Electronic structures and optical properties of dilute nitride quantum dots
School of Electrical and Electronic Engineering
Dilute nitride III-V compounds are potential candidate materials for the next generation of telecommunication optoelectronic device such as laser and photodetector. In this thesis, we have investigated the electronic structure and optical properties of quantum dots (QDs) using eight-band or ten-band k.p methods with the strain distribution calculated by the valence force field (VFF) model. Because of QD’s novel electrical and optical properties and quantum confinement effects, we simulate and study these QDs for better understanding. The optical gains are calculated using the zero-dimensional optical gain formula taking into consideration inhomogeneous broadening due to QD size fluctuation. With variations of pyramidal InAs(N) QD size, shape and nitrogen(N) composition, we find that when N composition is higher, the ground state (GS) of conduction band c1 is forced to be lower so that the c1-h1 GS transition energy is less, and the intersubband energy separation c2-c1 is greater so that the higher excited states transition is harder to occur. The incorporated N can facilitate to achieve shorter emission wavelength 1.3 μm or even 1.55 μm, with higher maximum optical gain and less detrimental effect induced by higher excited state transition. If we truncate the full pyramidal shape, the QD height in z-direction is reduced. This truncation changes the strain distribution and increases the confinement in z-direction resulting in greater GS transition energy, greater transition matrix element (TME), less detrimental effect of higher excited transition, higher saturation gain and differential gain. For pyramidal InAsPN/GaP(N) QDs, with GaP barrier, for smaller InAsPN QDs, the GS transition energy is smaller at a lower phosphorus (P) composition of InAsPN. But for larger InAsPN QDs, the GS transition energy increases as P composition increases due to the increased bandgap. To obtain laser materials with a lattice constant comparable to Si, we incorporated 2% of N into GaP barrier. So the conduction band offset and the quantum confinement are reduced resulting in a smaller transition energy and longer wavelength. Meanwhile the optical gains are less than those without N in the barrier at a low carrier density, but the maximum GS optical gain increases faster and surpass to reach a greater saturation optical gain when the carrier density increases.
DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics