ICMCTF2016 Session C2: Novel Aspects in Thin Film Characterization and Data Modeling

Although ellipsometry is very sensitive to surface overlayers, such as films or surface roughness, it does not give useful information directly. In order to get properties such as the refractive index and thickness of a surface structure from ellipsometry measurements, the surface must first be modeled and parameterized, and then this model fit to the experimental data (including error limits) using a non-linear fitting routine such as Levenberg-Marquardt. This procedure becomes very difficult in situations where the surface structure is very complicated, such as etched surfaces where the fraction of voids is a function of depth. In this talk, I will discuss the modelling of this type of surface using the incomplete Beta function to parameterize the void fraction as a function of depth. This function is particularly useful because its domain and range are 0 to 1, and virtually any reasonable profile can be specified with 2 adjustable parameters.

Applications for metal-insulator-metal (MIM) tunnel junctions include magnetic components, capacitors, waveguides, and rectifiers. Other applications in energy-harvesting and infrared detection depend on the inherently fast (femtosecond) transport characteristics. Charge carrier tunneling takes place through the insulating film part of the MIM placing two primary demands upon it. The first is that it must be very thin, in the nanometer range, due to the physics of tunneling. The second is that it must meet specific compositional and microstructural requirements to result in repeatable tunneling characteristics which underlie electrical performance. Other concerns are the formation of interfacial layers and adhesion. Different thicknesses of NiO and NiO/ZnO bilayer insulating layers were deposited by both magnetron sputtering and atomic layer deposition. Films were examined by Transmission Electron Microscopy and Ellipsometry. The focus of the work presented here is the ellipsometric characterization of these insulating layers which are in-homogeneous in optical properties in the growth direction. For film thicknesses increasing from nominal 10 nm to 30 nm the film index of refraction (n) decreased from approximately 2.25 to 1.85 and at the same time the film extinction coefficient (k) increased from 0.375 to 0.70 at a wavelength of 601 nm. Measurements were made at 367 wavelengths between 400 and 950 nm using a Woollam M-2000 ellipsometer. Results will be presented across the entire wavelength range along with TEM images and electrical performance results.

Characterization of nanoscale metal layers has been a challenging task[1]. With prevailing methods, nanolaminates with alternating layers are even more difficult to measure. Surface Plasmon Resonance, an optical phenomenon, which has been more commonly utilized in biochemical interaction characterization, is extremely sensitive to thin layers having plasmonic properties. Thin metal layers and nanolaminates can be effectively characterized in terms of thickness and optical properties with Multi-Parametric Surface Plasmon Resonance (MP-SPR) instruments, which measure the intensity of the reflected light for a wide range of angles at multiple wavelengths.

Atomic Layer Deposition (ALD) provides precise control of film thickness down to atomic scale. ALD enables fabrication of high quality coatings. Furthermore, it is highly desired to provide industry and R&D with a low cost thickness measurement solution with good precision for metals and complex nanolaminates.

Pure glass substrates were coated with target thickness of 11 nm Pt, or with a nanolaminate of alternating Al₂O₃ and Pt 5 nm each. In the nanolaminate preparation, one sensor was removed for analysis after each consecutive deposition step. The sensors were coated in a PICOSUN™ R-150 ALD reactor.

The coated substrates were analyzed using BioNavis MP-SPR Navi™ 200 OTSO instrument by recording intensity of the reflected light for angles 38 to 78 degrees at 785 and 670 nm wavelengths. The SPR curves (angular spectra) were analyzed using BioNavis LayerSolver software [2].

The Pt metal layer was on average 11 nm thick, with deviation of less than 1 nm (N = 3). The nanolaminates were also characterized, and it was possible to determine the thickness of the individual layers. MP-SPR is able to distinguish between Pt/Al₂O₃ (10 nm each) and a nanolaminate of Pt/Al₂O₃/Pt/Al₂O₃ (5 nm each). While both surfaces have same overall thickness of 20 nm, each of them produces a distinct shape in the SPR curve.

The SPR proved to be an effective tool for the nanoscale metal layer characterization. Even small deviations in the metal layer thickness are easily characterized due to SPR sensitivity to the plasmonic material thickness. It was similarly possible to characterize the nanolaminate thicknesses at each step and after whole deposition which, to our best knowledge, is not possible with other methods. The new characterization method, together with advanced ALD deposition method, allows new applications and innovations in optics, electronics and photovoltaics.

References

1. Hilfiker, J.N., et al., Thin Solid Films, 2008

2. Sadowski, J.W. et al., Optical Engineering, 1995

We have been developing a smart technology to achieve Al-doped ZnO (AZO) polycrystalline films with a well-defined (0001) orientation. We propose very thin “critical layer (CL)” made from Ga-doped ZnO films deposited by ion plating (IP) with direct current arc discharge technique that extremely affects the growth orientation of AZO films deposited by direct current magnetron sputtering (DC-MS). We deposited 490-nm-thick AZO films on glass substrates (@200 ºC). Al₂O₃ contents in the sputtering target was 0.5 wt.%.

We clarify the effect of the CL on the growth orientation of AZO films. We examined the location of the center of gravity of the diffraction peaks observed in the out-of-plane XRD-Reciprocal Space Maps (RSMs). We f ound that the AZO films with 10-nm-thick CL exhibit ed the evolution of the orientation distribution. For AZO film without CL, the analysis of the data obtained by XRD-RSM showed the peaks of the 10-11, 20-21 and 30-32 reflections together with those of the 0002, 0004 and 0006 reflections. Note that the center of gravity of the peak of the 10-11, 20-21 and 30-32 reflections was observed to locate approximately on the vertical line in the RSMs. This proves that the (10-11), (20-21) and (30-32) planes of the AZO films lie approximately parallel to the substrate surface. On the other hand, AZO films with CL that consisted of columnar grains with nearly perfect c-axis orientation showed a single (0001) orientation, and exhibited a small FWHMω of the ω rocking curves for 0002 of 1.81º as a result .

Then, we demonstrated the effects of the CL on electrical properties of AZO films. The use of CL enhanced Hall mobility (μ_H) of AZO films: m_H of 38.7 cm²/Vs for CL-free AZO films and of 50.1 cm²/Vs for AZO films with the CL. In addition, we found a slightly increase in carrier concentration. To obtain a better understanding of the drastic change in μ_H attributed to the presence of the CL, we calculated the contribution of grain boundary (GB) scattering to carrier transport (μ_opt/μ_GB), which is defined as the ratio of optical mobility (μ_opt), corresponding to intragrain carrier mobility, to the carrier mobility at the GBs (μ_GB), on the basis of Matthiessenʼs rule. We obtained the following results: AZO films with CL showed a μ_opt/μ_GB of practically zero together with higher μ_opt of 51.0 cm²/Vs, whereas CL-free AZO films with lower μ_opt of 47.2 cm²/Vs exhibited a large μ_opt/μ_GB of 0.22, resulting in a reduced μ_H compared with μ_opt.