Thank you Ben and hi everyone My name is Peter and I am PhD student at VUW and GNS Today I’ll talk about the structural, electronic and magnetic properties of ZnO doped with Gd by ion implantation technique. As this is the last talk for the day, you are welcome to ask any questions while I take you through the talk….
First of all what are the uses of ZnO….? We can see some of the common applications, such as in sun-screen, pharmaceuticals, ceramics, source of mineral Zn in food, and in chemicals However, ZnO is a very good semiconducting material, and it can play a big role in electronics Let’s see some of the basic properties of ZnO…..!
ZnO is a compound, wide band gap semiconductor with a hexagonal crystal structure It’s a transparent conducting oxide semiconductor which can be used as a flat screen display…. It usually has n-type conductivity, meaning the electrons are the major charge carriers…. This is due to the fact that ZnO hardly exists with a perfect stoichiometry of 50% Zn and O each… There could be slightly more number of Zn atoms, and missing oxygen atoms known as zinc interstitials and oxygen vacancies which are intrinsic defects in ZnO…. In recent years, high quality ZnO production has made easier to grow a thin film on ZnO template for device fabrication with no issue of lattice mismatching between the film and substrate… ZnO has also been reported as a promising host material for spintronic applications… As we know that the electron has got charge as well as a spin magnetic moment…. The semiconductor utilizes the charge whereas the magnetic materials exploi the spin In spintronics, both properties are combined in a single material, by doping a small amount of magnetic ions such as transition metals and rare-earth ions into a conventional semiconductor….. And what we get is a dilute magnetic semiconductors or DMS….. So, it can be used as a host material for dilute magnetic semiconductors for spintronic applications as a dilute magnetic semiconductor or DMS
There is a schematic of DMS system, and the common examples are GaAs, GaN doped with Mn, Gd etc. They can be used in magnetic RAM, spin transistors and diodes….. For practical applications, they must show the ferromagnetism above room temperature….. We have to be careful about the source of the room temperature ferromagnetism, because metallic clusters and secondary phases can also show the similar behavior….. So, it’s very sensitive to the defects which can incorporate during the sample growth or post-growth treatments….
Now why we chose rare earth elements for doping…because they have very high magnetic moment such as 7.94 uB for Gd ions… Gd doped ZnO has not been studied much, and a little is known about its various properties…. Also a very high magnetic moment in Gd doped GaN reported in a PRL paper further motivated us for the investigation… So we are trying to understand the fundamental properties which affect its electronic and magnetic properties…. The doping has been performed by “ion” implantation technique……which we already heard from Jerome in last talk
We have got the implanter where implantation can be done up to 100 keV We usually calculate the depth profiles of implanted atoms into the materials by DYNAMIC-TRIM program The figure shows the 40 keV Gd depth profiles for the fluences from 6.7e14 to 3e16 Gd/cm2
The samples were characterized by Rutherford backscattering spectrometry for compositional and depth profile analyses. This technique is based on the experiment Rutherford and his group performed on the Au-foils with alpha-particles.… As we know that, they were investigating the atomic structure and found that only few alpha-particles were backscattered contrary to their expectation, and the results were published in 1911, that is 100 years ago….. Just a reminder that there’ll be a talk on Rutherford by John Campbell on Wednesday morning, hopefully we’ll get to learn something new and interesting things about Rutherford…..
