HgCdTe Photodiode Structure With Cadmium-Dopant Gradient Control
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Solution Overview
Problem
Existing photodiode structures face challenges in efficiently channeling photogenerated carriers, particularly in achieving a controlled cadmium and electrical dopant concentration gradient for improved infrared radiation detection, which is difficult to achieve with multiple growth baths and can result in suboptimal crystallographic quality and electro-optical performance.
Innovation Solution
A method involving liquid phase epitaxy to form a photodiode structure with a cadmium concentration gradient and electrical dopant gradient in a HgCdTe layer, using a single bath with cadmium and dopant precursors, where the cadmium concentration decreases from the substrate interface, and the dopant concentration gradient extends over a specific distance, forming a pn or pin junction with a second semiconductor material, ensuring better carrier channeling and signal transformation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If liquid phase epitaxy is used to grow HgCdTe layers with controlled dopant diffusion, then the channeling of photogenerated carriers is improved, but the manufacturing precision required for concentration gradients increases
Solution Approach 1:
The patent changes physical and chemical parameters during the liquid phase epitaxy process, specifically controlling temperature, cadmium concentration, and dopant concentration in the solution to achieve desired concentration gradients in the grown layers
Solution Approach 2:
The substrate is pre-doped with electrical dopants before the epitaxy process begins, so that during liquid phase epitaxy, the dopants automatically diffuse into the growing layer to create the required concentration gradient without needing separate doping steps
2Reliability
If multiple epitaxial layers are grown to create doping gradients, then the electrical properties are improved, but the device complexity increases
Solution Approach 1:
The patent combines multiple functions into a single liquid phase epitaxy process: layer growth, dopant diffusion, and concentration gradient formation are all achieved simultaneously in one step rather than through multiple separate processes
Solution Approach 2:
The liquid phase epitaxy process serves multiple purposes: it grows the HgCdTe layer, introduces dopants, creates concentration gradients, and controls electrical properties all through a single universal process
3Reliability
If the first layer thickness is reduced to less than 6 microns, then the photodiode performance is improved, but the manufacturing control difficulty increases
Solution Approach 1:
The liquid phase epitaxy process allows for monitoring and control of growth conditions in real-time, enabling precise control of layer thickness through feedback on temperature, solution composition, and growth rate
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 enhances the signal-to-noise ratio and improves the Modulation Transfer Function (MTF) by maintaining precise concentration gradients, reducing dark current, and facilitating efficient charge carrier collection, resulting in better photodiode performance and imaging capabilities.
Implementation Method 1
the liquid phase epitaxy being performed at a temperature achieving diffusion of a part of the cadmium atoms and of the first electrical dopant from the top layer to the first layer
Implementation Method 2
The electromagnetic radiation that passes through the photodiode with a higher energy than the band gap value is captured and transformed into an electron-hole pair
Data Source
AI summary
A substrate to fabricate a photodiode structure has a top layer made from cadmium-doped semiconductor material. A first HgCdTe-base layer is formed by liquid phase epitaxy from the top layer with a bath containing an n-type electrically active dopant to electrically dope the first layer. The cadmium diffuses from the top layer to the first layer to form a decreasing cadmium concentration gradient from the interface with the top layer in a direction away from the interface. The cadmium concentration gradient causes a decreasing band gap width gradient in the first layer from the interface and causes an n-type dopant concentration gradient in the first layer from the interface.