Structure and electrical properties of polycrystalline SiGe thin films

Van der Pauw test patterns were fabricated in poly-{dollar}rm Sisb{lcub}1-x{rcub}Gesb{lcub}x{rcub}{dollar} (x = 0.05 and 0.1) films deposited by CVD. The SiGe films were implanted with P at doping concentrations ranged from {dollar}2times10sp{lcub}18{rcub}{dollar} to {dollar}1.5times 10sp{lcub}19{rcub}{dollar}/cm{dollar}sp3.{dollar} I-V measurements were conducted to evaluate the room temperature resistivity and activation energy of conduction. Hall measurements were carried out to determine the electron concentration and mobility. All electrical measurement results suggested that a significant fraction of P atoms segregated to grain boundaries. The concentration of trapped phosphorus atoms, was derived from "resistivity adjustment", "fitting Seto activation energy expression", and from Hall measurement. In all cases, a similar value {dollar}(2{lcub}-{rcub}4)times10sp{lcub}18{rcub}{dollar}/cm{dollar}sp3{dollar} was obtained. The mobilities derived from Hall measurements ranged from 4 to 10 cm{dollar}sp2{dollar}/V{dollar}cdot{dollar}sec. The segregation of P to grain boundaries in {dollar}rm Sisb{lcub}0.87{rcub}Gesb{lcub}0.13{rcub}{dollar} films was studied quantitatively by STEM x-ray microanalysis. Analysis showed that segregation was an equilibrium process with a segregation energy of 0.28 eV/atom. The amount of P segregated to boundary sites was found to vary with boundary structure. On the other hand, Ge did not segregate to grain boundaries in {dollar}rm Sisb{lcub}0.87{rcub}Gesb{lcub}0.13{rcub}{dollar} films. Since it is known that Ge segregate to planar heterointerfaces, this result was unexpected and it was speculated that segregated P atoms might interfere with Ge. In undoped polycrystalline {dollar}rm Sisb{lcub}1-x{rcub}Gesb{lcub}x{rcub}{dollar} films {dollar}(0.02le x le0.31),{dollar} Ge segregation was also absent. SIMS depth profile analysis showed that {dollar}rm Sisb{lcub}1-x{rcub}Gesb{lcub}x{rcub}{dollar} films uniformly contained between 10{dollar}sp{lcub}19{rcub}{dollar} to {dollar}4times10sp{lcub}19{rcub}{dollar}/cm{dollar}sp3{dollar} H, the concentration of which decreased with increasing deposition temperature. It is conceivable that H suppresses Ge segregation to grain boundaries as H was reported to suppress the segregation of Ge to planar interfaces. Conventional TEM and HREM were used to study the microstructures in SiGe thin films for Ge fractions of 2%, 13% and 31% and polysilicon control film. The small but systematic difference were observed in grain size, twin density and degree of twin symmetry films with different Ge fractions. The result showed that the atomic structure model for twin I boundary in polysilicon also applied in polycrystalline SiGe, but that the length of repeat unit varied with Ge content. The texture of {dollar}rm Sisb{lcub}1-x{rcub}Gesb{lcub}x{rcub}{dollar} thin films, x = 0.02, 0.13 and 0.31 respectively, and texture in polysilicon control wafer were studied by x-ray diffraction and electron diffraction. The results revealed that {dollar}<{dollar}110{dollar}>{dollar} was the preferred orientation for all films investigated, and the intensity of the texture increased with the Ge fraction. The values of width of the fiber axis (deviation from fiber axis) in the SiGe films with different Ge fractions were very similar, but about 26% higher than the value of control polysilicon film. In all cases investigated, electron diffraction patterns recorded at different thickness of wedged-shape samples showed that the {dollar}<{dollar}110{dollar}>{dollar} orientation was the major preferred orientation during the whole film growth.