High‑Performance Germanium pMOSFETs via Ozone‑Induced GeOₓ Passivation: A Comparative Analysis with Plasma Post‑Oxidation

Background

Conventional CMOS devices are nearing their physical limits, making it difficult to boost performance with silicon alone. Germanium, with its higher carrier mobility, has emerged as a promising channel material, but achieving a low‑density interface state (D_it) remains a challenge due to the instability of GeO₂ and dangling bonds. Numerous strategies have been explored to passivate the Ge/dielectric interface, such as pre‑depositing silicon monolayers or engineering self‑passivation via intentional GeO₂ growth. Oxidation techniques—including high‑pressure oxidation, ozone, H₂O plasma, and electron‑cyclotron‑resonance (ECR) plasma—have been employed to reduce D_it and enhance thermal stability.

Recent studies have demonstrated high‑performance Ge MOSFETs by post‑oxidation through an Al₂O₃/Ge interface. For example, a Ge CMOS inverter on Ge‑on‑insulator (GeOI) used rapid thermal annealing in pure O₂ to grow GeO_x after a 1‑nm Al₂O₃ layer. Ozone oxidation, which is more reactive than O₂, enables lower‑temperature growth, and has been shown to improve GeO_x thickness control and device performance. However, a systematic comparison between ozone and plasma post‑oxidation on the same Al₂O₃/n‑Ge interface has been lacking.

In this work, we fabricate Ge pMOSFETs using both OPO and PPO on Al₂O₃/n‑Ge interfaces and compare their electrical characteristics. All other process steps are identical. The influence of Al₂O₃ block‑layer thickness and the underlying mobility‑degradation mechanisms are also examined. Our results demonstrate that OPO is a superior passivation method for future Ge MOSFET technology.

Methods

Ge pMOSFETs were fabricated on 4‑inch n‑Ge (001) wafers (resistivity 0.14–0.183 Ω cm). After a dilute HF (1:50) clean, a ~1 nm Al₂O₃ film was deposited at 300 °C by plasma‑enhanced ALD (PEALD) using trimethylaluminium (TMA) and H₂O. The thin Al₂O₃/GeO₂ interfacial layer (denoted AlO_x/GeO_x) was then subjected to either 60 s of oxygen plasma (PPO) via a remote plasma source or to a 20‑min ozone (OPO) treatment in a 50 % O₃/O₂ mixture. Immediately after oxidation, a 5‑nm HfO₂ gate dielectric was deposited at 300 °C by ALD (60 cycles, using TDMAHf and H₂O). A 100‑nm TaN gate metal was sputtered, followed by lithography and etching. Source/drain regions were self‑aligned BF₂⁺ implanted (20 keV, 1 × 10¹⁵ cm⁻²), capped with 20 nm Ni, and activated by rapid thermal annealing at 450 °C for 30 s.

a Process flow for Ge pMOSFET fabrication with three passivation methods. b Schematic and c SEM image of the fabricated transistor.

Cross‑sectional TEM images reveal the gate stack TaN/HfO₂/AlO_x/GeO_x/Ge for PPO and OPO. In the PPO sample, a distinct AlO_x/GeO_x interlayer separates HfO₂ from Ge, whereas the OPO sample shows a more diffuse interface, consistent with literature reports on GeO₂ thickness.

Cross‑sectional TEM of the high‑k/metal gate stack with (a) OPO‑formed and (b) PPO‑formed AlO_x/GeO_x interfacial layers on n‑Ge (001).

Results and Discussion

Device characterization was performed with a Keithley 4200‑SCS system. All transistors have a 3 µm gate length. The PPO device (wafer A) shows higher saturated drain current but also a higher OFF‑state current compared to the OPO devices (wafers B and C). Importantly, the OPO devices operate in enhancement mode, whereas the PPO device remains in depletion mode, indicating residual p‑type character due to a higher D_it.

a I_DS–V_GS and b I_DS–V_DS for PPO (wafer A) and OPO (wafers B, C) Ge pMOSFETs.

Statistical analysis of the I_ON/I_OFF ratio and sub‑threshold swing (SS) confirms that OPO devices achieve up to four orders of magnitude higher I_ON/I_OFF and a steeper SS, reflecting a cleaner interface. Increasing the Al₂O₃ block thickness from 10 to 15 cycles improves the ON‑state current but reduces the I_ON/I_OFF ratio due to a larger equivalent oxide thickness (EOT).

Statistical plots of a SS and b I_ON/I_OFF for PPO versus OPO devices.

XPS measurements on dummy samples (without HfO₂ and TaN) reveal that PPO produces a GeO_x layer dominated by Ge¹⁺ and Ge³⁺ species, while OPO yields a higher oxidation state distribution, reducing the Ge¹⁺ component and indicating a more complete GeO₂ formation.

a Ge 3d XPS spectra for Al₂O₃/GeO_x/Ge under PPO, OPO‑10, and OPO‑15 conditions. b Peak fitting of the Ge 3d spectra.

Gate‑to‑source C‑V measurements at 1 MHz show that the OPO process results in a higher EOT compared to PPO, with the 15‑cycle OPO (wafer C) exhibiting the largest EOT. This is attributed to the thicker Al₂O₃ blocking layer. Despite the higher EOT, the OPO devices maintain superior SS and mobility.

Gate‑to‑source C‑V of PPO (wafer A) and OPO (wafers B, C) devices.

Total resistance (R_T) versus gate length analysis indicates that PPO devices have lower series and channel resistances, consistent with the C‑V data. However, the higher R_SD and R_CH for OPO devices are compensated by improved mobility.

R_T versus L_G for the three device types.

Effective hole mobility (μ_eff) was extracted from the total‑resistance slope. OPO devices exhibit higher peak μ_eff than PPO, with wafer C (15‑cycle OPO) achieving 283 cm²/Vs. Compared with literature, this represents a 2.37× improvement over silicon’s universal mobility at an inversion charge density of 5 × 10¹² cm⁻². The mobility advantage is attributed to reduced Coulomb scattering and better interface quality.

μ_eff versus Q_inv for PPO and OPO devices. The 15‑cycle OPO (wafer C) reaches a peak μ_eff of 283 cm²/Vs.

Conclusions

We have demonstrated that Ge pMOSFETs with GeO_x passivation fabricated via ozone post‑oxidation outperform those using plasma post‑oxidation. OPO yields higher I_ON/I_OFF ratios, steeper sub‑threshold swings, and superior peak μ_eff. Increasing the Al₂O₃ blocking layer thickness in the OPO process improves mobility at the cost of a larger EOT. These findings confirm that OPO is an effective passivation strategy for high‑quality Ge/oxide interfaces, making it a promising candidate for next‑generation Ge MOSFET technology.

Abbreviations

Al₂O₃:

Aluminum oxide

ALD:

Atomic layer deposition

BF₂⁺:

Boron fluoride ion

EOT:

Equivalent oxide thickness

Ge:

Germanium

GeO_x:

Germanium oxide

HF:

Hydrofluoric acid

HfO₂:

Hafnium dioxide

TEM:

Transmission electron microscope

MOSFETs:

Metal‑oxide‑semiconductor field‑effect transistors

OPO:

Ozone post‑oxidation

PPO:

Plasma post‑oxidation

Q_inv:

Inversion charge density

SS:

Subthreshold swing

XPS:

X‑ray photoelectron spectroscopy

μ_eff:

Effective hole mobility