Blue and green InGaN/GaN light-emitting diodes (LEDs) are crucial for solid-state lighting and display applications. Violet–blue LEDs have achieved a high EQE of ~80%. However, InGaN/GaN LEDs still suffer from strong efficiency loss in the green region, which is usually referred to as the “green gap.” The green gap issue has been related to Auger recombination, the quantum-confined Stark effect (QCSE), carrier leakage, random alloy fluctuations, and defects and carrier localization, among other causes. To achieve longer emission wavelengths, the indium composition must be increased, leading to several problems. First, high indium content increases lattice mismatch and introduces additional point defects during metalorganic chemical vapor deposition (MOCVD). High indium content also leads to strong polarization-related electric fields in the quantum wells (QWs), lowering the wave function overlap and decreasing the radiative and non-radiative recombination rates (i.e., from the QCSE). An important question is how much of the change in efficiency reduction from blue to green is due to intrinsic effects (i.e., wave-function overlap and phase-space filling (PSF)) vs. extrinsic effects (i.e., material degradation). In this work, we use small-signal electroluminescence (SSEL) to study the carrier dynamics in commercial c-plane LEDs with various emission wavelengths ranging from blue (~470 nm), to cyan (~500 nm), to green (~530 nm). The LED series consists of three wafers grown by Lumileds with identical active region designs (3X, 3 nm QWs) but different indium compositions. LEDs with mesa diameters of 100 µm and ground-signal-ground electrodes were fabricated. SSEL was then used to study the carrier dynamics of the LEDs. Each element in an equivalent circuit model was acquired via simultaneous fitting of the impedance and modulation responses, leading to the injection efficiency and total carrier recombination lifetime. Combining the total carrier lifetime with internal quantum efficiency (IQE) enables the separation of the radiative and non-radiative recombination lifetimes and rates as a function of carrier density (n) and current density (J). The results reveal that efficiency loss at longer wavelength is related to reductions in injection efficiency and radiative recombination, and an increase in Auger recombination. The interplay between the QCSE and carrier-current density relationship is shown to determine the relative importance of the radiative (B(n)*n2) and non-radiative recombination rates (A(n) and C(n)*n3) as the indium content is increased. Both the SSEL experiment and Schrödinger-Poisson calculations showed C(n)/B(n) is similar for the three wafers at high n, indicating that intrinsic material degradation is not significant. In addition, compared with QCSE, PSF, and material degradation, the increase of the carrier density at a given J for longer wavelengths is the dominant reason for the green gap. Thus, methods to decrease the carrier density in each QW are suggested to increase the EQE of green LEDs.

Carrier Dynamics in Blue, Cyan and Green Commercial InGaN/GaN LEDs Measured by Small-Signal Electroluminescence

Nonradiative recombination centers (NRCs) are known to negatively affect the efficiency of light-emitting diodes (LEDs). NRCs contribute not only to Shockley–Read–Hall (SRH) recombination, but also to trap-assisted Auger recombination (TAAR). The relationship between NRCs and the TAAR processes has been predicted theoretically and observed experimentally to a limited degree. However, some details about TAAR are still under debate, including its magnitude relative to intrinsic Auger and SRH recombination, its dependence on carrier density (e.g., ∝n2 or ∝n3), and its role in the green gap, particularly in high-quality commercial-grade LEDs. TAAR is typically assumed to be linearly dependent on trap density, directly tying the magnitude of TAAR to the quality of the sample. Previous studies have examined TAAR using time-resolved photoluminescence and optical differential lifetime measurements, which do not capture certain effects related to electrical injection. In this work, we report a quantitative study of TAAR in c-plane commercial-grade green LEDs grown by metal-organic chemical vapor deposition (MOCVD). We investigate a series of three wafers with identical active regions (3X, 3-nm-thick quantum wells) but different NRC trap densities (4.4e14 cm-3 < Nt < 1.5e15 cm-3). We use small-signal electroluminescence (SSEL) and deep-level optical spectroscopy (DLOS) to study the carrier dynamics and density of NRCs in the samples, respectively. SSEL is used to determine the carrier lifetime, which is used in conjunction with the internal quantum efficiency (IQE) to decouple the radiative and nonradiative recombination rates. The total recombination lifetime in higher quality growth is found to be longer than that in lower quality growth and is attributed to the longer nonradiative recombination time in higher quality samples. On the other hand, the radiative recombination rates for different growth qualities at a given carrier density (n) are identical, as expected from samples with the same active region design. We extract the relationship between the density of NRCs and the corresponding nonradiative recombination rates at a given carrier density (n). The difference in nonradiative recombination between the samples offers information about both SRH and TAAR with respect to growth quality. The linear dependence between the SRH recombination rate and the density of NRCs at low n was confirmed. However, TAAR was found to be sublinear with the density of NRCs, possibly due to the relatively small changes (~3X) in defect density (Nt) in these samples. We find that TAAR is best fit to an n2 dependence on carrier density. This is supported by the observation that all three samples have similar IQE at high carrier density, which would not be consistent with an n3 process. We also show that the TAAR recombination coefficients (D) are on the order of ~1e13 cm3 s-1, which is similar to that reported for violet LEDs and shows the weak dependence of TAAR on bandgap. Using the extracted TAAR coefficients, we calculate that TAAR accounts for about 10% of the total nonradiative recombination rate at moderate carrier densities. Thus, intrinsic Auger dominates the nonradiative recombination and TAAR is not a major contributor to the green gap in these commercial LEDs. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Trap-Assisted Auger Recombination in Commercial Green InGaN/GaN LEDs

KK: Micro-LEDs

technical paper

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

EMC is coordinated with the Device Research Conference (DRC). This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

EMC 2023

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. 

Xuefeng Li

2

Presentations

Carrier Dynamics in Blue, Cyan and Green Commercial InGaN/GaN LEDs Measured by Small-Signal Electroluminescence

Trap-Assisted Auger Recombination in Commercial Green InGaN/GaN LEDs

Stay up to date with the latest Underline news!

PRESENTATIONS

CONFERENCES

COMPANY

RESOURCES