Porous GaP LED Reflector for Light Extraction
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Solution Overview
Problem
GaP-based LEDs have low light extraction efficiency due to internal reflections, resulting in only about 14% of incident electrons being emitted as photons, and existing methods to enhance efficiency either do not address this effectively or increase the operating voltage.
Innovation Solution
A porous, light-diffusing n-type GaP layer is grown over the top p-type layer of the LED, with electrochemical etching creating pores that increase light scattering without affecting the forward voltage by making direct electrical contact to the underlying p-type layer through metal-filled vias or p-type doping, allowing light to escape without increasing the forward voltage drop.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a porous layer is added to increase light extraction efficiency, then light extraction efficiency is improved, but forward voltage increases due to increased electrical resistance
Solution Approach 1:
The device is segmented into distinct functional zones: a porous light-diffusing reflector layer for optical enhancement, and a nonporous current-blocking layer for electrical isolation. This segmentation allows the porous layer to improve light extraction without requiring current to pass through it, thereby avoiding the forward voltage penalty that would result from making the entire structure porous.
Solution Approach 2:
Different regions of the device are given different properties: the reflector layer is made porous to maximize light scattering and extraction, while the current-blocking layer remains nonporous to maintain low electrical resistance and block current effectively. This local differentiation of properties allows simultaneous optimization of optical and electrical performance.
2Illumination intensity
If the GaAs substrate is removed and replaced with transparent GaP substrate, then light emission is improved, but manufacturing complexity increases
Solution Approach 1:
The light-absorbing GaAs substrate is extracted and removed from the final device structure. It serves only as a temporary growth substrate during fabrication and is discarded before the product is completed. This allows the use of GaAs for convenient layer growth while achieving superior light emission properties with the transparent GaP substrate in the final device.
Solution Approach 2:
The GaAs substrate is used for preliminary layer growth and structure formation before being removed. All critical semiconductor layers are grown on the GaAs substrate using its favorable crystal structure, then the substrate is removed and replaced with the transparent GaP substrate that provides superior optical properties for the final emission phase.
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
The light extraction efficiency of GaP-based LEDs is increased by approximately 30% to about 18% without any increase in operating voltage, as the porous layer effectively scatters and reflects light to enhance emission, while maintaining minimal electrical resistance.
Implementation Method 1
the light that impinges on the porous layer is reflected and scattered, increasing the chances that the reflected light will exit out of the LED
Implementation Method 2
a light ray is internally reflected within the LED unless the light ray impinges on a wall of the LED within about 17 degrees of normal (the critical angle)
Implementation Method 3
The thick n-type GaP layer is then subjected to an electrochemical etch process that causes the n-type GaP layer to become porous, reflective, and light-diffusing
Implementation Method 4
The active layer is sandwiched between a p-type cladding layer and an n-type cladding layer, resulting in a heterostructure
Implementation Method 5
When light is generated by the active layer
Data Source
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AI summary
The invention relates to light emitting diodes and, in particular, to a technique for improving the light extraction efficiency without increasing the operating voltage by providing a light diffusing layer that adds virtually no forward voltage drop. In one embodiment, an AlInGaP LED includes a bottom n-type layer (16), an active layer (18), a top p-type layer (22), and a thick n-type GaP (24) layer over the top p-type layer. The thick n-type GaP layer is then subjected to an electrochemical etch process that causes the n-type GaP layer to become porous and light-diffusing. Electrical contact is made to the p-GaP layer under the porous n-GaP layer by providing metal-filled vias through the porous layer, or electrical contact is made through non-porous regions of the GaP layer between porous regions. The LED chip may be mounted on a submount with the porous n-GaP layer facing the submount surface. The pores and metal layer reflect and diffuse the light, which greatly increases the light output of the LED. Other embodiments of the LED structure are described.