Electrical and Structural Characterization of Germanium Nanowires

Available online at www.sciencedirect.com

Physics Procedia 28 (2012) 62 – 66

15th Brazilian Workshop on Semiconductor Physics

Electrical and Structural Characterization of Germanium Nanowires a

Luana Santos Araujo*, aHanay Kamimura, aOlivia Maria Berengue, aAdenilson José Chiquito a

NanOLaB, Departamento de Física, Universidade Federal de São Carlos, São Carlos 13565-905 , Brasil

Abstract Germanium nanowires were grown by thermal evaporation of germanium powder associated with the well known vapor-liquidsolid mechanism (VLS). The nanowires were investigated by x-ray diffraction (XRD), Raman spectroscopy and field emission gun scanning electron microscopy (FEG-SEM). Through the fabrication of a germanium nanowires-based device we have studied the electronic transport properties of these samples. The transport measurements revealed semiconductor – like features, characterized by the decrease of the resistance as the temperature decreases. The variable range hopping (VRH) was identified as the main transport mechanism in a large temperature range (77 K < T < 400 K) thus giving consistent support to the mechanisms underlying the observed semiconducting character. © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of Universidade Federal de © 2011 Published Juiz de Fora, Brazil.by Elsevier B.V. PACS: 81.07.Gf; 73.63.-b; 81.15.Gh Keywords: chemical vapor deposition; nanowires; germanium; eletronic transport; hopping

1. Introduction The nanoscience has been the fundamental basis for the technological advances aimed at miniaturization and development of optoelectronic devices. This potential to develop new devices is directly related to the unique characteristics presented by the nanostructures. The nanoscale size (< 100 nm) leads to different and sometimes surprising mechanical, optical and eletronic features. In fact the properties exhibited by the nanostructures are different from those found in three-dimensional materials, and deeply influenced by quantum mechanics. Among the various typ†es of nanostructures, nanowires and nanotubes are the most widely used for the development of new devices because they can have metallic or semiconducting properties, can be flexible or rigid, and also can selectively conduct heat [1,2]. Among the various nanostructures studied today, those based on germanium can be very interesting from a technological standpoint: they are semiconducting with a small and indirect gap (0,67 eV, easily doped with Boron (p type) and phosphorus (type n) thus enabling the development of nanodevices based on pn junctions (nanoleds, solar nanocells) [3,4,5]. Some successful routes for Germanium nanowires synthesis are known; however, they are still difficult to be implemented, have no morphology control and also involve high costs. Thus, to contribute to the

*

Corresponding author. Tel.: +55 16 3351 9729 E-mail address: [email protected]

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of Universidade Federal de Juiz de Fora, Brazil. doi:10.1016/j.phpro.2012.03.672

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development of a new simple route for the germanium samples synthesis, structural characteristics of these nanostructures synthesized by the vapor-liquid-solid method were studied. Combining X-ray diffraction (XRD), Raman spectroscopy measurements and field emission gun scanning electron microscopy (FEG-SEM), it was possible to characterize the sample's structure, composition, single crystalline character and vibrational spectrum of the samples. Additionally, electron transport mechanisms were investigated: the temperature dependent resistivity measurements unambiguously showed that samples presented the expected semiconducting behavior. Also, the variable range hopping was identified as the mechanism governing the electron transport in Ge nanowires. This characteristic was attributed to the disorder which leads to localized states. 2. Experiment Germanium nanowires were grown by a thermal evaporation process of germanium powder, in association with the well-known vapor-liquid-solid mechanism (VLS) [6]. The device (Fig.1) was fabricated as following: metallic electrodes were defined by using conventional lithographic techniques with Ti electrodes (50nm thick) over an oxidized n+ Si wafer (500 nm layer, n-type). Gold nanoparticles (10 nm) were deposited above titanium electrodes in order to nucleate the growth of the germanium nanowires. The catalyst (impurity) in the liquid phase is introduced in order to direct and confine the crystal growth in a specific orientation and within a restricted area. The germanium powder (Aldrich, purity> 99.99%) was placed in a quartz crucible and positioned in the central region of the tube furnace. The synthesis temperature was adjusted to 950 °C (heating rate of 32 °C/min) and stands for 20 minutes. The substrates with a previously deposited Au catalyst layer and Ti electrodes were positioned in a cooler region of the furnace (550 ° C 99.998 %) with a flow of 30 sccm.

