Pozyskanie przewag azotowej polarności GaN dla emiterów światła opartych na azotkach grupy trzeciej

Wide application of nitrides in everyday life, especially in light emitting diodes (LEDs), can give an impression that “everything” is already known about nitride emitters. However, in fact the situation is different. Despite unquestionable success of nitride laser diodes (LDs), none of the structures were ever obtained on N-face GaN substrates. Achievement of a LD is often regarded as an irrefutable evidence of high optical quality of obtained material and maturity of the used growth method. In this project we will explore completely new areas of nitrides growth along N-polar (000-1) GaN direction to present such a device for the first time as well as to open this crystallographic direction for more basic research targeting effects associated with polar crystals. Aim of the project is to understand and exploit advantages of the growth of III-nitrides on N-polar, i.e. (000-1), surface by plasma-assisted molecular beam epitaxy (PAMBE). Using recently defined new growth window, we will limit amount of incorporated point defects and present improved optical quality of N-polar structures. Combining improved active region with p-n junction on N-polar GaN, we will obtain LDs utilizing profitable built-in electric polarization. Crystal growth will be carried out using PAMBE. This ultra-high vacuum growth technique enables in-situ surface monitoring using reflection high electron energy diffraction and exceptional control over chemical composition of grown heterostructures. PAMBE for nitrides rely on supplying group-III materials from single element standard effusion cells and nitrogen in form of exited N2 molecules. The fact that it is a precursor-free technique significantly limits sources of foreign impurities introduced during crystal growth and enables clearer examination of point defects. The project will focus on three aspects associated with N polar nitride laser diodes. First, the “old” problem related to the reason of inherent low luminescence of N-face nitride structures, will be tackled. The correlation between poor light emission intensity and the presence of point defects, that are the suspects, has to be finally proven and shown. We will use temperature dependent photoluminescence (PL), time-resolved PL (TRPL) and positron annihilation (performed in collaboration with Prof. Filip Tuomisto group at University of Helsinki) to investigate the impact of specific growth conditions on the quality of nitride layers for both substrate polarities. Growth conditions for efficient N-polar quantum wells (QW) will be identified. Second, the doping efficiency for N-polar growth will be explored. Effect of growth conditions and surface preparation, including in-vacuum treatment, on the impurity and doping levels will be analyzed. Comparison between Si and Ge ntype dopant will be performed. Limit of Mg p-type doping on N-polar surface under different growth conditions will be confirmed. To realize the full potential standing in front of the N-face GaN better understanding of impurity incorporation mechanism is essential. Third exciting research area covered within the project is related to the design of the LD grown on N-polar surface. We will show that N-polar emitters can profit from built-in field direction. Due to that N-polar LDs can be constructed in a way so that p-type layers are located much further away from the QWs as compared to Ga-polar devices. Since p-type layers are responsible for majority of absorption in the structure, this should result in much lower internal optical losses. We believe that proposed studies will enable to obtain the first LD grown on N-face GaN. Careful analysis of the growth on N-polar GaN can present very interesting input to the general physics of crystal growth. This achievement will also confirm high quality of N-polar nitrides, what can result in further applications in electronics.