Book Description

The integration of photonics with electronics has emerged as a leading platform for microprocessor technology and the continuation of Moore's Law. As electronic device dimensions shrink, electronic signals encounter crippling delays and heating issues such that signal transduction across large on-chip distances becomes increasingly more difficult. However, these issues may be mitigated by the use of photonic interconnects combined with electronic devices in electronic-photonic integrated circuits (EPICs). The electronics in proposed EPIC designs perform the logic operations and short-distance signal transmission, while photonic devices serve to transmit signals over longer lengths. However, the photonic devices are large compared to electronic devices, and thus the two types of devices would ideally exist on separate levels of the microprocessor stack in order to maximize the amount of silicon substrate available for electronic device fabrication. A CMOS-compatible back-end process for the fabrication of photonic devices is necessary to realize such a three-dimensional EPIC. Back-end processing is limited in thermal budget and does not present a single-crystal substrate for epitaxial growth, however, so high-quality crystal fabrication methods currently used for photonic device fabrication are not possible in back-end processing. This thesis presents a method for the fabrication of high-quality germanium single crystals using CMOS-compatible back-end processing. Initial work on the ultra-high vacuum chemical vapor deposition of polycrystalline germanium on amorphous silicon is presented. The deposition can be successfully performed by using a pre-growth hydrofluoric acid dip and by limiting the thickness of the amorphous silicon layer to less than 120 nm. Films deposited at temperatures of 350° C, 450° C, and 550° C show (110) texture, though the texture is most prevalent in growths at 450° C. Poly-Ge grown at 4500 C is successfully doped n-type in situ, and the grain size of as-grown material is enhanced by lateral growth over a barrier. Structures are fabricated for the growth of Ge confined in one dimension. The growths show faceting across large areas, in contrast to as-deposited poly-Ge, corresponding to enhanced grain sizes. Growth confinement is shown to reduce the defect density as the poly-Ge grows. When coalesced into a continuous film, the material grown from 1 D confinement exhibits a lower carrier density and lower trap density than as-deposited poly-Ge, indicating improved material quality. We measure an increased grain size from as-deposited poly-Ge to Ge grown from ID confinement. Single-crystal germanium is grown at 450° C from confinement in two dimensions. Such growths exhibit faceting across the entire crystal as well as the presence of E3 boundaries ({111} twins), with many growths showing no other boundaries. These twins mediate the growth of the crystal, as they serve as the points for heterogeneous surface nucleation of adatom clusters. The twins can form after the crystal nucleates and are strongly preferred in order to obtain appreciable crystal growth rates. We model the growths from the confining channels in order to find the optimum channel geometry for large, uniform, single-crystal growths that consistently emerge from the channel. The growths from 2D confinement show lower trap density than those from 1 D confinement, indicating a further enhancement of the crystal quality due to the increased confinement. This method of single-crystal growth from an amorphous substrate is extensible to any materials system in which selective non-epitaxial deposition is possible.