Micro-LED Sidewall Geometry for Higher Light Extraction
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Solution Overview
Problem
Conventional micro-LEDs with small lateral dimensions suffer from low light extraction due to light being trapped within the semiconductor material, primarily due to refractive index differences, leading to inefficient light emission.
Innovation Solution
The implementation of a micro-LED device with a non-vertical sidewall and a low-index epitaxial layer, along with a mirror, enhances light extraction by directing light through the output facet, achieving a significant percentage of optical power escape, particularly through the use of optical interfaces and secondary micro-optics to control light trajectories and reflections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a conventional micro-LED with vertical sidewalls is used, then the device structure is simple and easy to manufacture, but light extraction efficiency is low due to light being trapped by total internal reflection at the semiconductor-air interface
Solution Approach 1:
The patent introduces non-vertical sidewalls with a specific angle γ (30°-60°) relative to the normal of the light-emitting layer. This asymmetric geometry modifies light trajectories by causing reflected light to strike the optical interface at angles that reduce total internal reflection, thereby improving light extraction efficiency while maintaining manufacturing feasibility through standard epitaxial growth and etching processes
Solution Approach 2:
The patent adds a dimensional parameter by introducing the sidewall angle γ as a new geometric degree of freedom. This angular dimension allows control over light reflection paths, enabling extracted light to escape through the output facet rather than being trapped, thus resolving the light extraction problem without complicating the manufacturing process
2Manufacturing precision
If the lateral dimension of the micro-LED is reduced to increase pixel density, then the display resolution improves, but light extraction efficiency decreases due to increased dominance of total internal reflection
Solution Approach 1:
The non-vertical sidewall geometry provides a scale-invariant solution that works effectively at reduced lateral dimensions. The fixed angle γ creates consistent light redirection paths regardless of device size, ensuring that even as pixels become smaller and more densely packed, the light extraction mechanism remains effective due to the angular control of reflection paths
3Loss of energy
If optical interfaces and non-vertical sidewalls are introduced to improve light extraction, then light extraction efficiency increases, but the device structure becomes more complex
Solution Approach 1:
The patent applies the non-vertical sidewall modification locally at the lateral boundaries of the micro-LED, while the central light-emitting region maintains its standard planar structure. This localized geometric modification targets the specific problem area (sidewall light trapping) without requiring complex changes to the entire device structure, thus improving light extraction with minimal increase in overall complexity
Solution Approach 2:
The patent modifies a single geometric parameter - the sidewall angle γ - within a specific range (30°-60°). This parameter change approach allows optimization of light extraction through angular control of light paths, while the simplicity of defining and manufacturing this single angular parameter keeps the overall device complexity manageable
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
This configuration significantly increases light extraction efficiency, with at least 50% of the optical power emitted escaping through the output facet, enhancing the display's brightness and color accuracy by directing light in desired trajectories.
Implementation Method 1
a second light in a second trajectory that reflects off the optical interface and the non-vertical sidewall and escapes through the output facet
Implementation Method 2
a method comprising selecting an angle γ of a non-vertical sidewall with respect to a normal of a light-emitting layer of a micro light emitting diode (micro-LED), the micro-LED comprising a layer having an optical thickness d between the light-emitting layer and a mirror, to direct a first light emitted by the light-emitting layer along a first trajectory that reflects off the non-vertical sidewall and through an output facet of the micro-LED
Data Source
AI summary
A micro light-emitting diode, micro-LED, is configured to provide increased light-extraction. The micro-LED (300) according to the present disclosure has a lateral dimension less than 20 micrometres and comprises: a light-emitting layer (106), an output facet (308), an optical interface (via a mirror 114) parallel to the light-emitting layer, and a non-vertical sidewall (102) forming an angle with respect to a normal of the light-emitting layer. The layer (106) emits light (310) comprising a first light (312) in a first trajectory that escapes the micro-LED through the output facet (308) without reflecting off the optical interface and a second light (314) in a second trajectory that reflects off the optical interface and the non-vertical sidewall (102) and escapes through the output facet. The disclosed micro-LEDs may further include one or more of: cavity effects, light guiding, total internal reflection at low-index layers, mirrors, and micro-optics.