This invention relates to optical detectors useful for lightwave communications, and in particular to a monolithic silicon photodetector which includes means for aligning an optical fiber for edge illumination of the detector.
Lightwave communication systems are currently enjoying an intensive development effort. In the present technology generally, silicon photodetectors are utilized to detect light produced by GaAlAs lasers at a wavelength of approximately 0.85 microns. The photodetectors are usually p-i-n or avalanche diodes which are illuminated through the top surface of the device (see, for example, Melchior, "Detectors for Lightwave Communication," Physics Today, Volume 30, pages 32-39 (November 1977)). While such systems are more than adequate, it is recognized that an increase in distance between repeaters over that presently available is highly desirable for more economical systems. This presently requires use of light at longer wavelengths.
Fiber attenuation in available fibers decreases as the wavelength of light increases and it is therefore desirable to provide devices which can detect the longer wavelengths. Presently available top-illuminated photodetectors are generally limited to the 0.8-0.9.mu. range since the detectable light absorbing region, which is defined by the depth of the depletion region in the device, is shallow and does not provide sufficient distance for light of longer wavelengths to be absorbed therein. That is, at wavelengths beyond 0.9 .mu.m, the quantum efficiency of conventional top-illuminated silicon detectors drops off rapidly due to the decrease in absorption coefficient with wavelength (see, e.g., Conradi, "Fiber Optical Transmission Between 0.8 and 1.4 .mu.m", IEEE Transactions on Electron Devices, Vol. ED-25, No. 2, pp. 180-191 (February 1978)).
If one wishes to provide detectors which can operate above 0.9 .mu.m, one has basically two alternatives: to construct special silicon detectors capable of absorbing and detecting light at longer wavelengths or to develop detectors in other material such as germanium and alloys of III-V compounds. Considerable effort is now being made in the latter approach since fiber attenuation in available fibers is minimized at a wavelength of approximately 1.3 .mu.m and absorption in silicon is limited to approximately 1.1 .mu.m due to its small bandgap (1.12 eV).
There are, however, definite advantages in retaining silicon as a photodetector material. These include the fact that silicon technology is well developed, the material has a greater inherent sensitivity than other materials being explored, and it permits integrating the detector with other elements on the same chip. It is possible to construct silicon photodetectors which are responsive to wavelengths of 1.0-1.1 .mu.m and therefore permit lower fiber attenuation while retaining silicon as the detector material. This construction basically involves making the optical path length longer than the diode depletion depth to accommodate the absorption requirements of the longer wavelengths. This can be accomplished, for example, by illuminating the detector at an edge so that light travels parallel to the device surface. This defines the detectable light absorbing region by the length of the depletion region which is considerably greater than the depth. However, since the thickness of the depletion region in the detector is very small, the vertical alignment of the fiber becomes critical. Alignment by normal mechanical means therefore becomes very difficult.
It is therefore a primary object of the invention to provide a silicon detector structure which is responsive to light at long wavelengths and includes means for precise alignment of the fiber with the detectable light absorbing region of the detector.