Characteristic Change of GeO2 / Ge Interface By Hf-Post Metallization Annealing
Authors: Fujiwara Haruka , Iwazaki Yoshitaka , Ueno Tomo 
Published: February 2020

Si has been widely used as primary semiconductor materials for electronic device fabrication. Due to the limitation of its electrical properties such as electron and hole mobilities, however, Ge would be one of the candidates for future electronic device materials because of its higher mobilities of both carieers. In addition, since Ge is the same Group IV semiconductor as Si, and physical as well as chemical properties are very similar, it can be considered that the manufacturing process of Si semiconductor devices can be followed, and the introduction cost can be suppressed. However, since GeO2 which composed of Ge-O chemical bonds is known to have water solubility and it reacts with Ge substrates at high temperature, it is well known that GeO desorption occurs at GeO2/Ge interface during higher temperature annealing process as well as during oxidation process itself, which causes deterioration of device characteristics. In this research, we attempted to improve interface characteristics by depositing Hf on the GeO2/Ge structure and applying heat treatment (PMA: Post Metallization Annealing).
P-type and n-type Ge (100) substrates were cleaned with acetone and ethanol, and native oxide films were removed with dilute-HF dip. GeO2/Ge structure with film thickness of about 20 nm was prepared by thermal oxidation at 500 °C for 30 minutes. A thin metal Hf film was deposited by the sputtering method, and heat treatment was performed at 300 °C for 20 minutes. As the electrode for electrical characteristics measurements, Al films were deposited by vapor deposition, and the interface characteristics and insulating property was evaluated by C-V, TDS and I-V measurement.
From C-V characteristics of GeO2 on both n-type and p-type Ge substrates with the measurement frequency of 1MHz, the injection type hysteresis, which means the existence of hole trap site in the GeO2 films adjacent to the GeO2/Ge interface, was observed in samples without PMA. By applying Hf-PMA, the hysteresis width has been drastically decreased. For C-V characteristics with changing the frequency of small AC signal during the measurements, on the other hand, less frequency dispersion can be seen from GeO2/n-Ge structure than that from GeO2/p-Ge samples. The difference of the calculated values of Dit distribution between GeO2/n-Ge and GeO2/p-Ge would be mainly due to the asymmetry of the Dit distribution in the bandgap of Ge, which is caused by the GeO desorption during the thermal oxidation at the GeO2/Ge interface. In fact, it is clearly shown that the decrease ratio in Dit value by the PMA process are almost the same between GeO2/n-Ge and GeO2/p-Ge.
From the TDS measurement, GeO desorption starts below 500 °C in samples without PMA, whereas two steps desorption can been seen in that with PMA. Especially, the desorption with higher temperature range start above 600 °C This indicates that it took more energy to release GeO, a slight amount of Hf buried the GeO2 film desorpted GeO by Hf-PMA and the bonding of Ge-O became stronger.
From the I-V measurement, we found that the leakage current was reduced and insulating property was improved by applying PMA. The insulating characteristics are more improved at positive bias for p-type and negative bias for n-type. This cause the band barrier of Metal/Oxide was more strengthened, the electron and the hole could not exceed the band barrier. Strengthening the Ge-O bond by applying PMA, the band barrier is also strengthened, and we consider that the leakage current is reduced.
It was confirmed that Hf-PMA is effective for both types of Ge substrate as a solution to reduce the Dit values due to the GeO desorption during the oxidation process at the GeO2/Ge interface. For further thinning in the future, it is necessary to optimize the deposition amount of Hf and the PMA temperature and time.