Category: Nanotechnology and Emerging Areas

Room Temperature ferromagnetism in Cu Doped ZnO for spintronics

Zaheer Ahmed Khan and Subhasis Ghosh

School of Physical Sciences, Jawaharlal Nehru University New Delhi 110067, India

E-mail: zaheer.jnu@gmail.com

Abstract— We report the growth of high quality Cu doped ZnO thin films by magnetron sputtering. Cu concentration has been varied over three orders of magnitude (0.01%–10%). Higher growth temperature ensures high level of Cu doping. The XPS measurements revealed that most of the Cu atoms have substituted into the ZnO lattice sites. Room temperature ferromagnetism, with magnetic moment initially increasing and then decreasing with Cu content, has been observed in the thin film. Oxygen vacancy and dopant activation play crucial role in the observed ferromagnetism.

Index Terms— RTFM, II-VI semiconductor.

I. INTRODUCTION

The possibility of achieving ferromagnetic properties in semiconductor and semiconductor based devices has led to emerge a new field of electronics known as spintronics and identifying suitable materials for spintronics is pursued vigorously. Spintronics aims at using both spin and charge of the electron to bring about novel devices such as spin transistor, spin polarized light emitting diode and other spin based devices1. For device applications an ideal DMS should exhibit room temperature ferromagnetism (RTFM) and should have a homogenous distribution of magnetic dopants without aggregations and precipitation of dopants in host semiconductor1. Dietl2 predicted that transition-metal-doped ZnO might display Curie temperatures above room temperature. The origin of RTFM in these systems is controversial. It is debated whether it is intrinsic or due to defects or due to clusters of the magnetic ions3. There are conflicting reports on RTFM in Cu doped ZnO (ZnO:Cu), some experiments found no ferromagnetic ordering in ZnO:Cu samples, whereas Chakraborti et.al 4 have provided evidences for RTFM in ZnO:Cu and claimed that substitution of Zn2+ (3d10) with Cu2+(3d9) is responsible for ferromagnetic ordering(FMO) in ZnO:Cu. The origin of FMO in ZnO:Cu remains inconclusive with different propositions. Moreover, there are several issues remain to be solved for the development of spintronics devices on ZnO based DMS. The RTFM is generally achieved either by post growth annealing or by high growth temperature. It is difficult to tailor and reproduce material properties using post growth

annealing as there are several uncontrolled factors, not suitable for device processing. There are conflicting claims regarding the bandgap in ZnO:Cu and role of oxygen vacancy(VO) on the FMO in ZnO:Cu5. In view of all these conflicting results regarding of RTFM in ZnO:Cu and role of VO on RTFM in ZnO:Cu, it is highly desirable to have better understanding of RTFM in ZnO:Cu. In this paper, we report on how to achieve highly transparent RTFM in ZnO:Cu thin films by optimizing different growth conditions.

II. EXPERIMENTAL DETAILS

ZnO:Cu thin films were deposited on quartz substrate using RF magnetron sputtering. Undoped and ZnO:Cu thin films were deposited at growth temperature varying from 100oC to 600°C. The target was 1 inch in diameter and 0.25 inches thick synthesized by conventional solid state reaction route made from commercially available (Aldrich, USA) ZnO (99.999%) and CuO (99.999%) powders, which were mixed in stoichiometric ratio, grounded for sixteen hours and sintered at 700 °C for twelve hours. The target substrate distance was kept fixed at 7cm and rate of deposition was kept fixed at 0.1Å/s for all films. Argon and oxygen were used as sputtering gases in ratio of 6:4. All these growth parameters have been optimized to get high quality undoped ZnO thin films. Thicknesses of the samples were measured by SOPRA GES5E spectroscopic ellipsometer. Grazing incidence X-ray diffraction studies were performed using PANalytical Xpert Pro system. Absorption spectra were taken by Shimadzu UV-2401PC spectrophotometer. Magnetic measurements were performed using a Quantum Design Ever Cool MPMS XL-7 superconducting quantum interference device (SQUID) magnetometer with the film parallel to applied field. The x-ray photoelectron spectroscopy (XPS) data were recorded using an Al K alpha laboratory x-ray source that was operated at 100 watts and an electron energy analyzer with five channeltrons from Specs GmbH, Germany. The data were recorded with 20 eV pass energy with 1 eV energy resolution. The chamber base pressure was 5×10-10 mbar.

III. RESULTS & DISCUSSIONS

XRD data ( not shown here) of the undoped and ZnO:Cu

thin films showed pronounced c-axis orientation resulting in

16th International Workshop on Physics of Semiconductor Devices, edited by Y. N. Mohapatra, B. Mazhari, M. Katiyar, Proc. of SPIE Vol. 8549, 85491Z · © 2012 SPIE · CCC code: 0277-786/12/$18 · doi: 10.1117/12.926802

Proc. of SPIE Vol. 8549 85491Z-1

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/22/2012 Terms of Use: http://spiedl.org/terms

strong (002) peak at 34.4o corresponding to wurtzite ZnO structure (SG: P63mc)3,. The absence of CuO planes indicate homogenous mixing of dopant atom with the host insulating matrix and negates the presence of any secondary phases of either metallic Cu or oxides of Cu.

