Carrier dynamics and high-speed modulation properties of tunnel injection InGaAs-GaAs quantum-dot lasers

We have performed pump-probe differential transmission spectroscopy (DTS) measurements on In/sub 0.4/Ga/sub 0.6/As-GaAs-AlGaAs heterostructures, which show that at room temperature, injected electrons preferentially occupy the excited states in the dots and states in the barriers layers. The relaxation time of these carriers to the dot ground state is >100 ps. This leads to large gain compression in quantum-dot (QD) lasers and limits the attainable small-signal modulation bandwidth to ~ 5-7 GHz. The problem can be alleviated by tunneling "cold" electrons into the lasing states of the dots from an adjoining injector layer. The design, growth, and steady-state and small-signal modulation characteristics of tunnel injection In/sub 0.4/Ga/sub 0.6/As-GaAs QD lasers are described and discussed. The tunneling times, directly measured by threepulse DTS measurements, are ~ 1.7 ps and independent of temperature. The measured small-signal modulation bandwidth for I/I/sub th/ * 7 is f/sub -3 dB/ = 23 GHz and the gain compression factor for this frequency response is (epsilon) = 8.2 * 10/sup -16/ cm/sup 3/. The differential gain obtained from the modulation data is dg/dn ~= 2.7 * 10/sup -14/ cm/sup 2/ at room temperature. The value of the K-factor is 0.205 ns and the maximum intrinsic modulation bandwidth is 43.3 GHz. Analysis of the transient characteristics with appropriate carrier and photon rate equations yield modulation response characteristics identical to the measured ones. The Auger coefficients are in the range ~ 3.3 * 10/sup -29/ cm/sup 6//s to 3.8 * 10/sup -29/ cm/sup 6//s in the temperature range 15 C