High-electron-mobility (370 cm2/Vs) polycrystalline Ge on an insulator formed by As-doped solid-phase crystallization

High-electron-mobility polycrystalline Ge (poly-Ge) thin films are difficult to form because of their poor crystallinity, defect-induced acceptors and low solid solubility of n-type dopants. Here, we found that As doping into amorphous Ge significantly influenced the subsequent solid-phase crystallization. Although excessive As doping degraded the crystallinity of the poly-Ge, the appropriate amount of As (~1020 cm−3) promoted lateral growth and increased the Ge grain size to approximately 20 μm at a growth temperature of 375 °C. Moreover, neutral As atoms in poly-Ge reduced the trap-state density and energy barrier height of the grain boundaries. These properties reduced grain boundary scattering and allowed for an electron mobility of 370 cm2/Vs at an electron concentration of 5 × 1018 cm−3 after post annealing at 500 °C. The electron mobility further exceeds that of any other n-type poly-Ge layers and even that of single-crystal Si wafers with n ≥ 1018 cm−3. The low-temperature synthesis of high-mobility Ge on insulators will provide a pathway for the monolithic integration of high-performance Ge-CMOS onto Si-LSIs and flat-panel displays.


Results and Discussion
We examined the C As dependence of the crystal quality of Ge using Raman spectroscopy (JASCO NRS-5100, spot diameter 20 μm, wavelength 532 nm). The samples with T anneal = 450 °C exhibit sharp peaks near 300 cm −1 , which correspond to crystalline (c-) Ge-Ge bonding in the whole C As range ( Fig. 2(a)). As shown in Fig. 2(b), annealing at 375 °C for 150 h crystallized the samples with C As ≤ 2.8 × 10 20 cm −3 , but not those with C As > 2.8 × 10 20 cm −3 . These results mean that excessive As lowers the crystallization rate. To analyze the Raman shift and the full width at half maximum (FWHM) of crystalline Ge (c-Ge) peaks, each spectrum was fitted as representatively shown in Fig. 2(c). The peak is fitted well enough to correctly calculate the FWHM and peak position. The Raman shift and FWHM results are summarized in Fig. 2(d). All peaks shifted to lower wavenumbers than that of a single-crystal bulk-Ge substrate, originating from the tensile strain. The peak shifts are almost constant with respect to C As while the peaks for T anneal = 450 °C shifted to the lower wavenumber than that for T anneal = 375 °C. The Raman shift had small variation (<0.5%), and therefore, seems to be the dominant difference with respect to the annealing temperature. This behavior suggests that the strain likely originates from the difference in the thermal expansion coefficients between Ge and the SiO 2 substrate. The FWHM is almost constant for C As ≤ 5.9 × 10 20 cm −3 and significantly increases for C As > 5.9 × 10 20 cm −3 . This indicates that excessive As negatively influences SPC-Ge crystallinity, as will become clear in the later-mentioned electron backscattering diffraction analyses. Thus, the Raman studies revealed that C As strongly influences the growth rate and crystal quality of SPC-Ge.
The C As dependence of the growth rate was evaluated using in situ optical microscopy during annealing. Figure 3(a) shows the typical growth evolution of SPC. Here we chose T anneal = 400 °C because it allowed for both domain visibility and practical observation time in a wide range of C As . The micrographs indicate that Ge nucleation occurs and the domain grows laterally with increasing annealing time. Eventually, the entire surface is covered with c-Ge for each sample, indicating that the SPC (lateral growth of domains) is saturated. The domain growth rate and saturated domain size vary significantly with C As (Fig. 3(b)). The medium C As sample (C As = 1.2 × 10 20 cm −3 ) exhibited the highest growth rate and the largest domain size among the three samples. Generally, impurity doping promotes semiconductor atom migration and enhances the recrystallization rate of amorphous films 43 . Conversely, excessive As reduces both nucleation and lateral growth rates ( Fig. 3(b)). This is likely because segregation of excessive As suppressed nucleation and growth. These behaviors have also been reported in Sn-and Sb-doped SPC-Ge 42,44,45 . Therefore, As doping in a-Ge greatly influences nucleation and lateral growth in subsequent SPC.
The inverse pole figures (IPFs) with grain boundaries in Ge were obtained using electron backscattering diffraction analyses (JEOL JSM-7001F with the TSL OIM analysis attachment). The grain size dramatically varies with C As (Fig. 4(a-d)). The average grain size increases with increasing C As and then begins to decrease ( Fig. 4(e)). This behavior agrees with that of the eventual domain size in optical micrographs (Fig. 3). Additionally, the grain size is significantly degraded by excessive As (C As = 1.8 × 10 21 cm −3 ). This behavior well accounts for the results www.nature.com/scientificreports www.nature.com/scientificreports/ of the Raman FWHM ( Fig. 2(d)). The lower T g provides a larger grain size, which agrees with the general tendency of SPC-Ge reflecting the reduction of nucleation frequency 17,39 . The sample with C As = 1.2 × 10 20 cm −3 and T g = 375 °C exhibited a grain size of approximately 20 μm, which is the largest among poly-Ge formed by SPC.
