Metallic Contact with Voids for Deep Ultraviolet LED
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Solution Overview
Problem
Developing ohmic contacts for semiconductor layers, particularly for deep ultraviolet LEDs made from group III-nitride materials, is challenging due to the difficulty in achieving low resistance and high reflectivity or transparency, especially for p-type contacts where the work function mismatch leads to Schottky barrier formation and high contact resistance.
Innovation Solution
A metallic contact structure with protruding metallic elements penetrating through a barrier layer into a two-dimensional or three-dimensional free carrier gas at the heterointerface, combined with voids that increase reflectivity or transparency, is used to form ohmic contacts on semiconductor layers, reducing contact resistance and enhancing light extraction efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a metal contact is formed on p-type AlGaN semiconductor layer, then electrical contact is established, but Schottky barrier formation occurs due to work function mismatch resulting in high contact resistance
Solution Approach 1:
The contact structure is segmented into multiple functional layers: a first metal layer with high work function (Ni, Pd, Pt) forms the primary ohmic contact interface with the p-type AlGaN layer, while a second metal layer with lower work function (Al, Ti, Mo) is deposited on top to reduce Schottky barrier formation. This segmentation allows each layer to perform its specific function optimally, with the first layer providing low resistance contact and the second layer preventing barrier formation.
Solution Approach 2:
The contact structure uses a composite material approach by combining two different metal materials with complementary properties. The first metal layer (Ni, Pd, Pt) provides high work function for ohmic contact, while the second metal layer (Al, Ti, Mo) provides low work function to prevent Schottky barriers. This composite structure leverages the advantages of both material types to achieve both low contact resistance and prevention of harmful Schottky barrier formation.
2Reliability
If annealing process is used to improve ohmic contact, then contact resistance is reduced, but diffusion and oxidation of TiN occurs requiring additional gold layer
Solution Approach 1:
The patent uses thin, easily deposited metal layers (particularly the second metal layer of Al, Ti, or Mo) that serve as sacrificial or protective elements. These layers are deposited in thin configurations (e.g., 5-50 nm) that allow them to perform their protective function during annealing without requiring thick, complex structures. The simple, inexpensive materials can be easily deposited and removed or integrated without adding significant complexity.
Solution Approach 2:
The patent controls the thickness parameters of each metal layer to optimize performance. The first metal layer is kept thin (5-50 nm) to maintain ohmic contact properties, while the second metal layer is even thinner (2-20 nm) to provide protection without interfering with the underlying contact. By precisely controlling these thickness parameters, the structure achieves effective protection during annealing while maintaining simple overall complexity.
3Reliability
If contact area is increased to reduce resistance, then current crowding increases and light extraction efficiency decreases
Solution Approach 1:
The contact structure applies local quality by creating spatially varying properties within the contact region. The first metal layer with high work function is localized at the interface with the p-type AlGaN layer where ohmic contact is most needed, while the second metal layer with low work function is positioned on top where it provides protection and modifies the electrical field distribution. This local differentiation allows the contact to reduce resistance through optimized interface properties while the upper layer prevents current crowding and maintains light extraction by distributing the current more evenly across the contact area.
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 results in lower contact resistivity and improved light extraction efficiency by increasing the contact area and reducing current crowding, while also addressing the challenge of forming effective ohmic contacts on high aluminum content semiconductor layers.
Implementation Method 1
a first metal layer... configured to form an ohmic contact to the two-dimensional free carrier gas at the heterointerface
Implementation Method 2
The voids can be configured to increase an overall reflectivity or transparency of the contact
Implementation Method 3
the barrier layer forms a two dimensional free carrier gas for the contact at a heterointerface of the barrier layer and the semiconductor layer
Data Source
AI summary
A contact to a semiconductor layer in a light emitting structure is provided. The contact can include a plurality of contact areas formed of a metal and separated by a set of voids. The contact areas can be separated from one another by a characteristic distance selected based on a set of attributes of a semiconductor contact structure of the contact and a characteristic contact length scale of the contact. The voids can be configured to increase an overall reflectivity or transparency of the contact.