pubs.acs.org/NanoLett
Orientation Controlled Schottky Barrier Formation at Au Nanoparticle-SrTiO3 Interfaces Ramsey Kraya,* Laura Y. Kraya, and Dawn A. Bonnell Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 ABSTRACT In this paper, the electrical transport of Au nanoparticle/SrTiO3 nanointerfaces has been studied. The fabrication method detailed creates atomically smooth SrTiO3 substrate and controlled Au nanoparticle morphologies to create two unique interfaces. The two interfaces are identifiable in atomic force microscope images allowing us to compare variations in the electronic structure using scanning force spectroscopy. By combining AC imaging with scanning force spectroscopy, the interfaces are effectively probed and left undisturbed. The ideality factor and Schottky barrier height are obtained and compared with one orientation exhibiting deviations from thermionic emission theory while the other showing strong similarities to large area Schottky contacts. It is thus shown that controlling the interface structure is of utmost importance to controlling nanoscale Schottky barriers. KEYWORDS Nanoprobe, nanointerfaces, metal nanoparticles, SrTiO3, Schottky barrier
INTRODUCTION
One of the major challenges facing the field is the ability to determine the interface structure of the contact while simultaneously studying its electronic transport characteristics. Scanning probe microscopes are the best tools to overcome these difficulties allowing for the identification of fabricated and controlled interface structures while simultaneously allowing for electronic structure to be determined. The main drawbacks of the scanning probe methods are the ability to image weakly bound particles without disturbing the interface while at the same time measuring the electronic structure. Strontium titanate is often considered the prototypical oxide material and an understanding of its properties is necessary for understanding more complex oxides. SrTiO3 is currently used as a superconductor, high-K dielectric layer, and semiconductor. When doped with Nb, it is considered an n-type oxide semiconductor23-33 and has many applications including thermoelectric devices. In this paper, we study in detail the formation of Schottky nanocontacts of Au metal nanoparticles on atomically smooth SrTiO3 substrates by means of AC scanning probe microscopy and conductive force spectroscopy. This method allows for the acquisition of IV curves at known loading forces with minimal disruption to the interfaces during imaging. The fabrication techniques developed allows for the formation of controlled and identifiable interfaces which are evident in AC mode AFM. In this way, it is possible to analyze the effect of interface structure on the formation of the Schottky barrier. For the nanocontacts studied, we find that a rectifying behavior is present for all nanointerfaces probed. Using thermionic emission theory, the Schottky barrier height is determined and the conformity of the interface to pure thermionic emission (the ideality factor) is determined. The reverse bias leakage current is also investigated as a function of interface structure. Finally, the results are analyzed in
A
s the demand for more and better functionality in electronic and optoelectronic devices continues to rise, significant effort has been directed toward investigating nanointerfaces and nanostructures. Nanoparticles,1 nanowires,2 and nanorods3 are a new class of materials that have been shown to exhibit excellent electrical, optical, mechanical, and thermal properties and can be used for building blocks for future electronic devices with applications ranging from chemical4 to biological sensors5 to building blocks for future semiconductor devices.6 Fundamental to the advancement of this field is an understanding of how electrical contacts scale down in this size regime. When materials with different atomic and electronic structures are integrated into small dimensions the control of the electrical integrity of the device becomes of utmost importance and is critical to the overall performance of the device. Metal-semiconductor interfaces represent the fundamental building block of all electronics whether at the microscale or nanoscale with carrier transport across these interfaces is governed by the band offset or the Schottky barrier height7 (SBH). The SBH is affected by size, orientation, and structure at the interface. The ability to systematically tune the height represents one of the biggest concerns and challenges of the nanoscale community. It has already been reported that small contacts behave differently than the correspondingly larger macroscopic contacts8-18 with theoretical investigations indicating that transport through the barrier (tunneling) will dominate transport over the barrier (thermionic emission) as size scales decrease.19-22
* To whom correspondence should be addressed. Received for review: 11/01/2009 Published on Web: 03/19/2010 © 2010 American Chemical Society
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terms of orientation of the nanoparticles on the SrTiO3(100) surface, establishing the existence of variations of barrier height and transport mechanisms based on the orientation of the nanoparticles on the surface, and thus the interface structure of the contact.