Figure 1. Schematic diagram of the Ge nanowires device fabricated with Ti electrodes and Au catalyst layer.

The as-grown Ge nanowires were analyzed by field emission gun scanning electron microscopy (FEG-SEM, Zeiss Supra 35), x-ray diffraction (XRD, Rigaku diffractometer model DMAX 2500PC, 40 kV, 150 mA, all measurements were performed using aluminum sample holders) with a Cu Ka radiation and Raman spectroscopy (Triple grating Jobin Yvon Spectrometer T64000 equipped with a liquid nitrogen cooled charge coupled device). The Raman measurements were performed in backscattering geometry configuration and the samples were excited by a 514.5 nm line of an Ar+ laser at room temperature. The used experimental settings present a spectral resolution about 2 cm-1. The transport measurements were carried out at different temperatures from 77K to 400K using a electrometer (Keithley 6517) and a closed-cycle helium cryostat (Janis CCS 400H) working at a pressure lower than 5x10-7 Torr.

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3. Results and Discussion

(a)

0.9

PDF 4-545 (220)

0.6 0.3

(311)

1.2

(111)

Intensity (arb. units) Intensity (arb. units)

For a first investigation of the as-synthesized samples XRD measurements were performed [Fig. 2 (a)].

0.0 20 1.0 0.8 0.6 0.4 0.2 0.0

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(b)

200

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Ge Nanowire λ = 514 nm T = 300 K

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500

(c)

300 nm

Figure 2. The structural data of the Ge nanowires: (a) The x-ray pattern of the nanowires grown on silicon substrates. (b) The Raman spectrum obtained from one Ge nanowire at room temperature showing the expected 300 cm-1 phonon mode which is related to the diamond structure of germanium phase. The 521 cm-1 peak corresponds to the Si substrate used to hold the sample. (c) The FEG-SEM image of the germanium nanowires grown on silicon substrates and catalyzed by gold nanoparticles.

The close agreement between the experimental data and the PDF 4-545 is an indication that the as-synthesized material is composed of atoms arranged in a germanium diamond-like structure (space group Fd-3m). The XRD pattern also shows that the samples probably have a preferred direction of growth in the (111) direction which is an evidence of a single crystalline character [Fig. 2 (a)]. We also performed Raman measurements in order to study the sample’s composition, structure and crystalline quality. The Raman spectrum of a single germanium nanowire is depicted in Fig. 2 (b). It is observed from the spectrum a single and sharp peak at 300 cm-1 which is commonly associated [7] to the germanium diamond structure, thus confirming the composition of the samples in agreement with the x-ray diffractogram shown in Fig. 2 (a). The presence of narrow and well-defined peaks points to a single crystalline characteristic of the samples which also agrees with the presence of a preferential growth direction (111) plane as obtained from the XRD data Fig. 2 (a). Additional data were obtained from FEG-SEM measurements, as shown in the inset of Fig. 2 (b). This analysis revealed that the nanowires have a great uniformity of diameters and lengths of several tens of micrometers Fig. 2 (c).

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Resistance (Ω)

Figure 3 depicts the temperature-dependent resistance measurement revealing a semiconducting character (resistance decreases as the temperature increases).

10

10

10

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10

6

Ge Nanowires

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Temperatura (K)

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VRH Experiments

ln ρ

20 16 ρ(T) = ρ0exp(T0/T)

12 0.20

0.24

0.28

1/T

1/4

0.32

(K

1/4

0.36

-1/4

)

Figure 3. Temperature-dependent resistivity measurement revealing a semiconducting character dominated by the variable range hopping instead of the expected simple thermal excitation law for a semiconductor.

