-
The crystal structures of the AlN samples were investigated by XRD. The XRD spectra were normalized with respect to the intensity of the (0006) peak of the sapphire substrate for the as-grown AlN sample. As shown in Fig. 1, the typical (002) peaks of AlN are observed, which can be indexed to h-AlN. No peaks related to metallic Al are found in the AlN patterns. As shown in the inset of Fig. 1, the intensity of (002) peak was reduced after ion implantation, indicating that the ion implantation process introduced lots of lattice defects in AlN:$ {\rm{O}}_{5\times10^{16}} $ and AlN:$ {\rm{O}}_{2\times10^{17}} $. The XRD results of the AlN samples are shown in Table 1. The AlN (002) peak shifts towards larger 2θ after implantation, suggesting a lattice constraction along the c-axis for AlN micro-crystals in AlN:$ {\rm{O}}_{5\times10^{16}} $ and AlN:$ {\rm{O}}_{2\times10^{17}} $, which is likely attributed to the substitution of N+ or Al3+ by the O+ ions due to the smaller covalent radius of O[13].
Figure 1. XRD patterns of as-grown AlN, AlN:$ {\rm{O}}_{5\times10^{16}} $, and AlN:$ {\rm{O}}_{2\times10^{17}} $. The inset shows the magnification of AlN (002) peaks.
Table 1. XRD results of the AlN samples.
Sample Peak positron/(°) c parameter/Å (002) as-grown AlN 35.95 4.992 AlN:$ {\rm{O}}_{5\times10^{16}} $ 35.96 4.991 AlN:$ {\rm{O}}_{2\times10^{17}} $ 35.98 4.988 The Raman spectra of as-grown AlN, AlN:$ {\rm{O}}_{5\times10^{16}} $, and AlN:$ {\rm{O}}_{2\times10^{17}} $ are shown in Fig. 2. For as-grown AlN, first-order phonon modes located at 250 cm−1 [$ E_{2} $(low)], 659 cm−1 [$ E_{2} $(high)], and 889 cm−1 [$ A_{1} $(LO)] were observed, which are in good agreement with the values measured for the AlN films[14−15]. Other five peaks (marked as asterisks) are attributed to vibrational modes from the sapphire substrate.
Figure 2. Raman spectra for as-grown AlN, AlN:$ {\rm{O}}_{5\times10^{16}} $, and AlN:$ {\rm{O}}_{2\times10^{17}} $. The asterisks represent vibrational modes related to the sapphire substrate. The green dotted lines represent the first-order phonon modes of AlN $ E_{2} $ (low), $ E_{2} $ (high), and $ A_{1} $ (LO), respectively. (color online)
For AlN:$ {\rm{O}}_{5\times10^{16}} $ and AlN:$ {\rm{O}}_{2\times10^{17}} $, the$ E_{2} $(high) peaks located at 659 cm−1 were also seen clearly in Fig. 2. The full width at half maximum (FWHM) of $ E_{2} $(high) peak for as-grown AlN, AlN:$ {\rm{O}}_{5\times10^{16}} $, and AlN:$ {\rm{O}}_{2\times10^{17}} $ are about 7.4, 11.1, and 12.1 cm−1, respectively. The $ E_{2} $(high) peak broadens after O+ implantation, suggesting that there are high-density defects in AlN:$ {\rm{O}}_{5\times10^{16}} $ and AlN:$ {\rm{O}}_{2\times10^{17}} $. With increasing O+ concentration, the intensity of $ E_{2} $(high) peak decreases, and the peak becomes asymmetric, indicating the lattice distortion of AlN wurtzite structure. What's more, the $ E_{2} $(low) and $ A_{1} $(LO) components in AlN:$ {\rm{O}}_{5\times10^{16}} $ and AlN:$ {\rm{O}}_{2\times10^{17}} $ disappear, broad bands emerge instead. In general, the broadening in Raman band and absence of local vibrational modes are associated with the lattice disorder, defects, and substitutional impurities, which can lead to photon confinement and loss of periodicity[16−17]. The vacancies and substitution of O+ ions for N+ or Al3+ ions are responsible for the changes in peak shape mentioned above.
