We have performed pump-probe differential transmission spectroscopy (DTS) measurements on In0.4Ga0.6As-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 In0.4Ga0.6As-GaAs QD lasers are described and discussed. The tunneling times, directly measured by three-pulse DTS measurements, are ∼ 1.7 ps and independent of temperature. The measured small-signal modulation bandwidth for I/Ith ∼ 7 is f-3dB = 23 GHz and the gain compression factor for this frequency response is ε = 8.2 × 10-16 cm3. The differential gain obtained from the modulation data is dg/dn ≅ 2.7 × 10-14 cm2 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 × 10-29 cm6/s to 3.8 × 10-29 cm6/s in the temperature range 15°C < T < 85°C, determined from large-signal modulation measurements, and these values are smaller than those measured in separate confinement heterostructure QD lasers. The measured high-speed data are comparable to, or better than, equivalent quantum-well lasers for the first time.
Bibliographical noteFunding Information:
Manuscript received November 4, 2002; revised March 10, 2003. The work was supported by the Army Research Office under Grant DAAD019-01-1-0331. The authors are with the Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109–2122 USA. Digital Object Identifier 10.1109/JQE.2003.814374
All Science Journal Classification (ASJC) codes
- Atomic and Molecular Physics, and Optics
- Condensed Matter Physics
- Electrical and Electronic Engineering