The electrical properties of the As-doped SPC-Ge layers were evaluated using Hall-effect measurements with the van der Pauw method (Bio-Rad HL5500PC). All samples showed n-type conduction owing to the self-organizing activation of As during SPC. Electron concentration n and electron mobility μ n depend on both T anneal and C As (Fig. 5(a,b)). We note that the maximum variation between samples prepared under the same conditions is approximately 20% in n and 5% in μ n , while the measurement variation was smaller than the marks in the figures for each sample. We first discuss the T anneal dependence of the electrical properties. Before PA, n for T anneal = 450 °C is higher than that for T g = 375 °C in the whole C As range (Fig. 5(a)). This behavior is consistent with the fact that higher temperatures cause higher solid solubility and activation of As in Ge 36 . T anneal = 450 °C exhibits a higher μ n than T anneal = 375 °C (Fig. 5(b)), whereas the grain size shows the opposite trend (Fig. 4(e)). According to the carrier conduction model proposed by Seto for polycrystalline semiconductors 46 , the energy barrier of the grain boundary E B decreases as the carrier density increases. The T anneal dependence of μ n is likely attributed to the fact that T anneal = 450 °C has higher n and therefore lower E B than T anneal = 375 °C. After PA at 500 °C, n for T anneal = 450 and 375 °C increases to a similar value for each C As (Fig. 5(a)). These results suggest that the activation rate of As in Ge is determined by the maximum process temperature. μ n is improved by PA for both T anneal (Fig. 5(b)). In particular, μ n for T anneal = 375 °C is higher than that of T anneal = 450 °C, which reflects the grain size (Fig. 4(e)). After PA, both n and μ n are maximized at around C As = 1.2 × 10 20 cm −3 where the grain size is maximum (Fig. 4(e)). The C As dependence of n is likely because the larger grain size provides the lower defect-induced acceptors and/or the less As segregation at grain boundaries. Although the C As dependence of μ n is consistent with the tendency of grain size, the dramatic improvement of μ n from C As = 1.0 × 10 19 cm −3 to C As = 1.2 × 10 20 cm −3 is difficult to explain only in terms of grain size.  www.nature.com/scientificreports www.nature.com/scientificreports/ To clarify the behavior, we quantified the trap-state density Q t in the grain boundaries and E B using the following equations 46 :  (d) Trap-state density Q t and energy barrier height E B of the Ge grain boundary as a function of C As . Here n and μ n were averaged over five measurements for each sample, where the variation was smaller than the marks. where T is the absolute temperature, L is the grain size, q is the elementary charge, m * is the effective mass, k is the Boltzmann constant and ε is the dielectric permittivity. The Arrhenius plot of µ n T 1/2 makes an almost-downward-sloping straight line for the whole C As region (Fig. 5(c)). Q t decreases with increasing C As , which suggests that As atoms passivate the grain boundary traps (Fig. 5(d)). Therefore, E B dramatically decreases by As doping at C As = 1.2 × 10 20 cm −3 , which reflects both the decrease of Q t and increase of n. On the other hand, Q t slightly increases with PA. This behavior is likely because PA increases lattice substitutional As and therefore reduces the extent to which As passivates the grain boundary. After PA, E B for C As = 1.2 × 10 20 cm −3 does not change, which reflects the balance between Q t and n, while μ n improves slightly (Fig. 5(b)). Considering that PA improves the activation rate of As, the μ n improvement is likely due to the decrease of carrier scattering by neutral As. The n and µ n values reached 5 × 10 18 cm −3 and 370 cm 2 /Vs, respectively. The µ n value further exceeds that of any other n-type poly-Ge layers formed on insulators and even that of single-crystal Si wafers with n ≥ 10 18 cm −3 (Fig. 6).

conclusions
As doping into a-Ge significantly influenced the subsequent SPC. Although excessive As doping degraded the crystallinity of poly-Ge, the appropriate amount of As (~10 20 cm −3 ) promoted the SPC and increased the Ge grain size. By combining slow annealing at low temperature (375 °C), the grain size reached approximately 20 μm, which is the largest among SPC-Ge. Moreover, neutral As atoms in Ge reduced Q t (2 × 10 12 cm −2 ) and E B (12 meV). These properties reduced grain boundary scattering and allowed for μ n of 370 cm 2 /Vs, which is the highest among n-type poly-Ge formed on insulators. These findings will provide a means for the monolithic integration of high-performance Ge-CMOS onto Si-LSIs and flat-panel displays.