IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 6, JUNE 1998 807 SiGe–Si Quantum-Well Electroabsorption Modulators O. Qasaimeh, P. Bhattacharya, Fellow, IEEE, E. T. Croke Abstract— We have measured the characteristics of SiGe–Si quantum-well (QW) waveguide electroabsorption modulators grown by molecular beam epitaxy (MBE). The modulation is based on the decoupling of the electron wavefunction from the shallow wells for electrons with a small band-bending. A 100- m-long modulator demonstrates contrast ratio and insertion loss of 1.43 and 28.5 dB, respectively. Index Terms— Electroabsorption, modulators, silicon-german- ium. L IGHT MODULATORS are of particular interest for Si- based optoelectronics and optical interconnects [1]–[3]. Quantum-well (QW) based modulators, based on the quantum- confined Stark effect (QCSE) [4], have been very successful with GaAs and InP-based semiconductors. On the other hand the QCSE in pseudomorphic SiGe–Si QW’s is hardly observed since the characteristic red-shift of the absorption edge due to QCSE is balanced by a blue-shift due to the sharp reduction of the exciton binding energy with applied electric field. The reduction in the exciton binding energy, which is normally not prevalent in most III–V QW’s, occurs due to the very small conduction band offset, [5]. However, we have recently reported on the design of a SiGe–Si QW modulator whose operation depends on the weak confinement of the electron wavefunction [6]. The calculated performance characteristics of the device were also reported. In this letter, we report the measured characteristics of the device grown by molecular beam epitaxy (MBE). The principle of operation of the SiGe–Si QW modulator is briefly reiterated. The small value of ( 20 meV) in type-I compressively strained SiGe–Si leads to a very shallow well for electrons [7]. Under flat-band conditions, the electron wavefunctions are weakly confined, while the hole wavefunctions are highly localized. With the application of a transverse electric field to the well, the electron states are extended in space while the hole states remain quasi-bound. As a consequence, the overlap integral between electron and hole wavefunctions will decrease, leading to a decrease in the absorption coefficient. The change in the absorption coefficient Manuscript received November 25, 1997; revised February 6, 1998. The work of O. Qasaimeh and P. Bhattacharya was supported by the Air Force Office of Scientific Research under Grant F49620-95-1-0013. O. Qasaimeh and P. Bhattacharya are with the Solid-State Electronics Laboratory, Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109-2122 USA. E. T. Croke is with Hughes Research Laboratories, Inc., Malibu, CA 90265 USA. Publisher Item Identifier S 1041-1135(98)03800-2. (a) (b) Fig. 1. (a) Design of ideal heterostructure for SiGe–Si QW modulator, and (b) doping profile of heterostructure grown by MBE as measured by SRA. will lead to a change in the refractive index near the bandedge and an electrooptic effect. From the Fermi Golden rule, the absorption coefficient for indirect bandgap QW’s can be written as [6] (1) where is the width of the QW, and is the photon energy. From (1), the absorption coefficient is found to be inversely 1041–1135/98$10.00  1998 IEEE