Recent advances in light-emitting diodes ( LEDs ) have led to a rapid growth in the lighting industry. At present, solid-state lighting technology has gradually penetrated into different market segments, such as automotive lighting, indoor and outdoor lighting , medical applications, and household items.
According to the latest report of the US Department of Energy, by 2020, the technology is expected to reduce energy consumption by 15% in the lighting industry, saving 30% in 2030 – that is, saving 261 TWh (terawatt hour) of energy in 2030, with current The price is calculated at more than $26 billion, which is equivalent to the current total energy consumption of 24 million American households. In addition, these savings in energy used in hybrid power plants will reduce emissions of approximately 18 million tons of CO2 greenhouse gases.
Although in many cases the initial cost of these devices is still higher than existing light source devices, the higher efficiency and longer life of LEDs makes them highly competitive. Strategies Unlimited estimates that 400 million LEDs will be sold worldwide in 2013. The McKinsey survey indicates that LEDs will have a 45% share of the global general lighting market in 2016 and close to 70% in 2020. By 2020, market capacity in this area is expected to increase from the current $26 billion to $72 billion.
The LED device is a complex multi-component system that adjusts performance characteristics to specific needs. The following sections discuss white LEDs and other applications.
LED development path
The electroluminescence phenomenon in inorganic materials is the basis of LED luminescence. HenryRound and Oleg Vladimirovich Losev reported LED luminescence phenomena in 1907 and 1927 respectively - current passing through makes silicon carbide (SiC) crystals emit light. These results have led to further theoretical studies of semiconductor and pn junction optoelectronic processes.
In the 1950s and 1960s, scientists began to study the electroluminescence properties of Ge, Si, and a series of III-V semiconductors (such as InGaP, GaAlAs). Richard Haynes and William Shockley demonstrated that electron and hole recombination in the pn junction leads to luminescence. Subsequently, a series of semiconductors were researched, and in 1962, the first red LED was developed by Nick Holonyak. Affected by it, George Craford invented orange LEDs in 1971. In 1972, yellow and green LEDs (both made up of GaAsP) were invented.
Intense research has rapidly commercialized LEDs that illuminate over a wide spectral range (from infrared to yellow), primarily for indicator lights on phones or control panels. In fact, these LEDs are very inefficient and have limited current density, making the brightness very low and not suitable for general illumination.
Blue LEDs
The development of high-efficiency blue LEDs took 30 years because there were no wide-bandgap semiconductors of sufficient quality to be applied at the time. In 1989, the first blue LEDs based on SiC material systems were commercialized, but because SiC is an indirect bandgap semiconductor, its efficiency is very low. Direct bandgap semiconductor GaN was considered in the late 1950s, and in 1971 Jacques Pankove demonstrated the first green-emitting GaN-based LED. However, techniques for preparing high quality GaN single crystals and introducing n-type and p-type dopants into these materials are still to be developed.
Technologies such as metal-organic vapor phase epitaxy (MOVPE) developed in the 1970s are a milestone for the development of high-efficiency blue LEDs. In 1974, Japanese scientist Isamu Akasaki began to grow GaN crystals in this way, and in 1986, in cooperation with Hiroshi Aman, the first high-quality device-level GaN was synthesized by the MOVPE method.
Another major challenge is the controlled synthesis of p-type doped GaN. In fact, in the MOVPE process, Mg and Zn atoms can enter the crystal structure of this material, but often combine with hydrogen to form an ineffective p-type dopant. Amano, Akasaki and co-workers observed that Zn-doped GaN emits more light after scanning electron microscopy.
In the same way, they proved that electron beam radiation has a beneficial effect on the doping performance of Mg atoms. Subsequently, Shuji Nakamura proposed to add a simple post-deposition step after thermal annealing to decompose the complexes of Mg and Zn, which can easily achieve p-type doping of GaN and its ternary alloys (InGaN, AlGaN).
It should be noted that the energy bands of these ternary systems can be adjusted by the composition of Al and In, which adds a degree of freedom to the design of blue LEDs, which is of great significance for improving its efficiency. In fact, the active layers of these devices currently consist of a series of alternating narrow bandgap InGaN and GaN layers and a wideband p-type doped AlGaN film (as a p-terminal confinement of carriers).
In 1994, Nakamura and its collaborators demonstrated the GaN double-heterostructure with Zn-doped InGaN active layer between n-type and p-type doped AlGaN, demonstrating for the first time an InGaN blue light with 2.7% external quantum efficiency (EQE). LEDs (Box 1 lists the main performance metrics for LEDs).
A schematic of the LED structure is shown in Figure 1a. These results are critical to today's LED-based lighting technology and have led to a revolution in the lighting industry. At the end of 2014, the Nobel Prize in Physics was awarded to Akasaki, Amano and Nakamura for their “invention of efficient blue LEDs for lighting and white light source energy savingsâ€.
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