. The two-stage change of PL intensity and linewidth against the gate bias voltage, shown in Fig. 2, corresponds to the transition from the electron localization in the insulating regime to the linear screening in the metallic regime through a non-linear screening regime. In the linear screening regime, the induced electrons effectively screen the random potential, whereas the electrons partially screen the random potential valley in the non-linear screening regime. The randomness of our sample may originate from electrons trapped in the surface state. A sample with a larger distance from the surface to the QW may reduce the random potential at low electron density.
This work was partly supported by CREST-JST.
[1] M. Yamaguchi et al., Phys. Rev. Lett., 100（2008）207401.
