3.4 Test and Analysis of the Properties of Group III Nitride Materials
27
the characterization of crystal structure and morphology is particularly important in
improving the quality of crystal growth. The following are some of the commonly
used methods for analyzing structures and morphologies in the application of Group
III nitride semiconductor materials.
High-resolution X-ray Diffraction (HRXRD): X-ray diffraction can characterize the crystal quality, dislocation density, composition, defects, stresses, strains,
and lattice constants in different epitaxial layers. It has the advantages of no damage
to the material, no pollution, high efficiency and high precision. The information of
the material is obtained by analyzing the diffraction pattern. X-rays with a wavelength of 0.01–10 nm are extremely penetrating. Its working principle is Bragg’s
law. When the angle between the incident X-ray and the crystal plane meets Bragg’s
law, the diffraction intensity reaches a maximum value:
2d sin θ = nλ
(3.13)
where θ is the angle between the incident X-ray and the crystal plane, that is, the
Bragg angle, d is the interplanar spacing, λ is the X-ray wavelength, and n is the
diffraction order.
X-ray double crystal diffraction is often used in practical applications. The three
commonly used scan modes are:
ω mode or the rocking curve: The detector is fixed, no slit is placed in front of the
detector, and the sample is rotated by ω. This mode is used to evaluate the quality of
the crystal, the shape and number of dislocations in the crystal, and the dislocation
density.
2θ mode: The detector and the X-ray source are fixed. The slit is added in front
of the detector, and the sample is rotated by 2θ. This mode can be used to study the
dispersibility of the crystal domain of the sample.
2θ-ω mode: On the basis of the ω scan, a slit is added in front of the detector.
The sample and the detector are rotated by ω and 2θ, respectively. The rotation angle
ratio is 1:2. The lattice constant can be calculated from the position of the diffraction
peak. Component information can be obtained for the alloy material. The thickness
of the epitaxial layer and the superlattice structure can also be measured.
Scanning Electron Microscopy (SEM): Scanning electron microscopy uses a
very fine electron beam to scan a sample to excite secondary electrons on the surface
of the sample. The number of secondary electrons is related to the angle of incidence
of the electron beam so that the surface structure can be obtained via the number of
the received electrons. Since the De Broglie wavelength of electrons is much smaller
than the wavelength of visible light, its resolution is much larger than that of an
optical microscope. The best resolution can reach 0.1 nm.
Transmission Electron Microscopy (TEM): The principle of transmission electron microscopy is the same as that of optical microscopy. It is a microscopic imaging
technique in which an electron beam is emitted by an electron gun, where an electromagnetic beam is used to focus the image. The accelerated and focused electron
27
the characterization of crystal structure and morphology is particularly important in
improving the quality of crystal growth. The following are some of the commonly
used methods for analyzing structures and morphologies in the application of Group
III nitride semiconductor materials.
High-resolution X-ray Diffraction (HRXRD): X-ray diffraction can characterize the crystal quality, dislocation density, composition, defects, stresses, strains,
and lattice constants in different epitaxial layers. It has the advantages of no damage
to the material, no pollution, high efficiency and high precision. The information of
the material is obtained by analyzing the diffraction pattern. X-rays with a wavelength of 0.01–10 nm are extremely penetrating. Its working principle is Bragg’s
law. When the angle between the incident X-ray and the crystal plane meets Bragg’s
law, the diffraction intensity reaches a maximum value:
2d sin θ = nλ
(3.13)
where θ is the angle between the incident X-ray and the crystal plane, that is, the
Bragg angle, d is the interplanar spacing, λ is the X-ray wavelength, and n is the
diffraction order.
X-ray double crystal diffraction is often used in practical applications. The three
commonly used scan modes are:
ω mode or the rocking curve: The detector is fixed, no slit is placed in front of the
detector, and the sample is rotated by ω. This mode is used to evaluate the quality of
the crystal, the shape and number of dislocations in the crystal, and the dislocation
density.
2θ mode: The detector and the X-ray source are fixed. The slit is added in front
of the detector, and the sample is rotated by 2θ. This mode can be used to study the
dispersibility of the crystal domain of the sample.
2θ-ω mode: On the basis of the ω scan, a slit is added in front of the detector.
The sample and the detector are rotated by ω and 2θ, respectively. The rotation angle
ratio is 1:2. The lattice constant can be calculated from the position of the diffraction
peak. Component information can be obtained for the alloy material. The thickness
of the epitaxial layer and the superlattice structure can also be measured.
Scanning Electron Microscopy (SEM): Scanning electron microscopy uses a
very fine electron beam to scan a sample to excite secondary electrons on the surface
of the sample. The number of secondary electrons is related to the angle of incidence
of the electron beam so that the surface structure can be obtained via the number of
the received electrons. Since the De Broglie wavelength of electrons is much smaller
than the wavelength of visible light, its resolution is much larger than that of an
optical microscope. The best resolution can reach 0.1 nm.
Transmission Electron Microscopy (TEM): The principle of transmission electron microscopy is the same as that of optical microscopy. It is a microscopic imaging
technique in which an electron beam is emitted by an electron gun, where an electromagnetic beam is used to focus the image. The accelerated and focused electron