Application of PL technology in LED material property test

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In recent years, due to the advantages of both in the white LED luminous efficiency, power consumption, life and environmental protection and other aspects have traditional light incomparable, the white LED is slowly replacing the incandescent lamp and fluorescent lamp, as governments have announced and proposed to ban incandescent bulbs time table, accelerates this trend.

With white light LED mechanism can be divided into three types as shown in Figure 1 (a) proposed by the Lan Guanglei Nichia chip plus Nd-YAG phosphor converted white LED[1,2]. (b) with a violet ray chip coupled with RGB tricolor phosphor converted to white LED, is still in the experimental stage. [3-5] (c) using three kinds of RGB epitaxial chip hybrid white LED[6,7]. At present, the products on the market are mostly blue chip and Nd-YAG phosphor converted to white LED, so how to improve the luminous efficiency of the Blu ray chip is very important for the development of white LED.

Figure 1 white LED mechanism (a) Blue LED +YAG Phosphor (b) UV LED + RGB Phosphor (c) RGB LED

The characteristics of semiconductor luminous efficiency of LED depends on the material itself, when LED injected additional carrier, composite carrier for additional radiation (additional band carrier light composite) composite and non radiative (phonon heat composite and Auger recombination) two mechanisms, in addition to the defect will capture the additional order carrier, to reduce the excess carrier recombination chance. So in recent years many research teams in order to study how to improve the luminous efficiency of LED, have borrowed technology and analysis to explore the luminescence mechanism of the fluorescence measurement.

Fluorescent light emitting mechanism

Fluorescence is a kind of electromagnetic radiation phenomenon. For any material, the incident photon energy is equal to or more than the band, it will take over the electronic band to the conduction band and valence electron excitation, when excited by the conductive back to the valence band will produce radiation, production process is mainly divided into three stages as shown in figure 2. (a) for excitation, the generation and excitation of the additional carrier (b) is the energy release and recombination, the energy release of the additional carrier of the excited state and the recombination (c), which is produced by fluorescence, and the fluorescence signal produced by the recombination.

Figure 2 fluorescence process

The fluorescence method is broadly divided into two categories, respectively, with higher than or equal to the bandgap energy of the photon irradiation samples to generate additional carrier, or electronic injection way to increase the carrier concentration to increase the probability of fluorescent photons produced, to enhance the intensity of the fluorescence signal measurement. The two methods are called photoluminescence (photoluminescence, hereinafter referred to as PL) and electro luminescence, LED light-emitting principle for electro luminescence, fluorescence measurement of electrical excitation however must be embedded electrodes, indicating that we must use the photoluminescence measurements process in electrodes embedded in the previous.

Since the laser can be used to provide enough power to stimulate the signal after the [8] incident light began using laser light source. When the excited state electron returns to the ground state, it will produce a photon, or it may produce a lot of phonons. Assuming that the light source is a continuous wave, the excited fluorescence can be treated as a steady state, and the test piece is continuously emitted by the light source to irradiate the [9], and the laser spectrum and the excited fluorescence spectrum are shown in figure 3.

Figure 3 laser spectroscopy and fluorescence excitation diagram

The as shown in Figure 4 by the Alexander Jablonski energy diagram Jablonski proposed by [10], and the absorption of incident photons incident light wavelength is related to the energy absorption, and the wavelength of the incident light source material about.

Figure 4 Jablonski energy diagram [10]

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