However, backscattering is not the only interaction can happen while the ions hit a sample… As shown in the figure the ions be recoiled or channel through… There can be X-rays, UV-visible photons emitted by excitations of core electrons, and if the ions can penetrate the nucleus, the nuclear reaction occurs and gamma-rays are released….. The RBS is used to identify the elements, measure their concentration and obtain the film thickness….. Let’s see the schematic diagram of RBS and assume the film is composed of two elements with different masses on a substrate….. The ions hit the sample with an energy E 0 and get scattered by an angle theta with an energy E 1 …. Kinematic factor, K is defined as the ratio of incoming and scattered particle energies, and is proportional to the masses of the interacting particles and scattering angle…. For different elements we get different K factors hence separate peak in the RBS spectrum which shows the backscattered energy and number of backscattered particles called as yield…. The elemental concentration is given by yield which is proportional to the cross section i.e. the scattering probability ….. As the ions travel deeper into the film they loose energy and can be used to calculate the film thickness…. In summary, the height of the peak gives the concentration and the width gives the film thickness… If the sample is crystalline, then for particular incident angles some of the ions can steer through the open spaces between atomic planes, which can cause drastic reduction in the number of backscattered particles…known as RBS channeling….. This can be used to study the crystalline quality and also locate the doped elements in a sample….
RBS measurements were performed at GNS Science Lower Hutt, and RUMP was used to extract the composition, depth profile etc from the spectra….. Figure shows the RBS spectrum with RUMP fitting for 3e16 Gd/cm2 …. What we can learn from the figure is that Gd atoms are situated in the near surface region …. The retained dose in the sample can also be obtained from the RBS spectra and RUMP which can be seen in the figure …. At higher fluences the retained dose decreases a bit which may be due to the preferential sputtering of already implanted Gd atoms during implantation ….
RBS channeling measurements were done along <0001> axis …. And Xmin, which is defined as the ratio of RBS yield in channeled and random condition, for Zn was found around 7% …. which suggests high quality of unimplanted ZnO …. For the ZnO implanted with 3.9e15 Gd/cm 2 the Zn X min was around 27% which implies moderate structural damages due to the implantation …. The Zn X min further goes down to 22% after vacuum annealing at 650 o C suggesting that the annealing helps to regain the crystalline quality as seen in the figure ….
To know more about the location of Gd atoms in the ZnO matrix the angular scan was performed around <0001> axis …. For Gd fluence of 3.9e15, the as implanted and vacuum annealed samples show similar trend suggesting both are subjected to similar disorders …. Split between Zn and Gd scans implies some of the Gd atoms are in random lattice sites …. We can estimate the Gd atoms at Zn substitutional lattice sites by using that formula …. Which tells that around 60% Gd atoms are in Zn substitutional lattice site for as implanted sample and it goes down to 47% up on vacuum annealing …. It is to be noted that some of the atoms may be also at interstitial site along with the channeling axis, which are shadowed by substitutional Gd atoms…. The diffusion of Gd atoms can cause the reduction of Gd atoms at the Zn substitutional sites ….
The structural properties were further studied using Raman spectroscopy …. Raman spectroscopy measures the change in photon energy as it hits the sample…. It was discovered by C.V. Raman for which he got a nobel prize for physics in 1930…. The Raman spectrum of un-implanted ZnO single crystal is shown in the figure …. The strong E2(high) and E2(low) are most common Raman peaks observed in high quality ZnO and assigned to O and Zn vibrational modes …. The peak at 575 cm-1 can be attributed to A1(LO) mode of ZnO and is usually observed due to implantation induced disorders …. It is also sometime correlated to the intrinsic structural defects in the material ….
We studied the magnetic properties using SQUID magnetometer …. And observed only diamagnetic response for unimplanted and as implanted ZnO …. However, we found the ferromagnetic signal after subjecting the sample in to vacuum annealing as we can see in the figure for 3.9e15 Gd/cm2 implanted ZnO …. What we can learn from the figure is that there is significant ferromagnetic ordering at room temperature …. And magnetic moment is roughly 4.5 uB at low temperature and 2 uB at room temperature ….
Now let’s see what happens if we implant more Gd atoms …. There is very small ferromagnetic ordering for vacuum annealed sample even at low temperature as can be seen in figure with red curve with respect to the hysteresis loop for 3.9e15 at low temperature …. The split between field cooled and zero field cooled curves suggest small ferromagnetic signal …. So, the ferromagnetism decreases at high fluences …. The distance between ions determines whether there’ll be a ferromagnetic or antiferromagnetic interaction It appears that when there are more and more Gd atoms, the antiferromagnetic coupling increases hence reducing the ferromagnetic interaction….
