GaN materials have been widely used in display, indication, backlight and solid-state lighting since the 1990s, and have formed a huge market. To date, gallium nitride (GaN)-based light-emitting diodes (LEDs) fabricated on three substrates (sapphire, silicon carbide, and silicon) have been commercialized. In recent years, GaN-based LED technology on silicon substrates has received much attention. 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 price ratio in power LED device applications.
Many research groups have grown GaN epitaxial films on Si substrates and some have obtained devices or studied the properties of Si-based GaN. In the preparation process of the LED, the GaN film was transferred to a new support substrate to prepare a vertical structure device, and the photoelectric performance was better than that of the ipsilateral structure device.
In this paper, the GaN epitaxial film grown on the Si substrate was transferred to the copper support substrate, the copper-chromium support substrate by electroplating, and transferred to the Si support substrate by pressure welding to obtain a vertical structure light-emitting device, and three kinds of samples were obtained. A comparative study of aging was conducted.
The epitaxial wafer used in the experiment is a 2 in (50.8 mm) blue InGaN/GaN multi-quantum well epitaxial wafer grown by MOCVD on a silicon (111) substrate, and its chip size is 1000 Lm@1000 Lm. It has been reported. The experiment prepared three epitaxial wafers grown in the same furnace. One of them was transferred to the Si substrate by pressure bonding technology and chemical etching method to obtain a light-emitting device, which is called sample A, and the other two were plated and chemically etched. The method discloses that the GaN epitaxial film is transferred to the plated copper substrate and the plated copper-chromium substrate, respectively, and the light-emitting devices are obtained, which are respectively referred to as sample B and sample C. The three kinds of samples were identical in the fabrication process of the other devices except that the epitaxial film transfer mode and the support substrate were different.
Because of the slight difference between individuals of similar samples, samples A, B, and C were initially tested, and representative chips were selected for experiments and tests. Each chip is packaged in a bare core. The chip current of 1000 Lm@1000 Lm usually operates at 350 mA. To accelerate aging, the sample A, B, C has a DC current of 900 mA at normal temperature. The current-voltage (I-V) characteristic curve, electroluminescence (EL) spectrum, and the relative light intensity of each sample at each current were measured using a power supply KEITHLEY 2635 and a spectrometer Compact Array Spectrometer (CAS) 140 CT. Wait.
3 Results and discussion
3. 1 I - V characteristic analysis
Table 1 shows the Vf and I r values â€‹â€‹of the three samples before aging, aging 80, 150 and 200 h. The aging conditions are 900 mA at normal temperature, where Vf is the voltage at 350 mA and I r is the leakage at 10 V in the reverse direction. The current value, usually the reverse leakage current I r is measured at 5 V in the reverse direction. For the comparison result, the more severe conditions are selected and measured in the reverse 10 V. Figure 1 shows the I-V characteristics of the three samples before aging, 80, 150 and 200 h after aging, as shown in Figures 1(a) ~ (d). Figure 1(a) shows that the three samples A, B, and C have good I-V characteristics before aging, and the turn-on voltage is about 2. 5 V, and the current is 10 - 9 A in the reverse 10 V. Magnitude. After aging for 200 h, the leakage current I r of the three samples in the reverse 10 V was significantly higher than that before aging. Table 1 shows that the B sample has the lowest leakage current at the same back pressure ( - 10 V) after 200 h of high current aging, the second sample is the second, the C sample is the largest, and the three samples are in the same inverse with the aging time. The difference in leakage current is increasing. The InGaN MQW LED has a slight increase in the forward voltage after aging because the large current is aged for a long time, causing partial oxidation of the exposed n-electrode (aluminum), resulting in a large contact resistance. The reason why the leakage increases after aging is as follows: The width of the InGaN LEDpn junction depletion layer is mainly determined by the carrier concentration of the p-type layer. After the chip is aged for a long time by a large current, the acceptor Mg is decomposed by the Mg-H complex. Reactivation, causing the p-type carrier concentration to increase, leading to a narrowing of the depletion layer, thinning of the barrier region during reverse biasing, increase of tunnel breakdown components, and increase of reverse current; in addition, the chip undergoes long-term aging after a large current After that, the defect density in the quantum well region increases, the defect in the reverse bias and the trap-assisted tunneling cause leakage current, and the thermal conductivity of the three samples B, A, and C decreases in turn, so the defects and trap density generated during aging are sequentially Reduced, so the leakage current of the three samples increased in sequence under the same back pressure (as shown in Table 1 and Figure 1).
