Crystal growth and characterization of GaCrN nanorods on Si substrate

ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 2962–2965

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Crystal growth and characterization of GaCrN nanorods on Si substrate H. Tambo, S. Kimura, Y. Yamauchi, Y. Hiromura, Y.K. Zhou, S. Emura, S. Hasegawa, H. Asahi  The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

a r t i c l e in f o

a b s t r a c t

Available online 20 January 2009

GaCrN nanorods were grown on GaN nanorods by RF-plasma-assisted molecular beam epitaxy. GaN nanorods were grown on Si (0 0 1) substrates with native SiO2. Cr doping into GaCrN nanorods was conducted at substrate temperatures of 800 and 550 1C. Cross-sectional transmission electron microscopy images revealed that the diameter of GaCrN nanorod gradually increases with growth proceeding at 550 1C, while the growth at 800 1C does not change the nanorod diameter. Lowtemperature growth enhances the growth perpendicular to the c-axis and decreases the growth along the c-axis. It was found that the solubility limit of Cr atoms in GaCrN is much higher for the lowtemperature growth than for the high-temperature growth. It was also found that the highest saturation magnetization is obtained at some optimum Cr cell temperature. & 2009 Elsevier B.V. All rights reserved.

PACS: 75.50.Pp 78.55.Cr 61.46.Hk Keywords: A1. Nanostructures A1. Solubility A3. Molecular beam epitaxy B1. Nitrides B2. Magnetic materials B2. Semiconducting III–V materials

1. Introduction Since the discovery of the carrier-induced ferromagnetism in InMnAs [1] and GaMnAs [2], diluted-magnetic semiconductors (DMSs) have caught a great deal of attention because of their potential applications which will open the way for spindependent photonic and electronic devices. However, these materials are impractical because of their low Curie temperatures much below the room temperature. On the other hand, we have succeeded in the growth of GaCrN layers by plasma-assisted molecular beam epitaxy (MBE) and the observation of ferromagnetism at temperatures as high as 400 K [3]. In order to obtain ferromagnetism, DMSs in wide gap semiconductors are necessary to make the concentration of magnetic atoms high because of short range of electron exchange between magnetic atoms [4]. However, ferromagnetic characteristics were observed even at above room temperature from GaCrN with relatively low Cr concentrations [5]. Recently, theoretical study by first principle calculations and Monte Carlo simulations were performed and the possibility of DMSs in wide gap semiconductors, in particular transitionmetal-doped GaN, was reported [6–8]. Their calculation shows that superparamagnetic blocking temperature (TB) of the DMS is enhanced and hysteretic magnetic response can be observed at finite temperature due to the formation of magnetic nano-clusters of magnetic impurities under the spin decomposition. The shape Corresponding author. Tel.: +81 6 6879 8405; fax: +81 6 6879 8409.

E-mail address: [email protected] (H. Asahi). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.01.068

of the cluster in the Konbu phase has observed higher TB and they expected that TB higher than room temperature can be obtained with ferromagnetic behavior. The Konbu phase like structures were observed experimentally for AlCrN and GeMn thin films, where ferromagnetic characteristics were also observed at above the room temperature [9,10]. On the other hand, we have succeeded in the growth of highly c-oriented hexagonal-GaN nanorods on Si (0 0 1) substrates with native SiO2 by RF-plasma-assisted MBE (RF-MBE) under nitrogen rich (N-rich) condition [11]. GaN nanorods show highly efficient photoluminescence due to their dislocation-free nature [12,13]. In this paper, we will report on the growth temperature dependence of the shape of GaCrN nanorods and the local structure around Cr atoms in GaCrN. The influence of the Cr concentration on the magnetic properties will also be discussed.

2. Experimental procedure GaCrN nanorods were grown on Si (0 0 1) substrates with native SiO2 by RF-MBE. The thickness of SiO2 thin layers is 2 nm or less. Elemental Ga, Cr and RF plasma-enhanced N2 were used as sources. After thermal cleaning of Si substrates at 830 1C for 15 min, low-temperature GaN buffer layers were first grown at 450 1C for 40 s. Then annealing was performed for 10 min in the plasma-activated nitrogen atmosphere at the same temperature as that of the growth temperature of GaN nanorods, i.e. 700 or 800 1C. Then GaN nanorods were grown followed by the growth of GaCrN nanorods. Two kinds of samples were grown. For the first