The electronic structures were investigated by X-ray absorption, and the measurements were conducted in Australian synchrotron, Melbourne…. The X-ray absorption near edge spectra (XANES) gives the information about the unoccupied electronic states in conduction band, and element specific details on valance state and its surroundings…. O K-edges, which are the transitions from O 2p to the conduction band for a ZnO thin film, and un-implanted and Gd implanted single crystals are shown in the figure …. …. The interesting observation is the pre-edge in ZnO film, which are usually assigned to the doping elements…. Which is missing in the bulk samples, suggesting low intrinsic defect density…. The Gd M-edge shows that most of Gd atoms are in 3+ valence state….
In summary, the RBS channeling shows that the majority of Gd atoms occupy Zn sub-lattices Raman spectroscopy reveals a peak assigned to radiation damages The room temperature magnetic moment decreases at higher annealing temperatures implies more contribution from non-ferromagnetic phase…. XANES measurement suggests the ZnO single crystals have low intrinsic defect density and the Gd ions are in 3+ valence state….
I am thankful to the funding agency MSI New Zealand and Australian Synchrotron for XANES measurement and MacDiarmid and GNS for my PhD scholarship…. Thanks to my supervisors Ben Ruck at VUW and John Kennedy at GNS …. We appreciate the help from Andreas Markwitz, Grant Williams and Sergey Rubanov for SQUID and TEM measurements….
Thanks for listening.
Thanks for listening.
16.40 o5 p murmu
P.P. Murmu NZIP 2011 Wellington, New Zealand Investigation of structural, electronic and magnetic properties of Gd implanted ZnO single crystals
<ul><li>II-VI compound semiconductor </li></ul><ul><li>Direct band gap semiconductor, E B ~ 3.37eV </li></ul><ul><li>Transparent c onducting o xide </li></ul><ul><li>Has n-type conductivity, due to zinc-interstitials and oxygen vacancies </li></ul><ul><li>Availability of high quality bulk substrate (low defect densities) for homo-epitaxial film growth </li></ul><ul><li>Suitable host material for spintronic applications ? </li></ul><ul><li>ZnO based dilute magnetic semiconductors </li></ul>ZnO properties Spintronics cartoon courtesy to T. Jungwirth c a Oxygen Zinc
Dilute Magnetic Semiconductors (DMSs) <ul><li>Semiconductor doped with magnetic elements (e.g. GaAs:Mn, GaN:Mn, GaN:Gd, TiO 2 :Co) </li></ul><ul><li>Applications: Magnetic RAM, </li></ul><ul><li>Spin FET, Spin LED etc. </li></ul><ul><li>Room temperature ferromagnetism </li></ul><ul><li>Origin: Metallic clusters / secondary phases ?? </li></ul><ul><li>Highly sensitive to defects </li></ul>Magnetic ions Cation Anion H. Toyosaki et al. Nat. Mater. 3, 221 (2004) Datta and Das, APL 56, 665 (1990)
ZnO- Rare earth (RE) doping <ul><li>Large magnetic moments e.g. 7.94 μ B for Gd </li></ul><ul><li>Very high magnetic moment (4000 μ B / Gd) in GaN:Gd * </li></ul><ul><li>Aim : to investigate the electronic and magnetic properties of ZnO:Gd prepared by ion implantation </li></ul><ul><li>* Dhar et al. PRL 94, 037205 (2005) </li></ul>Calculated and measured effective Bohr magneton for rare-earth ions
