Experimental studies of proton-implanted GaAs-AlGaAsmultiple-quantum-well modulators for low-photocurrent applications

We describe the first attempts to control photocurrent, and thus power dissipation, in surface-normal multiple-quantum-well (MQW) modulators. We have made detailed experimental studies of proton-implanted p-i-n GaAs-Al_xGa_1-xAs MQW modulators having barrier layers of x=0.3, 0.45, and 1.0. Structures were implanted to levels of 1×10¹² cm^-2, 1×10¹³ cm^-2, and 1×10¹⁴ cm ^-2. Photocurrent progressively decreased with increasing implant-dose and barrier mole fraction (x). Exciton linewidths showed a strong voltage and implant dose dependence, demonstrating a tradeoff between photocurrent and modulation performance. We obtained our best results with x=1.0 barriers. For example, 1×10¹³ cm^-2-implanted asymmetric Fabry-Perot modulators were realized in which the optical performance was similar to that of unimplanted devices. The photocurrent responsivity was, however, only 0.007 A/W at 12.5 V bias. We report measurements of carrier lifetime in these materials that show the reduction in photocurrent arises from a reduction in lifetime due to implant-induced damage. In addition, the reduced lifetime decreases the optically-excited quantum-well carrier population, leading to an increase in cw saturation intensity. Specifically, 1×10¹³ cm^-2-implanted devices with x=1.0 have a saturation intensity of roughly 45 kW/cm², while unimplanted devices have 3.5 kW/cm². Asymmetric self electro-optic effect devices (A-SEED's) are demonstrated, and power dissipation issues associated with the use of low-photocurrent modulators in integrated systems are discussed