Technical Co-Sponsors

1st Workshop

on Nanotechnology in Instrumentation and Measurement

NANOffIM 2015

“Measurements in the World of Nanosensing”

July 24-25, 2015

Ecotekne Conference Center, University of Salento

Lecce, Italy

PROCEEDINGS Editors: Aimé Lay-Ekuakille & Wen Jung Li

NANOfIM 2015 ISBN: 9 788896 496381

Surface-mediated electrical transport in single GaAs nanowires

Ilio Miccoli*,†, Frederik Edler, Herbert Pfnür, Christoph Tegenkamp

Institut für Festkörperphysik Leibniz Universität Hannover

Appelstrasse 2, D-30167 Hannover, Germany *Corresponding author: miccoli@fkp.uni-hannover.de

Paola Prete, Nico Lovergine† Institute of Microelectronics and Microsystems

(IMM-CNR), UOS Lecce, † and Dept. of Engineering for Innovation,

University of Salento, Via Monteroni, I-73100 Lecce, Italy

III-V semiconductor compound based nanowires (NWs) are expected to impact the fields of nano-electronic, nano-photonic, and photovoltaic devices. Self-assembly of crystal-phase controlled and high optical quality III-V NWs has been demonstrated. However, important physical and technological issues, such as carrier transport properties and reproducible incorporation of high dopant concentrations in NW materials, remain to be addressed for enabling robust nano-devices fabrication. In this work, we show the use of a multi-probe scanning tunneling microscope for the rapid electrical characterization of free-standing GaAs NWs, without any need for post-growth sample processing and contact fabrication. In particular, 2-probe I-V measurements were performed along the axis of a single 60-nm diameter unpassivated GaAs NW, and its resistance profile determined, obtaining high (in the range of G ) resistance values.. Due to its reduced radial dimension, the NW is expected to be completely depleted. Analysis of the NW resistance profile reveals instead, that carrier transport is mediated by the NW surface states. Finally, by using the substrate as a reference electrode and placing the other three STM-tips along the NWs, we demonstrate a 4-point probe geometry that can be used for the electrical characterization of highly doped NWs.

Keywords: scanning probe microscopy, III-V semiconductor nanowire, electrical resistivity, MOVPE self-assembly

I. INTRODUCTION The growth of ultra-narrow semiconductor nano-crystals (so-called nanowires, NWs) promises to realize quasi-1D electronic systems. These systems, with diameters in the sub-one hundred nanometer scale and lengths up to several microns are expected to impact on several nano-device fields, ranging from nanoelectronics [1,2,3] to nano-photonics [4], by offering both unprecedented materials properties and innovative device geometries. The distinctive gate-all-around architecture of NW-based field-effect transistors (FETs) is indeed, considered as one of the most promising roads toward the “ultimate” scaling of CMOS technology [5]. On the other

hand, exotic physical phenomena, e.g. the NW-driven light concentration due to resonant absorption effects, bodes to overcome the classical Schockley-Queisser limit of conventional bulk solar cells [6], paving the way to next-generation photovoltaics. In contrast to III-V NWs made by “top-down” methods, which often suffer from damages induced by chemical removal of material [7], “bottom-up” synthesis approaches, such as catalyst-assisted or self-catalyzed molecular beam epitaxy (MBE) [8] and metalorganic vapor phase epitaxy (MOVPE) [9], provide excellent control in structure and composition, damage-free surfaces and much larger flexibilities in device design. Dense and large arrays of self-assembled and vertically-oriented III-V NWs with atomically sharp facets can be thus realized [10,11]. Despite these striking advances, many issues still need to be solved to move the field from purely fundamental research, towards large-scale industrial applications, e.g. photovoltaics. These challenges include top-contact formation, efficient passivation of NW surfaces, as well as reproducible control of high dopant concentrations [12,13]. Technological progress in the field appears strongly limited by the lack of simple and precise electrical measurement methods. Conventional Hall measurements usually employed for determining carrier mobilities and concentrations in planar devices cannot be straightforwardly realized for NWs. Moreover, current-voltage (I-V) measurements are usually performed via the fabrication of NW field-effect transistor structures [13]. However, this comprises the transfer of the NW on an insulating substrate and successive contacting through multi-step and time-consuming electron or ion beam-assisted fabrication of nanometric-defined metal pads. In addition, electric transport properties could be significantly altered by NW-FET fabrication processes [14] and may thus substantially differ from the properties of the as-grown NWs.