Figure 3(a) shows the S parameters versus positron energy (S–E curves) measured for AlN films. For as-grown AlN, the positron energy of 0~2 keV is corresponding to the surface layer. The large S parameter is due to the positron annihilations and formation of positronium atoms at the surface[18]. In the suface layer, S parameters decrease with increasing positron energy. The S values saturate at positron energy of 2~10 keV, indicating that positrons annihilate in the AlN layer. The obtained bulk S parameter ($ S_{\rm{b}} $) for as-grown AlN is about 0.439. Then the S parameters decrease in the energy region $ E>10 $ keV due to the annihilation of positrons in the sapphire substrate. Compared to as-grown AlN, the O+ -implanted AlN films show much higher S parameters in the whole energy range, which is attributed to the annihilation of positrons in vacancy-type defects introduced by O+ implantation. The effective positron trapping centers in AlN are Al-related vacancies (negative charges). The defect types in AlN:$ {\rm{O}}_{5\times10^{16}} $ and AlN:$ {\rm{O}}_{2\times10^{17}} $ can be determined by the S parameters. The S values for the defect layer ($ S_{\rm{d}} $) in AlN:$ {\rm{O}}_{5\times10^{16}} $ (AlN:$ {\rm{O}}_{2\times10^{17}} $) is 0.461 (0.454), then the value of $ S_{\rm{d}} $/$ S_{\rm{b}} $ is approximately 1.050 (1.034), which indicates the implantation-induced defects in AlN:$ {\rm{O}}_{5\times10^{16}} $ and AlN:$ {\rm{O}}_{2\times10^{17}} $ are probably monovacancies (VAl) or divacancies according to previous reports[19−20].
Figure 3. (a) The parameter S as a function of positron energy (E) for the samples. Solid lines are fitted to the experimental data by VEPFIT software. (b) S depth distributions obtained from analysis of S–E curves for the samples.
To further investgate the positron annihilation characteristics in the AlN films, the S–E curves were fitted using VEPFIT software[21] to resolve the defect depth profiles and depth structures of implanted AlN layers. The S parameter measured at different positron energy $ E $ can be expressed by the following formula:
$$ S(E)=S_SF_S(E)+\sum\limits_{i=1}^nS_iF_i(E), $$ (1) where $ S_{S} $ and $ S_{i} $ are the S parameters to the positrons annihilated at the surface and in the ith layer, respectively; while, $ F_{S} $ and $ F_{i} $ denote the fraction of positrons annihilated at the surface and in the ith layer the with the relation of $F_S(E) + \sum_{i=1}^nF_i(E) = 1$, respectively. As shown in Fig. 3(a), the solid lines are in accord with the experiment data. According to the previous work[22], the region exposed to annihilated positrons was divided into two blocks for as-grown AlN, and three blocks for AlN:$ {\rm{O}}_{5\times10^{16}} $ (AlN:$ {\rm{O}}_{2\times10^{17}} $), respectively. The defect depth profiles for the AlN films are shown in Fig. 3(b). In the first block of ~60 nm, AlN:$ {\rm{O}}_{5\times10^{16}} $ and AlN:$ {\rm{O}}_{2\times10^{17}} $ exhibit high S values due to the ion implantation and high O+ concentration. However, the S value of AlN:$ {\rm{O}}_{5\times10^{16}} $ is much higher than that of AlN:$ {\rm{O}}_{2\times10^{17}} $, which was attributed to the reverse diffusion of implanted ions. In the implantation procedure, vacancy defect was formed, some O+ ions diffused towards the surface of AlN films and then gathered. At the lower implantation doses, less O+ ions occupied Al vacancies while more O+ ions occupied Al vacancies at higher doses, leading to more vacancy defects at lower implantation doses than higher doses before 60 nm of the film thickness. This anomalous results have also been observed in C+-implanted AlN films[9]. In the second block of ~350 nm, high S values for AlN:$ {\rm{O}}_{5\times10^{16}} $ and AlN:$ {\rm{O}}_{2\times10^{17}} $ are owing to the Al-related vacancies. The third block of ~950 nm depth corresponds to the AlN thickness, the S values tend to be the same due to the annihiltion of vast majority of positrons in the sapphire substrate.
