Electrical Conductivity of InN Nanowires and the Influence of the

NANO LETTERS

Electrical Conductivity of InN Nanowires and the Influence of the Native Indium Oxide Formed at Their Surface

2009 Vol. 9, No. 4 1567-1571

Florian Werner,* Friederich Limbach, Michael Carsten, Christian Denker, Joerg Malindretos, and Angela Rizzi IV. Physikalisches Institut, Georg-August-UniVersita¨t Go¨ttingen, Go¨ttingen D-37077, Germany Received December 5, 2008; Revised Manuscript Received February 10, 2009

ABSTRACT The electrical properties of InN nanowires were investigated in four-point probe measurements. The dependence of the conductance on the wire diameter allows distinguishing between “core” bulk (quadratic) and “shell” sheet (linear) contributions. Evidence of the formation of a thin In2O3 layer at the surface of the nanowires is provided by X-ray core level photoemission spectroscopy. The shell conductivity is therefore ascribed to an electron accumulation layer forming at the radial InN/In2O3 interface. Although conductance through the accumulation layer dominates for nanowires below a critical diameter of about 55 nm, the core channel cannot be neglected, even for small nanowires.

Group III nitrides represent a material class with promising electronic and optical properties. Among these, InN exhibits the narrowest band gap of about 0.67 eV at low temperatures, the lowest effective electron mass, and the highest peak drift velocity and electron mobility.1-4 In the search of new concepts for the reduction of power consumption in large scale integrated circuits, nanowires (NW) and in particular quantum wire transistors (QWT), in which the device size is even further reduced, are intensively studied. Moreover, due to their high surface to volume ratio, nanowires are potential candidates for sensor applications and they are also discussed in connection with next generation solar cells. A crucial role for applications of InN nanowires as novel nano electronic devices is played by a quasi-two-dimensional electron accumulation layer around the wire, since the Fermi level at the surface is pinned in the conduction band.5-7 To fully exploit the potential of InN nanowires, their physical properties have to be understood in depth. Although substantial progress has been made in the electrical characterization of InN nanowires, up to now their electrical properties have only been characterized under the influence of the contact resistances.8-11 In this work we report on the first four-point probe measurements of ultrathin InN nanowires with diameters below 150 nm. In particular, contributions to the conductivity from the core and from a quasi-twodimensional shell are separated. Evidence of a surface native In oxide is provided, and therefore the electron accumulation layer (shell sheet) is assumed to be at the InN/In oxide interface. * Corresponding author, [email protected]. 10.1021/nl8036799 CCC: $40.75 Published on Web 03/16/2009

 2009 American Chemical Society

The InN nanowires were grown by plasma-assisted molecular beam epitaxy under N-rich conditions on Si(111) substrates. The applied growth conditions of the InN nanowire ensembles correspond to the so-called bimodal regime, in which a significant number of long, round nanowires with small diameter can be found, while the rest of the nanowires on the sample is short and has a much larger diameter.12 Typical wires used for our measurements were between 800 nm and 1.5 µm long with radii in the range between 10 and 75 nm. The growth direction follows the c-axis of wurtzite InN. Raman spectroscopy measurements13 show that the nanowires are free of strain and of high crystal quality. High quality is also confirmed by high-resolution transmission electron microscopy (HR-TEM).12 The optical characterization by Raman and photoluminescence spectroscopy further reveals the contribution of an electron accumulation layer.13 By analysis of the low branch of the coupled plasmon-LOphonon mode the inhomogeneous electron concentration is shown to reach values on the order of 1019 cm-3 close to the nanowire surfaces.14 To obtain information about the chemical composition of the investigated nanowires, X-ray photoemission spectroscopy (XPS) analysis was performed on InN nanowire ensembles after they were exposed to air for about 1 day. The sample was annealed at about 120 °C for 3 h for partial desorption of water from the surface. Monochromatized Al KR was used for the photoexcitation, and a 126 mm mean radius hemispherical analyzer with a parallel electron detection system was used for the spectra acquisition. The binding energies were corrected with reference to the carbon

Figure 2. SEM image of a single InN nanowire contacted by electron beam lithography in a four-point probe geometry.

