Future green, violet and ultraviolet LED chip will become the focus of research

Introduction: at the end of last century, semiconductor lighting began to emerge and rapid development, one of the core premise is the preparation of growth and device structure of blue GaN based luminescent materials, and the future of material and device structure and technology level will determine the height of the semiconductor lighting technology.

This chapter will focus on the GaN based materials and devices derived from equipment, source materials, device design, chip technology, chip applications and other aspects of the analysis.

equipment

Under the condition that the bulk GaN single crystal material can not be prepared at present, MOCVD is the most important equipment of GaN material epitaxy. The current commercial MOCVD equipment market is dominated by the world's two giants, in this situation, China's MOCVD has made great progress, and the emergence of the 48 machine. But we still need to recognize the shortcomings of domestic MOCVD. For MOCVD, in general, the focus of research equipment is temperature control, commercial equipment is uniformity, repeatability, etc.. At low temperature, can the growth of high In composition InGaN, suitable for nitriding objects system materials used in orange yellow light, red light, infrared long wavelength, which covers the entire field of application of nitride white; and in the 1200oC-1500oC can grow under high temperature, high Al component of AlGaN, which should be extended to nitride UV field and power the field of electronic devices, the application range of gain greater expansion.

At present, foreign countries already have 1600oC high temperature MOCVD equipment, can produce high performance ultraviolet LED and power devices. China's MOCVD still need long-term development, expand the scope of temperature control of MOCVD; for commercial equipment not only to improve performance, but also to ensure uniformity and scale.

Source material

The source materials mainly include all kinds of gas materials, metal organic materials, substrate materials and so on. Among them, the substrate material is the most important, directly restricting the quality of epitaxial film. Currently, GaN based LED substrate is becoming more diverse, SiC, Si and GaN and other substrate technology gradually improved, some of the substrate from 2 inches to 3 inches, 4 inches or even 6 inches, such as the development of large size. But overall, the current price is still the highest performance SiC sapphire; but the price is expensive; the price and size advantage Si substrate and traditional integrated circuit technology between the temptation to make Si substrate is still one of the most promising technology roadmap.

The GaN substrate still need to improve the size and price reduction efforts, so that in the future high-end green laser and non polar LED applications display skills to the full metal organic materials; from imports to independent production, has made great progress; other materials have also made considerable progress in gas. In a word, China has made great progress in the field of source materials.

extension

The process of obtaining the structure of the device is the most technological process, which directly determines the internal quantum efficiency of LED. At present, the majority of semiconductor lighting chips adopt multi quantum well structure, the specific technical route is often constrained by the substrate material. However, the PSS technology is widely used in sapphire substrate, which can reduce the error density of epitaxial films and improve the internal quantum efficiency. The future of PSS technology is still an important substrate technology, and the size of the graphics gradually to the development of nanotechnology. The use of GaN homogeneous substrate can take non polar surface or semi polar surface epitaxy, quantum Stark effect caused by partial elimination of polarization field, has very important significance in the green, Huang Lvguang, orange light GaN based LED applications.

In addition, the current extension is generally the preparation of single wavelength quantum wells, the use of appropriate epitaxial technology, can be prepared by multi wavelength LED, which is a single chip white LED, which is also one of the promising technical route. Among them, the representative of the InGaN quantum wells in the phase separation, such as the realization of the high In component InGaN yellow light quantum dots and blue light quantum well combination of white light. In addition, the use of multiple quantum wells to achieve a wide spectrum luminescence mode, in order to achieve a single chip white light output, but the color rendering index is relatively low. Single chip white LED is a very attractive development direction. If we can achieve high efficiency and high color rendering index, it will change the technology chain of semiconductor lighting. In the aspect of quantum well structure, the introduction of the electronic barrier to block the electron leakage and improve the efficiency of luminescence has become the conventional method of LED epitaxial structure.

In addition, the optimization of the potential barrier and the potential well of quantum wells will be an important part of the process, how to adjust the stress, and to achieve the band clipping, can be prepared with different wavelength of LED. In the aspect of chip coverage, how to improve the quality of the p layer, the hole density of the p layer, the conductive properties and the droop effect under the large current is still a top priority.

chip

In chip technology, how to improve the light extraction efficiency and better heat dissipation scheme become the main chip design, and developed the corresponding vertical structure, surface roughening, photonic crystal structure, thin film, flip flip structure (TFFC), a new technology of transparent electrode. Among them, the thin film flip chip structure using laser stripping, surface roughening technology, can greatly improve the luminous efficiency.

Chip application

The fluorescence conversion efficiency of white LED technology scheme for the Blu ray excitation of LED yellow phosphor is low, RGB single chip and multi chip white phosphor free white LED technology has become the main trend in future, the low efficiency of the green LED has become the main limiting factors of RGB multi chip white, half polar or non-polar the green LED will become an important development trend. In the solution of white LED color rendering, the use of violet or ultraviolet LED excited RGB tricolor phosphors, high color rendering white LED technology, but it is necessary to sacrifice part of the efficiency. At present, violet or ultraviolet light efficiency of the chip has gained a great progress, the wavelength of 365nm UV LED external quantum efficiency of Nichia company is close to 50%. Future UV LED will get more applications, and there is no other UV light-emitting material instead of a great development prospects. Some developed countries have invested a lot of manpower and material resources to carry out the research of UVLED. In addition to the environment, it is difficult to compete with arsenic in the red and infrared light band, in addition to the environment, it is not clear whether the price or performance is competitive with the arsenide.

According to the above stated that, around the upper reaches of the materials and equipment of semiconductor lighting has gained great development, especially in terms of efficiency, the blue band has been close to the ideal efficiency of chip in semiconductor lighting price ratio has also declined substantially, the future of semiconductor lighting from the light efficiency to the light quality direction, which requires materials through blue chip at the same time, to the development of long and short wavelength direction, while the green, violet and ultraviolet LED chip will be the research focus.