Electrical Transport Properties of Au Nanoparticles and Thin Films on

Article pubs.acs.org/Langmuir

Electrical Transport Properties of Au Nanoparticles and Thin Films on Ge Probed Using a Conducting Atomic Force Microscope Erjuan Guo,†,‡ Zhigang Zeng,*,†,‡,§ Xiaobo Shi,†,‡ Xiao Long,†,‡ and Xiaohong Wang‡,∥ †

Department of Physics, College of Sciences, ‡Institute of NanoMicroEnergy, College of Science, §Shanghai Key Laboratory of High Temperature Superconductor, ∥Department of Chemistry, College of Sciences, Shanghai University, Shanghai 200444, China S Supporting Information *

ABSTRACT: In this work, gold nanoparticles (Au NPs) were distributed on an n-Ge substrate using the colloidal NP deposition method to form Au NP/Ge Schottky diodes (SDs), and the current transport properties of these nano-SDs were studied. The current density−voltage (J−V) characteristics were measured on each nanometer-sized Au particle using a conducting atomic force microscope (C-AFM). These Au NP/Ge diodes showed a rectifying behavior. According to the thermionic emission (TE) model, the effective Schottky barrier height (SBH) and ideality factors n were obtained. The SBH for the Au NP/Ge diodes ranges from 0.22 to 0.30 eV and the ideality factor ranges from 3.8 to 8.6. The current density and the barrier height increase while the ideality factor decreases with increasing Au NP diameters. This indicates that the tunneling effect is enhanced because of the narrowed depletion width and decreased size of the Au NP/Ge SDs. To compare the electrical behavior with Au NP/Ge diodes, the Au thin film/Ge diodes were also prepared and their SBHs were much larger because of the image-charge lowering effect and the tunneling effect in Au NP/Ge diodes.

1. INTRODUCTION The demand for electronic and optoelectronic devices that have higher function and precision is growing, and significant efforts have been directed toward investigating nanointerfaces and nanostructures.1−3 Nanoparticles (NPs),4 nanowires,5,6 and nanorods7 have exhibited excellent electrical, optical, and thermal properties and can be the cornerstone for future optoelectronic nanodevices and electronic devices.8,9 It is necessary for us to investigate those electrical properties of such nanostructures that can be applied to various metal−semiconductor (MS) junction-based optoelectronic devices, especially the relationship between transport properties and the device size.10 Until now, many kinds of nano-Schottky diodes (nano-SDs) that were fabricated in the nanometer size have been investigated, such as Pt/GaN nanowires,11 Al/GaN nanowires,12 Au/ZnO nanobelts,13 Au/CdS nanobelts,14 and Pt/ ZnO nanocolumns.15 In addition, nano-SDs in nanosized complex shapes have properties different from those of the thin film devices, which have been successfully proved by various theoretical and experimental studies. However, the mechanisms © 2016 American Chemical Society

related to the Schottky barrier height (SBH) and the ideality factor have not been fully explored on the nano-SD, which is one of the important evaluation indexes of high-performance nano-Schottky junctions. One of the objectives for researchers is to improve the measurement precision in scanning probe microscopy (SPM)the eye and hand of nanometer science and technologywhich provides unprecedented access to nanostructure materials for scientists. Atomic force microscopy has especially been proven to have a good adaptability. With the development of science and technology, we need to explore the features of nanoscale electronic and optoelectronic devices. In the past few decades, researchers have discovered many techniques for the formation of nano-structures.16 With the enhancement of processes, the future size of devices will continue to decrease. Received: June 15, 2016 Revised: September 18, 2016 Published: September 19, 2016 10589

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Figure 1. Schematic diagrams of current−voltage test for (a) Au NP/Ge diodes and (b) Au thin film/Ge diodes.

the Ge substrate using the boiling deposition method.19 The depositions were generally single dispersed particles, although some aggregates of two or more particles were also present. The roughness [root-mean-square (RMS) value] of the Ge surface was 1.18 nm, although the clean substrate surface had some scratches. From Figure 2b, it can be seen that the scratches have no influence on the distribution of Au NPs.

