Letter pubs.acs.org/NanoLett
Amphoteric Nature of Sn in CdS Nanowires Mengyao Zhang,†,‡ Marcel Wille,§ Robert Röder,§ Sebastian Heedt,∥ Liubing Huang,†,‡ Zheng Zhu,†,‡ Sebastian Geburt,§ Detlev Grützmacher,∥ Thomas Schap̈ ers,∥ Carsten Ronning,§ and Jia Grace Lu*,†,‡ †
Department of Physics and Astronomy, and ‡Department of Electrophysics, University of Southern California, Los Angeles, California 90035, United States § Friedrich-Schiller-Universität Jena, Institut für Festkörperphysik, Max-Wien-Platz 1, 07743 Jena, Germany ∥ Peter Grünberg Institut (PGI − 9), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany S Supporting Information *
ABSTRACT: High-quality CdS nanowires with uniform Sn doping were synthesized using a Sn-catalyzed chemical vapor deposition method. X-ray diffraction and transmission electron microscopy demonstrate the single crystalline wurtzite structure of the CdS/Sn nanowires. Both donor and acceptor levels, which originate from the amphoteric nature of Sn in II− VI semiconductors, are identified using low-temperature microphotoluminescence. This self-compensation effect was cross examined by gate modulation and temperature-dependent electrical transport measurement. They show an overall n-type behavior with relatively low carrier concentration and low carrier mobilities. Moreover, two different donor levels due to intrinsic and extrinsic doping could be distinguished. They agree well with both the electrical and optical data. KEYWORDS: CdS nanowire, VLS growth, doping, compensation, low-temperature photoluminescence
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electrical property characterizations and analysis. Synthesis and specific experimental techniques are described in the end section of Experimental Methods. The substrate surface was covered with a catkin-like yellow and gray layer of nanostructures after synthesis. A typical SEM image of a CdS nanowires ensemble is shown in Figure 1a, showing dendrite structures. For structural investigation of individual wires, the samples were transferred to a cleaned Si substrate by dry imprint. In the same batch, the diameters of wires are around several hundred nanometers, and the lengths vary from several nanometers to several hundred micrometers. A single CdS/Sn nanowire with diameter of 300 nm and length of 14 μm is shown in Figure 1b. The synthesized nanowire ensemble reveals either straight or tapered morphologies but both with clear hexagonal cross sections. In this report, we focus on the straight wires with homogeneous diameters. The image and the formation mechanism of the tapered nanowires are shown in the Supporting Information Figure S1. As known, the CdS nanowire growth is governed by the catalytic vapor− liquid−solid (VLS) mechanism. The Cd−Sn−S phase diagram shows that Sn and CdS have an eutectic point at 205 °C.17 Thus, Sn can form tiny alloyed seeds with the supplied Cd and S atoms, catalyzing the VLS growth. High-resolution TEM image taken on a single wire is exhibited in Figure 1c, and the corresponding fast Fourier
ingle crystalline semiconductor nanowires have emerged as ideal building blocks for the next generation nanoscale photonic and electronic devices. With a direct wide band gap, cadmium sulfide (CdS) nanowires are particularly attractive due to their unique properties and device applications, including but not limited to transistors, solar cells, photo detectors, light emitting diodes, and continuous wave lasers.1−8 Since intrinsic CdS is highly insulating, doping is crucial in order to tailor the material property and to control the device functionality. Stable p-type conductivity has been shown to be difficult due to the low solubility of the acceptor dopants, deep acceptor levels, and the compensation effect between acceptor dopants and the native donors.6 Several kinds of elements, such as Cl, N, Ga, and P, have proven to be effective dopants for obtaining n-type characteristics.9−12 Group IV elements have been reported to introduce compensation effect in II−VI and III-V semiconductor materials.13,14 However, the amphoteric nature of Sn in CdS has never been reported in literature, even though the studies of CdS/Sn system’s application as nanowire lasing or solar cell window have been carried out.7,8,15 Because the ionic radius of Sn2+ (1.02 Å) is close to that of Cd2+ (0.97 Å),16 Cd substitution is expected to be more preferential and thus leads to dominating n-type characteristics. In this work, the selfcompensation effect in Sn-doped CdS nanowires is investigated, and the band structure has been accurately determined from the results of the optical measurements with conclusive correlations to the temperature-dependent charge transport measurement. This article is organized to present first the nanowire structure characterization, followed by optical and © XXXX American Chemical Society
Received: September 20, 2013 Revised: December 19, 2013
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Figure 1. (a) SEM image of a CdS nanowire ensemble. (b) SEM image of a single nanowire with homogeneous diameter and a round catalyst on the tip. Inset shows an enlarged view of the section near catalyst. (c) HRTEM image of a CdS/Sn nanowire showing single crystalline structure with growth direction along[002] (c-axis). Inset displays a FFT pattern of the HRTEM image. (d) Point EDX spectrum on a single CdS nanowire shows distinct Cd and S peaks. Sn peaks are at the shoulder of the Cd peak. Cd/S/Sn ratio is quantified to be 48.1:50.2:1.7. The silicon signal originates from the Si wafer substrate. (e) XRD spectrum of a CdS/Sn wire ensemble indicates the wurtzite structure for CdS nanostructures and Sn with tetragonal phase.
