Yawar Abbasa,
Ayman Rezka,
Irfan Saadatb,
Ammar Nayfehb and
Moh'd Rezeq*ac
aDepartment of Physics, Khalifa University, Abu Dhabi, 127788, United Arab Emirates. E-mail: mohd.rezeq@ku.ac.ae
bDepartment of Electrical Engineering and Computer Science, Khalifa University, Abu Dhabi, 127788, United Arab Emirates
cSystem on Chip Center, Khalifa University, Abu Dhabi 127788, United Arab Emirates
First published on 16th November 2020
In this work, we investigate the time dependence of trapped charge in isolated gold nanoparticles (Au-NPS) dispersed on n-Si substrates, based on the electrical characteristics of nano metal–semiconductor junctions. The current–voltage (I–V) characteristics have been analysed on a single Au-NP at different time intervals, using conductive mode atomic force microscopy (AFM). The Au-NPs have been characterized for their morphology and optical properties using transmission electron microscopy (TEM), ultraviolet visible (UV-vis) spectroscopy and scanning electron microscopy (SEM). The tunneling current is found to be a direct function of the trapped charge in the NP, due to the charge screening effect of the electric field at the NP/n-Si interface. The evolution of the I–V curves is observed at different time intervals until all the trapped charge dissipates. Moreover, the time needed for nanoparticles to restore their initial state is verified and the dependence of the trapped charge on the applied voltage sweep is investigated.
For various applications, Au-NPs can be synthesized using either Ostwald ripening or chemical synthesis.15,17–19 In Ostwald ripening method, a very thin layer of gold is deposited on a suitable substrate followed by high temperature annealing process. The subsequent high temperature annealing process causes the formation of NPs or NCs due to the minimization of surface energy.20–22 Au-NPs fabricated using this technique are published in the literature.18,23 However, due to different sizes and distributions, due to the difficulty in size control of nanoparticles and diffusion of metal content at the interface, this method is not a suitable candidate for device applications. Alternatively, solution processed Au-NPs synthesis is used in memory application.24,25 By using solution-based synthesis, one can synthesize the targeted size of nanoparticles by choosing appropriate precursors as these customized colloidal solutions enable the use of NPs for different applications. The distribution of these NPs on the substrate can be controlled by different deposition methods like spin coating and dip coating.11,26,27 Recently, the solution processed Au-NPs have been used for electron storage centers by properly embedding NPs in the carefully engineered blocking and tunneling layers of Al2O3 on an n-Si substrate.26 To characterize individually or discretely dispersed NPs on the substrate, atomic force and scanning tunneling microscopies (AFM and STM) are considered to be among the best tools. Consequently, the tip size plays a vital role in investigating the electrical characteristics of individual NPs. Therefore, the smaller the dimension of the nanotip is, the more accurate local characteristic data can be obtained. As a result, the fabrication of such well-defined nanotips has received a lot of attention from researchers in this field.28–30
Earlier, we have shown the significant effect of individual NPs deposited on a thin film passivated n-Si substrate on the enhancement of the electric field, and, in turn, on the reverse tunneling current through the nano-Schottky junction.11 In this work, the effect of the charge trapped in the NP on the I–V characteristics of the nano Schottky junctions, formed between the NP and Si substrate, and the time evolution of the I–V curves during the charge decay is studied. Moreover, this will help to assess and optimize the use of these metal nanoparticles in future memory based devices.
The size and crystal structure of the NPs are accurately characterized using UV-vis method and HRTEM with atomic resolution. Fig. 2(a) shows the AFM image of well-separated Au-NPs on the n-Si substrate. The image is produced using an AFM tip with a resonance frequency of 261 kHz. The scan is carried out in the 1.4 μm × 1.4 μm region, with a scan frequency of 0.75 Hz. The sample is also characterized using SEM, as shown in Fig. 2(b), which again illustrates the individually dispersed Au-NPs. Fig. 2(c) shows the UV-visible spectroscopy of Au-NPs dispersed in citrate buffer solution. The spectrum shows the maximum absorbance peak at the wavelength of 523.5 nm, confirming the Au-NPs of size 15 nm.31,32
AFM and SEM micrographs confirm the nanoparticles are within an approximate diameter of 15 nm, and are well dispersed on the substrate. This discrete dispersion of Au-NPs is critical to investigating the electrical characteristics of the single nano-crystalline Au-NP. Fig. 2(d) shows the TEM image of Au-NP with a diameter of 14.9 nm. The crystal planes of Au-NP are shown with atomic crystal plane spacing (d-spacing) of 0.2317 nm.
The I–V measurements are carried out either on the Au-NP or directly on the n-Si substrate, using AFM nano-probe at different locations. Fig. 3 (a) and (b) shows the first and second consecutive voltage sweeps on Au-NP for the complete −2 V to +2 V sweep and 0 to +2 V sweeps respectively. Using a positive voltage sweep (reverse bias), the difference in tunneling current due to the charge trapping in Au-NP and the I–V curves is shown. On the other hand, during the negative sweeps (positive bias), the I–V curves are overlapping and the current is relatively small.
Fig. 3 The subsequent first and second sweeps on the Au-NP (a) from −2 V to +2 V (b) from 0 to +2 V the direction of arrows shows the direction of voltage sweep. |
From Fig. 3 we can conclude that the tunneling current of the second sweep is smaller than that of the first sweep for voltage sweeps beyond 1.0 V. This reduction in tunneling current is attributed to the effect of charge trapped in the Au-NP during the application of first voltage sweep, which leads to a screening of the electric field at the NPs/n-Si interface.
