Direct Measurement of Auger Electrons Emitted from a Semiconductor Light-Emitting Diode under Electrical Injection: Identification of the Dominant Mechanism for Efficiency Droop

We report on the unambiguous detection of Auger electrons by electron emission spectroscopy from a cesiated $\mathrm{InGaN}/\mathrm{GaN}$ light-emitting diode under electrical injection. Electron emission spectra were measured as a function of the current injected in the device. The appearance of high energy electron peaks simultaneously with an observed drop in electroluminescence efficiency shows that hot carriers are being generated in the active region (InGaN quantum wells) by an Auger process. A linear correlation was measured between the high energy emitted electron current and the “droop current”—the missing component of the injected current for light emission. We conclude that the droop phenomenon in GaN light-emitting diodes originates from the excitation of Auger processes.

Figure 1

(a) Energy levels for a biased LED structure emitting electrons in vacuum. (b) Schematics of hot electron generation into the $L$ valley by an $eeh$ Auger process: Left, the Auger electron is created in the $\Gamma$ band (intravalley process) and transferred to the $L$ valley; right, the Auger electron is created in the $L$ valley (intervalley process). (c) Schematics of the electron energy analysis setup.

Figure 2

Energy distribution curves for different injection currents. The baseline of each spectrum was shifted by the LED bias potential for that current (right-hand scale). Electron energy is referred to the vacuum level. When increasing injected current, high energy peaks appear around 2 eV signaling generation of hot carriers. The high energy thresholds of the different contributions to the energy distribution curves are labeled 1, 2, and 3 in the 64 mA spectrum.

Figure 3

Energy position of the three peak thresholds (labels 1, 2, and 3 in Fig. 2) under changing bias. As shown by the straight lines with slope 1, the hot electron energy increases with the applied bias due to a voltage drop at the $p$ contact.

Figure 4

(a) Integrated current of low (filled downward triangles) and high (filled upward triangles) energy peaks, and of the optical power (open circles) vs the injected current. The straight line is the expected optical output power in the absence of droop, extrapolated from low currents. (b) Integrated current of the high energy peak as a function of the supplementary current (SC, see text).