transform (FFT) pattern is shown in the inset. Both patterns reveal that these CdS nanowires are perfectly single crystalline, having a wurtzite structure and a growth direction along [002] (c-axis). The spacing between the (002) planes is the same as that in bulk material, indicating no distortion in the CdS crystal structure with the incorporation of Sn. The composition of the tip particles and wires were analyzed using point EDX measurements. The EDX spectrum recorded on one particular nanowire (shown in Figure 1d) reveals the presence of the Cd, S, and Sn elements in a stoichiometric ratio Cd/S/Sn = 50.2:48.1:1.7. The concentration of Sn is estimated to be in the range of 1−4 atom % based on several point EDX measurements on ten wires. The EDX mapping on a segment of one wire also indicates the distribution of Sn element along the wire, as shown in Supporting Information Figure S2a. On the other hand, the spectrum recorded on the spherical particle present at the tip of the nanowire shows that it is predominately
containing Sn (∼98 atom %, see Supporting Information Figure S2b), thus confirming the catalyst-assisted VLS growth. XRD pattern of a wire ensemble is plotted in Figure 1e. Most of the diffraction peaks can be assigned to the wurtzite phase of the CdS with a lattice constant of a = 4.14 Å and c = 6.72 Å. These values match well with the JCPDS data (card No. 653414). All the CdS peaks appear at the same positions of wurtzite CdS structure again proves that Sn doping does not alter the CdS crystal structure. The other peaks are attributed to Sn (JCPDS card no. 65-2631) mainly originating from the Sn particles on the wire tip and extra Sn particles formed during the fabrication. Optical characterization was performed via overview CL ensemble measurement and subsequent detailed PL and μ-PL measurement in order to obtain an approximated scheme of the impurity levels in the band gap. Figure 2a shows the SEM image of the nanowire ensemble, which was investigated with overview CL. The CL spectrum of the corresponding structure B
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Figure 2. (a) SEM and monochromatic CL images of a CdS/Sn nanowire ensemble. The near band edge emission (NBE) at 490 nm originates mainly from long and straight nanowires, whereas thick structures show predominantly luminescence at 520 nm corresponding to donor−acceptorpair (DAP) and a free to bound (e, A) transitions. Deep level emission (DLE) centers are homogeneously spread over the ensemble. (b) The lowtemperature (7 K) CL spectra of the corresponding ensemble in (a) reveals 3 emission regions, NBE (blue), DAP and (e, A) and their LO-phonon replica (green), and the DLE (red). (c) Photoluminescence spectra of CdS/Sn nanowire ensemble, taken at 4 K with a excitation density of 50 mW/ cm2 at 325 nm. Several transitions of the NBE are labeled, that is, the free exciton FXA, donor bound excitons (I2, I3), acceptor bound excitons (I1/ I1a), the surface exciton SX, and their phonon replica (ΔELO = 38 meV). The distance between DAP and (e, A) transition corresponds to the binding energy of an electron to a donor ED = 18 meV.
in CdS ELO = 38 meV.21 The occurrence of several phonon replica, where the first LO replica is more intense than the zero phonon lines, demonstrate strong electron−phonon coupling. The DAP nature of the feature at 2.399 eV was already shown in ref 7 for a single CdS/Sn nanowire and the nature of the (e, A) transition at 2.417 eV is proven by temperature-dependent PL investigations (see Supporting Information, Figure S5). An energetic distance between the two DAP and (e, A) series is observed, as illustrated in Figure 2c. Thus, the involved electron donor binding energy can be estimated to be around ED = 18 meV. Furthermore the PL spectra exhibit a highly featured NBE region. The high energetic shoulder of the predominant peak is the free exciton emission FXA at ∼2.552 eV22 accompanied by an exciton bound to ionized donors (DX, labeled I3) at 2.548 eV.21 The dominant peak near 2.545 eV is a superposition of the exciton bound to neutral donors (D0X, labeled I2) and the surface exciton SX.21,22 The peaks at the lower energy side are composed by two lines I1 (2.535 eV) and I1a (2.533 eV) corresponding to neutral acceptor-bound excitons and its acoustic phonon replica.23 The following five peaks correspond to longitudinal optical phonon replicas of I2B (B-Exciton bound to neutral donor), FXA, I2, and I1 with respective energy difference of 38 meV.21 These peaks identifications are confirmed by temperature-dependent PL (results shown in the Supporting Information Figure S3). With increasing temperature and enhanced thermal energy, the donor-bound excitons dissociate. Therefore, the I2 intensity gradually decreases, while the FXA transition becomes more intense. While the previous CL and PL spectra represent the integrated luminescence of an excited nanostructure ensemble, further information about single long and straight nanowires is made accessible by microphotoluminescence (μ-PL) measure-
in Figure 2b reveals three spectral emission regions: the near band edge (NBE) emission at 2.55 eV, caused by excitonic emission; the deep level emission (DLE) at ∼2.1 eV, attributed to several defects such as the carrier recombination at Cd interstitials;18 and the region between 2.45 and 2.2 eV, where the donor−acceptor-pair (DAP) and one free to bound transition (e, A) occur accompanied by their phonon replica.19 The DAP transition is the emission feature of an electron bound to a donor that recombines with a hole bound to an acceptor; and the free to bound transition (e, A) emerges from a free electron recombining with an acceptor bound hole. Thus, the energetic distance between these two series corresponds to the binding energy of an electron to a donor ED. The DAP as well as the (e, A) transition are absent in undoped CdS NWs, as shown in the Supporting Information (Figure S4), and therefore are most likely caused by the incorporation of Sn. The monochromatic CL images of the as grown CdS/Sn nanowire ensemble furthermore show that the NBE at ∼490 nm originates mainly from long and straight nanowires. The intense NBE compared to DLE in the thin nanowire structures, as shown in the monochromatic CL images, hints the high crystal quality of the nanostructures. Thicker structures show predominantly luminescence at 520 nm corresponding to DAP and (e, A) transitions. This suggests that more Sn is incorporated into thicker structures than in thin wires, resulting from different growth mechanism between them, which is nonVLS for thicker structures and VLS for thin wires.20 Low-temperature ensemble PL (4 K, λex = 325 nm, Iex = 50 mW/cm2) measurements allow a closer look to the NBE transitions, as assigned in Figure 2c, and the double structure containing the DAP and (e, A) transitions,19 which are strongly accompanied by phonon replica with a distinct spacing corresponding to the energy of a longitudinal optical phonon C
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Figure 3. (a) The temperature dependent μ-PL spectra of the CdS/Sn nanowire shown in the inset (scale bar = 10 μm) were obtained from the marked position with an excitation density of 3 mW/cm2 at 325 nm. The μ-PL spectra of a single nanowire reveal less intense DAP emission in comparison to NBE than the ensemble spectra in Figure 2c. The NBE shows a red shift of ΔEVar ∼ 1.4 meV due to Varshni effect with increasing sample temperature, whereas the DAP shifts to higher energies (ΔED ∼ 7 meV) due to thermal occupation of higher donor levels. (b) The band structure for CdS/Sn nanowires obtained from the optical data shows excitonic transitions: The free exciton FX, donor bound excitons DX and acceptor bound excitons AX, but also nonexcitonic transitions: free to bound transitions (h, D) and (e, A), or the bound to bound DAP transition. The energies of the transitions result from the band gap EGap = 2.58 eV and the corresponding binding energies (EBFX = 28 meV, ED = 18 meV, EA = 164 meV). The deep level defect luminescence appears at energies ranging from 1.8−2.2 eV.