In order to character the change of vacancy type, S–W analysis were performed for all AlN films as shown in Fig. 4. The S–W data for positron energies below 2 keV were subtracted. The fitting curves of the S–W data for the as-grown AlN, AlN:$ {\rm{O}}_{5\times10^{16}} $, and AlN:$ {\rm{O}}_{2\times10^{17}} $ are straight lines and approximately overlapped, indicating that these samples contain the same types of defects. Therefore, increasing the dose of O+ ions increases the density of the Al-related vacancies but does not change the defect type.
Figure 4. Plots of S vs W for the samples. The black solid, red dashed, and blue dotted lines are fits to the S–W data for the as-grown AlN, AlN:$ {\rm{O}}_{5\times10^{16}} $, and AlN:$ {\rm{O}}_{2\times10^{17}} $, respectively. (color online)
The variations of magnetization (M) as a function of magnetic field (H) for AlN:$ {\rm{O}}_{5\times10^{16}} $ and AlN:$ {\rm{O}}_{2\times10^{17}} $ by applying magetic field perpendicular ($ \bot $) to the sample plane are shown in Fig. 5(a). In our previous work, as-grown AlN exhibited an almost zero hysteresis phenomenon and showed diamagnetic character, indicating the absence of magnetic impurities in the as-grown AlN[9]. The ferromagnetic behavior has been observed for AlN:$ {\rm{O}}_{5\times10^{16}} $ and AlN:$ {\rm{O}}_{2\times10^{17}} $. The saturation magnetization ($ M_{\rm{S}} $) of the AlN:$ {\rm{O}}_{5\times10^{16}} $ is about 0.68 emu/g, much higher than that of AlN:$ {\rm{O}}_{2\times10^{17}} $, 0.09 emu/g. This might be due to the excessively high O+ dose would increase the number of O+ ions occupying adjacent Al3+ positions, resulting in antiferromagnetic coupling[7, 23]. This finding is consistent with the DBAR results. The magetic properties of AlN:$ {\rm{O}}_{5\times10^{16}} $ were measured by applying magnetic field parallel (//) and perpendicular ($ \bot $) to the sample plane, respectively, as shown in Fig. 5(b). Much stronger ferromagnetism of AlN:$ {\rm{O}}_{5\times10^{16}} $ are observed in the perpendicular diection, 0.68 emu/g than in parallel, 0.29 emu/g. The magnetic anisotropy can be considered as an evidence of intrinsic ferromagnetism in O+-implanted AlN films[24].
Figure 5. (a) Room-temperature out-of-plane M–H loops for AlN:$ {\rm{O}}_{5\times10^{16}} $ and AlN:$ {\rm{O}}_{2\times10^{17}} $. The diamagnetic background signals were subtracted. The magnetization values were normalized by accounting for the masses of the AlN films. (b) Magnetic anisotropy of AlN:$ {\rm{O}}_{5\times10^{16}} $. (1 Oe=1000/(4π) A/m)
To further investigate the origin of the ferromagnetism in O+-implanted AlN films, we calculated their electronic and magnetic properties using the DFT methods[9]. For the supercell containing a single Al vacancy, the total magnetic moment is 3.00 $ \mu _{\rm{B}} $. Furthermore, the calculated total energy of the spin-polarized state is approximately 556 meV lower than that of the non-spin-polarized state, indicating that single Al vacancy prefers the spin-polarized state. The magnetic coupling of Al divacancies (VAl–VAl) with different distances are also investigated[25]. The results suggest that the energy difference between antiferromagnetic and ferromagnetic phases rises sharply with the distance increase of two Al vacancies, leading to the crucial transformation of VAl–VAl coupling from antiferromagnetism to ferromagnetism. When the VAl–VAl distance is less than 9.96 Å, the magnetic coupling between them is tend to form antiferromagnetic interaction. Thus, for ion-implantation with higher dose, divacancies or vacancy clusters will weaken the ferromagnetism of AlN films, which is consistent with the results of magnetic measurements. According to the above results, it is proposed that the intrinsic ferromagnetism in the O+-implanted AlN films is related to the Al vacancies. The O+ ions occupying the adjacent Al3+ positions and Al divacancies or vacancy clusters jointly play important roles in regulating the magnetic property in the forms of antiferromagnetic alignment.
