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Magnetism in Band Gap Engineered Sputtered MgxZn(1-x)O Thin Films
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.ORCID iD: 0000-0002-6270-6741
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This dissertation presents a comprehensive study of the intrinsic room temperature ferromagnetism, RTFM, in technologically important thin films of ZnO, MgO, Mg@ZnO, the so-called d0–magnets that do not contain any intrinsic magnetic elements. We also present the first report on magnetism in Mn doped MgO films fabricated by dc magnetron sputtering. We have just published (April 2013 ‘on-line’) a state of the art review entitled ‘p-type ZnO Theory, growth, properties, and devices’ in the prestigious journal ‘Progress in Materials Science’, summarizing the recent advances of the studies on p-type ZnO thin films and pointing out the major challenges that remain in the field. The experimental work then focuses on the magnetic properties of band gap engineered Mg@ZnO films exploiting the fact that by substitutional doping of Mg for Zn in ZnO it is possible to tailor new materials with bandgap energy in the range 3.3 eV to 7.2 eV, thus extending the possibilities for new magnetic and optical device applications. In addition, we show that by doping Mn in MgO its magnetic properties can be enhanced to saturation values as high as 38.5 emu/cm3 in a 92 nm thick film. These studies involve extensive characterization of the high quality films in the thickness range of nanometers, using SQUID magnetometer for magnetic properties, XRD for structural analysis, Dual beam HRSEM/FIB and AFM for accurate film cross-sectioning and surface morphology, EDXS for elemental analysis, UV-VIS NIR for measuring the band gap of MgxZn(1-x)O films, Mg K-edge NEXAFS experiment in order to understand electronic structure of specific cations present in the thin films The origin of the observed room temperature feerromaganetism is attributed to cation vacancies and its consequences on the polarization about these vacancies in the oxides...

ZnO films are promising materials for optoelectronic device applications, and the fabrications of high quality p-type ZnO and p–n junction are the key steps to realize these applications. However, reliable p-type doping of the material remains a major challenge because of the self-compensation from native donor defects (VO and Zni) and/or hydrogen incorporation. Considerable efforts to obtain p-type ZnO by doping different elements with various techniques have resulted in remarkable progress in the field both from theoretical and experimental point of view. In our paper, we discuss p-type ZnO materials: theory, growth, properties and devices, comprehensively. We first discuss the native defects in ZnO. Among the native defects in ZnO, VZn and Oi act as acceptors. We then present the theory of p-type doping in ZnO, and summarize the growth techniques for p-type ZnO and the properties of p-type ZnO materials. Experimentally, besides the intrinsic p-type ZnO grown at O-rich ambient, p-type ZnO (MgZnO) materials have been prepared by various techniques using Group-I, IV and V elements. We pay a special attention to the band gap of p-type ZnO by band gap engineering and room temperature ferro magnetism observed in p-type ZnO. Finally, we summarize the devices based on p-type ZnO materials.

In presenting the current studies, we first focus on the sputtering process in order to produce high quality films. From a comparative study of RTFM, in MgO films deposited by sputtering from 99.999% pure metallic Mg, Vs MgO targets respectively on glass/Si substrates under identical ambience during deposition it is found that the metallic targets give the best magnetic properties (e.g: with maximum Ms values of ~13.75 emu/g vs ~ 4.2 emu/g respectively on Si substrates.(supplement 2). Furthermore, the Ms values are strongly film thickness dependent with Mg target while it is weakly so for films from MgO target. Also, the as deposited MgO films using metallic Mg target are found to be amorphous, while it is nanocrystalline when the films are sputtered off MgO targets. The overall Ms values are found to be dependent on the oxygen content in the atmosphere during deposition, increasing to 2.69 emu/g at a oxygen partial pressure of 40% of the total working gas pressure. On annealing to nanocrystallize these films in the temperature range 600 to 8000C strong XRD peaks corresponding to (200) orientation are observed, and Ms values decrease proportionately. (supplement 3).

With the above information on studies for optimizing the effect of sputtering gas, film thickness, and oxygen partial pressure, PO2, comprehensive investigations on band gap engineering and magnetism in MgxZn(1-x)O co-sputtered thin films from Mg and Zn targets are then closely examined. The optical band gap calculated from absorption spectra shows that the band gaps of Mg-doped ZnO thin films increased linearly from 3.33 to 4.074 eV. Our studies indicate that both the magnetic properties and the band gap of the film can be tailored by tuning the film thickness and PO2 in the working gas.

