Self-assembled nanowhiskers grown by MBE on InP(001) surface

Article abstract of DOI:10.1002/pssa.200669557

Ge and MnP nanowhiskers were synthesized by molecular-beam epitaxy technique on InP(001) surface concurrently. The growth of Ge nanowhiskers is found to be assisted by Mn-based nanocluster-mediated vapour-liquid-solid mechanism of growth, whereas the grow

Full text of DOI:10.1002/pssa.200669557

Original
Paper
phys. stat. sol. (a) 203, No. 11, 2793–2799 (2006) / DOI 10.1002/pssa.200669557
Self-assembled nanowhiskers grown by MBE
on InP(001) surface
Alexei Bouravleuv*, 1, 2, Kazuyuki Minami1, Takayuki Ishibashi1, and Katsuaki Sato1
1
Graduate School of Engineering, Tokyo University of Agriculture and Technology, 184-8588 Tokyo,
Japan
A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, Russian
2
Received 23 February 2006, revised 6 April 2006, accepted 26 June 2006
Published online 31 August 2006
PACS 75.75.+a, 81.07.Vb, 81.15.Hi, 81.16.Dn
Ge and MnP nanowhiskers were synthesized by molecular-beam epitaxy technique on InP(001) surface
concurrently. The growth of Ge nanowhiskers is found to be assisted by Mn-based nanocluster-mediated
vapour-liquid-solid mechanism of growth, whereas the growth of MnP nanowhiskers seems to be caused
by catalyst-free mechanism. Magnetic property measurements revealed that samples with prevailing Ge
nanowhiskers exhibit ferromagnetic behaviour up to room temperature.
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction
One-dimensional nanostructures such as nanorods, nanotubes, nanowhiskers have attracted much atten-
tion due to their potential application in nanoscale devices [1–5]. The standard techniques for its fabrica-
tion based on lithography techniques often have the disadvantage concerning with insufficient quality of
technological processes. Therefore methods based on the self-assembled growth of nanostructures, such
as bottom-up fabrication of nanowhiskers are a major focus of interest for the creation of novel nanode-
vices [2–6]. Among them a vapour-liquid-solid (VLS) growth proposed in the 1960 by Wagner and Ellis
is noteworthy [7]. According to this method, the formation of the nanowhiskers occurs due to the adsorp-
tion of the gas-phase reactants by liquid droplets that are formed from nanosized metal clusters [6]. The
sizes and the positions of nanowhiskers depend directly on the diameters of the metal clusters. However,
nanowhiskers can be contaminated due to the presence of the impurity metal on their tips. As a conse-
quence alternative techniques for the growth of nanowhiskers, which do not require any catalyst, also are
of importance for the technological aspects of nanoelectronics [8, 9].
The magnetic self-assembled nanowhiskers deserve special attention because their fabrication makes
possible to tune the Curie temperature, remanent magnetization and coercive saturation field by means of
the control over their sizes, shapes and distributions that in turn allows us to utilize such materials not
only for data storage and nanoscale spintronics applications, but also for the investigation of the funda-
mental magnetic properties of low-dimensional structures. Recently Mn-doped nanowhiskers have been
successfully synthesized using different methods [10–14]. Mn-doped ZnO and GaN nanowhiskers ex-
hibiting ferromagnetic behaviour were prepared by vapour-phase evaporation [10] and chemical vapour
deposition (CVD) [11], respectively. The similar magnetic properties have been shown by Mn+ ion im-
* Corresponding author: e-mail: alexei@cc.tuat.ac.jp, Phone: +81 42 388 74 32, Fax: +81 42 388 74 32
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A. Bouravleuv et al.: Self-assenbled nanowhiskers grown by MBE on InP(001) surface
planted ZnO nanowhiskers [12]. Mn-doped CdS and ZnS nanowhiskers have been fabricated using a
core/shell methodology [13]. Furthermore, MnP nanorods were synthesized via thermal decomposition
of continuously delivered metal-phoshine complexes [14].
In this paper, we report on the molecular beam epitaxial growth and the characterization of MnP and
Ge self-assembled nanowhiskers on the InP(001) substrates.
