MicroLED arrays

Light-emitting diodes (LEDs) are an attractive alternative display technology. The high emissivity and low power consumption of LEDs promise significant improvements over existing technologies in augmented reality and mobile device displays. In order to cover the full-color gamut for displays, LED pixels contain red, blue, and green single LEDs arranged in an array. Hence, the resolution of displays depends on the size and pitch of single LEDs. In efforts to improve resolution by reducing the size of the emitters, the irradiance is consequently reduced. For smaller conventional LEDs to achieve comparable irradiance, they require a higher-input power that can lead to greater heat dissipation requirements.

Hexagonal core-shell pillar

To combat the aforementioned issues that use conventional flat emitters, researchers have turned attention to three-dimensional microLEDs (μLEDs). Such emitters have the obvious advantage of increased surface area, where further the increased dimensionality allows for the formation of quantum wells on the sides of the emitter. The increased dimensionality also provides access to various crystal faces. In the case of GaN, it includes the m-plane, which has a minimal native electric field that reduces the influence of the quantum confined stark effect (QCSE), and thereby improves the recombination efficiency of excited carriers.

Compositional variation and defects

Spatial variation in the emission spectrum

A GaN/InGaN core-shell pillar is a typical example of a three-dimensional emitter. Observing the cathodoluminescence (CL) from such a pillar allows for the extraction of spectral information as a function of position. Spectral information can be spatially mapped in bands of interest that are easy to display, e.g., the GaN, InGaN, and point defect bands. Further, in this analysis example, compositional spatial mapping of the InGaN quantum well active layer reveals the In-fraction lies between 19 – 26% and concentrated at the vertices. The native non-linear least squares (NLLS) spectra fitting tool in Gatan Microscopy Suite® software is useful to estimate the compositional mapping and to determine the InGaN bandgap as a function of position. The NLLS fitting determined bandgap allows for the estimation of In concentration, x, and material composition is determined by observing the theoretical bandgap of InxGa1-xN as a function of x. Thus far, we have information on spectral CL emissions, but without information concerning the angles of emissions, such shaped specimens can emit anisotropically. To observe such anisotropy, the angle-resolved CL mode of operation can be used.

Angle-resolved cathodoluminescence

When light is emitted from a specimen at the focal point of the parabolic mirror, it reflects down the axis of the mirror. Mathematically, the paraboloidal shape is 1-to-1. Therefore, the position of the light’s reflection on the collimating mirror is unique to the angles of emission from the sample. Hence, acquisition of the angular emission information is done by simply imaging the backplane of the paraboloidal collection mirror. With a known mirror shape, you can transform the data into a polar representation for easier interpretation.

Emission selectivity and shape effects

In the case of the single out-of-array microLED pillar, the ARCL revealed an emission selectivity of about 2.4x stronger at ~70˚ as compared to the normal direction when integrating over the entire pillar. As one might expect, the direction of emission depends on the position of excitation. In fact, the emission is most intense in the direction opposite from the side of the pillar being excited and includes an interference pattern. The intensity directionality is likely due to the effect of total internal reflection from the index contrast at the pillar surface with vacuum, while the interference pattern is due to reflections from the sample surface.

microLED hexagonal pillar array

MicroLEDs are most frequently arranged in an array configuration. In this case, the array configuration is hexagonal with (edge-to-edge) pillar width ~800 nm and a (nearest neighbor) period of ~2 μm. Such an array using GaN opens the possibility of photonic influences on optical behavior. Therefore, it is important to observe them in such an array to determine the impact on light emission.

Effective compositional variation and defects

Spatial variation in the emission spectrum

An in-array single pillar was observed using the same data acquisition as the previously displayed out-of-array pillar. Again, we observe in this spectrum image, a spectrum for every point scanned, which allows for emission band mapping of the GaN, InGaN, and point defect bands, as well as an estimation of In content by position across the pillar surface. Here we see an estimate of indium content in InxGa1-xN content of between 11 and 19%.

Several interesting observations can be made in this comparison of in- vs. out-of-array microLED pillars. In the out-of-array device, the overall intensity is coming from the edge, as one might expect since the quantum well is being excited directly. While for the in-array device, the intensity is more evenly distributed across the surface. Observing spectra from various positions further reveals an enhancement in the overall intensity of the in-array device, especially in the 500 – 600 nm range where multiple smaller peaks are present. The differences are perhaps even more obviated by the color band images of the GaN, InGaN, and point defect bands. In these images, the array influence produces very different emission map for each excitation position.

Once the array's influence on light generation is understood, it is common to explore the emission anisotropy as a function of wavelength to determine whether a microLED is feasible for implementation in a display device. To do this, we employ the wavelength- and angle-resolved CL (WARCL) technique that includes wavelength information along with angular. This reveals any preferential direction of emission and allows for the observation of photonic bandgaps.

Here is a side-by-side comparison of WARCL frames at several wavelengths. The weaker emission in the direction normal to the pillar is present in both the out- and in-array LEDs. However, an interesting behavior is present for the in-array demonstrating the usefulness of the WARCL technique for microLED arrays.  

Further information on the photonic crystal behavior of micro-pillar arrays can be found here.