Structural and magnetic properties of GaSb:MnSb granular layers

Radiation Physics and Chemistry 80 (2011) 1051–1057

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Structural and magnetic properties of GaSb:MnSb granular layers E. Dynowska a,n, J. Bak-Misiuk a, P. Romanowski a, J.Z. Domagala a, J. Sadowski a,b, T. Wojciechowski a, S. Kret a, B. Kurowska a, A. Kwiatkowski c, W. Caliebe d a

Institute of Physics Polish Academy of Sciences, al. Lotnikow 32/46, PL-02668 Warsaw, Poland MAX-Lab, Lund University P.O. Box. 118, S-22100 Lund, Sweden c Institute of Experimental Physics, Faculty of Physics, University of Warsaw, ul. Hoza 69, PL-00681 Warsaw, Poland d Hasylab at DESY, Notkestr. 85, D-22607 Hamburg, Germany b

a r t i c l e i n f o

abstract

Article history: Received 19 July 2010 Accepted 2 February 2011 Available online 1 March 2011

The results of structural and magnetic characterization of GaMnSb layers grown on GaSb(0 0 1) and GaAs(1 1 1) substrates are presented. The presence of hexagonal, highly oriented MnSb inclusions embedded in GaSb matrix has been demonstrated. The lattice parameters of these inclusions were the same as those for bulk MnSb for the layers grown on GaSb(1 0 0) substrate while for the layers grown on GaAs(1 1 1) the MnSb inclusions were strained. The influence of a presence of MnSb clusters on the lattice parameter of GaSb matrix has been demonstrated. It was confirmed that in all cases the MnSb clusters exhibit a ferromagnetic behavior at room temperature. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Granular structures GaSb MnSb Ferromagnetism Spintronics X-ray diffraction

1. Introduction Ferromagnetic III–V semiconductors containing Mn have been extensively investigated due to their potential spintronic applications. The ferromagnetic phase transition temperature Tc recorded in the diluted GaMnAs material is about 190 K (Chen et al., 2009), which is remarkably high for a DMS material but still very low for application purposes. For GaMnSb the reported Tc is much lower (25 K) and the hysteresis loops are very small (Matsakura et al., 2000). One approach to increase the value of Tc is to incorporate the Mn into the host materials in the form of digital alloys. For both GaAs/Mn (Soo et al., 2003) and GaSb/Mn (Chen et al., 2002) a digital alloys ferromagnetism was observed above room temperature. Strong ferromagnetic behavior was also expected for a granular GaAs:MnAs (Couto et al., 2005; Kwiatkowski et al., 2007; Moreno et al., 2002; Yokoyama et al., 2005) or GaSb:MnSb (Abe et al., 2000; Akinaga et al., 2001; Ganesan and Bhat, 2008; Matsukura et al., 2000; Mizuguchi et al., 2000), materials containing inclusions of hexagonal MnAs or MnSb nanoclusters, respectively, embedded in the semiconductor matrix. It has been shown that high-temperature growth (performed above 830 K) of GaMnSb layers induces the formation of MnSb nanoclusters in the GaSb matrix, whereas the low growth temperature (below 520 K) suppresses the formation of MnSb clusters (Abe et al., 2000). Both MnAs and MnSb bulk compounds have hexagonal

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0969-806X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2011.02.005

structure of NiAs-type and high Tc temperatures: 310 and 580 K, respectively. These findings suggest that a granular structured GaSb:MnSb might be particularly interesting candidates for a system with increased Tc. The aim of this work was to produce and to characterize the structure and magnetic properties of granular GaSb:MnSb layers grown by the molecular beam epitaxy (MBE) method on different substrates.

2. Experimental The Ga1  xMnxSb layers with different x-value were grown by the molecular beam epitaxy (MBE) method on two kinds of substrates: on the (1 0 0)-oriented GaSb and on the (1 1 1)Aoriented GaAs covered by a thin GaSb buffer layer. The growth temperature of all layers was kept at 720 K. The nominal Mn contents x were defined as the ratio of Mn to Ga flux during growth. The full description of the technological parameters of the studied samples is given in Table 1. The visualization of inclusions inside the layers was performed by scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The magnetic properties were investigated by magnetic force microscopy (MFM). The crystal structure of the layers was studied by X-ray diffraction methods applying both the standard laboratory source and the synchrotron radiation. Both the out-of-plane (a?) and the in-plane (a99) lattice parameters of GaSb matrix were determined from the 2y/o scans of symmetrical and asymmetrical reflections by using a

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Table 1 Technological parameters of investigated GaSb:MnSb layers. Sample symbol

