Researchers in the field of integrated optics have long sought to develop methods for producing active devices, such as lasers or light-emitting diodes (LEDs), and passive devices, such as waveguides, couplers, switches, modulators, and the like on the same substrate. Because the appropriate active devices can only be manufactured in semiconductor materials, the above-referred to passive devices would therefore also have to be manufactured in or on a semiconductor substrate.
Both active devices and passive components, such as waveguides, have been manufactured in GaAs. However, GaAs lasers or LEDs emit light of a relatively short wavelength which is shorter than the infrared wavelengths currently considered most promising for fiber optical communication. These wavelengths of current interest for optical communication are in the region from about 1 .mu.m to about 1.6 .mu.m. In this wavelength regime, InP and InGaAsP LEDs and lasers have been developed. Consequently, attention is being paid to InP and InGaAsP as semiconductor materials for related integrated optics. In particular, work has been done to develop methods for fabricating optical strip waveguides in InP and InGaAsP.
In order to manufacture a strip waveguide, it is necessary to change the refractive index of some volume of material so that the effective refractive index of the material surrounding the waveguiding region is less than the effective refractive index of the waveguiding region. Such a structure can guide electromagnetic radiation of the appropriate wavelength by means of total internal reflection.
Several approaches have been used in the past to create the above referred to difference in the effective refractive indices. In insulators, such as LiNbO.sub.3 or LiTaO.sub.3, metal in-diffusion is a commonly practiced method. See, for instance, U.S. Pat. No. 4,284,663, issued Aug. 18, 1981 to J. R. Carruthers, I. P. Kaminow, and R. V. Schmidt for "Fabrication of Optical Waveguides by Indiffusion of Metals." The in-diffusion of metal ions, e.g., of Ti, into LiNbO.sub.3 or LiTaO.sub.3 is carried out at quite high temperatures, typically greater than about 800.degree. C.
InP and InGaAsP sample surfaces typically deteriorate when maintained at elevated temperatures. The deterioration is mainly due to loss of phosphorus from the sample. This loss is considerable at the high temperatures usually thought to be necessary to obtain acceptably short diffusion times, i.e., at temperatures comparable to those used in, e.g., LiNbO.sub.3. For this and other reasons it has hitherto been considered impractical to fabricate waveguides in InP and InGaAsP by means of metal indiffusion. Therefore, other approaches to waveguide formation have been used in these and other compound semiconductors.
One of these alternate approaches has been the use of rib waveguides in materials such as GaAs and InP. See, for instance, the article by Reinhart et al entitled "Transmission Properties of Rib Waveguides Formed by Anodization of Epitaxial GaAs on Al.sub.x Ga.sub.1-x As Layers," Applied Physics Letters, 24, pp. 270-272, Mar. 15, 1974. Rib waveguides can be formed by appropriately shaping the surface of a planar waveguide. See, for instance, U.S. Pat. No. 4,093,345 issued on June 6, 1978 to Ralph Andre Logan, Franz Karl Reinhart, and William Robert Sinclair. Stress-caused changes in the refractive index have also been used to create strip waveguides in InP. See, for instance, T. H. Benson et al., "Photoelastic Optical Waveguiding in InP Epitaxial Layers," 7th European Conference on Optical Communications, Sept. 8-11, 1981. Benson et al. defined strip waveguides in InP samples by depositing either a positive or a negative pattern of a thick (approximately 1 .mu.m) metal film on the substrate. By a positive pattern or mask, we mean herein a metal pattern that directly overlies the region to be transformed into a waveguide, and by a negative pattern or mask, we mean herein a metal pattern that overlies the regions of the substrate bordering the region to be transformed into a waveguide. The metal used by Benson et al typically was gold, and the pattern was typically created by standard photolithographic and etching techniques. A planar waveguiding layer was created by forming an n-type InP epitaxial layer on an n+ InP substrate. Lateral confinement of the radiation was achieved through the strain induced in the epitaxial layer by the thick metal film on cooling after evaporation of the metal.
The presence of a conductive layer, e.g., a metal layer, on a semiconductor surface is known to result in a change of the effective index of refraction of the near-surface semiconductor material. This effect has also been used to define waveguiding structures. See, for instance, "GaAs Electro-Optic Directional Coupler Switch," J. C. Campbell et al., Applied Physics Letters, 27, pp. 202-205, Aug. 15, 1975.
Thus, the prior art teaches several methods for manufacturing optical strip waveguides in InP and InGaAsP. However, these methods have drawbacks. For instance, rib waveguides typically have relatively high scattering loss off the edges of the rib. Stress-induced waveguides are in principle easy to manufacture, but in practice are difficult to manufacture reproducibly, and device characteristics are subject to change with time. And metal-loaded waveguides strongly attenuate the transverse magnetic (TM) mode of the electromagnetic radiation, and thus have restricted applicability, in addition to confining the radiation relatively poorly. For these and other reasons, a method for manufacturing optical strip waveguides in InGaAsP and InP, as well as other semiconductors, that is compatible with established processing techniques, is reliable, reproducible, and results in guides capable of guiding both transverse electric (TE) and TM modes with relatively little loss would be of considerable interest.