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.