Investigation of Al-PMA Effect on Al2O3/GeOX/Ge Gate Stack

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/GeO_X/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/GeO_X/Ge, it will be a useful method to improve MOSFET performance. In this study, we investigated Al-PMA effect on Al₂O₃/GeO_X/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/GeO_X 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/GeO_X/Ge gate stack similar to SiO₂/GeO_X/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.