dc.description.abstract | This dissertation describes InGaN light-emitting diodes (LEDs) of various structures prepared by low-pressure metal-organic chemical vapor deposition (MOCVD) for improving efficiency droop. The content is composed of two parts: One is investigation of the efficiency droop in c-plane LEDs and the other is developing the technologies of realizing semi-polar (1-101) LEDs on Si for low efficiency droop.
An n-type AlGaN/GaN/InGaN heterostructure layer is inserted below the multiple quantum wells (MQWs) to improve current spreading and efficiency droop. As indicated by one-dimensional simulation, an n-type Al0.1Ga0.9N/GaN/In0.06Ga0.93N heterostructure induces a higher electron concentration than either the n-AlGaN/GaN cladding layer or n-GaN/InGaN current spreading layer used in conventional LEDs. Experimental results show that the light output uniformity across a chip is indeed greatly improved. Consequently, the external quantum efficiency (EQE) and wall-plug efficiency is improved by about 18.2% and 22.2%, respectively, at an injection current of 350 mA. The efficiency droop at 46 A/cm2 is also improved from 20.9% to 12.5%.
As In content variation in the InGaN quantum well might worsen efficiency droop, inserting an In0.03Ga0.97N pre-layer in the quantum wells and using trimethylindium (TMIn) treatment is proposed to reduce In-rich clusters and improve droop effect. Experimental results indicate that the efficiency droop behavior of InGaN LEDs can be alleviated as expected by using an In0.03Ga0.97N/In0.13Ga0.87N/ GaN composite MQW active layer and TMIn treatment at the In0.03Ga0.97N/ In0.13Ga0.87N interface. Growth pit density, x-ray diffraction, reciprocal space mapping and photoluminescence (PL) measurements indicate that this approach reduces the occurrence of In-rich clusters in the MQWs and leads to reduction in the efficiency droop without sacrificing the EQE of the LEDs. The droop point of the EQE shifts from 44 A/cm2 to 86 A/cm2 compared to the reference sample. There is also an improvement in the efficiency droop at 176 A/cm2 from 13.7% to 5.5%. A
In order to decrease the electron overflow current, this study also propose a new InGaN-based LED structure, which has an extra p-GaN spacer layer between the last p-side GaN QW barrier layer and the AlGaN electron blocking layer (EBL). Based on the simulation, this structure is expected to exhibit reduced efficiency droop and enhanced internal quantum efficiency. Experimental results indicate that the EQE is improved by about 12.8 % at an injection current of 25 A/cm2. The experimental results agree very well with the simulation.
This dissertation also reports a novel method to produce a self-assembled double-island buffer layer for reducing threading dislocation density in GaN epilayers grown on (111) Si substrates by using in situ SiNx mask during MOCVD. The effect of dislocation density on efficiency droop can thus be analyzed based on these samples. Experimentally, this method effectively reduces the threading dislocation density from 9.6×109 cm-2 to 2.6×109 cm-2. The mechanisms of double-island formation as well as dislocation reduction are described based on transmission electron microscopy investigations. It is also shown that the InGaN LEDs fabricated on the double-island buffer layer exhibit a 27% improvement in their EQE. The efficiency droop at 100 A/cm2 is improved from 52.1% to 10.8%. Based on the temperature dependent PL results, threading dislocations seem to be the driving force of phase segregation in InGaN and cause In-rich regions in MQWs. Reducing threading dislocations will decrease the appearance of indium clusters in QWs and hence the efficiency droop effect.
The second part of this work aims at the fabrication of (1-101) semi-polar LEDs on 7°-off (001) Si substrates and their efficiency droop behavior. High quality semi-polar (1-101) GaN epilayers on V-grooved 7°-off (001) Si substrates have been realized as evidenced by the root-mean-square roughness of 0.3 nm as well as the (101) and (002) x-ray rocking curves of 523 and 581 arcsec, measured on a 1.2 m (1-101) GaN. Novel multi-stripe (1-101) blue LEDs show electroluminescence full- width at half-maximum of 35.9 nm. At 350 mA, the LEDs are nearly droop-free. | en_US |