Investigation of Cascaded GaN-based Optoelectronic Device with Tunnel Junction Layers Grown by MOCVD

Abstract

In this dissertation cascaded GaN-based optoelectronic devices with tunnel junction layers (TJLs) have been grown by metalorganic chemical vapor phase deposition (MOCVD) technique and related characterizations including optical and electrical properties were also studied In order to overcome the efficiency droop problem and and improve externl quantum efficiency of light emitting diodes (LEDs) firstly we fabricated GaN-based multiquantum well LED with tunnel junction (TJ)-cascaded active region It was found from x-ray diffraction (XRD) spectra that crystal quality of the TJ LED was almost identical to that of the conventional LED Compared with the conventional LED it was found that we could achieve 35% higher light output power from the TJ LED due to the repeated use of electron and holes for photon generation It was also found that external quantum efficiency (EQE) drooped by 36 9% and 35 0% for the TJ LED and the conventional LED respectively as we increased the injection current density to 80 A/cm2 Furthermore it was found that forward voltages measured with an injection current density of 20 A/cm2 were 8 94 V for the TJ LED The large forward voltage observed from the TJ LED should be attributed to the large TJ resistance Secondly to achieve high EQE and smaller drooping we must need to reduce the TJ resistance We fabricated the cascaded GaN LEDs with hybrid TJL Compared with the conventional LED it was found that we could enhance the light output power by 35% and 80% from the LED with TJL and the LED with hybrid TJL It was also found that the TJ resistances were 6 05×10-3 and 1 95×10-3 Ω?cm2 for the LED with TJL and the LED with hybrid TJL respectively It was also found that the use of hybrid TJL could result in smaller efficiency droop These improvements could all be attributed to the larger polarization charges induced at the AlGaN/InGaN interface which could enhance the tunneling current Furthermore it was found that the cascaded GaN LEDs with hybrid TJL were also reliable Thirdly we propose the use of TJ structure to cascade a green multiquantum well (MQW) active region and a blue MQW active region and the fabrication of GaN-based dual-color LEDs It was found that light output power observed from the TJ cascaded LED was only slightly smaller than the summation of those observed from the green LED and the blue LED This suggests that most of the injected carriers could tunnel through the TJ and could be repeatedly used for photon generation Furthermore it was found that the proposed dual-color LEDs was highly reliable Fourth we propose the use of tunnel junction (TJ) structure to cascade a green multiquantum well (MQW) active region and a blue MQW active region and the fabrication of white-light LED by using green/blue dual-color TJ LEDs coated with red phosphors It was found that the CIE color coordinates of “dual-color TJ LED + red phosphor” locates at (0 30 0 35) with the color temperature TC=4306 K and color-rendering index CRI =68 This suggests that green/blue dual-color TJ LEDs coated with red phosphors has high potential to become a way of the white LED Finnaly we report the fabrication of cascaded GaN-based dual-region solar cells (SCs) with hybrid TJL Compared with the conventional SCs it was found from XRD spectra that crystal quality of the TJ SCs was almost identical to that of the conventional SCs Compared with the single region SCs it was found that the use of dual region SCs with hybrid TJL could result in higher conversion efficiency The improvements show that hybrid TJL was useful to connect multiple active regions in series in solar cells

Date of Award	2016 Feb 3
Original language	English
Supervisor	Shoou-Jinn Chang (Supervisor)