This invention relates generally to semiconductor devices and more particularly to quantum well electro-optic semiconductor devices.
Electro-optic devices have been an important part of modern optical systems for both military and commercial applications. Such apparatus include semiconductor devices and are typically comprised of materials such as KDP and LiNbO.sub.3. Such materials, however, cannot be integrated onto a chip, but must normally be used as discrete elements in an optic system. As a result, the present day systems of this type are bulky, massive and expensive.
New developments in the technology of ultra-small semiconductor devices now make possible a new generation of devices in which several optical components are integrated onto a single chip. One such device is based on the technology relating to quantum wells in semiconductors and the quantum confined Stark effect which has been utilized in the prior art to implement both optical phase shifters and modulators.
Quantum wells in semiconductors as well as the concept of electron tunneling therebetween have been disclosed by R. Tsu and L. Esaki, Appl. Phys. Lett. 22, 562 (1973). The quantum confined Stark effect, moreover, has also been disclosed, a typical example being the publication of D. S. Chemla and D. A. B. Miller, J. Opt. Soc. Am., B. 2, 1155 (1985).
In a quantum well, a layer of semiconductor material, grown, for example, by molecular beam epitaxy, is sandwiched between cladding layers made from a different semiconductor with the resulting structure being in the form of a superlattice. Because of the small size of the quantum well, i.e. on the order of 100 .ANG., electronic motion in the direction perpendicular to the well is quantized, leading to the formation of electron and hole sub-bands in the conduction and valence bands, respectively. In addition, an electron - hole pair in a semiconductor can form a bound state, similar to a hydrogen atom, which is known as an exciton. Because their binding energies are larger in quantum wells than in bulk semiconductors, these excitons are stable at room temperature. This stability leads to the quantum-confined Stark effect.
In the quantum-confined Stark effect, the wavelength of the peak optical absorption feature in quantum wells associated with the creation of an electron-hole pair in a formation of an exciton shifts in response to an applied electric field. This shift occurs for two reasons. First, the difference between electron and hole sub-band energies in the quantum wells varies as a function of the applied field and secondly, the binding energy of the exciton varies with the applied field. Because of the interaction of electrons and holes in the quantum well with impurities and with phonons, the absorption feature associated with excitation formation has a finite spectral width. Therefore, the transmission through a quantum well sample at an operating wavelength near that of the excitonic absorption feature will vary as a function of applied bias when the feature is "tuned" through an operating wavelength. When such devices are utilized to optically modulate an optical signal, electric fields of the order of 100,000 V/cm are necessary.
Also because of the Kramers-Kronig relations describing the connection between the real and imaginary parts of the refractive index of the material, the shift in the peak absorption feature leads to a change in refractive index of a quantum well system at wavelengths which are somewhat larger than that of the peak absorption feature. Therefore, it is possible to obtain a sizable phase shift in a quantum well structure.
It is an object of the present invention, therefore, to provide an improved quantum well semiconductor device.
It is a further object of the invention to provide improved modulation of an optical beam with a quantum well semiconductor device.
It is a further object of the invention to provide improved phase shifting of an optical beam with a quantum well semiconductor device.
It is yet another object of the invention to implement an array of quantum well semiconductor devices for operating either as an electro-optic phase shifter or an electro-optic modulator.
It is a further object to provide an improved electro-optic device which can be integrated with other types of optical components in an optical system.