ARTICLE IN PRESS H. Tambo et al. / Journal of Crystal Growth 311 (2009) 2962–2965

kind of sample, both GaN and GaCrN nanorods were grown at 700 1C (Ts(GaN); Ts(GaCrN)=700 1C). For the other kind of sample, GaCrN nanorods were grown at 550 1C on the 800 1C-grown GaN nanorods (Ts(GaN)=800 1C; Ts(GaCrN)=550 1C). Cr cell temperature was varied in the range of TCr=880–930 1C for the Ts(GaCrN)=700 1C sample and 900–1030 1C for the Ts(GaCrN)=550 1C sample, respectively, while Ga flux (1.0  10 7 Torr), N2 flow rate (1.0 sccm) with RF plasma power (300 W) were fixed. Finally, GaN layer was deposited to prevent the oxidation of GaCrN nanorods surface. Cr concentrations of samples were estimated with electron probe microscope analyzer (EPMA) by comparing with a reference sample. Cr concentration of the reference sample was determined by fluorescence X-ray analysis. Cross-sectional transmission electron microscope (XTEM) and X-ray absorption fine structure analysis (XAFS) were used to investigate crystal structure. XAFS measurements were performed at BL-9A of the photon factory (PF). The Cr k-edge at room temperature was examined with a fluorescence mode using 19 SSD detectors. Magnetic properties were investigated using superconducting quantum interference device (SQUID) magnetometer.

3. Results and discussion XTEM images of GaCrN nanorods are shown in Fig. 1. This sample was prepared by the growth of GaN nanorods at 800 1C for 15 min and then GaCrN nanorods at 550 1C with Cr cell temperature of 930 1C for 60 min. From the magnified XTEM image of Fig. 1(b), it can be seen that c-oriented nanorods with a diameter of around 50 nm are vertically grown on SiO2/Si substrates. The XTEM image reveals that the diameter of GaCrN nanorod gradually increases with the proceeding of growth at 550 1C. The growth rates of GaN nanorods and GaCrN nanorods are about 3.6 and 2.8 nm/min, respectively. This result suggests that the low-temperature growth enhances the growth perpendicular to the c-axis (along the lateral direction) and decreases the growth

a

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along the c-axis. We have also found that these phenomena occur even for the GaN nanorods, which were grown under the same conditions only without Cr doping. It is considered that the diameter of nanorod is greatly dependent on growth temperature, which may correspond to V/III ratio [13]. In order to investigate substrate temperature dependence of local structure around Cr atoms in GaCrN, these samples were subjected to XAFS measurements. XAFS oscillations w(k) were extracted from the observed XAFS spectra. The w(k) function was multiplied by k3 to compensate the damping of the amplitude of XAFS oscillation. XAFS oscillation data k3w(k) from the GaCrN nanorods grown at 550 1C and 700 1C are presented in Fig. 2. Absolute values of Fourier transforms (FT) taken from the range of 1.5–14 A˚ 1 in k-space of XAFS oscillation are shown in Fig. 3. For comparison with the GaCrN nanorods, the NaCl-type CrN and Cr-substituted hexagonal-GaCrN thin film grown on Al2O3 (0 0 0 1) are also shown as Refs. [14,15]. Peak a in Fig. 3 corresponds to the first nearest neighbor N atoms. Peak b and g originated from the second nearest neighbor Cr atoms in NaCl-type CrN and Ga atoms in GaCrN. GaCrN nanorods grown at 550 1C (Fig. 3(c)) depicts a clearly different curve from that grown at 700 1C (Fig. 3(a)), even though both samples were grown at the same Cr cell temperature (TCr=930 1C). Fig. 3(c) curve is nearly the same as that for the substitutionally Cr-incorporated h-GaN thin film, shown in Fig. 3(e). The sample of Fig. 3(b) grown at 700 1C with low Cr concentration also shows a similar curve. Thus, we can conclude that phase separation does not occur in the GaCrN nanorods samples of Fig. 3(b) and (c), and that the solubility limit of Cr atoms in GaCrN nanorods is much higher for the 550 1C growth than that for the 700 1C growth. Furthermore, it was found that NaCl-type CrN is hardly formed in the nanorods growth than in the growth of thin film on MgO (0 0 1) substrate at 700 1C [14]. The fact that these GaCrN nanorods were grown under N-rich condition might be the reason for such difference [16]. In addition, peak a is clearly observed for the sample of Fig. 3(c). This may suggest the improvement in the quality of the crystal local structure as clearly seen by comparing with Fig. 3(e). On the other hand, in the sample of Fig. 3(a), peaks b and g are observed. It indicates that most of the doped Cr atoms

(a) Ts (GaCrN) = 700°°C ~0.6 %

(b)

100 nm k 3 χ (k) [a.u.]

Ts (GaCrN) = 700°C ~0.1 %

b

(c) Ts (GaCrN) = 550°C ~0.6 %

0

50 nm Fig. 1. XTEM image of GaCrN nanorods. (a) whole image of nanorods, (b) magnified image of GaCrN nanorods (Ts(GaN)=800 1C; Ts(GaCrN)=550 1C; TCr=930 1C).

2

4

6

8

10

12

14

k [Å-1] Fig. 2. k3w(k) function of the Cr k-edge XAFS for GaCrN nanorods with different Ts and Cr concentration.

