Magneto-optical spectroscopy of the wide band gap diluted magnetic semiconductor GaMnN

Materials Science and Engineering B 126 (2006) 240–244

Magneto-optical spectroscopy of the wide band gap diluted magnetic semiconductor GaMnN S. Marcet a,b , D. Ferrand a,b,∗ , S. Kuroda c , E. Gheeraert d , R.M. Galera e , J. Cibert e , H. Mariette a,b a

CEA-CNRS Group Nanophysique et Semiconducteurs, Laboratoire de Spectrom´etrie Physique, Universit´e Joseph Fourier BP87, 38402 Saint Martin d’H`eres, France b DRFMC-SP2M, CEA-Grenoble, France c Institute of Materials Science, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8573, Japan d Laboratoire d’Etudes des Propri´ et´es Electroniques des Solides, CNRS, BP166, 38042 Grenoble, France e Laboratoire Louis N´ eel, CNRS, BP166, 38042 Grenoble, France

Abstract Magneto-optical studies of an internal transition of Mn in the diluted magnetic semiconductor (Ga,Mn)N are reported. A strong zero-phonon line (ZPL) is observed at 1.413 eV, followed by weak phonon replicas. The sharpness of the ZPL in very diluted samples allows a precise study of its Zeeman effect in magnetic fields up to 11 T in both Faraday and Voigt configurations. Some consequences concerning the analysis of the Mn3+ (3d 4 ) configuration, the magnetic properties of the Mn system, and the spin–carrier coupling as revealed from magnetic circular dichroism at the bandgap, are briefly discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: Magnetic semiconductors; Wide-gap semiconductors; GaMnN; Magneto-optics

1. Introduction In the quest for carrier induced ferromagnetism at room temperature in diluted magnetic semiconductors, (Ga,Mn)N has been proposed as a good candidate [1]. This prediction is an extrapolation of the mean field model (previously applied to tellurides and arsenides) to wide-gap semiconductors. It assumes that Mn atoms are introduced in GaN, substituting Ga up to 5%, in the form of Mn2+ (d 5 configuration and spin 5/2), together with a large density of holes (3 × 1020 cm−3 ), and that the spin– carrier coupling follows the trend previously observed for other semiconductors. This prediction was followed by experimental reports of high Curie temperature [2], but a possible contribution of magnetic clusters could not be ruled out, and subsequent studies demonstrate a paramagnetic behavior down to low temperature, independently of the Mn configuration [3,4]. For example, in bulk materials with a very low Mn content, the Mn2+ (d 5 ) configuration has been found in nominally undoped n-type samples, and the Mn3+ (d 4 ) configuration upon codoping with

∗

Corresponding author. E-mail address: [email protected] (D. Ferrand).

0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.09.037

an acceptor impurity (Mg) [5]. In addition, several experimental and theoretical studies conclude that the peculiar configuration of Mn in GaAs (which leads to its acceptor character and the d 5 configuration) is not reproduced in GaN. We have applied magneto-optical spectroscopy to a series of well controlled (Ga,Mn)N samples. The zero-phonon absorption line due to d–d transition of the Mn impurity can be quite narrow in a sample with a low Mn content, so that its Zeeman effect can be analyzed with a better precision than in previous studies [6]. In addition, (Ga,Mn)N layers have been grown on transparent substrates, so that the magnetic circular dichroism (MCD) close to the absorption edge can be measured. 2. Experiment Mn-doped wurtzite GaN epilayers, 0.3 ␮m thick, have been grown using nitrogen plasma assisted molecular beam epitaxy, on GaN MOCVD buffer layers deposited on sapphire [7]. As demonstrated by X-ray diffraction and extended X-ray absorption fine structure (EXAFS), in well controlled samples, most of the Mn are incorporated in Ga substitutional sites, forming a diluted phase with no trace of clustering up to 6% Mn [8]. The Mn composition of all samples in the present study has been system-

