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We have demonstrated the nanopatterning of atomic layer deposited (ALD) Al:ZnO (AZO) films using electron beam lithography (EBL) for plasmonic waveguide applications. The influence of grains on repeatable planar nanostructures by nanolithography process was studied for annealed films in order to avoid effects of granularity. Our results demonstrate that the nanopatterning of AZO by the EBL technique is limited due to granularity of ALD grown AZO films. This finding suggests the limitations of ALD grown samples for optical applications where nanopatterns are fabricated by the EBL technique. Furthermore, the ALD grown films lose conductivity orders of magnitude on annealing.
©2012 Optical Society of America
1. Introduction
Al:ZnO has the potential to be utilized for several different optoelectronic and photonic applications. Currently there is a lot of semiconductor and plasmonic research being produced because of specific characteristics, such as transparency in the visible range, and conductive states which is a function of dopant concentration. Transparent conducting oxide (TCO) materials are emerging as novel metamaterials [1], apart from its usual demand as electrode materials in various optoelectronic devices. It is known that optical losses due to interband transitions are highly undesirable and severely limit potential applications of plasmonic metals, like Ag and Au. However, it has been recently found that wide band gap semiconductor such as Ga or Al doped ZnO (AZO) and other TCOs are potential candidates to replace silver and gold in plasmonic applications [1–4]. We have recently demonstrated that the doped degenerate wide-band-gap semiconductors can be efficient nanoplasmonic as well as plasmonic photovoltaic materials in the near-infrared spectral range because of strong confinement of surface plasmon polaritons (SPPs), low loss and metallic behavior [2,5,6]. Because semiconductors do not have bound-state absorption transitions in their band-gaps, their SPP losses can be even smaller in the near infrared (NIR) region than those in conventional plasmonic materials at corresponding visible wavelengths [7], and hence attracts a lot of attention for NIR plasmonic waveguide for telecom and transformative optics applications. However, grain structure in ZnO films and their influence on the nanopatterning of the films for waveguide applications are not studied yet.
In this report, we investigate the nanopatterning of atomic layer deposited (ALD) AZO films using electron beam lithography (EBL) for potential plasmonic waveguide applications. We have annealed the ALD grown samples at several temperatures to control the grains in order to study their influence on planar nanostructures with a repeatable nanolithography process. Our results demonstrate that the nanopatterning of AZO by the EBL technique is limited and controlled by the granularity of ALD grown AZO films, and could be improved by post annealing. In contrast, the films lose their conductivity (essential for plasmonic waveguides) on annealing.
2. Experimental details
AZO films were grown via alternate deposition through ALD cycles of diethyl Zinc (DEZ), H2O and trimethylaluminum (TMA). Al:ZnO (1:20) 100 nm was deposited on sapphire substrates using ALD (Cambridge Savannah S100). The growth temperature was kept at 200 °C, with a flow rate of 20 sccm of N2 gas. All samples were annealed in ambient atmospheric at different temperatures, 300 °C, 400 °C, 500 °C, and 600 °C. The Hall effect measurements were performed using a four-probe technique under a 4T magnetic field.
After annealing and cooling at room temperature, each sample was spincoated with 2% 996K PMMA diluted in chlorobenzene for 40 seconds at 6000 rpm. This resulted in a resist thickness around 50 nm. High-resolution lines, gratings, and Fresnel lens structures were exposed on the spin coated AZO (using EBL, Raith Pioneer). The Beam energy was 10 kV and working distance was kept at 10 mm. The aperture was 20 µm and the beam current was measured to be 0.126 nA. The base dose for lines, areas and curves elements were 700 pC/cm, 300 μC/cm2 and 300 pC/cm, respectively. All exposures were developed in MiBK: isopropyl alcohol (1:3) for 1 minute, and dipped in isopropyl alcohol (IPA) for 30 seconds and dried in N2. Once exposed, PMMA can be used as a masking layer in order to transfer the patterns into the AZO film. AZO can be etched by a variety of different methods, including reactive ion etching (RIE) and wet chemical etching. Several different wet chemical etchants for AZO have been studied such as HF and NH4Cl. The parameters that influence the etch rate are film temperature, concentration of etchant in the solution and film granularity. In the present experiment, we patterned PMMA which is coated on AZO film.
Atomic layer deposition is recently used to attain conformal films with a high degree of thickness control. The sequential exposure of reactants used in ALD [8,9] adjusts the composition of multicomponent materials by changing the number of cycles used for each precursor materials and deposits pinhole-free films on a variety of surfaces [10,11]. There is a need to control the grain structure and crystal orientations in order to fabricate low loss optical waveguide devices. This can be achieved by the ALD technique.
