Design of high-efficiency photonic devices based on wide gap semiconductors

The General Assembly of the United Nations has proclaimed 2015 the International Year of Light and Light-based technologies. If a few years ago lighting was synonym of incandescent lamp, now it is undeniable that we have entered a light-emitting diode (LED) era. From automotive to streetlights, from lights in our houses to the displays of TVs and smartphones, LED-based technology is making its way in the market. This proliferation would have been impossible without GaN-based LEDs, whose invention by Isamu Akasaki, Hiroshi Amano and Shuji Nakamura has been rewarded with the 2014 Nobel Prize in Physics. The fact that products based on LED technology are widely commercialized does not mean that there aren’t still gray areas in the understanding of their operating principles. Not surprisingly, one of the main targets of the EU Horizon 2020 research program is to achieve a 20% increase in energy efficiency. A critical contribution to research is given by computer-aided design, modeling, and simulation. My research activity was mainly focused on the numerical analysis of efficiency-limiting mechanisms in wide gap optoelectronic devices and, in particular, in GaN-based LEDs. The thesis is divided in two parts. In the first part, a short theoretical introduction on optoelectronic devices is presented, to provide an historical perspective. Chapter 1 illustrates LEDs physical properties while in Chapter 2 are discussed the peculiarities of vertical-cavity surface-emitting lasers (VCSELs). The simulation analysis is based on two commercial simulation tools: APSYS from Crosslight Software Inc. and TCAD Sentaurus from Synopsys Inc. Chapter 3 illustrates the physical models implemented in APSYS. In the second part, the research activity is discussed. LEDs intended to operate in the ultraviolet (UV) spectral range are of great interest for several applications. One material system with the potential of leading to efficient UV LEDs is zinc oxide and its ternary compounds (MgZnO and BeZnO). Two-dimensional numerical simulation, employed to assess a number of possible design approaches aimed at optimizing the internal quantum efficiency (IQE) of BeZnO-based light-emitting diodes is presented in Chapter 4. The realization of gallium-nitride GaN-based VCSELs emitting blue light has encountered some challenges associated with the difficulties of obtaining current transport close to the optical axis in the active region and of realize efficient distributed Bragg-reflector (DBR) mirrors in nitride-based materials. Therefore, a number of cavity structures have been realized or suggested, by different research groups, where a current-confining aperture, usually referred as current aperture, is used, on top of which a dielectric DBR mirror is deposited as the top mirror of the VCSEL. Chapter 5 shows numerically that many of these GaN-based VCSEL cavities balance dangerously close to the border between the guided and antiguided regime. In Chapter 6 GaN-based LEDs with color-coded QWs has been used to analyze the carrier injection and recombination phenomena in MQW-based LEDs. These devices have a MQW structure where each QW has a different indium content, corresponding to a different emission wavelength. Colorcoded devices have been used to study asymmetries in carrier distribution, i.e., to understand if in a MQW structure all the QWs contribute to light emission, to evaluate the relative contribution of each well to the overall light emission, and to understand how carriers are injected and recombine within the LED structure. The ongoing debate on efficiency droop in GaN-based LEDs attests the present limitations of both characterization techniques and numerical models of III-nitride active optoelectronic devices. Chapter 7 illustrates the main issues faced when performing a combined experimental and simulation-based analysis of InGaN/GaN blue LEDs with an example carried out on two sets of devices grown, characterized and simulated by the authors. In order to help discriminate the recombination coefficients, structures with different levels of threading dislocation densities (TDDs) have been considered. Recent experiments of electron emission spectroscopy (EES) on III-nitride LEDs have shown a correlation between droop onset and hot electron emission at the cesiated surface of the LED p-cap. The observed hot electrons have been interpreted as a direct signature of Auger recombination in the LED active region, as highly energetic Auger-excited electrons would be collected in long-lived satellite valleys of the conduction band so that they would not decay on their journey to the surface across the highly-doped p-contact layer. Chapter 8 discusses this interpretation by using a full-band Monte Carlo model based on first-principles electronic structure and lattice dynamics calculations. The results of our analysis suggest that Auger-excited electrons cannot be unambiguously detected in the LED structures used in the EES experiments.