However, the experimental curve does not follow the expected simple thermal excitation law for a semiconductor [8]. The observed behavior was well fitted to the variable range hopping (VRH) conduction mechanism due to Mott [9]. In this model, phonons are required to conserve energy during a hop from site to site: the higher phonon density at a higher temperature increases the hopping rate and thereby decreases the resistivity. The VRH mechanism is described by

⎛T ⎞ ρ (T ) = ρ 0 exp⎜ 0 ⎟ ⎝T ⎠

1/ 4

,

(1)

where T0 = 5,7α3/kBN(EF), N(EF) is the density of states at the Fermi level and α-1 the localization length. In this mechanism the conduction is governed by carriers with a small extra energy (kBT) in the vicinity of the Fermi level where the density of states remains almost a constant (m=1/4, considering a three-dimensional system). This condition is fulfilled when the temperature is sufficiently small or when the energy states are uniformly distributed. The agreement between theoretical and experimental curves (Fig. 3) confirms that the VRH process governs the transport in all temperature range measured (100 to 400K) providing us with a localization length of 42.3 nm. This length is in full agreement with the Bohr radius of germanium (~24 nm) and it is smaller than the cross section of the sample, also in agreement with the three-dimensional character of the sample. The question here is: where do the localized states leading to hopping come from? The surface of germanium nanowires are naturally covered by a thin oxide layer (mostly GeOx as follows from the X-ray data) during the

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synthesis process due to the presence of unavoidable oxygen molecules. The interface Ge/GeO2 is disordered due to its nature and then it induces a disordered potential that should affect the conduction of electrons inside the Ge nanowires. In fact, a small surface disorder could dominate nanowire bulk properties as the sizes of samples are scaled down (by controlling the Fermi level and screening out electrons from surface, for instance). Such disorder produces interface states (surface levels) which could also act as charge traps. When the system temperature is large enough to produce free electrons which begin to increase the conductivity of the samples, the presence of a disordered potential will make difference. Any electron subjected to a random potential is not able to move freely through the system if either the potential fluctuations exceed a critical value or the electron energy is lower than the characteristic value of the potential fluctuations. Taking into account the motion of charges in that disordered potential the conduction mechanism should change its character from a simply activated process to another one like the observed VRH. It should be notice that some authors have also observed VRH as the dominating transport mechanism in Ge films [10]. 4. Conclusions In summary, we have studied electronic properties of the Ge nanowires and the influence of the disorder on the electron transport. Such a disorder produces surface levels which could also act as charge traps leading the semiconducting Ge nanowires to present the variable range hopping conduction mechanism. Acknowledgment The authors thank the doctoral student Cleocir José Dalmaschio for analysis of field emission gun scanning electron microscopy and Dr. Fenelon M. Pontes for X-ray measurements and related analysis. This work was financed by the Brazillian agencies CNPq and FAPESP. References [1] G. Gu, M. Burghard, G. T. Kim, G. S. Düsberg, P. W. Chiu, V. Krstic, S. Roth, W. Q. Han, Journal of Applied Physics, 90, 11 (2001) [2] L. Vayssieres, Advanced Materials, 15, 464 (2003) [3] O. Hayden, A. B. Greytak, D. C. Bell, Advanced Materials, 17, 6, 701 (2005) [4] Andrew B. Greytak, Lincoln J. Lauhon, Mark S. Gudiksen, Charles M. Lieber, Applied Physics Letters, 84, 21 (2004) [5] E. Tutuc, J. Appenzeller, M. C. Reuter, S. Guha, Nano Letters, 6, 9, 2070 (2006) [6] D.Wang and H. Dai, Germanium nanowires: from synthesis, surface chemistry, assembly to devices. Appl. Phys. A, 2006 [7] X. Wang, A. Y. Shakouri, S. Bin, M. X. Meyyappan, J. Appl. Phys. 107, 014304 (2009) [8] P. Yu and M. Cardona, Fundamentals of Semiconductors, Springer, Berlin 2010. [9] N. F. Mott, Metal Insulator Transitions, Taylor and Francis, London 1990. [10] D. Yu, C. Wang, B. L. Wehrenberg, P. Guyot-Sionnest, Phys. Rev. Lett. 92 216802 (2004).