Electroluminescence in dopant-free GaAs/AlGaAs single heterojunctions: 2D free excitons, H-band, and the tidal effect

Abstract

Bright electroluminescence (EL) from dopant-free ambipolar lateral p-n junctions in GaAs/AlGaAs single heterointerface (SH) heterostructures is used to probe neutral free excitons arising from two-dimensional electron and hole gases (2DEGs and 2DHGs). The EL spectra reveal both the heavy-hole neutral free exciton (X$^0$) and the high-energy free exciton of the H band (HE). A combination of transition energies, lifetimes, spatial emission profiles, and temperature dependences points to a predominantly two-dimensional character for these excitons at the SH. For X$^0$, the EL peak energies (1515.5-1515.7 meV) lie slightly above the corresponding bulk GaAs photoluminescence (PL) line at 1515.3 meV, while time-resolved measurements yield markedly shorter lifetimes for EL than for PL (337 ps vs. 1610 ps), consistent with recombination in a confined interfacial layer. The HE exciton exhibits a Stark blueshift under forward bias below threshold, and its energies and lifetimes (down to 575 ps) are tuned by the topgate voltage; above threshold, HE emission is quenched in favor of X$^0$. Finally, the tidal effect $-$ a form of pulsed EL generated by swapping the topgate voltage polarity in ambipolar field-effect transistors $-$ produces an X$^0$ line at the same energy as in the lateral p-n junction and reproduces the characteristic nonmonotonic frequency dependence of the brightness previously observed in quantum-well heterostructures, again indicating a 2D-like origin. Taken together, these results show electrically generated and controllable 2D-like excitons (HE and X$^0$), thereby bridging 2D exciton physics and 2DEG/2DHG platforms in dopant-free GaAs/AlGaAs SH devices.

Figures (4)

• Figure 1: Sample A. (a) Cross-sectional schematic of a lateral p–n junction in a dopant-free FET with a GaAs/AlGaAs SH heterostructure. (b) Microscope image of fabricated device with associated circuit. $V_{\text{tgL}}$ and $V_{\text{tgR}}$ are the voltages applied to the left and right topgates respectively. $V_{\text{pn}}$ is the forward bias (or source-drain bias). The device is ambipolar, i.e. there are $p$-type and $n$-type phmic contacts on both sides of the p--n junction. (c) EL spectra from the $n$-side and $p$-side of the device superimposed onto the PL spectrum from the same heterostructure wafer. The X$^0$ peak is visible in all three spectra, all within a 0.4 meV window. Otherwise, the $p$-side EL is markedly different from PL and the $n$-side EL. (d) EL intensity maps of the device in two measurement configurations (PN and NP), illustrating that the $p$-side is always brighter than the $n$-side.
• Figure 2: EL from a lateral p–n junction at a SH interface (sample A). Normalized spectra with peak fits for: (a) the $n$-side and (b) the $p$-side. Corresponding exciton type and energies are listed in Table S2 of the supplemental material supplementalPRB-SH-2DPN. Spectra from the $p$-side as a function of $V_{\text{pn}}$ for: (c) subthreshold operation and (d) above-threshold operation. For (c), $V_{\text{pn}}$ is increased from 1.490 V to 1.500 V in steps of 2 mV. For (d), $V_{\text{pn}}$ is increased from 1.560 V to 1.800 V in steps of 40 mV.
• Figure 3: Sample A. (a) HE exciton lifetimes at different energies. (b) Fit (red solid line) to TREL experiments (blue circles) at the highest HE energy, using an exponentially-modified Gaussian (EMG). (c) Schematic of band structure, showing changes in the conduction band (CB) and the valence band (VB) at $V_{\text{tg1}}$ and $V_{\text{tg2}}$ on the $p$-side, with $\Delta E_1$ and $\Delta E_2$ corresponding to different $E_{\textsc{el}}$(HE). $z_1$ and $z_2$ are the corresponding spatial separations between the recombining electron and hole. (d) TREL and EMG fit for EL X$^0$ from the $p$-side.
• Figure 4: EL from the tidal effect in a SH heterostructure (sample B). (a) Electrical circuit. (b) Total EL intensity as a function of swapping frequency $f_{\text{s}}$ with $V_{\text{tg}}$ = 5 V. The dashed line is a fit to Eq. (1). (c) Tidal effect spectrum, using $f_{\text{s}}$ = 800 kHz and $V_{\text{tg}}$ = 5 V.