Deep UV LED Tunnel Junction Structure for Stable Hole Injection
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Solution Overview
Problem
Existing UV LEDs face challenges in achieving high efficiency and stability at wavelengths below 260 nm due to poor hole transport, electron overflow, and parasitic recombination, primarily because of asymmetric doping and difficulty in p-type doping, leading to reduced carrier injection efficiency.
Innovation Solution
The development of a light emitting device with graded p-doped and n-doped regions, incorporating a tunnel junction and non-uniform charge confinement structures, such as quantum wells and nanoclusters, grown using molecular beam epitaxy to enhance hole injection and transport, and reduce nonradiative recombination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If asymmetric doping is used in conventional UV LEDs, then device structure is simplified, but hole transport and carrier injection efficiency deteriorate
Solution Approach 1:
The patent applies local quality by implementing graded doping concentrations in specific regions. The p-doped region has a gradient from 1×10^19 to 1×10^20 cm^-3, and the n-doped region has a gradient from 1×10^19 to 5×10^19 cm^-3. This localized variation in doping quality optimizes hole transport in the p-region while managing electron injection in the n-region, resolving the contradiction between structural simplicity and transport efficiency.
Solution Approach 2:
The patent changes the doping concentration parameter continuously through graded regions rather than using abrupt asymmetric doping. The gradual transition of dopant concentrations (Mg for p-type, Si for n-type) across the AlGaN layers modifies the electrical parameters to enhance carrier injection and hole transport, thereby improving reliability without excessive structural complexity.
2Productivity
If p-type doping is enhanced to improve hole injection, then carrier injection efficiency improves, but parasitic recombination increases
Solution Approach 1:
The patent applies local quality by creating a graded p-doped region where the doping concentration varies spatially from 1×10^19 to 1×10^20 cm^-3. This localized gradient allows high hole injection efficiency near the active region while reducing parasitic recombination in lower-doped areas, optimizing the trade-off between productivity and energy loss.
Solution Approach 2:
The p-doped region is segmented into multiple layers with progressively increasing doping concentrations. This segmentation allows different portions of the p-region to serve different functions: lower-doped sections minimize parasitic recombination while higher-doped sections near the active region maximize hole injection, thus resolving the contradiction between carrier injection efficiency and parasitic recombination loss.
3Productivity
If tunnel junction thickness is reduced to improve carrier transport, then current spreading improves, but manufacturing precision requirements increase
Solution Approach 1:
The patent changes the thickness parameter of the tunnel junction to an optimized range of 3-7 nm, balancing current spreading efficiency with manufacturability. This specific thickness range allows sufficient carrier transport and current spreading while remaining within the capabilities of molecular beam epitaxy growth control, thus resolving the contradiction between productivity and manufacturing precision.
4Loss of energy
If MBE growth conditions are optimized to form nanoscale clusters, then nonradiative recombination is reduced, but growth process complexity increases
Solution Approach 1:
The patent changes the growth parameters (temperature, pressure, flux ratios) during MBE to promote the formation of nanoscale clusters in the AlGaN active region. These clusters reduce nonradiative recombination by providing radiative recombination centers. The parameter optimization is applied specifically to the active region growth step, minimizing overall process complexity while achieving the desired reduction in 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
The device achieves external quantum efficiency (EQE) greater than 5% and wall plug efficiency (WPE) of more than 4% at wavelengths below 260 nm, with stable emission peaks and reduced efficiency droop, significantly outperforming previous technologies.
Implementation Method 1
Tunnel junction structures have been investigated as an alternative to resistive p-AlGaN contact layers and absorptive p-GaN contact layers in UV LEDs. The reduced resistivity of the n-AlGaN contact layer helps to increase carrier injection to the active region
Implementation Method 2
a graded p-doped region, with a higher concentration of p-dopants towards one side of the p-doped region and a lower concentration of p-dopants towards an opposite side of the p-doped region
Implementation Method 3
The active region comprises quantum well heterostructures with barrier layer thicknesses varying from the n-doped region to the p-doped region in a configuration that enhances hole injection and transport
Implementation Method 4
by varying the growth conditions, nanoscale clusters can be formed in Al-rich AlGaN, which can provide strong carrier confinement and therefore effectively reduce nonradiative recombination
Implementation Method 5
The device achieves external quantum efficiency (EQE) greater than 5% and wall plug efficiency (WPE) of more than 4% at wavelengths below 260 nm, with stable emission peaks
Data Source
AI summary
The presented devices and methods are directed to efficient and effective photon emission. In one embodiment, high-performance tunnel junction deep ultraviolet (UV) light-emitting diodes (LEDs) are created using plasma-assisted molecular beam epitaxy. The device heterostructure was grown under slightly Ga-rich conditions to promote the formation of nanoscale clusters in the active region. The nanoscale clusters can act as charge containment configurations. In one exemplary implementation, a device operates at approximately 255 nm light emission with a maximum external quantum efficiency (EPE) of 7.2% and wall-plug efficiency (WPE) of 4%, which are nearly one to two orders of magnitude higher than previously reported tunnel junction devices operating at this wavelength. The devices exhibit highly stable emission originating from highly localized carriers in Ga-rich regions formed in the active region, with nearly constant emission peak with increasing current density up to 200 A/cm2, due to the strong charge carrier confinement related to the presence of nanoclusters (e.g., Ga-rich) and radiative emission originating from highly localized carriers in Ga-rich regions formed in the active region.