Effect of Defects on Magnetic Properties of O+-implanted AlN Films by Positron Annihilation Spectroscopy
-
摘要: 在氧离子注入的AlN薄膜AlN:$ {\rm{O}}_{5\times10^{16}}$(剂量为${5\times10^{16}}$ cm−2)和AlN:$ {\rm{O}}_{2\times10^{17}}$(剂量为${2\times10^{17}}$ cm−2)中观察到了室温铁磁性。所观察到的磁各向异性表明,氧离子注入的AlN薄膜中铁磁性是本征特性。在垂直于样品平面的外加磁场下,AlN:$ {\rm{O}}_{5\times10^{16}}$中的饱和磁化强度约为0.68 emu/g,高于 AlN:$ {\rm{O}}_{2\times10^{17}}$中的饱和磁化强度0.09 emu/g。这是由于过高的氧离子剂量使得更多的氧离子占据相邻的Al离子位置,导致了反铁磁耦合的出现。正电子湮没多普勒展宽谱表明在氧离子注入的AlN薄膜中存在大量的Al空位。第一性原理计算表明氧离子注入的AlN薄膜中的铁磁性主要来源于Al空位,而高浓度的Al空位形成双空位或空位团将会使得AlN样品从铁磁性到反铁磁性转变,最终使样品的铁磁性减弱。Abstract: Room-temperature ferromagnetism is observed in the O+-implanted AlN films with O+ doses of ${5\times10^{16}}$ cm−2 (AlN:$ {\rm{O}}_{5\times10^{16}} $) and ${2\times10^{17}}$ cm−2 (AlN:$ {\rm{O}}_{2\times10^{17}} $). The observed magnetic anisotropy indicates that the ferromagnetism is attributed to the intrinsic properties of O+-implanted AlN films. The out-of-plane saturation magnetization ($M_{\rm{S}}$) of the AlN:$ {\rm{O}}_{5\times10^{16}}$ is about 0.68 emu/g, much higher than that of AlN:$ {\rm{O}}_{2\times10^{17}} $, 0.09 emu/g, which is due to the excessively high O+ dose made more O+ ions occupy adjacent Al3+ positions in forms of antiferromagnetic coupling. Doppler broadening of positron annihilation radiation measurements demonstrate the existence of Al vacancies in the O+-implanted AlN films. The first-principles calculations suggest that the ferromagnetism originates mainly from the Al vacancies. Meanwhile, the formation of divacancies or vacancy clusters by high concentrations of Al vacancies will lead to the transformation of VAl–VAl coupling from ferromagnetim to antiferromagnetism, ultimately weakening the ferromagnetism of the sample.
-
Key words:
- positron annihilation spectroscopy /
- ion implantation /
- semiconductors /
- ferromagnetism
-
Figure 2. Raman spectra for as-grown AlN, AlN:$ {\rm{O}}_{5\times10^{16}} $, and AlN:$ {\rm{O}}_{2\times10^{17}} $. The asterisks represent vibrational modes related to the sapphire substrate. The green dotted lines represent the first-order phonon modes of AlN $ E_{2} $ (low), $ E_{2} $ (high), and $ A_{1} $ (LO), respectively. (color online)
Figure 5. (a) Room-temperature out-of-plane M–H loops for AlN:$ {\rm{O}}_{5\times10^{16}} $ and AlN:$ {\rm{O}}_{2\times10^{17}} $. The diamagnetic background signals were subtracted. The magnetization values were normalized by accounting for the masses of the AlN films. (b) Magnetic anisotropy of AlN:$ {\rm{O}}_{5\times10^{16}} $. (1 Oe=1000/(4π) A/m)
Table 1. XRD results of the AlN samples.