3. Results and discussion
The X-ray diffraction (XRD) patterns of the AZO films are shown in Fig. 1 . All films show polycrystalline nature even after the films are post-annealed up to 600 °C. However the diffraction peak (002) observed at 2θ = 34° for (002) orientation sharply increased with increasing annealing temperature demonstrating the enhanced crystallinity. All films show the wurzite structure and hexagonal phase of ZnO. The XRD graphs show the (002) peak along with (100) at 2θ ~32° and (101) orientations at 2θ ~37°. It is noted that no additional peaks were obverted on annealing the films up to 600 °C.
Fig. 1 XRD patterns of as-grown films (200 °C), and subsequently annealed at 300, 400, 500 and 600 °C of ALD grown AZO films.
Al:ZnO (1:20) films were grown via alternate deposition of diethyl Zinc (DEZ) of ALD cycles, H2O and ALD cycle of trimethylamine (TMA) at 200 °C on either pre-cleaned sapphire or glass substrates as shown in Fig. 2(a) . The films were found to be optically very smooth without having any microscopic defects. The AZO films are typically 80-100 nm thick. The films are atomically flat surfaces with root-mean-square (rms) value of surface roughness around 1 nm for as grown as well as for annealed films at various temperatures as shown in Figs. 2(b) to 2(e). The grain structure did not demonstrate any significant difference with increasing annealing temperature. The deduced surface roughness Vs. annealing temperature is shown in Fig. 2(f).
Fig. 2 (a) Pulse sequence used for one macro-cycle of Al:ZnO films by the ALD technique, and 3-diemesional AFM images of (b) as grown films at 200 °C, annealed films at (c) 400 °C, (d) 500 °C (e) 600 °C, and (f) shows the surface roughness of the film as a function of annealing temperature . The scale for the AFM images is 600 nm × 600 nm.
Figure 3(a) shows the FE-SEM image of the surface of the as grown films. The grains are mixtures of both elliptical as well as spherical in shapes. The surface roughness did not show any significant change as shown in Fig. 2(f) while the field emission scanning electron microscopic (FE-SEM) images show a clear coalescence of grains with increasing annealing temperature as shown in Figs. 3(b) to 3(d) at 600 °C,
Fig. 3 FE-SEM images of (a) as grown films at 200 °C, annealed films at (b) 400 °C, (c) 500 °C and (d) 600 °C. The scale bar for the SEM images is 200 nm.
Figure 4 shows the EBL patterns of periodic gratings in (a) as-grown films (200 °C), and subsequently annealed at (b) 400, (c) 500 and (d) 600 °C of ALD grown AZO films. It is anticipated that the surface roughness as well as line edge roughness LER) of AZO films will be reduced after post annealing, facilitating waveguide applications. Both surface and line roughness of optical waveguides are crucial for maintaining the % of efficiency, which is directly related to the scattering too.
Fig. 4 (a) EBL patterns of periodic gratings in (a) as-grown films (200 °C), and subsequently annealed at (b) 400, (c) 500 and (d) 600 °C of ALD grown AZO films.
It is interesting to note that the surface roughness remains around 1 nm as shown in the inset of Fig. 4(a), irrespective of annealing up to 600 °C. The surface roughness due to the grain structure is also visible in the gratings, Figs. 4(a) to 4(d). The gratings are well separated with pitch 200 nm and line width of 100 nm for (a) and (b) as shown it the inset, and the pitch is 100 nm for (c) and (d) . The central part of the current result is the line roughness of the gratings, which is limited by the grain size of the films. The grain size increases from about 30 nm for as-grown AZO film at 200 °C to about 50 nm on annealing at 600 °C. The waveguide gratings for as-grown films at 200 °C, and annealed at different temperatures (shown in Fig. 4) shows the fact that the line-edge roughness increases with the increase of annealing temperature. This is a deleterious effect for the waveguides due to power loss at the lines.
The above fact is further illustrated in Fig. 5 for the geometry or Fresnel rings fabricated by EBL for (a) as-grown films at 200 °C, and annealed at (b) 400, (c) 500 and (d) 600 °C of ALD grown AZO films. LER was measured from the SEM images. The LER is slightly decreased from as films were annealed until 500 °C as shown in the inset in Fig. 5(b). This is due to the improved crystallinity of the film and coalescence of grains. Because there are fewer grain boundaries, the probability of backscattering due to inelastic collisions is reduced. Backscattering may result in exposures in neighboring regions, which will distort features and produce roughness along the boundaries of the exposures. Annealing the films at high temperature resulted in a large increase in resistivity (see Fig. 6 ), orders of magnitude higher than the as grown sample. Because the sample became very insulating, charge accumulation took place during exposures. Charging of the substrate results in repulsive forces between the film surface and electron beam. Subsequently the roughness of the edges of the exposures increased for AZO films annealed at 600 °C. The edges of the Fresnel rings show irregularity and distinct granularity, which can drastically reduce the efficiency.
