Collaborative Research -Analysis of Defects and Their Causes in Bulk Aluminum Nitride Crystals

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Collaborative Research -Analysis of Defects and
Their Causes in Bulk Aluminum Nitride Crystals
Jharna Chaudhuri, DMR-0515858 Texas Tech
University, Lubbock, TX
Objective To investigate basic defect mechanisms
in AlN single crystal growth and oxidation of
AlN, to identify process conditions responsible
for initiation of defects, and to reduce or
control defect density by growth and processing
conditions. High resolution transmission electron
microscopy (HRTEM) provides key structural
information to establish relationships between
processing parameters and defects present as well
as structure/property relations. Simulations and
models based on diffraction theories are used to
identify the origin of defects.
interface
Figure1. High resolution bright field TEM image
from the interface region of an AlN single
crystal oxidized at 1000C indicating AlN is
perfect without any defects present, and Al2O3 is
crystalline with several large grains. Contrast
in AlN is due to local bending.
Figure 2. Inverse fast Fourier transform (IFFT)
of a HRTEM image from the interface region of AlN
oxidized at 800C indicating defects such as
dislocations, stacking faults and domains.
Regions A and B contain large and small angle
domains, respectively d dislocations.

• Results
• Oxidation of AlN at high temperature (1000 C),
  a thick crystalline oxide layer (450 nm), with
  several large grains, is formed with an abrupt
  compositional interface between Al2O3 and AlN
  (Figure 1), with the underlying AlN nearly
  defect-free. The oxide layer is mostly the stable
  a phase except at the surface where a cubic, or ?
  phase, is formed. In contrast, at lower oxidation
  temperature (800 C), an amorphous oxide is
  formed, and the nitride near the interface
  contains a high density of defects (dislocations,
  stacking faults and planar defects) as shown in
  Figure 2, along with a measurable amount of
  oxygen and nitrogen.
• Mechanism Oxidation of AlN produces oxygen and
  aluminum interstitials. At high temperature, the
  diffusion of these interstitials is rapid as is
  reaction kinetics involving these point defects
  allowing thermodynamic equilibrium to be reached
  throughout the nitride. In contrast, at low
  temperature, diffusion and reaction kinetics of
  the nitrogen and aluminum interstitials are much
  slower, leading to super-saturation of point
  defects in the nitride at the interface. This
  build up of excess point defects leads to
  precipitation of line and planar defects as
  observed near the interface.
• Significance
• This research has demonstrated for the first time
  defects and structures produced during the
  oxidation of single crystal AlN. The results
  obtained will be useful in the fabrication of
  high quality dielectric Al2O3 thin films. This
  basic materials research opens up new
  opportunities for making field effect
  transistors, and other electronic and optical
  devices.

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Collaborative Research -Analysis of Defects and
Their Causes in Bulk Aluminum Nitride Crystals
Jharna Chaudhuri, NSF Grant DMR-0515858 Texas
Tech University, Lubbock, TX
Figure 1. Ph. D. student Mr. Luke Nyakiti (left)
and MS student Mr. Rac Lee (right) are learning
high resolution transmission electron microscopy
work at CMM, University of Ill, Urbana-Champaign,
Ill.
Figure 2. Undergraduate student Ms. Jamie
Armstrong (right) and principal investigator Dr.
Jharna Chaudhuri (left) are discussing a
research project.

• Broader Impact
• One Ph. D., one MS and one undergraduate (female)
  students (Figures 1 and 2) involved in this
  research are advancing their education by
  discovery through experiments they design and
  perform, through training on sophisticated
  instrumentation, and application of
  state-of-the-art modeling techniques.
• This project has many learning opportunities for
  students beyond what is possible at Texas Tech
  University. Students are interacting closely
  with Professor Edgar and his students at Kansas
  State University. They are also getting trained
  in using high resolution transmission electron
  microscopy at the Center for Microanalysis of
  Materials (CMM), University of Illinois,
  Urbana-Champaign, IL, a DoE funded user facility.
• Based on the current research finding one paper
  has been presented in the Fourteenth
  International Materials Research Congress in
  August, 2005 and one paper has been submitted for
  publication. We are writing another paper on the
  effect of Si doping on crystalline quality of
  AlN.