Displays are moving towards higher efficiencies and sharper resolutions with the advent of applications such as augmented and virtual reality. This evolution requires shrinking of the individual pixels that emit light, however it has proven to be quite challenging. Due to their small size, individual devices need to be extremely bright and capable of providing strong luminescence while being self-emitting, which disqualifies existing technologies like organic light emitting diodes (OLEDs) and liquid crystal displays (LCDs). Inorganic semiconductor light emitting diodes (LEDs) can satisfy these requirements, however to date, their efficiency is significantly reduced as the device dimensions are lowered, and this reduction has been primarily attributed to the surface damage induced during LED fabrication.
The III-nitrides are more resilient to surface recombination as compared to other III-V compounds due to their lower surface recombination velocity, making them a suitable choice for micro-LEDs. However, attaining high-quality longer wavelength emission, such as green and red, is not trivial for InGaN-based devices due to the large lattice mismatch between the high indium composition InGaN active region, and the typical GaN substrates used. This mismatch can generate defects that adversely affect device performance, while strain-induced polarization fields would separate the electron and hole wavefunctions, lowering the probability of their radiative recombination. These factors have made the realization of efficient red microLEDs extremely difficult, as plotted in Fig. 1 showing the variation of external quantum efficiency (EQE) of red-emitting LEDs having different areas.
Fig. 1: Variation of external quantum efficiency with device area for some previously reported red-emitting InGaN LEDs from literature.
In this context, the research group led by Prof. Zetian Mi from the University of Michigan, Ann Arbor, demonstrated a red-emitting III-nitride based submicron-scale LED, having record efficiency for its size. This was achieved by incorporating an InGaN/GaN short-period superlattice (SPSL) beneath the InGaN active region of a III-nitride N-polar nanowire heterostructure. The SPSL helps in relaxing strain before the growth of the active region, allowing for higher indium incorporation and hence longer emission wavelengths. This advantage has been multiplied with the use of nanostructures, due to their defect-free nature and large surface area to volume ratio, that enables efficient strain relaxation.
The relevant research results are published in Photonics Research, Volume. 10, Issue 12, 2022 (A. Pandey, J. Min, Y. Malhotra, M. Reddeppa, Y. Xiao, Y. Wu, Z. Mi. Strain-engineered N-polar InGaN nanowires: towards high-efficiency red LEDs on the micrometer scale[J]. Photonics Research, 2022, 10(12): 2809).
N-polar nanowires were grown, with precisely controlled dimensions, spacing and morphology, using selective area growth, on a Veeco GEN930 plasma-assisted molecular beam epitaxy system. Scanning electron microscope images of nanowire arrays exhibit excellent morphology, shown in Fig. 2(a). A clear red-shift of the InGaN active region emission was observed in the photoluminescence spectrum for nanowires that incorporated the SPSL, as shown in Fig. 2(b), confirming its advantages.
Fig. 2: (a) SEM image of a nanowire array. (b) PL spectrum of nanowires grown with only an InGaN SPSL, only the InGaN dots and a combination of the two. (c) Variation of EQE and WPE with injected current density for the fabricated microLED. (d) EL spectra of the microLED at different injection currents.
Sub-micron scale LEDs, with an area of only 750 nm × 750 nm, that were fabricated using these nanowires displayed excellent diode characteristics and a peak EQE of 2.2%, measured at a current density of ~0.4 A/cm2 directly on wafer. The variation of EQE and wall-plug efficiency with injected current density is plotted in Fig. 2(c). Electroluminescence spectra of the device, shown in Fig. 2(d), exhibit peak emission wavelength at ~630 nm, when the device is operated at currents close to the peak of the efficiency.
This work highlights the crucial role of strain relaxation, achieved with a nanostructure SPSL, for attaining red emission. Further research on this topic, including modification of the device structure for longer wavelength emission at higher currents and optimization of the p-GaN to reduce electron overflow will enable high efficiency red LEDs at the micrometer scale, which provides new opportunities for future high-resolution, small-scale optoelectronic devices.
