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Graduate Exam Abstract


Lifang Xu

Ph.D. Final

April 2, 2007, 3:00 pm

Engineering B3

Study of carrier and gain dynamics in InGaAsN quantum well lasers


Abstract: This dissertation investigated the influence of incorporation of nitrogen on the carrier recombination dynamics, nonlinear gain dynamics and carrier capture and escape processes in InGaAsN quantum well lasers. Comprehensive studies have been carried out through temperature dependent steady state photoluminescence (PL) and time evolution of PL spectra,ultrafast pump-probe transmission measurement with selective pump and probe wavelengths. The experiments are complemented by 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. 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. 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-lh1) from the analysis of polarized photocurrent measurements. The fact that under high injection the carriers can occupy the light hole state affects the carriers' distribution at high injection; therefore likely impacting the device performance. 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 Ïesc is for the first time measured, which is about an order of magnitude smaller in InGaAsN than in InGaAs laser device. We show that the dependence of Ïesc 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. 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.

Adviser: Prof. Carmen Menoni
Co-Adviser:
Non-ECE Member: Prof. James Sites Department of Physics
Member 3: Prof. Mario Marconi, Electrical & Computer Engineering
Addional Members: Prof. Jon Pikal , University of Wyoming

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Program of Study: