Epitaxial Thin Films Of Half-Metallic Cr02 Deposited On Oxidized Si Substrates By PLD Using Cr203 Target Sudhanshu Dwivedi
Somnath Biswas
The LNM institute of Information Technology Jaipur-302031, India
The LNM institute of Information Technology Jaipur-302031, India
[email protected]
Ab stract-Epitaxial thin films of half-metallic compound crOz with (110) crystal orientation have been deposited on oxidized Si (100) substrates by pulsed laser deposition
(PLD)
technique using
KrF excimer laser and CrZ03 target. The deposited films were characterized
with XRD,
SEM,
FTIR, surface profilometry,
AFM and spectroscopic ellipsometry.
Keywords - Half-metals; chromium di-oxide (Cr02Y; pulsed laser deposition; epitaxial thin films.
I.
INTRODU CTION
Half-metallic Cr02 has drawn enormous research attention due to its potential applications in spintronic and magneto optical devices, as a detector in image analysis in spin polarized scanning tunneling microscopy (STM), magnetic random access memory (MRAM) in computers and several others [1-4]. Cr02 is a half-metallic ferromagnetic compound which exhibits metallic behavior for one spin sub-band while insulating or semiconductor for the other. Spin polarization of the order of 100% at very low temperatures and a high Curie temperature of around 395 K make it the most apposite for the above mentioned applications [2,3]. Experimentally, point contact Andreev reflection coupled with spin and energy resolved photoemission show nearly 100% spin polarization of conduction electrons near the Fermi energy level. The reported value of saturation magnetization per Cr site of 1.92 f.is at 10 K, is in good agreement with the ideal value of 2 f.is, and hence consistent with the half-metallic behavior [4]. �
Bulk Cr02 has a rutile type tetragonal crystal structure with lattice parameters a = 0.4421 nm and e = 0.2916 nm, cia "" 3 0.6596, with a lattice volume Va == de = 0.05699 cm and 3 density p = 4.91 g/cm [1]. In this arrangement of crystal structure, a body-centered tetragonal lattice is formed by the 4 Cr + ions. However, slight distortions of the octahedral array of 2 2 0 - anions happen in a way that each of the 0 - anions has the 2 4+ 4+ nearest Cr neighbors. Each Cr coordinates six 0 - anions in 4+ any oxygen octahedral Cr06, with Cr as the center in the octahedral site, with two short apical bonds (0.1890 nm) and four longer equatorial bonds (0.1910 nm) . However, Cr02 is a metastable compound and readily decomposes to Cr203 above 400°C under ambient conditions [5,6]. As a result, a crystalline phase based template similar to the phase of Cr02 becomes necessary for epitaxial stabilization of the films. This becomes further important when higher magnetic switching ratios are to
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be considered by exploiting the spin effects for device applications. Therefore, prevention of scattering while exploiting the polarization of conduction electrons becomes fundamental for construction of spintronic devices. As a result, high quality defect free epitaxial layers with high-quality crystalline templates become indispensable for smoother spintronic device applications [5]. PLD is a versatile method for depositing high quality epitaxial and multi-component thin films. It is a convenient tool for the formation of metastable phases under non equilibrium thermodynamic conditions. Here, we report on the deposition of epitaxial layers of Cr02 thin films by PLD on oxidized Si substrates without buffering or any other interfacial layer for epitaxial stabilization of the as-grown films. II. A.
EXPE RIMENTAL
RCA cleaning of Si substrate and growth of SiD2 layer
Firstly, the p-type Si (100) wafers with resistivity of 4-70cm were cleaned by the standard procedure of RCA cleaning. All the wafers were loaded in a quartz boat with their flat surface on the bottom side. A 2% HF solution was used for etching off the native oxide layer from the substrate surface, after that the wafers were rinsed thrice in deionized water and again dipped in 2% HF solution for 30 s. The wafers were then again rinsed in fresh deionized water. Next, the removal of organic contaminants was performed in �OH:H202:H20 solution and metallic contamination was removed with solution of HCl:H202:H20 and finally the wafers were dried in a rotor inside an enclosure. Just before the oxidation, the Si wafers were again subjected to HF etching to remove any native oxide layer that may have formed after RCA cleaning. Si wafers were kept in an oxidation furnace for a period of 3-4 mins at a temperature of 11OO°C to grow a 130 nm thick layer of Si02. �
B.
Pulsed Laser Deposition (PLD)
The PLD deposition was performed with a Coherent eompex Pro 20] model with a KrF excimer laser of wavelength 248 nm, repetition rate 10 Hz and pulse duration of 30 ns. The rotary pump of the deposition system was assisted by a turbo molecular pump to generate base pressures of the orders of
4xlO-5 mbar. The system had the capability of holding multiple targets and the maximum attainable substrate temperature was 800°e. The substrate temperatures used for the deposition were in the range of 50-400°e. The oxygen pressure was maintained 2 at 7xlO- mbar with the help of a gate valve and flow meter. 2 The laser fluence was kept at 3.3 J/cm and the laser energy at this laser fluence was 741 mJ. The target-to-substrate distance was maintained at 5.8 cm. III.
C HARACTE RIZATION OF T HIN FILMS
The as-deposited films were characterized by a number of methods including X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), surface profilometry, atomic force microscopy (AFM) and spectroscopic ellipsometry. A.
Figure L SE M image showing the vertical cross-section view of grown SiO, layer.
