SEMICONDUCTOR HETEROINTERFACE OPTICAL WAVEGUIDE

Abstract: Low loss semiconductor waveguides for supporting propagation of optical signals over a wide range of wavelengths are achieved by growing at least two epitaxial layers (12,13) of dopant material contiguous along a major surface of each layer to form a heterointerface (14) therebetween. At least one (12) of the epitaxial layers (12,13) includes a sufficient concentration of semiconductor material to cause strain via lattice mismatch substantially at and near the heterointerface (14). The strain induces a change in refractive index such that the heterointerface (14) exhibits a substantially higher refractive index than a portion of each epitaxial layer (12,13) proximate to the heterointerface. The resulting waveguide is capable of supporting propagation of optical signals substantially along the heterointerface (14). In one example, contiguous epitaxial layers of InP (13) and InGaP (12) form a waveguide for optical signals at wavelengths greater than 0.93 µ m . The concentration of Ga in the InGaP epitaxial layer (12) is varied from 10¹⁸ to 10²⁰ cm ⁻³ to increase the refractive index difference between the heterointerface (14) and the immediate surrounding layers (12,13).

Description: This invention relates to semiconductor waveguide devices. Semiconductor optical waveguides have been fabricated in the Group III-V systems by cladding an epitaxial layer of high refractive index material with one or more epitaxial or mass transported layers of lower refractive index material. For example, in the InGaAsP/InP system, a quaternary (InGaAsP) guiding layer is clad with lower index binary (InP) layers to form an optical waveguide. While the As content of the quaternary guiding layer can he raised to increase the refractive index of the guiding layer with respect to the cladding layers, this decreases the energy bandgap of the guiding layer and, thereby, increases the cutoff wavelength of the waveguide. As such, the resulting waveguide exhibits a limited transparency range in the window of wavelengths (1.3 to 1.6 µm.) of interest in lightwave communications. In addition to this limitation, quaternary layers such as InGaAsP are difficult to grow using procedures such as MOVPE because such growth requires precise, stable control of temperatures and flows of precursor materials used in the growth process. Other techniques have been employed to produce these semiconductor optical waveguides. For example, extremely thin layers of InP and InGaAs have been interleaved to create an "effective" quaternary guiding layer. While this approach is similarly limited with respect to transparency, it introduces a further requirement in the growth process, namely, the need for a manifold capable of switching rapidly between different precursor materials to create abrupt interfaces. IEEE Journal on Quantum Electronics, vol. QE-21, No. 8, Aug. 1985, ps.1147-1151 discloses the fabrication of two different semiconductor waveguides utilizing InP. The waveguides exhibit a rib structure and lattice matching. In addition, the waveguides are subjected to an applied voltage which varies the r ₄₁ electrooptic coefficient to cause phase differences to occur between orthogonal propagation modes (TE and TM). EP-A-143-070 discloses a rib waveguide formed using an InGaAsP/InP structure. Doping of the structure is performed to create a pn structure. Waveguiding is confined by the rib and is controlled by application of voltage through the Schottky electrodes. According to this invention there is provided a semiconductor waveguide device as claimed in claim 1. The invention enables low loss semiconductor waveguides to be provided for supporting propagation of optical signals over a wide range of wavelengths. In one example, contiguous epitaxial layers of InP and InGaP form a waveguide for optical signals at wavelengths greater than 0.93 µm. The concentration of Ga in the InGaP epitaxial layer is varied from 10¹⁸ to 10²⁰ cm ⁻³ to increase the refractive index difference between the heterointerface and the immediate surrounding layers. Electrode structures have been utilized in conjunction with the waveguide to produce active waveguide functions such as modulation and switching. Generally, the epitaxial layers in these active waveguides include dopants to make the layers highly resistive or semi-insulating. Illustrative embodiments of the invention will now be described with reference to the accompanying drawings, in which: FIG. 1 shows a cross-sectional view of a planar InGaP/InP (two layer) waveguide on an InP substrate; FIG. 2 shows a cross-sectional view of the waveguide of FIG. 1 including a ridge member; FIG. 3 shows a cross-sectional view of a dual waveguide structure on an InP substrate; and FIG. 4 shows a cross-sectional view of an active waveguide including electrodes disposed on the waveguide shown in FIG. 2. Semiconductor waveguides as described below include at least two contiguous epitaxial