2 Methods
Self-assembled (SA) nanowhiskers have been grown by conventional molecular beam epitaxy (MBE) on
InP(001) substrate using Knudsen cells for the Mn and Ge evaporation as well as cracking cell for the
decomposition of tertiarybutylphosphine (TBP) into P2 flux. The “epi-ready” InP(100) wafers were an-
nealed at 480 °C in vacuum inside a treatment chamber in a P2 flow for 20 minutes. The duration of
growth ranged from 30 minutes up to 2 hours at the temperatures between 430–545 °C which were con-
trolled using a temperature controller and an infrared pyrometer. The beam flux of Mn was adjusted in
the range between 0.5 – 0.9 × 10–8 Torr and that of Ge in the range between 0.9 – 1.5 × 10–8 Torr using an
ion gauge. The flow rate of TBP gas was set at 2.0 sccm by using a mass flow controller, whereas the
temperature of the cracking cell was kept in the range of 810 – 835 °C to assure efficient decomposition
of TBP into P2 during the MBE growth. The growth process was monitored using in-situ reflection high
energy electron diffraction (RHEED) measurements.
The morphology and microstructure of nanowhiskers were examined by scanning electron microscope
(SEM, Hitachi S-4500) and scanning transmission electron microscope (STEM, FEI TECNAI-F20).
Chemical compositions were measured using an energy-dispersive X-ray analysis (EDX) attached to the
SEM and STEM apparatus. The crystals were analyzed by X-ray diffraction technique using an X-ray
diffractometer (XRD, Philips X'Pert type). Temperature and magnetic-field dependences of magnetiza-
tion were measured by a superconducting quantum interference device (SQUID) and vibration sample
(VSM, TOEI VSM-5-19) magnetometers.
3 Results and discussion
3.1 Growth and structure of self-assembled nanowhiskers
The fabrication of self-assembled nanowhiskers was made possible by the development of the technol-
ogy of MnGeP2 thin film growth [15, 16], that is chalcopyrite-type ternary compound [15–17].
Contrary to usual above-mentioned metal catalyst vapour-liquid-solid (VLS) growth [3, 7], in our
experiments we did not use any preliminary tailor-made metal catalyst layer. SA nanowhiskers were
grown directly on InP(001) surface by MBE.
Figures 1a–d demonstrate a scanning electron microscopy images of SA nanowhiskers grown at dif-
ferent temperatures on InP(001) surface. At low temperatures around 435 °C the nanowhiskers with
typical diameters close to 20 nm and length up to 2 µm which are spaced at different intervals can be
obtained (Fig. 1a and b). These individual SA nanowhiskers have preferential direction which corre-
sponds to the 〈111〉 crystallographic orientation and seems to be dependent on the crystallographic form
and the orientation of the host substrate. Unfortunately, we cannot control the exact position of individual
nanowhiskers on the surface unlike the positions of nanowhiskers prepared according VLS growth mecha-
nism, since they can be preassigned by the distribution of preliminary fabricated nanosized metal cluster. In
spite of the fact, that individual nanowhiskers have a small dispersion in diameters, they differ greatly in
lengths which appear to be dependent on the time of growth. On further increasing the growth temperature
the microstructure and the distribution of nanowhiskers can be considerably modified (Fig. 1c and d).
Instead of widely distributed individual nanowhiskers, the arrays of SA nanowhiskers that have random
(Fig. 1c) or at higher temperatures solid (Fig. 1d) distribution on the InP(001) surface and different shape
with or without precipitated nanoclusters can be obtained.
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phys. stat. sol. (a) 203, No. 11 (2006)
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Fig. 1 SEM images of individual (a), (b) and arrays of
SA nanowiskers (c), (d) as well as MnP nanowhiskers (e),
(f) grown on InP(001) surface. The temperature of the
growth: (a), (b) 435 °C; (c) 500 °C; (d) 520 °C; (e), (f)
545 °C.
The phases of the samples with SA nanowhiskers were firstly identified by XRD analysis (Fig. 2).
X-ray diffraction θ–2θ scan data of the samples showed few dominant peaks inherent to all samples.
