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InSb nanowire field-effect transistors - electrical characterization and material analysis

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D. Candebat1, , Y. Zhao2 , C. Sandow1,3 , B. Koshel4 , C. Yang2,4 , J. Appenzeller1 Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47906, USA 2 Department of Physics, Purdue University, West Lafayette, IN 47906, USA 3 Forschungszentrum J¨ lich GmbH, IBN1-IT, 52425 J¨ lich, Germany u u 4 Department of Chemistry, Purdue University, West Lafayette, IN 47906, USA [email protected], Tel.: +1-765-496-8376

Introduction: With the smallest band gap and effective mass of known bulk semiconductors and an electron mobility at room-temperature in excess of 50,000 cm2 /Vs [1], Indium-Antimonide (InSb) holds the potential for low-power, high-speed device applications. At the same time there is a growing interest in improving scaling of devices employing ultra-thin body structures. A nanowire is the ultimate choice in this context allowing for unsurpassed control of the electrostatics inside the channel [2,3,4]. Exploring nanowires from InSb is thus an important task. Here we present the first electrical characterization of InSb nanowire field-effect transistors (NWFETs) including a careful analysis of the device characteristics in terms of the actual nanowire band gap. Nanowire Growth and Device Fabrication: Single-crystalline nanowires were synthesized via a vapor-liquidsolid process with Au catalysts on InSb substrates. Note that the surface oxide was removed prior to deposition of the gold particles. Growth was performed at 540 C at a partial pressure of 50 mTorr using hydrogen as the carrier gas. HRTEM imaging (see Fig. 1) shows initial evidence of growth occurring in the <110> growth direction and ° a native oxide of approximately 10 A. After releasing the wires from the growth substrate by means of sonication, they were deposited onto 20 nm SiO2 /Si substrates allowing for back gating of the nanowire devices. Metal source and drain contacts were patterned using e-beam lithography, e-beam evaporation of 80 nm of Nickel, and a lift-off process. The channel lengths of the devices varies between 1 µm and 30 µm. Fig. 2 shows an SEM image of a 60 nm diameter NWFET with multiple metal contacts. Results and Discussion: The first task at hand when characterizing a novel nanomaterial is to check the electrical device characteristics against the expected performance assuming bulk-type materials properties. The most relevant parameter in this context is the band gap. InSb is expected to exhibit an energy gap of Eg 170 meV. We first want to consider the transfer characteristics shown in Fig. 3 which provide initial evidence of the material quality - in particular indicating that indeed a band gap of just a few kB T prevails in our nanowire devices. A strong ambipolar behavior and a small Ion /Ioff -ratio of around 10 are in good agreement with a small Eg -value. In fact, we extract a band gap of around 160 meV from the on/off-ratio if ideal onedimensional ballistic transport in the quantum capacitance limit is assumed: Ion /Ioff = 1/2 · exp(Eg /2kB T ). Operation in the quantum capacitance limit implies a one-to-one movement of the bands in the channel with changing Vgs . While in the presented long-channel devices the existence of scattering and non-ideal gate control may seem to make the simple model we employed inapplicable, it is the one-dimensional nature of a small gap semiconducting nanowire that ensures a fair description for small drain voltages in the present case. Typical output characteristics for the n-branch are shown in Fig. 4 and exhibit a smooth linear region, clear evidence of current saturation at Ion-max 7.5 µA, and ambipolar characteristics. Collectively, Figs. 3 and 4 show that we have successfully fabricated the first high quality InSb NWFETs. To further substantiate our claim of having fabricated small band gap nanowire devices and to explain the significant current for Vgs below threshold at small Vds as well as a finite slope in the saturation region, we are employing the model from above once more. In the off-state, we assume that one one-dimensional mode contributes in the conduction and valence band, making the nanowire behave as a shunt resistor Rshunt . This assumption is justified when considering the mode spacing in a material with an effective mass around 0.014 m0 [5] . In the on-state, we distinguish between two nanowire regions as shown in Fig. 2; the gated side's current is modulated as a transistor while the shunted current remains the same as in the device off-state. This model explains indeed our experimental observations in Fig. 4 qualitatively for the entire curve-set and even quantitatively for small enough Vds values in the device off-state. Fig. 5 shows the result of our model including the shunt resistance from above. A finite slope in the saturation region of the Id - Vds characteristics as well as a substantial residual current level in the device off-state are clearly visible. Since the agreement between our model and the experimental data-set is expected to be best for small Vds in the device off-state, we have further explored this region for NWFETs with different channel lengths. Fig. 6 shows Rshunt for in total 11 NWFETs. We note that Rshunt reaches a minimum value of around 150 k - 200 k for decreasing channel length. Since decreasing the channel length is a means to reach the ballistic transport limit in our devices, we have compared the value of Rshunt with the expected shunt resistance Rshunt h/4e2 exp(Eg /2kB T ) from our model. We extract a band gap of 160 meV to 180 meV again in agreement with our expectations and the initial on/off-current analysis. Conclusion: In summary we have demonstrated for the first time the fabrication and characterization of InSb NWFETs. Clear evidence of a small band gap semiconductor with device characteristics consistent with an Eg value of around 160 meV to 180 meV have been provided using a simple - yet novel - data analysis. Our data can consistently be explained assuming two distinct regions in the device with one being a gate-independent shunt resistance. The presented device characteristics are a starting point to develop an understanding of InSb NWFETs that can lead to new device architectures such as tunneling field-effect transistors, highly linear amplifiers, and other high-speed, low-power applications.