143NANOfIM 2015 ISBN: 9 788896 496381

Fig. 1. (a) Schematics of the procedure for approaching a STM W-tip on a single NW. (b) Sequence of SEM micrographs showing a STM W-tip moving (within the tunneling contact regime) along the NW sidewall by constant-current mode.

Recently, the technique of scanning tunneling microscopy (STM) has been proven to be a versatile and flexible tool for directly measuring the IV-characteristic of single NWs without any additional sample processing [15]. In these first experiments, the current was directly applied by placing a single STM probe on the catalyst-droplet of a free-standing NW, while the highly doped substrate played the role of back-contact. However, in this way the IV-characteristic is often dominated by the metal-catalyst/NW Schottky interface, and thus the intrinsic NW transport properties remain inaccessible [16,17,18]. In the present work, we show how to circumvent these issues by directly contacting the tungsten-STM probe on the lateral facet of a GaAs NW, with the NW sample tilted by 45-degree. We performed 2-terminal I-V measurements on the as-contacted GaAs NW. We will show that unpassivated GaAs NW appear completely depleted in spite of intentional doping, most likely because of its reduced dimensions. Systematic transport measurements recorded by varying the probe spacings along the NW can be explained by a surface mediated carrier transport mechanism. In order to exclude contributions from contact resistances we finally demonstrate a 4-point probe contact assembly realized with a 4-tip STM/SEM, similarly to a recent work performed by Korte et.al. [19] that further confirms the capabilities and correctness of the proposed approach.

II. EXPERIMENTAL GaAs NWs were epitaxially grown on semi-insulating (111)B-GaAs substrates by Au-catalyst assisted MOVPE, as previously reported [9]. NW growth was performed in an Aixtron 200RD reactor at a pressure of 50 mbar and a substrate temperature of 400°C, using high-purity trimethylgallium (TMGa) and tertiarybutylarsine (TBAs) as Ga and As precursors, respectively. Colloidal Au NPs in aqueous solution were used as metal-catalysts for the VLS growth of the core GaAs NWs. A TMGa molar flow rate of 12.2 µmol/min (corresponding to a total Ga molar fraction of ~4.3 10-5) was employed in the vapor phase, while the V:III precursors concentration ratio in the vapor was maintained at 10:1. Te-doping was attempted during the NW growth by introducing di-isopropyltellurium (DiPTe)

diluted in H2 with a molar flow of 0.075 µmol/min (molar fraction ~2.6 10-7). All transport experiments were performed under ultra high vacuum (UHV) conditions at room temperature by means of a commercial 4-tip STM system (from Omicron NanoScience) in combination with a high-resolution (4 nm resolution) SEM column. Details regarding the positioning of the tips are given below (Fig. 1). Electrochemically etched (NaOH, 0.25 mm diameter) W-wires were used as probe tips. Before loading into the STM microscope the samples were cleaned by deep immersion (30 sec) in a low concentrated HF solution (2% in water) and then immediately loaded (within 10 sec) into the microscope UHV chamber (3 10-10 mbar) to prevent re-oxidation of the GaAs NW surface.