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atically checked by secondary ion mass spectroscopy (SIMS), using a Mn-implanted GaN layer as a reference. Near infrared optical absorption of Mn-doped GaN was measured by Fourier transform infrared spectroscopy (FTIR) at 300 and 4 K using a tungsten lamp and a silicon photodetector [9]. The propagation of the light was oriented along the growth axis (c-axis). Then, very diluted samples have been selected to avoid Mn interactions, and the absorption of the ZPL has been measured in magnetic field up to 11 T, using dispersive spectroscopy with a tungsten lamp, a grating monochromator and a CCD camera. We describe here the results obtained on a sample with 0.03% Mn. The wavevector of the light was oriented along the c-axis, in both Faraday and Voigt configurations, i.e., with magnetic field parallel and perpendicular to the c-axis. MCD spectroscopy at the band gap edge with the magnetic field applied parallel to the c-axis, in Faraday configuration, was performed on non-dope GaN and on a 1.7% Mn-doped GaN. The circular polarization of the transmitted light was analyzed using a 50 kHz photoelastic modulator and a linear polarizer. The transmitted light was recorded simultaneously by a chopper, a lock-in amplifier and a photomultiplier tube. 2.1. Near infrared absorption Fig. 1 shows the absorption coefficient for different Mn compositions, after carefully removing the Perot–Fabry in-

Fig. 2. Zero-phonon line of internal Mn transition at 2 K, in Faraday configuration in magnetic field up to 11 T (step 1 T) measured by standard dispersive spectroscopy.

terference fringes. The spectra exhibit a zero-phonon line at 1.413 eV and weak phonon replicas on the high energy side. These features increase and broaden when the Mn content increases. This infrared absorption was observed in previous studies [10–12] and was attributed to an internal transition of Mn3+ (d 4 ), and ascribed to the spin-allowed internal transition from 5 T2 to 5 E within the 3d 4 configuration of the Mn3+ neutral acceptor A0 . X-ray absorption near-edge spectroscopy (XANES) performed in some of the present samples has confirmed the d 4 configuration of the Mn impurity [13]. 2.2. ZPL absorption in magnetic field

Fig. 1. Near infrared absorption coefficient of Mn-doped GaN with Mn content from 0.03 to 5.7%, measured by FTIR spectroscopy.

Fig. 2 shows the absorption of the ZPL in Faraday configuration at 2 K. Phonon replicas could not be observed with our dispersive optical set-up. At low magnetic field, line 1 slightly shifts towards lower energy. Above 5 T, a second line (line 2) grows up on the lower energy side whereas the intensity of line 1 decreases. ZPL absorption in the same configuration at 10 K is shown in Fig. 3. Then, line 2 could be observed even at 0 T, as a shoulder. Both peaks have the same intensity at 11 T. Lines 1 and 2 are thermally activated, respectively at high and at low magnetic field, which may result from a crossing (or a weak anticrossing) between the different spin levels of the ground states at 7 T. Measurements in Voigt configuration at 4.2 K are shown in Fig. 4. The ZPL slightly shifts towards lower energy at

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Fig. 5. Magnetic field dependence of the magnetization and circular dichroism measured close to the band gap on a non-doped GaN layer and a 1.7% Mn-doped GaN layer. No hysteretic behavior could be observed at 2 K.

Fig. 3. Zero-phonon line of internal Mn transition at 10 K, in Faraday configuration in magnetic field up to 11 T (step 1 T) measured by standard dispersive spectroscopy.

low magnetic field. Increasing the magnetic field above 5 T has no influence on the absorption. This suggests no crossing of the ground state in this configuration. It confirms that this direction is an easy axis, as observed in magnetization [14]. In both configurations, the circular dichroism was measured to be at most of the order of 0.2% in a field of 11 T and a temperature of 1.7 K. 2.3. MCD at the band gap edge Fig. 5 reports the degree of circular polarization P defined as P=

Iσ+ − Iσ− Iσ+ + Iσ−

(1)

and the magnetization of a 1.7% Mn-doped GaN layer. The MCD signal is one order of magnitude larger than in GaN. Furthermore, there is a strong temperature dependence and a saturation at high magnetic field. Both MCD temperature and magnetic field dependence are similar to the magnetization ones, with a paramagnetic behavior and no hysteresis. It clearly reveals an interaction between the d-electrons of Mn and the s-, p-electrons of the crystal host. 3. Model and discussion

Fig. 4. Zero-phonon line of internal Mn transition at 4 K, in Voigt configuration in magnetic field up to 11 T (step 1 T) measured by standard dispersive spectroscopy.