Morphological and electrical properties can be determined simultaneously by using an atomic force microscope (AFM) probe coated with a conductive material while a bias voltage is applied to the tip or sample.17 The tip can be brought in contact with the nanostructures of interest with a known and sustained load during the acquisition of the J−V curves. In the present work, the Au NPs/Ge SDs were fabricated to study the nanoscale Schottky characteristics by using the conductive mode of AFM (C-AFM). Compared with the studies in which direct contact of conductive tip and semiconductor is used to investigate the nano-Schottky behavior,18 our method excludes the tunneling effect between the conducting probe and the semiconductor surface; thus, the results are more accurate. The effective SBHs and the ideality factor n were calculated from these different sized Au NP/Ge diodes using the thermionic emission (TE) model. The SBH decreases whereas the ideality factor increases with the decrease in the NP size. In addition, the Au/n-type Ge nanodiode has a lower SBH value than conventional Au thin film/n-type Ge diodes.

2. EXPERIMENTAL SECTION 2.1. Preparation of Au NPs and Au Thin Films. All n-type Ge(111) substrates were first cleaned in an ultrasonic bath with deionized water followed by the acetone solution, and then immersed in a buffered oxide etch (BOE) solution (HF/NH4F = 1:6) for 30 s to etch off the surface oxide layer. Immediately after the surface cleaning, a Ti film with a thickness of ∼100 nm was deposited on the back of the Ge substrate by e-beam evaporation under ultrahigh vacuum (10−8 Torr). It was then followed by heat treatment at 673 K for 60 min in an Ar atmosphere to form Ohmic back contact. According to the colloidal NP deposition method,19 we placed 4−15 μL drops of 1 wt % gold colloid solution (purchased from Hualan Chem, China) on the Ge wafer and allowed them to boil. Then, differently-sized Au NP/Ge SDs were fabricated. To compare the electrical behavior with these Au NP/Ge diodes, the Au thin film/Ge diode was prepared and investigated by sputtering a Au thin film with a thickness of 30 nm (PVD75, Kurt J. Lesker, USA) as Schottky front contacts. 2.2. Measurements. The topographies and electrical properties of Au NP/Ge diodes were determined using a C-AFM (Dimension Edge, Bruker Corporation) operating in the contact mode, with Pt-/Ircoated tips (SCM-PIC), as shown in Figure 1a. The current−voltage (J−V) curves of the Au thin film/Ge diodes were measured with a Keithley semiconductor characterization system (SCS-4200) using a 92 mm tungsten probe, as shown in Figure 1b.

Figure 2. AFM images of (a) the Ge substrate and (b) Au NPs on Ge. (c) Section profile of the Au NPs indicated in (b) with a red arrow. (d) Schematic diagram of the broadening effect of the AFM tip.

Figure 2c shows the section profile of Au NPs on the Ge substrate marked by the red arrow in Figure 2b. The heights of these Au NPs are 35, 27, and 18 nm. However, their lateral width is much larger than their height because of the broadening effect of the AFM tip. As shown in Figure 2d, when an AFM probe goes over a sphere that is attached to a surface, the size of the tip will cause a broadening feature of the sphere, and the nanospheres often look larger than they are. However, the height measured by the line profile is correct.20 We evaluated the contact area between the Au NPs and the Ge substrate from the height of the Au NPs. 3.2. Analysis of Nanoinhomogeneity Effects on Nanoscale Au/Ge SD Parameters. To date, MS contacts and the current characteristics of nano-Schottky junctions have been investigated by many researchers.21−23 To analyze the nano-Schottky contacts, a model of implanting metal NPs on a semiconductor surface was developed. For real MS contacts, the interfaces have various randomly distributed geometrical

3. RESULTS AND DISCUSSION 3.1. Morphology of Au NPs on a Ge Substrate Probed using an AFM. Surface morphologies of the n-type Ge(111) and Au NPs on the n-type Ge wafer were examined using the AFM tapping mode in the scan range of 5 × 5 μm2, as shown in Figure 1. It can be seen that the height of the Au NPs ranges from 1 to 40 nm. The nanosized Au particles were dispersed on 10590

