Effect of electric current on optical response of viscous electron-hole plasma

Technical Summary: Effect of Electric Current on Optical Response of Viscous Electron-Hole Plasma

Problem Statement
While the effects of electric fields on the optical properties of bulk semiconductors (e.g., Stark effect, Franz-Keldysh effect) and the impact of electric current via Joule heating are well-established, the specific influence of electric current on the optical response of a dense, hydrodynamic electron-hole (e-h) plasma remains an area of active investigation. In hydrodynamic regimes, where momentum-conserving collisions dominate over disorder scattering, mutual Coulomb drag between electrons and holes can significantly alter carrier dynamics. Previous work by the authors demonstrated that electric current flowing through a mesoscopic channel could induce magnetoexcitons and trions via e-h drag. However, a clear distinction between the effects of a longitudinal current flowing within the channel and the effects of an electric field (specifically the Hall field) generated by a perpendicular current was needed. This study addresses the influence of the Hall voltage and the resulting effective Hall current on the photoluminescence (PL) of a viscous e-h plasma in a uniformly n-doped GaAs quantum well, specifically distinguishing current-induced accumulation effects from pure electric field effects.

Methodology
The study utilizes a mesoscopic channel (5 µm wide, 100 µm long) fabricated from a 14 nm GaAs quantum well (QW) grown via molecular beam epitaxy. The structure features Si $\delta$-doping to achieve a sheet electron density of $9.1 \times 10^{11} \text{ cm}^{-2}$ and a mobility of $2.0 \times 10^6 \text{ cm}^2/\text{Vs}$ at 1.4 K.

Experiments were conducted at 4 K using scanning magneto-photoluminescence microscopy with a 9 T magnetic field applied perpendicular to the QW plane. A 440 nm laser (2.82 eV) was focused to a ~1 µm spot in the center of the channel to generate e-h pairs. The laser energy exceeds the barrier gap, leading to the injection of holes into the QW, where they form a hydrodynamic e-h plasma with nearly equal electron and hole concentrations.

Two distinct experimental configurations were employed to isolate variables:

1. Longitudinal Current: A DC current flows parallel to the channel (through the channel itself).
2. Perpendicular Current (Hall Configuration): No current flows inside the channel. Instead, a DC current flows perpendicular to the channel through lateral potentiometric contact pads. This setup generates a Hall voltage across the channel due to the Lorentz force acting on the background electrons, creating an "effective Hall current" in the illuminated region via interband recombination.

Key Results

• Longitudinal Current Effects: When a current flows parallel to the channel, the electron flow drags photogenerated light holes via Coulomb interaction, while heavy hole dynamics remain largely unchanged. This leads to the accumulation of light holes in the region opposite to the electron flow. Consequently, a double photoluminescence line emerges, consisting of a higher-energy line attributed to light-hole excitons ($X_{LH}$) and a lower-energy line attributed to positively charged light-hole trions ($X^+_{LH}$).
• Hall Current Effects (Perpendicular Configuration): In the absence of longitudinal current, but with a perpendicular current generating a Hall voltage, the same double PL line ($X_{LH}$ and $X^+_{LH}$) appears in the illuminated region. The effective Hall current, driven by the recombination of photogenerated carriers, exerts a Coulomb drag on the holes. This drag inhibits the diffusion of light holes in the direction opposite to the effective current, causing their accumulation.
• Distinction from Electric Field Effects: In regions of the channel where no effective Hall current flows (dark regions or sections where the current path is interrupted), the PL spectrum is dominated by heavy-hole transitions ($HH_0$, $HH_1$). The energy of these Landau levels shifts slightly due to the electric field created by the Hall potential difference, but no exciton/trion doublet forms. This confirms that the double line is a result of current-induced charge accumulation (drag), not merely the presence of an electric field.
• Polarity Dependence: Reversing the polarity of the perpendicular current reverses the direction of the effective Hall current and the resulting accumulation of light holes, confirming the causal link between current direction and the specific PL features observed.

Significance and Claims
The paper claims to provide experimental evidence for two distinct photoluminescence mechanisms within the same mesoscopic GaAs system, driven by different physical origins:

1. Electric Field Effect: In the absence of a Hall current, changes in PL energy are caused by the electric field generated by the Hall potential difference (affecting heavy holes).
2. Current-Induced Effect: The appearance of the exciton/trion double line is caused by the accumulation of light holes mediated by Coulomb drag from an effective Hall current.

The authors assert that this work demonstrates electrical control over exciton complex populations in a uniformly n-doped quantum well without a p-n junction. By manipulating the current geometry and magnetic field, the local optical response can be controlled. Furthermore, the study posits that the optical response (specifically the emergence of the exciton/trion double line) serves as a local probe for hydrodynamic electron-hole transport, as its appearance is directly linked to current-driven hole accumulation facilitated by strong electron-hole scattering. The findings distinguish the phenomenon from standard LED operation, noting that here, charge accumulation and subsequent emission occur in a uniformly doped material driven by e-h drag rather than a p-n junction potential.