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Carrier injection rate in tunnel injection quantum well structures used in semiconductor lasers was investigated through theoretical analysis. Carrier capture time from 3D-state to 2D-state in a conventional quantum well was several ten picoseconds and it is almost uncontrollable characteristics. The tunnel injection structure can be designed to control the carrier injection rate. The carrier transition time from carrier reservoir-region of the tunnel injection structure to the active well was a few picoseconds when the structure was designed for high speed transition. As results, carrier injection to ground state of the active well takes less than 10 picoseconds. The change of the radiative recombination rate in the active well also defines the optimal design of the tunnel injection structure. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A quantum well design for controlling of the carrier dynamics was investigated for high speed direct modulation semiconductor lasers. The proposed Well-in-Well structure decreases the carrier relaxation time into an active well by designing the wave function which relates to the LO phonon scattering rate. By analyzing the scattering rate of each energy state, the electron relaxation time was estimated to be a few picoseconds which is 2–10 times smaller than that of a conventional quantum well structure. The result is desirable for improving the modulation damping effect which limits the modulation band width. f-3dB is expected above 50GHz. The effect of an asymmetrical Well-in-Well structure was also discussed.

Carrier relaxation in tunnel-injection quantum structures are analyzed using thousands-sets of rate equations of each quantum state. The results suggest that modulation-bandwidth of TI-QW lasers is broaden by suppression of lack of carriers.

A quantum well design for controlling of the carrier relaxation process and reducing of the gain damping effect was investigated for improvement of high speed direct modulation semiconductor lasers. The proposed Well-in-Well structure decreases the carrier relaxation time into an active well by carefully designed wave function which relates to the LO phonon scattering rate. The decrease of relaxation time and the increase of modulation bandwidth was analyzed numerically. The reduction of damping factor γ was also demonstrated by experimental measurement of the relative intensity noise spectrum.

A quantum-well design for controlling carrier dynamics was investigated for high-speed direct-modulation semiconductor lasers. The proposed well-in-well (WWell) structure decreases the carrier relaxation time into an active well by designing the wave function that is related to longitudinal optical phonon scattering rate. The electron relaxation time was analyzed numerically and was estimated to become 1/2–1/10 times smaller than that of a conventional quantum-well structure. This reduction of the relaxation time is effective for reducing the optical gain damping. The advantage of an asymmetrical WWell structure is also shown.

A large kink in the light output versus current characteristics was observed in semiconductor lasers with a tunnel injection quantum well structure. The output power was changed from 13 to 1.9 mW under a small current increase of 0.06I th (30 mA). The peak-to-valley (P/V) ratio of the output power was as high as 6.8. The kink current and the P/V ratio changed greatly within a monolayer thickness variation of the tunnel injection well. The kink mechanism that can be assumed was presented on the basis of the threshold current and the threshold voltage characteristics of the experimental results.