Mn content x

Substrate

Thickness of the Growth Thickness of rate the buffer layer GaMnSb layer (ML/s) (nm) (nm)

E-1 E-2 E-3 HT-4 HT-5 HT-6

0.01 0.06 0.08 0.01 0.03 0.07

GaAs(1 1 1)A GaAs(1 1 1)A GaAs(1 1 1)A GaSb(1 0 0) GaSb(1 0 0) GaSb(1 0 0)

120 80 10 – – –

630 830 630 630 630 630

2 2 2 2 2 2

high resolution (HRXD) Philips material research diffractometer (MRD) in double and triple crystal configuration, equipped with a standard laboratory source of Cu Ka1 radiation. The determination of the lattice parameters of MnSb inclusions was possible with the use ˚ at the W1 of monochromatic synchrotron radiation (l ¼1.54056 A) beamline at DESY-Hasylab. The suitable choice of reflections for determination of the lattice parameters of MnSb inclusions was conditioned by the substrate orientation. For layers grown on (1 0 0)-oriented substrates the 20.2 and 30.0 reflections were used for that purpose while in the case of (1 1 1)-oriented substrates – the 00.4 and 10.5 reflections were measured, respectively.

3. Results and discussion Two kinds of samples were investigated: first kind of GaMnSb layers was grown on a substrate consisting of a thin GaSb buffer layer on the GaAs(1 1 1)A, and the second one – on the GaSb(1 0 0) substrate. The samples from the first group, denoted by E-1, E-2, and E-3, have a nominal Mn concentrations x¼0.01, 0.06, and 0.08, respectively. The samples belonging to the second group grown with nominal Mn concentrations x ¼0.01, 0.03, and 0.07 are denoted by HT-4, HT-5, and HT-6 (see Table 1). It was shown that the growth temperature 720 K induces the formation of MnSb clusters in all studied samples. 3.1. Structural results obtained for samples GaMnSb/GaSb/ GaAs(1 1 1)A The SEM studies evidenced on the sample surfaces the objects of different shapes and sizes depending on the samples (Fig. 1), interpreted as other phase inclusions. Generally, the sizes of these inclusions ranged from 80 to 600 nm. The lattice parameters of these layers were calculated from 2y/o scans of the 333 symmetrical and 331 asymmetrical reflections performed with the use of HRXD techniques. The results showed that all the layers were practically relaxed. As it is seen in Table 2 the lattice parameters of the GaMnSb matrix, within the experimental error, were close ˚ to that of bulk GaSb (aGaSb ¼6.0964 A). The presence of MnSb inclusions was demonstrated by X-ray diffraction with the use of synchrotron radiation. Typical diffraction pattern obtained in the 2y/o mode (coupling on 333 peak of GaAs substrate) is shown in Fig. 2. The strongest peaks in this pattern correspond to the GaAs substrate and GaSb layer, moreover, the three orders of reflections from (0 0 .1) lattice planes of hexagonal MnSb are also visible. It means that (0 0 .1) planes from MnSb inclusions are parallel to the (1 1 1) planes of the substrate. In other words, the c-axis of MnSb crystals is perpendicular to the substrate. A small amount of MnSb clusters was grown with (1 0 .1) lattice planes parallel to the (1 1 1) planes of the substrate, which confirms the presence of 10.1 and 20.2 reflections in the pattern. Nevertheless, the MnSb inclusions grow mainly on the {1 1 1} planes of GaSb matrix.

Fig. 1. The SEM pictures obtained for GaSb:MnSb layers grown on GaAs(1 1 1) substrates with nominal Mn contents: (a) E-1, x¼ 0.01, (b) E-2, x¼ 0.06, and (c) E-3, x¼ 0.08.

The 2y/o scans of symmetrical 00.4 and asymmetrical 10.5 reflections from MnSb inclusions allowed estimating values of ˚ their lattice parameters: a ¼4.1070.04 A˚ and c ¼5.848 70.004 A. Such result, within the experimental error, was the same for all samples grown on the GaAs(1 1 1) substrate. These values differ from those characteristic for bulk MnSb (a¼4.128 A˚ and ˚ – one can conclude that the MnSb inclusions in the c¼5.789 A) layers were strained. This supposition has been confirmed by the

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Table 2 The results of X-ray measurements for GaSb:MnSb layers grown on GaAs(1 1 1)A. Sample symbol

Mn content x

Lattice parameter of the ˚ GaSb matrix a (A)

Lattice parameters of the ˚ MnSb inclusions a, c (A)

E-1

0.01

6.0966 7 0.0003

E-2

0.06

6.0972 7 0.0003

E-3

0.08

6.0972 7 0.0003

a ¼4.10 7 0.04; c ¼ 5.848 70.004 a ¼4.10 7 0.04; c ¼ 5.848 70.004 a ¼4.10 7 0.04; c ¼ 5.848 70.004