III. RESULTS AND DISCUSSION Fig. 1 shows the procedure for approaching by the STM tip the lateral facet of a single free-standing NW. As sketched in Fig. 1a the substrate was tilted by 45-degree which enabled us to access the side facets of the NW. Moreover, the high-resolution SEM column allows a precise navigation of the W-tip in proximity of the GaAs NW. In order to avoid irreversible damages to the W-probe, each tip was first approached via feedback-controlled-loop mode into a tunneling contact with the GaAs substrate and then moved towards the NW pedestal. Finally, the NW sidewall was climbed up by the W-tip with the constant-current mode permanently active. Once the desired position on the NW sidewall was reached, electric contact was realized by a defined push-down of the tip with the calibrated piezos in the feedback-off mode. Using the procedure described above, two W-tips were accurately placed on the NW sidewalls, one near the NW pedestal while the other was moved along the NW facet (see inset of Fig. 2 for reference). The corresponding I-V measurements are shown in Fig. 2 and reveal a clear ohmic behavior within the investigated voltage range (see full symbols in Fig. 2). Space-charge limited transport, giving rise to non-linear IV-curves [20,21], turned out to be negligible in our case. For comparison, we also measured the I-V curve of the substrate (dashed black line in Fig. 2), corresponding to a resistance

144NANOfIM 2015 ISBN: 9 788896 496381

Fig. 2. Two point probe I-V characteristic of a single GaAs NW obtained by placing one STM tip at the base of GaAs NW, while a second tip is moved along the NW sidewall. The dashed black line refers to the case where both probes are placed on the GaAs substrate free surface in between the NWs.

of about Rsub=22 M . Note that this value is obviously the sum of the GaAs substrate and the contact resistance of the 2-terminal setup. Resistance values measured from the NW are two orders of magnitude higher, i.e. the contribution of the contact resistance, inherent of the 2-terminal measurement, can be neglected and thus, it does not severely influence our conclusions. The high resistances measured on individual NWs suggest that the GaAs NW is insufficiently-doped and that Fermi-level pinning at the NW surface causes a carrier depletion. In order to get a better understanding of our findings, we estimated the NW resistance for the adopted configuration as the following:

with the NW bulk resistance given by

in which ND is the donor dopant concentration of the Te-doped GaAs NW and

is the electron mobility according to Hilsum et al. [22]. For further information on eq. 2 the reader is referred to [23]. Since the NW is slightly tapered and conically shaped as sketched in Fig. 2 [9], the NW radius at a position l along ___

Fig. 3. Theoretical resistance RNW-bulk and effective radius reff as a function of the dopant concentration inside the GaAs NW. The solid red-curve is plotted by using eq. 2.

the NW axis can be expressed as:

where rAu=(32±6) 10-7 cm, hNW=4 10-4 cm, and t=6.5 10-3 are the Au-catalyst radius, the NW height and the NW tapering coefficient, respectively. In order to account for the effect of Fermi level pinning due to mid-gap surface states in GaAs [24], in eq. (2) we introduced a position-dependent effective radius reff(l) for the transport of charged carriers that is equal to minus the depletion layer width

where denotes the barrier height. Referring to a typical electron mobility L=9400 cm2/Vs, and a dielectric constant

r=13 for n-doped GaAs [22], RNW-bulk has been evaluated, based on eq. (2), for a carrier concentration between 1017 cm-3 and 1019 cm-3 (Fig. 3). Interestingly, the expected NW resistance is always smaller than 1 M as long as the NW doping level remains larger than , while the effective NW radius reff(l) rapidly shrinks from the rAu value (at the NW tip) for to 1 nm when

. If then nm and the NW is completely depleted, i.e. no longer conductive. Noteworthy, in our measurements resistance values in the G range were measured, which (according to our bulk resisitivity model) would be reached for an effective radius of only 0.1 nm (i.e, smaller than GaAs lattice constant of 0.56 nm), which underlines the breakdown of our bulk resistance model.