In summary, RTFM ordering in the thin films originates from cation vacancies which couple ferromagnetically and establish long range magnetic order. The ferromagnetic ordering in these materials is shown to arise from defects situated at the cation sites. Electronic structure studies of some selected films disclose that the unoccupied O 2p states exist and this unoccupied state results from cation vacancy (VMg).

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , xii, 80 p.
Keyword [en]
Magnetron co-sputtering, MgO, MgxZn(1-x)O, MnxMg(1-x)O, thin films, Room-temperature ferromagnetism, band gap engineering, and intrinsic defects
National Category
Materials Engineering Other Physics Topics
Identifiers
URN: urn:nbn:se:kth:diva-122488ISBN: 978-91-7501-807-2 (print)OAI: oai:DiVA.org:kth-122488DiVA: diva2:622669
Public defence
2013-06-14, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 14:00
Opponent
Supervisors
Funder
EU, European Research Council
Note

QC 20130524

Available from: 2013-05-24 Created: 2013-05-22 Last updated: 2013-05-24Bibliographically approved
List of papers
1. P-Type ZnO materials: Theory, growth, properties and devices
Open this publication in new window or tab >>P-Type ZnO materials: Theory, growth, properties and devices
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2013 (English)In: Progress in Materials Science, ISSN 0079-6425, E-ISSN 1873-2208, Vol. 58, no 6, 874-985 p.Article in journal (Refereed) Published
Abstract [en]

In the past 10 years, ZnO as a semiconductor has attracted considerable attention due to its unique properties, such as high electron mobility, wide and direct band gap and large exciton binding energy. ZnO has been considered a promising material for optoelectronic device applications, and the fabrications of high quality p-type ZnO and p-n junction are the key steps to realize these applications. However, the reliable p-type doping of the material remains a major challenge because of the self-compensation from native donor defects (VO and Zni) and/or hydrogen incorporation. Considerable efforts have been made to obtain p-type ZnO by doping different elements with various techniques. Remarkable progresses have been achieved, both theoretically and experimentally. In this paper, we discuss p-type ZnO materials: theory, growth, properties and devices, comprehensively. We first discuss the native defects in ZnO. Among the native defects in ZnO, VZn and O i act as acceptors. We then present the theory of p-type doping in ZnO, and summarize the growth techniques for p-type ZnO and the properties of p-type ZnO materials. Theoretically, the principles of selection of p-type dopant, codoping method and XZn-2VZn acceptor model are introduced. Experimentally, besides the intrinsic p-type ZnO grown at O-rich ambient, p-type ZnO (MgZnO) materials have been prepared by various techniques using Group-I, IV and V elements. We pay a special attention to the band gap of p-type ZnO by band-gap engineering and room temperature ferromagnetism observed in p-type ZnO. Finally, we summarize the devices based on p-type ZnO materials.

Keyword
Band gap engineering; Doping different elements; Exciton-binding energy; Growth techniques; High electron mobility; Hydrogen incorporation; Room temperature ferromagnetism; Self-compensation, Binding energy; Defects; Electron mobility; Energy gap; Growth (materials); Materials; Optoelectronic devices; Semiconductor doping; Semiconductor quantum wells; Zinc, Zinc oxide
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-122618 (URN)10.1016/j.pmatsci.2013.03.002 (DOI)000320141400002 ()2-s2.0-84876727540 (Scopus ID)
Note

QC 20130524

Available from: 2013-05-24 Created: 2013-05-24 Last updated: 2017-06-13Bibliographically approved
2. Comparative study of room temperature ferromagnetism in MgO films deposited by sputtering Mg and MgO targets
Open this publication in new window or tab >>Comparative study of room temperature ferromagnetism in MgO films deposited by sputtering Mg and MgO targets
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(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-122626 (URN)
Note