First of them, the broad amorphous-like peak at 2θ ~ 24° is a contribution from the glass plate used as a
sample holder. The peaks at 2θ ~ 30.5° and 2θ ~ 63.4° bear on 002 and 004 diffraction of InP sub-
strates, respectively. The diffraction peak observed at 2θ ~ 66.04° may be assigned to the reflection from
MnGeP2(008) [15, 16]. Other peaks, which seem to be related to the MnP 111, MnP 202 and MnP 121
Fig. 2 X-ray diffraction pattern of samples with SA
nanowhiskers grown at 435 °C (1) (see Fig. 1a and b);
520 °C (2) (see Fig. 1d) and MnP nanowhiskers grown at
545 °C (3).
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A. Bouravleuv et al.: Self-assenbled nanowhiskers grown by MBE on InP(001) surface
Fig. 3 Low-resolution STEM images of Ge (a) and MnP (b)
nanowhiskers grown at 500 °C (see Fig. 1c). (EDX analysis of
the nanowhiskers has been performed for the depicted points).
were also detected. At elevated temperatures the signal most likely assigned to MnP 111 became extinct
thereby indicating that this peak can be result of the reflection from MnP nanowhiskers.
In order to verify this supposition, we have performed the growth of MnP nanowhiskers by the same
above-described technique but without using Ge K-cell. The nanowhiskers obtained are shown on
Fig. 1e and f. They have lengths up to 6 µm and larger dispersion in sizes. Furthermore, they are similar
in structure to individual nanowhiskers grown at low temperatures, because they do not have any termi-
nated nanocluster, as opposed to the SA nanowhiskers grown at higher temperatures. From these facts it
transpires that the growth of MnP nanowhiskers is apt to be caused by non-catalytic growth mechanism
which has not been thoroughly investigated [8, 9]. XRD diffraction pattern of the sample with MnP is
presented on Fig. 2. Diffraction peaks that may be related to MnP 111 and MnP 202 have been also
found and in doing so they substantiated our supposition. However, it should be noted, that the growth of
nanowhiskers can be accompanied by the growth of thin film. In this connection some of these peaks
may be assigned not only to nanowhiskers but also to the different phases of thin film.
Therefore, we have performed TEM and EDX study of the samples. For these investigations
nanowhiskers were transferred from the host substrate onto Cu TEM grid. Energy dispersive X-ray
analysis showed that two different types of nanowhiskers can be obtained concurrently: Ge nanowhiskers
(Fig. 3a) and MnP nanowhiskers with atomic ratio Mn:P ~65:20% (Fig. 3b points 1 and 2).
In as much as the amount of Ge nanowhiskers at high temperatures is more than MnP as well as taking
into account the XRD data, the formation of MnP nanowhiskers seems to be dominated at low tempera-
tures (Fig. 1a and b), whereas that of Ge nanowhiskers prevails at high temperatures (Fig. 1c and d). In
Fig. 4 TEM image of Ge nanowhisker with Mn-based nanoclus-
ter. Insets: HRTEM image of Ge nanowhiskers and the diffraction
pattern of Mn-based nanocluster.
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Fig. 5 (online colour at: www.pss-a.com) Temperature de-
pendence of magnetization measured with an applied field of
100 Oe for (1) the sample with SA nanowhiskers grown at
520 °C (Fig. 1d); (2) MnGeP2 thin film grown on GaAs(001)
substrate with Ge buffer layer [16] and (3) SA nanowhiskers
grown at 520 °C peeled off from the InP(001) substrate.
turn, despite the fact that we did not use an tailor-made catalyst compound, the growth of Ge nanowhisk-
ers is found to be amenable to VLS mechanism of growth [6, 7]. The role of the catalyst for the growth
in this case plays Mn-based nanoclusters with atomic composition of elements close to Mn:P:Ge
78:5:6% (Fig. 3a point 2 and Fig. 4) which seem to be self-assembled at an initial stage of the growth.
TEM as well as SEM analysis has shown that most of Ge nanowhiskers are straight and uniform in di-
ameters along their length. High-resolution TEM image demonstrated on Fig. 4 indicates that Ge
nanowhiskers are single crystalline with lattice fringe spacing 0.327 nm which corresponds to (111)
plane. As a consequence the Ge nanowhiskers are found to be grown along 〈110〉 crystallographic axis
(Fig. 4).
