UV LED Mesa Sidewalls at Brewster Angle for Light Extraction
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Solution Overview
Problem
Short-wavelength ultraviolet light-emitting diodes (UV LEDs) face limitations in efficiency and reliability due to high defect levels in nitride semiconductor systems, particularly at wavelengths shorter than 270 nm, leading to poor photon-extraction efficiency and low wall-plug efficiencies, despite advancements in low-defect AlN substrates.
Innovation Solution
The UV LEDs are structured with mesas having lateral surfaces angled at the Brewster angle to enhance lateral emission, combined with reflectors to redirect photons vertically, optimizing the emission efficiency of both TM- and TE-polarized photons, and optionally using substrate thinning or texturing to further improve photon extraction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If UV LEDs are configured to emit light at wavelengths shorter than 240 nm, then the emission wavelength is reduced, but the photon extraction efficiency deteriorates due to increased TM-polarized photons that are difficult to extract
Solution Approach 1:
The patent introduces a third dimension by forming mesas with angled sidewalls instead of flat surfaces. The sidewalls are angled at approximately the Brewster angle (e.g., 20-30 degrees from vertical) to enable TM-polarized photons to escape laterally from the sidewall surfaces, adding a lateral extraction dimension to the conventional vertical extraction path
Solution Approach 2:
The patent changes the geometric parameter of the emitting surface by forming angled sidewalls at the Brewster angle. This parameter change optimizes the extraction of TM-polarized photons by matching the angle of incidence to the Brewster angle, where TM-polarized light experiences minimal reflection and maximal transmission
2Device complexity
If conventional flat surface UV LEDs are used, then the device structure is simple, but the photon extraction efficiency is poor due to the large index of refraction of AlN resulting in a small escape cone
Solution Approach 1:
The patent transforms the flat two-dimensional emitting surface into a three-dimensional mesa structure with angled sidewalls. This dimensional change creates additional emission surfaces oriented at the Brewster angle, enabling efficient extraction of TM-polarized photons that would otherwise be trapped by the large refractive index mismatch
Solution Approach 2:
The patent introduces asymmetry by forming mesas with angled sidewalls rather than symmetric flat surfaces. The asymmetric angled geometry is specifically designed to match the Brewster angle for TM-polarized light, creating preferential extraction paths that break the symmetry of conventional flat LED surfaces
3Reliability
If AlN substrates are used to reduce defects, then the internal efficiency is improved, but the photon extraction efficiency remains poor due to substrate absorption and the p-contact absorbing half of the generated photons
Solution Approach 1:
The patent segments the light extraction function by creating separate extraction paths: TE-polarized photons continue to exit vertically from the top surface, while TM-polarized photons are directed to exit laterally from the angled sidewalls. This segmentation allows each polarization mode to utilize its optimal extraction path, minimizing total energy loss
Solution Approach 2:
The patent adds a lateral extraction dimension through angled sidewalls to complement the vertical extraction path. This dimensional addition provides an alternative escape route for TM-polarized photons that would otherwise be absorbed by the p-contact or substrate, reducing overall energy loss
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 approach significantly increases the external quantum efficiency and light output of UV LEDs, particularly at short wavelengths, by efficiently transmitting TM-polarized photons from angled sidewalls and allowing vertical emission of TE-polarized photons, thereby improving the overall photon-extraction efficiency.
Implementation Method 1
At least a portion of a sidewall of the light-emitting device structure is angled with respect to a normal to the top surface of the substrate at an angle of approximately a Brewster angle of the light emitted by the multiple-quantum well layer
Implementation Method 2
combined with reflectors to redirect photons vertically
Data Source
AI summary
In various embodiments, device structures configured to emit ultraviolet light have lateral surfaces that form angles to the substrate normal of approximately the Brewster angle corresponding to the light-emitting portion of the device structure. The device structures may include one or more mesas disposed over a shared substrate or handle wafer.