Copper-doped AlN polycrystalline powders A class of room-temperature
Solid State Communications151(2011)
499–502
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Copper-doped AlN polycrystalline powders:A class of room-temperature ferromagnetic materials
H.Li a,b,X.L.Chen a,?,B.Song c,H.Q.Bao a,W.J.Wang a
a Beijing National Laboratory for Condensed Matter Physics,Institute of Physics,Chinese Academy of Sciences,P.O.Box603,Beijing100190,China
b College of Chemistry and Molecular Engineering,Peking University,Beijing100871,China
c Academy of Fundamental an
d Interdisciplinary Sciences,Harbin Institut
e o
f Technology,Harbin150080,China
a r t i c l e i n f o Article history:
Received16November2010 Received in revised form
8December2010
Accepted10December2010
by Xincheng Xie
Available online12January2011 Keywords:
A.Diluted magnetic semiconductor D.Room temperature ferromagnetism a b s t r a c t
Cu-doped AlN polycrystalline samples were synthesized by a solid-state reaction.X-ray diffraction analysis revealed the hexagonal structure of the doped samples.Photoluminescence measurements reveal the substitution of Cu for Al in the AlN lattice.Clear hysteresis loops are observed in the M–H curves for the samples at300K,revealing room temperature ferromagnetism of the samples.Our experimental results verify that room temperature ferromagnetism is an intrinsic property of Cu-doped AlN.
?2010Elsevier Ltd.All rights reserved.
1.Introduction
Recently,considerable efforts have been paid to investigate diluted magnetic semiconductors(DMSs)[1–4].Since Dietl et al.[5] predicted the existence of RTFM order in transition metal(TM)-doped semiconductors,intense investigations have been devoted to DMSs such as TM-doped GaN[6],ZnO[7]and SiC[8–10]. However,there are still open issues about DMSs,for example, the origin of the nature of FM and the micro-origin mechanism. For example,some reporters have revealed RTFM was due to secondary magnetic phases originating from the magnetic element dopant[11–13].To clarify the origin of the FM order in DMSs, it is urgent to study DMSs with a nonmagnetic element as dopant.Cu is such an element,for Cu and all its related possible binary and ternary phases are not ferromagnetic[14].Thus, Cu-doped non-magnetic semiconductors are ideal systems for detecting the possible long-range magnetic order while ruling out the contribution from the secondary magnetic phase if any. Unfortunately,the study of Cu-doped DMSs,especially Cu-doped AlN,is still limited.One possible reason is that the Cu element has little solubility in the AlN lattice.
As one of the most important direct wide-band-gap semicon-ductors,AlN(Eg=6.28eV at RT)is a promising DMS matrix
?
Corresponding author.Tel.:+861082649039;fax:+861082649646.
E-mail address:chenx29@https://www.sodocs.net/doc/2d10850058.html,(X.L.Chen).material owing to its outstanding intrinsic characteristics such as superior properties of electric breakdown field(1.17×107V cm?1), electron mobility(1100cm2V?1s?1),and thermal conductivity (3.4W cm?1K?1).Therefore,AlN-based DMSs are expected to have potential applications in energy-saving,high-voltage,high RF power electronic and light-emitting devices[15].More impor-tantly,AlN based DMSs show a high resistance nature,which is quite helpful in making clear if RTFM can be realized in high resis-tance non-magnetic semiconductors[16,17].Recently,RTFM was observed in Cu-doped AlN nanorods reported by Ji and cowork-ers[18].Nevertheless,RTFM was also observed in undoped AlN nanorods synthesized at the same conditions[18],doubting the intrinsic origin of the RTFM in Cu-doped AlN nanorods.Therefore, the study of Cu-doped AlN is seriously demanded for probing long-range FM order in the AlN lattice doped with a nonmagnetic ele-ment and to study the origin of the RTFM.
In this study,Cu-doped AlN polycrystalline powders with low Cu concentration(in the order of1019cm?3)were synthesized by a solid-state reaction(SSR).A strong emission band with peak center located at426nm is observed in the photoluminescence (PL)spectrum of Cu-doped AlN,indicating the Cu substitution of Al in the AlN lattice.RTFM is observed in the synthesized Cu-doped AlN.Our results indicate the RTFM is an intrinsic property of Cu-doped AlN.The observation of RTFM with such low Cu content implies Cu itself cannot induce such robust RTFM.Defects and grain boundaries maybe also play an important role in the observed RTFM as plenty of defects and grain boundaries are observed in the doped samples.
