Comparison of stepped-well and square-well multiple-quantum-well optical modulators

The responsivity and transmission performance of 60-period p-i-n multiple-quantum-well (MQW) optical modulators having single or double steps in the quantum well (QW) has been experimentally compared to a square-QW control sample. It has been confirmed that significantly increased shift of the exciton with applied electric field [the quantum confined Stark effect (QCSE)] is obtained in the stepped QWs as compared to a conventional square QW. However, no meaningful increase in optical modulation performance is found, due to broadening of the exciton transition in the stepped QWs. All MQWs contained nominal 45 Å Al0.3Ga0.7As barriers. Single-step QWs consisted of a 15 Å GaAs region with an 85 Å Al0.1Ga0.9As step. Double-step QWs consisted of a central Al0.15Ga0.85As plateau, with 20 Å GaAs regions on either side. The square QW consisted of 90 Å of GaAs. Excellent agreement between measured QCSE and tunneling resonance calculations was found. Our results indicate that stepped MQW devices are intrinsically more susceptible to growth induced degradation than square MQW modulators.