III. RESULTS AND DISCUSSION

In this section, the structural and optical properties of as-grown MQWs without epitaxial growth of p-GaN layer and Sample B with a full LED structure are carefully studied. The x-ray rocking curves for GaN (0002) and (10−12) planes of MQWs and Sample B are shown in Figs. 2(a) and 2(b), respectively. The full width at half maximum (FWHM) values of the GaN (0002) plane for the MQWs and Sample B are 252 and 231 arcsec, respectively. Meanwhile, the FWHM values of the GaN (10−12) plane for the MQWs and Sample B are 265 and 247 arcsec, respectively. It is known to us that the FWHM value for the GaN (0002) plane is related to the densities of screw dislocation in GaN films, which mainly generated from the different step heights of the substrates. While the FWHM value for GaN (10−12) is related to the densities of the mixed and pure edge dislocations that generated from the coalescence process among the misoriented individual islands.^{Reference Li, Jiang, Fana, Fang and Wang30,Reference Liu, Tsai, Yen, Chen, Tsai, Huang, Chen, Hsu and Liu31} The densities of dislocations in MQWs and Sample B can be approximately evaluated as:^{Reference Liu, Tsai, Yen, Chen, Tsai, Huang, Chen, Hsu and Liu31}

(1)$${D_{{\rm{dis}}}} = {{{\beta ^2}} \over {9{b^2}}}\quad {\rm{,}}$$

where D _dis is the density of dislocation, β is the FWHM value, and b is the length of the Burger vector of the corresponding dislocations. The FWHM values of (0002) and (10−12) for both InGaN/GaN green LEDs and MQWs are listed in Table I. Based on Eq. (1) and Table I, the densities of screw, pure edge, and mixed dislocations in n-GaN after the growth of the p-GaN layer are decreased with the values of 16 and 13%, respectively, when compared with those of the MQWs without growth of p-GaN layer. Meanwhile, as the increase in p-GaN layer growth temperature, the crystalline quality of n-GaN films gradually increases, as shown in Table I. We ascribe it to the high growth temperature of p-GaN that enhances the migration distance of Ga atoms incorporating into the lattice of the defect locations, suppresses the edge and screw dislocations, and therefore improves the GaN crystalline quality ultimately. The higher the p-GaN growth temperature is, the better the n-GaN crystalline quality will be. Meanwhile, the crystalline quality of these InGaN/GaN green LEDs is comparable with that of the state-of-the-art InGaN/GaN blue LEDs grown on sapphire and patterned sapphire substrates.^{Reference Wang, Zhou, Lin, Hong and Li32}

FIG. 2.

The comparison of (a) (0002) and (b) (10−12) rocking curves for MQWs and Sample B.

TABLE I.

The FWHM values of PL, EL, and XRD for MQWs, Samples A, B, and C.

Figure 3(a) shows the comparison of PL spectrum for both the MQW structure without growth of p-GaN layer and Sample B. The FWHM value of the PL peak for MQW structure is 29 nm, while for Sample B is 33 nm (Table I), which is attributed to the stacked growth of p-GaN layer on the MQWs that increases the light emitting energy level, leading to the broadening in PL peaks ultimately.^{Reference Lin, Ma, Hsu, Feng, Cheng, Liao, Yang, Chou, Lee and Chyi33} This is quite different from the blue MQWs and blue LEDs, and the peak of PL spectrum for blue LEDs is just a little broadening due to the lower indium composition in the InGaN well layer.^{Reference Lin, Ma, Hsu, Feng, Cheng, Liao, Yang, Chou, Lee and Chyi33} It is believed that the p-GaN epitaxial growth makes a negative effect on the green MQW structure in green LEDs, where high indium composition in InGaN wells is existed. The p-GaN layer growth would exacerbate indium composition fluctuation to a certain extent and deteriorate the crystalline qualities and optical properties of the MQWs ultimately.^{Reference Dupuis, Limb, Liu, Ryou, Horne and Yoo34,Reference Lv, Wang, Wang, Hao and Luo35} However, the comparison test of the blue LEDs shows a contrary result, because the exacerbated fluctuation indium in the blue MQWs is beneficial to the carrier confinement that enhances the radiation recombination efficiency to some extent.^{Reference Dupuis, Limb, Liu, Ryou, Horne and Yoo34,Reference Lv, Wang, Wang, Hao and Luo35}

FIG. 3.

