J. Phys. Chem. C 2009, 113, 4495–4501
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Controlling the Smoothness of Optically Transparent Gold Films by Temperature Tuning Erin S. Carmichael†,‡ and M. Gruebele*,†,‡,§ Beckman Institute of AdVanced Science and Technology, Department of Chemistry, and Department of Physics, UniVersity of Illinois, Urbana, Illinois 61801 ReceiVed: September 5, 2008; ReVised Manuscript ReceiVed: January 13, 2009
We investigate the effect of temperature tuning on the deposition of gold films onto sapphire. Transparent conductive films with roughness as low as 0.4 Å rms at 10 nm thickness allow STM imaging of aerosoldeposited carbon nanotubes. A Monte Carlo lattice model explains the experimentally observed surfaceroughening trends and shows that the interplay of deposition-induced kinetic roughening and thermal annealing and roughening can be optimized. Intrinsic surface roughness (e.g., stepping), previously postulated to play a role in film roughness, is not found to be important. Introduction Ultrathin gold films are potentially desirable surfaces for combining scanning tunneling microscopy (STM) with optical excitation of surface-deposited nanostructures. If the tradeoff between optical transparency, conductivity, and smoothness can be optimized, such substrates allow rear illumination to reduce tip heating, are sufficiently transparent to avoid substrate heating, and yet maintain the conductivity necessary for STM. Semiconductor surfaces have been used in optical STM studies,1,2 including single carbon nanotube near-infrared absorption experiments,3,4 but their band gaps are too small to allow excitation at visible or ultraviolet wavelengths. The substrates that do have large gaps generally require nontrivial ultrahigh vacuum surface preparation.5 Thin metallic films have been studied widely, both theoretically and experimentally, because of their interesting electrical,6,7 optical,8-11 and morphological properties.12,13 Ultrathin gold films in particular do not oxidize under ambient conditions and can be functionalized with alkanethiols.14 Smooth gold films on transparent flat substrates have been achieved using a variety of deposition techniques.15-19 However, at thicknesses required to yield atomically flat surfaces (typically 1 µm),20 there is insufficient transmission of visible light. For ultrathin films, roughness has generally exceeded values acceptable for molecular detection via STM,15,16 although one recent study has reported small terraces that are atomically flat on 10 nm thick gold films.21 Atomically flat terraces of sufficient size are required to reliably detect small molecules. A larger root mean squared (rms) roughness is admissible if one wishes to detect nanostructures such as carbon nanotubes or quantum dots. Here we investigate the growth of thin gold or niobium/gold films on two different flat sapphire substrates, using different deposition techniques and temperature profiles. We model the deposition process with a simple lattice model and Monte Carlo simulations. Our goal is to elucidate the effect of temperature profiles on the relationship between film thickness and rms film roughness. Although the thinnest films do not produce atomically flat terraces, we demonstrate a roughness sufficiently small to image single-walled carbon nanotubes deposited by aerosol * To whom correspondence should be addressed. † Beckman Institute of Advanced Science and Technology. ‡ Department of Chemistry. § Department of Physics.
spraying,22 thus enabling absorption spectroscopy studies of the type previously carried out on Si(100) surfaces.23 Two types of roughening are considered in our lattice Monte Carlo model: kinetic roughening results from the random initial deposition process, while thermal roughening results from diffusion processes subsequent to deposition. Moderate thermal motion can also serve to anneal kinetic roughening before thermal roughening occurs at higher temperatures. We confirm a transition from kinetic roughening at low temperatures to thermal roughening at high temperatures, observed on glass and Si surfaces by Cattani et al.12 We show that the roughening can be controlled to produce a local minimum in the rms roughness as low as 0.06 nm over regions exceeding 20 nm. Monte Carlo simulations reproduce the thickness-roughness trends observed in our data and demonstrate that the shape of the temperature profile dominates the rms roughness as a function of deposited thickness d. In the work of Cattani et al. random surface roughness was implicated as a cause for kinetic roughening. Here we show that reduced lattice mismatch on different sapphire planes and improved lattice matching by a niobium adhesion layer21 can be used to control roughness. However, random substrate roughness, such as that caused by adatoms or steps, does not have a significant effect on the resulting thin film rms by the time 10 nm thickness is reached, either in the experiment or in the Monte Carlo simulations. Experimental Methods Experiments were conducted on polished r-plane and c-plane sapphire substrates that were cleaned by sonication for 30 min each in acetone, 2-propanol, and methanol and subsequently annealed in a furnace at 1400 °C for 70 min. The surfaces were atomically flat except for steps as shown in Figure 1. Gold deposition was performed using three different techniques, with different temperature profiles. Electron beam evaporation (on a Temescal evaporator) was carried out at base pressures below 10-3 Pa (≈10-5 torr). DC magnetron sputtering (AJA International ATC2000) was done under 0.6 Pa (≈5 mtorr) of argon at a flow rate of 20 standard cubic centimeters per minute at room temperature. In both of these cases temperature profiles were controlled by deposition rate and method, with e-beam evaporation producing the largest temperature increases without any temperature delay, and sputtering producing a delayed onset of heating during deposition. Thermal evaporation (Common
10.1021/jp807907x CCC: $40.75 2009 American Chemical Society Published on Web 02/25/2009
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Carmichael and Gruebele
Figure 1. AFM images (1 µm2) of Au films of varying thickness deposited by sputtering on annealed r-Al2O3. Deposited at 0.3 Å/s at room temperature. Four nanometer vertical color scale.
