Nils Asmus Kristian Kaufmann
Nowadays, light emitting diodes (LEDs) and laser diodes (LDs) are part of our daily life. More and more devices incorporate InGaN-based optoelectronic devices. In fact, since the first demonstration of a candela-class InGaN-based LED in the beginning of the nineties, those LEDs have quickly become popular. The efficiency of blue InGaN LEDs is nowadays very high and when coated with a yellow phosphor, they can efficiently emit white light. Such white LEDs are more and more used for different lighting applications, from home lighting to car lighting. Another application of blue InGaN emitters that is found in many households is the Blu-ray disc, based on a 405 nm LD. The great success of the III-nitrides in the blue range created a strong interest to extend the wavelength range into the green, as efficient green emitters are missing. However, their performance is still low compared to their blue counterparts. The goal of this dissertation is to investigate issues that limit the performance of green InGaN LEDs and LDs. To achieve efficient green emission, the growth temperature needs to be reduced to incorporate more indium into the active region. In this context, the present study is divided into two parts. In the first one, the growth regime of GaN as a function of the temperature is investigated and the physical origin of the surface features is discussed. When lowering the temperature, the so called Ehrlich-Schwöbel barrier, a barrier at the step-edges, causes a different attachment probability for the adatoms arriving from the different sides. This asymmetry can lead to the appearance of three different kinetic surface morphologies: hillocks, fingers, or step-bunching. After presenting the theory, the three different regimes are demonstrated for GaN, and the growth parameters that allow to control the morphology are investigated. Indeed, the different morphologies usually reported for the different growth techniques can be attributed to the different growth parameters commonly used. In the end of the first part, the influence of the underlaying morphology on the properties of the active region of an LED is discussed. In the second part, the impact of the thermal budget on the properties of InGaN quantum wells (QWs) is discussed. Active regions with a high In content can degrade easily when the p-type layer is grown on top, due to being exposed to a too high thermal budget. This is especially critical for green LDs which require thick cladding layers, i.e. a long growth time. The origin of the degradation and the parameters influencing it are discussed first: when an In-rich QW is annealed at a too high temperature, its emission intensity is reduced and dark spots appear on PL maps. The annealing temperature causing this degradation depends on the indium content, the QW thickness and the number of QWs. It is shown that these dark spots exhibit a peculiar emission spectra, with a characteristic emission at 2.75 eV. This emission is tentatively attributed to nitrogen vacancies that are created by the formation of metallic indium clusters in the active region. Then the focus is laid on the beneficial effects that can take place when an In-rich active region is exposed to a moderate thermal budget. Apart from reducing the linewidth of the emission, a strong improvement in photoluminescence intensity can be achieved. This improvement increases for high indium content QWs, i.e. when the growth temperature is low. In fact, this improvement is more pronounced for molecular beam epitaxy grown structures whose active regions are grown at a much lower temperatures than their metal organic vapor phase epitaxy counterparts. This is tentatively ascribed to the reduction of point defects by thermal annealing. As a consequence, a moderate annealing can be beneficial for both to reduce the non-radiative recombinations and the linewidth. It is finally proposed to perform a short annealing step of high indium content QWs before growing the p-type (cladding) layer at low temperature.
EPFL2013