Figure 4. (a) Source−drain current versus voltage of a CdS/Sn single nanowire field effect transistor show good linearity under 0, 20, and 40 V gate voltages. Inset: SEM image of this single wire with Ti/Au contact. Upper left inset: SEM image of single wire FET. Lower right inset: Source-drain current versus gate voltage plots at room temperature under source-drain voltage Vds = 2, 4, and 8 mV, respectively. The transconductance is estimated to be 1.80 × 10−12 A/V at Vds = 2 mV. Carrier concentration n = 8.10 × 1016 cm−3 and mobility μe = 2.22 cm2/V·s are obtained. (b) Temperature dependence of conductivity depicts the thermal activation of electrons from various donor levels.
with N ∼ 0.1. Thus, a macroscopic amount of electrons occupy these higher donor states. Summarizing the optical investigations allows an estimation of the energy levels of the CdS/Sn nanowires, which is sketched in Figure 3b with the NBE consisting of excitonic emission, the (e, A), DAP double structure and the DLE. We also incorporated the free to bound (h, D) transition in the sketch, although it was not observed in the measurements. To examine in depth the electronic structure of CdS/Sn nanostructures, gate-dependence measurements were performed via a back gate FET configuration. The source-drain current versus voltage (Ids−Vds) characteristics of a single CdS/ Sn nanowire with diameter of 700 nm at room temperature under different gate voltages is plotted in Figure 4a. The linear Ids−Vds curve suggests good ohmic contact. It is found that the source-drain current of the FET increases consistently with increasing gate voltage (Vg), indicating an overall n-type nature
ments at different temperatures. Figure 3a shows the μ-PL spectra, obtained from the “cross” position on the nanowire in the inset with the NBE, which is 2 orders of magnitude larger than the double DAP and (e, A) structure. These data also indicates a weaker Sn incorporation in long and straight nanowires compared to thick structures and is in good agreement with the monochromatic CL images. With increasing temperature, the NBE reveals a red shift to lower energies due to Varshni shift;24 whereas, the double structure of DAP and (e, A) shifts to higher energies by ΔED ∼ 7 meV. This blue shift is likely to originate from a thermal occupation of higher donor levels, which can be attributed to intrinsic and tininduced donor states. The ionization of donor−acceptor pairs with a corresponding activation energy of 18 meV leads to a Boltzmann factor of N ∼ 3 × 10−3 at 35 K, but the thermal ionization is significantly smaller than the thermal occupation of higher donor states due to lower energy of ΔED ∼ 7 meV D
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ature, and the temperature dependence of the conductivity in this region is governed by that of the mobility. In summary, uniform Sn-doped CdS nanowires were achieved from in situ Sn-catalyzed VLS mechanism. Assynthesized CdS/Sn nanowires exhibit wurtzite structure and high crystal quality. Low-temperature CL, PL and μ-PL spectra suggest that the incorporation of Sn introduces both donor and acceptor levels but reveals overall a predominant n-type character. The transport properties via back gate transistor configuration confirm this behavior, showing compensated carrier concentration with correspondingly low carrier mobilities. Both optical and electrical measurements agree well. The energy levels for amphoteric Sn states in CdS nanowire are determined from the experimental results. The elucidation of the electronic band structure is essential for the design and development of the nanoelectronic and nanophotonic application based on the nanowire system. Experimental Methods. Tin-doped CdS nanowires were synthesized in a horizontal tube furnace by chemical vapor deposition. A 460 mg mixture of CdS, SnO, SnO2, and graphite powders with a molar ratio of 1:1:1:0.1 was placed in an alumina boat located in the center of the furnace. A Si growth substrate was placed at the downstream edge of the furnace in a temperature region of 300−400 °C. The furnace was pumped to ∼0.1 Torr and flushed with Ar flow (200 sccm). The growth took place in a two-step process. The furnace was first maintained at 600 °C for 1 h and then maintained at 1000 °C for 30 min. The pressure was kept at 1 atm and Ar flow was kept at 50 sccm during the growth process and turned off after the furnace was cooled down to the room temperature. The morphology and crystal structure of the synthesized nanostructures were characterized by scanning electron microscopy (SEM, JEOL 7001), transmission electron microscopy (TEM, JEOL 2100F), and X-ray diffraction (XRD, Rigaku Ultima IV). The stoichiometry was examined using energy dispersive X-ray spectroscopy (EDX) equipped in SEM. Optical characterization of a CdS/Sn nanowire ensemble was performed using cathodoluminescence (JEOL JSM-6490 SEM equipped with a GATAN MonoCL3+ system). The CL system allows low-temperature measurements using a cryostage and two detection modes (monochromatic and panchromatic mode). Single CdS/Sn nanowires were transferred onto clean SiO2/Si substrate by dry imprint for further optical measurements. The temperature-dependent ensemble photoluminescence as well as microphotoluminescence (μ-PL) measurements of single nanowires were performed using a HeCd 325 nm cw laser for excitation, while the sample was mounted in a Janis ST-500 helium flow cryostat with a cryostat temperature range from 4 K to room temperature. The luminescence light was collected by spherical fused silica lenses for ensemble PL measurement and dispersed by a 500 mm monochromator and detected with a liquid nitrogen-cooled CCD.1 The gate-dependence measurements were performed using a back gate configuration. The as-grown CdS/Sn nanowires were collected from the Si growth substrate and dispersed in isopropyl alcohol by sonication. The suspension was then dropped onto a degenerately doped p-type silicon wafer covered with 500 nm SiO2. Photolithography and electron beam evaporation were used to define the metal electrode contacts (Ti 20 nm/Au 200 nm) on single CdS/Sn nanowires. Rapid thermal annealing was carried out in a N2 flow chamber at 300 °C for 30 s in order to improve the metal−
