The quality of light emitting diodes (LEDs) has increased to a point where solid state lighting is becoming fairly common. Despite this, greater understanding of the effect of the device structure and the electric fields within them is helpful to continue improving device efficiency and uniformity and in reducing costs. In this thesis the optical and electronic properties of InGaN/GaN LEDs have been studied with a combination of luminescence spectroscopy, microscopy, conductivity mapping and efficiency measurements.A study was made of the effects of the various electric fields, and the interplay between them, on LED luminescence and conductivity. Cathodoluminescence (CL) mapping shows die to die variation across large wafers revealing the powerful effects of a induced electric field on spectral intensity/position/width, in uncontacted devices. Micron scale spots in the LED material, lower in luminescence intensity and which trap charge, were revealed by CL/EBIC mapping with the origin attributed to cluster point defects in the active region. Depth resolved CL and CL under bias reveal the extent of asymmetry in carrier transport in the p/n type GaN around the active region. LEDs grown with different active region temperature profiles were studied. Devices exposed to high temperature after quantum well growth (2T) were found to have a uniform spatial luminescence and a peak efficiency that is higher and occurs at a lower current density (0.1 W/A @ 1 Acm¯²). By contrast those with a low temperature cap (Q2T) exhibit dark spots in the luminescence, and a lower peak efficiency at a higher current density (0.04 W/A @ 10 Acm¯²). The effect of improvement in LED design and material quality on the device efficiency, uniformity and spectral characteristics was studied.The addition of an Al₀.₂₃Ga.₇₇N electron blocking layer (EBL) was found to reduce the size and strength of the dark spots by about a factor of 2, while an additional In₀.₀₅Ga₀.₉₅N underlayer (UL) removed the dark spots entirely and shifted the luminescence peak by around 100 meV. The effect on the electroluminescence efficiency of the reduction in template dislocation density was found to depend strongly on the drive current density, with defect non-radiative recombination more important at low currents. Overall device efficiency was shown to be improved with an EBL and UL. The most efficient devices were those with the 2T type growth but the relative improvements are larger in LEDs grown with the Q2T method.Together, the results present a number of factors limiting the performance of current LEDs and suggest potential routes for improvement and optimisation.
|Date of Award||14 Oct 2016|
- University Of Strathclyde
|Sponsors||EPSRC (Engineering and Physical Sciences Research Council)|
|Supervisor||Robert Martin (Supervisor) & Kevin O'Donnell (Supervisor)|