Charge transport mechanisms and memory effects in amorphous TaN x thin films
© Spyropoulos-Antonakakis et al.; licensee Springer. 2013
Received: 30 April 2013
Accepted: 28 August 2013
Published: 17 October 2013
Amorphous semiconducting materials have unique electrical properties that may be beneficial in nanoelectronics, such as low leakage current, charge memory effects, and hysteresis functionality. However, electrical characteristics between different or neighboring regions in the same amorphous nanostructure may differ greatly. In this work, the bulk and surface local charge carrier transport properties of a-TaN x amorphous thin films deposited in two different substrates are investigated by conductive atomic force microscopy. The nitride films are grown either on Au (100) or Si  substrates by pulsed laser deposition at 157 nm in nitrogen environment. For the a-TaN x films deposited on Au, it is found that they display a negligible leakage current until a high bias voltage is reached. On the contrary, a much lower threshold voltage for the leakage current and a lower total resistance is observed for the a-TaN x film deposited on the Si substrate. Furthermore, I-V characteristics of the a-TaN x film deposited on Au show significant hysteresis effects for both polarities of bias voltage, while for the film deposited on Si hysteresis, effects appear only for positive bias voltage, suggesting that with the usage of the appropriate substrate, the a-TaN x nanodomains may have potential use as charge memory devices.
KeywordsNitrides TaN x thin films Amorphous semiconductors Nanoelectronics Memory effects Conductive-AFM
Nowadays, there is an urgent need of efficient dielectrics with minimum leakage current as electronic devices are shrinking to only few nanometers. A class of nanomaterials that display these characteristics is amorphous semiconductors . Generally, amorphous semiconducting nanostructures display some advantageous electrical characteristics compared with their crystalline counterparts. In particular, due to their disordered structure, amorphous materials typically have a high density of localized defect states, resulting in significant charge trapping and much lower leakage current . Moreover, amorphous nanomaterials can be produced at relatively low temperatures, while a lower strain is expected between the embedded nanoparticles and the matrix due to their flexible amorphous structure . In addition, very recent works have demonstrated that some amorphous or polycrystalline nitrides, like CuN, AlN, and NiN, exhibit resistive switching behavior capable for fabricating resistance-switching random access memory devices [4–7]. However, the research for switching resistive materials had been focused almost only on metal oxides, e.g., TiO2[8, 9], NiO [10, 11], ZnO , and Ta2O5[13–16], as their electrical properties are well known and their preparation methods are relatively easy and well established. On the contrary, metal nitrides, even though they exhibit intriguing electrical properties, remain largely unexplored in this field.
Low-power memristive behavior with outstanding endurance has been already demonstrated in tantalum oxide [13–15], alongside with efforts to maximize its performance with nitrogen doping . A promising material in this point of view is amorphous tantalum nitride (a-TaN x ). Tantalum nitride is proved to be a mechanically hard and a chemically inert material, combining both high thermal stability and low temperature coefficient of resistance [17, 18]. TaN x appears with many crystalline phases that are well studied [19, 20]. For example, the metallic TaN may have potential applications as Cu diffusion barriers , thin film resistors , and superconducting single-photon detectors , while nitrogen-rich Ta3N5 is used as photocatalytic material for water splitting [24, 25]. On the other hand, the amorphous phase (a-TaN x ), which is the most common phase of the as-prepared TaN x at relatively low temperatures [26–28], has received very low attention. Early electrical studies on a-TaN x films by Chang et al. showed that there was increasing resistivity of films, as the nitrogen concentration in the gas environment increased , while Kim et al.  indicated that a-TaN x could prevent copper diffusion more effectively than the crystallized Ta2N film by eliminating grain boundaries.
