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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 21 (2014) 1–6

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Analysis on the local structure and its implicationon the magnetic properties of Si1�xMnx thin films

Tiecheng Li a, Liping Guo a,n, Congxiao Liu b,n, Jihong Chen a, Guoliang Peng a,Fengfeng Luo a, Zheng Jiang c, Yuying Huang c

a Key Laboratory of Artificial Micro- and Nano-structure of Ministry of Education, Hubei Nuclear Solid Physics Key Laboratory and Schoolof Physics and Technology, Wuhan University, Wuhan 430072, Chinab Department of Physics, Chemistry, and Mathematics, Alabama A&M University, 4900 Meridian Street, Huntsville, AL 35810, USAc Shanghai Synchrotron Radiation Facility, Shanghai 201204, China

a r t i c l e i n f o

Available online 30 January 2014

Keywords:Diluted magnetic semiconductorsMagnetron sputteringPost thermal annealingX-ray absorption fine structure

01/$ - see front matter & 2014 Elsevier Ltd.x.doi.org/10.1016/j.mssp.2014.01.014

esponding authors.ail addresses: guolp@whu.edu.cn (L. Guo),o.liu@aamu.edu (C. Liu).

a b s t r a c t

Si1�xMnx diluted magnetic semiconductor films were deposited on p-type Si(1 0 0)substrate by radio frequency magnetron sputtering method. Post thermal annealing wasperformed in an argon atmosphere at 1073 K for 300 s and at 1473 K for 120 s. The as-grown sample exhibits ferromagnetism at room temperature. Ferromagnetism isenhanced after annealing treatment. High resolution transmission electron microscopyshows that only Mn4Si7 compound formed in all samples. X-ray diffraction patterns andFast Fourier Transform image indicate Mn atoms incorporated into Si lattice uponannealing. X-ray absorption fine structure suggests the formation of substitutional–tetrahedral interstitial Mn–Mn and tetrahedral interstitial–substitutional–tetrahedralinterstitial Mn–Mn–Mn complexes in the 1473 K annealed sample, which possesses thestrongest ferromagnetism.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

As a new type of semiconductor utilizing both thecharge and spin degrees of freedom for electrons, dilutedmagnetic semiconductors (DMSs) have attracted muchattention of research [1–5]. While most of the workinvolving DMSs has been focused on Mn-doped III–V[6,7] or II–VI [8] group materials, fewer investigationswere directed toward the synthesis of transition metaldoped Si [5,9–11].

Nonequilibrium techniques have been used to incorpo-rate Mn atoms into Si [12,13] since equilibrium solubilityof manganese in Si is rather low. By using magnetronsputtering method, Si heavily doped with Mn has beensuccessfully prepared and the sample showed a curietemperature above 250 K [14]. Zhang et al. preparedcrystalline Mn0.05Si0.95 films by vacuum deposition with

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post-crystallization processing. The Curie temperature ofthe films was more than 400 K [15]. Ferromagnetic beha-vior was also observed above room temperature in singlecrystal Si through Mn ion implantation [16]. It was foundthat the ferromagnetism was affected by concentration ofMn atoms, thermal annealing conditions, and carriers ofthe Si substrate. However, the mechanism of ferromagnet-ism in Si1�xMnx DMS is not clear.

X-ray absorption fine structure (XAFS) analysis hasbeen proved to be a powerful tool to obtain informationabout the local structure of transition metals in DMS[17,18]. In the present study, microstructure of the as-grown and annealed Si1�xMnx films was investigated byXAFS. Possible origin of ferromagnetism is discussed basedon the XAFS results.

2. Experimental

Si1�xMnx thin films were deposited on p-type Si(1 0 0)wafer at room temperature by radio frequency (RF) mag-netron sputtering method. The silicon substrates were

T. Li et al. / Materials Science in Semiconductor Processing 21 (2014) 1–62

ultrasonically cleaned in acetone and alcohol in sequenceprior to the deposition. In the target, small Mn pellets withthe purity of 99.9% were uniformly distributed on a singlecrystalline Si(1 0 0) disk, with a Si/Mn area ratio of 19:1.The Ar pressure was 1 Pa during deposition. With the RFsputtering power of 10 W, the average deposition rate wasaround 300 nm h�1.

