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.
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.
Leading the way to LEDs
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 led to further theoretical studies of semiconductor and p-n 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 p-n 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 light from 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 symmetrical double heterostructure design of Zn-doped InGaN active layers between n-type and p-type doped AlGaN, demonstrating for the first time an external quantum efficiency (EQE) of 2.7%. InGaN blue 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 savingâ€
LED performance indicators
Quantum efficiency: The quantum efficiency (IQE) within a material is the ratio of the number of electron-hole recombination (ie, photons produced) to the total amount of recombination (radiation versus non-radiation). This indicator determines the luminous efficiency of semiconductor materials. Semiconductor LED performance is typically expressed using external quantum efficiency (EQE), the product of IQE and extraction efficiency. The extraction efficiency specifically refers to the portion of the generated photons that escapes from the LED. EQE depends on semiconductor layer defects that directly affect IQE and device configurations that affect extraction efficiency.
Luminous efficacy: Luminous efficiency is the efficiency of the light source to emit visible radiation, and the unit is generally lm W?1. The light source is 100% electric energy with monochromatic green light (frequency of 450x1012 Hz, corresponding wavelength of about 555 nm, the most sensitive light of human eyes, Figure 2b is the corresponding eye sensitivity curve), and its maximum luminous efficiency reaches 683 lm W?1 . White light sources for illumination typically require a broader emission spectrum than the entire visible range, so their luminous efficiency is significantly lower than their maximum. The electrical energy is converted into radiation other than the sensitivity curve of the eye and cannot be used for illumination. This type of radiation should be minimized.
Correlated colour temperature: The reference source used to compare different lighting technologies is black body radiation in thermal equilibrium. According to Planck's law of radiation, the emission spectrum of a black-body incandescent lamp depends on its temperature. The color point corresponding to the radiation at different temperatures is represented by a CIE diagram, which is called the Planckian trajectory. The black point curve of Planckian locus) (Fig. 2f, h). The color temperature (CCT) of white light can be roughly divided into “warm white†(2,500-3,500 K) and “natural white†(3,500–4,500 K) along different positions of Planck's trajectory. , “cold white†(4,500–5,500 K) and daylight (5,500–7,500 K).
Colour rendering index: The color rendering index (CRI) is a dimensionless indicator that describes the ability of a white light source to develop color in an accurate and comfortable manner relative to human visual perception, while considering the reference light source. (Under the same CCT, blackbody radiation was tested at CCT < 6,000 K or natural light CCT > 6,000 K). CRI is usually defined as the average of the color of eight test color samples (R1-R8), with a nominal range from 0 to 100. For high CRI, an additional R9 value is used, indicating a deep red color. CRI = 100 means that all color samples illuminated by the test source have the same color as the same sample illuminated by the reference source.
figure 1. Design of blue InGaN LED chips
a. The first blue InGaN/AlGaN LED schematic.
b. A schematic diagram of a flip-chip LED chip with an inverted structure and a contactless front surface. Two contact points are soldered to the substrate adjacent to the LED.
c. The highest level of film-type flip-chip LED schematic and top view of the LED device. The effective layer simplification of these three schematics represents a double heterostructure, single or multiple quantum well structure InGaN/AlGaN.
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