0038-1098/$–see front matter?2010Elsevier Ltd.All rights reserved. doi:10.1016/j.ssc.2010.12.016
500H.Li et al./Solid State Communications151(2011)499–502
2.Experiment
High purity AlN(99.999%)and Cu(99.999%)powders with molar ratios of7:3and2:1were used as starting reaction materials. In a typical run,AlN and Cu powders with predetermined molar ratios were first uniformly mixed and ground together in an agate mortar,followed by pouring into a TaC crucible.Secondly, the TaC crucible was put into an induction heating furnace.The schematic diagram for the furnace can be found in Ref.[19]. Then,the furnace was evacuated to a vacuum of about2.0×10?3Pa,followed by charging to0.6atm high purity N2(99.999%), which is particularly useful for inhibiting the evaporation of metal elements of high volatility.Next,the TaC crucible was heated for6h at2073K.Finally,the power of the induction heating furnace was shut off and the TaC crucible was cooled down to RT naturally.After etched in hot(100±10°C)HCl and HNO3aqueous solutions followed by washing with distilled water and drying,Al1?x Cu x N polycrystalline powders were obtained and characterized.For comparison,undoped AlN also underwent the same experimental process.The chemical compositions of doped and undoped AlN powder samples were determined by an inductively coupled plasma-atomic emission spectrometry(ICP-AES).Raman spectra of the samples were collected at RT by a multi-channel modular triple Raman system(JY-64000)using the 532nm line of a solid-state laser as the excitation source.The phase analysis of the undoped and Cu-doped AlN powder samples was characterized by an X-ray diffraction(XRD)diffractometer performed on an X’Pert PRO with Cu Kαradiation at40kV and 40mA in a step mode.PL spectra of the undoped and Cu-doped AlN samples were collected at RT excited using the line of an Xe lamp as the excitation source.Magnetic properties of the undoped and Cu-doped AlN samples were measured on a superconducting quantum interference device(SQUID,MPMS-7)magnetometer.A transmission electron microscope(TEM,JEM-2010)was used to investigate the detailed microstructures.
3.Results and discussion
The Cu content in the doped samples is0.037at.%(2.768×1019cm?3,labeled as sample‘‘A’’)and0.063at.%(4.713×1019cm?3,labeled as sample‘‘B’’),determined by the ICP-AES.We tried to obtain samples with higher Cu content but failed probably due to Cu solubility in the AlN lattice.The phase analysis of the obtained undoped and doped AlN polycrystalline powders was characterized by a high-resolution XRD diffractometer in a step mode(Fig.1).The diffraction peaks in the PXRD patterns of samples ‘‘A’’and‘‘B’’are well indexed based on a hexagonal AlN cell with a=3.1114?and c=4.9792?(ICDD-PDF:25-1133,space group: P63mc),indicating the hexagonal wurtzite structure of doped samples.No secondary phases,such as Cu,Cu x N y,and Al x Cu y,are detected by the XRD diffractometer within the resolution of the XRD diffractometer,implying possibly the single phase nature of the doped samples.The XRD data were further analyzed by the Rietveld technique using the FULLPROF program[20].The lattice constants for samples‘‘A’’and‘‘B’’are:a=3.1121(6)?,c= 4.9799(6)?and a=3.1116(9)?,c=4.9796(9)?,in good agreement with the known values of AlN:a=3.1114?,and c=4.9792?(ICDD-PDF:25-1133,space group P63mc).Thus,the doped samples have a hexagonal wurtzite structure.
The photoluminescence(PL)spectra of undoped and doped samples were collected at RT(Fig.2).No emission peak is observed in the spectral range of370–450nm in the PL spectrum of AlN. However,a strong blue emission peak located at～426nm,due to the intracenter transitions in the Cu doped AlN lattice[21],is observed in the RTPL spectra for the doped samples.Transition metals(TM)prefer to incorporate at the substitution cation
sites Fig.1.PXRD patterns of Cu-doped AlN samples.(a)PXRD pattern of the sample
with a Cu concentration of0.037at.%(sample‘‘A’’)and(b)PXRD pattern of the
sample with a Cu concentration of0.063at.%(sample
‘‘B’’).