Fig.1 I-V characteristic curve before and after aging of three samples
Table 1 Vf and I r values â€‹â€‹of three samples before and after aging
3. 2 EL spectral analysis
Figure 2 is an electroluminescence (EL) spectrum at 1, 10, 100, 500, 800, 1000, and 1200 mA before and after 168 h of continuous aging at 900 mA for three samples at normal temperature [Figure 2 ( a1 ) ~ ( a3) And the relationship between the EL wavelength and current of the three samples before and after aging [Fig. 2(b1) ~ (b3)], the solid line in the figure indicates the spectrum before aging, and the broken line indicates the spectrum after aging. Figure 2 (a1) ~ ( a3) shows the EL spectrum before and after aging by normalization. The EL spectrum waveforms of the three samples before and after aging are not significantly changed except for the red shift of the peak wavelength at high current. Figure 2 (b1) ~ (b3) shows that the wavelengths of the three samples before and after aging vary significantly with the change of current. The wavelength of B sample before and after aging is almost the same as the current, but the wavelength is slightly the same after aging. There is an increase. A, B, C Three samples have different thermal conductivity of the substrate. The junction temperature of each sample is different when aging. Therefore, the wavelength drift of C at the same current after aging is the largest, the second sample is the second, and the B sample is the smallest. In addition, since the three sample substrate materials and the chip transfer method are different, the stress state of the GaN epitaxial film on the new substrate is different after transfer. Literature studies have shown that the tensile stress of the entire GaN layer is reduced after the transfer of GaN from the silicon substrate to the new silicon substrate by pressure welding and chemical etching, and the compressive stress of the quantum well InGaN layer is increased. The GaN stress relaxation of the film transfer by the electroplating method is more thorough, so that the quantum well is subjected to a larger compressive stress, and the generated polarized electric field is larger, thereby causing the band to be inclined more, and thus the photons are released when the carriers are recombined. The energy is reduced and the EL wavelength is longer. Therefore, in the EL spectrum before and after aging, the A sample with the lowest pressure on the silicon substrate is the shortest, the C sample is the second, the B sample is the longest, and the B sample and the C sample are very close. Figure 2 also reflects the maximum redshift of the B sample from small currents to large currents before and after aging, which may be related to the following aspects: On the one hand, the junction temperature increases, making the GaN band gap smaller, causing the wavelength to red shift, another In view of the fact that the stress relaxation of the B sample is the most thorough, the B-sample quantum well is subjected to the largest compressive stress, so the polarization effect of the multi-quantum well region of the B sample is the strongest, and the polarization effect produces a strong built-in electric field, which leads to a remarkable quantum. Limits the Stark effect, causing a red shift in the wavelength of the light.
Fig. 2 EL spectra of three samples at 900 mA ambient temperature aging before and after 168 h [ ( a1 ) ~ ( a3 ) ] and the relationship between the three samples before and after aging with current [ [ b1) ~ ( b3 ) ]
3. 3 power-current (L-I) relationship analysis
Figure 3 shows the relative light intensity of each sample as a function of aging time at 350 mA. All three samples have a light intensity of 100% before aging. It can be seen from Fig. 3 that the light intensity of the three samples A, B, and C increases first and then decreases with the increase of aging time, and the light intensity increases most after A aging for 2 h, followed by aging. The intensity of the light began to decrease, while the B and C samples were aged for 32 h, and the light intensity began to decrease at 10 h, and the downward trend was slower than that of the A sample. Moreover, it can be seen that after the aging of 900 mA at room temperature, the light intensity at 350 mA of all three samples of A, B, and C passes through a maximum value and then decreases, and the C sample decreases most, A. The light intensity of the B sample 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 which is passivated due to the formation of the Mg-H complex with H, and the activation rate of Mg is low, resulting in a low hole concentration, and in the high current aging, there is Part of the Mg-H bond is interrupted and the acceptor Mg is activated, so that the hole concentration is increased, the carrier concentration may become more matched, and the luminous efficiency becomes higher. On the other hand, aging causes the density of non-radiative recombination centers such as dislocations and defects in the GaN material to increase, whereby the luminous efficiency is lowered and the light intensity is lowered. These two mechanisms compete with each other. In the early stage of aging, the Mg acceptor activation mechanism dominates. Therefore, the light intensity of the three samples increases at the same current. As the aging progresses, the non-radiative complex center proliferation mechanism such as dislocations and defects gradually takes up. Leading, so the intensity of the three samples decreases after a period of high current aging. The speed of light decay of the three samples may be different because the stress state of the three sample quantum wells and the thermal conductivity of the supporting substrate are different, resulting in different degrees of non-radiative recombination.
Figure 3 The relative light intensity at 350 mA current as a function of time after aging at 900 mA (100% before aging)
By comparing the aging of GaN-based blue LEDs epitaxially grown on silicon substrates onto copper substrates, copper substrates and copper-chromium substrates, the results show that the EL wavelength of the copper substrate is the longest at the same current because of electroplating. The stress relaxation of the GaN epitaxial film is more thorough after transfer to the copper substrate. By aging the three different substrate LED devices, the main factor affecting the reliability of the LED may be its stress state. The I-V characteristics, L-I characteristics and EL spectra of the three substrate LEDs before and after aging were studied. It is found that the copper substrate devices have better aging properties.
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