Sample Peak positron/(°) c parameter/Å (002) as-grown AlN 35.95 4.992 AlN:$ {\rm{O}}_{5\times10^{16}} $ 35.96 4.991 AlN:$ {\rm{O}}_{2\times10^{17}} $ 35.98 4.988 -
[1] ZANG H, SUN X, JIANG K, et al. Advanced Science, 2021, 8(18): 2100100. doi: 10.1002/advs.202100100 [2] LIU F, WANG T, GAO X, et al. Science Advances, 2023, 9(31): eadf8484. doi: 10.1126/sciadv.adf8484 [3] WOLF S, AWSCHALOM D, BUHRMAN R, et al. Science, 2001, 294(5546): 1488. doi: 10.1126/science.1065389 [4] FRAZIER R, STAPLETON J, THALER G, et al. Journal of Applied Physics, 2003, 94(3): 1592. doi: 10.1063/1.1586987 [5] KUMAR D, ANTIFAKOS J, BLAMIRE M, et al. Applied Physics Letters, 2004, 84(24): 5004. doi: 10.1063/1.1763216 [6] ZHU Z, OU Z, CHEN Y, et al. Journal of Alloys and Compounds, 2023, 170986. doi: 10.1016/j.jallcom.2023.170986 [7] RAN F Y, SUBRAMANIAN M, TANEMURA M, et al. Applied Physics Letters, 2009, 95(11): 112111. doi: 10.1063/1.3232238 [8] XU Y, YAO B, LIU D, et al. CrystEngComm, 2013, 15(17): 3271. doi: 10.1039/c3ce26920e [9] YE R, LIU J, ZHANG H, et al. Applied Physics Letters, 2019, 115(26): . doi: 10.1063/1.5131036 [10] NATH D, CHAKRAVARTY S, GUPTA M, et al. Journal of Alloys and Compounds, 2023, 171727. doi: 10.1016/j.jallcom.2023.171727 [11] TUOMISTO F, MAKKONEN I. Reviews of Modern Physics, 2013, 85(4): 1583. doi: 10.1103/RevModPhys.85.1583 [12] AGARWAL S, LIEDKE M, JONES A, et al. Science Advances, 2020, 6(31): eaba8437. doi: 10.1126/sciadv.aba8437 [13] LIU C, MENSCHING B, VOLZ K, et al. Applied Physics Letters, 1997, 71(16): 2313. doi: 10.1063/1.120059 [14] DAVYDOV V Y, KITAEV Y E, GONCHARUK I, et al. Phys Rev B, 1998, 58(19): 12899. doi: 10.1103/PhysRevB.58.12899 [15] SOARES M, LEITÃO J, DA SILVA M, et al. Optical Materials, 2011, 33(7): 1055. doi: 10.1016/j.optmat.2010.09.005 [16] LUGHI V, CLARKE D R. Applied Physics Letters, 2006, 89(24): 241911. doi: 10.1063/1.2404938 [17] SHAH A, AHMAD J, AHMAD I, et al. Applied Surface Science, 2014, 317: 262. doi: 10.1016/j.apsusc.2014.08.112 [18] ZHANG B, YAO B, LI Y, et al. Applied Physics Letters, 2011, 99(18): 182503. doi: 10.1063/1.3657412 [19] UEDONO A, ITO K, NAKAMORI H, et al. Journal of Applied Physics, 2007, 102(8): 084505. doi: 10.1063/1.2798586 [20] LI X, CHEN Z, LIU C, et al. Journal of Applied Physics, 2015, 117(8): 085706. doi: 10.1063/1.4913523 [21] VAN VEEN A, SCHUT H, CLEMENT M, et al. Applied Surface Science, 1995, 85: 216. doi: 10.1016/0169-4332(94)00334-3 [22] OKUMURA H, UEDONO A. Japanese Journal of Applied Physics, 2023, 62(2): 020901. doi: 10.35848/1347-4065/acb898 [23] XIONG J, GUO P, GUO F, et al. Materials Letters, 2014, 117: 276. doi: 10.1016/j.matlet.2013.12.018 [24] VENKATESAN M, FITZGERALD C, LUNNEY J, et al. Phys Rev Lett, 2004, 93(17): 177206. doi: 10.1103/PhysRevLett.93.177206 [25] YE R, LIU J, ZHANG H, et al. Room Temperature Ferromagnetism Induced by Al-vacancy in N+ Implanted AlN Films[Z].