Correlative Microscopy of Optical Materials
Characterization of LED by Correlative Light and Electron Microscopy (CLEM)
- Correlative Microscopy of Optical Materials - Characterization of a LED by Correlative Light and Electron Microscopy (CLEM)
- Fig. 1: pcLED device with lens: brightfield image without emission (a), optical emission (b) and thermal image (c) at 70mA driving current.
- Fig. 2: Cross section of a pcLED imaged by CLEM: brightfield LM (a), optical emission (b), BSE image (c), EDS mapping (d), and overlay of optical emission with BSE contrast (e).
- Fig. 3: Optical interference filter on top of the ceramic: BSE image of cross section (a), element distribution along the line (b), brightfield LM (c) and optical emission (d) from top of the LED polished slightly tilted.
Modern optical and optoelectronic devices are composed of different optical materials and photonic structures which need to be analyzed at the microscale. Comprehensive micro-characterization including functional and structural properties mostly requires a combination of different microscopic techniques applied to these devices. In this article the characterization of optical materials with Correlative Light and Electron Microscopy (CLEM) is outlined for the example of a Light Emitting Diode (LED).
The so-called second semiconductor revolution is currently ongoing and characterized by solid state lighting which replaces the classical lighting technologies of incandescent, fluorescent, or discharge lamps. The main advantages of solid state lighting with inorganic LEDs are a high luminous efficiency, long lifetime, and small size.
LEDs are semiconductor diodes with a well-defined band gap and basically emit monochromatic radiation. To achieve white light, the emission of two (yellow and blue) or three (red, green and blue) LEDs can be combined. But next to other disadvantages this multispectral approach has poor color rendering properties. Therefore white LEDs generally consist of a blue or UV LED combined with a light converting inorganic phosphor [1,2]. This optical material converts short-wavelength radiation of the LED into broad emission bands which cover a large spectral range. Those phosphor converted LEDs (pcLEDs) can achieve high color rendering indices of >90.
Figure 1 shows a pcLED (Luxeon Rebel, 700 mA) with and without emission together with a thermographic image of the device at a low operation current of 70 mA. In figure 1b it can be observed that the white light is composed of blue and yellow because of slight color over angle variations. The temperature distribution (fig. 1c) shows that the LED produces an appreciable amount of heat even under this low operation conditions. Therefore thermal management is very important to ensure a low junction temperature.
By integration of photonic microstructures the performance of LEDs can further be improved. For example dielectric mirrors can increase the output of the device or by interference filters the color properties as well as the optical coupling can be improved [3-5].
For microscopic characterization of optical materials, information from light microscopy (LM) as well as scanning electron microscopy (SEM) is required.
LM gives a quick overview of morphology and optical properties like color, transmission or reflectivity. If the device under test is investigated during operation, optical activity or inhomogeneities can be characterized, too. In SEM the sample structure can be analyzed with high resolution regarding surface morphology or material contrast. Also chemical compositions can be measured. SEM is important especially for characterization of sub-wavelength structures as they cannot be detected in LM as a matter of principle.
Therefore, the combination of LM and SEM results in complementary information about functional and structural properties of the inspected optical material. This information can be merged at the same location by CLEM which is an efficient method for comprehensive micro-characterization . The following results are acquired with an upright compound LM and a field emission SEM. Both systems are combined with each other by "Shuttle & Find", a 2D CLEM solution from Zeiss . It is based on a sample holder which allows a fast change of the sample from one microscope to the other still keeping the same locations of interest by software algorithms.
Functional and Structural Setup of a pcLED
The application of CLEM to optical materials is shown in the following for an LED. A polished cross section is prepared from a commercially available pcLED with applications in automotive, indoor and outdoor lighting. The preparation was carried out without removing the electrical contacts in the device so that light emission from the sample still can be enabled by an external current source. Successively, the sample was characterized in LM and SEM with the results shown in figure 2.
The brightfield LM image (fig. 2a) illustrates morphology and color of the different layers in the sample. In figure 2b electric current flows through the sample exciting optical emission. This image was taken without external illumination and only depicts light distribution within the LED. Here the principle setup of a pcLED is visible illustrating the functions of different optical materials. Blue light is emitted from a thin active layer below which no optical transmission is possible. Phosphor conversion can be observed in the materials above the active layer where blue light is converted to red and yellow emission. The total emission from top of the LED is formed by addition of those spectra and results in a warm white light.
Material contrast in the cross section is visualized with backscattered electrons (BSE) in the SEM (fig. 2c). There the microstructure is imaged with a higher resolution and the different layers can be separated with respect to materials. The chemical composition of the layers is detected by energy dispersive x-ray spectroscopy (EDS) in SEM. It can be measured that the top layer consists of a Ce-doped YAG ceramic which has a broad yellow emission whereas the grains below that layer are Eu-activated CaAlSiN3-particles in SiO2 that are responsible for the proportion of red in emission. The active region is a GaN semiconductor contacted by Gold and Copper layers. Figure 2d shows an EDS mapping of the same region; for clarity reasons only selected element distributions are included. Functional behavior and microstructure of the different phosphor materials can finally be correlated as shown in the overlay of figure 2e.
Sub-Wavelength Optical Structures
By zooming in on top of the YAG ceramic a multi-layer structure can be observed in the SEM. Figure 3a shows a BSE image of these layers which have a periodic distance of about 150 nm. EDS analysis identifies the structure as an alternating SiO2/Nb2O5 system. The EDS line scan in figure 3b is obtained at 4.4 keV electron energy and illustrates the element distribution. Note that at this energy the electron interaction volume is not small enough to resolve a single layer; the line scan shows the convolution with the neighbor layers, too. The system of transparent layers with different refractive indices of 1.46 (SiO2) and 2.35 (Nb2O5) forms an optical interference filter which partially reflects the blue light back into the ceramic phosphor.
The function of this filter can be observed in figure 3c+d. which show a top view on a similar LED. The lens is removed and the sample is polished under a flat angle tilt. In the top right corner the filter is still complete whereas in the bottom left it is entirely removed and only the ceramic surface is visible. In figure 3c the filter reflects the proportion of blue from the brightfield illumination and the ceramic surface appears yellow. When emission is observed (fig. 3d), it can be seen that the color changes and without filter a higher proportion of blue is emitted from the device than in the filter area. This illustrates how the filter increases phosphor converted light emission by partially reflecting the blue component.
CLEM offers additional gain for microscopic investigation of optical materials. Complementary results of optical and structural information can be joined together as shown at the example of a white LED. This allows detecting the influence of material properties on the characteristics of an optical device which can be investigated even under operation. Therefore the method is suitable for development of optical devices as well as for reliability analysis, e.g. observation of degradation processes.
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Prof. Dr. Christian Thomas (corresponding author via e-mail request button below)
Dipl.-Ing. Tekie Ogbazghi
Hamm-Lippstadt University of Applied Sciences
Micro- and Nanotechnology