Fig. 2. The 2y/o diffraction pattern performed with the use of synchrotron radiation for GaSb:MnSb layer on GaAs(1 1 1) substrate (sample E-2).

results of TEM studies. However, it does not influence the strain state of GaSb matrix (see Table 2). According to the TEM results the distribution of inclusions depends on the Mn content: for the layers with small Mn contents (0.01–0.03) the inclusions were detected on the interface only (Fig. 3a), while for the samples with higher Mn concentration (0.06–0.08) the inclusions were detected also inside the layers. They are strained by surrounded matrix — a set of dislocations is clearly visible around these inclusions (Fig. 3b). Fig. 3c shows the highly oriented growth of the MnSb inclusion inside the GaSb matrix: the (0 0 .1) lattice planes of the inclusion are parallel to the (1 1 1) lattice planes of the matrix. Fig. 3. TEM pictures: (a) small Mn content (x ¼0.03), (b) large Mn content (x¼ 0.08), and (c) (0 0 .1) lattice planes of the inclusion are parallel to the (1 1 1) lattice planes of the matrix.

3.2. Structural results obtained for samples GaMnSb/GaSb(1 0 0) The SEM studies confirmed the granular character of these layers also (Fig. 4). The sizes of the inclusions depended on the sample – they were larger for layers with higher Mn content: the smallest size was 50 nm and the highest one – 600 nm. The lattice parameters of the investigated layers were calculated from the symmetrical 004 and asymmetrical 224 2y/o scans performed with the use of the HRXD methods. It was confirmed that all GaMnSb layers were pseudomorphic i.e. their in-plane lattice parameter (a99) value was equal to that of the substrate. The 004 2y/o scans for HT-4, HT-5 and HT-6 samples are shown in Fig. 5. One can see that diffraction peaks related to the layers are shifted into higher Bragg angles and this shift is more pronounced for samples with a higher Mn concentration. As a consequence, the relaxed lattice parameters of GaSb:MnSb layers are slightly lower than that of bulk GaSb. The precise results are presented in Table 3.

The evidence of the MnSb clusters existing in the layers was possible owing to the use of synchrotron radiation. Typical symmetrical 2y/o diffraction pattern taken for the sample HT-6 is presented in Fig. 6. The strongest peaks are related to the GaSb substrate, the significantly weaker peaks were identified as originating from (1 0 .2), (2 0 .3), and (3 0 .4) lattice planes of MnSb hexagonal phase. The occurrence of such sequence of MnSb peaks indicates a block structure of the MnSb clusters. Assuming that MnSb inclusions grew in a similar manner as those in the case of the first type of samples, i.e. on {1 1 1} lattice planes of the matrix, we were able to find them in the vicinity of the 004 reflection of GaSb (Fig. 7) measured in the rocking curve mode (o scan). It was possible because the lattice spacing of 20.2 ˚ which is very close to that reflection for MnSb is equal to 1.521 A, ˚ On the other hand, the angle between of 004 for GaSb (1.524 A). (1 0 .1) planes of MnSb structure and (1 0 0) planes of the

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Fig. 5. The HRXD 004 2y/o scans for HT-4, HT-5, and HT-6 samples.

Table 3 The results of X-ray measurements for GaSb:MnSb layers grown on GaSb(1 0 0).

Fig. 4. The SEM pictures obtained for GaSb:MnSb layers grown on GaSb(1 0 0) substrates with nominal Mn contents: (a) HT-4, x ¼ 0.01, (b) HT-5, x¼ 0.03, and (c) HT-6, x¼ 0.07.

substrate is about 3.61 if the MnSb inclusions grow on the {1 1 1} planes of the GaSb matrix. All the peaks on both sides of central 004 GaSb peak originated from (1 0 .1) planes of different MnSb blocks, which are tilted with respect to each other and to the substrate orientation. The symmetrical character of this rocking curve is connected with the symmetry of the {1 1 1} lattice planes with respect to the [1 0 0] direction in cubic crystals. The 2y/o

Sample symbol

Mn content x

Lattice parameter of the ˚ GaSb matrix a (A)

Lattice parameters of the ˚ MnSb inclusions a, c (A)

HT-4

0.01

6.0946 7 0.0002

HT-5

0.03

6.0950 7 0.0002

HT-6

0.07

6.0934 7 0.0002; 6.0922 7 0.0002

a¼ 4.126 70.003; c¼ 5.789 7 0.003 a¼ 4.126 70.003; c¼ 5.789 7 0.003 a¼ 4.126 70.003; c¼ 5.789 7 0.003