It is well known for 1-D and 2-D nanostructures, i.e., nanowires, nanorods, and thin films, that the electrical properties may differ greatly from point to point within regions separated by several nanometers, due to differences in charge concentration, defect density, surface band bending, etc. [31, 32]. It is also established that the large surface-to-volume ratio of these nanostructures results in increasing contribution of the surface and space-charge-limited current to the total current . Hence, local measurements with the conductive atomic force microscopy (C-AFM) technique are of high importance, because C-AFM is capable of resolving the electrical properties at the nanoscale.
In this letter, the local charge carrier transport mechanisms and memory effects of a-TaN x thin films deposited either on Au (100) or Si  substrates by pulsed laser deposition (PLD) at 157 nm  are investigated by C-AFM, and the influence of the space charge layer in conductivity along with a pronounced current hysteresis is revealed. For the sample’s characterization, atomic force microscopy (AFM), focused ion beam (FIB), transmission electron microscopy (TEM), micro-Raman spectroscopy, and energy-dispersive X-ray spectroscopy (EDXS) are used.
a-TaN x films are prepared by PLD at 157 nm (LPF 200, Lambda-Physik, (since 2006 Coherent, Santa Clara, CA, USA)) in a vacuum stainless steel chamber at ambient temperature under 105 Pa of research grade (99.999%) N2 gas. The pulsed discharged molecular fluorine laser at 157 nm has been used previously in various applications where high energy per photon is required [34–36]. A high-purity tantalum foil (99.9%, Good-Fellow, Huntingdon, UK) of 0.5 mm in thickness is used as the ablation target. The films are efficiently deposited using relative low laser energy per pulse (30 mJ) with 15-Hz repetition rate. The pulse duration is 15 ns at full width at half maximum. The Au (100) or Si  substrate is placed approximately 3 to 5 mm away from the target material and perpendicular to the optical axis of the laser beam in axial ablation geometry. In previous works, PLD at 157 nm has been used to grow metal nitrides efficiently [37–39].
An AFM (d’Innova, Bruker, Madison, WI, USA) is operated at ambient conditions to evaluate the morphology and roughness of the as-deposited a-TaN x films. The AFM images are acquired in tapping-mode using a phosphorus-(n)-doped silicon cantilever (RTESPA, Bruker, Madison, WI, USA) with a nominal spring constant of 40 N/m at approximately 300-kHz resonance frequency and nominal radius of 8 nm. The AFM images are obtained at different scanning areas at a maximum scanning rate of 0.5 Hz with an image resolution of 512 × 512 pixels. FIB technique with a Pt protection layer is used to determine the film thickness, while TEM (operated at 200 kV; Jeol 2100, JEOL Ltd., Akishima-shi, Japan) is carried out to reveal the different structures in TaN x deposited on Si. In order to be examined in the microscope, the samples are transferred to a lacey-carbon-coated Cu grid. Additionally, EDXS with a Si(Li) detector from JEOL is performed to detect the nitrogen and oxygen content, while micro-Raman spectroscopy using the 488-nm line of an Ar+ laser, which irradiates a sample area of 1 μm2 with a power of 3 mW, is performed at room temperature to identify the possible crystalline or amorphous phases.
Results and discussion
Other possible charge carrier transport mechanisms from the bulk of the film could be thermionic emission of charge carriers across the metal-dielectric interface or field emission by electron tunneling from the metal or charge traps to the quasi-conduction band of the amorphous semiconductor . These mechanisms have also exponential like I-V behavior.
However, after the examination of the I-V curves in Figure 5, it is found that they exhibit a power-like (Figure 5a) to almost linear (Figure 5c) response rather than the characteristic exponential behavior of the above charge carrier transport mechanisms, indicating that the current contribution from the bulk is small compared to the space charge and surface current. The I-Vs in Figure 5a are fitted well by a power law I ∝ V m , with m = 2.7 to 5.5, indicating that the predominant charge carrier transport mechanism is the space-charge-limited current [47–50]. Due to the band bending of the quasi-conduction band near the metal-dielectric interfaces, a space charge layer is formed near the surface of the dielectric where electrons are depleted. Hence, under a voltage threshold, the electrons injected from the gold electrode are combined with the holes which are present in the space charge layer resulting in the decrease of free carriers. With the increase of voltage bias, the holes are fully filled after a voltage threshold, causing the rapid increase of free carriers. Similar results are obtained for the I-V characteristics under negative bias, where m = 2.3 to 3.4, Figure 5b.