GNS ion implanter – New Zealand unique facility Gd depth profiles calculated with DYNAMIC-TRIM for Gd implanted at 40 keV into ZnO in the fluence range from 6.7x10 14 to 3.0x10 16 ions.cm -2 Implantation parameters: Energy: up to 100 keV Ions: 12 C + , 13 C + , 14 N + , 15 N + , & Vacuum: < 2x10 -7 mbar
Rutherford Backscattering spectrometry (RBS) E. Rutherford, Philos. Mag. 6, vol. 21 (1911) p. 669-688 He ++ He ++ H. Geiger (left) and E. Rutherford (right) Au-foil " The making of the Rutherford documentary " by Dr John Campbell , 10.45 am Wed 19 th Oct
Rutherford Backscattering spectrometry (RBS) <ul><li>Elastic interaction: RBS, channeling, recoiling ions </li></ul><ul><li>Inelastic interaction: X-rays, visible-UV photons, Nuclear reaction </li></ul><ul><li>RBS: kinematic factor, K α mass </li></ul><ul><ul><ul><li>cross section, σ α yield </li></ul></ul></ul><ul><ul><ul><li>energy loss, Δ E α depth </li></ul></ul></ul><ul><li>RBS and channeling: drastic reduction </li></ul><ul><ul><ul><li>in backscattered particles </li></ul></ul></ul>Schematic illustration of RBS mechanism High energy light ion interaction with target atoms
Rutherford Backscattering spectrometry (RBS) <ul><li>RBS performed with 2 MeV He + </li></ul><ul><li>Rutherford Universal Manipulation Program (RUMP) used to retrieve the composition, depth profile etc. </li></ul><ul><li>For the 1x10 16 Gd cm -2 and higher fluence implantation, preferential sputtering occurs </li></ul><ul><li>Leads to a slightly lower retained dose for the implantation at higher fluences </li></ul>Zn Gd O surface 3 MeV Particle Accelerator at GNS Science, Lower Hutt RBS spectrum and RUMP fitting Fluence vs. retained dose
RBS-Channeling Random and <0001>-aligned RBS spectra of 3.9x10 15 Gd cm -2 implanted and annealed ZnO (magnified Gd peaks in inset) <ul><li>RBS/C along <0001> </li></ul><ul><li>Zn minimum yield ( Zn X min ) of ~7% indicates the high crystalline quality of un-doped ZnO </li></ul><ul><li>For 3.9x10 15 Gd.cm -2 , Zn X min in as implanted ZnO is ~ 27%, which suggests the implantation causes only moderate structural damages </li></ul><ul><li>Vacuum annealing at 650 o C, the X min for Zn goes down to ~ 22%, which implies the annealing helps to regain the crystalline quality </li></ul>
RBS-Channeling Angular scan around <0001> for 3.9x10 15 Gd cm -2 (a) as-implanted and (b) annealed ZnO <ul><li>Angular scans around <0001> </li></ul><ul><li>Zn and Gd scans followed same trend, suggests both subjected to similar disorders </li></ul><ul><li>Split between the scans implies some of Gd atoms at random sites </li></ul><ul><li>Using, Gd Zn = (1- Gd X min ) / (1- Zn X min ), around 60% Gd atoms estimated to occupy the substitutional lattice sites </li></ul><ul><li>A small fraction of interstitial Gd atoms align with <0001> (a shadow-effect) </li></ul><ul><li>Gd Zn reduces to 47 % up on annealing, possibly due to the diffusion of Gd atoms </li></ul>
Raman Spectroscopy <ul><li>Raman effect: inelastic scattering of light (provides information about vibrational modes in a material) </li></ul><ul><li>Discovered by C.V. Raman in 1928 </li></ul><ul><li>E 2 (high) and E 2 (low) modes from O and Zn vibrations </li></ul><ul><li>575 cm -1 attributed to the A 1 (LO) mode of ZnO </li></ul><ul><li>A 1 (LO) mode primarily observed due to implantation induced disorder </li></ul><ul><li>also correlated to the structural defects such as oxygen vacancies (V o ) and zinc interstitials (Zn i ) or their complexes </li></ul>C.V. Raman demonstrating his Nobel Prize (1930) winning spectrometer Raman spectra of unimplanted, 3.9x10 15 Gd cm -2 implanted and annealed ZnO
