Eu-doped Gallium nitride (GaN) is a promising candidate for GaN-based red light-emitting diodes, which are needed for future micro-display technologies. Introducing a superlattice structure

comprised of alternating undoped and Eu-doped GaN layers has been observed to lead to an order-of-magnitude increase in output power; however, the underlying mechanism remains unknown. Here,

we explore the optical and electrical properties of these superlattice structures utilizing terahertz emission spectroscopy. We find that ~0.1% Eu doping reduces the bandgap of GaN by ~40 

meV and increases the index of refraction by ~20%, which would result in potential barriers and carrier confinement within a superlattice structure. To confirm the presence of these

efficiency in the Eu-doped devices. We argue that the benefits of the superlattice structure are not limited to Eu-doped GaN, which provides a roadmap for enhanced optoelectronic

functionalities in all rare-earth-doped semiconductor systems.

The ubiquity of light-emitting diodes (LEDs) in our everyday life is undeniable, and micro-LEDs are now being rapidly developed to usher in a new wave of display technologies. One obstacle

to realizing these technologies is the lack of an efficient red LED based on GaN. Currently, most full-color LED displays are made by combining green and blue GaN-based LEDs with traditional

red GaAs-based LEDs using the pick-and-place technique1. A long-standing issue with fabricating GaN-based red LEDs is the large Indium content required in the InGaN layers, which introduces

strain and reduces efficiency due to the quantum-confined Stark effect and indium phase separation2,3,4,5,6. For micro-LEDs in general, InGaN faces two additional challenges associated with

the reduction in device size: a sharp decrease in external quantum efficiency (EQE) due to leakage current at the device walls and significant blue shifts in peak emission wavelength due to

band-filling effects7. The leakage current increases for smaller devices8,9, due to the non-radiative recombination at the sidewalls of the micro-LED structures. Despite these challenges,

blue and green GaN-based micro-LEDs with EQEs of 13% and 25%, respectively, for an active area of 1 µm2 were reported10,11. However, the EQE of red micro-LED remains below 5% for active

regions smaller than 100 µm211,12,13,14. Once these challenges are overcome, micro-LEDs will facilitate the development of innovative technologies such as augmented reality and transparent

displays15.

Eu-doped GaN-based LEDs represent an alternative path towards full-color monolithic displays and offer favorable properties over InGaN LEDs, especially for micro-LED applications. Figure 1

shows monolithically grown blue (InGaN) and red (Eu-doped GaN) micro-LEDs with a 20 µm width. For Eu-doped GaN LEDs, the red emission at ~620 nm originates from intra-4f transitions within

the Eu3+ ions16. As with all rare earth (RE) ions, Eu3+ ions exhibit sharp and spectrally stable emission regardless of the host system or excitation conditions. Additionally, Eu-doped

materials have been shown to be less influenced by non-radiative sidewall defects due to their short carrier diffusion lengths17,18. They have also been used to realize monolithically

stacked full-color LEDs on a single chip19. These properties make Eu-doped GaN a promising candidate for micro-LED applications.

a Schematic for monolithically grown blue (InGaN) and red (Eu-doped GaN) micro-LEDs. b Prototype of a monolithically grown two-color micro-LED array with 20 µm wide pixels.

The output power of Eu-doped GaN devices now exceeds 1.2 mW with EQEs as high as 9.2%. This was achieved using a superlattice structure consisting of alternating GaN and Eu-doped GaN layers,

resulting in a 25-fold increase16,19. While this performance far exceeds that of devices grown using single active layers of Eu-doped GaN, the exact origin of the increased output power and

EQE remains unknown16. Using atomic force and transmission electron microscopy, it was shown that the size and density of threading dislocations were significantly reduced in superlattice

samples, which would reduce leakage current and could partially explain the improved electrical properties of the superlattice structure20. In addition, these results also showed that the

lattice expanded within the Eu-doped layers relative to the undoped layers. A similar enhancement in luminescence and device performance was reported for superlattice devices consisting of

alternating Si/Er-Si layers, which also outperformed monolayer-based devices by over an order of magnitude, at 80 K21,22,23,24,25. In this case, the enhancement has simply been explained as

an increase in the efficiency of the energy transfer between the host and the RE ions or the selective formation of highly efficient defect centers26. However, there may be a deeper

underlying mechanism for this enhancement.