For the deposited films, rapid thermal annealing (RTA)was carried out in pure argon atmosphere at a tempera-ture of 1073 K for 300 s (sample marked as M1), and at1473 K for 120 s (sample marked as M2), respectively. Forcomparison, the as-grown film was marked as M0. Scan-ning electron microscope (FEI SironMP SEM system),energy dispersive spectroscopy (EDS, EDAX genesis 7000system), atomic force microscope (AFM, Shimadzu SPM-9500J3), X-ray diffraction (XRD, Bruker-axs D8 with a Cu Kαradiation), and high resolution transmission electron micro-scopy (HRTEM, JEM-2010) were employed to obtain thestructural information of the films. Magnetic measurementwas carried out at room temperature by a high-sensitivity(10�11 A m2) alternating gradient magnetometer (AGM, PMC2900-04C). X-ray absorption near-edge structure (XANES) andextended X-ray absorption fine structure (EXAFS) spectrawere measured at the Mn K-edge using the X-ray fluorescencedetection method, at BL14W1 station, Shanghai SynchrotronRadiation Facility (SSRF), Shanghai, China. During the experi-ment, the storage ring was operated at 3.5 GeV with a typicalcurrent of 300 mA.

3. Results and discussion

Determined by EDS, the relative content of Manganesein as-grown Si1�xMnx film is 8% (x¼0.08). Si/Mn composi-tion in the annealed samples is presumably the same asthe as-grown sample. The cross-section SEM micrographof the as-grown Si1�xMnx sample is shown in Fig. 1(a). Theimage indicates that the film thickness is about 2.84 μm.

Fig. 1(b)–(d) shows the surface morphology of allsamples. The root-mean-square (RMS) roughness valuesdetermined by AFM are 8.72 nm, 5.69 nm, and 137.43 nmfor samples M0, M1, and M2, respectively. Annealing at1073 K for 300 s caused recrystallization of the as-grownsample (Fig. 1(c), M1) and remarkably reduced the surfaceroughness. On the other hand, there are many bubble-likehumps on the surface of M2, probably due to the peelingoff of the film during the high temperature annealing at1473 K.

For all samples, room-temperature ferromagnetismwas observed with magnetic hysteresis (Fig. 2). Ferromag-netism is enhanced after annealing treatment, evidencedby the broader hysteresis loops and increased saturationmagnetization (Ms) of samples M1 and M2. The values ofMs for the samples M0, M1, and M2 are 0.012 Am2 kg�1,0.017 Am2 kg�1, and 0.038 Am2 kg�1, respectively. Thesevalues are comparable to the results reported already [14].

Fig. 3 shows XRD θ–2θ scan of the samples. The patternshave been normalized according to the intensity of Si(4 0 0)peak (not shown in the figure). The appearance of Si(2 0 0)forbidden peak results from stacking faults in the recrys-tallization process [19]. Several peaks star-marked in the

figure originated from manganese silicides with Si compo-sition around 1.7 [20], such as Mn4Si7, which exhibit weakferromagnetism only at low temperature [21].

Fig. 4(a) shows the HRTEM micrograph of 1473 Kannealed sample. Fig. 4(b) is the corresponding fast Four-ier transform (FFT) image for the marked region in Fig. 4(a). The diffraction spots in the FFT image originated fromMn4Si7 phase. Since Mn4Si7 is not ferromagnetic at roomtemperature, to explain the observed room-temperatureferromagnetism, Zhang, Liu et al. [14,15] proposed thepossibility of manganese atoms incorporated into thesilicon lattice. As pointed out in the literature [22–24],theoretical investigations demonstrated that the substitu-tional and interstitial sites are energetically stable andpossibly lead to ferromagnetism in MnxSi1�x DMSs, whichcould be verified by XANES and EXAFS.