Fig. 5 EBL patterns of Fresnel rings (a) as-grown films at 200 °C, and annealed at (b) 400, (c) 500 and (d) 600 °C of ALD grown AZO films.
Fig. 6 The resistivity and optical band gap of ALD grown films as a function of annealing temperature.
Figure 6 shows the effect of annealing on resistivity and optical band gap of ALD grown films. The resistivity decreases as annealing temperature increases for AZO films. The resistivity remains the lowest at 7.9 × 10−4 Ω-cm for the as-grown AZO films. However, the resistivity exponentially decreases up to 0.82 Ω-cm after annealing at 500 °C. This value is orders of magnitude higher than that of the as-grown films grown at 200 °C. In a high absorption region, Tauc [12] and Davis and Mott [13] showed that the absorption coefficient and photon energy are related by the following equation:
where A is a constant, Eg is the band gap of the material and n has different values depending on the optical absorption process. It was found that n = 1/2 is the best fit to our results, and is characteristic of the direct band absorption without phonon mediation. The optical band gap was determined from the plots of (αhν)2 versus hν plot. It is interesting to note that the optical energy band gap (Eg) decreases with increasing annealing temperature (Fig. 5). The band gap reduction at higher anneal temperature may be due to the sub-band gap defect produced due to the formation of Al2O3 clusters in the band. However, neither the stress induced distortion in the band nor the grain boundary scattering is responsible for increased resistivity by the annealing process.Alternately, the energy band gap broadening is related to the carrier concentration (ne) through the equation:
where h is Planck’s constant and m0* is the electron effective mass in conduction band. For ZnO, the reported effective masses of electrons and holes are 0.28 m0 and 0.59 m0, respectively [14], n is determined to be 2 × 1020 cm−3 for as grown AZO at room temperature using 4T applied magnetic field. However, the carrier concentration significantly decreases to ~8 × 1017 cm−3 with post annealing at 600 °C of ALD grown AZO films. We consider here the plasma frequency, ωpl predicted by the Drude theory as ω2pl = ne2/ε0m0*, where ε0 is the permittivity of free space, e is the electron charge. The density of free charges ne is directly proportional to the ωpl. In view of the above, the annealed films should have diminishing or no ωpl due to reduced carrier concentration as described above. Formation of AZO films via ALD allows easily and continuously adjustable electron density and, hence, free-electron plasma frequency. At a high annealing temperature oxygen might be absorbed by reaction with Al, forming Al2O3 complex and decreasing the resistivity of AZO film. This is an opposite case in which the filling of conduction band by electrons in n-type ZnO semiconductor with H2 incorporation [15]. Considering the number of Al atoms deposited using TMA during one ALD cycle on ZnO terminated surface is ~3 × 1014 cm−2, which indicates that the average spacing between Al atoms is ~5.5 A°. In contrast, the spacing of Al atoms in AZO films deposited by other techniques using bulk target is about 1 nm with a doping concentration of the order of 1021 cm−3. Hence, it is expected that the ALD grown AZO films would be energetically favorable to form stoichiometric Al2O3 clusters. These defects are the trapping centers for electrons for excessive dopant concentration, and results in decrease in the carrier concentration, hence higher resistivity in AZO films, and consisted with previous observation on clusters in ZnO [16,17].4. Conclusion
In summary, we have demonstrated the nanopatterning of atomic layer deposited Al:ZnO films using electron beam lithography for plasmonic waveguide applications. The influence of grains on repeatable planar nanostructures by nanolithography process was studied for annealed films in order to avoid effects of granularity, which is one of the major reasons of loss in the waveguides. Our results demonstrate that the nanopatterning of AZO by the EBL technique is limited due to granularity of ALD grown AZO films. This finding suggests the limitations of ALD grown samples for optical applications where nanopatterns are fabricated by the EBL technique. Furthermore, the ALD grown films lose conductivity orders of magnitude on annealing. This research may open up venues for various optical, opto-electronic and plasmonic applications.
Acknowledgments
This work is supported by the DoD (CEAND) Grant Numbers W911NF-11-1-0209 and W911NF-11-1-0133 (US Army Research Office), NSF-CREST (CNBMD) Grant number HRD 1036494, NSF-RISE Grant number HRD-0931373, and NSR-MRI.
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