layers 1, * and 1 Glnur Aygn 1 Physics Department, Izmir - PDF document

Oral Presentation, Theme B : Nanotubes, Nanowires, Nanofilms Interfacial and Structural Properties of sputtered HfO 2 layers 1, * and ������������ 1 Gülnur Aygün 1 Physics Department, Izmir Institute of Technology, Gülbahçe Campus, TR-35430 Urla- ����� Abstract -Magnetron sputtered HfO 2 layers formed on heated Si substrate were studied. It is shown that formation of a SiO x suboxide layer at the HfO 2 /Si interface is unavoidable. XRD spectra show that the deposited HfO 2 film has (111) monoclinic phase, also supported by FTIR spectra. Atomic concentration and chemical environment of Si, Hf and O have been measured as a function of depth starting from the film surface by XPS technique. It is shown that HfO 2 layers of a few nanometers are formed at the top surface. After then, Si-Si bonds are detected just before Si suboxide layer and then the Si substrate is reached by XPS. It is inferred that the highly reactive sputtered Hf atoms consume the oxygen atoms from the underlying SiO 2 to form HfO 2 leaving Si-Si bonds behind. Hafnium oxide (HfO 2 ) is one of the best materials to be parameters of Cauchy dispersion relation. Thickness and replaced by SiO 2 [1]. Since interfacial properties play crucial refractive index of HfO 2 is, respectively, 7.3 nm and 1.98 role in the electrical properties of the devices, the film system (Figure 1 b). A very thick SiO x interfacial layer of 20.8 nm is is better to be analyzed in depth by chemical diagnostic formed between the Si substrate and HfO 2 film. techniques like XPS [2]. Si and Si-oxide O 1s Spectra HfO 2 thin film was grown on chemically cleaned p-type Si Total Region (b) (100) substrate having a resistivity of 7– ���� -cm by reactive 18 Depth, Etching Cycle 528 16 DC magnetron sputtering technique in high vacuum. Target to Binding Energy, eV 530 (a) 14 532 12 534 10 substrate distance was 7.4 cm of magneto sputtering 18 536 16 8 14 12 Depth, Etching Cycle 8 10 538 6 6 4 2 4 deposition chamber. O 2 , Ar gases were 12, 30 sccm 2 0 96 98 100 102 104 106 108 respectively. 99.9% pure Hf target was pre-sputterred for 3 Binding Energy, eV min to remove the possible surface contaminations. Substrate temperature and DC power were kept at 200 o C and 30 W 100 Si respectively for 5 min sputtering time for this growth process. 80 HfO Hf 4f spectra Concentration, % SiO 2 Figure 1 (a) shows FTIR spectrum taken between 400 and x (c) 60 -1 of Hf oxide layer. Since OH peaks (around 3300– 18 16 4000 cm Depth, Etching Cycle 14 40 (d) 12 Hf 3400 cm -1 ) and hydrocarbon absorption peaks (around 3000– 10 SiO 8 2 20 6 4 1600 cm -1 ) cannot be detected, this region is not shown. C 2 0 13 15 17 19 21 23 25 0 2 4 6 8 10 12 14 16 18 Binding Energy, eV Depth, Etching Cycle 2.15 2 � o (a) 0.000 Figure 2. Interfacial characterization by XPS depth profiling feature. (111) monoclinic phase of HfO 2 Absorption Intensity, a.u. Si Si -0.001 2.10 Absorption, a.u. HfO Refractive index 2 -0.002 crystalline HfO 2 Figure 2 shows the depth profile results from XPS spectrum. -0.003 phonon Elemental Si 0 is started to be seen at the very surface layers 2.05 10 20 30 40 50 60 bonds 300 400 500 600 700 800 900 2 � �� degrees C-O Wavelength, nm Si-O Hf silicate Hf-O stretching and continued till the depth of the oxide. The existence of this vibration 2.00 Hf-O mode thick layer was also supported by FTIR. Shift of Si peaks (b) 1.95 infers as the oxide containing Hf-O-Si, i.e. Hf silicate 300 400 500 600 700 800 900 1200 1000 800 600 400 Wavenumber, cm -1 Wavelength, nm (HfSi x O y ). The energy state of O centered ~530 eV for the Figure 1. Structural characterization by FTIR, XRD, SE. very surface layer is attributed to oxygen bonds in HfO 2 . At deeper layers, the peak is shifted to higher binding energy The broad and very intense absorption band can be levels to ~531.5 eV which is explained as the detection of O decomposed into two intensive peaks around 1060 and 1020 bonds in non-stoichiometric HfSi x O y [2]. After the very cm -1 corresponding to Si–O stretching vibration mode and surface layer, the main peak is decomposed into two sub- hafnium silicate (HfSi x O y ), respectively [2]. The intensive peaks, centered ~531.5 and 533 eV corresponding to Hf peak infers that there is a very thick hafnium silicate layer. silicates (HfSi x O y ) and SiO x . Simultaneously, the peaks are However, having close location but not around the exact shifted to 533 eV corresponding to Si-suboxide till the 12 th position of natural oxide of SiO 2 (1075 cm -1 ) implies that this etching cycle. Hf 4f has a spin-orbit splitting of 1.6 eV at interfacial oxide layer is in the suboxide form of Si, i.e. SiO x 16.90 and 18.50 eV for 4f 7/2 and 4f 5/2 respectively for HfO 2 , with x<2. The main peaks around 595, 550, 527, 506, and 720 all levels containing Hafnium oxide can be decomposed into cm -1 are due to Hf-O chemical bonds. It can be concluded that two contributions. However, these peaks shift to higher the presence of sharp phonon bonds in the low wavenumber binding energies in the vicinity of Si, i.e. to 17-17.5 eV, for Hf region (600–400 cm -1 ) is the result of the crystalline structure 4f 7/2 , which can be attributed to HfSi x O y . It is shown that of HfO 2 film. The C-O vibration mode detected at its expected reactive sputtered Hf atoms consume the oxygen atoms from position ~670 cm -1 is a result of measurement in air the underlying SiO 2 to form HfO 2 leaving Si-Si bonds behind. environment. The broad peak around 28 o of inset XRD picture This study was supported by a TUBITAK project shows that the microcrystalline structure, predicted also from (107T117). the sharpness of the phonon spectrum in the 400 to 600 cm -1 of FTIR spectrum, has been just started to be formed . The *Corresponding author: gulnuraygun@iyte.edu.tr other two intensive peaks around 33.3 o and 62.1 o are the signals coming from the Si (100) substrate. [1] G.D.Wilk, R.M. Wallace, J. Anthony J. Appl. Phys. 89, 5243- The film systemwas modeled for SE as Air/HfO 2 /SiO 2 /c- 5275 (2000). Si(100) in 300–850 nm. Thickness of SiO 2 film, and thickness [2] G. Aygun and I. Yildiz, J. Appl. Phys. 106, 014312 (2009) and and refractive index ( n ) of HfO 2 thin film were fitting references therein. 48 6th Nanoscience and Nanotechnology Conference, �zmir, 2010