| dc.description.abstract | In this dissertation, the growth and characterization of multiple MgxNy/GaN buffer layers and InGaN/GaN blue light-emitting diodes have been studied. The improvement of both LED light output power and efficiency droop are investigated. In addition, we also discuss the optoelectronic characteristics of nitride-based solar cells with MQW absorption layer. The primary results obtained in this dissertation are summarized as follow: (a) It was found that the GaN grown on MgxNy buffer layer showed lower dislocation density (~2.2 × 108 cm-2), higher carrier mobility (~630 cm2/Vs), lower background carrier
concentration (~5.1× 1016 cm-3) and narrower FWHM of (002) (~228 arcsec) and (102) (~248 arcsec) in DCXRD, as compared with conventional GaN grown on low-temperature (LT) GaN buffer layer. The blue LEDs grown on 12-pairs MgxNy (120 sec)/GaN buffer layers could also reduce the reverse leakage current and enhance the LED output power from 5.36 mW to 5.97 mW at a 20 mA current injection, as compared with conventional LEDs. (b) For GaN-based LEDs, nitride-based asymmetric two-step LEDs with a In0.08Ga0.92N shallow step was proposed and fabricated. By inserting an In0.08Ga0.92N shallow step, it was found that LED output powers can be improved from 2.9 mW to 6.6 mW under a 20 mA current injection. The improvement of output power could be attributed the fact that the significant carrier localization effect in the asymmetric two-step LEDs can lead to higher IQE. Under high injection current density, LEDs with InN/GaN multilayer wells (MLWs) structure can improve both the light output power and efficiency droop, compared to the conventional InGaN MQW LED. Under an injection current density of 500 A/cm2, we can enhance LED output power from 71.0 mW to 89.4 mW, compared to conventional LED. As compared to the EQE at an injection current density of peak value, the EQE values at an injection current of 500 A/cm2 are approximately reduced by 47 % and 21 % for the conventional LED and
InN/GaN MLWs LED, respectively. These improvements could be attributed to the InN/GaN MLWs can significantly reduce the threading dislocations generated from active region, improve the interfacial quality between QW and QB, enhance the localized state and release strain in InGaN layer grown on GaN. (c) For GaN-based solar cells, The MQW structure should be able to maintain the material quality of high-indium-content InGaN alloys, leading to better device performance than devices with a single InGaN layer as the active layer. The short-circuit current density (JSC) and open-circuit voltage (VOC) can be modulated by different arrangement of blue and green QW in MQW absorption layer. The optimal electrical characteristics of solar cell with JSC = 0.473 mA/cm2, VOC = 1.30 V, fill factor (FF) = 0.630 and conversion efficiency = 0.39 % with AZO transparent contact layer (TCL) can be obtained.
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