On the contrary, the a-TaN x film deposited on Si, despite it is thicker than the film deposited in Au, displays much lower voltage threshold, lower total resistance, and parabolic to almost linear current behavior for higher bias voltages, Figure 5c. This is attributed to the presence of tantalum nanoparticles, as those identified in Figure 3d, which provide additional free charge carriers after a proper value of the applied field, changing the conductive behavior from almost parabolic, m = 1.8, to almost ohmic, m = 1.3 to 1.5, Figure 5c [49, 50]. The threshold value of the applied field is much lower compared to the a-TaN x deposited on Au, considering the lower threshold bias voltage and the thickness of the film. Furthermore, all the I-V characteristics under negative bias show a quite high leakage current with a very noisy profile, although the mean current still has a linear dependence to the voltage bias (Figure 5d). This high flow of electrons under negative voltage bias may be attributed to the usage of a low work function bottom electrode (Ag, φ = 4.5 eV) compared with the high work function electrode (Au, φ = 5.1 eV) that is used in the other device. The charge transport at the metal-dielectric interface depends on the Schottky barrier height (SBH) which is defined as φb = φm - χ, where φm and χ are the metal work function and electron affinity of the dielectric, respectively. Hence, in the case of an n-doped dielectric, lower metal work function results in lower SBH and easier charge transport through the barrier.
Hysteresis and the calculated resistivity ratio at 3.5-V bias voltage
Point contact (Figure6, curves 1 to 7)
Hysteresis [ δI(nA)] at 3.5 V
Resistivity ratio at 3.5 V
1 a-TaN x on Au
2 a-TaN x on Au
3 a-TaN x on Au
4 a-TaN x on Au
5 a-TaN x on Si
6 a-TaN x on Si
7 a-TaN x on Si
In summary, it is found that the conduction on metal/a-TaN x /metal devices through the amorphous film is dominated by the space-charge-limited current and the current contribution from the bulk is small compared to the space charge and surface current. The conduction of the devices is also expected to be greatly influenced by the eventual presence of Ta nanoparticles embedded in the amorphous matrix and the choice of the metal electrodes, as it is shown in the case of the a-TaN x films deposited on Si. Large variations between neighboring nanodomains on the same film are found. These variations in conductivity between nanodomains of the same film establish the importance of C-AFM technique as a diagnostic tool in nanoelectronics. Finally, significant current hysteresis effects are demonstrated, indicating the possible use of a-TaN x in memory applications, especially for a-TaN x deposited on Au where bipolar memory effects are observed.
The authors would like to acknowledge NHRF/TPCI for the financial support from the internal funding sources.
