Spontaneous magnetization of InN nanocrystals induced by oxygen

Xiuqing Meng1*, Ning Tang1, Qinglin Xia2, Zhuo Chen3

1 Research Center for Light Emitting Diodes (LED)，Zhejiang Normal University, Jinhua,

Zhejiang Province 321004, China2 Department of Physics, Central South University, Changsha 410083, China3Department of Physics, Beijing Institute of Technology, Beijing 100081, China

AbstractIn this study, undoped InN nanocrystals were fabricated by nitridizing In2O3

nanocrystals. The structural and morphological characteristics of the nanocrystals

indicate that they are single crystals. InN nanocrystals exhibit ferromagnetic behavior

at room temperature without detectable phase segregation or clusters. Oxygen defects

incorporated during the phase transmission are probably the intrinsic causes for the

room-temperature ferromagnetism (FM) order. The results provide useful information

in understanding the true magnetic origin in nanostructured diluted magnetic

semiconductors (DMSs) and reveal the potential of undoped semiconductors as the

frontiers of DMSs given that no spurious FM signal from metal segregation occurs.

Keywords: InN nanocrystals, nitridizing, single crystals, room-temperature

ferromagnetism

*Corresponding author. Tel: +86-579-82297911, Fax: +86-579-82297913E-mail: xqmeng@semi.ac.cn(X. Meng). jbli@semi.ac.cn (J. Li)

Introduction

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Diluted magnetic semiconductors (DMSs) have attracted great interest since the

first theoretical prediction of room-temperature ferromagnetism (FM) in magnetically

doped III–V nitrides [1]. Indeed, several groups have reported FM far above room

temperature in GaN films doped with various magnetic transition metal ions prepared

via a variety of techniques, including metal-organic vapor-phase epitaxy and plasma-

assisted molecular beam epitaxy [2–4]. Compared with other III–V compounds, InN

has several unique properties, such as high Hall mobility [5], high carrier drift

velocity [6], smallest effective mass, and least dependent of band gap energy on

temperature [7]. The low spin-orbit coupling strength in InN (0.003 eV compared

with 0.34 eV for GaAs) indicates a long electron spin diffusion length and spin

lifetime in this material, which make InN a promising candidate for spintronic

applications [8]. Theoretically, various methods can help in obtaining room

temperature FM; for example, doping is a commonly used method [9]. Unfortunately,

most of the few reported doped InNs only show no or low temperature FM,

particularly in Mn-doped InN samples [10, 11]. On the contrary, several studies have

found that room temperature FM can be induced by defects [7, 12, 13]. However,

although the authors claim that the defects are responsible for the room temperature

FM in intrinsic InN, these studies have controversial results as to whether the In or N

vacancies are the real origins of FM. In the current study, InN nanocrystals were

successfully obtained by nitridizing In2O3, and room temperature FM was obtained.

Based on the analysis of the structural and optical properties of InN nanocrystals, the

FM order probably originated from oxygen-related defects.

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Experimental details

InN nanocrystals were synthesized in two steps. First, In2O3 nanocrystals were

prepared via a simple sol-gel method followed by nitridizing the as-synthesized In2O3

nanocrystals with NH3 flow at 550 °C. In the first step, 50 mL of absolute ethanol was

mixed with 1.91 g of indium nitrate hydrate (In(NO3)3 · 41/2H2O) in a three-neck

flask with vigorous stirring and refluxing at 60 °C for 3 h. The flask was then cooled

to 5 °C, and an additional 0.02 mol of C4H13NO was injected into the solution

dropwise. The reaction continued for another 30 min, which yielded a transparent gel

that was then aged overnight and annealed at 500 °C and 900 °C sequentially under

the protection of a nitrogen atmosphere for 1 h. The as-obtained In2O3 nanocrystals

were subsequently nitridized under NH3 flow at 600 °C for 5 h to form InN

nanocrystals.

Structural and morphological studies were done via X-ray diffraction (XRD) by

using Cu Kα irradiation on an 800 W Philips 1830 powder diffractometer and high-

resolution transmission electron microscopy (HRTEM) measurements performed

using Hitachi S-4800 microscope instruments with an accelerating voltage of 15 kV.

The optical properties were observed with a xenon lamp as the excitation source. The

magnetization measurements were obtained using a Quantum Design superconducting

quantum interference device system.