EXPERIMENTAL AND METHODOLOGY Single-side epitaxially polished Nb-doped (0.2 atom % Nb) SrTiO3 (100) (Princeton Scientific) were rinsed with acetone and ethanol and air-dried with nitrogen. The substrates were then annealed in flowing oxygen for 1 h at 1000 °C to remove carbon and point defects. The substrates were removed from the furnace and placed on a hot plate and heated to 250 °C where 100 nm gold nanoparticles (citrate stabilized in H2O solution, British Biocell) were boiled onto the surface of the substrate.34 Interparticle distance was controlled by diluting the solutions with water so that aggolermations were limited and so that measurements were between a single nanoparticle and the SrTiO3 and not affected by the depletion region of other nanoparticles. Following the deposition of nanoparticles, the sample was reinserted in the furnace and heated to 900 °C for 1 h. The final annealing step was found to create truncated octahedron gold nanoparticles on the surface. SEM images show the truncated octahedron shape to be the most prevalent shape on both graphite and SrTiO3 using this heat treatment method. The truncated octahedron morphology, the Wulff shape of gold nanoparticles, has been shown both experimentallyandtheoreticallytobethemoststablemorphology.35-39 With this fabrication process that includes multiple annealing steps we are able to fabricate two unique contacts, Type (I) a TiO2 terminated (100) SrTiO3 surface in contact with Au (111) hexagonal facets and Type (II) a TiO2 terminated (100) SrTiO3 in contact with Au (100) square facets. The prepared samples were attached to aluminum pucks using silver paint to create an Ohmic contact. Experiments were performed using an Asylum MFP3D atomic force microscope composed of a closed loop in the X, Y, and Z-direction SFM head, a high voltage controller, and a high gain current amplifier. For these experiments, we used Pt/ Ir-coated conducting tips (Nanosensors, PPP-NCHPt) with a nominal force of 250 nN applied during conductive force spectroscopy measurements. The experiments were performed at room temperature and only after the system was sufficiently stabilized. The samples were loaded onto the AFM, the hood was closed, and the system was left on for 12-15 h. This allowed for thermal drift control and overall system stabilization. Transport measurements were performed in a four step process (Figure 1). First, topography images in AC mode were acquired to select a clean area. This method allowed for imaging without disrupting the interface between the nanoparticles and the substrate surface. Once a desired nanoparticle was identified, the tip was retracted from the surface and the drive voltages applied to oscillate the tip for © 2010 American Chemical Society
FIGURE 1. Schematic representation of the experimental setup. During imaging, the oscillation of the tip is used as feedback, and for spectroscopy (when current-voltage data is extracted) the deflection of the cantilever is used.
FIGURE 2. Truncated octahedron with the hexagonal facet up (I), and the square side up (II) The perimeter is the outline that would be visible in an AFM image (assuming a hemispherical tip shape), six sides for the hexagonal face and eight sides for the square face.
imaging were set to zero. Next, the tip was brought into contact with the nanoparticles of interest with a known and sustained load during the acquisition of the IV curves. Finally, topography images were acquired to check for changes in the height of the measured nanoparticle. Maintaining the integrity of the interface between the nanoparticle and the SrTiO3 substrate is vital to obtain reproducible results. Therefore, the load supplied to the nanoparticles must not be too large to cause a deformation. This ability to withstand a set load varies from nanoparticle to nanoparticle, but as long as the height of the nanoparticle does not change after I-V curves are extracted then the curves are taken as reliable data. Images were taken imaging in the repulsive regime with slow scan rates (0.4-0.7 Hz) to more clearly identify the faceting of nanoparticles. Figure 2 shows two examples of faceted nanoparticles on the surface, one type with six sides and the other with eight sides. By analyzing a two-dimensional image of the truncated octahedron-shaped nanoparticles (Figure 3), we can see that for the hexagonal facet to be in contact with the surface an outline of a six-sided particle should be evident and for the square facet, 8 sides should be evident. In our AFM images the two shapes are clearly visible. Although some of the nanoparticles in the images appear roughly circular, the two high-magnification images are representative of the nanoparticles analyzed in this paper, that is, the shapes are clearly outlined by a hexagon or an octagon. By this method, we are able to clearly identify two different orientations on the SrTiO3 surface. 1225
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FIGURE 3. AC mode AFM images (a-d) of the gold nanoparticles on SrTiO3 with hexagons (for the Type I interfaces) and octagons (for the Type II interfaces) clearly outlining the nanoparticles TABLE 1. Values for E As a Function of Dopant Concentration for SrTiO3a
Once the I-V curves are identified, the tip is brought into contact with a single particle with no other particle within the vicinity by a few hundred nanometers. The voltage is swept from -10 to 10 V while the current is recorded with a sampling frequency of 2 kHz. To determine whether thermionic emission or thermionic-field emission (thermally assisted tunneling where the electrons near the top of the barrier tunnel through to the metal) should be expected in this type of contact we started with the general equation governing thermionic emission and thermionic-field emission40 and substituted material values for SrTiO3 into the equation
( )]
( )[
I ) Is exp
eVD eVD 1 - exp E kBT
ND 18
10 cm 1018 cm-3 1018 cm-3
E00 kBT
(2)
and E00 is given by
E00 )
( )
hq ND 4π m*εs
1/2
) 18.5 × 10-15
© 2010 American Chemical Society
( ) ND mcεsr
eV
kBT/E00 (at T ) 300 K)
E
33.3626 10.5502 3.3363
0.0259 eV 0.0259 eV 0.0266 eV
where m* ) mcm is the effective mass of electrons in the semiconductor, m is the free electron mass, and mc is the ratio of the two; εs ) εsrε0 is the permittivity of the semiconductor, εsr is the relative permittivity, ε0 is the vacuum permittivity; ND is the dopant concentration in units of m-3, Is is the saturation current, eVd is the applied voltage in units eV, and kB is the Boltzmann constant in units eV/K. E00 is a parameter that plays an important role in tunneling theory. The ratio kBT/E00 is a measure of the relative importance of thermionic emission and tunneling. We should expect field emission if kBT , E00, thermionic-field emission if kBT ≈ E00, and thermionic emission if kBT . E00. If kBT . E00, E reduces to E ≈ kBT, reducing eq 2 to the thermionic emission model, eq 5. For SrTiO3, E ≈ kBT for the values of ND listed below in the Table 1. Therefore thermionic emission models are adequate for analyzing metal contacts to SrTiO3 over a large dopant concentration profile. As can be seen, charge transport depends on the dopant concentration, dielectric constant of the semiconductor, and the effective mass of the charge carriers in the semiconductor. Thermionic emission current across an intimate, abrupt Schottky contact is derived as
(1)
( )
7.744 × 10 0.0025 eV 0.0077 eV
-4
a Since the values are ≈ kBT, thermionic emission models are valid to apply for the listed ND values.
