Ge pMOSFETs with GeOx Passivation Formed by Ozone and Plasma Post Oxidation
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A comparison study on electrical performance of Ge pMOSFETs with a GeOx passivation layer formed by ozone post oxidation (OPO) and plasma post oxidation (PPO) is performed. PPO and OPO were carried out on an Al2O3/n-Ge (001) substrate followed by a 5-nm HfO2 gate dielectric in situ deposited in an ALD chamber. The quality of the dielectric/Ge interface layer was characterized by X-ray photoelectron spectroscopy and transmission electron microscopy. The PPO treatment leads to a positive threshold voltage (VTH) shift and a lower ION/IOFF ratio, implying a poor interface quality. Ge pMOSFETs with OPO exhibit a higher ION/IOFF ratio (up to four orders of magnitude), improved subthreshold swing, and enhanced carrier mobility characteristics as compared with PPO devices. A thicker Al2O3 block layer in the OPO process leads to a higher mobility in Ge transistors. By comparing two different oxidation methods, the results show that the OPO is an effective way to increase the interface layer quality which is contributing to the improved effective mobility of Ge pMOSFETs.
KeywordsGermanium Passivation Ozone Plasma Post oxidation Metal-oxide-semiconductor field-effect transistor (MOSFET)
Atomic layer deposition
Boron fluoride ion
Equivalent oxide thickness
Transmission electron microscope
Metal-oxide-semiconductor field-effect transistors
Ozone post oxidation
Plasma post oxidation
Inversion charge density
X-ray photoelectron spectroscopy
Effective hole mobility
With conventional complementary metal-oxide-semiconductor (CMOS) devices approaching its physical limit, performance enhancement is hard to realize for high-speed semiconductor devices with silicon (Si) as the channel material. Replacing substrate or channel material with other material with high mobility is an imperative option. Germanium (Ge) has been considered as a promising alternative channel material owing to higher carrier mobility than that of Si. The MOSFET usually needs a high-quality oxide/semiconductor interface to reach high effective mobility. However, for quite a long history, Ge MOSFETs suffered from the high interface state density (Dit) caused by the poor thermal stability of GeO2 and dangling bonds . Thus, plenty of research has been carried out on Ge interface passivation.
Several approaches to achieving a high-quality Ge/dielectric interface layer have been reported, such as Si passivation by uniformly depositing several Si monolayers on Ge substrate before dielectric epitaxy or self-passivation by forming GeO2 on purpose [2, 3]. In order to form a high-quality GeO2 layer, there are many oxidation processes to reduce Dit and improve thermal stability including high-pressure oxidation , ozone oxidation , H2O plasma , and electron cyclotron resonance (ECR) plasma post oxidation .
In recent years, plenty of works have been reported that high-performance Ge MOSFET can be realized by post oxidation through Al2O3/Ge interface. In 2014, a Ge CMOS inverter was realized on a Ge-on-insulator (GeOI) substrate with GeOx grown by rapid thermal annealing in pure oxygen ambient after 1 nm Al2O3 was deposited on Ge . In ref. , Ge pMOSFETs and nMOSFETs with GeOx passivation were fabricated with oxygen plasma post oxidation and temperature dependence of GeOx thickness and electrical performance were also discussed. Thermal oxidation of Ge by ozone can be performed at a lower temperature, for ozone is more reactive than oxygen . The impact of temperature on GeOx thickness grown by ozone on Ge surface was demonstrated. Ge pMOSFETs with GeOx passivation fabricated by ozone post oxidation was also reported .
In this work, Ge pMOSFETs with GeOx passivation are fabricated using ozone post oxidation (OPO) and oxygen plasma post oxidation (PPO) of the Al2O3/n-Ge interface. A comparison study on the electrical performance of Ge pMOSFETs with OPO and PPO is carried out. All the processes except passivation are precisely controlled to be the same. The post oxidation was carried out after the Al2O3 block layer deposition that is different from  in which the post oxidation was after HfO2 deposition. The mobility degeneration mechanism of Coulomb and roughness scattering is investigated. The impact of the thickness of the Al2O3 block layer on device performance is also discussed. Overall, we demonstrate that OPO is a promising passivation technique for future Ge MOSFET fabrication.
Results and Discussion
Calculated properties of Ge pMOSFETs in three passivation conditions
Dit(1012 cm–2 eV−1)
Key device performance of Ge pMOSFETs in this work vs. other published results with OPO
ION @ VDS = − 0.5 V
VGS − VTH = − 0.8 V (μA/μm)
ION/IOFF @ VDS = − 0.5 V
μeff @ peak (cm2/V × S)
μeff @ Qinv = 5 × 1012 cm−2 (cm2/V × S)
~ 0.9 × 103
~ 2.3 × 103
This work wafer C
~ 4.8 × 103
Ge pMOSFETs are realized with GeOx passivation, which is formed by OPO or PPO treatment of Al2O3/n-Ge in PEALD. The OPO devices exhibit the better transfer and output characteristics, the higher ION/IOFF ratio, the improved subthreshold swing, and the higher peak μeff compared to the PPO devices. For the 15-cycle OPO process, a thicker Al2O3 layer leads to a higher EOT value and an improved μeff in devices compared to the 10-cycle case. All the results in this work indicate that the OPO is an effective passivation way to achieve a high-quality Ge/dielectric interface and thus can be a promising candidate passivation technique for future Ge MOSFET fabrication.
The authors acknowledge support from the National Natural Science Foundation of China under Grant No. 61534004, 61604112, 61622405, 61874081, and 61851406.
Availability of Data and Materials
The datasets supporting the conclusions of this article are included in the article.
YX carried out the experiments and drafted the manuscript. GQH, YL, HL, YBW, and YX designed the experiments. GQH and YL helped to revise the manuscript. JPA and YH supported the study. All the authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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