PhD Thesis Defenses

Carrier dynamics in blue and green InGaN LED structures


This thesis focuses on effects that are critical to achieving high internal quantum efficiency (IQE) in GaN-based light-emitting diodes (LEDs) that emit in a broad spectral range, from violet to green-yellow. These effects include interwell carrier transport in multiple quantum well (QW) structures, lateral transport in the QW plane, and radiative and nonradiative recombination.

The investigation is conducted with the time-integrated and time-resolved near- and far-field photoluminescence (PL) spectroscopy. Measurements are performed on polar single and multiple InxGa1-xN QW structures of different alloy compositions, which are supplemented with a self-consistent solution of one-dimensional Schrödinger and Poisson equations and an evaluation of the carrier density dynamics.

Interwell carrier transport is studied to determine the conditions required for a uniform interwell carrier distribution in an LED active region. Such a distribution would decrease the detrimental impact of the nonradiative Auger recombination and increase the IQE. Since the hole transport is the bottleneck for this process, ambipolar interwell transport, determined by the slower holes, is studied. Standard time-resolved PL measurements are performed on multiple QW structures with a different number of In0.12Ga0.88N QWs and different barrier parameters in terms of thickness and material. Photoexcited carrier transport over the multiple QW structure is monitored by measuring PL rise times from a deeper detector QW. Such measurements make it possible to distinguish the interwell transport mechanism at high temperatures (e.g., thermionic emission – ns range) and low temperatures (e.g., ballistic – sub-ps range). In standard InGaN/GaN structures, the interwell hole transport is found to be inefficient. Studies of transport and IQE in structures with InGaN barriers of different compositions, as well as thin GaN or AlGaN interlayers between the QWs and barriers, allowed the design of structures with fast, efficient interwell transport and high IQE. These measurements are performed for blue LED structures; however, the conclusions could be extended to QWs emitting at longer wavelengths, for which the issue of the nonuniform interwell carrier distribution is even more severe.

Studies of the carrier recombination and IQE are performed on single QWs with a focus on long wavelength (green, green-yellow) emitting structures, in which the IQE is much smaller than for the violet and blue-emitting wells. Radiative and nonradiative carrier recombination times are determined at different temperatures, revealing a record-high IQE of ∼60% in the green-yellow QWs.

Since nonradiative recombination is often assigned to extended defects, near-field spectroscopy is applied to study the impact of V-defects related to dislocations in polar GaN-based structures. The parameters of PL spectra, as well as radiative and nonradiative recombination times, show large spatial variations. The increased nonradiative recombination related to the dislocations is revealed only in their immediate vicinity, suggesting that their impact on the IQE and device performance, contrary to common belief, should be small.