of this Sn-doped CdS nanowire. This is consistent with lowtemperature PL result revealing intense donor bound exciton and DAP emissions. The conductivity at zero Vg at room temperature is estimated to be lower bounded at ∼2.89 × 10−2 S·cm−1 using equation σ = 1/ρ = L/(R·A) where A is the cross area of the nanowire, L is the channel length, and R is determined by dIds/dVds. The conductivity is enhanced compared to the undoped CdS nanowire conductivity (10−7− 10−6 S·cm−1).10 From Figure 4a lower right inset, the fieldeffect electron mobility (μe = gm(L2/CVds))25 is estimated to be 2.22 cm2/V·s, where the channel transconductance gm = ∂Ids/ ∂Vg = 1.80 × 10−12 A/V at Vds = 2 mV, and the channel length L = 22 μm. The gate capacitance C for a cylindrical wire lying on a dielectric substrate is determined by C = (2πε0εL)/[ln(2h + r)/r], with r representing the radius of the nanowire, h representing the thickness of the gate oxide (500 nm), and effective dielectric constant ε is 2.20.26 The carrier concentration n is calculated to be 8.10 × 1016 cm−3 from the threshold voltage Vth, as n = (Vth/e)[C/(Lπr2)], where Vth = −55 V is extrapolated from the reverse extension of the Ids−Vg curves at different Vds. The small electron concentration manifests the self-compensation effect in the material. Another five wires have been measured and the obtained mobility lies in the range of 0.12−2.64 cm2/V·s. The mobility in CdS/Sn wires is considerably smaller than that of the bulk material (∼340 cm2/V·s), however it is comparable to that of doped CdS nanowires with back gate configuration reported in the previous studies.10,27 The small mobility further validates the self-compensation effect in the nanowire, originating from the amphoteric nature of Sn in CdS, which gives rise to an increased number of ionized donors and acceptors. Both contribute to enhanced scattering, leading to the low mobility. The logarithmic scaled conductivity of a CdS wire FET as a function of reciprocal temperature is plotted in Figure 4b. The temperature-dependent behavior corresponds to the electronic structure after the incorporation of Sn. The curve exhibits three distinct regions: (I) 300−230 K (II) 230−80 K (III) 80−40 K. In regions I and III, the conductivity shows Arrhenius-type behavior, σ ∝ exp(−(Ea/kT)), where Ea is the activation energy required to promote an electron into the conduction band. The activation energy extracted from the slope in region III below 80 K is about 20 ± 0.2 meV, attributing to the shallow donor levels due to the intrinsic and extrinsic dopants. For compensated semiconductors, at low temperature the Fermi level is fixed at the shallow donor energy level. Thus the activation energy is simply the ionization energy ED, that is, σ ∝ exp(−(ED/kT)).28 It is in good agreement with the ionization energy of the donor state (∼18 meV) determined from the energy difference between the free to bound (e, A) and DAP transitions, as discussed earlier on the low temperature PL analysis. In region I for temperature above 230 K, the activation energy extracted from the slope is about 110 ± 10 meV. This temperature dependence change could be contributed from electron excitation from deeper donor level located ∼110 meV below the conduction band edge, which is consistent with the donor level of sulfur vacancy (∼100 meV).22 On the other hand, in the intermediate temperature regime, region II shows a conductivity increase with decreasing temperature at the hightemperature side, while inverse behavior is observed at the lowtemperature side. This exemplifies a characteristic extrinsic saturation region, where all shallow donors are ionized and the carrier concentration in the band is independent of temperE
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(12) Wu, B.; Jiang, Y.; Wu, D.; Li, S.; Wang, L.; Yu, Y.; Wang, Z.; Jie, J. J. Nanosci. Nanotechnol. 2011, 11 (3), 2003−2011. (13) Fistul, V. I. Impurities in Semiconductors: Solubility, Migration and Interactions; CRC Press: Boca Raton, FL, 2004. (14) Panchuk, O.; Savitskiy, A.; Fochuk, P.; Nykonyuk, Y.; Parfenyuk, O.; Shcherbak, L.; Ilashchuk, M.; Yatsunyk, L.; Feychuk, P. J. Cryst. Growth 1999, 197 (3), 607−611. (15) Jafari, A.; Zakaria, A.; Rizwan, Z.; Ghazali, M. S. M. Int. J. Mol. Sci. 2011, 12 (9), 6320−6328. (16) Roy, P.; Srivastava, S. K. J. Phys. D: Appl. Phys. 2006, 39, 4771− 4776. (17) Zargarova, M. I.; Alieva, S. B.; Allazov, M. R. Russ. J. Inorg. Chem. 1985, 30, 726−729. (18) Aguilar-Hernández, J.; Contreras-Puente, G.; Morales-Acevedo, A.; Vigil-Galán, O.; Cruz-Gandarilla, F.; Vidal-Larramendi, J.; Escamilla-Esquivel, A.; Herńandez-Contreras, H.; Hesiquio-Garduno, M.; Arias-Carbajal, A.; Chavarría-Castaneda, M.; Arriaga-Mejía, G. Semicond. Sci. Technol. 2003, 18, 111−114. (19) Colbow, K. Phys. Rev. 1966, 141 (2), 742−749. (20) Perea, D. E.; Hemesath, E. R.; Schwalbach, E. J.; Lensch-Falk, J. L.; Voorhees, P. W.; Lauhon, L. J. Nat Nanotechnol. 2009, 4 (5), 315− 319. (21) Thomas, D. G.; Hopfield, J. J. Phys. Rev. 1962, 128 (5), 2135− 2148. (22) Xu, X.; Zhao, Y.; Sie, E. J.; Lu, Y.; Liu, B.; Ekahana, S. A.; Ju, X.; Jiang, Q.; Wang, J.; Sun, H.; Sum, T. C.; Huan, C. H. A.; Feng, Y. P.; Xiong, Q. ACS Nano 2011, 5 (5), 3660−3669. (23) Henry, C. H.; Faulkner, R. A.; Nassau, K. Phys. Rev. 1969, 183 (3), 798−806. (24) Varshni, Y. P. Physica 1967, 34 (1), 149−154. (25) Dürkop, T.; Getty, S. A.; Cobas, E.; Fuhrer, M. S. Nano Lett. 2003, 4 (1), 35−39. (26) Wunnicke, O. Appl. Phys. Lett. 2006, 89 (8), 083102−3. (27) Jie, J. S.; Zhang, W. J.; Jiang, Y.; Lee, S. T. Appl. Phys. Lett. 2006, 89 (22), 223117−3. (28) Shklovskii, B. I.; Efros, A. L. Electronic Properties of Doped Semiconductors; Springer: Berlin, 1984.
semiconductor contacts. The gate voltage was applied to the back gate underneath the SiO2 dielectric layer (500 nm thick), modulating the current flow in the nanowire. The temperaturedependent measurements were carried out in a standard voltage-driven DC-setup inside a He-4 continuous-flow cryostat. The transistor sample was bounded onto a standard chip-carrier and mounted into the cryostat. The cryostat temperature was first cooled down to 4K, and then heated in controllable steps with a heater-coil close to the sample.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information and figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. M.Z. and M.W. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to Professors Hans Lüth and Richard Thompson for insightful discussions. This material is based upon work supported by the Center for Energy Nanoscience funded by the U.S. Department of Energy, Office of Science, Energy Frontier Research Center (EFRC) program under Award Number DE-SC0001013. Furthermore, we like also to thank the Deutsche Forschungsgemeinschaft (DFG) for funding this study under grant FOR1616.