layers grown on a semiconductor substrate and exhibiting a lattice mismatch at the heterointerface defined therebetween. While a specific arrangement of layers is shown in each of the FIGS., it should be understood that the arrangements shown are for descriptive purposes alone and should not, therefore, be interpreted as limiting. For example, the arrangement of layers on any substrate may be sequentially reversed or juxtaposed. It should also be noted that layer dimensions such as thickness and width have not been made to scale in the FIGS. The views of the waveguides depicted in FIGS. 1 through 4 are cross-sectional in a plane transverse to the direction of propagation for optical signals. As shown in FIG. 1, semiconductor waveguide includes two epitaxial layers grown on a semiconductor substrate. Epitaxial layer 12 comprised of InGaP is grown on substrate 11 which is comprised of InP. Epitaxial layer 13 comprised of InP is grown directly on epitaxial layer 12. In the semiconductor waveguide device, heterointerface 14 is created along the surface where epitaxial layer 13 is contiguous with epitaxial layer 12. Similarly, heterointerface 15 is created along the surface where epitaxial layer 12 is contiguous with substrate 11. In this structure, lattice mismatch occurs at heterointerfaces 14 and 15. The degree of lattice mismatch is primarily dependent upon Ga concentration in epitaxial layer 12 which is controlled to be between For the range of Ga concentration above, it is possible to produce a lattice mismatch less than 0.2% and, thereby, create an effective refractive index difference of approximately 0.3 or less. The refractive index difference is measured between the heterointerface and regions in each epitaxial layer parallel to the heterointerface and substantially adjacent thereto. Although it is possible to produce even higher refractive index differences by increasing Ga concentration, it is important to avoid increasing Ga concentration beyond the level at which surface morphology is severely disturbed. Optical signals are guided along and in substantially close proximity to the heterointerface. For the example shown in FIG. 1, it is clear that there are two heterointerfaces and therefore two parallel waveguides. While this is true in theory, it is not the case in practice because there are a high number of defects present generally of the major surface of substrate 11 at heterointerface 15 which contribute to significant optical scattering and loss. Therefore, the waveguide device shown in FIG. 1 is preferably referred to as a single waveguide device at heterointerface 14. Also, it is important to note that, as stated above, the compositions of epitaxial layers 12 and 13 can be juxtaposed so that the InP epitaxial layer is disposed on substrate 11 and the InGaP epitaxial layer is disposed on the InP epitaxial layer. In this juxtaposed relationship, it is clear that the waveguide structure includes a single heterointerface and is thereby a single waveguide structure. While any epitaxial growth technique (e.g., MBE, LPE, VPE) may be employed to fabricate the waveguide devices described herein, it should be noted that LPE may unduly limit the length of waveguide device achievable. Using techniques other than LPE offers the potential for growing waveguide devices to the full extent of the semiconductor substrate wafer, that is, at least 2-3 cm. length. The waveguide shown in FIG. 1 was grown by atmospheric pressure MOVPE in horizontal reactors. For a straight cylindrical reactor with a susceptor heated by rf induction as described in Electronics Letters , Vol. 22, pp. 1186 et seq. (1986), typical growth conditions were: carrier gas flow at 8 l/min., trimethylindium flow at 6.7 x 10⁻⁵ moles/min. and phosphine flow at 9 x 10⁻³ moles/min . Trimethylgallium flow varied from 0 to 3 x 10 ⁻⁷moles/min . Trimethylindium and trimethylgallium bubblers were kept at 10 ° C and -15 ° C, respectively. At a growth temperature of 650 ° C, the growth rate of InP was 2 µm/hr . For the waveguide shown in FIG. 1, an exemplary thickness for epitaxial layer 13 measured perpendicular to heterointerface 14 was between 0.3 and 2.0 µm ., while an exemplary thickness for epitaxial layer 12 was between 2 and 3 µm .. For the waveguide device shown and described above, it has been possible to guide optical signals at 1.32 µm . with a mode width at half maximum intensity of 3.0 µm . Loss at this wavelength has been measured as approximately 1.25 dB/cm. The waveguide device shown in FIG. 2 incorporates a layer structure similar to that shown in FIG. 1. The waveguide is grown directly on substrate 21. Epitaxial layer 22 is grown on substrate 21 creating heterointerface 25 therebetween; epitaxial layer 23 is grown on epitaxial layer 22 creating heterointerface 24 therebetween. A ridge member is included in epitaxial layer 23 to provide lateral confinement (transverse mode control) for optical signals propagating in the waveguide device. For