3.2 Magnetic properties
Since the major contribution to the magnetic properties of the sample with the individual SA nanowhisk-
ers grown at low temperatures seems to result from MnGeP2 thin film [16] most attention has been con-
centrated on the investigation of samples grown at high temperatures, that is, the samples with prevailing
concentration of Ge nanowhiskers (Fig. 1c and d). The measurements of the temperature and magnetic
Fig. 6 (online colour at: www.pss-a.com) Hysteresis
loops for the sample with SA nanowhiskers grown at
520 °C (Fig. 1d) measured at different temperatures:
(1) 295 K; (2) 70 K; (3) 30 K; (4) 5 K.
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A. Bouravleuv et al.: Self-assenbled nanowhiskers grown by MBE on InP(001) surface
Table 1 Potential second phases that can be formed during the growth of SA nanowhiskers.
phase
nature of magnetism
applicable magnetic
temperature (K)
ref.
Mn
MnO
MnO2
Mn3O4
MnP
MnP
Mn3P
Mn2P
antiferromagnetic
antiferromagnetic
antiferromagnetic
ferromagnetic
100
122
84
1443
291
50
115
12
12
12
12
18
18
18
18
ferromagnetic
antiferromagnetic
antiferromagnetic
antiferromagnetic
ferromagnetic
ferromagnetic
ferromagnetic
103
MnxGe1–x
Mn5Ge3
MnGeP2
25–116
296
320
19
20,21
17
field dependences of magnetization have been carried out using SQUID magnetometer for both SA
nanowhiskers on InP substrates and nanowhiskers which have been peeled off from wafers (Figs. 5
and 6). The magnetization of sample with SA nanowhiskers is observed to exhibit ferromagnetic behav-
iour up to room temperature (Fig. 5, curves 1) therewith the field dependences of magnetisation reveal
hysteresis loops with coercive fields Hc are equal to 100 Oe and 4560 Oe measured at 295 K and 5 K,
respectively (Fig. 6). Besides, the SA nanowhiskers peeled off from the host substrate exhibits the same
ferromagnetic behaviour (Fig. 5, curves 3). Table 1 demonstrates the potential secondary phases that
could form during the growth process. According to these data, ferromagnetic properties of the samples
are attributable to the existence of MnGeP2, MnP or Mn5Ge3 phases, but SA nanowhiskers exhibit behav-
iour of magnetisation that is distinct from that of MnGeP2 thin film [16] (Fig. 5, curves 1). Therefore
ferromagnetic properties of SA nanowhiskers may be caused by most likely the presence of Mn5Ge3
phase [21] and yet we have to take into account that MnP can also be responsible for this behaviour, in
spite of the fact that magnetic properties of MnP nanowhiskers demonstrated in [14] differ from it.
4 Conclusion
We have grown self-assembled Ge and MnP nanowhiskers by molecular-beam epitaxy technique on
InP(001) surface. It has been shown, that the growth of Ge nanowhiskers was appeared to be assisted by
Mn-based nanocluster-mediated VLS mechanism of growth, whereas the growth of MnP nanowhiskers
seemed to be caused by catalyst-free mechanism. Hence, new method for the creation of nanowhiskers
with using Mn-based nanocluster as a catalyst is found. The investigation of the magnetic properties
revealed that samples with prevailing Ge nanowhiskers exhibit ferromagnetic behaviour up to room
temperature. Although the nature of this ferromagnetism is still an open question that requires an addi-
tional clarification such structures would be expected to exhibit the size-depended magnetic properties
which are of particular interest for nanometer scale spintronics applications.
Acknowledgements This work has been supported by 21st Century COE Program “Future Nano-Materials” of
TUAT, Japanese Society for Promotion of Science (Grant P 06112 and Grant-in-Aid for Scientific Research B
17360009) and Inter-University Cooperative Research Program of Tohoku University. We sincerely thank S.Mitani
from Tohoku University, H. Sosiati and N. Kuwano from Kyushu University for their help in measurements.
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