LED material characteristics detection technology——PL technology

Preface

In recent years, white light LEDs have gradually replaced incandescent bulbs and fluorescent lamps because they have unparalleled advantages over traditional light sources in terms of luminous efficiency, power consumption, lifespan and environmental protection. This trend has been further accelerated as governments around the world have announced and proposed timetables for banning incandescent bulbs.

The mechanism of producing white light LED can be divided into three types as shown in Figure 1. (a) Nichia proposed to convert blue epitaxial chips with Nd-YAG phosphors into white light LEDs [1, 2]. (b) Purple epitaxial chips with RGB phosphors are used to convert white light LEDs, which is still in the experimental stage. [3-5] (c) Use RGB epitaxial chips to mix white light LEDs [6, 7]. Currently, most products on the market are blue epitaxial chips with Nd-YAG phosphors converted into white light LEDs, so how to improve the luminous efficiency of blue epitaxial chips is crucial to the development of white light LEDs.

Figure 1. The mechanism of white light generation by LEDs (a) Blue LED + YAG Phosphor (b) UV LED + RGB Phosphor (c) RGB LED

The luminous efficiency of semiconductor LEDs depends on the characteristics of the material itself. When LEDs are injected with extra carriers, the recombination of the extra carriers is divided into two mechanisms: radiative recombination (extra carriers in the energy band recombine to emit light) and non-radiative recombination (phonon recombination to release heat and Ogier recombination). In addition, defect energy levels between energy bands will also capture extra carriers, reducing the chance of extra carrier recombination. Therefore, in recent years, many research teams have used fluorescence measurement technology to analyze and explore the luminescence mechanism in order to study how to improve the luminous efficiency of LEDs.

Fluorescence mechanism

Fluorescence is a phenomenon of electromagnetic radiation emission. For any material, when the incident photon energy is equal to or exceeds the energy band, it will excite the valence band electrons to cross the energy band to the conduction band. When the excited electrons return from the conduction band to the valence band, radiation emission will occur. The generation process is mainly divided into three stages as shown in Figure 2. (a) is excitation, the generation and excitation of additional carriers (b) is energy release and recombination, the energy release and recombination of the excited additional carriers (c) is the generation of fluorescence, and the fluorescence photon signal generated after recombination.

Figure 2 Fluorescence generation process

There are two ways to generate fluorescence, which are to generate additional carriers by irradiating the sample with photons with energy higher than or equal to the energy gap, or to increase the carrier concentration by electron injection to increase the probability of generating fluorescent photons, thereby increasing the intensity of the measured fluorescence signal. These two methods are called photoluminescence (hereinafter referred to as PL) and electroluminescence. The principle of LED light emission is electroluminescence, but the measurement of electroluminescence must be embedded in the electrode, which means that photoluminescence must be used for measurement in the process before the electrode is embedded.

Since lasers can be used to provide sufficient power excitation signals [8], the incident light begins to use laser light sources. When the excited electrons return to the ground state, a photon is generated, and many phonons may also be generated. Assuming that the light source used is a continuous wave, the fluorescence excited by it can be regarded as a steady state. The sample is irradiated by the light source and emits fluorescence continuously [9]. The laser spectrum and the excited fluorescence spectrum are shown in Figure 3.

Figure 3 Laser and excitation fluorescence spectrum

As shown in Figure 4, from the Jablonski energy diagram proposed by Alexander Jablonski [10], it can be seen that the absorption of incident light is related to the wavelength of the incident photon, that is, the energy. Therefore, the absorption of the material is related to the wavelength of the incident light source.

图4 Jablonski energy diagram ［10］

When the sample absorbs incident light, it excites electrons to a higher energy state. After a period of time, the electrons release energy to a lower energy state. Impurities and defects form various energy levels in the energy gap, and the corresponding energy is emitted by radiative recombination processes such as photoluminescence, or absorbed by non-radiative recombination processes [8][11], such as phonon emission, defect capture, or the Ogier effect [12].

In addition to the above-mentioned fluorescence caused by the conversion of energy bands such as the conduction band and the valence band, defects can also cause fluorescence, as shown in Figure 5. EC, EV and ED are the conduction band, valence band and defect band respectively. The defect band is distributed between EC and EV, and the position and number depend on the quality of the material. In Figure 5, (a) is the recombination of electron-hole pairs between energy bands, (b) and (c) are both recombination of defects, (b) is the electrons in the conduction band captured by the defects between energy bands, (c) is the electrons captured by the defects and the recombination of the valence band, and the emitted fluorescence band depends on the distance between the energy bands before the electrons and holes recombine.

Figure 5 Radiative recombination (a) Recombination of electron-hole pairs between energy bands (b) If there is a defect between energy bands, the electron will be captured by the defect (c) The electron captured by the defect will electrokinetically recombine with the valence band

Photostimulated fluorescence measurement

The main structure of the PL spectrometer includes an excitation source, a signal receiver (spectrometer), a signal processor (computer) and a cryogenic system. The structure diagram is shown in Figure 6.

Figure 6 PL spectrometer architecture

Since the energy band of blue LED is about 2.75 eV, the excitation source is selected as a laser with a wavelength greater than its energy band of 325 nm (energy 3.8 eV), 375 nm (energy 3.3 eV) and 405 nm (energy 3 eV). The scanning range of the spectrometer is between 350 nm and 700 nm. In addition, since temperature has a great influence on the fluorescence intensity of radiation recombination, the measurement environment must be temperature controlled. Taking the measurement of blue LED as an example, the PL fluorescence spectrum is shown in Figure 7. The excitation source is a laser with a wavelength of 405 nm. The peak position of the blue LED is 461 nm, and the half-height width is 25.2 nm.

Figure 7 PL fluorescence spectrum

Advantages of PL in LED material analysis

The fast PL measurement feature can adapt to the production speed of the LED production line, and the non-contact and non-destructive measurement can ensure that the sample will not change its original characteristics during the measurement process. With mapping technology or changing the signal receiver to CCD, the spatial distribution characteristics of the sample can be obtained, and the uniformity of the process can be learned to feedback the MOCVD process. During measurement, no electrodes are required to monitor the changes in each step of the manufacturing process. This is the advantage of introducing PL measurement technology into the LED Wafer production line.

于LED元件设计及验证方面，以蓝光LED常用的材料氮化铟镓为例，由于在晶格常数与能阶宽度图中，连接氮化镓与氮化铟兩点的抛物曲线便是氮化铟镓，随着氮化铟镓中的铟含量增加，其能阶宽度变小［13，14］，所以可由PL萤光光谱波峰的位置，得知氮化铟镓中的铟含量，可借由调变激发源的雷射强度与量测萤光光谱强度可拟合出LED发光效率的相关系数，进而求出LED的内部发光效率以提供元件设计之验证，量测时不需电极，在制程时任一步骤，皆可调变制程参数，或选用不同制程方式，比较PL萤光光谱以优化出最佳制程条件等优势。

in conclusion

PL is a fast, non-contact, non-destructive measurement technology that can measure the spatial distribution of samples. It has good applications in both mass production and product development.