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H. Tambo et al. / Journal of Crystal Growth 311 (2009) 2962–2965

in the high Cr content samples segregate to form NaCl-type CrN as a secondary phase. In order to obtain further information, we have fitted the XAFS data with McKale database. Table 1 shows the fitted values of the physical parameters for each FT peak using McKale database. The fitting range for each peak is from 1.5 to 14 A˚ 1 in the k-space in Fig. 2. From Fig. 3(a), the best-fitted scattering atom for peak b is Cr, and the fitted value of the coordination number and bond distance from absorption Cr atoms are 3.2 and 2.93 A˚, respectively. This bond distance value is corresponding to CrN with NaCl structure. Besides, for peak g, they are 9.0 and 3.23 A˚, respectively. This is from Ga atoms in h-GaCrN nanorods. The coordination numbers and relative intensities of peaks in Fig. 3(a) show that NaCl-type CrN coexists, in this case.

α

β

γ

Ts (GaCrN) = 700°C ~0.6 %

(a)

Fig. 4 shows the saturation magnetization value (Ms) obtained from the magnetization versus magnetic field (M–H) curve at 300 K for the samples grown at Ts(GaCrN)=550 1C as a function of Cr concentration. To calculate the Ms per cm3, the volume of nanorods was simply assumed as those having no air gaps and thin film volume value was used. The magnetic field was applied parallel to the sample plane. The sample of Fig. 3(c) grown with Cr cell temperature of 930 1C showed the largest Ms. For this sample, the Cr concentration is approximately 0.6%. With decreasing Cr cell temperature, the concentration of Cr atoms that substitute Ga sites and the Ms are decreased. On the other hand, for the Cr cell temperature above 930 1C, the Ms also decreased, since the antiferromagnetic CrN secondary phase come to form. Therefore, it was found that the optimum Cr concentration is 0.6% under the present growth conditions. The values of Ms for samples of Fig. 3(a) and (b) were also calculated and were 0.2 and 0.3 emu/cm3, respectively. It is considered that the origin of this small Ms of the former is that the Cr atoms form CrN with antiferromagnetic phase partially, as already shown in the XAFS analysis. On the other hand, for

1.2 Ts (GaCrN) = 700°C ~0.1 %

1.0

Ms [emu/cm3]

FT Intensity [a.u.]

(b)

Ts (GaCrN) = 550°C ~0.6 %

(c) CrN (d)

0.6 0.4 0.2

GaCrN thin film

(e)

0.8

0.0

0

1

2 3 4 Radial Distance [Å]

5

0.1

6

Fig. 3. FT intensity of the Cr k-edge for GaCrN nanorods with different Ts and Cr concentration.

2

4

6

8

1 Cr Concentration [%]

2

4

6

Fig. 4. Ms obtained from the M–H curve at 300 K for as a function of Cr concentration.

Table 1 Fitted parameters for each sample. N, R, DW and MF are coordination number, radial distance, Debye-Waller factor and mean free path, respectively. R (A˚)

DW

MF

R factor (%)

9.0 3.2

3.23 2.93

0.09 0.03

7.0 8.0

0.96

Ga Cr

8.9 –

3.19 –

0.08 –

7.0 –

1.27

0.6

Ga Cr

9.0 –

3.22 –

0.06 –

6.0 –

1.45

(d) CrN

–

Cr

12.3

2.94

0.03

4.5

0.96

(e) GaCrN thin film

1.0

Ga Cr

6.2 0.2

3.20 2.98

0.05 0.03

– –

3.20

Sample

Cr concentration (%)

Scattering atom

(a) GaCrN nanorods

0.6

Ga Cr

(b) GaCrN nanorods

0.1

(c) GaCrN nanorods

Data for (d) and (e) are from Refs. [14,15].

N

ARTICLE IN PRESS H. Tambo et al. / Journal of Crystal Growth 311 (2009) 2962–2965

Fig. 3(b) sample, the amount of Cr itself that incorporated into the crystal is small.

4. Summary We have grown GaCrN nanorods on Si (0 0 1) substrates with native SiO2 by RF-MBE. XTEM image reveals that the diameter of GaCrN nanorod gradually increases with proceeding of the growth at 550 1C. It was found that the growth rate of GaN nanorods at 800 1C and GaCrN nanorods at 550 1C are approximately 3.6 and 2.8 nm/min, respectively. From these results, it was suggested that the low-temperature growth enhances the growth perpendicular to the c-axis (along the lateral direction) and decreases the growth along the c-axis. XAFS analysis also revealed that the solubility limit of Cr atoms in GaCrN is much higher for the 550 1C growth than that for the 700 1C growth. SQUID measurements showed that an optimum Cr concentration existed. With decreasing Cr cell temperature, the concentration of Cr atoms that substitute Ga sites and the Ms are decreased. On the other hand, for the Cr cell temperature above the optimum Cr cell temperature, the Ms also decreased.

Acknowledgments This work was supported in part by the grant-in-aid for Scientific Research, the grand-in-aid for Creative Scientific Research and the Special Education and Research Expenses from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The XAFS measurements were performed at Photon Factory through Proposal no. 2007G215. The authors

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