Synthesis of indium nanowires by oblique angle deposition

Synthesis of indium nanowiresby oblique angle deposition

Aniruddha MondalPaul Samy Chinnamuthu

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Synthesis of indium nanowires by obliqueangle deposition

Aniruddha Mondal and Paul Samy ChinnamuthuNational Institute of Technology Agartala, Department of Electronics and Communication

Engineering, Jirania, Tripura (West) 799055, [email protected]

Abstract. Indium nanowires were synthesized by the oblique angle deposition technique, usingan e-beam evaporator. The shape of the deposited nanowires changes with the vapor flux angle,and the x-ray diffraction spectrum shows scattering from tetragonal phase indium. Depositionrates ranging from 0.5 to 5 A◦/s were used for 200 and 500 nm thick deposits. Nanowiresformed at the highest deposition rate and were up to 4.5 μm in length. Raman scatteringshows enhanced intensity and optical absorption occurs in the infrared region. C© 2011 Society of

Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3630050]

Keywords: e-beam evaporator; field emission scanning electron microscope; indium nanowire;Raman spectroscopy; optical absorption.

Paper 11073RR received Jun. 27, 2011; revised manuscript received Aug. 3, 2011; accepted forpublication Aug. 8, 2011; published online Sep. 9, 2011.

1 Introduction

Metal nanowires with different shapes and sizes exhibit novel properties of interest for applica-tions in optoelectronic devices.1,2 The available reports on one-dimensional (1D) nanostructuregrowth are restricted by template growth technique.3,4 Gold is the most commonly used materialfor particle assisted 1D nanostructure growth. The growth of 1D structure in oblique angle de-position (OAD) is dominated by vapor flux angle.5 The conditions of limited adatom diffusionin OAD enhance atomic shadowing and create an inclined 1D structure. A 1D nanostructure ofdifferent shapes has great attention for fabricating electronics, optoelectronics, electrochemical,and electromechanical devices.6 Substrate temperatures, deposition rate, angular distribution ofthe deposition flux, and background pressures are critical parameters for nanowires morphology.The morphology of the nanowires depends on the adatom mobility, which is invariably relatedto the deposition rate. Under conditions of low adatom mobility, the vapor flux arriving at anincreasingly vapor flux angle would enhance atomic shadowing. This will lead to an increasinglyporous nanowire structure of isolated grains,7,8 because atomic shadowing produces areas thatvapor flux cannot reach directly and the adatom mobility is too low for surface diffusion to fillthe voids.

There are a number of nanolithographic3 techniques for the fabrication of a controlled 1Dstructure. But the techniques are not cost effective for large area 1D nanostructure growth,whereas the unconventional methods based on the chemical synthesis are cost effective andhave potential for large volume growth.9 It is very difficult to control the morphology of thenanostructure in a chemical synthesis method. The OAD is comparatively low in cost thannanolithography and suitable for large area controlled growth.10

1934-2608/2011/$25.00 C© 2011 SPIE

Journal of Nanophotonics 053522-1 Vol. 5, 2011


mailto: [email protected]

http://dx.doi.org/10.1117/1.3630050

Mondal and Chinnamuthu: Synthesis of indium nanowires by oblique angle deposition

2 Experimental Work

Indium (purity 99.999%) was deposited on the substrate using an e-beam evaporation systemat a base pressure of ∼10−6 Torr on 2 cm × 2 cm silicon (100) substrate for the differentOADs. The distance between the substrate holder and metal source was kept at 700 mm.The substrates were used at different orientations set between 40

◦and 85

◦with respect to the

perpendicular line between the metal source and the planer substrate holder. Three differentdeposition rates 0.5, 2, and 5 A◦/s were used for the 200 and 500 nm film depositions, measuredwith the help of a quartz crystal monitor. The depositions of the films were done at chambertemperature.

3 Characterization

The samples were characterized by using x-ray diffraction (XRD, CCRF, D8 Discover, BrukerAXS) with copper Kα radiation. The nanowire structure of the deposited indium was observedusing field emission scanning electron microscope (FE-SEM) (S-4800, Hitachi). The chemicalcompositions of the nanowires were analyzed using the energy dispersive x-ray (EDX) method.The Raman study has been done by using excimer laser of 25 mW with an excitation wavelengthof 1064 nm. The optical absorption measurement has been done on indium nanowires and thinfilm deposited on silicon substrates by a UV-visible near-infrared spectrophotometer (Lambda950, Perkin Elmer).

