Advances in quantum tunneling models for semiconductor optoelectronic device simulation

The undiscussed role of solid-state optoelectronics covers nowadays a wide range of applications. Within this scenario, infrared (IR) detection is becoming crucial by the technological point of view, as well as for scientific purposes, from biology to aerospace. Its commercial and strategic role, however, is confirmed by its spreading use for surveillance, clinical diagnostics, environmental analysis, national/private security, military purposes or quality control as in food industry. At the same time solid-state lighting is emerging among the most efficient electronic applications of the modern era, with a billion-dollar business which is just destined to increase in the next decades. The ongoing development of such technologies must be accompanied by a sufficiently fast scientific progress, which is able to meet the growing demand of high-quality production standards and, as immediate but not obvious consequence, the need of performances which would be the highest possible. One issue affecting both kinds of applications we mentioned is the quantum efficiency, no matter the signal they produce is coming from absorbed or emitted photons. At any rate, the balance between the stimulus coming from the surrounding environment is and the generated electrical current is absolutely crucial in each modern optoelectronic device. More in depth, since IR detectors are asked to convert photons into electrons, device designers must ensure that mechanisms concurring to this conversion should be dominant with respect to any opponent phenomenon. Symmetrically, light-emitting diodes should realize the inverse process, where electrons are converted into photons. In real life this mechanism never take place in a one-to-one electron-photon correspondence. Indeed tunneling, a quantum effect related to the probabilistic nature of particles and, thus, also of charges, contributes to unbalance this correspondence by degrading the signal produced within the device active region. In IR photodetectors this translates into of a current even in absence of light (and, by virtue of this fact, this current is known as "dark current") while in light-emitters tunneling is responsible for leakages that may undermine the quantum efficiency and the power consumption also below the optical turn-on. The present dissertation is part of such framework being the result of studying and modeling different tunneling mechanisms occurring in narrow-gap infrared photodetectors (IRPDs) for mid-wavelength IR (MWIR) applications (3 to 5 um) and in wide-gap blue LEDs (around 450 nm) based on nitride material system. This study has been possible thanks to the collaboration with several academic institutions (Boston University, Padua and Modena e Reggio Emilia Universities) and two important German industries, AIM Infrarot Module and OSRAM Opto Semiconductors, which provided the case-study devices here analyzed. After reviewing basic concepts of solid-state physics, the first part of this work deals with the description of the above cited optoelectronic devices, along with their constituent materials: the HgCdTe alloy, in the case of photodetectors, and GaN and its ternary alloys with In and Al, for what concerns blue LEDs. Since the literature focusing on this research area is still not mature enough, in the second part different tunneling mechanisms and models are proposed, described in detail and then tested for the first time, as in the case of a novel formulation intended for direct tunneling in IRPDs or the description of defect-assisted tunneling in LEDs which also includes elements coming from the microscopic theory of multiphonon emission (MPE) in solids. Simulations are carried out by means of several numerical simulation approaches, using either commercial TCAD (Technology Computer Aided Design) tools and codes developed ad hoc for this purpose. The encouraging and fully satisfying results of numerical modeling here proposed confirm, on the one hand, the widely accepted relevance of tunneling in modern electronics and, on the other hand, also propose a new perspective about possible tunneling mechanism in optoelectronic devices and their appropriate physical, mathematical and numerical investigation tools. Furthermore, the role of device modeling does not end here because many physical details and technological information can be inferred from simulations, with enormous beneficial effects for the electronic industry and the quality improvement of its fabrication processes such those invoked above.