Study of carrier and gain dynamics in InGaAsN quantum well lasers

Abstract

The goal of this dissertation is to unveil the role of nitrogen incorporation on important processes that shape the high-speed device performance of InGaAsN laser diodes. This is done through the investigation of the carrier recombination dynamics, nonlinear gain dynamics and carrier capture and escape processes in InGaAsN quantum well lasers. The carrier dynamics is investigated through a comprehensive temperature dependent steady state photoluminescence (PL) and time evolution of PL spectra. The gain dynamics is obtained from the analysis of ultrafast pump-probe transmission measurement with selective pump and probe wavelengths. The experiments are complemented with theoretical model simulations of the gain spectra. The analysis of the steady state and time dependent PL measurements provided insightful understanding on the effect of nitrogen incorporation on the conduction band effective mass, the electronic structure of the quantum well and the main carrier recombination channels. A detailed lineshape analysis of the temperature dependent PL spectra was carried out. The analysis extracts the binding energy of the e1-hh1 ground-state exciton which equals 10±1 meV and 18±1 meV for InGaAs and InGaAsN (N=0.5%) single QW sample, respectively. By using a fractional dimension exciton binding energy model, an electron effective mass of me*=(0.11±0.015)m0 is determined for the highly strained In0.4Ga0.6As0.995N0.005/GaAs QW. We show by simulation of the gain model analysis that the enhanced me* increases the transparency carrier density. It also affects the differential gain dG/dN through the Fermi factor and the momentum matrix element. The former leads to an increase in dG/dN while the latter to a decrease. From the time evolution of the PL, two recombination channels are present at early stages of carrier recombination. These two transitions are identified as the first quantized electron state to heavy-hole state (e1-hh1) and electron to light-hole state (e1-1h1) from the analysis of polarized photocurrent measurements. At longer time delays, the dilute nitride QW exhibits carrier localization at low temperatures and dominant nonradiative recombination at higher temperatures. This behavior contrast with that of the host matrix InGaAs in which the carrier lifetime behavior indicates radiative recombination dominates. Through single color pump-probe experiments, carrier heating, two photon absorption (TPA) are found to be the two main factors contributing processes to the nonlinear gain compression. In InGaAsN laser carrier heating has more significant effect on gain compression in the gain regime. The relaxation time constant associated with carrier heating in both InGaAs and InGaAsN lasers have similar values, 2-3 ps. The carrier escape times Tesc is for the first time measured. Tesc is dominant by hole escape and is about an order of magnitude smaller in InGaAsN than in InGaAs laser device. We show that the dependence of Tesc on carrier density in dilute nitrides is only explained if tunneling through the triangular biased quantum well is considered. This mechanism is most important at low carrier densities. Exciton effects are also proven to be important in affecting the escape time. Together with time-resolved photoluminescence measurements, the carrier transport is found to be dominant over the carrier capture process and no significant change has been found for both InGaAs and InGaAsN lasers. These experimental values allow us to understand the impact of nitrogen incorporation on the high-frequency modulation and bandwidth limitations of InGaAsN quantum-well lasers.