- Rockett A: The Materials Science of Semiconductors. Berlin: Springer; 2008.View ArticleGoogle Scholar
- Vieira EMF, Diaz R, Grisolia J, Parisini A, Martín-Sánchez J, Levichev S, Rolo AG, Chahboun A, Gomes MJM: Charge trapping properties and retention time in amorphous SiGe/SiO2 nanolayers. J Phys D Appl Phys 2013, 46: 095306.View ArticleGoogle Scholar
- Diaz R, Grisolia J, BenAssayag G, Schamm-Chardon S, Castro C, Pecassou B, Dimitrakis P, Normand P: Extraction of the characteristics of Si nanocrystals by the charge pumping technique. Nanotechnology 2012, 23: 085206.View ArticleGoogle Scholar
- Chen C, Yang YC, Zeng F, Pan F: Bipolar resistive switching in Cu/AlN/Pt nonvolatile memory device. Appl Phys Lett 2010, 97: 083502.View ArticleGoogle Scholar
- Kim HD, An HM, Kim TG: Ultrafast resistive-switching phenomena observed in NiN-based ReRAM cells. IEEE Trans Electron Devices 2012, 59: 2302–2307.View ArticleGoogle Scholar
- Lu Q, Zhang X, Zhu W, Zhou Y, Zhou Q, Liu L, Wu X: Reproducible resistive-switching behaviour in copper-nitride thin film prepared by plasma-immersion ion implantation. Phys Status Solidi A 2011, 208: 874–877.View ArticleGoogle Scholar
- Choi BJ, Yang JJ, Zhang MX, Norris KJ, Ohlberg DAA, Kobayashi NP, Medeiros-Ribeiro G, Williams RS: Nitride memristors. Appl Phys A 2012, 109: 1–4.View ArticleGoogle Scholar
- Yang L, Kuegeler C, Szot K, Ruediger A, Waser R: The influence of copper top electrodes on the resistive switching effect in TiO2 thin films studied by conductive atomic force microscopy. Appl Phys Lett 2009, 95: 013109.View ArticleGoogle Scholar
- Nardi F, Deleruyelle D, Spiga S, Muller C, Bouteille B, Ielminim D: Switching of nanosized filaments in NiO by conductive atomic force microscopy. J Appl Phys 2012, 112: 064310.View ArticleGoogle Scholar
- Wang W, Dong R, Yan X, Yang B: Memristive characteristics in semiconductor/metal contacts tested by conductive atomic force microscopy. J Phys D-Appl Phys 2011, 44: 475102.View ArticleGoogle Scholar
- Chiu FC, Li PW, Chang WY: Reliability characteristics and conduction mechanisms in resistive switching memory devices using ZnO thin films. Nanoscale Res Lett 2012, 7: 178.View ArticleGoogle Scholar
- Lin CC, Chang YP, Lin HB, Lin CH: Effect of non-lattice oxygen on ZrO2 based resistive switching memory. Nanoscale Res Lett 2012, 7: 187.View ArticleGoogle Scholar
- Yang JJ, Zhang MX, Strachan JP, Miao F, Pickett MD, Kelley RD, Medeiros-Ribeiro G, Williams RS: High switching endurance in TaO x memristive devices. Appl Phys Lett 2010, 97: 232102.View ArticleGoogle Scholar
- Strachan JP, Torrezan AC, Medeiros-Ribeiro G, Williams RS: Measuring the switching dynamics and energy efficiency of tantalum oxide memristors. Nanotechnology 2011, 22: 505402.View ArticleGoogle Scholar
- Strachan JP, Medeiros-Ribeiro G, Yang YY, Zhang MX, Miao F, Goldfarb I, Holt M, Rose V, Williams RS: Spectromicroscopy of tantalum oxide memristors. Appl Phys Lett 2011, 98: 242114.View ArticleGoogle Scholar
- Cheng CH, Chen PC, Wub YH, Wu MJ, Yeh FS, Chin A: Highly uniform low-power resistive memory using nitrogen-doped tantalum pentoxide. Solid-State Electron 2012, 73: 60–63.View ArticleGoogle Scholar
- Bozorg-Grayeli E, Li Z, Asheghi M, Delgado G, Pokrovsky A, Panzer M, Wack D, Goodson KE: High temperature thermal properties of thin tantalum nitride films. Appl Phys Lett 2011, 99: 261906.View ArticleGoogle Scholar