Copyright © 2009 IEEE.

Reprinted from Device Research Conference Digest

InSb-nanowire

InSb Wire

1 m

Ni electrode resistive region gated region SiO2 gate oxide

InSb wire

Au particle

10 nm

Ni electrode

Fig. 1. HRTEM image of an InSb nanowire with diameter in the 10 nm range. The gold catalyst particle and the InSb lattice are clearly visible.

Fig. 2. Left: SEM image of 60 nm InSb NWFET and multiple Ni contacts with channel lengths of 1.8 µm. Right: Visualization of two distinct regions of nanowire. The red area is under full gate control while the "resistive region" acts like a shunt.

8

10

-5

8 6 4 2

Lg = 1800 nm, tox = 20 nm

Lg = 1800 nm, tox = 20 nm

6

Id [ A]

V = 3V to 6V, step: 1V gs V = 7V to 12V, step: 1V gs

Id [A]

10

-6

4

8 6 4 2

Finite slope

2

10

-7

Vds = 0.2V to 1.2V, step: 0.2V 0 2 4 V gs 6 [V] 8 10

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

V [V] ds

Fig. 4. Experimental n-branch output characteristics for the same nanowire as in Figure 3 show smooth linear region, saturation, and ambipolar characteristics. Significant current for Vgs below threshold at small Vds as well as a finite slope in the saturation region can be explained by the unmodulated part of the wire acting as shunt resitance.

3.0

Fig. 3. Experimental transfer characteristics for 60 nm InSb NWFET with channel length of 1.8 µm. An Ion /Ioff -ratio ratio of 10 and ambipolar behavior indicates a small band gap.

12 10 8

Rshunt [M ]

Eg = 0.2 eV

V = 0.05V to 0.15V, gs step: 0.05V V = -0.05V to 0.00V, gs step: 0.05V shunt resistance

2.5 2.0 1.5 1.0 0.5 0.0

Id [ A]

6 4 2 0 0.00

0.05

0.10

0.15

0.20

0.25

0

5

10

15

20

25

30

V [V] ds

Fig. 5. Simulation from our model showing a finite slope in the current saturation region and a resistance for minimal Vds . Vds and Vgs scales differ from experimental results due to the assumption of ballistic transport and the differences in oxide thicknesses.

channel length [ m]

Fig. 6. Rshunt versus channel length for 11 nanowire FETs. For decreasing channel length, the shunt resistance Rshunt reaches a minimum value of around 150 k to 200 k.

[1] [2] [3] [4] [5]

R EFERENCES Litwin-Staszewska, E., W. Szymanska, and P. Piotrzkowski, The Electron Mobility and Thermoelectric Power in InSb at Atmospheric and Hydrostatic Pressures, Phys. Status Solidi (b), vol. 106, no. 2, pp. 551-559, 1981. C. Thelander, P. Agarwal, S. Brongersma, J. Eymery, L. F. Feiner, A. Forchel, M. Scheffler, W. Riess, B. J. Ohlsson, U. G¨ sele, and L. o Samuelson, Nanowire-based one-dimensional electronics, Mater. Today, vol. 9, no. 10, pp. 2835, Oct. 2006. C. Auth and J. D. Plummer, Scaling theory for cylindrical, fully-depleted, surrounding-gate MOSFETs, IEEE Electron Device Lett., vol. 18, no. 2, pp. 7476, Feb. 1997. J. Knoch, W. Riess, and J. Appenzeller, Outperforming the conventional scaling rules in the quantum-capacitance limit, IEEE Electron Device Lett., vol. 29, no. 4, pp. 372374, Apr. 2008. Zwerdling, Solomon and Lax, Benjamin and Roth, Laura M., Oscillatory Magneto-Absorption in Semiconductors, Phys. Rev., vol. 108, no. 6, pp. 1402-1408, Dec. 1957.

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