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Study on Aging Performance of 1W Silicon Substrate Blue LED for Different Substrates
- May 08, 2017 -

GaN materials since the 20th century since the 90's gradually in the display, instructions, backlight and solid-state lighting and other fields widely used, has formed a huge market. Up to now, gallium nitride (GaN) light-emitting diodes (LEDs) prepared on three substrates (sapphire, silicon carbide and silicon) have been commercialized. In recent years, silicon substrate GaN-based LED technology concern. Because silicon (Si) substrate has the advantages of low cost, large crystal size, easy processing and easy transfer of epitaxial film, it has excellent performance and low cost in power LED device application.

Many research groups have grown GaN epitaxial films on Si substrates and have obtained some of the devices or have investigated the Si-based GaN-related properties. In the preparation of LED, the GaN film is transferred to the new support substrate to prepare the vertical structure of the device, compared with the same side of the structure of the device better optical performance.

In this paper, the GaN epitaxial film grown on the Si substrate was transferred to the copper supporting substrate, the copper chrome supporting substrate and the method of welding by means of welding to the Si supporting substrate. The vertical structure light emitting device was obtained, and the three kinds of samples Conducted a comparative study of aging.

Experiment

The experimental epitaxial wafers were a 2in (50.8mm) blue In GaN / GaN multi-quantum oxide epitaxial wafers grown on a silicon (111) substrate by MOCVD, with a chip size of 1000Lm @ 1000Lm, and the growth method has been reported. The epitaxial wafers grown with the furnace were prepared. One of them was transferred to the Si substrate by means of pressure welding and chemical etching. The light-emitting device was called the sample A and the other two were electroplated and chemically etched The GaN epitaxial film was transferred to a plated copper substrate and an electroplated copper-chrome substrate, respectively, and a light-emitting device was referred to as Sample B and Sample C, respectively. Three samples in addition to the epitaxial film transfer mode and support the substrate is not the same, the other device manufacturing process is the same.

As a result of similar differences between individuals of similar samples, so the samples A, B, C for the initial test, were selected representative of the chip for experiments and testing. Each chip is a bare core package. Usually the size of 1000Lm @ 1000Lm chip operating current of 350mA, in order to accelerate the aging of the samples A, B, C at room temperature through the DC current 900mA. The current-voltage (I-V) characteristic curve, electroluminescence (EL) spectrum, the relative light intensity of each sample at each current were measured before and after aging with the power supply KEITHLEY2635 and the spectrometer Compact Array Spectrometer (CAS) 140CT.

Results and discussion

I-V characteristics analysis

Table 1 shows the Vf and Ir values for aging at 80, 150 and 200 hours before aging of the three samples. The aging conditions are 900mA at room temperature, where Vf is the voltage at 350mA and Ir is the leakage current at reverse 10V, Leakage current Ir is measured in reverse 5V, for comparison results, select more severe conditions, measured at reverse 10V. Figure 1 shows the I-V characteristic curves of the aged, aged 80, 150 and 200 hours before aging, as shown in Fig. 1 (a) to (d), respectively. Figure 1 (a) shows that A, B, C three samples have good I-V characteristics before aging, the opening voltage of about 2.5V, reverse 10V current in the order of 10-9A. After aging 200h, the leakage current Ir of the three samples in the reverse direction was significantly higher than that before aging. Table 1 shows that the leakage current of the B sample is the smallest at the same back pressure (-10V) after aging at 200 h after high current. The A sample is the second, and the C sample is the largest, and with the aging time, the three samples are under the same back pressure The leakage current difference is getting bigger and bigger. In GaN MQW LED after the aging of the positive voltage slightly increased, because the large current aging for a long time so that the bare n electrode (aluminum) local oxidation resulting in greater contact resistance caused. The reason for the large leakage after aging is that the width of the In GaN LED pnjunction depletion layer is mainly determined by the p-type carrier concentration. After the aging of the chip after aging for a long time, due to the decomposition of the Mg-H complex, Activation, making the p-type carrier concentration increased, resulting in depletion layer narrowing, reverse bias when the barrier area thinning, tunnel breakdown components increased, reverse current increases; In addition, the chip after a long time after aging , The defect density of the quantum well region increases, the defects in the reverse bias and the trap-assisted tunneling cause leakage current, and the thermal conductivity of the samples of B, A and C decreases in turn. Therefore, the defects and trap density , So that the leakage current of the three samples increases in the same back pressure (as shown in Table 1 and Fig. 1).