QS 2013

Available from: 2013-05-24 Created: 2013-05-24 Last updated: 2013-05-24Bibliographically approved
3. Magnetism of amorphous and nanocrystalized dc-sputter-deposited MgO Thin Films
Open this publication in new window or tab >>Magnetism of amorphous and nanocrystalized dc-sputter-deposited MgO Thin Films
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2013 (English)In: NANOMATERIALS, ISSN 2079-4991, Vol. 3, no 3, 486-497 p.Article in journal (Refereed) Published
Abstract [en]

We report a systematic study of room-temperature ferromagnetism (RTFM) in pristine MgO thin films in their amorphous and nano-crystalline states. The as deposited dc-sputtered films of pristine MgO on Si substrates using a metallic Mg target in an O-2 containing working gas atmosphere of (N-2 + O-2) are found to be X-ray amorphous. All these films obtained with oxygen partial pressure (P-O2) similar to 10% to 80% while maintaining the same total pressure of the working gas are found to be ferromagnetic at room temperature. The room temperature saturation magnetization (MS) value of 2.68 emu/cm(3) obtained for the MgO film deposited in P-O2 of 10% increases to 9.62 emu/cm3 for film deposited at P-O2 of 40%. However, the MS values decrease steadily for further increase of oxygen partial pressure during deposition. On thermal annealing at temperatures in the range 600 to 800 degrees C, the films become nanocrystalline and as the crystallite size grows with longer annealing times and higher temperature, MS decreases. Our study clearly points out that it is possible to tailor the magnetic properties of thin films of MgO. The room temperature ferromagnetism in MgO films is attributed to the presence of Mg cation vacancies.

Keyword
room temperature ferromagnetism, Mg vacancy, magnetron sputtering, O-2 content; annealing
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-122628 (URN)10.3390/nano3030486 (DOI)000342201300009 ()
Note

QC 20150626. Updated from manuscript to article in journal.

Available from: 2013-05-24 Created: 2013-05-24 Last updated: 2017-02-22Bibliographically approved
4. Room Temperature Ferromagnetism and Band Gap Investigations in Mg Doped ZnO RF/DC Sputtered Films
Open this publication in new window or tab >>Room Temperature Ferromagnetism and Band Gap Investigations in Mg Doped ZnO RF/DC Sputtered Films
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2013 (English)In: Oxide semiconductors and thin films: symposia held November 25-30, 2012, Boston, Massachusetts, U.S.A., Materials Research Society, 2013, Vol. 1494, 115-120 p.Conference paper, Published paper (Refereed)
Abstract [en]

Mg@ZnO thin films were prepared by DC/RF magnetron co-sputtering in (N2+O2) ambient conditions using metallic Mg and Zn targets. We present a comprehensive study of the effects of film thickness, variation of O2 content in the working gas and annealing temperature on the structural, optical and magnetic properties. The band gap energy of the films is found to increase from 4.1 to 4.24 eV with the increase of O2 partial pressures from 5 to 20 % in the working gas. The films are found to be ferromagnetic at room temperature and the saturation magnetization increases initially with the film’s thickness reaching a maximum value of 14.6 emu/cm3 and then decreases to finally become diamagnetic beyond 95 nm thickness. Intrinsic strain seems to play an important role in the observed structural and magnetic properties of the Mg@ZnO films. On annealing, the as-obtained ‘mostly amorphous’ films in the temperature range 600 to 800°C become more crystalline and consequently the saturation magnetization values reduce.

Place, publisher, year, edition, pages
Materials Research Society, 2013
Series
Materials Research Society Symposium Proceedings, ISSN 0272-9172 ; 1494
Keyword
Ambient conditions, Annealing temperatures, Intrinsic strain, Magnetron co-sputtering, Room temperature, Room temperature ferromagnetism, Structural and magnetic properties, Temperature range
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-122631 (URN)10.1557/opl.2013.260 (DOI)2-s2.0-84889244913 (Scopus ID)9781605114712 (ISBN)
Conference
2012 MRS Fall Meeting, Boston, MA, United States, 25 November 2012 through 30 November 2012
Note

QC 20130524

Available from: 2013-05-24 Created: 2013-05-24 Last updated: 2017-03-01Bibliographically approved
5. Room Temperature Ferromagnetism and Band Gap Engineering in Mg Doped ZnO RF/DC Sputtered Films
Open this publication in new window or tab >>Room Temperature Ferromagnetism and Band Gap Engineering in Mg Doped ZnO RF/DC Sputtered Films
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2013 (English)In: Oxide thin films and heterostructures for advanced information and energy technologies: April 1-5, 2013, San Francisco, California, USA, Materials Research Society, 2013, Vol. 1557, 1-6 p.Conference paper, Published paper (Refereed)
Abstract [en]

