AlN Crystal Impurity Control and Annealing for Deep-UV Transparency
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Solution Overview
Problem
The commercial feasibility of aluminum nitride (AlN)-based semiconductor devices is limited by the scarcity and high cost of low-defect single crystals, and existing techniques fail to achieve adequate UV transparency, particularly at deep-UV wavelengths due to contamination and point defects during the growth process.
Innovation Solution
The production of highly UV-transparent single-crystal AlN is enabled through vapor-phase growth, impurity control, post-growth temperature control, and isothermal or quasi-isothermal annealing techniques, which result in low UV absorption coefficients and uniform optical properties across deep-UV wavelengths without the need to eliminate carbon and oxygen to impractical levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional sublimation-recondensation growth is used to produce AlN single crystals, then crystal growth rate can be increased, but UV transparency deteriorates due to contamination and point defects
Solution Approach 1:
The patent applies parameter changes by precisely controlling the nitrogen partial pressure (0.1-10 Torr) and temperature gradient (5-20°C/mm) during sublimation-recondensation growth. These parameter optimizations enable high growth rates while minimizing contamination and point defects, thereby maintaining UV transparency despite increased productivity
Solution Approach 2:
The patent utilizes an inert nitrogen atmosphere during the sublimation-recondensation process to prevent oxidation and contamination of the AlN crystal. By maintaining a controlled nitrogen environment with specific partial pressures, the method prevents harmful impurities from incorporating into the crystal lattice, thus preserving UV transparency while enabling faster growth
2Reliability
If oxygen and carbon impurities are eliminated to achieve high UV transparency, then manufacturing complexity and cost increase significantly
Solution Approach 1:
The patent applies preliminary action by optimizing the composition and purity of the starting AlN powder before the sublimation-recondensation process. By pre-characterizing and selecting powders with specific oxygen and carbon content ranges, the method prevents excessive impurity incorporation from the outset, achieving UV transparency without requiring extremely complex purification procedures
Solution Approach 2:
The patent maintains continuous control of impurity levels throughout the entire growth process by sustaining optimized nitrogen partial pressure and temperature conditions. This continuous control prevents impurity accumulation and ensures consistent UV transparency without requiring intermittent complex intervention or post-growth purification steps
3Productivity
If rapid cooling is applied after growth to increase productivity, then manufacturing efficiency improves, but crystal cracking occurs due to thermal stress
Solution Approach 1:
The patent applies dynamics by implementing a staged cooling process that transitions from rapid cooling to slow cooling at specific temperature thresholds. The system dynamically adjusts the cooling rate based on the crystal's thermal state, enabling high initial cooling rates for productivity while switching to slow cooling near room temperature to prevent thermal stress cracking
Solution Approach 2:
The patent employs periodic action through a multi-stage cooling schedule with distinct rate phases. The cooling process alternates between rapid cooling phases (for efficiency) and slow cooling phases (for stress relief), creating a periodic pattern that maintains both productivity and crystal integrity throughout the cooling cycle
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 produces AlN single crystals with UV absorption coefficients below 10 cm−1 for wavelengths between 230 nm and 280 nm, facilitating improved performance of optical devices like LEDs and lasers with consistent optical properties and reduced risk of cracking during cooling.
Implementation Method 1
The most effective method of growing AlN bulk single crystals is the 'sublimation-recondensation' method that involves sublimation of lower-quality (typically polycrystalline) AlN source material and recondensation of the resulting vapor to form the single-crystal AlN
Implementation Method 2
vapor including, consisting essentially of, or consisting of aluminum and nitrogen is condensed on the seed crystal, thereby forming a single-crystalline AlN boule
Data Source
AI summary
In various embodiments, single-crystal aluminum nitride boules and substrates are formed from the vapor phase with controlled levels of impurities such as carbon. Single-crystal aluminum nitride may be heat treated via quasi-isothermal annealing and controlled cooling to improve its ultraviolet absorption coefficient and/or Urbach energy.