Fabrication and characterization of lasers and phase modulators with strained quantum well and quantum box active regions.

Abstract

As an optical transmitter in telecommunication systems, a semiconductor laser can be either directly modulated or can serve as a coherent light source for an external modulator. While direct modulation of injection lasers is easily achieved, it suffers from gain compression and relaxation oscillation problems under high speed operation. Although external modulators are less susceptible to these problems, there are problems associated with modulation depth and power consumption which need to be addressed. The primary research interests of this dissertation work were to use low dimensional quantum confined heterostructures in the design of semiconductor lasers and phase modulators, and to investigate the possibility of monolithic integration of these devices. In order to provide high quality material required for this work, an extensive study of molecular beam epitaxy (MBE) growth has been performed. Very high quality strained InGaAs/GaAs and InGaAs/InAlAs/InP quantum well layers have been grown by MBE. A number of design and fabrication issues affecting modulation bandwidths, threshold currents, characteristic temperatures and reliability of conventional quantum well lasers have been studied and characterized. Measurements for DC and high frequency characterization of these devices have been performed. Very low threshold current density (37.5 A/cm$\sp2$ per well) and large modulation band-widths (20 GHz) were obtained. A carrier relaxation "bottleneck" in the conventional SCH laser structure is seen as a serious limitation to the modulation speed of conventional MQW lasers. To overcome this problem, a novel quantum well laser where the injected electrons by-pass this bottleneck by resonant tunneling is proposed and experimentally demonstrated. The characterization of two different and novel material structures, the (111)B-oriented InGaAs/InAlAs quantum well (QW) system on InP substrate and low dimensional structures--quantum boxes (QB), for phase modulators have been carried out. The electro-optic coefficients of (111)B and QB material systems have been measured with a greatly enhanced $r\sb{l}$ of 7 and 4.5 times that of bulk GaAs, respectively. To reduce the optical coupling losses of these external modulators, the monolithic integration of the laser and modulator is also experimentally investigated.