The origin of these enhancements may be related to reports in other RE-doped semiconductor systems used for different applications24,27,28,29,30,31,32,33,34. For example, the doping of RE

ions, such as Eu, Nd, Er, Tb, and Sm into semiconductor nanoparticles such as TiO2 and ZnO has been used for nearly two decades to modify the bandgap of the nanoparticles themselves, which

makes the host materials more suitable for certain applications, such as photovoltaics27,28,30,31. Dopant concentrations typically exceed 2%; however, this behavior has also been observed

for dilute dopant levels of ~0.5%. Since the optical band gap and index of refraction are related quantities35, it is not surprising that several other groups have also reported an increase

in the index of refraction due to dilute doping25,36, which could be used to fabricate integrated waveguides for telecommunication applications36,37. However, there are no reports, to the

best of our knowledge, on the simultaneous measurement of bandgap and index of refraction changes due to RE doping, or the use of superlattice structures to enhance optoelectronic properties

of RE-based devices due to the carrier confinement and waveguiding that should result. To this end, probing the carrier behavior in the Eu-doped GaN superlattice structure is necessary to

understand the device performance and further optimize these devices.

Here, we employed terahertz (THz) emission spectroscopy (TES) to study the dynamics of photoinduced carriers in the Eu-doped superlattice structures as compared to single-layer Eu-doped GaN

samples. When photocarriers are excited in semiconductors by femtosecond (fs) optical pulses, they are accelerated by a built-in electric field. This acceleration generates THz radiation

that reflects the carrier movement within the first few picoseconds after excitation. The waveforms and amplitudes can provide physical information on a wide range of device materials and

structures, such as the semiquantitative estimation of semiconductor surface/interface potentials, in a non-contact and non-destructive manner38,39,40,41. This technique is particularly

useful for wide bandgap semiconductor evaluation as it allows for the direct measurement of the material’s bandgap41,42. In many cases, measuring the bandgap energy using room-temperature

photoluminescence (PL) is difficult in semiconductors with a high concentration of impurities or dopants. However, TES solely results from excitation to the conduction band and is not

influenced by impurity levels and provides a new platform to discuss the ultrafast photocarrier dynamics of wide bandgap semiconductors43,44,45,46,47. Thus, TES is an ideal tool to study the

mechanism behind the enhanced properties of dilutely doped RE-based superlattice optoelectronic devices, and Eu-doped GaN serves as an example system for the utility of these properties.

The structures of all samples used in this study are shown in Fig. 2. Measurements were taken on undoped GaN (Fig. 2a), a thick Eu-doped GaN sample (Fig. 2b), GaN doped with low and high Eu

concentrations (Fig. 2c), and a superlattice structure consisting of 40 pairs of alternating Eu-doped GaN (3 nm) and GaN (10 nm) layers (Fig. 2d). These samples are labeled ud-GaN,

GaN:EuTDS, GaN:Eulow and GaN:Euhigh, and superlattice, respectively.

a ud-GaN, (b) GaN:EuTDS, (c) GaN:Eulow,high, (d) superlattice. All structures were grown on the low-temperature-grown (LT)-GaN, which were grown on c-plain sapphire substrates.

The bandgap of a semiconductor material can be determined with meV precision by evaluating the excitation wavelength dependence of the emitted THz signal within 10 ps of excitation43,48.