Fig. 6. The 2y/o diffraction pattern performed with the use of synchrotron radiation for GaSb:MnSb layer on the GaSb(1 0 0) substrate (sample HT-6).

scan in the vicinity of asymmetrical 224 GaSb reflection allowed to detect the 30.0 MnSb reflection because in this orientation of inclusions the lattice planes {1 0 .0} of MnSb are parallel to the {1 1 2} planes of GaSb matrix. Next, from the 2y/o scans of 20.2 and 30.0 MnSb reflections the values of the lattice spacing ˚ were calculated. These values d20.2 ¼1.521 A˚ and d30.0 ¼1.191 A,

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Fig. 7. The GaSb 004 o scan for sample HT-6 in the large range of o angles. The block structure of MnSb inclusions is demonstrated.

were the same within the accuracy 70.001 A˚ for all blocks and for all samples. Finally, the lattice parameters of the hexagonal unit cell of MnSb inclusions in GaSb:MnSb layers grown on GaSb(1 0 0) substrate, a¼ 4.12670.003 A˚ and ˚ were determined, which leads to the concluc¼ 5.78970.003 A, sion that the MnSb clusters in these kind of samples are relaxed. All results of X-ray measurements for the samples E-4, E-5, and E-6 are presented in the Table 3. A decrease in the lattice parameters of GaSb matrix can be attributed to the presence of Mn atoms in Ga position (Ganesan and Bhat, 2008), or to strain effects. The TEM studies revealed two characteristic kinds of MnSb inclusions in the GaMn:MnSb layers grown on the GaSb(1 0 0) substrates: the trapezoidal objects grown near the layer surface (Fig. 8a,c) and the rectangular objects inside the layers (Fig.8b,d). The block structure can be observed in Fig. 8c.

3.3. Magnetic properties The surface topography and magnetic properties were examined for all samples by AFM and MFM measurements. Fig. 9a shows the AFM result obtained on the part of HT-4 sample of the size 10 mm  10 mm, and Fig. 9b shows magnetic structure of the same region. These results are qualitatively the same and typical for the samples grown on the GaSb(1 0 0) substrates (HT-4, HT-5, and HT-6). The inclusions have approximately a spherical or a cylindrical shape and their size typically ranges from 100 to 500 nm. Some of them are elongated in the directions, which are parallel to the sample edges. These inclusions are sources of strong magnetic contrasts (10–20 Hz in this case). This suggests that the inclusions are ferromagnetic at room temperature. Typically magnetization vectors of the inclusions lie in the plane of sample surface, or they are slightly inclined relatively to the surface. The magnetization direction and sense of individual grains is thermodynamically stable in the time scale of the order of 1 h, but sometimes, as it is visible, magnetization of individual inclusions changes its sense during the scan. The AFM results for sample E1 are presented in Fig. 10. This image shows some elevations on the layer surface. They look like pyramids with triangular base, and sizes of the order of 2 mm. There are also ferromagnetic inclusions similar to those observed on the HT-4, but in this case a density of inclusions is smaller and

Fig. 8. The TEM picture showing the characteristic shape of MnSb inclusion near the layer surface, sample HT-4: (a), (b) full view and (c), (d) magnification of an upper part of the objects.

magnetization of some of them is perpendicular to the sample surface. The detailed magnetic properties of MnSb hexagonal inclusions in a granular GaSb:MnSb material will be reported elsewhere.

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Fig. 9. The AFM and MFM results for the sample HT-4: (a) topography of the surface and (b) magnetic structure of the same region.

Fig. 10. The AFM and MFM results for the sample E-1: (a) topography of the surface and (b) magnetic structure of the same region.

Acknowledgments 4. Conclusions All investigated GaMnSb layers grown by the described MBE method exhibit characteristic granular structure consisting of MnSb clusters embedded in the GaSb matrix. It was proved that these inclusions have hexagonal, NiAs-type structure, with the lattice parameters equal or close to those corresponding to bulk MnSb. These clusters have the single-crystalline character with lattice parameter c parallel to the /1 1 1S directions of the GaSb matrix. There are also some differences depending on the substrate orientation: the clusters created in the layers grown on the GaSb(1 0 0) substrates were relaxed but consist of several mutually tilted grains (block structure). On the other hand, the inclusions inside the layers obtained on the GaAs(1 1 1) substrates were strained, but they did not exhibit a block structure. The lattice parameter of GaSb matrix differed from that for bulk crystal and it was various for each sample. The reason for such behavior is not yet clear. The MnSb inclusions in the studied sample are ferromagnetic at room temperature.

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