In this work, we focus on the positive voltage (reverse bias) sweep where the tunneling current is dominant. Notably, in forward bias (negative voltage sweep) the current is very small, as shown in Fig. 3(b), due to forward conduction barrier and high contact resistance.11
Fig. 4 shows the I–V measurements of the sample when placing the AFM nano-probe directly on the n-Si surface. The (I–V) curves for consecutive positive voltage sweeps (0.0 to +2.0 V) are shown in Fig. 4, where the AFM tip is in direct contact with the n-Si substrate, and shows no change in the current during successive sweeps on n-Si substrate. It has been demonstrated that the tunneling current is a direct function of the electric field at the nano metal/Si junction, and the I–V behavior is reproducible when using the same Au AFM probe on a clean n-Si surface.12,33 Therefore, the drop in the tunneling current at the Au NP/n-Si interface at subsequent voltage sweeps, as shown Fig. 3, is a result of the reduction of the interface electric field due to the charge screening effect from the Au-NP. This effect is not observed in the absence of NPs, as shown in Fig. 4.
Fig. 4 The I–V characteristics by placing tip directly on the surface and the direction of arrows show the direction of voltage sweeps. |
For a further understanding of the tunneling current in Fig. 3, the I–V characteristics on the Au-NP at different voltage ranges and time intervals are measured. The results showed the modulation and irreversibility of I–V curves with sequential sweeps are only observed when the probe is placed on Au-NPs. However, there is no change in I–V characteristics with multiple voltage sweeps when the AFM tip is directly in contact with the n-Si substrate, as shown in Fig. 4. To assess the impact of different voltage sweep ranges on the charge storage, different voltage sweep ranges are carried out, as shown in Fig. 5, where Fig. 5(a), (b) and (c) shows the consecutive I–V curves on Au-NPs with voltage sweeps in the range of 0 to +0.6 V, 0 to +1.0 V and 0 to +1.2 V respectively. Fig. 5(b) shows the sweeps up to +1.0 V do not pump enough charge into the Au-NP to impact the I–V characteristics, therefore, this value (+1.0 V) is considered the minimum bias to pump enough charge to create this effect. For the consecutive voltage sweep of 0 to +1.2 V we observe a slightly smaller current level as compared to that of its previous sweep as shown in Fig. 5(c). The shift in overlapping of first and second consecutive sweeps shows a small amount of charge accumulation in the Au-NP.
Fig. 5 The voltage-dependent I–V measurements with (a) voltage sweep of 0–0.6 V (b) voltage Sweep of 0–1 V and (c) the sweep of 0 to 1.2 V. |
The time dependence of the I–V curves with a sweep range of 0 to +2.0 V, carried out at different time intervals, is shown in Fig. 6. Fig. 6(a) shows multiple I–V sweeps, with intervals (Δt) of 15 seconds (s), 30 s, 45 s and 60 s between the successive sweeps. At Δt = 60 seconds from the previous sweep, we can readily observe that the I–V curve coincides with the initial (first) I–V sweep where there was no charge stored; indicating the decay of the charge stored due to the earlier sweep.
Fig. 6 The time-dependent I–V measurements with (a) the complete charging and discharging process (b) the time-dependent voltage sweeps of 0 to +2 V (c) consecutive I–Vs with 1 minute time interval. |
It is worth mentioning that while each I–V measured during a sweep reads the current charge state of the Au-NP, it also injects charge into the Au-NP (if the sweep is larger than +1.0 V). Therefore, if an immediate readout sweep is carried out, a lower current state is obtained due to the charge pumping introduced by the preceding sweep. If the sweep is done after longer intervals, the charge decay states (higher I–V current) are observed and with given enough time it will get back to the non-charged state of high current. This is also captured in Fig. 6(b), which shows a different data set depicting the initial I–V curve, the immediate I–V measurement at Δt = 0 s, Δt = 30 s and again Δt = 0 s from the previous sweep. In Fig. 6(b), the immediate I–V measurement after any sweep (i.e. Δt = 0 s and after 30 s dark green color) overlaps with the lowest current level, indicating the charging effect of any full range sweep (sweep voltage >1.0 V). Fig. 6(c) shows two overlapping time-delayed I–V sweeps at the highest current, indicating no charge in the Au-NP, where the second sweep is carried out after 60 s from the first sweep. This indicates there is a charge decay time of ∼60 s. This evolution in I–V behavior can be understood considering the charge screening effect from the NPs on the current reduction in Fig. 3, as explained earlier. However, during the charge decay from the NP, the screening effect becomes less, i.e. higher electric field, hence higher tunneling current. When the NPs is completely discharged the electric field is back to the maximum value and the initial tunneling current is observed, as in Fig. 6(a) and (c).
Finally, close observation of the difference in I–V behaviors in Fig. 4 (when the AFM probe is placed directly on n-Si surface) and Fig. 3(b) (when the AFM probe is placed directly on n-Si surface), shows that charge trapping is strictly attributed to the existence of NP itself. On the other hand, if there is oxidation underneath Au NP, on Si surface, charge trapping effect will have been evident in Fig. 4 as well. Furthermore, the time evolution of the I–V curves in Fig. 6 and the restoration of the NP initial state after waiting along enough time (one minute) also supports the NP charging effect. This indicates that the effect of oxidation on n-Si underneath Au-NPs is negligible, as the n-Si is passivated with hydrogen after HF cleaning.
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