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.
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.
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