SQUID results <ul><li>Diagmagnetic response observed in unimplanted and as-implanted ZnO </li></ul><ul><li>Annealing enhanced the ferromagnetism in low fluence Gd implanted and annealed ZnO </li></ul><ul><li>Ferromagnetism at room temperature !! </li></ul><ul><li>FC curve shows combination of ferromagnetic and paramagnetic ordering </li></ul><ul><li>Clustering or secondary phases ?? </li></ul><ul><li>Zero field-cooled (ZFC) magnetisation curve suggests contribution from super-paramagnetic particles </li></ul>Hysteresis loop of 3.9x10 15 Gd cm -2 implanted and annealed ZnO at 5 K & 300 K FC and ZFC magnetisation curves of 3.9x10 15 Gd cm -2 implanted and annealed ZnO at 100 Oe
SQUID results <ul><li>For 3.0x10 16 Gd cm -2 implanted and annealed ZnO v ery small ferromagnetic ordering (even at 5 K) </li></ul><ul><li>Ferromagnetism decreases at higher fluences </li></ul><ul><li>Ferromagnetic interaction depends on the ions separation, and can have ferromagnetic or antiferromagnetic coupling </li></ul><ul><li>Antiferromagnetic interaction among Gd atoms at high concentration may be responsible </li></ul>(b) FC and ZFC magnetisation curves of 3.0x10 16 Gd cm -2 implanted and annealed ZnO (a) Hysteresis loops of 3.9x10 15 & 3.0x10 16 Gd cm -2 implanted and annealed ZnO at 5K
XANES results <ul><li>O K-edge (~ 538 eV) observed due to electronic transition from O 2p states to conduction band </li></ul><ul><li>A pre-edge feature appears in ZnO thin film </li></ul><ul><li>Usually assigned to intrinsic defects such as oxygen vacancies and zinc interstitials </li></ul><ul><li>Gd atoms in 3+ state </li></ul>(a) O K-edge for as-deposited ZnO film, un-implanted and Gd-implanted ZnO single crystals (b) Gd M-edge for Gd-metal and Gd-implanted ZnO single crystals
Conclusion <ul><li>RBS channeling along <0001> show ~60% Gd occupation in Zn sub-lattices </li></ul><ul><li>A small fraction of the ions may be at interstitialy shadowed region aligned with <0001> </li></ul><ul><li>Radiation damages related A 1 (LO) peak observed in Gd implanted ZnO </li></ul><ul><li>Ferromagnetic ordering at room temperature </li></ul><ul><li>Moment decreases at higher temperatures </li></ul><ul><li>FC/ZFC curves suggest presence of small clusters </li></ul><ul><li>O K-edge shows intrinsic defect related pre-edge feature </li></ul><ul><li>Gd valency: 3+ </li></ul>
<ul><li>Ministry of Science and Innovation </li></ul><ul><li>MacDiarmid scholarship </li></ul><ul><li>GNS Science scholarship </li></ul><ul><li>Australian synchrotron </li></ul>Acknowledgement Funding Supervisors <ul><li>Dr A. Markwitz (GNS, New Zealand) </li></ul><ul><li>Dr G.V.M. Williams (VUW, New Zealand) </li></ul><ul><li>Dr S. Grenville (IRL, New Zealand) </li></ul><ul><li>Dr S. Rubanov (University of Melbourne, Australia) </li></ul><ul><li>Dr A. Suvorova (UWA, Australia) </li></ul>Collaborators <ul><li>Dr B.J. Ruck (VUW, New Zealand) </li></ul><ul><li>Dr J. Kennedy (GNS, New Zealand) </li></ul>