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Effects of Structural and Compositional Characteristics on the Electrical and Optical Properties of InGaN/GaN Multi Quantum Wells

Effects of Structural and Compositional Characteristics on the Electrical and Optical Properties of InGaN/GaN Multi Quantum Wells
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The atomic structure, composition and epitaxial quality in the vicinity of the interface between GaN and InGaN in the multi-quantum wells (MQWs) determine the optical, electrical, and mechanical properties of the optoelectronic devices. GaN-based light-emitting diodes (LEDs) contain a quantum confinement structure with different semiconductor layers, ie., InGaN/GaN, InGaN/AlGaN and AlInGaN/AlGaN heterostructure. There are three confinement dimensions of 1, 2 and 3 dimension as a function of structural and compositional states of surrounding material. Generally, pseudomorphic heterostructure has the 1D quantum wells (quantum confinement in one direction) which are made of a thin layer of a narrow band gap semiconductor (InGaN), surrounded by two wide band gap semiconductor layer (GaN). This confinement plays a significant role on the electronic properties and the optical properties. However, these materials related GaN were strongly affected polarization field on the physical properties of heterostructure resulting in the separation of the electron and hole in quantum well. Recently, in order to increase the efficiency, the control for the presence of piezoelectric fields in quantum well has been studied. This piezoelectric field strongly depends on the lattice mismatch between InGaN well layer and GaN barrier layer. The lattice constant of InGaN well layer increase as the In content increases, resulting in biaxial compressive strain. Therefore, when the devices design based on this material, structural and compositional conditions must be considered. To do this, we have to understand correlations between structural, compositional, and as well as their optical and electrical properties. The emission mechanism of a InGaN alloy, the spontaneous emission from them is assigned as being due to the recombination of excitons spatially localized at potential minima, its lateral size varied from less than 60 to 300 nm caused by the potential inhomogeneity. In fact, when the InGaN layer was observed by transmission electron microscopy (TEM), the size of irregularly broken layer is similar to the above mentioned dimensions. The irregularly broken layers were referred as to a quantum disk (QD) or segmented quantum well. The origin of these potential minima is mainly due to compositional fluctuation and phase separation of InGaN alloy. Generally, in InGaN based LED structure, there is the difference of energy for emission and adsorption, which is referred to the Stoke shift due to the potential inhomogeneity. These Stoke shift phenomenon is strong evidence for the potential fluctuation in the InGaN quantum well layer. In order to elucidate the mechanism of carrier localization, various studies, thus, have been carried out in last decade. There are several mechanisms for carrier localization, such as 1~2 monolayer well-width thickness fluctuation, complete phase separation and locally compositional and strain fluctuation. In the case of discontinuous InGaN well layer was observed by TEM, which is well explained from above mentioned. In order to verify possibility for the origin of carrier localization, such as well-width (thickness) fluctuation of InGaN quantum well layer, we have observed various types of InGaN well layers in terms of interface roughness and differences in interface characteristics as a function of position at the MQWs by STEM-HAADF and APT. The well-width fluctuation of the interface between GaN and InGaN was investigated from the thickness of the monolayer (1 or 2 monolayers) up to that of the InGaN well layer. The root mean square (RMS) roughness of the upper interface gradually increases from 0.349 to 0.466 nm at the p-GaN, and the upper interface is rougher than the lower interface. The correlations between compositional, structural and optical properties of InGaN/GaN MQWs LED structure with different strain states by strain relief layer (superlattice) were investigated. The measured lower and upper interface roughness of sample with superlattice are lower than those of sample without superlattice. These results indicated that a strain controlled layer superlattice decreased the upper and lower interface roughness of InGaN well layer. Generally, the effect of QCSE became weak due to the carrier screen effects at low injection current. Therefore, at sample without supperlattice, the blue-shift of peak wavelength is smaller than that of sample with superlattice. On the other hand, as the injection current increases over 20 mA, the band-filling effect increased because the local potential minimum in the potential fluctuation of the InGaNwell layer increased. The FWHM of the sample without superlattice increased significantly with increasing injection current, in constant to slightly increase of FWHM in the sample with superlattice. This significant spectral broadening in the sample without superlattice is further evidence for band filling of localized states at high currents. In addition, the difference of PL and PLE of sample without superlattice is larger than that of sample with superlattice. This phenomenon was explained by stoke shift which also one of evidences for the potential fluctuation induced by composition fluctuation. In addition, the piezoelectric field in InGaN quantum well layer changed by In-composition fluctuation, this turn will change the effective energy band gap across the wall. That is, total net energy band gap varies across the well due to the localized changes in piezoelectric field. The size of localized In-composition fluctuation in sample without superlattice decreases, while the area of In fluctuation increases in sample with superlattice, resulting in a decrease of QCSE. Therefore, the blue shift of sample with superlattice increases at low current larger than sample with superlattice. The degree of efficiency droop in the sample without superlattice is larger than sample with superlattice as the injection current increase. The difference of droop characteristics is caused by the difference of resistivity for the carrier delocalization effect.
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