Figure 1. In 3d, N 1s, and O 1s core-level spectra measured under an emission angle of 50°. The intensity is normalized to the C 1s measured intensity (not shown here). The core levels are fitted with Voigt functions; the fitted background is subtracted from the data. The continuous line across the experimental points is the result of the fit. The single core level components are indicated by the dashed lines.

contamination peak at 284.6 eV and they are affected by an error of (0.3 eV due to energy calibration. The In 3d, N 1s, and O 1s core-level XPS spectra taken under an emission angle of 50° to enhance the contribution from the lateral surface of the nanowires are shown in Figure 1 together with the results of the core-level fit. The In 3d doublet shows a clear contribution from two components. The 3d5/2 component at the lowest binding energy (444 eV) is due to the In-N bond; the one shifted by 1 eV toward higher binding energies is assigned to the In2O3 bond.15,16 The resulting spin-orbit splitting is 7.6 eV, and the branching ratio is 0.7, which is close to the expected value arising from the level multiplicity. The measured N 1s core level also shows two distinct components. The one at 396.5 eV binding energy is assigned to the In-N bond.16 The higher binding energy component at 397.7 eV arises from the N-Si bond in a SiNx phase formed by reaction of the nitrogen with the clean Si(111) substrate during growth. The measured O 1s is best fitted by two components at 530.7 and 532.5 eV. The lowest binding energy component (530.7 eV) is assigned to the In-O bonding,16 while the one at 532.5 eV is due to oxygen physisorbed at the surface and not completely removed by the mild annealing. Due to the morphology of the InN nanowire ensemble, we do not attempt any quantitative estimation of the In2O3 shell thickness from the XPS analysis. Since cross-section HR-TEM of a single wire out of the ensemble does not show 1568

a clear signature of such an oxide layer, it is reasonable to assume a thickness of about 1 nm, judging from the resolution of the HR-TEM image (not presented here). This estimation is also compatible with the significant intensity of the In-N component measured in the N 1s peak. A set of 10 four-point contacted wires with different radii was prepared for the electrical measurements. Macroscopic Ti/Au contact pads with a size of 100 µm × 100 µm and alignment markers were defined by optical lithography on a carrier substrate of thermally oxidized silicon (thickness of the oxide 500 nm). After the growth, the wires were mechanically transferred from the sample to the patterned carrier. Using the markers to identify the exact location of the nanowires by scanning electron microscopy (SEM), the wires were contacted by electron beam lithography. Before evaporation of the contact stripes, the areas of the nanowires below the intended contact position were cleaned in an argon plasma. The rest of the wire in between the contacts was protected by the unexposed resist and therefore was not affected by the cleaning process. The as-deposited Ti/Au (10 nm/50 nm) contacts to the wires show Ohmic behavior up to several hundreds of millivolts, well above the potentials applied in the electrical measurements. The electrical measurements were performed at room temperature in the dark with an Agilent 4155C semiconductor parameter analyzer. Under the assumption of a homogeneous wire material, the four-point probe measurement of the wire in Figure 2 with a diameter of 30 nm provides a value of the normalized resistance ∆Rw/∆L ) 5.1 Ω/nm, where Rw is the resistance measured between the voltage probes. On the other hand in a two-point probe geometry several contributions to the measured total resistance Rt have to be distinguished: Rt ) Rc1 + Rc2 + Rw, where Rci are the contact resistances and Rw is the resistance of the wire between the contacts. Under the assumption that all contact resistances are equal, the intrinsic electrical properties of the wires can be derived by measuring several wires of similar diameters but different lengths L (transmission line method). The measured resistance can then be expressed as Rt ) 2Rc + (∆Rw/∆L) L. Two-point probe measurements in all six possible combinations have been performed, and the results are plotted in Figure 3 as a function of the distance between the contacts. By combining all six measurements with the Nano Lett., Vol. 9, No. 4, 2009