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REFERENCES
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Amphoteric Nature of Sn in CdS Nanowires Mengyao Zhang,†,‡ Marcel Wille,§ Robert Röder,§ Sebastian Heedt,∥ Liubing Huang,†,‡ Zheng Zhu,†,‡ Sebastian Geburt,§ Detlev Grützmacher,∥ Thomas Schap̈ ers,∥ Carsten Ronning,§ and Jia Grace Lu*,†,‡ †
Department of Physics and Astronomy, and ‡Department of Electrophysics, University of Southern California, Los Angeles, California 90035, United States § Friedrich-Schiller-Universität Jena, Institut für Festkörperphysik, Max-Wien-Platz 1, 07743 Jena, Germany ∥ Peter Grünberg Institut (PGI − 9), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany S Supporting Information *
ABSTRACT: High-quality CdS nanowires with uniform Sn doping were synthesized using a Sn-catalyzed chemical vapor deposition method. X-ray diffraction and transmission electron microscopy demonstrate the single crystalline wurtzite structure of the CdS/Sn nanowires. Both donor and acceptor levels, which originate from the amphoteric nature of Sn in II− VI semiconductors, are identified using low-temperature microphotoluminescence. This self-compensation effect was cross examined by gate modulation and temperature-dependent electrical transport measurement. They show an overall n-type behavior with relatively low carrier concentration and low carrier mobilities. Moreover, two different donor levels due to intrinsic and extrinsic doping could be distinguished. They agree well with both the electrical and optical data. KEYWORDS: CdS nanowire, VLS growth, doping, compensation, low-temperature photoluminescence
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electrical property characterizations and analysis. Synthesis and specific experimental techniques are described in the end section of Experimental Methods. The substrate surface was covered with a catkin-like yellow and gray layer of nanostructures after synthesis. A typical SEM image of a CdS nanowires ensemble is shown in Figure 1a, showing dendrite structures. For structural investigation of individual wires, the samples were transferred to a cleaned Si substrate by dry imprint. In the same batch, the diameters of wires are around several hundred nanometers, and the lengths vary from several nanometers to several hundred micrometers. A single CdS/Sn nanowire with diameter of 300 nm and length of 14 μm is shown in Figure 1b. The synthesized nanowire ensemble reveals either straight or tapered morphologies but both with clear hexagonal cross sections. In this report, we focus on the straight wires with homogeneous diameters. The image and the formation mechanism of the tapered nanowires are shown in the Supporting Information Figure S1. As known, the CdS nanowire growth is governed by the catalytic vapor− liquid−solid (VLS) mechanism. The Cd−Sn−S phase diagram shows that Sn and CdS have an eutectic point at 205 °C.17 Thus, Sn can form tiny alloyed seeds with the supplied Cd and S atoms, catalyzing the VLS growth. High-resolution TEM image taken on a single wire is exhibited in Figure 1c, and the corresponding fast Fourier
ingle crystalline semiconductor nanowires have emerged as ideal building blocks for the next generation nanoscale photonic and electronic devices. With a direct wide band gap, cadmium sulfide (CdS) nanowires are particularly attractive due to their unique properties and device applications, including but not limited to transistors, solar cells, photo detectors, light emitting diodes, and continuous wave lasers.1−8 Since intrinsic CdS is highly insulating, doping is crucial in order to tailor the material property and to control the device functionality. Stable p-type conductivity has been shown to be difficult due to the low solubility of the acceptor dopants, deep acceptor levels, and the compensation effect between acceptor dopants and the native donors.6 Several kinds of elements, such as Cl, N, Ga, and P, have proven to be effective dopants for obtaining n-type characteristics.9−12 Group IV elements have been reported to introduce compensation effect in II−VI and III-V semiconductor materials.13,14 However, the amphoteric nature of Sn in CdS has never been reported in literature, even though the studies of CdS/Sn system’s application as nanowire lasing or solar cell window have been carried out.7,8,15 Because the ionic radius of Sn2+ (1.02 Å) is close to that of Cd2+ (0.97 Å),16 Cd substitution is expected to be more preferential and thus leads to dominating n-type characteristics. In this work, the selfcompensation effect in Sn-doped CdS nanowires is investigated, and the band structure has been accurately determined from the results of the optical measurements with conclusive correlations to the temperature-dependent charge transport measurement. This article is organized to present first the nanowire structure characterization, followed by optical and © XXXX American Chemical Society
Received: September 20, 2013 Revised: December 19, 2013
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Figure 1. (a) SEM image of a CdS nanowire ensemble. (b) SEM image of a single nanowire with homogeneous diameter and a round catalyst on the tip. Inset shows an enlarged view of the section near catalyst. (c) HRTEM image of a CdS/Sn nanowire showing single crystalline structure with growth direction along[002] (c-axis). Inset displays a FFT pattern of the HRTEM image. (d) Point EDX spectrum on a single CdS nanowire shows distinct Cd and S peaks. Sn peaks are at the shoulder of the Cd peak. Cd/S/Sn ratio is quantified to be 48.1:50.2:1.7. The silicon signal originates from the Si wafer substrate. (e) XRD spectrum of a CdS/Sn wire ensemble indicates the wurtzite structure for CdS nanostructures and Sn with tetragonal phase.