Fig.2.Typical PL spectra of undoped AlN and Cu-doped AlN samples measured at
RT.
with four nitrogen atoms in tetrahedral configuration as nearest
neighbors,when doped into the AlN lattice[22].Thus,Cu ions are
expected to substitute Al ions with four nitrogen ions as nearest
neighbors.However,Cu is preferred to form Cu2+ions,resulting in
the formation of nitrogen vacancies in the lattice to keep the charge
in equilibrium.
Raman spectroscopy is a useful tool for characterization of the
structural quality by analyzing the phonon frequency,frequency
shift and the width of the Raman scattering peak[23,24].Hexag-
onal wurtzite AlN belongs to the P63mc space group,with all the
atoms occupying the C3v sites.Six Raman-active modes are pre-
dicted by group theory:1A1(TO)+1A1(LO)+1E1(TO)+1E1(LO)+ 2E2(high).Five intense peaks located at246,609,656,668,and
910cm?1are clearly seen in the undoped and doped samples
(Fig.3),well consistent with the reported E2(low),A1(TO),E2
(high),E1(TO),and E1(LO)phonon modes of AlN[23].Raman scat-
tering peaks for the doped samples are slightly shifted to higher
frequencies compared with those of undoped AlN,clearly seen in
the inset of Fig.3.We attribute the slight peak shift to the disor-
der of the AlN lattice due to the incorporation of Cu,for the fre-
quency shift is correlated with residual stress in the lattice and a
similar frequency shift phenomenon was also observed in Cu(Sc)-
doped AlN nanorods and nanowires due to the Cu(Sc)substitu-
tion of Al in the AlN lattice[18,25].The value of full width at half-
maximum(FWHM)of the E2(high)peak for undoped AlN,samples
H.Li et al./Solid State Communications 151(2011)499–502
501
Fig.3.RT Raman spectra of undoped AlN and samples ‘‘A’’and ’’B’’.The inset shows RT Raman spectra of undoped AlN and samples ‘‘A’’and ’’B’’in the spectral range of 580–700cm ?1.
‘‘A’’and ‘‘B’’is 37,40,44cm ?1,respectively (Fig.3).These values are near 50cm ?1for that of AlN bulk single crystals with high defect density [26],suggesting possible existence of high density point defects in our samples [27].Doped samples have more de-fects than undoped AlN,implying the formation of point defects may be related to the Cu doping.Our doped samples are Al-rich characterized by the energy dispersive X-ray spectroscope (EDX),proving lots of N vacancies existed in the sample.The formation point defects may be related to the substitution of Al and the sample preparing process.The molar ratios of AlN and Cu in the starting materials are very large (see the experimental process),however the final isolated products only contain a small amount of Cu (0.04–0.06at.%),suggesting vacancy formation when Cu is sublimating during the reaction process.Therefore,the Cu substi-tution of Al is further revealed by the Raman method.Further,lots of defects existed in the doped samples revealed by the Raman method.
Fig.4shows the high-resolution transmission electron mi-croscopy (HRTEM)of AlN and sample ‘‘A’’.In the HRTEM image of AlN,a clear lattice stripe is seen in the HRTEM image,indicating the single crystalline nature of the AlN.However,in the Cu-doped AlN,several lattice stripe areas and amorphous areas coexisted in the HRTEM image,showing much higher quantity of the grain bound-ary and defect density in the doped sample than AlN.
Magnetic properties of undoped and doped samples were measured on a superconducting quantum interference device (SQUID,MPMS-7)magnetometer.The magnetization (M )versus
magnetic field (H )curves for undoped and doped samples were measured at 150and 300K,as shown in Fig.5.Samples ‘‘A’’and ‘‘B’’have hysteresis loops at 150and 300K,revealing the ferromagnetic order of samples ‘‘A’’and ‘‘B’’in the whole measured temperature range and above the RT value of Tc .However,the AlN that had undergone the same experimental condition has no hysteresis loop Fig.5,different to that reported for AlN nanorods with clear hysteresis at RT [18].The coercive fields (Hc )and the saturation magnetizations (M s )are about 80Oe,3.8×10?3emu /g and 90Oe,5.3×10?3emu /g for samples ‘‘A’’and ‘‘B’’at 300K (the insets of Fig.5(a)and (b)).Temperature dependence of magnetization (M –T )curves of Cu-doped AlN samples were measured in zero-field-cooled (ZFC)and field-cooled (FC)processes under an applied field of 500Oe from 5to 300K.ZFC and FC measurements reveal FM ordering of Cu-doped AlN and diamagnetic ordering of AlN.
