Efficiency is increased to about 100 times the Kyoto University developed half green polar surface LED

Figure 1: light.

The research team composed of the Japan Federation of Kyoto University associate professor Kawakami Yachi and Nichia chemical industries such as the day before developed with the past by making the crystal growth in the bottom half polar surface and the production of green light emitting diode (LED) compared to the same type of external quantum efficiency is improved by about 100 times (green LED released data (Figure 1)). The bottom plate is GaN Bulk, and the active layer is InGaN (Figure 2). When the driving current is 20mA, the power is 1.9mW, the external quantum efficiency is 4.1%, the luminous wavelength is about 530nm, the driving current is 200mA, the power is 13.4mW, the external quantum efficiency is 2.8%, the luminous wavelength is 520nm. However, compared with the ordinary sapphire substrate and the SiC substrate on the polar surface of the class InGaN green LED, weak power.

The external quantum efficiency is improved by decreasing the piezoelectric field

The nonpolar surface is the surface in the normal direction of the polar surface, while the semi polar surface is the surface between the polar surface and the non-polar surface (Figure 3). For example, the InGaN class LED made in the bottom of the non-polar surface and semi polar surface, compared with the production in the polar surface of LED, has the advantages of high luminous efficiency, can reduce the wavelength variation caused by the increase of drive current. This is mainly due to the weakening of the InGaN (Piezoelectric Field) as an active layer. While the ordinary blue LED and green LED active layer used for the quantum well structure of the class InGaN light-emitting devices, usually the use of sapphire substrate and SiC polar surface. At this time, the lattice constant of InN (lattice constant) is larger than the lattice constant of GaN, so the piezoelectric polarization (piezoelectric polarization) will produce the piezoelectric field. However, the electric field will pull up the distance between the electrons and holes in the active layer, which leads to the decrease of the recombination probability, and thus the luminous efficiency will decrease. In addition, since the emission wavelength is closer to the long wavelength side, if the amount of In increases, with the increase of the amount of In, the crystal strain will increase, which will enhance the electric field generated by the piezoelectric polarization. Therefore, the longer the wavelength, the lower the external quantum efficiency.

In addition, the semi polar and nonpolar InGaN class LED also has the characteristics of polarization. For example, if used as a liquid crystal backlight, it will be able to reduce the loss of polarization filters.

Figure 2: schematic diagram of the structure of the element

Reduction of dislocations and defects

Compared with the past semi polar surface class InGaN green LED, the reason why the power can be improved, there are 2 main reasons. First, it is possible to carry out epitaxial growth on the semi polar surface of the GaN Bulk substrate with a higher crystalline quality than in the past, and the two is to select the crystal surface with weak electric field strength (1122) in the semi polar plane. In the past, it was difficult to crystallize the orientation of the crystal plane of the semi polar and non polar surfaces, and it is easy to produce defects and dislocation density (dislocation density). The defects and dislocation density were effectively reduced by improving the growth conditions. Details of manufacturing methods are not disclosed. As the bottom plate of the GaN, the first use of the HVPE method, and then the use of chemical mechanical polishing (CMP) cutting, made of (1122) Bulk type GaN backplane. Thereafter, the organic metal vapor phase growth method (MOCVD) was used to cascade the active layers. In the manufacturing process, it is reported that the HVPE and MOCVD devices can be used in the past. GaN floor by the ancient river machinery metal production.

Figure 3: semi polar and non polar

Such as the use of semi polar and non-polar surface of the bottom of the blue LED and green LED research, University of California professor Nakamura Shuji and other research teams are doing. (reporter: root Jin Zhen)