- Kwon J, Chabal YJ: Thermal stability comparison of TaN on HfO2 and Al2O3. Appl Phys Lett 2010, 96: 151907.View ArticleGoogle Scholar
- Yu L, Stampfl C, Marshall D, Eshrich T, Narayanan V, Rowell JM, Newman N, Freeman AJ: Mechanism and control of the metal to insulator transition in rocksalt tantalum nitride. Phys Rev B 2002, 65: 245110.View ArticleGoogle Scholar
- Chun WJ, Ishikawa A, Fujisawa H, Takata T, Kondo JN, Hara M, Kawai M, Matsumoto Y, Domen K: Conduction and valence band positions of Ta2O5, TaON, and Ta3N5 by UPS and electrochemical methods. J Phys Chem B 2003, 107: 1798–1803.View ArticleGoogle Scholar
- Zhao Y, Lu G: First-principles simulations of copper diffusion in tantalum and tantalum nitride. Phys Rev B 2009, 79: 214104.View ArticleGoogle Scholar
- Malmros A, Andersson K, Rorsman N: Combined TiN- and TaN temperature compensated thin film resistors. Thin Solid Films 2012, 520: 2162–2165.View ArticleGoogle Scholar
- Engel A, Aeschbacher A, Inderbitzin K, Schilling A, Il’in K, Hofherr M, Siegel M, Semenov A, Hübers HW: Tantalum nitride superconducting single-photon detectors with low cut-off energy. Appl Phys Lett 2012, 100: 062601.View ArticleGoogle Scholar
- Ishikawa A, Takata T, Kondo JN, Hara M, Domen K: Electrochemical behaviour of thin Ta3N5 semiconductor film. J Phys Chem B 2004, 108: 11049–11053.View ArticleGoogle Scholar
- Li Y, Takata T, Cha D, Takanabe K, Minegishi T, Kubota J, Domen K: Vertically aligned Ta3N5 nanorod arrays for solar-driven photoelectrochemical water splitting. Adv Mater 2013, 25: 125–131.View ArticleGoogle Scholar
- Sreenivasan R, Sugawara T, Saraswat KC, McIntyre PC: High temperature phase transformation of tantalum nitride films deposited by plasma enhanced atomic layer deposition for gate electrode applications. Appl Phys Lett 2007, 90: 102101.View ArticleGoogle Scholar
- Langereis E, Knoops HCM, Mackus AJM, Roozeboom F, van de Sanden MCM, Kessels WMM: Synthesis and in situ characterization of low-resistivity TaN x films by remote plasma atomic layer deposition. J Appl Phys 2007, 102: 083517.View ArticleGoogle Scholar
- Fang Z, Aspinall HC, Odedra R, Potter RJ: Atomic layer deposition of TaN and Ta3N5 using pentakis(dimethylamino)tantalum and either ammonia or monomethylhydrazine. J Cryst Growth 2011, 331: 33–39.View ArticleGoogle Scholar
- Chang CC, Jeng JS, Chen JS: Microstructural and electrical characteristics of reactively sputtered Ta-N thin films. Thin Solid Films 2002, 413: 46–51.View ArticleGoogle Scholar
- Kim SM, Lee GR, Lee JJ: Effect of film microstructure on diffusion barrier properties of TaN x films in Cu metallization. Jpn J Appl Phys 2008, 47: 6953–6955.View ArticleGoogle Scholar
- Lv Y, Cui J, Jiang ZMM, Yang XJ: Nanoscale electrical property studies of individual GeSi quantum rings by conductive scanning probe microscopy. Nanoscale Res Lett 2012, 7: 659.View ArticleGoogle Scholar
- Wang SJ, Cheng G, Cheng K, Jiang XH, Du ZL: The current image of single SnO2 nanobelt nanodevice studied by conductive atomic force microscopy. Nanoscale Res Lett 2011, 6: 541.View ArticleGoogle Scholar
- Talin AA, Léonard F, Swartzentruber BS, Wang X, Hersee SD: Unusually strong space-charge-limited current in thin wires. Phys Rev Lett 2008, 101: 076802.View ArticleGoogle Scholar