transform (FFT) pattern is shown in the inset. Both patterns reveal that these CdS nanowires are perfectly single crystalline, having a wurtzite structure and a growth direction along [002] (c-axis). The spacing between the (002) planes is the same as that in bulk material, indicating no distortion in the CdS crystal structure with the incorporation of Sn. The composition of the tip particles and wires were analyzed using point EDX measurements. The EDX spectrum recorded on one particular nanowire (shown in Figure 1d) reveals the presence of the Cd, S, and Sn elements in a stoichiometric ratio Cd/S/Sn = 50.2:48.1:1.7. The concentration of Sn is estimated to be in the range of 1−4 atom % based on several point EDX measurements on ten wires. The EDX mapping on a segment of one wire also indicates the distribution of Sn element along the wire, as shown in Supporting Information Figure S2a. On the other hand, the spectrum recorded on the spherical particle present at the tip of the nanowire shows that it is predominately
containing Sn (∼98 atom %, see Supporting Information Figure S2b), thus confirming the catalyst-assisted VLS growth. XRD pattern of a wire ensemble is plotted in Figure 1e. Most of the diffraction peaks can be assigned to the wurtzite phase of the CdS with a lattice constant of a = 4.14 Å and c = 6.72 Å. These values match well with the JCPDS data (card No. 653414). All the CdS peaks appear at the same positions of wurtzite CdS structure again proves that Sn doping does not alter the CdS crystal structure. The other peaks are attributed to Sn (JCPDS card no. 65-2631) mainly originating from the Sn particles on the wire tip and extra Sn particles formed during the fabrication. Optical characterization was performed via overview CL ensemble measurement and subsequent detailed PL and μ-PL measurement in order to obtain an approximated scheme of the impurity levels in the band gap. Figure 2a shows the SEM image of the nanowire ensemble, which was investigated with overview CL. The CL spectrum of the corresponding structure B
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Figure 2. (a) SEM and monochromatic CL images of a CdS/Sn nanowire ensemble. The near band edge emission (NBE) at 490 nm originates mainly from long and straight nanowires, whereas thick structures show predominantly luminescence at 520 nm corresponding to donor−acceptorpair (DAP) and a free to bound (e, A) transitions. Deep level emission (DLE) centers are homogeneously spread over the ensemble. (b) The lowtemperature (7 K) CL spectra of the corresponding ensemble in (a) reveals 3 emission regions, NBE (blue), DAP and (e, A) and their LO-phonon replica (green), and the DLE (red). (c) Photoluminescence spectra of CdS/Sn nanowire ensemble, taken at 4 K with a excitation density of 50 mW/ cm2 at 325 nm. Several transitions of the NBE are labeled, that is, the free exciton FXA, donor bound excitons (I2, I3), acceptor bound excitons (I1/ I1a), the surface exciton SX, and their phonon replica (ΔELO = 38 meV). The distance between DAP and (e, A) transition corresponds to the binding energy of an electron to a donor ED = 18 meV.
in CdS ELO = 38 meV.21 The occurrence of several phonon replica, where the first LO replica is more intense than the zero phonon lines, demonstrate strong electron−phonon coupling. The DAP nature of the feature at 2.399 eV was already shown in ref 7 for a single CdS/Sn nanowire and the nature of the (e, A) transition at 2.417 eV is proven by temperature-dependent PL investigations (see Supporting Information, Figure S5). An energetic distance between the two DAP and (e, A) series is observed, as illustrated in Figure 2c. Thus, the involved electron donor binding energy can be estimated to be around ED = 18 meV. Furthermore the PL spectra exhibit a highly featured NBE region. The high energetic shoulder of the predominant peak is the free exciton emission FXA at ∼2.552 eV22 accompanied by an exciton bound to ionized donors (DX, labeled I3) at 2.548 eV.21 The dominant peak near 2.545 eV is a superposition of the exciton bound to neutral donors (D0X, labeled I2) and the surface exciton SX.21,22 The peaks at the lower energy side are composed by two lines I1 (2.535 eV) and I1a (2.533 eV) corresponding to neutral acceptor-bound excitons and its acoustic phonon replica.23 The following five peaks correspond to longitudinal optical phonon replicas of I2B (B-Exciton bound to neutral donor), FXA, I2, and I1 with respective energy difference of 38 meV.21 These peaks identifications are confirmed by temperature-dependent PL (results shown in the Supporting Information Figure S3). With increasing temperature and enhanced thermal energy, the donor-bound excitons dissociate. Therefore, the I2 intensity gradually decreases, while the FXA transition becomes more intense. While the previous CL and PL spectra represent the integrated luminescence of an excited nanostructure ensemble, further information about single long and straight nanowires is made accessible by microphotoluminescence (μ-PL) measure-
in Figure 2b reveals three spectral emission regions: the near band edge (NBE) emission at 2.55 eV, caused by excitonic emission; the deep level emission (DLE) at ∼2.1 eV, attributed to several defects such as the carrier recombination at Cd interstitials;18 and the region between 2.45 and 2.2 eV, where the donor−acceptor-pair (DAP) and one free to bound transition (e, A) occur accompanied by their phonon replica.19 The DAP transition is the emission feature of an electron bound to a donor that recombines with a hole bound to an acceptor; and the free to bound transition (e, A) emerges from a free electron recombining with an acceptor bound hole. Thus, the energetic distance between these two series corresponds to the binding energy of an electron to a donor ED. The DAP as well as the (e, A) transition are absent in undoped CdS NWs, as shown in the Supporting Information (Figure S4), and therefore are most likely caused by the incorporation of Sn. The monochromatic CL images of the as grown CdS/Sn nanowire ensemble furthermore show that the NBE at ∼490 nm originates mainly from long and straight nanowires. The intense NBE compared to DLE in the thin nanowire structures, as shown in the monochromatic CL images, hints the high crystal quality of the nanostructures. Thicker structures show predominantly luminescence at 520 nm corresponding to DAP and (e, A) transitions. This suggests that more Sn is incorporated into thicker structures than in thin wires, resulting from different growth mechanism between them, which is nonVLS for thicker structures and VLS for thin wires.20 Low-temperature ensemble PL (4 K, λex = 325 nm, Iex = 50 mW/cm2) measurements allow a closer look to the NBE transitions, as assigned in Figure 2c, and the double structure containing the DAP and (e, A) transitions,19 which are strongly accompanied by phonon replica with a distinct spacing corresponding to the energy of a longitudinal optical phonon C
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Figure 3. (a) The temperature dependent μ-PL spectra of the CdS/Sn nanowire shown in the inset (scale bar = 10 μm) were obtained from the marked position with an excitation density of 3 mW/cm2 at 325 nm. The μ-PL spectra of a single nanowire reveal less intense DAP emission in comparison to NBE than the ensemble spectra in Figure 2c. The NBE shows a red shift of ΔEVar ∼ 1.4 meV due to Varshni effect with increasing sample temperature, whereas the DAP shifts to higher energies (ΔED ∼ 7 meV) due to thermal occupation of higher donor levels. (b) The band structure for CdS/Sn nanowires obtained from the optical data shows excitonic transitions: The free exciton FX, donor bound excitons DX and acceptor bound excitons AX, but also nonexcitonic transitions: free to bound transitions (h, D) and (e, A), or the bound to bound DAP transition. The energies of the transitions result from the band gap EGap = 2.58 eV and the corresponding binding energies (EBFX = 28 meV, ED = 18 meV, EA = 164 meV). The deep level defect luminescence appears at energies ranging from 1.8−2.2 eV.