In GaMnAs, there is a common agreement that the Mn neutral acceptor state assumes the Mn2+ (3d 5 + h) with five electrons localized on 3d orbitals and one hole delocalized on a shallow acceptor level [15]. The binding energy is about 110 meV. In (Ga,Mn)N, a broad absorption band has been observed, extending from 2.1 eV to the band gap, which is about 3.5 eV [11,12], and ascribed to a band-level transition between the Mn 3d orbitals and the valence or conduction band, depending on the Fermi level position. A large ionization energy is deduced from this observation. This suggests a very strong localization of the hole around the Mn ion, and leads to the assumption of a Mn3+ (d 4 ) configuration.

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But some differences between Mn3+ in GaN and its counterpart Cr2+ in II–VI’s, which is in the d 4 configuration, have to be pointed out. The vibronic band is dominant in Cr2+ absorption spectra [16], while only weak phonon replicas are observed in (Ga,Mn)N. The integrated area of the ZPL divided by the integrated area of the phonon replicas or vibronic band is in the range of 10−4 for Cr2+ in II–VI hosts and is about 0.4 for (Ga,Mn)N. The integrated cross-section is defined as
1 k(E)dE (2) xMn where xMn is the Mn content in the Ga1−x Mnx N sample, and k(E) is the optical absorption coefficient measured at photon energy E with non-polarized light. The integrated cross-section the ZPL (1.1 × 10−23 m2 eV/Mn) is three orders of magnitude larger than for Cr2+ ions (3.7 × 10−27 m2 eV/Cr in ZnSe). However, the total integrated absorption divided by the Cr or Mn content is of the same order of magnitude: most of the absorption is contained in the ZPL. This weak electron-phonon coupling suggests a weak delocalization of the hole from the Mn3+ 3d levels. It may result from the p–d hybridization which is actually revealed in the band-structure calculation [13]. The calculation of the d 4 crystal field levels by the model developed in [16] for Cr in II–VI’s has been adapted for Mn in GaN in [6]. Tetrahedral crystal field, static tetragonal Jahn– Teller distortion, wurtzite trigonal crystal field and spin–orbit coupling are taken into account. The tetrahedral crystal field splits the free ion state 5 D (L = 2, S = 2) into a doublet 5 E and a ground triplet state 5 T2 . The Jahn–Teller distortion lifts the degeneracy of the 5 T2 into a doublet and a ground singlet state. The trigonal distortion, the spin–orbit coupling and the Zeeman term reveal the fine structure of the fivefold degeneracy of the ground state. This model can be applied to the measured layer because at such a low concentration, all Mn are isolated. The sharpness of the absorption lines in the present study gives us a better accuracy and points out some discrepancies with the calculated spectra reported in [6]. For instance, the calculated spectra in [6] exhibit two lines at 0 T, separated by about 1 meV, while we clearly observe a single line with a width of 1 meV. Hence a better check of the crystal field model and a better determination of the relevant parameters will be possible. Also, an estimation of the spin–carrier coupling should be achieved. Assuming a constant absorption edge shape, the magnetic circular polarization of the transmitted light is [17] P =−

dk(E) E dE 2

(3)

Using the mean field expression, for B  c, the exchange Zeeman splitting E is E = N0 (α − β)xMn Sz

(4)

where α and β stand for the exchange integrals with the conduction and valence band respectively, N0 the cation density and Sz the average spin of Mn3+ ions. The difference of s, p, d

