[Science] a text to read about those things LED

Light emitting diode (LEDs), the latest developments in the rapid growth of the lighting industry. At present, solid state lighting technology gradually penetrated into different segments of the market, such as automotive lighting, indoor and outdoor lighting, medical applications, and daily necessities.

The United States Department of energy's latest report, to 2020, the lighting industry is expected to reduce energy consumption by 15%, saving 30% of light in 2030 2030 261TWh (TWH) can save energy, to the current price to calculate the value of more than $26 billion, equivalent to the sum of the twenty-four million household energy consumption current. In addition, these savings of energy for hybrid power plants will reduce emissions of about eighteen million tons of CO2 greenhouse gases.

Although in many cases, the initial cost of these devices is still higher than the existing light source equipment, but LEDs higher efficiency and longer life to make it very competitive. Strategies Unlimited estimates that in 2013 the global sales of 400 million LED lights, McKinsey survey shows that in 2016 the global LEDs lighting market share in the world will reach 45% in 2020 will be close to 70%. By 2020, the market capacity is expected to rise from about $26 billion to $72 billion.

LED device is a complex multi component system, which can adjust the performance characteristics according to specific requirements. The following sections will discuss white LED and other applications.

The development road of LED

The electroluminescence of inorganic materials is the basis of LED luminescence, Henry Round and Oleg Losev in 1907 and in 1927 reported the phenomenon of LED Luminescence - current through the silicon carbide (SiC) crystal luminescence. These results lead to further theoretical study on the optoelectronic processes of semiconductors and p-n junctions.

In twentieth Century 50, in 60s, scientists began to study the Ge, Si and a series of III-V semiconductor (such as InGaP, GaAlAs) electroluminescent properties. Richard Haynes and William Shockley show that the recombination of electrons and holes in the p-n junction leads to luminescence. Subsequently, a series of semiconductors were studied, and eventually developed the first red LED by NickHolonyak in 1962. Under its influence, in 1971 George Craford invented the orange light LEDs, in 1972 and successively invented the yellow and green LEDs (composed of GaAsP).

Intense research has led to the commercialization of LEDs, which is widely used in the wide spectral range (from infrared to yellow), and is mainly used in the telephone or control panel. In fact, these LEDs efficiency is very low, limited current density, so that the brightness is low, is not suitable for general lighting.

Blu ray LEDs

The development of efficient blue light LEDs has taken 30 years, because there is no available quality broadband gap semiconductor. In 1989, the first blue light LEDs based on the SiC material system was commercialized, but because SiC is an indirect band gap semiconductor, its efficiency is very low. By the end of 1950s it has been considered that the use of direct bandgap semiconductor GaN, 1971 JacquesPankove demonstrated the first launch of green GaN based LED. However, the preparation of high quality GaN single crystals as well as the introduction of n- and p- doping techniques in these materials are still to be developed.

The development of metal organic vapor phase epitaxy (MOVPE) technology in 1970s is of great significance for the development of high efficiency blue LEDs. In 1974, the Japanese scientist Isamu Akasaki began to use this method to grow GaN crystals, and in collaboration with Hiroshi Aman in 1986 through the MOVPE method for the first time the synthesis of high-quality device level GaN.

Another major challenge is the controllable synthesis of p- doped GaN. In fact, in the process of MOVPE, Mg and Zn atoms can enter into the crystal structure of the material, but often combine with hydrogen to form an invalid p- type doping. Amano, Akasaki and their collaborators observed that Zn doped GaN emitted more light after scanning electron microscopy.

In the same way, they show that the electron beam radiation has a beneficial effect on the doping properties of Mg atoms. Subsequently, ShujiNakamura proposed a simple post deposition step after thermal annealing to decompose the complex of Mg and Zn, which can easily realize the p- type doping of GaN and its three alloy (InGaN, AlGaN).

It should be pointed out that the band of these three systems can be adjusted by the composition of Al and In, so that the design of the blue LEDs adds a degree of freedom, which is of great significance to improve its efficiency. In fact, the active layer of these devices is usually composed of a series of alternating narrow band gap InGaN and GaN layers and p- doped AlGaN thin films (p- terminal) as the carrier.

In 1994, based on the symmetry between n- type and p- type AlGaN doped Zn doped InGaN active layer heterostructure design of Nakamura and its partners, first demonstrated with 2.7% external quantum efficiency (EQE) of InGaN blue LED (box 1 lists the main performance index of the LEDs definition).

The schematic diagram of the LED is shown in Figure 1a. These results are critical to the LED based lighting technology that is being used today, and hence the revolution in the lighting industry. By the end of 2014, the Nobel prize in physics was awarded to Akasaki, Amano and Nakamura, in recognition of their "invention for efficient lighting and white light energy efficient Blu ray LED".

LED performance index

Quantum efficiency Quantum efficiency: the material internal quantum efficiency (IQE) is the ratio of the electron hole recombination (i.e., the number of photons produced) to the total amount of radiation (non radiative).

The index determines the luminous efficiency of semiconductor materials. Semiconductor LED performance is usually used