4 Results and Discussion

4.1 Indium Nanowires Formation and FE-SEM Images

The FE-SEM images of the samples prepared at 0.5 A◦/s deposition rate for 40◦

to 85◦

OADare shown in Fig. 1. The probable shape of the maximum number of grains for the particularangle of deposition is furnished by the schematic diagram. At the deposition rate of 0.5 A◦/s,the deposited film is a collection of different shaped nanowires for 200 nm deposition. Someof them are very low aspect ratio. The grown nanowires have lengths within the range of 75 to150 nm and diameter 40 to 75 nm, for 200 nm deposition at 85

◦OAD [cross-sectional view

shown in Fig. 2(a)]. The length of the nanowires increased with the material deposition thickness

(a) 40o OAD @ 0.5

Ao/S

(b) 60o OAD @ 0.5

Ao/S

(c) 70o OAD @ 0.5

Ao/S

(d) 80o OAD @

0.5 Ao/S

(e) 85o OAD @ 0.5

Ao/S

200 nm

deposition

1 µm 1 µm 500 nm 500 nm 500 nm

Schematic

Diagram

1 µm 1 µm 500 nm 500 nm 500 nm

Fig. 1 Top view FE-SEM images of the samples deposited at angles of 40 deg to 85 deg withthe corresponding schematic diagram.




(a)

(c) (d)

200 nm deposition @ 0.5 Ao /s

500 nm deposition @ 0.5 Ao /s

500 nm deposition @ 2 Ao /s

500 nm deposition @ 5 Ao /s

(b)

Fig. 2 85◦

OAD: (a) 200 nm deposition at the rate of 0.5 A◦/s, (b) 500 nm deposition at the rateof 0.5 A◦/s, (c) 500 nm deposition at the rate of 2 A◦/s, and (d) 500 nm deposition at the rate of5 A◦/s.

from 200 to 500 nm for the same deposition rate. Nanowires 300 to 600 nm long and 40 to75 nm thick have been obtained for 500 nm deposition at 0.5 A◦/s deposition rate, shown inFig. 2(b) (tilt view).

At 2 A◦/s deposition rate, the length of the nanowires within the range of 300 nm to 1 μmand thickness 75 to 150 nm are shown in Fig. 2(c) [cross-sectional view]. The lengths of a largenumber of nanowires are between 900 nm and 1 μm. For higher rate of deposition (5 A◦/s), theusual nanowire structure disappears and whiskers are formed. The thicknesses of the whiskersare less than 120 nm and their length varies from 3 to 4.5 μm. Some of the nanowires havelengths between 1 and 1.5 μm [Fig. 2(d), cross-sectional view].

But in the case of 80◦

OAD [Fig. 2(d)] at 5 A◦/s, a large number of nanowires are formedcompared with 85

◦OAD [Fig. 2(d)] with a thickness range of 100 to 300 nm and length 1.2 to

4 μm [Fig. 3(a)], because of less diffusion length of adatoms on the silicon substrate. The shapesof the nanowires change with the deposition rates. At lower deposition rates both hexagonaland cylindrical shaped nanowires are formed, whereas the smooth walled cylindrical shaped

(a) 80o OAD @ 5 Ao/s (b)

85o OAD @ 5 Ao/s

Fig. 3 (a) 80◦

OAD deposition at a rate of 5 A◦/s and (b) 85◦

OAD deposition at a rate of 5 A◦/s.




nanowires formed at higher deposition rate. A typical cylindrical shaped nanowire is shown inFig. 3(b) for 5 A◦/s deposition rate at 85

◦OAD.

The adatom mobility of indium is high at room temperature11 due to its low melting point(156

◦C), which is very high at the time of e-beam evaporation. Hi-Lin12 reported the disordered

indium nanowire growth by dc magnetron sputtering at room temperature. The well collimatedand very high mobility indium vapor flux was deposited as more or less parallel nanowire seedson the silicon substrate. So nanowires are not growing roughly parallel to each other. Also, dueto the selective crystallinity of the deposited indium (Figs. 1 and 2), the nanowires are randomlyorientated over the entire surface of the substrate.

4.2 Morphological and Crystallographic Changes of Indium Nanowireswith Increasing Vapor Flux Angle

The XRD analysis of the deposited nanowires shows the scattering from 101, 002, 112, 200,and 103 crystals planes (JCPDS Card 05-0642) of the tetragonal phase of indium, respectively.The intensity of scattering from the 103 plane increased more rapidly compared to that of the101 plane with increasing vapor flux angle, indicating the crystallographic change of the indiumnanowires for the angle of deposition from 60 deg to 85 deg [Fig. 4(a)]. The EDX analysis[Fig. 4(b)] of the deposited films shows the presence of pure indium on the substrates.