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Langmuir ⎤ ⎛ Φ ⎞⎡ ⎛ qV ⎞ J = AT 2 exp⎜ − B ⎟⎢exp⎜ ⎟ − 1⎥ ⎝ kT ⎠⎣ ⎝ kT ⎠ ⎦

sizes, shapes, and local work functions, such as metal islands24 or patches; hence, the spot field (, ) along the interface of an MS contact is inhomogeneously distributed because its direction and intensity are random, complex, and instant (Figure 3a). If real SDs contain nanoislands or patches (Figure

(1)

with ⎛ Φ ⎞ J0 = AT 2 exp⎜ − B ⎟ ⎝ kT ⎠

where A is the Richardson’s constant, T is the absolute temperature, k is the Boltzmann constant, V is the applied voltage, and J0 is the saturation current density. The barrier height ΦB depends on the applied voltage V and can be written as ΦB = Φ0B + (1 − 1/n)qV + ···

(2)

Here, the derivative β = ∂ΦB/∂V = 1 − 1/n (higher-order terms are assumed to be constant and hence neglected) and Φ0B is the zero-bias SBH. On the clear surfaces, the lowering of barrier height by the image-charge lowering effect of Schottky barrier has to be considered and can be written as

Figure 3. (a) Schematic image before the metal and n-type semiconductor close contact with the additional electric field, (b) contact interface between the metal and the semiconductor with the additional electric field, and (c) energy diagram of the Schottky junction without the additional electric field.

⎤1/4 ⎡ q3N D ΔΦB = −q⎢ 2 3 3 (Vd − V )⎥ ⎦ ⎣ 8π εr ε0

(3)

where Vd is the diffusion voltage and the image force vanishes when V equals to Vd, flattening the bands. In addition, provided only the image force determines the voltage dependence of SBH, the ideality factor can be written as

3b), of which the potential barrier heights are ΦB1 and ΦB2, then this is characterized by the effective SBH ΦBA and the characteristic length xm from the metal surface (Figure 3c). The effective SBH ΦBA is in the range of ΦB1 + ΔΦB1 ≤ ΦBA ≤ ΦB2 − ΔΦB2, which is closely related to the inhomogeneity of the heterogeneous contacts. The characteristic length xm is in the interval x1 ≤ xm ≤ x2. An energy diagram of the real MS contact is shown in Figure 3c (dashed line). The depletion layer in the semiconductor of the Schottky junction interface largely determines its rectifying behavior. At the interface, the SBH ΦB is the measure of the energy distance between the Fermi level EF and the conduction band for the ntype semiconductor. According to the TE model, the J−V characteristics can be written as

−1 ⎡ ΔΦB(0) ⎤ n = ⎢1 − ⎥ 4qVd ⎦ ⎣

(4)

where ND is the donor concentration, εr is the semiconductor static dielectric constant, and Vd is the diffusion potential. ΔΦB(0) is the zero-bias SBH caused by the image-force lowering effect. According to the image-force lowering effect of barrier height, the (J−V) characteristics of a real inhomogeneous SD can be expressed as eqs 1 and 2 in forward bias25

Figure 4. (a) J−V curves obtained on a single Au NP marked with a cross in the inset and (b) J−V curves obtained on the Au film. The insets of (a) and (b) show the atomic force microscopy topography (500 × 500 nm2) of the Au NPs and the Au thin film on Ge substrates, respectively. 10591

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Figure 5. Plots of the (a) Au NP/Ge diode and (b) Au film/Ge diode obtained by fitting the J−V curves according to the TE equation. The diameter of the Au NP in the Au NP/Ge diode was 30 nm, and the thickness of the Au film in the Au film/Ge diode was also 30 nm. (c) In the interface, the depletion width reduces from the Au thin film to the Au NPs.