X-ray photoelectron spectroscopy (XPS) technique was use to elucidate upon the bonding characteristics and oxidation states of copper in the film. A detailed high resolution scans of the Cu peaks as shown in Fig. 1 indicates the nature of the bonding of oxygen atoms to the copper atoms in the 1% Cu doped thin film. The scan shows Cu 2p3/2 peak at 933.23 eV and Cu 2p1/2 peak at 952.83 eV which corresponds to a mixed oxidation state of +1 or +2, which is believed to be facilitated by the presence of point defects. Thus, the XPS measurements revealed that most of the Cu atoms have substituted into the ZnO lattice sites.

The absorption spectra of the undoped and ZnO:Cu thin films(not shown here) have revealed high quality of the thin films with excitonic peak observed even in heavily doped ZnO:Cu samples. Transmission spectra for Cu-doped samples revealed that all samples except the sample with highest Cu content (10%) show almost 90% transmissions in the infrared to UV region.

The result of magnetization as a function of applied field at 300 K is shown in Fig. 2. All the RF sputtered samples with Cu content ≥ 1%, grown at 600°C show FMO at room temperature. The moment per Cu atom at 300 K first increased and then decreased with Cu concentration, as observed in PLD grown samples4. The consistent drop in moment per Cu atom of dopant at higher Cu concentration could be due to either (i) antiferromagnetic ordering between Cu ions occurring at shorter separation, or (ii) breaking of FMO due to alloy disorder in higher alloy composition. Comparing the optical and magnetic data, it is worthwhile to

note that the drop in magnetization is observed after 2.5% doping, but it is interesting to note that maximum magnetization is observed when the band gap starts decreasing due to alloying. The Bohr magnetron value of 0.25μB/Cu atom is similar with values obtained in PLD grown epitaxial layers4 and an order of magnitude higher than the value reported by Herng et al5.

The comparison of room temperature magnetization data

of three 2.5% Cu doped samples, (i) first sample grown at 100°C without post growth annealing, (ii) second sample grown at 100°C and subsequently annealed at 600°C in oxygen atmosphere and (iii) third sample grown at 600°C without post growth annealing, revealed that RTFM was observed in second and third samples, but no RTFM was observed in first sample. Moreover RTFM was not observed in any of the samples with Cu content varying from 1 to 10%,

grown at 100oC. When the ZnO samples are grown at relatively low temperature, the incorporation and activation of dopants remain incomplete and there will be high concentration of Vo. Hence, non-observation of RTFM in samples grown at lower temperature may be due to either or both of these reasons. As the sample is annealed at higher temperature (~600oC) in oxygen atmosphere, substantial number of Vo will be filled and more Cu will substitute Zn. It is difficult to conclude whether filling of Vo or dopant activation or both are responsible for observation of RTFM in sample grown at 100oC and subsequently annealed at 600oC. But, RTFM is increased by one order of magnitude in samples grown at 600oC without post growth annealing. Hence, combining this observation with absorption and X-ray data, it

-0.3

-0.1

0.1

0.3

-10 -5 0 5 10

Zn0.99Cu0.01OZn0.975Cu0.025OZn0.925Cu0.075OZn0.90Cu0.10O

Field (KOe)

Mom

ent μ

B / C

u0.1

1

10

0 5 10 Cu cons. (%)

Mom

ent x

10-1

μ B /C

u

Figure 2 Magnetization as a function of applied field at 300 K in ZnO:Cu thin films for different concentrations of Cu. Top inset shows the variation of magnetic moment with Cu concentration.

5700

5900

6100

6300

6500

930 940 950 960

Zn 0.99Cu 0.01O

2p1/

2

2p3/

2

Binding energy (eV)

Inte

nsity

(a.u

.)

Figure 1 X-ray photoelectron spectroscopy high resolution scan of Cu 2p3/2 and 2p1/2 peaks of Zn0.99Cu0.01O thin film.

Proc. of SPIE Vol. 8549 85491Z-2

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/22/2012 Terms of Use: http://spiedl.org/terms

can be emphasized that dopant activation is more important than Vo for RTFM in ZnO:Cu. Moreover, there are conflicting claims regarding the exact role of VO on FMO in ZnO:Cu. Herng et al5 have shown that VO is essential for FMO in ZnO:Cu, and argued that spins at Cu sites are parallely aligned due to mediation through VO orbitals. Whereas, Ye et al6 have shown that VO is undesirable for RTFM in ZnO:Cu because removing one O atom from CuO4 tetrahedran leads to breaking of Cu-O bond, which should result weakening of FMO.

IV. CONCLUSIONS

In conclusion, we have shown that room temperature

ferromagnetism can be observed in highly transparent ZnO:Cu thin films grown at higher growth temperature. It is further observed that the moment per Cu atom first increases and then decreases with Cu concentration. It is important to optimize growth conditions and concentration of Cu to have maximum magnetization in ZnO:Cu. .

V. REFERENCES [1] S. A. Wolf et al, Science, 294, 1488 (2001). [2] T. Dietl, Nat. Mater.2, 646 (2003). [3] U. Ozgur et al, J. Appl. Phys. 98, 041301 (2005). [4] D. Chakraborti, et.al Appl. Phys. Lett. 90,062504, (2007). [5] T. S. Herg et al, Phys. Rev. Lett. 105, 207201 (2010). [6] L.H. Ye., et.al, Phys. Rev. B, 73, 033203 (2006).

Proc. of SPIE Vol. 8549 85491Z-3

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/22/2012 Terms of Use: http://spiedl.org/terms