Investigation of Al-PMA Effect on Al2O3/GeOX/Ge Gate Stack
Authors: Nagatomi Yuta , Nagaoka Yuichi , Yamamoto Keisuke , Wang Dong , Nakashima Hiroshi 
Published: February 2020

To realize high mobility Ge channel metal-oxide-semiconductor field-effect transistor (MOSFET), formation of gate stack with low interface state density (D
it) is extremely important. Recently, it has been reported that Al2O3/GeOX/Ge gate stack fabricated by atomic layer deposition (ALD) and plasma post oxidation using electron cyclotron resonance (ECR) had obtained low D
it less than 1011 eV-1cm-2 [1]. On the other hand, our group has previously reported that Al introduction into SiO2/GeO2/Ge gate stack by post metallization annealing (Al-PMA) is effective to decrease both D
it and slow trap in the lower half of the band gap [2,3]. If Al-PMA is also effective on Al2O3/GeOX/Ge, it will be a useful method to improve MOSFET performance. In this study, we investigated Al-PMA effect on Al2O3/GeOX/Ge gate stack.

Ge-MOS capacitors were fabricated by the following procedure. First, a p-type (100) Ge substrate was dipped in dilute HF solution to remove native oxides. Then 1 nm-Al2O3 was deposited at 300°C using ALD. Subsequently, ECR plasma post oxidation was carried out at RT for 1 min. The oxidation conditions were microwave power of 500 W, gas pressure of 1.15×10-1 Pa, and O2/Ar flow rate of 3/18 sccm. Then 4.1 nm-Al2O3 and 1 nm-SiO2 were deposited using ALD and rf magnetron sputtering, respectively. Subsequently, post deposition annealing (PDA) was carried out at 400°C for 30 min, and 100 nm-Al as a gate electrode was deposited by vacuum evaporation. Al-PMA was carried out at 400°C for 30 min. Finally, the electrode was patterned with an area of 2.25×10-4 cm2. For a comparison, the MOS capacitor without Al-PMA was also prepared.

Figures 1 and 2 show C-V and J-V characteristics of two types of samples, respectively. The equivalent oxide thickness (EOT), hysteresis (HT), flat band voltage (V
fb), and effective breakdown field (E
B) are summarized in Table 1. We can find that V
fb is significantly shifted to the positive direction by Al-PMA. This is considered the effect of interface dipole. Generally, it is known that dipole is formed at high-k/SiO2 interface [4-7]. Therefore, the dipole is also created at the 1 nm-SiO2/4.1 nm-Al2O3 interface, leading to negative shift of V
fb [7]. From the reduction of EOT by Al-PMA, it is considered that Al near the Al/SiO2 interface was oxidized, and changed to Al2O3. As a result, the dipole which is the opposite direction of one described above is formed, leading to positive shift of V
fb. We can also find that HT is remarkably reduced by Al-PMA. It agrees well with the previous study of SiO2/GeOX system [2], suggesting that reduction of D
it and/or slow trap by Al introduction to the gate insulator. On the other hand, low leakage current has been maintained even after Al-PMA. Figure 3 shows energy distributions of D
it estimated by deep level transient spectroscopy [3]. It is clearly observed that both D
itand slow trap in the lower half of the band gap are reduced by Al-PMA.

From the results mentioned above, we can conclude that Al-PMA is also effective for Al2O3/GeOX/Ge gate stack similar to SiO2/GeOX/Ge. We will report Al-PMA effect for Ge p-MOSFET at the conference.

References:

[1] R. Zhang et al., APL. 98 (2011) 112902. [2] K. Hirayama et al., JJAP. 50 (2011) 04DA10. [3] D. Wang et al., JAP. 112 (2012) 083707. [4] Y. Yamamoto et al., JJAP. 46 (2007) 7251. [5] K. Iwamoto et al., APL. 92 (2008) 132907. [6] P. D. Kirsch et al., APL. 92 (2008) 092901. [7] K. Kita et al., APL. 94 (2009) 132902.