Light Wavelength Effects on Charge Trapping and Detrapping of AlOx Embedded ZrHfO High-K Stack

When the MOSFET with the SiO₂ gate dielectric is scaled down to the nanosize, the leakage current becomes very large and the dopant can easily diffuse to the channel region to degrade the device’s performance and reliability [1]. To solve these problems, a physically thicker high dielectric constant (high-k) film with the same equivalent oxide thickness (EOT) as the SiO₂ film is used. For the conventional floating gate (FG) memory device, a poly-Si layer is embedded in the silicon dioxide (SiO₂) gate dielectric to trap charges. Because of the large band offsets between Si and SiO₂, a large operating voltage is required to achieve the acceptable write/erase efficiency and the tunnel oxide layer has to be thick enough to retain the trapped charges [2]. Since the high-k dielectric has lower band offsets with Si than the SiO₂ film has, it can be used as the tunnel oxide to solve the above power consumption and charge retention problems.  Separately, it has been proved that the Zr-doped HfO₂ (ZrHfO) has better bulk and interface properties, such as a higher crystallization temperature, lower interfacial layer thickness, and lower interface density of states, than the undopd HfO₂ has [3]. Also, the memory functions of the nc-ZnO or nc-MoO_x embedded capacitor are affected by light exposure [4,5]. In this study, MOS capacitors made of the AlO_x embedded ZrHfO high-kgate dielectric are fabricated and studied for the light wavelength effects on the memory functions. 

The ZrHfO/AlO_x/ZrHfO tri-layer gate dielectric stack was sputter deposited on the HF pre-cleaned p-type Si (100) wafer (10¹⁵ cm^-3) in a one-pump-down process without breaking the vacuum. The bottom (tunnel oxide) and top ZrHfO (control oxide) layers were sputtered from a composite Zr/Hf (12/88 wt %) target in an Ar/O₂ (1:1) mixture at 5 mTorr and 60 W for 2 and 10 min, respectively. The embedded AlO_x film was deposited from the Al target in Ar/O₂ (1:1) at 5 mTorr and 80 W for 1 min. The post deposition annealing (PDA) step was done with rapid thermal annealing (RTA) at 800^°C in the N₂ ambient for 3 min. An ITO film was sputtered on the high-k surface and wet etched into gate electrodes. An Al film was sputter deposited on the backside of the wafer for ohmic contact. The final post metal annealing (PMA) condition was 400°C, H₂/N₂(10/90) for 5 min. 

Fig. 1 shows C-V hysteresis curves of the control sample, i.e., without the embedded AlO_x layer and the AlO_x embedded ZrHfO capacitor measured at two different gate voltages (V_g) sweeping ranges in dark. The memory window of the control sample is very small, i.e., the shift the flat band voltages (DV_FB’s) in the C-V hysteresis curves are almost negligible. However, the AlO_x embedded sample has much larger memory windows, i.e., the counter-clockwise hysteresis curves with large DV_FB’s.  The DV_FB increases with the increase of magnitude of the beginning V_g. Therefore, the memory window is mainly contributed by the trap of holes to the embedded AlO_xfilm.

Fig. 2 Shows the C-V hysteresis curves of the AlO_x embedded capacitor with the ITO gate electrode measured in dark and under the red and blue LED light exposures. The V_FB’s of both forward and backward curves under red and blue light exposures shift to the positive V_g direction compared with those in the dark. The curves under the blue light exposure shift slightly more toward the positive V_g direction than that under the red light exposure. This phenomenon is caused by the electron-hole pairs generation in the Si substrate under the light exposure [4]. The difference in the magnitude of the V_FB shift is due to the band energy difference of the two lights. The blue light has a wavelength of 470 nm equivalent to 2.6 eV and the red light has a wavelength of 625 nm equivalent to 1.98 eV. The nc-MoO_x embedded ZrHfO capacitor showed the similar C-Vcurve shift phenomenon [5].

Figure 3 shows the J-V curves of the same AlO_x embedded sample as those of Fig. 2 in dark and under red and blue light exposures. The V_g was swept from -5V to +5V. In the negative V_g region, the leakage current in the dark is slightly larger than that under the light exposure condition. In the positive V_g region, the leakage current under the light exposure condition is much larger than that in the dark especially at V_g > 2V. In addition, two peaks are observed in the J-V measured in dark. The first peak is contributed by the release of trapped holes due to change of V_g polarity and the second peak is due to the Coulomb blockade effect. Under the light exposure conditions, only the first peak is observed. The Coulomb blockade effect disappears because of the easy transfer of charges through the embedded thin AlO_x layer. The leakage current change is related to the thickness of the gate dielectric film and the charge trapping capability of the embedded AlO_xfilm. 

This research was supported by NSF CMMI 0926379 and NSF CMMI 0926420 projects.

1. C. H. Yang, Y. Kuo, and C. H. Lin, Appl. Phys. Lett.,  96, 192106 (2010).

2. C. H. Lin and Y. Kuo, J. Appl. Phys., 110, 024101 (2011).

3. Y. Kuo, ECS Trans., 3(3), 253 (2006).

4. B. Luo, C. H. Lin, and Y. Kuo, ECS Trans., 41(3), 93 (2011).

5. X. Liu, Y. Kuo, and T. Yuan, MRS Proc., 1562, mrss13-1562-dd10-05-cc06-05 (2013). (2013).