Certain applications require III-Nitride LEDs to be efficient at high current density. This is challenging because of the phenomenon of efficiency droop, whereby the internal quantum efficiency (IQE) of a III-Nitride LED decreases at high current density.
By way of background, IQE is understood conventionally as a competition between three effects: (1) low-current nonradiative recombinations, (2) radiative recombinations, and (3) high-current nonradiative recombinations. Low-current nonradiative recombinations are widely understood to be induced by defects in the crystal, and to be a type of Shockley-Read-Hall (SRH) recombination. If n and p are the densities of electrons and holes in the active region, they can be expressed as G_SRH=2 Anp/(n+p). In the typical case where n=p, this is equivalent to G_SRH=An. Radiative recombinations involve an electron and a hole, and can be expressed as G_Rad=Bnp=Bn{circumflex over ( )}2 (when n=p). High-current non-radiative recombinations are widely believed to be caused by Auger scattering, which can be expressed as G_droop=(c1 n{circumflex over ( )}2p+c2 np{circumflex over ( )}2)=C n{circumflex over ( )}3. While others have proposed alternative models, in all embodiments, the droop current effectively scales with a high power of the carrier density (often with an exponent of 3, or more).
The IQE can be expressed as a combination of these different recombinations, i.e. IQE=Bn{circumflex over ( )}2/(An+Bn{circumflex over ( )}2+Cn{circumflex over ( )}3). This is the well-known “ABC model.” Often, coefficients A B and C are empirical and are chosen to match experimental data. The value of A, in particular, scales with the concentration of point defects. This modeling results in the well-known bell-shaped efficiency curve shown on FIG. 1. Although DAVID ET AL., All-optical measurements of carrier dynamics in bulk-GaN LEDs: Beyond the ABC approximation, Appl. Phys. Lett. 110, 253504 (2017) (David17) refines this model to better match experimental data, the model shares the same basic features—i.e., at low current, G_SRH dominates, at intermediate currents G_Rad dominates, and finally, at high current, G_droop dominates.
It is important to note that, in these proposed models, the nonradiative recombinations at low- and high-current density (respectively G_SRH and G_droop) are understood to be caused by distinct and unrelated physical phenomena. Low-current recombinations are caused by defects in the crystal (i.e. an extrinsic process), while high-current recombinations are caused by phenomena unrelated to defects (i.e., an intrinsic process). Accordingly, reducing G_SRH is believed to be a distinct challenge from reducing G_droop.
The independence of G_SRH and G_droop can be quantified numerically using the equations mentioned above. FIG. 2A shows modeled IQE using a basic ABC model for various values of A. The parameters are as follows: B=1.6e-13 cm3 s-1, C=3.16e-33 cm6 s-1, and A varies between 1E3 and 1E6 s-1. The values of B and C are representative of a 4 nm InGaN single quantum well (QW) with 13% In concentration. As A increases, the low-current efficiency and the peak efficiency decrease. However, the difference vanishes at high current density. For instance, at J=100 A·cm-2, all curves have the same IQE within +/−1% absolute—a very small variation. FIG. 2B further exemplifies the relative variation of IQE as a function of the relative defect density at 100 A·cm-2. Here, the baseline for relative comparison corresponds to the case A=1E6 s-1. When A is reduced tenfold, the relative IQE increases by less than 2%.
Although A could be high enough (for instance A=1E7) such that the high-current efficiency at 100 A·cm-2 would start to be impacted because SRH recombinations would become strong enough to be significant, this would correspond to a low-quality crystal having a low peak IQE (e.g. 30% for A=1E7). But for technologically-relevant devices, IQE>50% is desirable. Accordingly, the disclosure herein relates to the regime of sufficiently-good devices having sufficient crystal quality, where the effect of SRH recombinations is very small at high current density. In such crystals, the conventional understanding of IQE provides no motivation to improve high-current IQE by reducing point defects. This may correspond to LEDs having a peak IQE above 50% (or above 60%, 70%, 80%).
This understanding also applies to more sophisticated versions of the ABC model. For instance, [David17] showed that screening effects should be included in B and C (as will be further detailed later in this Application)—namely B and C are not strictly constant, but have a current dependence caused by screening. Including these screening effects slightly modifies the ABC model. For this improved ABC model, the influence of A on IQE is shown on FIG. 3. Screening effects change the high-efficiency shape of the IQE curve. However, even with an understanding of screening effects, the conventional belief that the effect of A on high-current IQE is very small remains.
This conventional belief also applies to different LED designs. For instance, increasing the In concentration can lead to lower values of A, B and C (which roughly scale together)—but the relative effect of A at high current density remains very small. Likewise, droop onset can be delayed by using thick active regions with more quantum wells. Here again however, SRH recombinations are believed to have little effect in the droop regime.
In summary, it is widely believed that the nonradiative recombinations at low-current density and high-current density are caused by distinct and unrelated physical phenomena—namely, low-current recombinations are caused by defects in the crystal (extrinsic process), whereas high-current recombinations are caused by phenomena unrelated to defects (intrinsic process). Thus, according to the conventional understanding, for a sufficiently good LED, a large variation in defect density translates into a very small improvement in high-current-density IQE. For example, a substantial 10× reduction in defect density causes less than 2% absolute (or 5% relative) increase in IQE at 100 A·cm-2. Accordingly, efforts to improve the efficiency of a III-Nitride LEDs at high current density and avoid current droop have been focused on other strategies, such as increasing the thickness of the active region and improving the carrier spreading across the active region. These approaches have been met with limited success. Therefore, the need to improve the efficiency of a III-Nitride LEDs at high current density remains. The present invention fulfills this need, among others.
USPTO Backgrounds
USPTO Backgrounds