FEFF 8.2 program enabling theoretical calculation of theXANES spectra for different models was used to study localchemical environments of Mn atoms in our samples. FEFF[25] is an automated program for ab initio multiplescattering calculations of XAFS, XANES, and various otherspectra for clusters of atoms. The complete code wastermed FEFF for the central role of the effective, curved-wave backscattering amplitude, feff(π,k,R), in the theory.In our case, a cluster of 140–160 silicon atoms wasconstructed from perfect silicon matrix, and the clusterradius was set to 1 nm. For the substitutional model(Mnsub), the central Si atom was replaced by one Mn atom,while for the interstitial model (Mnint), the central Mnatom located either in the tetrahedral position (Mnt-int)or the hexagonal position (Mnh-int). The illustrations ofMn4Si7, Mnt-int, Mnh-int, and Mnsub models are shown inFig. 5. The Hedin–Lundqvist exchange correlation poten-tial, the self-consistent-field (SCF), the XANES, and the fullmultiple scattering (FMS) cards were applied in all cases.

Shown in Fig. 6 are measured XANES spectra of thesamples and the standard metallic Mn spectrum, as well asthe calculated XANES spectra for Mnint, Mnsub, and Mn4Si7models. The main absorption peak C in the calculatedspectra corresponds to 1s-4p electron transition, whilepre-edge peak A and peak B in Mnsub are due to 1s-eg and1s-4s transitions. For our samples, as the annealingtemperature increased, peak 1 in the spectra of the filmsshifted toward higher energy, indicating a gradual conver-sion to Mnsub atoms from small metal Mn clusters, whichcould not be detected by XRD. On the other hand, theemerged weak peak 2 in sample M2 is thought to originatefrom 1s-4s transition of Mnint atoms, including Mnt-int

and Mnh-int atoms, implying that 1473 K annealing pro-moted the motion of Mn atoms into the interstitial posi-tions. Moreover, while the shoulder peak 3 in all samplesis attributed to peak B of Mn4Si7, the slight shift of peak 4in samples M1 and M2 is probably due to a combinedeffect of Mnsub, Mnint, and Mn4Si7. The XANES resultssuggest that both Mnsub and Mnint atoms exist mostly inM2, the sample annealed at 1473 K. In contrast, only Mnsub

atoms and a small quantity of Mn4Si7 compound weredetected in M1, the 1073 K annealed sample.

ATHENA is widely used for XAFS data processing andARTEMIS is a program for the analysis of EXAFS data [26]using theoretical standards computed by the popular FEFF

Fig. 1. (a) SEM cross-section area image for the as-grown Si1�xMnx film. (b)–(d) AFM morphologies of samples: (b) M0; (c) M1; (d) M2.

Fig. 2. Magnetic hysteresis loops for the as-grown and annealed samplesmeasured at room temperature.

Fig. 3. XRD patterns for the as-grown and annealed samples.

T. Li et al. / Materials Science in Semiconductor Processing 21 (2014) 1–6 3

program [25]. ATHENA and ARTEMIS were applied in theanalysis of EXAFS data for our samples and each spectrumwas analyzed in the same manner. Fig. 7 shows the raw χ(k)k2 EXAFS data of the investigated samples and theFourier transform with the best fit, providing informationabout the Si shell and Mn shell around the Mn dopantatom. Only the first shell, i.e., the Mn–Si bond, is visible for

samples M0 (Fig. 7(b)) and M1 (Fig. 7(c)). Most likely, thisis due to the amorphous structure of M0 and the Mnsub

atoms in M1. However, both the first and second shells, i.e.,the Mn–Si and Mn–Mn bonds, show up in the spectrum ofM2 (Fig. 7(d)), with an average coordination number of 4.0and 2.1, respectively. This is consistent with a structureclose to Mn–Si–Mn clusters. The EXAFS data thus infersthat annealing at 1473 K helped the formation of Mn–Si–Mn clusters, creating Mnsub–Mnt-int dimers and evenMnt-int–Mnsub–Mnt-int complexes.