Figure 4. (a) Source−drain current versus voltage of a CdS/Sn single nanowire field effect transistor show good linearity under 0, 20, and 40 V gate voltages. Inset: SEM image of this single wire with Ti/Au contact. Upper left inset: SEM image of single wire FET. Lower right inset: Source-drain current versus gate voltage plots at room temperature under source-drain voltage Vds = 2, 4, and 8 mV, respectively. The transconductance is estimated to be 1.80 × 10−12 A/V at Vds = 2 mV. Carrier concentration n = 8.10 × 1016 cm−3 and mobility μe = 2.22 cm2/V·s are obtained. (b) Temperature dependence of conductivity depicts the thermal activation of electrons from various donor levels.
with N ∼ 0.1. Thus, a macroscopic amount of electrons occupy these higher donor states. Summarizing the optical investigations allows an estimation of the energy levels of the CdS/Sn nanowires, which is sketched in Figure 3b with the NBE consisting of excitonic emission, the (e, A), DAP double structure and the DLE. We also incorporated the free to bound (h, D) transition in the sketch, although it was not observed in the measurements. To examine in depth the electronic structure of CdS/Sn nanostructures, gate-dependence measurements were performed via a back gate FET configuration. The source-drain current versus voltage (Ids−Vds) characteristics of a single CdS/ Sn nanowire with diameter of 700 nm at room temperature under different gate voltages is plotted in Figure 4a. The linear Ids−Vds curve suggests good ohmic contact. It is found that the source-drain current of the FET increases consistently with increasing gate voltage (Vg), indicating an overall n-type nature
ments at different temperatures. Figure 3a shows the μ-PL spectra, obtained from the “cross” position on the nanowire in the inset with the NBE, which is 2 orders of magnitude larger than the double DAP and (e, A) structure. These data also indicates a weaker Sn incorporation in long and straight nanowires compared to thick structures and is in good agreement with the monochromatic CL images. With increasing temperature, the NBE reveals a red shift to lower energies due to Varshni shift;24 whereas, the double structure of DAP and (e, A) shifts to higher energies by ΔED ∼ 7 meV. This blue shift is likely to originate from a thermal occupation of higher donor levels, which can be attributed to intrinsic and tininduced donor states. The ionization of donor−acceptor pairs with a corresponding activation energy of 18 meV leads to a Boltzmann factor of N ∼ 3 × 10−3 at 35 K, but the thermal ionization is significantly smaller than the thermal occupation of higher donor states due to lower energy of ΔED ∼ 7 meV D
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ature, and the temperature dependence of the conductivity in this region is governed by that of the mobility. In summary, uniform Sn-doped CdS nanowires were achieved from in situ Sn-catalyzed VLS mechanism. Assynthesized CdS/Sn nanowires exhibit wurtzite structure and high crystal quality. Low-temperature CL, PL and μ-PL spectra suggest that the incorporation of Sn introduces both donor and acceptor levels but reveals overall a predominant n-type character. The transport properties via back gate transistor configuration confirm this behavior, showing compensated carrier concentration with correspondingly low carrier mobilities. Both optical and electrical measurements agree well. The energy levels for amphoteric Sn states in CdS nanowire are determined from the experimental results. The elucidation of the electronic band structure is essential for the design and development of the nanoelectronic and nanophotonic application based on the nanowire system. Experimental Methods. Tin-doped CdS nanowires were synthesized in a horizontal tube furnace by chemical vapor deposition. A 460 mg mixture of CdS, SnO, SnO2, and graphite powders with a molar ratio of 1:1:1:0.1 was placed in an alumina boat located in the center of the furnace. A Si growth substrate was placed at the downstream edge of the furnace in a temperature region of 300−400 °C. The furnace was pumped to ∼0.1 Torr and flushed with Ar flow (200 sccm). The growth took place in a two-step process. The furnace was first maintained at 600 °C for 1 h and then maintained at 1000 °C for 30 min. The pressure was kept at 1 atm and Ar flow was kept at 50 sccm during the growth process and turned off after the furnace was cooled down to the room temperature. The morphology and crystal structure of the synthesized nanostructures were characterized by scanning electron microscopy (SEM, JEOL 7001), transmission electron microscopy (TEM, JEOL 2100F), and X-ray diffraction (XRD, Rigaku Ultima IV). The stoichiometry was examined using energy dispersive X-ray spectroscopy (EDX) equipped in SEM. Optical characterization of a CdS/Sn nanowire ensemble was performed using cathodoluminescence (JEOL JSM-6490 SEM equipped with a GATAN MonoCL3+ system). The CL system allows low-temperature measurements using a cryostage and two detection modes (monochromatic and panchromatic mode). Single CdS/Sn nanowires were transferred onto clean SiO2/Si substrate by dry imprint for further optical measurements. The temperature-dependent ensemble photoluminescence as well as microphotoluminescence (μ-PL) measurements of single nanowires were performed using a HeCd 325 nm cw laser for excitation, while the sample was mounted in a Janis ST-500 helium flow cryostat with a cryostat temperature range from 4 K to room temperature. The luminescence light was collected by spherical fused silica lenses for ensemble PL measurement and dispersed by a 500 mm monochromator and detected with a liquid nitrogen-cooled CCD.1 The gate-dependence measurements were performed using a back gate configuration. The as-grown CdS/Sn nanowires were collected from the Si growth substrate and dispersed in isopropyl alcohol by sonication. The suspension was then dropped onto a degenerately doped p-type silicon wafer covered with 500 nm SiO2. Photolithography and electron beam evaporation were used to define the metal electrode contacts (Ti 20 nm/Au 200 nm) on single CdS/Sn nanowires. Rapid thermal annealing was carried out in a N2 flow chamber at 300 °C for 30 s in order to improve the metal−
