In a wide range of electronic and optoelectronic applications, there is required an arrangement of one or more semiconducting material layers that are preferably crystalline, herein defined as polycrystalline or monocrystalline. But for many such applications, the device structure employing the material layers cannot accommodate the high temperatures required to produce crystalline layers. For example, in the integration of photonic devices with integrated circuits, it is desired to integrate CMOS electronics with CMOS-compatible photodetectors and modulators operating in the C telecommunications band of 1520 nm-1560 nm. Germanium is particularly well-suited for such optoelectronics devices, but the growth of a crystalline Ge layer on, e.g., a single-crystal Si substrate, conventionally requires a growth temperature above 600° C. by an epitaxial process. Back-end production of Ge optical devices, after CMOS circuitry fabrication, dictates the use of low processing temperatures, e.g., ≦450° C., as well as non-epitaxial growth techniques on an amorphous material surface, for photonic device integration with CMOS circuitry. At such low temperatures, a Ge layer formed by conventional methods is characterized by a small-grain polycrystalline morphology, not the single-crystal morphology that is characteristic of high-temperature epitaxial growth.
It has been suggested to employ the resulting small-grain polycrystalline germanium, rather than single-crystal germanium, for photodetector fabrication in an optoelectronic system, but such devices have been demonstrated to suffer from the high defect density that is characteristic of polycrystalline Ge. The small-grain Ge that is conventionally produced at low temperatures therefore does not enable the required electronics and photonics integration.