Khomenkova et al. Nanoscale Research Letters 2011, 6:172 http://www.nanoscalereslett.com/content/6/1/172 phase. The tetragonal phase is more preferred, since it has a higher dielectric constant (about 25). It was clearly demonstrated that the Si incorporation plays the major role for the improvement of the thermal stability of the HfO-based layers [21]. The high-frequency capacitance-voltage (C-V) study of metal-insulator-semiconductor (MIS) capacitors contain- ing pure HfO2 layers (as-deposited as well as annealed at 800°C for 15 min) was performed for the samples grown at different temperatures. In most cases, the C-V curves of the annealed samples demonstrated less stretch-out effect compared to the as-deposited films due to lower number of interface states. However, the significant negative shift of flat-band voltage (Vfb) up to -2 V indicates the existence of considerable amount of positive oxide charges in the films. The introduction of positive charges can be caused by the formation of SiOx interfacial layer between HfO2 layer and Si substrate as a result of oxygen diffusion towards Si wafer under annealing treatment [21]. In addition, the presence of oxygen vacancies inside HfO2 films that is most com- mon in high-k gate dielectrics also gives rise to positive charge [22]. One of the limiting factors for oxygen diffusion inside HfO2 films can be Si incorporation in HfO2-based layers. It was supposed that due to covalent nature of Si-O bonds, the formation of oxygen interstitials and vacan- cies will be prevented, which in turn can give rise to an improvement in the electrical properties of high-k mate- rials. In this regard, the effect of the Si content on the electrical properties of our high-k films was investigated. Figure 1a, b represents the C-V curves of MIS struc- tures containing pure HfO2 and HfSiO films (RSi = 6% and 12%) measured at 100 kHz. As evident from the Page 3 of 8 figure, pure HfO2 and HfSiO (RSi = 12%) layers grown at 45°C exhibit irregular C-V curves at 100 kHz. They show existence of humps, which are the characteristic features of slow traps present at the insulator/semicon- ductor interface, i.e., defects that are distributed away from the interface to the insulator. Hence, electron emission and capture produce broad time constant dis- persion giving rise to hysteresis in the C-V curves. In addition, the C-V curves demonstrate negative Vfb shift indicating the existence of fixed insulating charges in these layers. Similar effect was observed for the HfO2- based layers grown at 100°C (not shown here). In contrast, HfSiO samples grown with RSi = 6% exhi- bit regular C-V curves. The extremely low hysteresis, along with a sharp transition from accumulation to depletion, demonstrates the high quality of interfacial as well as bulk properties of this layer. We have further investigated the effect of deposition temperature on the C-V characteristics of HfSiO layers grown with RSi = 12%. As can be seen from Figure 1b, the samples deposited at higher temperature (TS = 400°C) show better C-V characteristics than their coun- terparts grown at TS = 45°C and 100°C. Therefore, we can conclude that higher deposition temperature is pre- ferable for the Si-rich HfO2-based layers. Moreover, as one can see from Figure 1a, b, the C-V characteristics of HfSiO layer grown with RSi = 12% at Ts = 400°C are similar to the case of HfSiO films grown at RSi = 6% and Ts = 45°C. However, in the former case, the hyster- esis effect is negligible compared to the last one. In this regard, one can deduce that the HfSiO layer grown with the RSi = 12% at Ts = 400°C is more suitable for the fab- rication of the structures even at high-temperature deposition, whereas the material with the lower Si Figure 1 C-V characteristics of MIS structures containing pure HfO2 and HfSiO films. High-frequency C-V characteristics of pure and Si-rich HfO2 single layers versus Si content in the films (a) and deposition temperature (b) measured at 100 kHz. The C-V curves were normalized to their respective accumulation capacitance. All the high-k films were annealed at 800°C for 15 min. Deposition temperature is mentioned in the figures.