of the Au thin film on the Ge diode were also detected. The topography image of the Au film on the Ge substrate is shown in the inset of Figure 4b. It also exhibits the typical rectifying behavior for the Au thin film/Ge SD. Keeping the same height of the Au NPs and the thin film, it is shown that the current density of the Au NP/Ge diode is much larger than that of the Au thin film/Ge diode; moreover, the turn-on voltage of the Au NP/Ge diode is also larger than that of the Au thin film/Ge diode. From eq 6, we can deduce the value of SBHs and the ideality factor. Figure 5 shows the plots of the measured J−V curve and the fitted results on the Au NP/Ge diode and the Au thin film/ Ge diode. As we can see from the fitting results, the TE model shows excellent agreement with the measured J−V curves. Therefore, we can explore the electrical properties of Au NP/ Ge diodes using the TE model. The effective SBHs were 0.302 eV for the Au NP/Ge diode and 0.77 eV for the Au thin film/ Ge diode. The schematic in Figure 5c depicts the energy band structure through the transition from a Au thin film/Ge planar interface to a Au NP/Ge interface. The electrostatic equilibrium will be formed only when enough electrons transfer from the semiconductor side to the metal side. However, because the Au NP size is limited, unlike the Au thin film planar contacts, the amount of charge required to transfer to the metal surface is less compared to that of planar contacts.26 As a consequence, this causes a narrower depletion region, and the tunneling effect will be more significant for the NP diode contacts. In addition, the image charge in the Au NPs can be generated by the existence of electrons, which induces an electric field in the n-type Ge. The image force induced by the Coulomb attraction between the electron and the image charge distorts the potential barrier by applying a constant external electric field, as shown in Figure 3c. Therefore, the ideal SBH ΦB drops

⎤ ⎛ Φ ⎞⎡ ⎛ qV ⎞ J = AT 2 exp⎜ − B ⎟⎢exp⎜ ⎟ − 1⎥ ⎝ kT ⎠⎣ ⎝ kT ⎠ ⎦ ⎛ Φ0 + ΔΦ ⎞⎡ ⎛ qV ⎞ ⎤ B ⎟⎢exp⎜ ⎟ − 1⎥ = AT 2 exp⎜ − B ⎦ kT ⎝ ⎠⎣ ⎝ kT ⎠ ⎛ Φ0 + ΔΦ (0) + βqV ⎞⎡ ⎛ qV ⎞ ⎤ B ⎟⎢exp⎜ ⎟ − 1⎥ = AT 2 exp⎜ − B ⎝ ⎠ ⎦ ⎣ kT kT ⎝ ⎠ ⎛ Φ0 + ΔΦ ⎞⎡ ⎛ qV ⎞ ⎛ (n − 1)qV ⎞⎤ B ⎟ − exp⎜ − ⎟⎢exp⎜ = AT 2 exp⎜ − B ⎟⎥ ⎝ ⎠ ⎝ ⎠⎦ kT nkT nkT ⎝ ⎠⎣ ⎛ Φ ⎞ ⎛ qV ⎞ ⎟ ≈ AT 2 exp⎜ − BA ⎟ exp⎜ ⎝ nkT ⎠ ⎝ kT ⎠ (5)

when qV ≫ kT, the eq 4 can be derived as ⎛ Φ ⎞ ⎛ qV ⎞ ⎟ J = AT 2 exp⎜ − BA ⎟ exp⎜ ⎝ nkT ⎠ ⎝ kT ⎠ ⎛ qV ⎞ ⎟ = JS exp⎜ ⎝ nkT ⎠

(6)

with ⎛ Φ ⎞ JS = AT 2 exp⎜ − BA ⎟ ⎝ kT ⎠

where ΦBA is the effective Schottky barrier height at the zerobias voltage. 3.3. Size Dependency of Transport Properties in the Au NPs/n-Ge Diodes. As shown in Figure 4a, the rectifying behavior of the J−V curve was obtained on the Au NP/Ge SD, marked by a cross in the topography image inset of Figure 4a with the sample bias from −1.5 to 3 V. The J−V characteristics 10592

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Figure 6. (a) Measured J−V curves for the AFM tip on various Au NPs/Ge diodes and directly on the Ge substrate with contact areas of 254, 415, 706, 1133, and 314 nm2. Experimental SBH (b) and ideality factor (c) versus the diameter of the Au NPs. (d) Plot for SBH versus ideality factor of Au NPs/n-Ge SD at room temperature.