It is important to exclude impurity phase induced RTFM in doped samples with such low Cu content.To investigate the FM origin,we first take into account the purity of the starting materials.Even if some tiny parasitic phase is not detected with such low content,the FM signal cannot be ascribed to the impurity magnetic phase since all possible phases in the Al–Cu–N ternary system are non-FM.Other possible contaminations are Ta and C due to the use of a TaC crucible.In recent years,ZnO and AlN doped with C has been predicted to show RTFM [28,29].Thus,it is quite important to exclude RTFM induced by C.The magnetic ordering for the C-doped AlN was characterized.No hysteresis is observed at 150and 300K,excluding the RTFM order induced by the C element.The magnetic properties of Ta-doped AlN with a Ta content of 0.027wt%were also characterized.No ferromagnetic signal is observed,excluding that the RTFM order comes from the Ta element.At the same condition,we measured the magnetic order of AlN co-doped with C and Ta synthesized as in the process for the synthesis of Cu-doped AlN in the TaC crucible.No FM signal is observed.Thus,Ta/C/TaC has no contribution to the observed RTFM.Furthermore,no RTFM was observed in the AlN,excluding the RTFM induced by oxygen in Cu-doped AlN samples as oxygen has an equal chance to incorporate into AlN and Cu-doped AlN in the synthesis process.Therefore,possible secondary phases and possible contamination induced RTFM in the Cu-doped AlN sample is impossible.
These results provide convincing evidence that RTFM is only explained by the intrinsic features of the doped sample.The 3d states of the substituted Cu 2+are split into e and t 2irreducible states,which further split into spin-up and spin-down states by the exchange interaction in the tetrahedral configuration field [22].There are nine electrons localized in 3d states of Cu 2+in the AlN lattice,indicating that only one electron in the 3d states has net spin.As the Cu content is low in the doped AlN sample,FM coupling has little possibility to form through the direct exchange
5 nm 5 nm
Fig.4.HRTEM of (a)AlN and (b)sample ‘‘A’’.
502H.Li et al./Solid State Communications 151(2011)
499–502
Fig.5.M –H curves of Cu-doped AlN samples measured at 150and 300K.(a)M –H curves of the doped sample with a Cu concentration of 0.037at.%(sample ‘‘A’’)and undoped AlN measured at 150and 300K.(b)M –H curves of the doped sample with a Cu concentration of 0.063at.%(sample ‘‘B’’)and undoped AlN measured at 150and 300K.
between Cu ions.As mentioned above,lots of N vacancies existed in the doped samples evidenced by our characterizations.Recently,defects are found to play a great role in the FM ordering of all inorganic nanomaterials [30].Thus ,the N vacancy mediated Cu net spin may be the possible reason for the robust RTFM order of doped samples,in the case like the SiC based DMSs [10].This study will urge an effort to reinvestigate the magnetic properties of TM-doped AlN since the role of defects was not taken into consideration in previous studies.However,more detailed research is still required to investigate the nature of the FM origin in Cu-doped AlN.
In conclusion,Cu-doped AlN polycrystalline samples were successfully synthesized by a solid-state reaction.Cu-doped AlN shows ferromagnetism at room temperature.The ferromagnetism is an intrinsic property of Cu-doped AlN,indicating that Cu is a good element to induce ferromagnetic order in the AlN lattice.It is found that defects may play a vital role in the robust room temperature ferromagnetism.
Acknowledgements
This work was financially supported by 2009Ludo Frevel Crystallography Scholarship Award (The International Centre for Diffraction Data,ICDD,USA),and the National Natural Science Foundation of China (Grant Nos.50472075,59972040,50702073,59925206,and 50772012),‘‘863’’program (Grant No.2006AA03A107),and ‘‘973’’program (Grant No.2007CB936300).Appendix.Supplementary data
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