most applications, a ridge member having a lateral dimension between 4 and 20 µm . is sufficient for transverse mode control. The ridge member extends over and parallel to the heterointerface to define the direction of propagation for optical signals in the waveguide. Standard photolithographic and etching techniques known to those skilled in the art are employed in the fabrication of such ridge members. For example, deposition of a silicon nitride mask to define the ridge member followed by appropriate wet (e.g., HCl, bromine methanol) or dry (e.g., plasma) etching step is suitable for fabricating a ridge member. The waveguide device shown in FIG. 3 includes three epitaxial layers (layers 32, 33, and 34) defining two heterointerfaces (35 and 36) therebetween on substrate 31. It should be noted that the composition of the layers may be juxtaposed without affecting the intermediate heterointerfaces. By properly controlling the thicknesses of epitaxial layers 32, 33, and 34, it is possible to operate the waveguide device shown in FIG. 3 as a vertical coupler in which optical signals propagating along the waveguide defined in the vicinity of heterointerface 35 are coupled into the waveguide defined in the vicinity of heterointerface 36 and vice versa. The degree of coupling is dependent upon the relative separation of the waveguides as well as the degree of transverse mode confinement for each waveguide. In contrast to the passive waveguide structures shown in FIGS. 1 through 3, an active waveguide device is shown in FIG. 4. This device incorporates the waveguide structure and ridge member shown in FIG. 2 and includes a pair of electrodes for applying an electric field perpendicular to heterointerface 46. Electrode 45 is disposed directly over the ridge member whereas electrode 44 is disposed adjacent and parallel to the ridge member. By proper selection of the location for these electrodes, it is possible to direct an electric field perpendicular to heterointerface 46 in the region directly under the ridge and electrode 45. While other electrode configurations are applicable to the active waveguide structure in FIG. 4, the configuration shown is preferred because it permits independent electric fields to be applied when the waveguide is incorporated in an integrated optoelectronic circuit with other devices. For the device shown in FIG. 4, it is preferred that the semiconductor layers exhibit highly resistive or semi-insulating properties. An important feature of the waveguide device shown in FIG. 4 is that the propagation velocities of optical and electrical signals are well matched thereby avoiding the complex electrode structures frequently used for velocity matching in other dielectric waveguide media. Waveguides of the type shown in FIG. 4 are suitable for modulation, multiplexing, demultiplexing and switching applications. It is understood that the waveguide shown in the FIGS. may be useful as filters by disposing gratings and/or grating resonators on a surface of an epitaxial layer along the direction of optical signal propagation in the waveguide.

Claims: A semiconductor waveguide device comprising a first epitaxial layer (12), a second epitaxial layer (13), said first epitaxial layer being contiguous with said second epitaxial layer along a major surface of each epitaxial layer to form an interface (14) therebetween, and CHARACTERIZED BY an optical waveguiding region induced along the interface of the two epitaxial layers and by a sufficient concentration of dopant material in at least one of the epitaxial layers (12) to cause strain via lattice mismatch substantially at or near the interface, which strain creates a zone of substantially higher refractive index at and near the interface of the epitaxial layers, so as to induce the waveguiding region. A device as claimed in claim 1 wherein said first epitaxial layer comprises InGaP, said second epitaxial layer comprises InP, and said dopant material comprises gallium in sufficient concentration to achieve the lattice mismatch desired to control the refractive index profile of the device in the waveguiding region. A device as claimed in claim 1 or 2 wherein at least one of said epitaxial layers (23) comprises a ridge member aligned substantially along a direction of propagation for said optical signals. A device as claimed in claim 3 comprising first and second electrodes (44,45) for applying an electric field perpendicular to said interface (46) and substantially along a direction of propagation for said optical signals, said first electrode (45) being disposed on and along said ridge member (43), and second electrode (44) being disposed adjacent to said ridge member and substantially parallel to said first electrode.

Inventors: ALFERNESS, RODNEY CLIFFORD, CHARLES, HOWARD JOYNER, JR., DENTAI, ANDREW GOMPERZ
Applicants: AMERICAN TELEPHONE AND TELEGRAPH COMPANY
Application year: 1988
Main classification: G02B6
IPC codes: C30B25, C30B29, G02B6, H01S5, G02F1