Tunnel oxide thickness dependence of activation energy for SiGe quantum dot flash memory

For nonvolatile memory devices, a long retention time is very important. Nanocrystal floating gate has been demonstrated to lead to an improvement for retention time compare to conventional continuous floating gate. There is a controversy about where the electrons are stored, in the semiconductor nanocrystals or at trap site. Some reports claim that the electrons are stored in deep traps below Si/SiGe conduction band (1), meanwhile, other results suggest that the electrons are stored in the Si/SiGe conduction band (2). In this paper, we present our studies of activation energy for SiGe nanocrystal flash memory devices as a function of tunnel oxide thickness to try to clarify this issue. P-MOS capacitors with SiGe nanocrystal floating gate on SiO ₂ were fabricated on n-type Si (100) substrate; a schematic structure is shown in Fig. 1. Two types of sample were prepared: sample I and II have different tunnel oxide thickness, 30Å and 90Å, respectively. Fig. 2 shows AFM images of self-assembled SiGe QDs on SiO ₂ with 16% Ge concentration, an average size of 7-10 nm and a density of ∼ 2×10 ¹¹ cm ^-2. The devices were tested at 27°C, 50°C and 100°C. The activation energy of retention time can be estimated from the Arrhenius plots (Fig. 3 and 4). For 30Å tunnel oxide devices, we get activation energies of 0.33eV-0.46eV. However, we do not see any temperature dependence of charge loss rate for those devices with 90Å tunnel oxide, which means that the activation energy is almost zero. The nature of charge storage site can be determined from the activation energy (Fig. 5). After programming, the electrons may be stored in the deep traps below the SiGe conduction band initially, and eventually are released from those traps to the SiGe conduction band. Obviously, the thin (30Å) tunnel oxide can not block the electrons in SiGe conduction band tunneling to the substrate. The activation energy here can be interpreted as the deep trap level below SiGe conduction band (0.33-0.46eV), which agree with the results reported before (3), as well as our calculations. Our density functional theory calculations of the local density of states (LDOS) of the interface of Si nanocrystals and SiO ₂ indicate a trap ∼0.3eV below the Si conduction band due to band-centered Si interstitials (Fig. 6). But for thick tunnel oxides, even after electrons are released from the deep traps, they still can be stored in the SiGe conduction band due to 90Å thick tunnel oxide, and this will result in an activation energy ∼ 0eV. Therefore, the programmed electrons are stored in the deep tap states in SiGe quantum dots, and those states are localized at few hundred meVs below the SiGe conduction band.