Results and discussion

Phase identification was performed on the XRD system. Figure 1 summarizes the

XRD patterns of as-synthesized In2O3 and nitridized InN samples. Figure 1 (a) shows

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that the as-synthesized samples were indexed as cubic In2O3 (JCPDS No.71-2195), in

which the lattice parameter a = 10.1 Å calculated from the XRD diffractograms is

consistent with that of the bulk. After nitridation, the sample obtained from pristine

In2O3 was indexed as hexagonal InN (JCPDS No. 50-1239) with lattice constants of a

= 3.54 Å and c = 5.70 Å. No secondary phase related to In or In2O3 was observed in

the XRD pattern of InN, which implies that all In2O3 nanocrystals were converted to

InN nanocrystals.

Figure 1 XRD patterns of the as-synthesized In2O3 and nitrified InN nanocrystals. The In2O3

samples are indexed to be cubic phase while the InN samples are indexed to be hexagonal phase.

To verify further the structures and morphologies of the samples, HRTEM

measurements were performed using Hitachi S-4800 microscope instruments with an

accelerating voltage of 15 kV. Figures 2 (a) and (b) show the TEM images of samples

before and after the nitridation, respectively. The nanocrystals of In2O3 and InN are of

spherical/cubic shape with sizes in the range of 50 nm to 100 nm. Although nitridation

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greatly affects the lattice structures of In2O3, the morphology does not change much

after forming InN. Therefore, InN nanocrystals still retain their spherical shapes, as

shown in Figure 2 (b). However, the lattice structure of the samples was significantly

changed, in which the HRTEM image in Figure 2 (c) clearly indicates that the sample

is single-crystalline and free of secondary crystalline phases and that no sign of

segregation of impurities or clusters could be detected. These nanocrystals, which

are

Figure 2 TEM and corresponding HRTEM images of In2O3 ((a) and (c)) and InN

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((b) and (d)) nanocrystals, respectively.

bounded by (100) facets, also showed clear lattice fringes with an interplanar

spacing of ~2.5 Å, which is consistent with that of the bulk. Figure 2 (d) shows that

the clear lattice fringes of the HRTEM images of InN, which imply that In2O3 was

successfully converted to InN. The interplanar distance of 1.43 Å indicates that the

InN samples had a wurtzite (WZ) structure. Any sign of impurities or clusters, which

are indicative of obtaining high-quality InN nanocrystals, was not observed.

Figure 3 shows the magnetization results for the InN nanocrystals.

The typical magnetization versus temperature curve is presented in

Figure 3 (a). Field-cooled (FC) and zero-field-cooled (ZFC)

magnetization measurements were performed from 10 K to 300 K.

The FC results were obtained by measuring the magnetic moment of

the sample in a magnetic field of 1000 Oe during cooling. The ZFC

results were obtained by first cooling the sample to 10 K in a zero

field and then warming it in the same field as that of the FC

measurement. ZFC magnetization showed stronger temperature

dependence than FC magnetization below 300 K. The divergence

between the FC and ZFC curves indicates that the InN nanocrystals

are ferromagnetic in the whole temperature range and have a Curie

temperature well above 300 K. Figure 3 (b) shows the magnetization

versus applied magnetic field curve measured at 300 K after

subtracting the diamagnetic background. The well-defined

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hysteresis loops showed that the nanocrystals are clearly

ferromagnetic at room temperature. The inset in Figure 3 (b)

presents the magnified parts of the hysteresis loops, in which the

saturation magnetization, coercive field, and remanence

magnetization were 0.0064 emu/g, 299 Oe, and 0.0059 emu/g,

respectively. The absence of any detectable traces of secondary

phases or clusters from the XRD and HRTEM results clearly confirms

that the FM signal was not produced by secondary phases in the

sample.

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Figure 3 (a) The FC-ZFC M-T curve of the InN nanocrystals. (b) M-H curve of the sample

taken at 300 K after the necessary background diamagnetic subtraction; the magnetic field used is

from 0 up to 0.3 T.

Contamination during sample preparation or annealing also

could be ruled out because the experimental conditions are

precisely controlled. Therefore, in conclusion, FM originates from

intrinsic InN. However, studies have different attitudes toward

controlling the ferromagnetic properties of intrinsic InN. Song et al.

[13] argued that surface defects such as N vacancies cause the enhancement of FM

order, whereas Xie et al. found defects of In vacancies that are responsible for the

magnetism [6]. Therefore, further research is necessary to explore if N or In vacancy

or other defects is the main causes for the magnetism. In the current study, evidence

of these causes for the magnetism was provided by photoluminescence (PL)

measurements.