- Skordoulis C, Sarantopoulou E, Spyrou S, Cefalas AC: Amplification characteristics of a discharge excited F2 laser. J Modern Opt 1990, 37: 501–509.View ArticleGoogle Scholar
- Sarantopoulou E, Cefalas AC, Dubinskii MA, Nicolaides CA, Abdulsabirov RY, Korableva SL, Naumov AK, Semashko VV: VUV and UV fluorescence and absorption studies of Nd3+ and Ho3+ ions in LiYF4 single crystals. Opt Commun 1994, 107: 104–110.View ArticleGoogle Scholar
- Cefalas AC: Current trends in 157 nm dry lithography. Appl Surf Sci 2005, 247: 577–583.View ArticleGoogle Scholar
- Sarantopoulou E, Kollia Z, Drazic G, Kobe S, Antonakakis NS: Long-term oxidization and phase transition of InN nanotextures. Nanoscale Res Lett 2011, 6: 387.View ArticleGoogle Scholar
- Spyropoulos-Antonakakis N, Sarantopoulou E, Kollia Z, Drazic G, Kobe S: Schottky and charge memory effects in InN nanodomains. Appl Phys Lett 2011, 99: 153110.View ArticleGoogle Scholar
- Spyropoulos-Antonakakis N, Sarantopoulou E, Kollia Z, Samardzija Z, Kobe S, Cefalas AC: Thermionic field emission in gold nitride Schottky nanodiodes. J Appl Phys 2012, 112: 094301.View ArticleGoogle Scholar
- Stoehr M, Shin CS, Petrov I, Greene JE: Raman scattering from epitaxial TaNx (0.94 ≤ x ≤ 1.37) layers grown on MgO(001). J Appl Phys 2007, 101: 123509.View ArticleGoogle Scholar
- Lima LPB, Diniz JA, Doi I, Miyoshi J, Silva AR, Godoy FJ, Radtke C: Oxygen incorporation and dipole variation in tantalum nitride film used as metal-gate electrode. J Vac Sci Technol B 2012, 30: 042202.View ArticleGoogle Scholar
- Henderson SJ, Hector AL: Structural and compositional variations in Ta3N5 produced by high-temperature ammonolysis of tantalum oxide. J Solid State Chem 2006, 179: 3518–3524.View ArticleGoogle Scholar
- Harrell WR, Frey J: Observation of Poole-Frenkel effect saturation in SiO2 and other insulating films. Thin Solid Films 1999, 352: 195–204.View ArticleGoogle Scholar
- Tiggelaar RM, Groenland AW, Sanders RGP, Gardeniers JGE: Electrical properties of low pressure chemical vapor deposited silicon nitride thin films for temperatures up to 650°C. J Appl Phys 2009, 105: 033714.View ArticleGoogle Scholar
- Frenkel J: On pre-breakdown phenomena in insulators and electronic semi-conductors. Phys Rev 1938, 54: 647–648.View ArticleGoogle Scholar
- Sze SM: Physics of Semiconductor Devices. 2nd edition. New York: Wiley; 1981.Google Scholar
- Vila M, Román E, Prieto C: Electrical conduction mechanism in silicon nitride and oxy-nitride-sputtered thin films. J Appl Phys 2005, 97: 113710.View ArticleGoogle Scholar
- Crunteanu A, Dumas-Bouchiat F, Champeaux C, Catherinot A, Blondy P: Electrical conduction mechanisms of metal nanoclusters embedded in an amorphous Al2O3 matrix. Thin Solid Films 2007, 515: 6324–6327.View ArticleGoogle Scholar
- Aw KC, Ooi PC, Razak KA, Gao W: A transparent and flexible organic bistable memory device using parylene with embedded gold nanoparticles. Mater Electron: J Mater Sci 2013, 24: 3116–3125.View ArticleGoogle Scholar
- Son DI, Park DH, Choi WK, Cho SH, Kim WT, Kim TW: Carrier transport in flexible organic bistable devices of ZnO nanoparticles embedded in an insulating poly (methylmethacrylate) polymer layer. Nanotechnology 2009, 20: 195203.View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.