Solid-phase crystallization of ultra-thin amorphous Ge layers on insulators

Abstract

A simple method to form ultra-thin (< 20 nm) semiconductor layers with a higher mobility on a 3D-structured insulating surface is required for next-generation nanoelectronics. We have investigated the solid-phase crystallization of amorphous Ge layers with thicknesses of 10−80 nm on insulators of SiO2 and Si3N4. We found that decreasing the Ge thickness reduces the grain size and increases the grain boundary barrier height, causing the carrier mobility degradation. We examined two methods, known effective to enhance the grain size in the thicker Ge (>100 nm). As a result, a relatively high Hall hole mobility (59 cm2/Vs) has been achieved with a 20-nm-thick polycrystalline Ge layer on Si3N4, which is the highest value among the previously reported works.

Interfacial Nucleation Control in Amorphous GeSn Thin Films Using Bilayer Structure

“…Figure d shows that the grain size increases with increasing x and subsequently begins to decrease. This trend is similar to that of thick GeSn layers (≥100 nm) and is explained by the balance between growth enhancement by small amounts of Sn and the subsequent growth inhibition by precipitated Sn exceeding the solid solution limit. , The grain size reached 12 μm, which is larger than the grain size of most poly-Ge (Sn) thin films (<100 nm thickness) formed by SPC. ,− Figure e shows that p decreases with increasing x and subsequently increases, reflecting the reduction in grain boundaries responsible for acceptor defects and the subsequent passivation of acceptor defects by Sn. , μ p peaked at x = 3%, although the grain size was not the maximum. This behavior is similar to that of thick GeSn layers (≥100 nm) and is attributed to the reduction in the energy barrier height at grain boundaries due to the amount of Sn exceeding the solid solution limit .…”

“…In the bottom structure, the incubation time and grain size are comparable to that of the conventional single-layer structure, suggesting a dominance of heterogeneous nucleation at the substrate interface in the single-layer structure. 2,40,41 The middle structure exhibited a long incubation time owing to suppressed nucleation at the substrate and surface; however, it provided an extremely slow lateral growth rate. Reflecting the balance between nucleation frequency and lateral growth, the top structure exhibited the largest grain size.…”

“…The smaller grain size in the SPC of thinner Ge films is possibly due to more pronounced heterogeneous nucleation at the substrate interface. 2,40,41 In this study, we investigated the effects of the deposition temperature and Sn doping in a-Ge layers on the nucleation frequency during SPC. Based on these findings, we achieved selective nucleation at the film surface by bilayering the a-GeSn layer deposited at different temperatures.…”

“…Among the aforementioned methods, SPC has made remarkable progress in recent years. Densification and elemental doping in amorphous Ge (a-Ge) precursors have dramatically increased the grain size of the poly-Ge layers after SPC. − Reflecting the reduced grain boundary scattering, high Hall carrier mobilities (electron: 380 cm 2 V –1 s –1 , hole: 690 cm 2 V –1 s –1 ) have been achieved. , However, thicker (>100 nm) Ge films were essential for achieving such high carrier mobilities because thinner Ge films exhibit poorer crystallinity such as smaller grain size. − Furthermore, acceptor defects in Ge complicate the low carrier concentration of poly-Ge layers. , These features hamper TFT operation because the off-current reduction requires full depletion, which can be achieved by a thin Ge film and low carrier concentration . We have achieved p-channel accumulation-mode TFTs using 50 nm thick GeSn .…”

“…Although their field-effect mobility was the highest among low-temperature poly-Ge TFTs, both low carrier concentration and high mobility in thinner films are highly desirable to fully exploit the potential of Ge-TFTs. The smaller grain size in the SPC of thinner Ge films is possibly due to more pronounced heterogeneous nucleation at the substrate interface. ,, In this study, we investigated the effects of the deposition temperature and Sn doping in a-Ge layers on the nucleation frequency during SPC. Based on these findings, we achieved selective nucleation at the film surface by bilayering the a-GeSn layer deposited at different temperatures.…”