Table 1. Electrical Properties of Au NP/Ge Nano-SD from Their J−V Characteristics diode diameter (nm)

2.6

3.3

5.2

6.5

18

20

23

30

38

ideality factor (n) SBH ΦBA (eV)

5.23 0.217

5.32 0.227

6.53 0.236

5.28 0.258

6.4 0.25

4.4 0.29

5.8 0.28

3.8 0.3

3.2 0.35

to the effective SBH, as the barrier decreases by ΔΦB. As a result, the Au NP/Ge diode is vulnerable to the image-force lowering effect of barrier height from the edges of the diode, which induces a decrease in the Schottky energy barrier for charge carrier TE ones.27 It is generally known that the imageforce lowering effect of the barrier height in NPs is more obvious than that in thin film SDs. As a consequence, the SBH of Au NP/Ge diode is much lower than that of the Au thin film/Ge diode because of the significant tunneling effect and the image-force lowering effect of the barrier height. The measured J−V curves for the AFM tip on various Au NP/Ge diodes and directly on the Ge substrate with different contact areas are shown in Figure 6a. The J−V curves reveal that the current density at the expected voltage increases with the decrease in the Au NP size. It implies a strong effect of the size on nanocontacts. When the Pt-coated AFM tip directly contacts the Ge substrate, the current for this kind of Pt/Ge nanodiode is very small and close to 0 nA. In addition, different normal forces of the AFM tip applied on samples can also influence the I−V characteristics,28 as shown in Figure S1. The I−V characteristics of the SDs change significantly with the normal forces. In addition, the current signal will be unstable

and weak if the force is too small or too large. It must be pointed out that precise values of the normal force cannot be read on the equipment because the values of the force are all relative data, which are given by set points in volts. In our experience, the tip contacts the Au NPs well but does not scrape off them when the set point is 2 V. At the same time, reliable and repeatable data can be obtained. The electrical contact properties of the AFM tip on the Au film were also compared with those of the AFM tip on the Au NPs (30 nm)/ Ge diode, as shown in Figure S2. When the tip was in direct contact with the Au NPs, the current responses were much faster and the I−V curves were nearly symmetric, not showing an obvious rectifying behavior. Therefore, we can postulate that the electrical properties of the AFM tip on Au NP/Ge diodes are dominated by the Schottky contact rather than the AFM tip/Au NPs or the AFM tip/Ge substrate. The estimated values of the different Au NP/Ge diode parameters are tabulated in Table 1 by using the TE model. The effective SBH and the ideality factor of Au NP/Ge diodes with the size ranging from 2.6 to 38 nm are shown in Figure 6b,c, respectively. In Figure 6b, the error bars show a deviation from the repeated measurements. For the small-sized NPs 10593

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be explained by the lateral inhomogeneities of the barrier height. The plot of SBH versus ideality factor of Au NPs/n-Ge SD at room temperature is shown in Figure 6d. When the size of a diode is reduced, the barrier height of the diode generally decreases and its ideality factor becomes larger. The dashed line in Figure 6d is a linear least-squared fit of the data points, and the extrapolation to n = 1 gives an image-force lowered barrier height of 0.38 eV. In other words, even when n = 1, the barrier height (0.38 eV) of the Au NP/Ge SD is still much less than the effective barrier height (0.77 eV) of the Au thin film/Ge SD. This implies that the image-force lowering effect of the barrier height has a great influence on the nano-SD or the zerobias barrier height is affected significantly by the contact area of the diode at the nanoscale.