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coupling (α − β) can be deduced. But this determination is not very precise due to at least two reasons. First, the absolute value of the magnetization in a thin layer of quite diluted magnetic semiconductor is not easy (the contribution from the substrate is much larger). Hence a calculation of the magnetization from the crystal field model would give a useful check. Also, in the wurtzite structure, two optically active excitons (A and B) are expected [18], with opposite MCD: in the present samples, due to a large MCD linewidth, the contribution of both the so-called A and B absorption edges cannot be distinguished, so that the estimation should lead to underestimated values. So far, (α − β) is roughly estimated at 0.2 meV [19] but this analysis is still in progress. 4. Conclusion (Ga,Mn)N epilayers have been investigated by magnetooptical spectroscopy. Near infrared absorptions are in agreement with the assumption of the incorporation of Mn as Mn3+ neutral acceptor. Nevertheless, the low electron–phonon coupling shows a peculiar behavior of Mn atoms which could be interpreted as a weak localization of the hole on the Mn3+ 3d levels. Following this assumption, the calculation of Mn3+ energy levels and the magnetization is under consideration to determine more accurately the exchange constant (α − β). References [1] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [2] S. Sonoda, S. Shimizu, T. Sasaki, Y. Yamamoto, H. Hori, J. Cryst. Growth 237-239 (2002) 1358. [3] M. Zajac, R. Doradzinski, J. Gosk, J. Szczytko, M. Lefeld-Sosnowska, A. Twardowski, M. Palczewska, E. Grzanka, W. Gcbicki, Appl. Phys. Lett. 78 (2001) 1276. [4] Y.L. Soo, G. Kioseoglou, S. Kim, S. Huang, Y.H. Kao, S. Kuwabara, S. Owa, T. Kondo, H. Munekata, Appl. Phys. Lett. 79 (2001) 3926. [5] J. Gosk, M. Zajac, A. Wolos, M. Kaminska, A. Twardowski, I. Grzegory, M. Bockowski, S. Porowski, Phys. Rev. B 71 (2005) 094432. [6] A. Wolos, A. Wysmolek, M. Kaminska, A. Twardowski, M. Bockowski, I. Grzegory, S. Porowski, M. Potemski, Phys. Rev. B 70 (2004) 245202. [7] S. Kuroda, E. Bellet-Almaric, R. Giraud, S. Marcet, J. Cibert, H. Mariette, Appl. Phys. Lett. 83 (2001) 7284. [8] X. Biquart, O. Proux, J. Cibert, D. Ferrand, H. Mariette, R. Giraud, B. Barbara, J. Supercond. 16 (2003) 792–795. [9] E. Gheeraert, T. Yamamoto, S. Kuroda, S. Marcet, H. Mariette, D. Ferrand, J. Cibert, in: Proceedings of the International Conference on Crystal Growth ICCG, Grenoble, J. Cryst. Growth 275 (2005) 2233. [10] R.Y. Korotkov, J.M. Gregie, B.W. Wessels, Appl. Phys. Lett. 80 (2002) 1731. [11] T. Graf, M. Gjukic, M.S. Brandt, M. Stuzmann, O. Ambacher, Appl. Phys. Lett. 81 (2002) 5159. [12] A. Wolos, M. Palczewska, M. Zajac, J. Gosk, M. Kaminska, A. Twardowski, M. Bockowski, I. Grzegory, S. Porwski, Phys. Rev. B 69 (2004) 115210. [13] A. Titov, X. Biquard, D. Halley, S. Kuroda, E. Bellet-Amalric, H. Mariette, J. Cibert, A.E. Merad, G. Merad, M.B. Kanoun, E. Kulatov, Yu.A. Uspenskii, Phys. Rev. B. 72 (2005) 115209. [14] R. Giraud, S. Kuroda, S. Marcet, E. Bellet-Almaric, X. Biquard, B. Barbara, D. Fruchard, D. Ferrand, J. Cibert, H. Mariette, Europhys. Lett. 65 (2004) 553.

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