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The PL properties of InN remain unclear until now. The emission peak at 0.7 eV

is considered as an intrinsic emission [14], whereas the wider gap emissions are

usually considered as defect-related emissions. For example, the commonly accepted

opinion that the enlarged gap emission is related to oxygen, and M. Yoshimoto

observed varied band gap emissions ranging from 1.55 eV to 2.27 eV with the

increase in oxygen concentrations ranging from 1% to 6 % [15]. Motlan also reported

that oxygen incorporation is one of the causes for the increase in gap. Therefore, the

larger values of the emission gap are considered as oxygen defect-related emissions,

in which different electron concentration results in the variation of gap emission

between 0.7 and 2.0 eV [16, 17]. Davydov et al. also reported that the sample with a

band gap ranging from 1.8 eV to 2.1 eV contained up to 20% of oxygen [18], which is

much higher than for samples with a narrow band gap. From these observations,

oxygen is probably the cause of the high concentration of defects, and in this case, an

increase in the band gap in wide gap samples results from the formation of

oxynitrides, which have a much larger band gap than that of InN [17]. Figure 4 shows

a strong wide gap emission at 889 nm (corresponding to 1.5 eV) in the samples

observed. When the samples were converted from In2O3 via nitridation, oxynitrides

could possibly form during nitridation; thus, oxygen-related defects are dominant for

the wide gap emission. Furthermore, room temperature FM probably originates from

oxygen-related defects.

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Figure 4. PL spectra of InN nanocrystals, a strong and intense peak located at 889 nm is

observed.

Conclusions

In conclusion, In2O3 nanocrystals were successfully converted into InN by

nitriding In2O3 in an NH3 atmosphere at moderate temperatures, in which the InN

nanocrystals were described as single crystals with hexagonal structures. Room

temperature FM was observed in these InN nanocrystals, and based on PL

measurements, oxygen-related defects were speculated to be related to the origin of

FM. The observation of room temperature FM in the InN nanocrystals provided

evidence for the defect-induced room temperature FM in undoped narrow gap

semiconductors and also provided a method to obtain room temperature FM

semiconductors by varying or modulating the defect density, which implies the

properties of nanostructured DMSs are very sensitive to the fabrication processes.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant

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No. 11104250, 61274099), the Science Technology Department of Zhejiang Province

(Grant No. 2012C21007), Zhejiang Provincial Science and Technology Key

Innovation Team (2011R50012) and Zhejiang Provincial Key Laboratory (No.

2013E10022).

References

[1] Dietl, T., Ohno, H., Matsukura, F., Cibert, J., Ferrand, D. (2000) Zener Model

Description of Ferromagnetism in Zinc-Blende Magnetic Semiconductors. Science

11

287, 1019-1022. http://www.sciencemag.org/content/287/5455/1019.short7

[2] Reed, M. L., El-Masry, N. A., Stadelmaier, H. H., Ritums, M. K., Reed, M. J.,

Parker, C. A., Roberts, J. C., Bedair, S. M. (2001) Room temperature ferromagnetic

properties of (Ga, Mn)N. Appl. Phys. Lett. 79, 3473-3475.

http://dx.doi.org/10.1063/1.1419231

[3] Dhar, S., Brandt, O., Trampert, A., Daweritz,L., Friedland, K. J., Ploog, K. H.,

Keller, J., Beschoten, B., G¨untherodt, G. (2003) Origin of high-temperature

ferromagnetism in (Ga,Mn)N layers grown on 4H–SiC(0001) by reactive molecular-

beam epitaxy. Appl. Phys. Lett. 82, 2077-2079. http://dx.doi.org/10.1063/1.1564292

[4] Thaler, G. T., Overberg, M. E., Gila, B., Frazier, R., Abernathy, C. R., Pearton, S.

J., Lee, J. S., Lee, S. Y., Park, Y. D., Khim, Z. G., Kim, J., Ren, F. (2002) Magnetic

properties of n-GaMnN thin films. Appl. Phys. Lett. 80, 3964-3966.

http://dx.doi.org/10.1063/1.1481533

[5] Guo,Q. X., Nisho, N., Ogawa, H., Yoshida, A. (1999) Jpn. J. Appl. Phys., Part 2

38 (1999) L490.