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It has been found by the model calculation that theformation energy of Mn–Mn dimers is lower than the sumof the separate constituents [27,28]. In p-type Si the moststable configuration involving up to two Mn atoms is theMnsub–Mnt-int complex, which shows a ferromagnetic spinalignment [22,23]. In addition, first-principles calculationreveals that the Mnt-int atoms assemble together via anintervening Mnsub atom, forming ferromagnetic Mnt-int–

Mnsub–Mnt-int complexes [24]. The theoretical predictionsare in good agreement with our experimental results.

Fig. 4. (a) HRTEM image of the sample M2; (b) corresponding FFT imagefor the marked region.

Fig. 5. Illustration of Mn4Si7, Mnt-

A comparison of the atomic order surrounding implantedMn in our samples with those reported [18,29,30] indicatesthat the presence of a second Mn atom in the vicinity of thecentral Mn enhanced magnetism. It is believed that Mn atomsprefer an interstitial position in Si without pressure [27], whileMnsub structure is stable under applied pressure. We believethat pressure was produced by stacking faults in the process

int, Mnh-int, and Mnsub models.

Fig. 6. Comparison of XANES spectra with calculated models.

Fig. 7. (a) The raw χ(k)k2 data of the investigated samples. (b)–(d) Fourier transform of the data with the best fit: (b) M0; (c) M1; (d) M2.

Table 1Fitted parameters of the experimental EXAFS spectra and the Crystal-lographic data. N is the coordination number. R is the bond length. s2 isthe Debye–Waller-like factor as a measure of local disorder.

Sample Bond N R (nm) s2 (10�2 nm2)

M0 Mn–Si 2.770.4 0.23970.002 0.005270.0024M1 Mn–Si 3.771.2 0.24570.004 0.003370.0058M2 Mn–Si 4.072.2 0.23570.006 0.007370.0104

Mn–Mn 2.173.3 0.29570.011 0.000870.0178Mnsub Mn–Si 4 0.235

Mn–Si 12 0.384Mnt-int Mn–Si 4 0.235

Mn–Si 6 0.272Mnh-int Mn–Si 6 0.225

Mn–Si 8 0.353

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of RTA at 1073 K for 300 s, leading to the formation of Mnsubatoms in sample M1. In contrast, RTA with a much highertemperature of 1473 K and a relatively shorter time of 120 sresulted in less stacking faults and lower pressure. Thus insample M2 some Mn atoms tend to stabilize in tetrahedralinterstitial positions, and the Mnt-int atoms assemble togethervia Mnsub atoms, forming ferromagnetic Mnt-int–Mnsub andMnt-int–Mnsub–Mnt-int complexes, accounting for theenhanced ferromagnetism in the sample [22,24]. For thesamples M0 and M1, since there is no neighbored Mn atomsaround the central Mn atom, hole-mediated indirect exchangeinteraction is a possible mechanism for the room-temperatureferromagnetism [17,31] Table 1.

4. Conclusion

In summary, Si1�xMnx films deposited by RF magnetronsputtering showed ferromagnetism at room temperature.Mn4Si7 was the only Mn/Si compound observed in thesamples. SampleM2 exhibited the maximalMs value. XANESand EXAFS analysis reveals a structure close to Mnsub–Mnt-intand Mnt-int–Mnsub–Mnt-int complexes in this sample. Theresult indicates that the exchange interaction betweenthe interstitial and substitutional Mn atoms gives rise tothe ferromagnetism in such a Si1�xMnx DMS system. For themuch weaker room-temperature ferromagnetism in samplesM0 and M1, RKKY type hole-mediated indirect exchangeinteraction is considered to be the reason.

Acknowledgements

The present work was supported by National NaturalScience Foundation of China (Grant no. 11075119, 11275140and J1210061), and the Fundamental Research Funds for theCentral Universities (Grant no. 20102020201000034). CL alsoacknowledges support from the 2013 CETPS Seed Grant ofAlabama A&M University.

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