145NANOfIM 2015 ISBN: 9 788896 496381

In the following we will thus consider a contribution from the surface. It is known that surface transport channels can greatly contribute to total conductance and may easily become comparable to bulk conductivity in semi-insulating GaAs wafers [25]. Assuming that the current flows solely through the surface states of the GaAs NW eq. (2) could be re-written as

where is now the surface conductivity of the GaAs NW. A determination of is easily obtained now by using eq. (6) to best-fit the experimental data points reported in Fig. 4, which shows how the NW resistance increases as a function of the position of the second STM probe along the NW axis. Full symbols in Fig. 4 are directly deduced by I-V curves shown in Fig. 2. Noteworthy, from the best-fitting curve of experimental data, we get a value of around

for for the NW surface conductivity. This results matches perfectly with the surface conductance (at 300 K) of a semi-insulating GaAs wafer, measured via a guard-ring configuration [25]. The little deviation from the expected curve of some of the experimental data points in Fig. 4 may be related to surface roughness inhomogeneities which are not considered in this simple model. Nonetheless, our data clearly reveal that surface transport channels are more dominant than bulk channels in our unpassivated GaAs NW. For the sake of clarity, we note that the typical resistance of present GaAs NWs is about three orders of magnitude smaller than values reported in previous investigations and obtained for a NW-FET configuration [14, 26]. This difference can be probably ascribed to the NW-FET fabrication process, including rapid thermal annealing steps and evaporation of metal pads, which can irreversibly alter the NW surface states and therefore its final conductance. With the in-situ 4-tip approach used in this study, similar post-processing steps were not required, thus supporting our conclusion of a surface-mediated transport channel. As mentioned, the contact resistances in this study can be neglected in our 2-terminal configuration. However, this will no longer be the case for highly-doped NWs. In this case 4-point probe transport measurements are mandatory. Fig. 5 demonstrates that the adopted tilted-sample geometry is suitable for the approach of at least three STM tips onto a single free-standing GaAs-NW. If the substrate is used as the fourth contact, a 4-point geometry can be achieved, and will be used in the near future for the characterization of highly doped NWs. Finally, we would like to point out here that Te-doped GaAs NWs have been successfully reported in the case of Au-catalyzed MBE and MOVPE growth [15,27]; this suggests that the insufficient (<7 1017 cm-3) Te-doping observed in present NWs cannot (most likely) be ascribed to a VLS-related limiting mechanism.

Fig. 4. Resistance profile of a single free-standing GaAs NW. The experimental data points are obtained by I-V curves shown in Fig. 2, while the solid-red line is the best-fitting curve of experimental data, obtained by using eq. 6.

Fig. 5. FE-SEM micrograph of three STM probes contacting a single free-standing GaAs NW.

Furthermore, GaAs:Te layers have been grown by MOVPE using DiPTe as dopant precursor, and show carrier concentrations around 1017 cm-3. Highly Te-doped (up to 1020 cm-3) GaAs samples are also grown using diethyl-tellurium as precursor [28]. The origin of the limited Te incorporation in present GaAs NWs remains thus to be understood.

IV. CONCLUSIONS AND OUTLOOK We have demonstrated that electrical measurements on

single free-standing GaAs NWs can be rapidly performed by means of a multi-probe scanning tunneling microscopy without any additional sample processing. The extremely high resistances found for present GaAs NWs is ascribed to the Fermi level surface-pinning which results in a complete carrier depletion for NW bulk at low (<7 1017 cm-3) carrier concentrations. The as-measured residual conductance of present NWs is explained instead as

146NANOfIM 2015 ISBN: 9 788896 496381

due to GaAs surface states. In order to further confirm and clarify the details of this transport mechanism, temperature- dependent transport measurements are envisaged.

ACKNOWLEDGMENT The financial support through the DFG is gratefully acknowledged. The authors wish also to acknowledge F. Marzo for assistance during growth experiments and S.Korte from “Forschungszentrum Jülich” for useful discussions during the DPG Spring Meeting (Berlin-TU, 15. - 20. March 2015).

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