Mg doped ZnO thin films were prepared by DC/RF magnetron co-sputtering in (Ar+O2) ambient conditions using metallic Mg and Zn targets. We present a comprehensive study of the effects of film thickness on the structural, optical and magnetic properties. Room temperature ferromagnetism was observed in the films and the saturation magnetization (Ms) increases at first as the film's thickness increases and then decreases. The Ms value as high as ∼15.76 emu/cm3 was achieved for the Mg-doped ZnO film of thickness 120 nm. The optical band gap of the films determined to be in the range 3.42 to 3.52 eV.

Place, publisher, year, edition, pages
Materials Research Society, 2013
Series
Materials Research Society Symposium Proceedings, ISSN 0272-9172 ; 1577
Keyword
Magnetrons, Metallic films, Ambient conditions, Band gap engineering, Magnetron co-sputtering, Mg-doped ZnO, Room temperature ferromagnetism, Sputtered films
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-122632 (URN)10.1557/opl.2013.509 (DOI)2-s2.0-84900342947 (Scopus ID)9781632661562 (ISBN)
Conference
2013 MRS Spring Meeting, San Francisco, CA, United States, 1 April 2013 through 5 April 2013
Note

QC 20130524

Available from: 2013-05-24 Created: 2013-05-24 Last updated: 2017-03-01Bibliographically approved
6. Electronic Structure of Room temperature ferromagnetic MgO and MgZnO Thin Films
Open this publication in new window or tab >>Electronic Structure of Room temperature ferromagnetic MgO and MgZnO Thin Films
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(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-122633 (URN)
Note

QS 2013

Available from: 2013-05-24 Created: 2013-05-24 Last updated: 2013-05-24Bibliographically approved
7. Suppression of Ferromagnetic Ordering in thicker co-sputtered Mn doped MgO Films
Open this publication in new window or tab >>Suppression of Ferromagnetic Ordering in thicker co-sputtered Mn doped MgO Films
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2013 (English)In: Nanostructured metal oxides for advanced applications: symposium held April 1-5, 2013, San Francisco, California, U.S.A, Materials Research Society, 2013, Vol. 1552, 83-88 p.Conference paper, Published paper (Refereed)
Abstract [en]

We report on preliminary studies of low (14 at.%) and high (53at.%) concentration Mn doped MgO films deposited by co-sputtering from metallic Mn and Mg targets. The structural, surface morphologies and magnetic properties of the films of different thickness were studied. All the as grown films are found to be amorphous and film surfaces are found to be flawless and homogeneous. We observe at room temperature robust ferromagnetic loops with a saturation magnetization value that is a function of film thickness reaching a maximum of ~38.5 emu/cm3 in the Mn0.53Mg0.47O film at a thickness of ~92 nm. In thicker films room-temperature ferromagnetic ordering is suppressed and eventually at a thickness around 120nm the expected diamagnetism of the bulk appears. The origin of ferromagnetism may be attributed to cation defects at the Mg-site.

Place, publisher, year, edition, pages
Materials Research Society, 2013
Series
Materials Research Society Symposium Proceedings, ISSN 0272-9172 ; 1552
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-122634 (URN)10.1557/opl.2013.611 (DOI)2-s2.0-84892653717 (Scopus ID)9781605115290 (ISBN)
Conference
2013 MRS Spring Meeting, San Francisco, CA, United States, 1 April 2013 through 5 April 2013
Note

QC 20130524

Available from: 2013-05-24 Created: 2013-05-24 Last updated: 2017-03-01Bibliographically approved
8. Room Temperature Ferromagnetism and Aging effects in MnxMg(1-x)O thin films deposited in (Ar+O2) atmosphere
Open this publication in new window or tab >>Room Temperature Ferromagnetism and Aging effects in MnxMg(1-x)O thin films deposited in (Ar+O2) atmosphere
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(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-122636 (URN)
Note

 QS 2013

Available from: 2013-05-24 Created: 2013-05-24 Last updated: 2013-05-24Bibliographically approved

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