| Keywords [eng] |
Green-gap, GaN, InGaN, Gallium Nitride, Indium Gallium Nitride, Doping, Magnesium, p-type doping, LED, diffusion, diffusion coefficient, light induced transient grating, LITG |
| Abstract [eng] |
Gallium nitride is a semiconductor material that became important in various fields, particularly in the production of light-emitting diodes (LEDs). By alloying GaN with other group III elements, such as indium or aluminum, the bandgap energy can be tuned in a wide range covering the visible range, enabling the manufacture of LEDs emitting blue, green, and even red light from the same material family. However, certain challenges remain that need to be overcome before these devices can reach their full potential. One such challenge is the decrease in external quantum efficiency as the indium content is increased, commonly known as the "green-gap". The cause of this phenomenon is not yet fully understood, but several theories have been proposed, including the complex character of carrier transport in disordered material and non-radiative Auger recombination of carriers. Furthermore, the built-in electric field plays an important role in the carrier dynamics. The internal electric field causes spatial separation of electron and hole wavefunctions, thus limiting the probability of carrier recombination. There are several strategies to mitigate this effect. The most popular one is to use thin quantum wells, but in this case one has to operate LEDs at high carrier densities, which results in IQE limiting processes. Another idea is to grow LEDs on non-polar GaN surfaces, but after many attempts it has been concluded that it is extremely difficult to obtain nonpolar InGaN of sufficient structural quality. Lastly, it has been proposed to screen the field by adding additional p-doped layers into the LED structure. This idea has been demonstrated to work; however, the p-type layers must be close to InGaN QW and doping with Mg requires high temperatures. This requirement leads to heating of InGaN layers during LED structure formation, which can lead to degradation of the active medium due to diffusion of indium atoms. In this thesis, we aim to investigate p-type doping in GaN to address these issues and advance our understanding of the underlying mechanisms, ultimately contributing to the development of more efficient and reliable LED devices. Key goal is to investigate how different growth regimes impact carrier transport in the p-type GaN layers, in attempt to obtain doped p-type GaN layers at temperatures as low as possible. In particular, my objectives are (i) to compare the ambipolar diffusion coefficient, D, and lifetime of nonequilibrium carriers, τR, in a number of Mg-doped GaN layers, (ii) to deduce the equilibrium hole density, p0, from the dependence of D on the carrier density, and (iii) to identify the growth conditions most suited for the growth of p-type GaN contact layers. The layers were grown at different temperatures and doped using different dopant flowrates. The measurements were conducted using light-induced transient grating (LITG) technique to determine the carrier diffusion coefficient and non-equilibrium charge carrier lifetime dependances on photoexcited carrier density, and time-integrated photoluminescence (TIPL) to determine photoluminescence quantum yield (PLQY) of the layers. Both LITG and TIPL measurement methods are non-destructive and require no contacts, which seems attractive having in mind that electrical measurements of p-type GaN is a rather complicated task. |