(a) The PL spectrum for MQWs and Sample B and (b) the comparison of ω–2θ rocking curves for Sample A, Sample B, and Sample C.

It is not hard to find that the p-GaN epitaxial growth temperature makes a tremendous effect on the status of MQWs in LED wafers. In this work, the crystalline qualities of InGaN/GaN MQWs influenced by the growth temperature of p-GaN layer are studied by ω–2θ rocking curves, as shown in Fig. 3(b). On the one hand, if the epitaxial growth temperature for p-GaN is as low as 920 °C, one can note the broadened satellite peaks in typical ω–2θ rocking curves of Sample A when compared with that of Sample B grown at 950 °C. On the other hand, if the p-GaN epitaxial growth temperature is increased to 980 °C, one can also clearly identify the broadened in satellite peaks for MQWs in Sample B as compared with that of Sample C. Furthermore, the FWHM values of satellite (SL)-3 peaks are 260, 220, and 272 arcsec for Sample A, Sample B, and Sample C, respectively, which indicates the poorer crystalline quality of the MQWs in Sample A and Sample C when compared with that of Sample B. We attribute these results to the following reason. If the growth temperature of p-GaN layer is too low, it may not supply enough energy to enhance the crystalline quality of InGaN/GaN MQWs even though the fluctuation of indium composition in InGaN wells is suppressed. On the contrary, if the growth temperature of p-GaN layer is too high, the fluctuation of indium composition in InGaN wells will happen, and the crystalline quality of InGaN/GaN MQWs is significantly reduced. The optical properties of InGaN/GaN MQWs influenced by the growth temperature of p-GaN layer are also studied. If the p-GaN epitaxial growth temperature is decreased to 920 °C, as shown in Sample A, it also shows poorer optical properties of as-grown InGaN/GaN green LEDs with a broadened PL peak. This may be ascribed to the poorer Mg doping rate at lower growth temperature, which leads to the poorer quality of as-grown p-GaN and eventually results in the decline of effective recombination efficiency. Meanwhile, the broadened emission peaks of PL spectra for Sample C prove that the increase of the p-GaN growth temperature would further exacerbate the fluctuation of indium composition in the original high stressed green QW (Table I).