four-point probe measurement, the contact resistances can be determined separately, yielding 130, 370, 590, and 4500 Ω. In contrast, values of Rc ≈ 850 Ω for all the contacts and ∆Rw/∆L ) 7.6 Ω/nm for the wire are obtained by the transmission line method, or Rc ≈ 210 Ω and ∆Rw/∆L ) 6.1 Ω/nm if only data points corresponding to the lowest contact resistances are included (black circles in Figure 3). A possible reason for the varying contact resistances is the thin layer of indium oxide, forming on the surface of the nanowires when they are exposed to atmosphere. The thin In oxide layer forms a tunnel barrier of slightly varying thickness, explaining both the high contact resistances and the strong variation of the resistances measured. This is in good agreement with electrical two-point measurements depending on the pretreatement of the wires. Without argon plasma etching, resistances of up to several hundreds of megaohms were measured, while the highest resistance measured on argon-cleaned wires was lower than 20 kΩ. The spread in contact resistances for the nanowire shown in Figure 3 is representative of the 10 nanowires characterized in this work. The normalized resistance ∆Rw/∆L or its inverse, the normalized conductance g ) (∆Rw/∆L)-1, are key parameters for determining different contributions to the nanowire resistance due to its geometry. In particular the question is addressed, whether conductance through the wire will dominantly occur through the bulk or through an electron accumulation layer. The contribution of an interface electron accumulation layer to the conductance of InN nanowires has been reported recently.8,10,17 To explain the carrier transport through the nanowires, we therefore consider both the core conductance (3D) and a contribution from a shell (2D) represented by the accumulation layer, assuming both electron densities (bulk and sheet) to be independent of the diameter of the nanowire. g)

π 2 2π L ) r + r Rw F3D F2D

(1)

where Rw is resistance of the wire, L is length of the wire over which the voltage drop is measured, r is radius of the wire, F3D is core resistivity, and F2D is resistivity of the electron accumulation layer. A rough estimate of the influence of the bulk and a 2D channel on the carrier transport is the exponent β in the relation g ∝ rβ (with the normalized conductance g ) ∆L/ ∆Rw). From eq 1 a value of β ≈ 1 is expected for a pure 2D channel, and β ≈ 2 for bulk conductivity only. The normalized conductance is plotted as a function of r in Figure 4a and a value of β ) 1.6 ( 0.2 is derived, indicating that a two-dimensional electron accumulation layer is indeed present, but much less pronounced than initially expected and reported by other groups.8,10 However, the β-exponent is only a rough approximation, as for larger diameters, the bulk conductance will dominate; hence the calculated β-exponent will increase. A more quantitative estimate can be obtained directly from the plot of the normalized conductance g versus the radius Nano Lett., Vol. 9, No. 4, 2009

Figure 3. Six two-point probe measurements on different pairs of contacts to the wire along with a linear fit to extract the contact resistance and the resistance of the wire (red dotted line). For the dashed blue line the three data points with the highest resistances involving contact 1 have been discarded. The black straight line represents the data obtained from the four-point probe measurements, not including the contact resistances.