Study on Aging Performance of 1W Silicon Substrate Blue LED for Different Substrates

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Fig.1 I-V characteristic curves of three samples before and after aging

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Table 1 Vf values and Ir values of the three samples before and after aging

EL spectral analysis

Figure 2 shows the electroluminescent (EL) spectra of the samples at 1,10, 100, 500, 800, 1000 and 1200 mA before and after 900 hours of continuous aging at 900 mA at room temperature [Fig. 2 (a1) to (a3)] and three (Fig. 2 (b1) to (b3)], the solid line in the figure shows the spectrum before aging, and the dotted line indicates the spectrum after aging. Figure 2 (a1) ~ (a3) shows the EL spectrum before and after aging, and the EL spectrum of the current before and after aging of the three samples has no obvious change except that the peak wavelength of the high current is red. Figure 2 (b1) ~ (b3) shows that the wavelengths of the three samples before and after aging are significantly different from those of the current. The wavelengths of the B samples before and after aging are almost the same as those of the current, but only after the aging There is an increase. A, B, C three samples due to the difference between the thermal conductivity of the substrate, the aging of the sample temperature is not the same, so after aging the same current wavelength drift C sample maximum, A sample followed, B sample minimum. In addition, due to the three kinds of sample substrate and the chip transfer method is not the same, so that after the transfer of GaN epitaxial film on the new substrate by the stress situation is not the same. The literature shows that the tensile stress of the GaN layer is reduced and the compressive stress of the quantum well In GaN layer is increased after the GaN is transferred from the silicon substrate to the new silicon substrate by welding and chemical etching. The stress relaxation of the thin film transfer is more thorough, so that the quantum well is subjected to a greater compressive stress, and the resulting polarized electric field is larger, resulting in a greater inclination of the band, so that the release of photons Energy is reduced, the performance of the EL wavelength longer. Therefore, the A samples were printed on the silicon substrate in the E-spectrum before and after aging, the wavelength of the A sample was the shortest, the C sample was the second, the B sample was the longest, and the B sample and the C sample were very close. Figure 2 also reflects the redshift of the wavelength of the B sample from small current to high current before and after aging, which may be related to the following aspects. On the one hand, the junction temperature increases so that the GaN band gap becomes smaller and the wavelength is redshift. As a result of the stress relaxation of the B sample, the B-sample quantum well is the most compressive stress, so the B-sample multi-quantum well region has the strongest polarization effect, and the polarization effect produces a strong built-in electric field. This electric field leads to significant quantum Limit the Stark effect, causing a redshift of the wavelength of the light.

Study on Aging Performance of 1W Silicon Substrate Blue LED for Different Substrates

 

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Figure 2 three samples 900mA ambient temperature aging 168h before and after the EL spectrum [(a1) ~ (a3)] and before and after aging three kinds of sample wavelength with the current changes [(b1) ~ (b3)]

Power-current (L-I) relationship analysis

Figure 3 is 350mA current under the relative light intensity of the sample with the aging time of the relationship between the three samples are aged before the light intensity of 100%. It can be seen from Figure 3, A, B, C three kinds of samples with the aging time increased with the first increase and then reduce, which A sample after 2h after the increase in light intensity, followed by aging The light intensity began to decrease, and B, C samples were aged at 32h, 10h light intensity began to decline, and the trend of decline slower than the A sample. And can be seen at room temperature 900mA aging after A, B, C three samples 350mA under the light intensity has been a maximum and then reduced, C samples reduced the most, A times, B sample light intensity value is reduced , But still larger than the value before aging. The reason for this phenomenon is that the GaN grown by the MOCVD method has a partial acceptor Mg that is passivated by the formation of Mg-H complex with H, and the activation rate of Mg is very low, resulting in a low hole concentration. Part of the Mg-H bond is interrupted so that the acceptor Mg is activated, so that the hole concentration increases, the carrier concentration may become more matched, the luminous efficiency becomes higher. On the other hand, the aging causes the density of nonradiative recombination centers such as dislocations and defects in the GaN material to be lowered, resulting in a decrease in luminous efficiency and a decrease in light intensity. These two mechanisms compete with each other. At the beginning of aging, the Mg-acceptor activation mechanism dominates, so that the intensity of the three samples increases with the same current. With the aging process, the non-radiative complex center hyperplasia mechanism Dominant, so the high current aging after a period of time after the three samples are reduced light intensity. The difference in light failure of the three samples may be due to the fact that the stress states of the three sample quantum wells and the thermal conductivity of the supporting substrate are not the same as those of the nonradiative compound center.

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Figure 3,350mA current relative light intensity at room temperature 900mA aging after the change over time (100% of the light intensity before aging)

Conclusion

The results show that the EL wavelength of the copper substrate is the longest at the same current, because the electroplating of the device is carried out on the silicon substrate, copper substrate and copper-chromium substrate GaN-based blue LED. After the transfer to the copper substrate, the stress relaxation of the GaN epitaxial film is more thorough. Through the aging of three different substrate LED devices can be seen that the main factors affecting the reliability of LED may be its stress state. The I-V characteristics, L-I characteristics and EL spectra of the three substrates before and after aging were studied. The results show that the copper substrate devices have better aging         

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