Wealth Scientific) was performed at pressures below 10-3 Pa (≈10-5 torr) with samples held constant at 0 °C to provide a low-temperature baseline. In all cases, deposition thickness was measured using a quartz crystal monitor, and deposition rates were varied over a range near 1 Å/s. The films were evaluated using a Digital Instruments (DI) Dimension 3100 atomic force microscope (AFM). rms roughness was calculated for 1 µm × 1 µm AFM scans using the DI NanoScope software. Care was taken to compare images with similar tip resolution to provide reliable relative changes in roughness. Subsequent analysis by scanning tunneling microscopy (STM) was performed at room temperature in our homebuilt ultrahigh vacuum system3,4 (6 nm region of interest for conductive applications; however, a niobium layer is helpful. We conclude that roughening of thinnest films is affected by lattice mismatch on a length scale less than the atomic size, not by topographic roughness on an atomic or greater length scale. For thicker films, only thermal roughening has an effect. The experimental results and simulations suggest that to get the best annealing effect before thermal roughening begins, the deposition rate and temperature profile must be matched. As demonstrated by Figure 9, changing different deposition parameters one at a time can shift or stretch the rms(t) profile. However, the dip in roughness due to annealing occurs at reasonable film thickness only under optimal conditions. When the maximum allowed temperature is decreased (Figure 9A), the roughness peak grows because initial kinetic roughening becomes a larger factor. Although both high and low temper-
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Carmichael and Gruebele films have minimal absorption (14% upper limit, partly scatter). As d increases beyond 10 nm, the transmittance begins to drop off rapidly, which agrees well with previous results. In this regime, most of the light not transmitted is reflected back. The variation in optical behavior as a function of wavelength has been attributed to surface plasmon resonance in the gold particles.10 This is demonstrated in Figure 3B for a 10 nm thick film. The peaks and valleys shift as film thickness is varied, and for 10 nm films the optimal window for illumination occurs between ∼500-650 nm, where the absorption is at a minimum. Conclusion
Figure 9. (A) Simulated roughness with delayed heating temperature profile and varying maximum temperatures: 1.5 kT0 (red), 2 kT0 (black), and 2.5 kT0 (blue). (B) Varied deposition rates: kdep (black), 3kdep (red), and 1/3kdep (blue). (C) Varied heating delays rates: 0.003kdelay (red), 0.002kdelay (black), and 0.001kdelay (blue).
ature will cause roughening, an increase in particle deposition rate can always make the surface rougher (Figure 9B) as previously demonstrated by Xiao et al.13 Figure 9C shows changes in the rms(t) profile as the temperature profile is stretched. As expected, faster heating causes a smaller initial peak because kinetic roughening is diminished but a quicker increase and leveling off due to thermal roughening. Thus relative heating and deposition rates can control at what thickness the minimum roughness occurs. For optical experiments that require conductivity, the desirable zone is d ≈ 10 ( 3 Å, as discussed next. For ultrathin metal films to be imaged by STM, they must be thick enough to be conductive, with a resistance smaller than that of the tunneling barrier (60% optical transmission and