[6] Foutz, B. E.，O’Leary, S. K., Shur, M. S., Eastman, L. F. （1999）Transient

electron transport in wurtzite GaN, InN, and AlN. J. Appl. Phys. 85, 7727-7734.

http://dx.doi.org/10.1063/1.370577

[7] Xie, Q. Y., Gu, M. Q., Huang, L., Zhang, F. M., Wu, X. S. (2012) Defect-induced

room temperature ferromagnetism in un-doped InN film. AIP Advances 2, 012185-1-

6. http://dx.doi.org/10.1063/1.3698320

[8] Prinz, G. (1998) Magnetoelectronics. Science 282, 1660-1663.

12

DOI:10.1126/science.282.5394.1660

[9] Rajaram, R., Ney, A., Solomon, G., Harris Jr, J. S., Farrow, R. F.C., Parkin, S. S.

P. (2005) Growth and magnetism of Cr-doped InN. Appl. Phys. Lett. 87, 172511-

172513. http://dx.doi.org/10.1063/1.2115085

[10] Ney, A., Rajaram, R., Farrow, R. F. C., Harris Jr., J. S., Parkin S. S. P. (2005) J.

Supercond.: Incor. Nov. Magn. 18, 41.

[11] Ney, A., Rajaram, R., Arenholz, E., Harris Jr., J. S., Samant, M., Farrow, R. F.

C., Parkin, S. S. P. (2006) Structural and magnetic properties of Cr and Mn doped

InN. J. Magn. Magn. Mater. 300, 7-11. http://www.sciencedirect.com/science?

_ob=ArticleListURL&_method=list&_ArticleListID=-

829429657&_sort=r&_st=13&view=c&md5=a0de275af323e7dbd14549ead969c330

&searchtype=a

[12] Lei, M., Fu, X. L., Yang, H. J., Wang, Y. G., Zhang, Y. B., Huang, Y. T., Zhang,

L. (2012) Synthesis of N-deficient indium nitride nanowires and their room-

temperature ferromagnetism. Mater. Lett. 73, 166-168.

http://www.sciencedirect.com/science?

_ob=ArticleListURL&_method=list&_ArticleListID=-

829430679&_sort=r&_st=13&view=c&md5=8a085eb8e4d0c2f9579def7192fe5e37&

searchtype=a

[13] Song, B., Zhu, K. X., Liu, J., Jian, J. K., Han, J. C., Bao, H. Q., Li, H., Liu, Y.,

Zuo, H. B., Wang, W. Y., Wang, G., Zhang, X. H., Meng, S. H., Wang, W. J., Chen,

X. L. (2000) J. Mater. Chem. 20, 9935.

13

[14] Wu, J. Q. (2009) When group-III nitrides go infrared: New properties and

perspectives. J. Appl. Phys. 106, 011101-1-29. http://dx.doi.org/10.1063/1.3155798

[15] Yoshimoto, M., Yamamoto, H., Huang, W., Harima, H., Saraie, J., Chayahara,

A., Horino, Y. (2003) Widening of optical bandgap of polycrystalline InN with a few

percent incorporation of oxygen. Appl. Phys. Lett. 83, 3480-3482.

http://dx.doi.org/10.1063/1.1622445

[16] Motlan, Goldys, E. M., Tansley, T. L. (2002) Optical and electrical properties of

InN grown by radio-frequency reactive sputtering, J. Cryst. Growth 241, 165-170.

http://www.sciencedirect.com/science?

_ob=ArticleListURL&_method=list&_ArticleListID=-

829437443&_sort=r&_st=13&view=c&md5=1f339a8f0189bf80dca3a5115788a56e&

searchtype=a

[17] Bhuiyan, A. G., Hashimoto, A., Yamamoto, A. (2003) Indium nitride (InN): A

review on growth, characterization, and properties. J. Appl. Phys. 94, 2779-2808.

http://dx.doi.org/10.1063/1.1595135

[18] Davydov.V. Y., Davydov, Yu., Klochikhin, A. A., Emtsev, V. V., Kurdyukov, D.

A., Ivanov, S. V., Vekshin, V. A., Bechstedt, F., Furthmüller, J., Aderhold, J., Graul, J.,

Mudryi, A. V., Harima, H., Hashimoto, A., Yamamoto, A., Haller, E. E. (2002) Band

Gap of Hexagonal InN and InGaN Alloys. Phys. Status Solidi B 234 (2002) 787-795.

DOI: 10.1002/1521-3951(200212)

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