Eur. Phys. J. B (2013) 86: 52 DOI: 10.1140/epjb/e2012-30566-3
THE EUROPEAN PHYSICAL JOURNAL B
Regular Article
Tuning optical and ferromagnetic properties of thin GdN films by nitrogen-vacancy centers Reddithota Vidyasagar1,a , Shinya Kitayama1, Hiroaki Yoshitomi1 , Takashi Kita1 , Takahiro Sakurai2 , and Hitoshi Ohta3 1 2 3
Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, 1-1 Rokkodai, 657-8501 Kobe, Japan Center for Support to Research and Education Activities, Kobe University, 1-1 Rokkodai, 657-8501 Kobe, Japan Molecular Photoscience Research Center and Graduate School of Science, Kobe University, 1-1 Rokkodai, 657-8501 Kobe, Japan Received 9 July 2012 / Received in final form 22 August 2012 c EDP Sciences, Societ` Published online 13 February 2013 – a Italiana di Fisica, Springer-Verlag 2013 Abstract. AlN/GdN/AlN double heterostructures were grown on c-sapphire substrates using a reactive rf sputtering method under high vacuum conditions. The optical absorption spectrum of the GdN shows a clear fundamental band edge of GdN around 800 nm; this transition is attributed to the minority spin band energy of GdN at the X point. Nitrogen vacancy centers cause a blue-shift of the optical band edge of GdN, which could be ascribed to both the band filling, and the electron-hole interactions resulting from the free carriers generated by nitrogen vacancies. Temperature-dependent magnetization measurements demonstrate a clear change in the magnetization values of GdN with respect to the N2 partial pressure. Nitrogen vacancy centers in the thin GdN film raise the Curie temperature from 31 K to 39 K, which has been accurately measured by the Arrott plots.
1 Introduction Controlling the optical and ferromagnetic properties of rare-earth nitrides by nitrogen vacancy centers has been identified as an important field of research [1,2]. Recently, it has attracted more and more attention with its applications to spin based devices and quantum computation [3,4]. According to earlier reports, GdN is considered a ferromagnetic semiconductor where both semiconducting and ferromagnetic characteristics are exhibited simultaneously [5,6]. GdN can support a partially correlated electronic system due to its fcc structure and its highly localized, half-filled 4f shell with a spin moment of up to 7 μB /Gd3+ . However, earlier theoretical and experimental investigations on its electrical and magnetic properties showed conflicting results ranging from metal to insulator, and from paramagnetic to ferromagnetic [7–9]. In point of fact, growing GdN without lattice imperfections and extrinsic defects was difficult due to its high reactivity towards oxygen when exposed to air [10,11]. Very recently, it has been found that the temperature dependent optical and magnetic properties of thin GdN films show a red shift in both the majority and minority spin gaps; there is also a clear spin splitting in the band structure
Contribution to the Topical Issue “Excitonic Processes in Condensed Matter, Nanostructured and Molecular Materials”, edited by Maria Antonietta Loi, Jasper Knoester and Paul H. M. van Loosdrecht. a e-mail:
[email protected]
of GdN. These results demonstrate that GdN is an intrinsic ferromagnetic semiconductor [5,12]. To understand the optical and ferromagnetic properties of thin GdN films, detailed temperature-dependent optical and magnetic measurements on a stoichiometric structure are required. In this paper, we describe optical and magnetic properties of epitaxial AlN/GdN/AlN double heterostructures grown by a reactive rf sputtering technique under high vacuum conditions. The GdN layer has been grown at different reactive gas ratios (Ar : N2 ) of 9 : 6, and 9 : 3.6, which provides different nitrogen deficient conditions The optical and magnetic properties depending on the nitrogen deficient conditions have systematically been investigated by optical absorption spectroscopy and superconducting quantum interference device (SQUID) measurements.
2 Experimental AlN/GdN/AlN double heterostructures were grown on c-sapphire (0001) substrates at 500 ◦ C using reactive rf magnetron sputtering under high vacuum conditions, and the base pressure of the chamber was less than 2.5 × 10−6 Pa [12]. We used 6 N pure mixed gases of Ar and N2 for the reactive growth, and the total sputtering pressure was 5 Pa. Here, we used different pressure ratios (Ar : N2 ) of 9 : 6, and 9 : 3.6 in order to control the nitrogen deficiency of the GdN. A 10 nm thick GdN layer was grown in between a 100 nm thick AlN buffer layer and a 100 nm thick AlN capping layer, which effectively restricts
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Fig. 1. (Color online) (a): Absorbance spectrum of 10 nm thin GdN film at different N2 pressures. (b): Tauc plots for 10 nm thin GdN film at different N2 pressures.
3 Results and discussion
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oxygen contamination. The growth rates for AlN and GdN were 32.3 and 2.0 nm/s, respectively. The secondary ion mass spectroscopic (SIMS) analysis of the AlN/GdN/AlN double heterostructure confirmed the oxygen content was less than 1.0 × 1019 cm−3 (not shown here). The optical and magnetic properties of the heterostructures were characterized by using an ultra violet-visible-near infrared spectrophotometer (model: Solid spec 3700) and a SQUID magnetometer (Quantum design MPMS-XL).