(3.3−8.7 nm), we can measure only once; then, we cannot receive stable J−V information. However, we can repeat the test several times for the larger-sized Au NPs. This may be caused by the oxide layer on the Au NPs, which is easier to generate on smaller-sized NPs.29−31 On the other hand, it becomes more difficult to locate the AFM tip on smaller particles because of the unavoidable thermal drift of the tip position.30,32 As seen in Figure 6b, the barrier height increases with the decreasing diameter, in contrast to larger diameter diodes.33 In other words, with the decrease in the contact size, a charge can be injected by various transport pathways, such as tunneling through a Schottky barrier and TE.34 The Debye screening length is proportional to the characteristic length of the semiconductor. At the MS interface, we can go through theoretical calculations for the electronic structure when the contact size is smaller than the characteristic length of the semiconductor, which indicates that the size of the interface determines the thickness of the barrier layer in the interface. As the electric field increases gradually and the barrier layer becomes thinner significantly at the MS interface, the effects of the image force and tunneling become remarkable.35−37 In addition, the current density decreases as the size increases. Such a distinct difference with respect to the NP diameter and the special phenomenon are caused by decreasing the thickness of the Schottky barrier, and consequently, the tunneling effect is enhanced. As a consequence of the reduction in the SBH, the Au NP/Ge SD is vulnerable to the Fowler−Nordheim tunneling and the image force. With the decrease in the contact area, the current density increases, which indicates that the effects of lowering image-force and increasing tunneling as the contact area decreases. These results provide insights into the electrical transport nature of the nano-Schottky contact, which has potential applications in nanocatalysis and renewable energy conversion for MS hybrid nanostructures.38 The ideality factors, n, for the different sizes of Au NPs on the n-Ge substrate were determined to range from 3.8 to 8.6. Here, n is a measure of the conformity of the diode to the pure TE model; if n = 1, the transport properties are of pure TE. However, because of the particular distribution of interface states and lateral barrier inhomogeneities caused by grain boundaries, a mixture of different phases, and so on,39−41 n has usually a greater value. Our results indicate that the ideality factor of Au NP/Ge diodes show a large departure from unity. For various Schottky metals (Pt, Au, Cu, and Pb) on n-Si,42−45 as well as for Pt and Au on n-ZnO nanowires, the same results have been reported previously.22,46,47 Smit et al. suggested that when the interface area is smaller than the semiconductor depletion width, nano-Schottky contacts exhibit an enhanced tunneling effect.35,37 On the basis of the experimental results on the relationship between the ideality factor and the Au NP diameters, as shown in Figure 6c, it can be found that the ideality factors gradually increase with the decrease in the diode size because of the enhancement of the tunneling effect. Thus, a significant tunneling effect may increase the ideality factor of the Au NP/Ge nano-SD. It should be mentioned that our results exclude the tunneling effect between the conducting probe and the semiconductor surface, compared with studies that use direct contact of the conductive tip and the semiconductor.18 The SBH and the ideality factors of the Au NP/Ge SDs differ from one diode to another, even if the diodes are identically prepared. These variations are correlated in that the SBHs become lower with the increase in the ideality factors. This can

4. CONCLUSIONS In summary, the Au NP/Ge SDs having different particle sizes were successfully fabricated using the colloidal NP deposition method. Current transport characteristics of nano-SDs with different sizes were investigated. The J−V curves were obtained for the Au NP/Ge diode using a C-AFM. The current density increases with decreasing contact areas. As the size of the Au NP/Ge diode decreases, the SBH decreases and the ideality factor increases because of the enhancement of the tunneling effect induced by the narrowed depletion width. The Au NP/ Ge nano-SD exhibits a lower SBH than the conventional Au thin film/Ge diode, presumably because of the tunneling effect and the image-force lowering effect with the down-scaling. The zero-bias barrier height is affected significantly by the contact area of the diode at the nanoscale and it should be given serious consideration when we evaluate the barrier height of the nanoSD using C-AFM.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02232. I−V characteristics on the Au nanoparticle/Ge diode (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (61204129 and 61571280) and the Shanghai Natural Science Foundation (12ZR1444000) and partly supported by the Shanghai Key Laboratory of High Temperature Superconductors (No. 14DZ2260700). We also acknowledge the Instrumental Analysis & Research Center of Shanghai University for providing measurement services. We would also like to thank Prof. Dr. Yulong Jiang and Guodong Zhu for their discussion and viewpoints.

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REFERENCES

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DOI: 10.1021/acs.langmuir.6b02232 Langmuir 2016, 32, 10589−10596

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DOI: 10.1021/acs.langmuir.6b02232 Langmuir 2016, 32, 10589−10596