of the wires (see Figure 4a). As expected from eq 1, a parabola function can be fitted to the data. The coefficients of the parabola correspond to core and shell channel resistivities of F3D ) (1.1 ( 0.2) × 10-3 Ω cm and F2D ) (820 ( 150) Ω, respectively. The bulk resistivity obtained in this way is significantly larger than the values published earlier by other groups, which are around F ≈ 4 × 10-4 Ω cm9,18 and is 1 order of magnitude lower than the value measured for epitaxial layers in a study, in which the bulk and the surface contributions have been determined separately.19 This is expected, as the former were derived by assuming carrier transport only through the bulk, without separating 3D and 2D contributions. A higher current (lower resistivity) is therefore assumed through the bulk channel. Ignoring the contribution of the 2D conductive channel our measurements yield (4.6 ( 0.8) × 10-4 Ω cm in agreement with the results published earlier. For InN layers, a surface electron accumulation layer has been explained by In-In bonds on clean polar InN surfaces and is predicted to be absent under specific conditions on nonpolar surfaces.6 Studies on oxidized InN layers also indicate that the electron accumulation layer is strongly influenced by oxides on the surface, which modify the surface Fermi level pinning position and hence the near surface carrier density.20 In a recent study the universality of the InN electron accumulation layer at InN surfaces has been proposed.7 In our experiments the conductivity of the InN nanowires surrounded by a native In2O3 clearly exhibits a contribution from a high density electron accumulation layer, which is therefore assumed to form at the InN/In2O3 interface. The F2D determined above allows an estimation of the average mobility of the interface accumulation layer. By assuming a range of sheet carrier concentrations of 1013 cm-2 up to 1014 cm-2,14 the resulting mobility is between 760 and 76 cm2/ (V s), which is comparable with the value of about 500 cm2/ (V s) derived for the electron surface accumulation layer in InN layers.19 1569

Figure 4. (a) Plot of the normalized conductance g vs the radius r of the wire, derived from four-point measurements on various nanowires of different aspect ratios. The error bars of the thin wires are of the order of the symbol size and have not been plotted for clarity. A parabola (red line) is fitted to the data points to determine the resistivities of bulk and surface. Inset: the same data points in logarithmic scale with a power law fit (blue dashed line) to determine the exponent β ) 1.6 ( 0.2 in the relation g ∝ r β. (b) Sketch of the InN nanowire structure as derived from the electrical measurements and from photoemission data.

The assumption of an interface instead of a surface accumulation layer is further supported by the observation that electrical contacts to the untreated surface often result in a high contact resistance. Theoretical calculations so far are concerned with the InN/vacuum interface.6 A theoretical knowledge of the electronic properties of the InN/In2O3 interface would be of great interest, due to its relevance for the applications of InN nanoscale devices. From our experiments a critical diameter dsc ) 55 ( 15 nm can be obtained, for which the shell and core resistances are equal. For larger diameters, carrier transport will mainly take place in the core, while for d < dsc transport through the electron accumulation layer will dominate. Most nanowires used in this study have diameters of around 30 nm. In this range we obtain normalized resistances of (∆R/∆L)3D ) 15 ( 2 Ω/nm and (∆R/∆L)2D ) 9 ( 2 Ω/nm for the core and shell, respectively, clearly demonstrating a strong contribution of the electron accumulation layer at the interface. It demonstrates that the contribution of the core to the conductance exists also for small nanowires. Within the range of diameters easily achievable experimentally (d ≈ 15-200 nm), both contributions will not differ by more than 1 order of magnitude. Some authors have reported about a resistance anomaly in thin InN nanowires, where the resistance of the wires decreases as the cross section is reduced.9,21 This has been explained with the presence of a high density electron accumulation layer at the surface of thin InN nanowires,17 under the assumption that surface conduction plays a major 1570