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3.1 Optical properties of GdN thin films Figure 1a shows the optical absorbance spectra for 10 nm thick GdN films deposited at two different partial pressures of nitrogen. These spectra were recorded in the wavelength range of 350–2600 nm using the spectrophotometer. The spectral trends have similar line shapes with deviations in the absorbance edge around 800 nm. In addition to that, the optical absorbance has been slightly increased in the infrared regime. The absorbance edge shifts (blue shift) towards the lower wavelength side when the nitrogen partial pressure is increased. This change in the optical absorbance spectrum can be interpreted by band filling effects and electron-hole interactions resulting from free carrier absorption effects generated by nitrogen vacancies in GdN [13]. By using these optical absorbance spectra, the optical direct band gaps for the GdN thin films have been estimated using a Tauc plot, which is shown in Figure 1b. From Figure 1b, it is clear that the evaluated optical bandgap is reduced from 1.03 (Ar : N2 = 9 : 6) to 0.95 eV (Ar : N2 = 9 : 3.6) with the increase in the density of nitrogen vacancies in GdN. Interestingly, these optical band gaps are quite large compared to the half-metallic band gap of 0.6 eV estimated from theoretical calculations [14]. Thus, the half-metallic nature of GdN has completely been abandoned. Figure 2 shows the temperature-dependent optical direct bandgap of the thin GdN film deposited at Ar : N2 = 9 : 3.6. A remarkable magneto-optical effect has been
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observed. From Figure 2, it is obvious that the band gap shrinks at the X point with the temperature below 38 K; the dramatic reduction in the optical bandgap below 38 K is approximately 70 meV. This well-known red shift in the optical bandgap is ascribed to the alignment of magnetic spins via the long range correlation of spins below its critical temperature for ferromagnetic materials [5,12]. Shrinkage in the optical bandgap pertaining to the spin disorder scattering by thermodynamic fluctuations of the magnetic spins is anomalously reduced when the magnetic moments are aligned in parallel. When the temperature is increased from 38 K to 100 K, the optical bandgap gradually decreases from 1.96 eV to 1.87 eV. This trend is attributed to a well-known phenomenon in semiconductors [15]. Observation of red-shift in GdN is important evidence for an intrinsic ferromagnetic semiconductor. 3.2 Magnetic properties of thin GdN films Figure 3a presents the temperature-dependent magnetization for the 10 nm GdN films grown at the two different nitrogen partial pressures. It is obvious that the magnetization values of the nitrogen deficient GdN film
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Fig. 3. (Color online) (a): Temperature dependent magnetic moment of 10 nm GdN thin film at Ar : N2 = 9 : 3.6 and Ar : N2 = 9 : 6. (b): Magnetic hysteresis loop of 10 nm thin GdN grown at Ar : N2 = 9 : 3.6 ccm. (c) and (d): Arrott plots for 10 nm thin GdN grown at Ar : N2 = 9 : 6 and 9 : 3.6 ccm.
(Ar : N2 = 9 : 3.6) are comparatively higher than the other film. Meanwhile, the magnetization curve shifts to the higher temperature side with the decrease in the nitrogen partial pressure from 6 to 3.6 ccm. This shift can be attributed to the enhancement of the Curie temperature (Tc ). Figure 3b shows that the magnetichysteresis loop measured at 4.2 K for the 10 nm GdN film grown at Ar : N2 = 9 : 3.6. The saturation magnetization (Ms ) and remanent magnetization (Mr ) are found to be 4.42 μB /Gd3+ and 3.06 μB /Gd3+ , respectively. From the intercepts in the x-axis in Figure 3b, it can be seen that the coercivity is 44 Oe, and this value is comparatively less than the previous reports of Natali et al. [16]. Even trace amount of oxygen contamination in GdN will increase the coercive field to thousands of Oe, hence Tc decreases. The very small coercive field in the present study attests to the absence of significant quantities of oxygen; this has been noted in our earlier reports from SIMS data [5]. One of the standard experimental methods for determining the Curie temperature accurately is the use of Arrott plots, in which the square of the magnetization M in a field B is plotted as a function of B/M at different temperatures, as shown in Figures 3c and 3d. The intercept (1/χ) (χ is the susceptibility) on the B/M axis vanishes at 31 K and 39 K, which indicates that Tc has been projected as 31 K and 39 K for the GdN films deposited at Ar : N2 = 9 : 6 and 9 : 3.6, respectively. It has been reported earlier that the Curie temperature of GdN can be
enhanced by adding charge carriers; those charge carriers could be controlled by the nitrogen content of GdN films during growth process [17]. Furthermore, recent reports on the enhancement of Tc could be due to structural distortion of the lattice caused by nitrogen vacancies [14]. The lattice distortion in GdN is caused by the electron doping (nitrogen vacancies), and is very likely to strengthen the antiferromagnetic superexchange over the nearest-neighbor ferromagnetic exchange, resulting in AFM behavior. The exchange coupling (J1 and J2 ) between the antiferromagnetic GdN-II and ferromagnetic GdN results in an effective magnetic hardening [18,19]. Hence, the enhanced Curie temperature (Tc ) value obtained in the present study could be attributed to the differences in Gd-N ratio, defects associated by nitrogen vacancies (structural distortion and exchange interactions), and biaxial compressive strains in the GdN layer.
4 Conclusions AlN/GdN/AlN heterostructures were grown on c-sapphire substrates using a reactive rf sputtering technique under high vacuum conditions. The absorbance spectrum shows a prominent fundamental absorbance edge around 800 nm. A broad absorbance signal has been observed at the longer wavelengths arising from free carriers generated
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by nitrogen vacancies in GdN. The temperature dependent bandgap demonstrated a remarkable red-shift below Tc . It is obvious that the presence of nitrogen vacancies in the GdN layer has significantly increased the magnetization values. The Arrott plot reveals that the Curie temperature (Tc ) of GdN has been enhanced from 31 K to 39 K by an increase in the nitrogen deficiency. These results confirm that GdN is an intrinsic ferromagnetic semiconductor, and that Tc can be controlled by nitrogen vacancies.
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