CARRIER THERMALIZATION IN SUB-3-DIMENSIONAL ELECTRONIC SYSTEMS - FUNDAMENTAL LIMITS ON MODULATION BANDWIDTH IN SEMICONDUCTOR-LASERS

Carrier equilibration is essential for semiconductor laser operation s ince carriers are injected into the active region at energies higher t han the effective band edges. While the threshold current of the laser diode can be minimized by quantum confinement in extra dimensions, th e quantum effects in carrier capture and thermalization become more pr onounced. In this paper, a full treatment of the carrier thermalizatio n in electronic systems of reduced dimensionality for injection condit ions relevant to laser operation is given based on ensemble Monte Carl o simulations and the fundamental limits on modulation bandwidth are d iscussed. Results are presented for quantum wells, quantum wires, and quantum dots. The peculiarities of the relaxation process in each stru cture are elucidated. It is shown that the relaxation times increase f rom approximate to 1 ps in bulk, to approximate to 10 ps in quantum we lls, approximate to 50 ps in quantum wires, and approximate to 200 ps in quantum dots. Since the intraband relaxation times determine the ex tent of gain nonlinearities in semiconductor lasers, the maximum modul ation bandwidth imposed by the intrinsic process of carrier relaxation can be calculated via the dependence of the optical gain on the photo n density in the laser structure. For a graded-index quantum-well lase r structure, the calculated value of the nonlinear gain coefficient is 1.1X10(-17) cm(3) with the maximum -3-dB modulation bandwidth of 78 G Hz for a 100-mu m cavity length. The nonlinear gain coefficient in qua ntum wires is enhanced in comparison with quantum wells, although the differential gain may be increased by as much as an order of magnitude with the exact value of the modulation bandwidth dependent on the det ails of the design of the quantum-wire laser.