Fluorescence microscopy: observation of spatial variations and the effect

The spatial distributions of the emission from the LED structures were characterized using uniform excitation with the fluorescence microscope set up described in chapter 4. The excitation source is filtered using a 390nm-420nm bandpass filter and incident on the LED material through the transparent, polished sapphire substrate. This selected wavelength band generates carriers only inside the 475 nm emitting QWs as the bandgap of GaN corresponds to a wavelength of 360 nm. The luminescence from the sample is filtered with a short wavelength cut-off filter with a cut-off wavelength of 450 nm and imaged onto a CCD camera. Figures 5.1-a, -b and -c show the fluorescence images from the 5QW, 10QW and commercial samples respectively. A strong spatial variation in the emission is observed with a characteristic wave-like pattern at tens of microns scale for the 5QW/10QW samples. This emission pattern of bright spots and patches from QWs grown by the quasi-two-temperature (Q2T) method has also been reported previously and studied by cathodo-luminescence mapping on similar material [1], where it was established that this profile originates from the growth of the template.

The emission wavelength from the samples also seems to be different; although the peak wavelength of the emission under electrical excitation (EL) does not vary by more than 12 nm across all the samples as it will be shown later (see figure 5.7 for example). The 10QW (Figure 5.1-b) sample appears to emit at longer wavelengths (green) than the 5QW due to a yellow band emission that is detected under electrical and optical excitations and will be discussed later. The combination of both blue and yellow wavelengths is detected as green through the CCD Camera under low optical excitation densities (~20 mW/cm2_{) when the}

amount of light from both emissions is comparable. Differences in the emission with the commercial sample are due to the higher intensity of the emission from this sample which is converted to a lighter blue colour by the CCD camera.

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Figure 5.1: Fluorescence microscopy images of a) 5QW sample, b) 10QW sample and c) commercial sample. All images were taken through the polished substrate under the same illumination conditions.

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In the last section of chapter 4 it was shown that the optical excitation induces a photo-voltage across the junction. This potential is local to the illuminated region as the lateral resistance of the p-type GaN is very high. In the presence of a metal providing an ohmic contact with high conductivity, the voltage will be shared across the metal surface and thus the light emission under the metal will be as appropriate to that voltage. Thus, it is expected that in the presence of defects or shunting paths the photo-generated potential on the metal will be reduced and consequently the emission under FM will be reduced under the entire contact rather than just locally where the shunting is dominant.

To study this effect, the 5QW piece processed with the quick evaluation method (see chapter 3) is excited and imaged under open circuit conditions. The partially (about 60% across the wavelength range of interest, i.e. 390-420nm) reflective Pd produces a double pass of the incident light thereby increasing absorption and the photovoltage under the metal. For a given optical intensity it was found that the luminescence under some Pd contacts was lower than in the un-metallized region (Fig 5.2 a and c). Under higher illumination intensity the metalized region produced higher luminescence intensity than the un-contacted region. Figure 5.2 shows images of such a poor contact (top-left) and a regular contact (bottom-left) from the 5QW sample. To understand the reason for this observation the I-V and electrical light-current (L-I) characteristics of the particular contacts were measured and shown in figure 5.2-e and –f, respectively. The slope of the poorer contact is less steep than the slope of the good contact. This is indicative of a higher diode ideality factor in the electrical characteristic for voltages of ~2V. In this voltage range, the minimum value of the ideality is 1 which corresponds to radiative recombination. Higher values are indicative of the dominance of non- radiative recombination. Under higher excitation levels the non-radiative current paths become saturated and the photovoltage can rise. This is also a confirmation that in the presence of an ohmic contact metallization the photovoltage is shared across the whole contact [2].

The analysis of the electrical and optical performance of the contacts from the fluorescence pictures depicted in figure 5.2 also suggests a quick way for yield analysis. Under open circuit conditions the photo-voltage, and therefore the

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emission from the sample will depend on the electrical qualities of the full area under contact. So fluorescence imaging of the processed LEDs can quickly locate the devices that will have the best performance. This and the analysis of the spatial variation of the wafer also under fluorescence microscopy could identify defects and problems in the fabrication of devices.

Figure 5.2: a) Fluorescence microscope images of the 5QW sample at low excitation levels a), c), and at high excitation levels b), d) of two partially reflective 250µm diameter Pd contacts. The reduced intensity for a) and c) has corresponding poorer electrical and optical characteristics. e) IV characteristics of the contacts, f) LI characteristics from the same contacts.

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