30
3 Properties and Testing of Group III-Nitride LED Materials
3.4.3 Other Photoelectric Test Methods
In addition to the above analytical methods, there are also many other methods for
analyzing materials using the photoelectric properties of materials.
Photoluminescence Spectroscopy (PL): Photoluminescence, as its name
implies, utilize the way that light is used to excite the semiconductor illumination.
The PL spectrum [15] is one of the commonly used optical characterization methods
in the semiconductor field. Its basic principle is to illuminate the sample with a
laser with photon energy greater than the forbidden band width. Such a process will
generate electrons and excite them from valence band to the conduction band to form
unbalanced carriers. The excited electrons will then transit to a lower energy level
and recombine with the holes. Meanwhile, the recombination energy is released in
the form of photons. The detector receives photons and converts them into electrical signals. A distribution curve of luminous intensity versus photon energy or
wavelength is commonly plotted or so called a photoluminescence spectrum.
The mechanisms of electron-hole recombination luminescence include direct radiation recombined luminescence between bands, indirect radiation recombination
between bands, radiation recombination between energy band and impurity level,
donor-acceptor recombination, free exciton radiation recombination, and binding
sub-radiation recombination and deep level impurity radiation recombination. These
luminescence mechanisms all form corresponding spectral lines in the PL spectrum.
In addition to radiation recombination luminescence, there are many non-radiative
recombination that are detrimental to device illumination. At present, many PL test
equipment can be used at low temperatures such that the non-radiative recombination
is greatly reduced. Measurement at low temperature is advantageous for observing
the fine structure of the luminescence peak. Information on the energy band structure, defects, and impurities of the semiconductor can be obtained by analysis of the
photoluminescence spectrum.
Cathodoluminescence Spectroscopy (CL): The principle of cathode fluorescence measurement is similar to that of photoluminescence. It uses a high-energy
electron beam as an excitation source to excite a semiconductor material to emit
a characteristic fluorescence spectrum with energy less than the band gap. Photoluminescence is non-destructive but cathode fluorescence can damage the surface
of the sample. The incident electron energy is generally 1–20 keV. When testing
the sample, the sample can be at room temperature or be cooled by liquid nitrogen.
The cathode fluorescence spectrum can be used to study the luminescence properties of luminescent semiconductors, especially for the luminescence properties
of various semiconductor quantum wells, quantum wires, quantum dots and other
nanostructures.
Raman: Raman spectroscopy is a scattering spectrum based on Raman scattering.
When light interact with medium molecules, the molecules are forced to vibrate to
produce scattering. The frequency of the scattered light is generally the same as the
frequency of the incident light. This scattering is called Rayleigh scattering and is an
elastic scattering. The scattering of the scattered light frequency different from the
3 Properties and Testing of Group III-Nitride LED Materials
3.4.3 Other Photoelectric Test Methods
In addition to the above analytical methods, there are also many other methods for
analyzing materials using the photoelectric properties of materials.
Photoluminescence Spectroscopy (PL): Photoluminescence, as its name
implies, utilize the way that light is used to excite the semiconductor illumination.
The PL spectrum [15] is one of the commonly used optical characterization methods
in the semiconductor field. Its basic principle is to illuminate the sample with a
laser with photon energy greater than the forbidden band width. Such a process will
generate electrons and excite them from valence band to the conduction band to form
unbalanced carriers. The excited electrons will then transit to a lower energy level
and recombine with the holes. Meanwhile, the recombination energy is released in
the form of photons. The detector receives photons and converts them into electrical signals. A distribution curve of luminous intensity versus photon energy or
wavelength is commonly plotted or so called a photoluminescence spectrum.
The mechanisms of electron-hole recombination luminescence include direct radiation recombined luminescence between bands, indirect radiation recombination
between bands, radiation recombination between energy band and impurity level,
donor-acceptor recombination, free exciton radiation recombination, and binding
sub-radiation recombination and deep level impurity radiation recombination. These
luminescence mechanisms all form corresponding spectral lines in the PL spectrum.
In addition to radiation recombination luminescence, there are many non-radiative
recombination that are detrimental to device illumination. At present, many PL test
equipment can be used at low temperatures such that the non-radiative recombination
is greatly reduced. Measurement at low temperature is advantageous for observing
the fine structure of the luminescence peak. Information on the energy band structure, defects, and impurities of the semiconductor can be obtained by analysis of the
photoluminescence spectrum.
Cathodoluminescence Spectroscopy (CL): The principle of cathode fluorescence measurement is similar to that of photoluminescence. It uses a high-energy
electron beam as an excitation source to excite a semiconductor material to emit
a characteristic fluorescence spectrum with energy less than the band gap. Photoluminescence is non-destructive but cathode fluorescence can damage the surface
of the sample. The incident electron energy is generally 1–20 keV. When testing
the sample, the sample can be at room temperature or be cooled by liquid nitrogen.
The cathode fluorescence spectrum can be used to study the luminescence properties of luminescent semiconductors, especially for the luminescence properties
of various semiconductor quantum wells, quantum wires, quantum dots and other
nanostructures.
Raman: Raman spectroscopy is a scattering spectrum based on Raman scattering.
When light interact with medium molecules, the molecules are forced to vibrate to
produce scattering. The frequency of the scattered light is generally the same as the
frequency of the incident light. This scattering is called Rayleigh scattering and is an
elastic scattering. The scattering of the scattered light frequency different from the