We further study on the optoelectronic properties of the InGaN/GaN green LED devices that affected by the p-GaN epitaxial growth temperature. Figure 4(a) shows the comparison of V–I curves for LEDs of Sample A, Sample B, and Sample C, respectively. At an injection current of 20 mA, the forward voltages are of 3.43, 3.21, and 3.01 V for these three samples, respectively. One can clearly find that the increase of the p-GaN epitaxial growth temperature obviously benefits to the improvement of electronic properties of InGaN/GaN green LEDs. On the one hand, when the growth temperature of p-GaN layer is increased, the N vacancies and H atoms together with other impurities incorporated into p-GaN are dramatically reduced, while the activation rate of Mg atoms is also greatly improved.^{Reference Ju, Zhu, Kim, Lee and Lee36,Reference Taniyasu, Carlin, Castiglia, Butté and Grandjean37} On the other hand, the series resistance rate of device is decreased significantly and the conductivity of layer rather than InGaN/GaN MQWs may be improved to some extent with the increase in the growth temperature.^{Reference Ju, Zhu, Kim, Lee and Lee36,Reference Taniyasu, Carlin, Castiglia, Butté and Grandjean37} As a result, these lead to the decrease of forward voltage. Although the higher p-GaN epitaxial growth temperature improves the electrical properties of InGaN/GaN green LED devices, it damages the MQW structure because the FWHM of EL peak for Sample C LEDs increases 5 nm compared with that of Sample B LEDs, and the emission intensity decreases to 69% for Sample C LEDs when compared with that of Sample B LEDs [Fig. 4(b) and Table I]. Meanwhile, with the decrease in the p-GaN growth temperature, the worse Mg doping efficiency leads to the poorer light output power, which leads to the decline of emission peak intensity. In this case, the peak intensity for Sample A LEDs is just about 78% when compared to that for Sample B LEDs. These comparisons indicate that the growth temperature of p-GaN layer plays a great impact on the MQW structures. Although the enriched indium is beneficial to the carriers' confinement state and enhances the recombination, it is also very easy to induce V-defects that decrease the conductivity in the InGaN/GaN and the effective recombination of electrons and holes.^{Reference Yong, Soh, Zhang, Chow and Chua38–Reference Oh, Kwon, Park, Baek, Park, Lee and Jung42} The negative effects of excessive p-GaN epitaxial growth temperature on the MQWs can be seen from the changes in the reverse current (reverse voltage is –7 V), the reverse current value of Sample C is 48 μA, while the reverse current for the Sample B LEDs is only 0.01 μA. This increase of reverse current is related to the MQW structure, which releases the stress by the formation of the more defects under the higher p-GaN epitaxial growth temperature. Figure 4(c) shows the light output power for LEDs dependence on the forward current. As for Sample A LEDs and Sample B LEDs, the light output power of Sample B LEDs is higher than that of Sample A LEDs, since the lower growth temperature results in the poorer Mg doping rate, leads to the worse effective recombination, and therefore results in the decline of the light output power ultimately. In the Sample B LEDs and Sample C LEDs case, the light output power of Sample B LEDs is higher than that of Sample C LEDs, which is attributed to the too high growth temperature of p-GaN. This deteriorates the crystalline quality of MQWs, leading to the poor conductivity of InGaN/GaN MQWs and low effective recombination between holes and electrons, and eventually poor optoelectronic properties of LEDs. Based on these results, we therefore can make a conclusion that the p-GaN layer growth temperature of 950 °C is most suitable for obtaining highly efficient InGaN/GaN green LEDs.

FIG. 4.

(a) V–I, (b) EL, and (c) L–I curves for various LED devices.

Based on the above discussion, the mechanisms for the effect of p-GaN growth temperature on the properties of InGaN/GaN green LEDs can be deduced. As for the p-GaN layer grown at low temperature, the Mg doping rate is very poor, and many defects may form during the p-GaN growth, as illustrated in Fig. 5(a). These defects may capture the carriers, increase the absorption of lights when they pass through the p-GaN layer, and eventually reduce the light extraction efficiency of InGaN/GaN green LEDs. In this case, the defects caused by the fluctuation of indium composition in InGaN wells are very few due to their low growth temperature. Therefore, the absorption of lights in InGaN/GaN MQW layers is very few. While for the p-GaN layer grown at too high temperature, the high-quality p-GaN layer can be grown due to the fact that the N vacancies and H atoms together with other impurities incorporated into the p-GaN layer are dramatically reduced, and the activation rate of Mg atoms is also greatly improved. Therefore, the formed defects by the Mg doping in the p-GaN layer are significantly reduced, and the lights absorbed by the p-GaN layer are greatly reduced. However, in this case, the serious fluctuation of indium composition in InGaN well would take place, and many defects are formed in InGaN/GaN MQWs, as shown in Fig. 5(b). This process also increases the absorption of lights and reduces the light extraction efficiency of InGaN/GaN green LEDs. Conclusively, the growth temperature of the p-GaN layer plays an important role in achieving highly efficient InGaN/GaN green LEDs.

FIG. 5.

Schematic diagram for p-GaN grown at too (a) low temperature and (b) high temperature.