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Chapter 1 Physics of III-Nitride Light-Emitting Diodes 1.1History of III-Nitride LEDs Light emission caused by electrically pumping was first reported by Henry in 1907 [1]. He found that the silicon carbide (SiC) crystal emitted yellowish light when a voltage was applied between two-point contacts on the SiC surface. The explanation of this phenomenon named electroluminescence was proposed by Kurt Lehovec in 1951 [2]. The electroluminescence process includes carrier injection over the p-n barrier and carrier recombination across the SiC bandgap. The appearance of quaternary alloy AlGalnP in the 1980s is the next important improvement in visible light LED technology [3-6]. The (AlxGai_x)o.5lno.5P heterostructure can be made to emit from red to green. AlGalnP-based LEDs are now the dominant emitters of red/orange region. However£¬ the emission is limited to long wavelength because the bandgap turns into indirect when the A1 concentration is higher than 53%. In addition£¬ higher aluminum content leads to the weaker electronic confinement£¬ resulting in a strong temperature sensitivity for the internal quantum efficiency. Significant improvement in crystal quality of GaN grown on sapphire contributed to the blue LED development. A low-temperature AIN buffer prior to GaN was adopted by using metalorganic chemical vapor deposition (MOCVD) in 1986 [7]. Later£¬ a low-temperature GaN buffer was grown to obtain high-quality GaN film on sapphire using MOCVD and molecular beam epitaxy (MBE) [8£¬9]. By employing postgrowth low-energy electron beam irradiation treatment£¬ a breakthrough of p-type Mg-doped GaN was reported in 1989£¬which encouraged the birth of first p-n junction GaN-based LED. The conductivity of GaN doped with Zn or Mg increased sharply after being irradiated by low energy electron beam. The mechanism of increased conductivity was revealed£¬ and thermal annealing could result in a similar effect on Mg- or Zn-doped GaN [10]. The dopant is passivated by Mg-H complex formation during crystal growth£¬ while low energy electron beam irradiation or thermal annealing can destroy the Mg-H complex. Thermal annealing process is suitable for mass-production£¬ which paves the way to the industrialization of GaN-based p-n junction devices. The high-brightness InGaN/GaN blue LEDs were grown on sapphire using MOCVD in 1993 [11]£¬ followed by efficient InGaN/AlGaN blue LED in 1994. Considering the pioneering and critical contributions by Isamu Akasaki£¬ Hiroshi Amano and Shuji Nakamura£¬ they were awarded Nobel Prize in Physics in 2014. 1.2Mechanisms of III-Nitride LEDs LED is a kind of optoelectronic device based on the p-n junction£¬ which consists of the p-type doped region and the n-type doped region. When two semiconductor materials form a p-n junction£¬ electrons diffuse from the n-region to the p-region£¬ and holes diffuse from the p-region to the n-region. Thus£¬ space charge region (depletion region) is formed at the interface between the p-region and n-region. There exists a built-in electric field whose direction is from the n-region to the p-region. The drift process takes place under the effect of the built-in electric field. When the applied bias is zero£¬ the diffusion and drift of carriers could finally reach a dynamic balance. Figure 1.1a shows the energy band diagram of p-n junction without applied bias. The electrons flow from the n-region with a high Fermi level to the p-region with a low Fermi level. Meanwhile£¬ the holes flow from the p-region to the n-region. Therefore£¬ Epn keeps moving downward and Eyv keeps moving upward until En equals Epv. Figure 1.1b shows the energy band diagram of p-n junction when a forward bias is applied. We can find that the value of Fn is higher than that of The electric field induced by the forward bias is opposite to the built-in electric field£¬ resulting in alleviation of electric field in the depletion region. Therefore£¬ diffusion current is higher than drift current. Figure 1.1c shows the energy band diagram of p-n junction when a reverse bias is applied. The value of Epp is higher than that of Epn. Besides£¬ the electric field generated by the reverse bias possesses the same direction as the built-in electric field£¬ which enhances the drift process and leads to a larger drift current than diffusion current. The light-emitting region in the initial LED based on homogeneous junction structure is determined by the diffusion length of electrons and holes. Because the carrier diffusion length is generally on the order of micrometers£¬ this will lead to the low electron and hole concentration radiative recombination and thus the low quantum efficiency. The double heterostructure£¬ which consists of a wide bandgap p-type semiconductor£¬ wide bandgap n-type semiconductor and thin interlayer of narrow band gap semiconductor between them£¬ can improve the quantum efficiency of LEDs. Wide bandgap p-type and n-type semiconductor materials provide holes and e