The parameter E is given by
E ) E00 coth
E00 -3
1/2
eV (3)
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( )[
I ) Is exp
( )]
eVD eVD 1 - exp nkBT kBT
(4)
(
(5)
Is ) AA//T2 exp -
eΦeff kBT
)
where A is the diode area, Φeff is the effective barrier height, and A** is the effective Richardson constant. The Richardson constant is determined from the following equation: A** ) 4πm*k2/h3, where h is Planck’s constant. By fitting the linear portion of the forward biased current-voltage (I/V) curves of the real Schottky contacts to the relation derived for thermionic emission, the effective barrier height and ideality factor, n, can be determined. An ideality factor between 1 and 2 would reaffirm that transport is dominated by thermionic emission while higher values would indicate a strong tunneling component.
FIGURE 4. Typical I-V curves for the Type I interfaces (blue) and the Type II interfaces (green) (80 nm particles). Notice the suppression of the reverse bias leakage current to -10 V for the Type I interfaces as has been measured for macroscopic metal/SrTiO3 interfaces.
RESULTS As can be seen from Figure 5, the type I interface exhibited ideality factors between 1 and 2. The general trend of the data is that as the nanoparticle size decreased we see a decrease in the calculated barrier height and less conformity to pure thermionic emission. The largest nanoparticles, 90 nm heights, exhibited ideality factors closest to 1, and the 70 nm particles began to show greater deviations from these values. For the type II interface, there was much less variation in the barrier heights over the range of sizes with the typical barrier height for type II close to the minimum of type I. The ideality factors are larger than 2 indicating a strong tunneling component to the overall charge transport characteristics. Because of this added transport component, the calculated barrier height would intuitively be calculated as lower using thermionic emission models. The larger the nanoparticle, the closer to conformity to pure thermionic emission and the higher the measured barrier. For both types of interfaces, as the nanoparticle size decreased the measured SBH decreased. The largest nanoparticles (90 nm) showed the closest conformity to thermionic emission. Sample reverse bias leakage currents (RBLC) of the Type II interface was higher (Figure 4), while the Type I interface exhibited no RBLC down to values of -10 V, similar to macroscopic results for metal/SrTiO3 contacts.41 Overall, the type II interfaces exhibited much larger reverse bias leakage currents and stronger deviations from pure thermionic emission.
FIGURE 5. Barrier height and ideality factors for the measured interfaces. Red represents 70 nm particles, green 80 nm, and blue 90 nm. Hexagon-shaped data points are Type I interfaces, and square-shaped data points are Type II interfaces.
sion transport properties while the other exhibited a large tunneling component. The reasons for this differing behavior can be explained by the concept of edge effects.42-45 Discontinuities at the interfaces lead to increased electric field intensities. Therefore the electric field will be larger at the edges of the nanoparticle facet in contact with the substrate, while at the center of the nanoparticle facet the electric field should be similar to that of an infinite plain of charge. This results in greater band bending at the edge and increased charge flow overall through the interface. This effect increases as the size of the interface decreases due to the increase in the edge/area ratio of the contact. Edges that meet at sharp angles will have increased electric field intensities than more rounded edges.46 This explains the data obtained from the 2 interfaces where the square facets of the nanoparticle in contact with the SrTiO3 surface
DISCUSSION AND CONCLUSIONS In conclusion, we have calculated the SBH for two controlled interfaces between Au metal nanoparticles and SrTiO3 substrates. One type exhibited pure thermionic emis© 2010 American Chemical Society
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exhibited larger deviations from macroscopic behavior than did the Type I interfaces. This work has shown that interfaces play a much larger role in determining transport properties than does nanoparticle size; two nanoparticles of the same size can exhibit a great divergence in transport characteristics if the interface structures vary, while two nanoparticles of varying size exhibit only minor changes in transport properties if the interface structure is similar. Thus, it is of utmost importance that controlling interface structure be the focal point in fabrication nanoscale electronic devices.
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Acknowledgment. This work was supported in part by the U.S. Department of Energy under Grant DE-FG0200ER45813-A000 and partially supported by the NSF IGERT. Instrumentation use in the Nano/Bio Interface Center (IGERT DGE02-21664) and the Laboratory for Research on the Structure of Matter (DMR05-20020) are gratefully acknowledged. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
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