Ammonia Preflow Nucleation for AlN Buffer Layers on Silicon
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Solution Overview
Problem
Current methods for growing high-quality epitaxial gallium nitride (GaN) layers on silicon substrates are hindered by lattice constant and thermal expansion coefficient mismatches, leading to structural defects, cracking, and poor optical properties due to the use of existing buffer layers like AlN and AlGaN.
Innovation Solution
A method involving a silicon substrate with a single polarity aluminum nitride (AlN) buffer layer and an aluminum gallium nitride (AlxGa1-xN) layer, where ammonia preflowing ensures nitrogen atoms bond to the silicon surface, reducing dislocation density and enabling smooth interface morphology, thus compensating for lattice mismatch and thermal expansion differences.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If buffer layers of AlN and AlGaN are grown between silicon substrate and GaN layers, then lattice mismatch and thermal expansion differences are compensated, but structural defects such as discontinuities, dislocations and faults occur in the epitaxial GaN layers
Solution Approach 1:
The patent applies preliminary action by performing an ammonia preflow treatment before growing the AlN buffer layer. This preflow creates a nitrogen-terminated silicon surface that serves as an ideal nucleation site for AlN, ensuring proper crystal orientation and reducing dislocation density before the main buffer layer growth begins
Solution Approach 2:
The patent changes the surface termination parameter of the silicon substrate from its native state to nitrogen-terminated state through ammonia preflow. This parameter change fundamentally alters how the AlN buffer layer nucleates and grows, leading to fewer structural defects while maintaining lattice mismatch compensation
2Ease of manufacture
If GaN is grown directly on silicon substrate, then manufacturing cost is reduced, but the GaN layers crack upon cooling due to lattice constant and thermal expansion coefficient differences
Solution Approach 1:
The patent uses an AlN buffer layer as an intermediary between the silicon substrate and GaN layers. This intermediary layer compensates for the lattice mismatch and thermal expansion coefficient differences, preventing cracks while maintaining cost-effectiveness by using silicon instead of sapphire substrates
Solution Approach 2:
The ammonia preflow is performed as a preliminary action before AlN buffer layer growth to ensure proper surface termination. This preliminary treatment ensures the AlN layer forms with correct orientation and minimal defects, enabling the buffer layer to effectively mediate between silicon and GaN
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 allows for the growth of high-quality GaN layers with reduced structural defects and improved optical properties, suitable for use in LEDs, by creating a compressive stress that counteracts tensile stress in GaN layers, ensuring 2-dimensional growth and reduced substrate bowing.
Implementation Method 1
A method is disclosed for beginning the growth of AlN that results in a smooth interface between AlN and Si(111). The method grows a single polarity buffer layer of AlN having a low dislocation density. The properties of the aluminum nitride nucleation layer are determined in large part by the conditions under which the growth of the AlN is initiated and by how the silicon substrate is treated prior to growth of the AlN.
Implementation Method 2
ammonia preflowing ensures nitrogen atoms bond to the silicon surface, reducing dislocation density and enabling smooth interface morphology
Implementation Method 3
Because of the great difference between the lattice constants and thermal expansion coefficients of GaN and silicon, GaN is not well suited for epitaxial growth directly on a silicon substrate. GaN has a much larger coefficient of thermal expansion than does silicon. So as the layers of GaN grown on silicon at high temperature cool to room temperature, the smaller lattice distance of the GaN crystals relative to the silicon crystals becomes even more pronounced.
Implementation Method 4
The AlN buffer layer overlies the Si(111) surface of the substrate and is between 205 to 250 nanometers thick... creating a compressive stress that counteracts tensile stress in GaN layers
Implementation Method 5
The second buffer layer of aluminum gallium nitride is disposed between the buffer layer of aluminum nitride and the upper layer of gallium nitride... allowing high quality epitaxial GaN layers with fewer structural defects to be grown over the buffer layers
Data Source
AI summary
A silicon wafer used in manufacturing crystalline GaN for light emitting diodes (LEDs) includes a silicon substrate, a buffer layer of aluminum nitride (AlN) and an upper layer of GaN. The silicon wafer has a diameter of at least 200 millimeters and an Si(111)1×1 surface. The AlN buffer layer overlies the Si(111) surface. The GaN upper layer is disposed above the buffer layer. Across the entire wafer substantially no aluminum atoms of the AlN are present in a bottom most plane of atoms of the AlN, and across the entire wafer substantially only nitrogen atoms of the AlN are present in the bottom most plane of atoms of the AlN. A method of making the AlN buffer layer includes preflowing a first amount of ammonia equaling less than 0.01% by volume of hydrogen flowing through a chamber before flowing trimethylaluminum and then a subsequent amount of ammonia through the chamber.