# Presentation: 862879

## Analysis of Defects and Their Causes in Bulk Aluminum Nitride CrystalsJ.H. Edgar, Kansas State University, DMR-0408874

- 	Aluminum nitride crystals are being developed as substrates for group III-nitride semiconductor devices such as short wavelength (ultraviolet and blue) light emitting diodes, laser diodes and high power-high frequency electronics. In this research, the effects of process conditions and impurities on the types and densities of defects in the aluminum nitride crystals are being established. Using oxidation and etching, the distribution of the defects is determined, and in collaboration with J. Chaudhuri at Texas Tech University, the defects are being identified. Oxygen is a common impurity that creates point defects that can coalesce and form larger, extended defects. By identifying the types, distributions, and root causes of defects in the AlN crystals, processing strategies for making more perfect crystals can be developed. For example, the AlN can be purified, sintered, and compacted by partially subliming the source before commencing crystal growth. This greatly reduces the oxygen concentration in the source (by >50 times) and increases the final grain size.

- 25 mm dia, 8 mm thick AlN boule seeded on tungsten, and grown by sublimation.

- Etch pits formed at defects on an aluminum nitride crystal.  The area is 75 μm x 50 μm.

**Notes:**
Notes   Applications of short wavelength emitters and detectors include sensors for biological and chemical toxins and ultraviolet water sterilization. Applications of high frequency –high power electronics include cell phone base stations.  
Explanations  The left hand image is a polycrystalline AlN boule seeded on tungsten, approximately 25 mm in diameter and 8 mm thick.  The dark color is due to point defects – most likely Al vacancies, and impurities, of which oxygen is present in the highest concentrations.  These two types of defects are commonly present together as oxygen incorporation increase the Al vacancy concentration to maintain system charge neutrality.  
The right hand image is a scanning electron micrograph of an AlN facet after it has been oxidized and subsequently etched in molten potassium hydroxide.  Etching is selective at the defect.  Work is underway to identify the type of defect at the etch pit.
The paper accepted for publication is: Thermal oxidation of polycrystalline and single crystalline aluminum nitride wafers, Z. Gu, J.H. Edgar, S.A. Speakman, D. Blom, J. Perrin, and J.Chaudhuri, accepted for publication in J. Electron. Mater. 
Significance   By identifying the types, distributions, and root causes of  defects in the AlN crystals, processing strategies for making more perfect crystals can be developed.  For example, the AlN  can be purified, sintered, and compacted by partially subliming the source before commencing crystal growth.  This greatly reduces the oxygen concentration in the source (by >50 times) and increases the final grain size.  
Future plans  Currently, the maximum grain size in our polycrystalline boules is approximately 3 mm in diameter, with an etch pit density on the order of 105 cm-2. Further source purification should make larger crystals possible, probably grains up to 10 mm in diameter with defect densities of 104 cm-2 using the current furnace. 
Such crystals would be sufficiently large enough to make prototype devices. Defect densities on the order of 104 cm-2 would be a factor of 104 to 105 less than what is currently available.  This should be particularly beneficial for improving the efficiency and maximum power possible for AlN-rich AlGaN light emitting diodes and laser diodes operating in the sub 300 nm wavelength regime. Lower defect densities are expected to result in more unform material properties and more reproducible device fabrication.
Additional plans include examining the changes in the lattice parameters of AlN caused by the incorporation of oxygen, carbon, and silicon, and identifying specific types of defects by TEM.

## Analysis of Defects and Their Causes in Bulk Aluminum Nitride CrystalsJ.H. Edgar, Kansas State University, DMR-0408874

- Fifteen 7th and 8th grade girls participated in the workshop *Crystals and Crystal Growth* as part of their participation in project GROW: G*irls Researching Our World*. This program is designed to encourage girls to consider science, technology, engineering, and mathematics as education options.  They learned how symmetry, habit, and facets distinguish crystals from other materials.  They constructed polyhedrons to illustrate different crystal symmetries.  They examined a variety of simple crystals, to determine their basic structure.
- Undergraduate student Megan Winter developed procedures to grow crystals from simple, readily available chemicals in aqueous solutions.  The procedures can be used for teaching K-12 students about the properties of crystals and illustrating methods of crystal growth.

- Education and Outreach

- Crystals with different
- symmetries grown from
- solution to illustrate the
- fundamental properties
- of crystals.

**Notes:**
Fifteen 7th and 8th grade girls participated in the workshop on crystals and crystal growth.  The essential features of the crystals, such as their symmetry, crystal habits, common reoccurring angles, regular features, and flat, smooth facets were pointed out.  The girls constructed three-dimensional tetrahedrons, cubes, octahedrons, and hexagons out of construction paper to illustrate the different types of common symmetries found in crystals.  They examined several crystals (potassium alum, chrome alum, sodium chlorate, and potassium ferricyanide) under a microscope to identify the crystal symmetry group to which they belong.  Nucleation and crystal growth were observed by preparing saturated aqueous solutions which were allowed to evaporate.  The applications of crystals in industry (electronics, pharmaceutics, and chemical commodities) were described.  The girls were given recipes for growing rock candy that they could try at home.

Natalie Lewis, a middle school science and mathematics teacher from the Kansas City school district helped with this activity.  

The figure shows three types of crystals grown by evaporation from aqueous solutions.  They are (from top to bottom) chrome alum (purple) potassium ferricyanide (red), and nickel sulfate (green).