III-nitride quantum dots for application in opto-chemical sensors
This thesis work has focused on the synthesis of (In)GaN-based quantum dot (QD) structures by plasma-assisted molecular-beam epitaxy (PAMBE), deposited in both polar (0001) and semipolar (11-22) crystallographic orientations, for application as optical transducers for chemical sensors for detection of pH levels, and hydrogen or hydrocarbon concentrations in gas or liquid environments. In the first part of this work, I describe the synthesis of semipolar-oriented two-dimensional layers: binary alloys (AlN, GaN and InN) and ternary alloys (AlGaN and InGaN), which are required for the reference contact of the transducers and set the basic know-how to understand the transition from two-dimensional growth to three-dimensional QD nanostructures. It is particularly relevant the study of indium kinetics and indium incorporation during the PAMBE growth of InGaN(11-22) layers. Similarly to (0001)-oriented InGaN, optimum growth conditions for this semipolar crystallographic orientation correspond to the stabilization of 2 ML of In on the growing InGaN surface, in excellent agreement with first-principles calculations. The limits of the growth window in terms of substrate temperature and In flux lie at same values for polar and semipolar materials. However, I observe an inhibition of the In incorporation in semipolar layers even for substrate temperatures below the segregation threshold for polar InGaN. In a second stage, I report the successful fabrication of superlattices (SLs) of GaN/AlN and InGaN/GaN QDs, both in polar and semipolar orientations. Photoluminescence and time-resolved photoluminescence confirmed the reduction of the internal electric field in the semipolar GaN/AlN QDs in comparison with polar structures. On the other hand, semipolar InGaN QDs must face the challenge of In incorporation in this crystallographic orientation. To overcome this problem, the influence of the growth temperature on the properties of the polar and semipolar InGaN QDs has been studied, considering growth at high temperature (TS = 650–510 °C, where In desorption is active) and at low temperature (TS = 460–440 °C, where In desorption is negligible). I demonstrate that low-TS growth conditions are not compatible with polar plane whereas they provide a favorable environment to semipolar plane to enhance the internal quantum efficiency of InGaN nanostructures. Finally, I have synthesized a number of GaN/AlN and InGaN/GaN QD optical transducers, grown in polar and semipolar orientations. In each case, the growth conditions to attain the targeted spectral range (emission at 420-450 nm with buffer transparent for wavelengths shorter than 325 nm) were identified. The influence of an external electric field on the luminescence of the transducers confirmed that the best performance (larger variation of the luminescence as a function of bias) was provided by InGaN/GaN QD structures. With this feedback, the specifications of the targeted opto-chemical transducer structures have been established (5 InGaN/GaN QD layers on Al0.35Ga0.65N:Si). Then, I have synthesized a number of InGaN/GaN opto-chemical transducers in order to get an insight on the reproducibility, limitations and critical steps in the fabrication process. Using these samples, we have achieved an integrated sensor system based on polar InGaN QD SLs, and the system was useful for monitorization of the pH value of water.