Abstract

Analytical Method Once data has been collected for a sample, it is analyzed using the modeling program SIMNRA. After a model that accurately simulates the data is constructed, the results are compiled to illustrate what was learned. These reports, which are sent to our colleagues in India, focus on five main areas of interest: elements found in both the thin film and the substrate, unexpected elements, substrate roughness, film homogeneity and our conclusions.

Tandem Electrostatic Particle Accelerator A National Electrostatics Corporation PelletronTM Accelerator was utilized to accelerate He+ ions at 2.975 MeV on a sample in order to measure the stoichiometry and thickness of a thin semiconducting film via Rutherford Backscattering Spectroscopy. This non-destructive analytical technique is the analytical method of choice due to the ability to measure layer thickness and the high sensitivity for heavier elements. This study was done at the Hope College Ion Beam Analysis Laboratory (HIBAL).

Above is the Hope College Accelerator Laboratory

Rutherford Backscattering Spectroscopy Rutherford Backscattering Spectroscopy (RBS) is a powerful analytical technique capable of measuring the layer structure and composition of materials. When ions from the beam described above scatter off a nuclei in the sample, they lose a specific amount of energy depending on the mass of the target atom. They also lose energy as a result of the distance the particle travels through the sample before and after if backscatters. This is due to the Coulombic forces acting on the particle as it travels through the sample. This principle allows RBS to differentiate between layers. Scattered particles from the middle of the sample will appear at a lower energy than those of the same element on the surface due to the energy losses of the particles as they pass through the foremost layer. Once the particles are backscattered, a silicon surface detector registers the particles and transmits a voltage which corresponds to the energy of the backscattered particle through a series of amplifiers and pre-amplifiers. In an analog to digital converter the voltage is then converted into a number which is sorted into energy ranges (channels) by a computer. From here the data is uploaded into the modeling program SIMNRA for analysis.

The ion beam scatters from atoms in the solid sample..

Continuing Work • Constructing models of combined spectra from two separate points on the sample.

• Understanding thin film deposition techniques and their effects on layer homogeneity and thickness.

• Calculating the electrical properties of thin semiconducting films using the results from RBS analysis.

Acknowledgments

Analysis of Thin Semiconducting Films’ Thickness and Stoichiometry

Zachary J. Diener & Matthew P. Weiss

Mentors: Dr. Stephen K. Remillard, Dr. Paul A. DeYoung and R. Reena Philip

Department of Physics, Hope College, Holland, MI 49423

Energy-Dispersive X-Ray Spectroscopy

Background

Modeling & SIMNRA Analysis

Concluding Remarks

In order for our collaborators in India to determine the electrical properties of thin semiconducting films, the layer structures and corresponding chemical composition need to be characterized. • The layer structures and corresponding chemical composition reveal what events may have happened during a film’s fabrication such as clumping of elements on the surface of the film.

• Films were fabricated by three methods: Chemical Bath Deposition (CBD) for the SnS and ZnO groups, Vacuum Co-Evaporation (VCE) for the GS and AIGS groups, and Sputtering for the (InSn)O group.

• All deposition methods use glass or silicon substrates which function as a support because the films can be as thin as 1 micrometer.

The electrical properties of a semiconductor can only be determined if the sample’s thickness and stoichiometric makeup are known. The composition of thin films can be measured using Energy Dispersive X-ray Spectroscopy (EDS) in a Scanning Electron Microscope (SEM). However due to the low stopping power of electrons, EDS is limited to analysis of the surface. When compared, EDS results are complementary to those determined by Rutherford Backscattering Spectroscopy (RBS). RBS provides depth-sensitive compositional analysis due to the large stopping power of alpha particles compared to protons. Unlike EDS, RBS allows for the simultaneous analysis of both the stoichiometric makeup and thickness. Semiconducting thin films composed of AgIn1-xGaxSe2, CuGaxSe2 and Ag(InGa)5Se8 deposited on glass or silicon substrates through a variety of techniques were analyzed. Modeling of some samples was straightforward; however in other samples the modeling was complicated due to various inhomogeneities.

Layer Differentiation of Al2O3 & Al2O3 Au Film

Due to the energy loss of particles as they pass through the gold film the Al peak begins at a lower energy. This energy loss ~25KeV correlates to the thickness of the Au Film

Original energy of Al peak

Here is a model for the thin film (InSn)O. From this fit, stoichiometric ratios, elemental layering and the thicknesses of the layers can be determined (~3.74 µm)

The table above shows the percentage of the atoms for each element that exist in the samples. Results from four thin films are presented, highlighting only a fraction of the large quantity of data collected. The elements highlighted in blue are the elements making up the substrates, while the elements highlighted in green are the elements making up the films. These results from EDS are more sensitive to concentrations of lighter atoms than RBS and are utilized to speed up the process of modeling spectra.

Non-uniformities pertaining to the layer thickness and composition of the film greatly complicate linear thickness calculations, as do un-anticipated layers, elemental clumping, and oxidation.

• Stephen Remillard • Paul DeYoung • Dave Daughtrey This work is supported by the National Science Foundation under grant no. PHY-0969058 and Hope College Division of Natural and Applied Sciences.

• Hope Physics Department • Nuclear Group • Microwave Group

When films were found to be homogeneous in layer composition and had the expected layer structure, linear thickness was quite easy to calculate. (InSn)O was one such film. As stated above, non-uniformities cause the calculation to become much more complicated. When a film was found to contain non-uniformities the results was sent to the film makers so they can adjust the fabrication method to achieve the desired result.

Film O Si Na Ca Mg Al K Ag In Sn (Tin) Ga Se Cu Sr Zn

ZnOSr 55.53 22.81 9.86 2.58 1.75 0.61 0.78 --- --- --- --- --- --- 0.17 5.91

GaS 53.4 25.37 8.77 2.85 1.88 0.71 0.71 --- --- --- 6.23 0.09 --- --- ---

AIGS III 3.94 86.6 --- --- --- --- --- 1.41 1.26 --- 4.28 2.51 --- --- ---

(InSn)O 60.26 10.19 1.31 1.5 0.8 0.54 --- --- 22.21 3.18 --- --- --- --- ---