Realization of High-Hole Mobility and Low-Hole Concentration in Polycrystalline Ge Layers By Hydrogen Passivation
Authors: Nozawa Koki , Suemasu Takashi , Toko Kaoru 
Published: December 2024

Introduction

Polycrystalline Ge thin films are highly promising for next-generation semiconductor devices, such as thin film transistors and solar cells. To optimize their grain size and mobility, we meticulously controlled the density of amorphous Ge, a key precursor for solid-phase growth, by adjusting the deposition temperature (T
d)[1,2]. Despite these advances, the synthesized films still exhibit high concentrations of defect-induced acceptors (hole concentration p > 1017 cm−3), presenting significant challenges in Fermi level control, crucial for device applications. In this study, we aimed to synthesize low-defect polycrystalline Ge thin films through hydrogen passivation of solid-phase crystallized Ge. We achieved a significant reduction in p while maintaining high mobility, utilizing hydrogen plasma treatment on large-grain polycrystalline Ge films under optimized conditions.

Experimental Procedures

Ge was deposited onto quartz glass substrates to form 100 nm thick amorphous Ge films. These were annealed at 450 ℃ in a nitrogen atmosphere for solid-phase crystallization. Afterward, the polycrystalline Ge films underwent hydrogen plasma treatment at different powers (W
P = 150, 300, 450 W) to facilitate hydrogen diffusion. Vacuum post-annealing (PA) was then performed to further enhance hydrogen activation.

Results and Discussion

We examined how hydrogen plasma irradiation time (t
H) affects the electrical properties of samples deposited at room (T
d = 50 ℃) and elevated temperatures (T
d = 125 ℃) for each W
P setting. Figures 1(a) and (b) show that the Ge layer's grain size after annealing varies significantly with T
d; the T
d = 125 ℃ sample had a grain size over an order of magnitude larger than the T
d = 50 ℃ sample. Figures 1(c) and (d) reveal the plasma treatment's impact on electrical properties: hydrogen introduction decreased p under both conditions. The T
d = 50 ℃ samples saw a reduction in p to 5.0 × 1017 cm−3, while the T
d = 125 ℃ samples progressively lowered p with t
H to a minimum of 1.2 × 1015 cm−3. These findings highlight a limit to defect compensation through plasma treatment and emphasize the crucial role of initial Ge film quality in achieving low p-values. Furthermore, an increase in W
P correlated with a reduction in p, likely due to deeper hydrogen penetration into the Ge matrix. Additionally, μ was impacted differently in each scenario: in the T
d = 50 ℃ sample, μ decreased due to hydrogen passivation. In contrast, for the T
d = 125 ℃ samples, μ initially decreased following plasma irradiation but subsequently increased with extended t
H. Notably, a plasma treatment at W
P = 300 W for 20 minutes achieved a balance, maintaining high μ (155 cm² V−1 s−1) while also achieving a low p (2.3 × 1015 cm−3).

We also applied low-temperature PA to samples that had undergone hydrogen plasma treatment. Figure 2(a) reveals that PA at 200 ℃ resulted in further reductions in p and significant enhancements in μ. Conversely, PA at 300 ℃ reversed the improvements, aligning p and μ values with those seen prior to plasma treatment. These observations suggest that PA at appropriate temperatures promotes the passivation of acceptor defects and grain boundary trapping, whereas higher PA temperatures facilitate hydrogen desorption from Ge.

We standardized the PA temperature at 200 ℃ and varied t
PA (Figure 2(b)). An increase in t
PA initially led to a decrease in p, which then began to rise, while μ initially rose and subsequently fell. This dynamic suggests an equilibrium between the effects of defect passivation and hydrogen desorption. For a t
PA of 4 hours, we achieved optimal properties with a low p (4 × 1014 cm−3) and high μ (370 cm² V−1 s−1), indicating an effective balance between these critical parameters.

Conclusion

Figure 3 presents a comparative analysis of the electrical properties of undoped polycrystalline Ge, GeSn, and SiGe thin films, each less than 150 nm in thickness, grown on insulating substrates utilizing various deposition techniques. Universally, polycrystalline Ge layers demonstrate p exceeding 1017 cm−3 across all applied methods, attributable to intrinsic acceptor defects. Adding minor Sn concentrations to Ge modestly improves characteristics, reducing p and enhancing μ. Meanwhile, increasing Si in SiGe films—especially over 50%—substantially lowers p but significantly diminishes μ due to increased carrier effective mass and grain boundary barriers. Remarkably, the polycrystalline Ge layers developed in this study not only exhibited the lowest p observed in polycrystalline Ge-based thin films but also maintained enhanced μ. This constitutes a significant advancement, considerably enhancing the electrical properties traditionally associated with conventional Ge thin films and represents a pivotal development in semiconductor material engineering.

Figure 1