Khomenkova et al. Nanoscale Research Letters 2011, 6:172 http://www.nanoscalereslett.com/content/6/1/172 The high-k layers (pure and Si-rich HfO2) were used as alternative tunnel layers. At the beginning, the effect of the deposition conditions and postdeposition annealing treatment was investigated separately for high-k and SRSO layers to obtain the optimal fabrication conditions for each material. Subsequently, the different stack structures were fabricated, and their structural and elec- trical properties were analyzed versus annealing treatment. Experimental procedure The structures investigated in the present study were grown on p-Si (100) substrates (resistivity of approxi- mately 15 Ω cm) by RF magnetron sputtering. Prior to deposition, the substrates were subjected to standard RCA cleaning, dipped in a diluted hydrofluoric solution (10%), dried in N2, and immediately transferred to the vacuum chamber of the deposition setup. Single HfO- based and SRSO layers were developed to find the opti- mal conditions for fabrication of p-Si/tunnel layer/charge storage layer/control layer stack memory structures. Four-inch HfO2 (99.9%) and SiO2 (99.995%) targets were used as starting targets to grow high-k (HfO- based) and low-k (pure or Si-rich SiO2) layers, respec- tively. The HfO-based layer was grown by sputtering either pure HfO2 or composed HfO2 + Si targets. The different Si content in the high-k layers was achieved by the variation of the number of Si chips topped on HfO2 target. In this study, Si surface ratio was RSi = 6% or 12%. The RF power density applied on HfO2 cathode, the argon flow, and the total plasma pressure were 0.74 W/cm2, 1.5 standard cubic centimeters per minute (sccm), and 40 μbar, respectively. The substrate tem- peratures were 45°C, 100°C, and 400°C. The pure or Si-rich SiO2 layers were grown in the same chamber by sputtering of SiO2 target using either standard or reactive approaches. The deposition of pure SiO2 layers was performed in pure argon plasma (stan- dard approach) with argon flow of 3.2 sccm. The Si-rich SiO2 layers were fabricated by reactive approach. The SiO2 target was sputtered in the mixed argon-hydrogen plasma. The argon and hydrogen flows were kept at 1.6 and 5.0 sccm, respectively. The RF power density applied on SiO2 cathode and the total plasma pressure were 0.74 W/cm2 and 20 μbar, correspondingly, for both pure and Si-rich SiO2 layers. The substrate tem- peratures were 45°C, 100°C, and 400°C. The deposition conditions, mentioned above, were also used for the fabrication of trilayer structures where (1) the tunnel layer is HfO-based material (either pure or Si- rich HfO2), (2) the charge storage layer is Si-rich SiO2, and (3) the control layer is SiO2 or HfO-based layer. To study the effect of the postdeposition processing on the thermal stability of the high-k layers as well as Page 2 of 8 on the formation of Si-ncs inside SiO2 ones, both sin- gle-layer and trilayer structures were annealed in a con- ventional furnace within the temperature range of 800°C to 1,100°C for 10 to 30 min under nitrogen flow. In some cases, an additional annealing in forming gas at 400°C for 60 min was also used to passivate dangling bonds, if any. After this, the Al contacts were deposited by means of thermal evaporation of Al target on the back and face sides of the structures, followed by an annealing of the final structures at 400°C for 20 min in forming gas flow. The combination of different methods allows informa- tion about film properties to be obtained. Thus, infrared attenuated total reflectance (ATR) was used to study the structure and chemical composition of the films. ATR- FTIR spectra were measured in the range 600 to 4,000 cm-1 by means of a 60° Ge Smart Ark accessory inserted in a Thermo Nicolet spectrometer (Nexus model 670) (Thermo Nicolet Corporation, Madison, USA). X-ray diffraction (XRD) data were obtained using a Phillips X’PERT PRO device http://www.panalytical.com/ with Cu Ka radiation (l = 0.1514 nm) at a fixed grazing angle incidence of 0.5°. An asymmetric grazing geometry was chosen to increase the volume of material interact- ing with X-ray beam as well as to eliminate the contri- bution from Si substrate. The electrical properties of the samples were studied at different frequencies using an HP 4192A LF Impedance Analyzer http://www.home. agilent.com/. Results and discussion The fabrication of an NVM cell requires a perfect con- trol of four main parameters: (1) the tunnel oxide thick- ness, (2) the nanocrystal density, (3) the nanocrystal diameter, and (4) the control oxide thickness. In these regards, properties of the samples were analyzed at dif- ferent fabrication steps and applied to get an insight on the formation and quality of the structures. Prior to describing the electrical properties of the trilayer stack structures, let us consider separately the parameters of single pure and Si-rich HfO2 layers as well as Si-rich SiO2 layers. HfO-based tunnel layers In our previous study [21], the thermal stability of amorphous structure and the chemical composition of pure HfO2 layers grown by RF magnetron sputtering after annealing at 800°C to 850°C for 15 min in nitrogen flow have been discussed in detail. Besides, the forma- tion of monoclinic HfO2 phase after treatment at higher annealing temperature (TA = 900°C to 1,100°C) was also demonstrated. However, HfSiO layers were found to be stable at 950°C, whereas the increase of annealing tem- perature (TA) led to the formation of tetragonal HfO2