role for small diameter wires. We cannot confirm this anomalous diameter dependence of the resistance in our nanowires. InN nanowires of different lengths and radii were investigated by electrical four-point probe measurements and the dependence of the conductance on the geometry of the nanowires was derived, distinguishing between contributions originating from the bulk and from a quasi-two-dimensional conduction channel. The resistivity of the bulk of the nanowires was found to be Fbulk ) (1.1 ( 0.2) × 10-3 Ω cm with a significant contribution to the conductance by an electron accumulation layer that dominates for diameters d < dsc ) 55 ( 15 nm. A thin layer of In2O3 at the InN nanowire surface is identified by XPS on InN nanowire ensambles and lets us conclude that the 2D channel is formed at the interface between the InN and the surface In oxide. The contribution of the surface oxide to the contact resistance is crucial for experimental studies whenever four-point probe measurements are impracticable. Furthermore, there have been reports on superconductivity in semiconductor InN layers and a recent work has shown that this is not an intrinsic property of the InN but rather a contribution from traces of indium oxide.22 So the knowledge about the surface native In oxide could also be important in the analysis of possible anomalies in the temperature-dependent resistivity of InN/ In2O3 nanowires. Acknowledgment. This work was supported by the ERANET project “NanoSci-ERA: NanoScience in the European Research Area” of the EU FP6. We thank H. Schumann and M. Seibt for the TEM characterization. We thank R. G. Ulbrich for stimulating discussions and for critical reading of the manuscript. References (1) Walukiewicz, W.; Ager, J. W., III.; Yu, K. M.; Liliental-Weber, Z.; Wu, J.; Li, S. X.; Jones, R. E.; Denlinger, J. D. J. Phys. D: Appl. Phys 2006, 39, R83-R99. (2) Klochikhin, A. A.; Davydov, V. Y.; Emtsev, V. V.; Sakharov, A. V.; Kapitonov, V. A.; Andreev, B. A.; Lu, H.; Schaff, W. J. Phys. ReV. B 2005, 71, 195207. (3) Bhuiyan, A. G.; Hashimoto, A.; Yamamoto, A. J. Appl. Phys. 2003, 94, 2779. (4) Bellotti, E.; Doshi, B. K.; Brennan, K. F.; Albrecht, J. D.; Ruden, P. P. J. Appl. Phys. 1999, 85, 916. (5) Mahboob, I.; Veal, T. D.; Piper, L. F. J.; McConville, C. F.; Lu, H.; Schaff, W. J.; Furthmueller, J.; Bechstedt, F. Phys. ReV. B 2004, 69, 201307(R). (6) Segev, D.; van de Walle, C. Europhys. Lett. 2006, 76, 305–311. (7) King, P. D. C.; et al. Appl. Phys. Lett. 2007, 91, 092101-092103. (8) Richter, T.; Blo¨mers, C.; Lu¨th, H.; Calarco, R.; Indlekofer, M.; Marso, M.; Scha¨pers, T. Nano Lett. 2008, 9, 2834–2838. (9) Chang, C. Y.; Chi, G. C.; Wang, W. M.; Chen, L. C.; Chen, K. H.; Ren, F.; Pearton, S. J. Appl. Phys. Lett. 2005, 87, 093112. (10) Calleja, E.; Grandal, J.; Sanchez-Garcia, M. A.; Niebelschutz, M.; Cimalla, V.; Ambacher, O. Appl. Phys. Lett. 2007, 90, 262110. (11) Chen, J.; Cheng, G.; Stern, E.; Reed, M.; Avouris, P. Nano Lett. 2007, 7, 2276–2280. (12) Denker, C.; Malindretos, J.; Werner, F.; Limbach, F.; Schuhmann, H.; Niermann, T.; Seibt, M.; Rizzi, A. Phys. Status Solidi C 2008, 5, 1706–1708. (13) Segura-Ruiz, J.; Garro, N.; Cantarero, A.; Denker, C.; Werner, F.; Malindretos, J.; Rizzi, A. Phys. Status Solidi C 2008, 5, 1678–1681. (14) Segura-Ruiz, J.; Garro, N.; Cantarero, A.; Denker, C.; Malindretos, J.; Rizzi, A. Phys. ReV. B 2009, 79, 115305. (15) Piper, L. F. J.; Veal, T. D.; Walker, M.; Mahboob, I.; McConville, C. F.; Lu, H.; Schaff, W. J. J. Vac. Sci. Technol., A 2005, 23, 617– 620. Nano Lett., Vol. 9, No. 4, 2009

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