Lowering Metal–Ge Contact Resistance with ZnO Interlayers and Argon Plasma: Achieving Ohmic Al Contacts on n‑Ge

Background

Germanium offers superior carrier mobilities compared to silicon, making it a prime candidate for advanced MOSFETs [1, 2]. While progress in strained Ge channels, surface passivation, and source/drain contacts has led to ultra‑scaled Ge p‑FinFETs that outperform Si counterparts [3–11], n‑channel Ge devices still struggle with poor interface quality, limited n‑type dopant activation, and strong FLP that raises the Schottky barrier height to ~0.5 eV for most metals on n‑Ge [13–15].

Inserting a thin IL such as TiO₂ [16] or ZnO [17] can depin the Fermi level by blocking metal‑induced gap states (MIGS) [18, 19, 20]. ZnO is particularly attractive because it has a small conduction band offset with Ge, enabling a thinner depletion region and potentially lower R_c than TiO₂ (which has a positive offset) [16]. Additionally, ZnO can be readily n‑doped via oxygen vacancies (V_o), further reducing the depletion width. Conventional vacancy creation by nitrogen annealing risks interdiffusion and dopant migration, necessitating a low‑temperature approach [23–25].

This work explores FLP mitigation using atomic layer deposition (ALD)‑grown ZnO ILs and investigates the impact of Ar plasma treatment on the contact resistance of Al/ZnO/n‑Ge.

Methods

Metal contacts were fabricated on lightly doped (≈3 × 10¹⁶ cm⁻³) and heavily doped n‑Ge(001) wafers. The latter were produced by 30 keV P⁺ implantation (1 × 10¹⁵ cm⁻²) followed by 600 °C, 60 s rapid thermal annealing. After cleaning with deionized water and dilute HCl, the wafers were transferred to an ALD chamber (Beneq TSF‑200) to deposit ZnO at 150 °C using DEZn and H₂O precursors. Three ZnO thicknesses (1, 2, 3 nm) were examined; thicknesses were verified by spectroscopic ellipsometry (SE). Al contacts were sputtered and patterned by lift‑off. Some ZnO films received Ar plasma treatment (50 W, 60 sccm, 45 s). Contact resistance was extracted using the circular transmission line method (CTLM); exposed ZnO was fully etched to isolate devices [16]. Electrical measurements employed a Keithley 4200 SCS. Structural and chemical analyses used HRTEM, XPS, and UV‑VIS spectroscopy (PerkinElmer LAMBDA 950) to determine the band gap of ZnO.

Results and Discussion

Material Characterization of Al/ZnO/n‑Ge

Figure 1 shows XPS valence band spectra for bulk ZnO, ZnO/Ge interface, and pure Ge, enabling determination of the valence band offset (VBO) of 2.33 eV. Combined with the ZnO band gap of 3.21 eV (Fig. 2a), the conduction band offset (CBO) at ZnO/Ge is 0.22 eV (Fig. 2b), confirming effective FLP suppression. HRTEM (Fig. 3) reveals a uniform, amorphous 3 nm ZnO layer that conforms to the Al and Ge surfaces, with a thin GeO_x interfacial layer (<3 nm) formed at the deposition temperature of 150 °C.

Ar plasma treatment increases the density of oxygen vacancies, as evidenced by the shift of the O 1s peak to lower binding energy in XPS (Fig. 4). This vacancy enrichment reduces the tunneling barrier and series resistance at the Al/ZnO interface, directly contributing to lower R_c [36]. Post‑plasma VBO decreases by 0.05 eV (Fig. 5), indicating a CBO of 0.17 eV and further FLP mitigation.

Electrical Performance of Al/ZnO/n‑Ge Contacts

Current‑voltage measurements (Fig. 6a) show that Al/n‑Ge devices without ZnO exhibit rectifying behavior due to FLP, whereas Al/ZnO/n‑Ge devices display improved reverse current and reduced barrier height. A 2 nm ZnO thickness balances MIGS suppression and series resistance, yielding the optimal performance. Ar plasma treatment of the 2 nm ZnO layer enhances both forward and reverse currents (Fig. 6b), confirming the role of vacancy‑induced doping.

Ohmic behavior is achieved for Al/2 nm ZnO/n‑Ge across a range of plasma exposure times (15–60 s). The total resistance R_tot decreases with plasma duration, reaching a specific contact resistivity of 3.66 × 10⁻³ Ω cm² after 45 s (Fig. 8). The sheet resistance of the Ge channel remains unchanged, indicating that the plasma selectively dopes ZnO rather than the Ge substrate.

For heavily doped n⁺-Ge, the Al/2 nm ZnO contact (plasma‑treated 45 s) exhibits excellent ohmic behavior (Fig. 9). Extracted values are a sheet resistance of 64 Ω/□ and a specific contact resistivity of 2.86 × 10⁻⁵ Ω cm², outperforming previously reported metal/Ge contacts such as Ni/GeSn, Ni/Ge, and Ti/TiO₂/GeO₂/Ge [40–43]. The superior performance is attributed to the thin, highly doped ZnO IL and the reduced FLP.

Conclusions

Insertion of a thin ZnO IL effectively depins the Fermi level in Al/ZnO/n‑Ge structures, as confirmed by XPS band alignment. Ar plasma treatment further increases oxygen vacancy concentration, enhancing n‑type doping of ZnO and dramatically reducing contact resistance. Ohmic Al contacts are achieved on both lightly and heavily doped n‑Ge, with specific resistivities of 3.66 × 10⁻³ Ω cm² (n‑Ge) and 2.86 × 10⁻⁵ Ω cm² (n⁺-Ge) when ZnO is plasma‑treated at 50 W for 45 s.