of this Sn-doped CdS nanowire. This is consistent with lowtemperature PL result revealing intense donor bound exciton and DAP emissions. The conductivity at zero Vg at room temperature is estimated to be lower bounded at ∼2.89 × 10−2 S·cm−1 using equation σ = 1/ρ = L/(R·A) where A is the cross area of the nanowire, L is the channel length, and R is determined by dIds/dVds. The conductivity is enhanced compared to the undoped CdS nanowire conductivity (10−7− 10−6 S·cm−1).10 From Figure 4a lower right inset, the fieldeffect electron mobility (μe = gm(L2/CVds))25 is estimated to be 2.22 cm2/V·s, where the channel transconductance gm = ∂Ids/ ∂Vg = 1.80 × 10−12 A/V at Vds = 2 mV, and the channel length L = 22 μm. The gate capacitance C for a cylindrical wire lying on a dielectric substrate is determined by C = (2πε0εL)/[ln(2h + r)/r], with r representing the radius of the nanowire, h representing the thickness of the gate oxide (500 nm), and effective dielectric constant ε is 2.20.26 The carrier concentration n is calculated to be 8.10 × 1016 cm−3 from the threshold voltage Vth, as n = (Vth/e)[C/(Lπr2)], where Vth = −55 V is extrapolated from the reverse extension of the Ids−Vg curves at different Vds. The small electron concentration manifests the self-compensation effect in the material. Another five wires have been measured and the obtained mobility lies in the range of 0.12−2.64 cm2/V·s. The mobility in CdS/Sn wires is considerably smaller than that of the bulk material (∼340 cm2/V·s), however it is comparable to that of doped CdS nanowires with back gate configuration reported in the previous studies.10,27 The small mobility further validates the self-compensation effect in the nanowire, originating from the amphoteric nature of Sn in CdS, which gives rise to an increased number of ionized donors and acceptors. Both contribute to enhanced scattering, leading to the low mobility. The logarithmic scaled conductivity of a CdS wire FET as a function of reciprocal temperature is plotted in Figure 4b. The temperature-dependent behavior corresponds to the electronic structure after the incorporation of Sn. The curve exhibits three distinct regions: (I) 300−230 K (II) 230−80 K (III) 80−40 K. In regions I and III, the conductivity shows Arrhenius-type behavior, σ ∝ exp(−(Ea/kT)), where Ea is the activation energy required to promote an electron into the conduction band. The activation energy extracted from the slope in region III below 80 K is about 20 ± 0.2 meV, attributing to the shallow donor levels due to the intrinsic and extrinsic dopants. For compensated semiconductors, at low temperature the Fermi level is fixed at the shallow donor energy level. Thus the activation energy is simply the ionization energy ED, that is, σ ∝ exp(−(ED/kT)).28 It is in good agreement with the ionization energy of the donor state (∼18 meV) determined from the energy difference between the free to bound (e, A) and DAP transitions, as discussed earlier on the low temperature PL analysis. In region I for temperature above 230 K, the activation energy extracted from the slope is about 110 ± 10 meV. This temperature dependence change could be contributed from electron excitation from deeper donor level located ∼110 meV below the conduction band edge, which is consistent with the donor level of sulfur vacancy (∼100 meV).22 On the other hand, in the intermediate temperature regime, region II shows a conductivity increase with decreasing temperature at the hightemperature side, while inverse behavior is observed at the lowtemperature side. This exemplifies a characteristic extrinsic saturation region, where all shallow donors are ionized and the carrier concentration in the band is independent of temperE
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(12) Wu, B.; Jiang, Y.; Wu, D.; Li, S.; Wang, L.; Yu, Y.; Wang, Z.; Jie, J. J. Nanosci. Nanotechnol. 2011, 11 (3), 2003−2011. (13) Fistul, V. I. Impurities in Semiconductors: Solubility, Migration and Interactions; CRC Press: Boca Raton, FL, 2004. (14) Panchuk, O.; Savitskiy, A.; Fochuk, P.; Nykonyuk, Y.; Parfenyuk, O.; Shcherbak, L.; Ilashchuk, M.; Yatsunyk, L.; Feychuk, P. J. Cryst. Growth 1999, 197 (3), 607−611. (15) Jafari, A.; Zakaria, A.; Rizwan, Z.; Ghazali, M. S. M. Int. J. Mol. Sci. 2011, 12 (9), 6320−6328. (16) Roy, P.; Srivastava, S. K. J. Phys. D: Appl. Phys. 2006, 39, 4771− 4776. (17) Zargarova, M. I.; Alieva, S. B.; Allazov, M. R. Russ. J. Inorg. Chem. 1985, 30, 726−729. (18) Aguilar-Hernández, J.; Contreras-Puente, G.; Morales-Acevedo, A.; Vigil-Galán, O.; Cruz-Gandarilla, F.; Vidal-Larramendi, J.; Escamilla-Esquivel, A.; Herńandez-Contreras, H.; Hesiquio-Garduno, M.; Arias-Carbajal, A.; Chavarría-Castaneda, M.; Arriaga-Mejía, G. Semicond. Sci. Technol. 2003, 18, 111−114. (19) Colbow, K. Phys. Rev. 1966, 141 (2), 742−749. (20) Perea, D. E.; Hemesath, E. R.; Schwalbach, E. J.; Lensch-Falk, J. L.; Voorhees, P. W.; Lauhon, L. J. Nat Nanotechnol. 2009, 4 (5), 315− 319. (21) Thomas, D. G.; Hopfield, J. J. Phys. Rev. 1962, 128 (5), 2135− 2148. (22) Xu, X.; Zhao, Y.; Sie, E. J.; Lu, Y.; Liu, B.; Ekahana, S. A.; Ju, X.; Jiang, Q.; Wang, J.; Sun, H.; Sum, T. C.; Huan, C. H. A.; Feng, Y. P.; Xiong, Q. ACS Nano 2011, 5 (5), 3660−3669. (23) Henry, C. H.; Faulkner, R. A.; Nassau, K. Phys. Rev. 1969, 183 (3), 798−806. (24) Varshni, Y. P. Physica 1967, 34 (1), 149−154. (25) Dürkop, T.; Getty, S. A.; Cobas, E.; Fuhrer, M. S. Nano Lett. 2003, 4 (1), 35−39. (26) Wunnicke, O. Appl. Phys. Lett. 2006, 89 (8), 083102−3. (27) Jie, J. S.; Zhang, W. J.; Jiang, Y.; Lee, S. T. Appl. Phys. Lett. 2006, 89 (22), 223117−3. (28) Shklovskii, B. I.; Efros, A. L. Electronic Properties of Doped Semiconductors; Springer: Berlin, 1984.
semiconductor contacts. The gate voltage was applied to the back gate underneath the SiO2 dielectric layer (500 nm thick), modulating the current flow in the nanowire. The temperaturedependent measurements were carried out in a standard voltage-driven DC-setup inside a He-4 continuous-flow cryostat. The transistor sample was bounded onto a standard chip-carrier and mounted into the cryostat. The cryostat temperature was first cooled down to 4K, and then heated in controllable steps with a heater-coil close to the sample.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information and figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. M.Z. and M.W. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to Professors Hans Lüth and Richard Thompson for insightful discussions. This material is based upon work supported by the Center for Energy Nanoscience funded by the U.S. Department of Energy, Office of Science, Energy Frontier Research Center (EFRC) program under Award Number DE-SC0001013. Furthermore, we like also to thank the Deutsche Forschungsgemeinschaft (DFG) for funding this study under grant FOR1616.
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REFERENCES
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