Figure 2 shows the XRD pattern of the Cr02 film deposited on Si02 at a substrate temperature of 350 0e.
X-ray Diffraction
The crystalline phase of the grown thin films were analyzed with an X-ray diffractometer of X'Pert-Pro using CuKa radiation of 0.1540 nm
Cr02(110)
.
B.
Scanning Electron Microscopy (SEM)
Scanning electron microscopy was performed with RAITH150 model operated at the voltage of 10 KV with beam current range of 5 pA - 20 nA and beam energy of 100 eV - 30 keV. The RAITH150 model had the capability of imaging down to as small as 10 nm sized structures. Different magnifications and scales (20-100 nm) were used for imaging the thin films. Since Cr02 is an insulating material, hence this was coated with an ultra-thin layer of Au of the order of 3-5 nm by e-beam deposition to enhance the number of electrons getting backscattered after striking the thin film.
10
Fourier Transform Infrared Spectroscopy (FTlR)
Atomic Force Microscopy (AFM)
40
45
�
55
00
FTIR spectrum for the sample has been shown in Figure 4 and a detailed assignment of the various bands has been presented in Table l. The band at around 537 cm-1 in the sample has been assigned to Cr02 angle bending ( V5) . The band at 717 cm-1 resembles closely with that caused due to Cr02 bending (V2 ) . Another band at 873 cm-1 is closer to stretching vibrations ( V3 ) of Cr02 as has been reported in literature [3,7]. The strong band at 1094 cm-1 in our samples is assigned to the transverse-optic (TO) mode of 4- and 6- fold Si04 rings with a Si-O-Si angle of 143° [8]. The other bands including the broad band at 1376 cm-I, are not yet identified. Comparatively broader bands in these samples in comparison to reported data in bulk Cr02, suggests the existence of amorphous phase in the as-deposited Cr02 films.
Spectroscopic Ellipsometry
Spectroscopic ellipsometry was used to determine the thickness of the deposited layers. Spectroscopic ellipsometer from SENTECH Instruments GmbH SE 800 was used for this pupose. IV.
35
The SEM images in Figure 3 show the morphological features in the deposited samples (350°C substrate temperature). It can be clearly observed that the particles are uniformly distributed over the whole area of the sample, which matches well with the morphological data obtained from AFM.
Surface Profilometry
The surface morphological features in the deposited samples were determined with an Agilent Technologies 5100 Scanning Probe Microscope. F.
30
The diffractogram reveals a single crystalline nature of the deposited Cr02 layer with an (110) orientation as determined from standard JCPDS files (9-332).
Surface profilometry (Ambios-XP2) was used to determine the thickness of the as-deposited Cr02 films. E.
�
Figure 2. X-ray diffractogram of CrO, thin film deposited at substrate temperature of350 0c.
FTIR spectrometer from Perkin Elmer, Spectrum BX II was used to determine the bonds present in the samples. D.
�
Diffraction angle 26
For determining the thickness of grown Si02 layer, vertical cross-section imaging was also performed. C.
15
RE SULTS AN D DrSCU SSIONS
Vertical cross-section imaging of the grown Si02 layer (Figure 1) reveals its thickness to be around 128 nm This value is in good agreement with that obtained from spectroscopic ellipsometry, which determined a thickness value of 134 nm .
.
220
rings -
1376
-
1740 em"
-
-
2350 em"
-
em
"
-
AFM image of the deposited Cr02 films is shown in Figure 5. The RMS roughness and average grain size was determined to be 0.22 run and 24 run, respectively at a scanning area of 2 300 run (Figure 5a). Figure 5b estimates the RMS roughness and average grain size values to be 1.3 run and 60 run, 2 respectively in a scanning area of 50 run .
Figure 3. SE M images of Cr02 samples.
Thickness of the as-deposited Cr02 films was determined with the help of a stylus surface profilometer. The average thickness of the films was found to be �65 run.
1740
%T
2350
537
Figure 5, AFM images of the Cr02 films showing morphology of the films in a 2 2 scanning area of (a) 300 nm and (b) 50 nm
V.
1094 500
1COO
1500
2OC()
Wavenumbers cm'
2500
In conclusion, epitaxial Cr02 thin films on amorphous Si02 layers have been deposited using PLD with Cr203 targets. The deposited films show crystalline orientation along (110) planes. We plan to further extend our work for the optimization of the deposition process. This would be an important step towards fabrication of Cr02 based spin-FET devices.
1
Figure 4.FTIR spectrum of Cr02 film.
TABLE 1.FTIR bands of Cr02 films. Bands
Cr02 angle bending (V5)
Observed
As reported
position
in literature
537
em
'!
558
em
'!
717 em'!
700 em'! [3]
Cr02 stretching (V3)
873 em'!
887 em' [3,7]
1094
em
'!
A CK NOWLEDGME NT
The authors sincerely thank the Center of Excellence in Nanoelectronics (CEN) at l IT Bombay, for providing the instrumental facilities under the Indian Nanoelectronics Users' Program (INUP) of the Department of Information Technology (DIT), Govt. of India.
[3]
Cr02 bending (V2)
TO mode of 4and 6- fold Si04
C ONCLUSION
!
1090
em
'!
REFERENCE S
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[8]
J. M. D. Coey and M. Venkatesan, "Half-metallic ferromagnetism : E xample of Cr02 (invited)," J. Appl. Phys. , vol. 91,pp. 8345-8350, May
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[3]
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[4]
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