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3 Properties and Testing of Group III-Nitride LED Materials
beam is projected onto an ultra-thin sample with a periodic structure, and the electrons are diffracted to form a diffraction spectrum. After the diffraction spectrum
is recombined and imaged in the image plane, different types of TEM images can
be obtained. Transmission electron microscopy is currently the highest spatial resolution microscope with a spatial resolution of up to one crystal lattice. It can be
used to analyze the fine structure inside the material, to observe the defects such as
dislocations in the material and the state of the heterojunction interface.
Atomic Force Microscopy (AFM): Atomic force microscopy is based on an
improved scanning tunneling microscope (STM). It overcomes the shortcomings of
scanning tunneling microscopes that are not suitable for non-conductive samples.
AFM is the most commonly used method for characterizing the surface topography
of semiconductor materials. Using a sharp tip to move on the surface of the sample,
the surface topography of the sample is imaged by a force test between the tip and
the sample. The AFM has a cantilever beam that is sensitive to weak forces. The
cantilever beam is fixed at one end and has a sharp tip at the other end. When the
needle scanning, the interaction force between the tip and the sample causes the
cantilever beam to deform. The back of the tip is machined into an optical mirror
that is reflected onto the photodetector when it is incident on the back of the cantilever
beam. The deformation of the cantilever beam causes the position of the spot on the
detector to shift, reflecting information on the surface topography of the sample. The
main parameter of the AFM test is the surface roughness, which characterizes the
degree of fluctuation of the surface flatness. With high lateral and vertical resolution,
the general lateral resolution can reach 0.1~0.2 nm, and the vertical resolution can
reach 0.01 nm.
3.4.2 Surface and Film Composition Analysis
GaN materials are mainly obtained by epitaxial growth on a substrate such as
sapphire. The analysis methods for the film and the surface are as follows.
Auger Electron Spectroscopy (AES): When the electrons in the atom are excited
to form holes, the electrons are unstable at high energy levels and will transition to low
energy levels. When the energy is radiated in the form of photons, characteristic Xrays are formed. When the energy of the transition is absorbed by another electron and
is emitted, Auger electron is formed. Auger electron spectroscopy uses the interaction
between the incident electron beam and the matter to excite the inner electrons of
the atom. The energy released by the outer electrons to the inner electron transition
causes the other electron outside the nucleus to be excited into free electrons, i.e.,
Auger electrons. The electron energy analyzer is used to collect and analyze the
energy of Auger electrons to form a curve of the number of Auger electrons as a
function of electron energy, that is, the Auger electron spectrum. Auger electrons
can escape the solid surface with only a few layers of atoms on the surface, so
Auger electron spectroscopy is an analytical method for solid surface properties.
3 Properties and Testing of Group III-Nitride LED Materials
beam is projected onto an ultra-thin sample with a periodic structure, and the electrons are diffracted to form a diffraction spectrum. After the diffraction spectrum
is recombined and imaged in the image plane, different types of TEM images can
be obtained. Transmission electron microscopy is currently the highest spatial resolution microscope with a spatial resolution of up to one crystal lattice. It can be
used to analyze the fine structure inside the material, to observe the defects such as
dislocations in the material and the state of the heterojunction interface.
Atomic Force Microscopy (AFM): Atomic force microscopy is based on an
improved scanning tunneling microscope (STM). It overcomes the shortcomings of
scanning tunneling microscopes that are not suitable for non-conductive samples.
AFM is the most commonly used method for characterizing the surface topography
of semiconductor materials. Using a sharp tip to move on the surface of the sample,
the surface topography of the sample is imaged by a force test between the tip and
the sample. The AFM has a cantilever beam that is sensitive to weak forces. The
cantilever beam is fixed at one end and has a sharp tip at the other end. When the
needle scanning, the interaction force between the tip and the sample causes the
cantilever beam to deform. The back of the tip is machined into an optical mirror
that is reflected onto the photodetector when it is incident on the back of the cantilever
beam. The deformation of the cantilever beam causes the position of the spot on the
detector to shift, reflecting information on the surface topography of the sample. The
main parameter of the AFM test is the surface roughness, which characterizes the
degree of fluctuation of the surface flatness. With high lateral and vertical resolution,
the general lateral resolution can reach 0.1~0.2 nm, and the vertical resolution can
reach 0.01 nm.
3.4.2 Surface and Film Composition Analysis
GaN materials are mainly obtained by epitaxial growth on a substrate such as
sapphire. The analysis methods for the film and the surface are as follows.
Auger Electron Spectroscopy (AES): When the electrons in the atom are excited
to form holes, the electrons are unstable at high energy levels and will transition to low
energy levels. When the energy is radiated in the form of photons, characteristic Xrays are formed. When the energy of the transition is absorbed by another electron and
is emitted, Auger electron is formed. Auger electron spectroscopy uses the interaction
between the incident electron beam and the matter to excite the inner electrons of
the atom. The energy released by the outer electrons to the inner electron transition
causes the other electron outside the nucleus to be excited into free electrons, i.e.,
Auger electrons. The electron energy analyzer is used to collect and analyze the
energy of Auger electrons to form a curve of the number of Auger electrons as a
function of electron energy, that is, the Auger electron spectrum. Auger electrons
can escape the solid surface with only a few layers of atoms on the surface, so
Auger electron spectroscopy is an analytical method for solid surface properties.