This behavior was used to determine the influence of dilute Eu doping on the bandgap of GaN. Figures 3a–c shows the time evolution of the THz emission obtained from the ud-GaN, GaN:Eulow,

and GaN:Euhigh samples, where the THz signals are shown for different excitation wavelengths, at room temperature. Since the bandgap of ud-GaN is ~361 nm at room temperature49,50, the THz

signals should be strong for excitation wavelengths of 6.25% Eu, Sm and Pm68. Experimentally, however, these quantities are not measured or reported simultaneously.

Experimental results that demonstrate a bandgap modification are usually obtained by observing a red-shift in the absorption spectra of the host material, where impurity states and phonon

modes can make the analysis complex27,28,29,76. In addition, the primary reason for the RE doping is usually to enhance the properties of the host, and not to enhance the emission from the

RE ions themselves28,32,33. Conversely, the index of refraction is generally determined by fitting the results of reflectivity or ellipsometry measurements, for example in Er-doped GaAs,

Er-doped GaN or Er-doped Si24,36,37,72. The benefit of using THz measurement techniques to evaluate RE-doped semiconductor systems, as was shown here, is that the bandgap, refractive index,

and carrier concentration of RE-doped semiconductors can be determined, as well as barrier potentials for electrons within superlattice structures, using only THz techniques.

Regarding the performance of RE-doped GaN-based optoelectronics devices, the observed bandgap reduction and refractive index increase have significant benefits, particularly when a dilutely

doped superlattice structure is implemented. While Er-doped Si superlattice structures have been shown to exhibit a higher performance than bulk Er-doped Si, the underlying principle behind

this behavior has not been addressed in detail21,22,23,24,25. The results from this work could provide new insight into the origin of the enhancement due to the superlattice structure. In

addition, RE doping has been shown to increase the refractive index of host materials experimental approaches in Er-doped GaN36,37,74,75 or through first-principles calculations68. The

optical bandgap and index of refraction are related quantities35, which suggests that RE doping will reduce the bandgap of host semiconductors. In this case, the bandgap reduction in the

RE-doped superlattice structure induces the carrier confinement within the active layers, which enhances the carrier recombination and luminescence efficiency16,60,61. Additionally, the

higher index of refraction will result in a waveguiding effect37,72,75, increasing the extraction efficiency. These benefits are achieved even at a very low doping concentration of ~0.1%,

which also facilitates high crystal-quality growth. Moreover, the reduction of threading dislocations due to the alternating strain between Eu-doped layers and GaN layers was already

observed20. Taking all of these effects into consideration, it seems that introducing a superlattice structure with dilute Eu doping naturally enhances all of the properties needed to

produce high brightness, high-efficiency red GaN LEDs. Critically, we argue that these enhanced optical and electrical properties are not limited to the Eu-doped GaN system. Er-doped GaN,

for example, is known to have a higher refractive index than GaN, and GaN/GaN:Er waveguides and optical amplifiers that take advantage of this have already been reported36,37,74,75. By

introducing multiple alternating layers, an efficient Er-doped GaN LED operating at the telecom C-band could be achieved.

The use of carrier confinement by introducing a MQW structure is the standard way to increase carrier recombination in commercial LEDs1,2,3,4,5,6. However, although introducing an MQW

structure consisting of alternating Eu-doped GaN or Er-doped GaN layers and AlGaN barrier layers was shown to improve the PL intensity and the excitation efficiency of devices77,78,79,80,81;

the simple superlattice devices have consistently been found to significantly outperform the MQW devices16,77,78. One reason could be that the optimal growth conditions of the AlGaN barrier

layers needed to form intentional QWs are substantially different from those optimized for the RE-doped GaN layers. Therefore, a sacrifice must be made regarding the quality of either the

RE-doped GaN layers or the AlGaN layers. In addition, Xie et al. reported that large potential barriers in MQW structures can interfere with hole transport and cause poor LED performance65.

For the dilutely doped superlattice structure, the potential barriers for the electrons and holes were found to be