Inte

nsity

(a.u

.)

30

Si (1

11) In

(101

)In

(101

)In

(101

)In

(101

)

(a)

35 40

In (0

02)

60

2

In (0

02)

70 0

80 0

85

45 5

0 Inclination

( Degrees )

0 Inclination5

0 Inclination

0 Inclination

50 55

Si (3

11)

In (2

00)

In (1

12)

Si (3

11)

In (2

00)

In (1

12)

Si (3

11)In

(200

)In

(112

)

60 6

In (1

03)

In (1

03) 6

6

In (1

03) 6

In (1

03)

65

65

65

65 (b)

θ

Fig. 4 (a) XRD analysis of samples grown at angles of 60 deg to 85 deg using OAD and (b) EDXanalysis of deposited films.

4.3 Raman Analysis

The room temperature Raman scattering from the samples has been recorded by using ex-cimer laser at the excitation of 1064 nm. Figure 5 shows the intensity of Raman scatteringcollected from indium nanowires deposited on silicon substrates. The Raman scattering on thegrown samples shows the enhanced scattering from indium nanowires prepared at 40

◦to 85

◦

OAD. The Raman spectrum consists of two separate peaks for silicon and indium at 532 and693 cm−1, respectively. The sharp peak at 532 cm−1 is for silicon, whereas the spectrum is inbroad nature at 693 cm−1. A faint peak at 693 cm−1 for the sample deposited at 85

◦OAD shows

the improvement of crystal quality of the indium nanowires compared with the samples prepared




400 800 1200 1600 2000 2400

0.0

0.2

0.4 (693 cm-1)Si-Si

Inte

nsity

(a.u

.)

Raman Shift (cm-1)

40 60 70 80 85

In-InEnhancement Of Raman Intensity with the increase of Deposition Angle

200 nm deposition

@ 0.5 Ao/S

Fig. 5 Room temperature Raman spectra for indium nanowires/silicon for 40◦–85

◦OAD for

200 nm deposition at 0.5 A◦/s.

at lower deposition angles. The broad Raman spectrum may be due to the noncrystalline natureof indium nanowires. The crystal quality of the indium nanowires can be improved by increasingthe substrate temperature at the time of deposition.13

4.4 Optical Absorption

Figure 6 shows the room temperature optical absorption of indium nanowire structure onsilicon deposited at 85

◦OAD [Fig. 1(e)] and 200 nm indium thin film on silicon. The samples

were examined under the wavelength range of 300 to 2000 nm. The sharp peaks at 907 nm(∼1.37 eV) and 908 nm (∼1.36 eV) have been observed for nanowire structure and thin filmindium, respectively. The observed peaks show good agreement with the measured value 1.37 eVat 298 K for diffusely reflecting indium thin film prepared through indium evaporation onquartz substrate.14 The enhancement of optical absorption has been observed for the nanowires,attributed to large scattering inside the nanowire structure. We have observed a small blueshiftof 0.01 eV of the absorption peak of the indium nanowire structure compared to thin film, due

900 902 904 906 908 910 912

1.37 eV

Abs

orpt

ion

(a.u

.)

Wavelength (nm)

Indium NW @ 850OAD Indium Thin Film

1.36 eV

Fig. 6 Room temperature optical absorption for indium nanowires and thin film on silicon for200 nm deposition at 0.5 A◦/s.




to the change of lattice constant (because of particle surface stress)15 of indium and, hence, theband transition.

5 Conclusion

In this paper, we have investigated the formation of indium nanowires at a high temperaturewith the help of OAD technique, using an e-beam evaporator. The particular vapor flux angleand the deposition rate are the key factors for longer nanowire formation in the OAD technique.The nanowires of different lengths with 120 nm width were formed at 5 A◦/s for 85

◦OAD. The

atomic shadowing effect takes the main role for the nanowire formation at a lower deposition rate,whereas the atomic shadowing effect will be minimized at a higher deposition rate. The enhancedRaman scattering has been observed from the indium nanowires. The nanowire structure onsilicon shows a sharp infrared optical absorption at ∼1.37 eV.

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Synthesis of indium nanowires by oblique angle deposition