Micro-LED Mesa Sidewall Epitaxy for Directional Light Extraction
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Solution Overview
Problem
Micro-LEDs, particularly AlGaInP-based red light-emitting diodes, face challenges in improving collected light extraction efficiencies due to high surface recombination rates, total internal reflection, and optical crosstalk, which reduce their internal and external quantum efficiencies as device sizes decrease.
Innovation Solution
The use of reduced quantum well sizes and a combination of high-refractive index and low-refractive index passivation layers, along with micro-lenses, to enhance light extraction by increasing the optical distance between the active region and sidewall metal reflectors, reducing surface plasmon resonance, and improving collimation of emitted light.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If device size is reduced to increase packing density, then high resolution is achieved, but surface recombination rate increases reducing quantum efficiency
Solution Approach 1:
The patent applies local quality by creating a mesa structure with different refractive index regions: the core mesa region maintains high refractive index for light generation, while the sidewall passivation layer provides a lower refractive index region. This local differentiation addresses surface recombination at the sidewalls without affecting the overall device size and packing density.
Solution Approach 2:
The patent uses composite materials by combining the semiconductor mesa structure with a passivation layer of different refractive index. This composite structure reduces surface recombination at the sidewalls while maintaining the small device size needed for high packing density, thereby preserving quantum efficiency.
2Manufacturing precision
If device size is reduced to increase packing density, then high resolution is achieved, but total internal reflection increases reducing light extraction efficiency
Solution Approach 1:
The patent creates a local refractive index gradient at the sidewalls through the passivation layer. This local quality change allows light to escape more efficiently at the sidewall regions without requiring larger device dimensions, thus maintaining high packing density while improving light extraction.
Solution Approach 2:
The patent changes the refractive index parameter at the sidewall region by introducing a passivation layer with lower refractive index than the mesa core. This parameter change reduces total internal reflection at the sidewalls, improving light extraction efficiency without increasing device size.
3Manufacturing precision
If device size is reduced to increase packing density, then high resolution is achieved, but optical crosstalk increases reducing directional control
Solution Approach 1:
The patent applies local quality by confining light emission to a specific directional pattern through the mesa structure geometry and sidewall passivation. This localized light control prevents optical crosstalk between adjacent pixels while maintaining high packing density, as each micro-LED emits light in a controlled direction rather than omnidirectionally.
Solution Approach 2:
The patent uses the vertical dimension of the mesa structure to control light directionality. By creating a three-dimensional mesa geometry with specific sidewall angles and heights, the patent directs light emission vertically while suppressing lateral emission, thereby reducing optical crosstalk between adjacent pixels in the high-density array.
4Reliability
If passivation layer thickness is increased to reduce surface recombination, then quantum efficiency improves, but optical distance increases reducing extraction efficiency
Solution Approach 1:
The patent optimizes the thickness parameter of the passivation layer to achieve the right balance. By controlling the passivation layer thickness to be sufficient for reducing surface recombination but not excessively thick, the patent maintains both high quantum efficiency and good light extraction efficiency. The specific thickness is tuned based on the refractive index difference and device geometry.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Significantly increases the collected light extraction efficiency, improves tolerance to lens width variations, and reduces optical crosstalk, resulting in enhanced light emission directionality and overall efficiency of micro-LEDs.
Implementation Method 1
an insulator layer that includes an undoped semiconductor passivation layer grown on sidewalls of the semiconductor mesa structure
Implementation Method 2
a dielectric passivation layer characterized by a refractive index lower than a refractive index of the undoped semiconductor passivation layer
Implementation Method 3
reducing surface plasmon resonance
Implementation Method 4
a reflective metal layer deposited on the dielectric passivation layer
Implementation Method 5
a micro-lens configured to collimate the visible light emitted by the active region
Implementation Method 6
an active region configured to emit visible light
Implementation Method 7
The sidewalls of the semiconductor mesa structure may be outwardly tilted from the p-type semiconductor layer to the n-type semiconductor layer
Data Source
AI summary
A micro-light emitting diode includes a semiconductor mesa structure that includes at least a portion of an n-type semiconductor layer, an active region configured to emit visible light, and a p-type semiconductor layer. The micro-LED device also includes an insulator layer that includes an undoped semiconductor passivation layer grown on sidewalls of the semiconductor mesa structure, and a dielectric passivation layer characterized by a refractive index lower than a refractive index of the undoped semiconductor passivation layer. The micro-LED device further includes a reflective metal layer deposited on the dielectric passivation layer, and a micro-lens configured to collimate the visible light emitted by the active region, where a ratio between a width of the micro-lens and a width of the active region may be greater than about 1.5.