The open counter is an electron counter invented in 1979 by Dr. Masayuki Uda. It lies at the heart of the AC Series. The papers shown here consist of an original paper describing the open counter published in 1981 after the patent application [1] and a paper published in 1985 describing the first practical measurements of work function, ionization potential, and film thickness using the open counter [2].

[1] H. Kirihata, M. Uda, “Externally quenched air counter for low-energy electron emission measurement”, Rev. Sci. Instrum, 52, 68 (1981);
https://doi.org/10.1063/1.1136448

[2] M. Uda, “Open Counter for Low Energy Electron Detection”, Jpn. J.Appl.Phys. 24,284 (1985);
https://iopscience.iop.org/article/10.7567/JJAPS.24S4.284/meta#

Feel free to reference these when writing papers using AC Series measurement data. A number of related papers are also listed here. Feel free to click on and review the individual articles. Note that the following doctoral theses are available at the Waseda University repository: They should be used together as a general overview in Japanese.

[3] Y. Nakajima, “Production of Two Types of Open Counters and Their Application to Industry” (2004)
https://waseda.repo.nii.ac.jp/?action=repository_action_common_download&item_id=20573&item_no=1&attribute_id=20&file_no=3

[4] D. Yamashita, “Study on surface analysis using improved photoemission yield spectrometer in air” (2015)
https://waseda.repo.nii.ac.jp/?action=repository_action_common_download&item_id=16919&item_no=1&attribute_id=20&file_no=1

The open counter^*1 is an ultra-sensitive electron detecting device invented in 1979 by Dr. Uda and Dr. Kirihata. It is capable of counting individual electrons within the atmosphere [1], [2]. Dr. Uda and his colleagues combined this open counter with a spectral ultraviolet irradiation optical system to perform various measurements using a technique later called photoemission yield spectroscopy in air^*2. Since 1986, RIKEN KEIKI has manufactured and sold atmospheric photoemission yield spectrometers incorporating this open counter as models in the AC Series.

Fig. 1: The prototype open counter built by Dr. Uda and Dr. Kirihata [3]

Photoemission yield spectroscopy is a technique for analyzing sample surfaces based on the relationship between the number of photoelectrons emitted and the ultraviolet light used to irradiate the surfaces. Photoelectrons are normally counted using an electron multiplier. The electron multiplier is capable of counting individual electrons used in photoelectron spectroscopy like XPS or UPS. However, this can operate only in a vacuum. This means photoemission yield spectroscopy using an electron multiplier requires the sample to be placed in a vacuum for measurement.

Ammeters were also used to measure the photoelectrons. Ammeters can operate in both atmospheric conditions and in a vacuum. However, this technique offers low sensitivity and requires a large volume of electrons to be emitted from the sample. The most sensitive ammeters currently available, for example, detect currents down to a minimum of 1 fA (femtoampere, 10^-15 A). 1 fA represents an extremely small current. However, if we convert this to the number of electrons flowing, it is equivalent to 6,242 electrons per second^*3. In other words, photoemission yield spectrometers that measure electrons using an ammeter require at least 6,242 electrons per second to be emitted from the sample surface to detect electrons. Extracting large numbers of electrons from the sample surface for the purpose of measurement raises the risk of changing the sample surface. A procedure that alters the sample surface during measurement not only makes it impossible to evaluate repeatability, but raises doubts about the validity of the measurement results obtained.

Partly for these reasons, photoemission yield spectroscopy in air did not really take off until the invention of the open counter and the launch of the AC Series. The invention of the open counter led to commercially available products that allow photoemission yield spectroscopy in air. Photoemission yield spectroscopy is now widely used for measuring work function, ionization potential, and ultra-thin surface film thickness, even by materials and device researchers and engineers with no special training in photoelectron measurement.

[5] A. Koyama, M. Kawai, H. Zenba, Y. Nakajima, A. Yoneda, and M. Uda, “Electron counting by a double cylindrical open counter in mixtures of N2 and inert gases of various concentrations” Nucl. Instr. and Meth. in Phys. Res. A422 (1999) 309. ; https://www.sciencedirect.com/science/article/abs/pii/S0168900298009644

The papers [1] and [2] describe the results of measuring photoelectrons emitted from aluminum. The paper discusses the work function by creating a graph (photoelectron yield spectrum) plotting the photon energy of the irradiated ultraviolet rays against the photoemission and using this to determine the photoemission threshold energy.

Fig. 2: Photoemission behavior from an aluminum plate [1]

This shows that the work function for a scratched aluminum surface is smaller than that for an aluminum surface left to stand in the atmosphere. These results are measured in greater detail and reported on later [6]. Paper [6] suggests that the work function decreases due to changes to the lattice spacing of the aluminum during the initial oxidation process. In the paper, the authors discuss the changes in the local density of states (LDOS) of the aluminum surface exposed to the air estimated experimentally from the photoelectron yield spectrum compared to the results of molecular orbit calculations.

Fig. 3: Changes in local density of states (LDOS) of an aluminum surface exposed to the air [6]

Note that the phenomenon in which the work function of a scratched surface changes can be observed using a Kelvin probe. It has also been reported to occur in zinc and cadmium in addition to aluminum [7].

Fig. 4: Changes in work function of a metal with a scratched surface [7]. The vertical axis represents the work function. The horizontal axis represents the time elapsed after scratching. □ indicates measurements obtained with photoemission yield spectroscopy ● indicates measurements obtained with a Kelvin probe.

These measurement and analytical techniques have contributed immensely to the development of organic materials. They are widely used in measuring the ionization potential of materials such as organic light-emitting diode materials and Perovskite solar cell materials. In papers [8] and [9], the authors estimate ionization potential and density of states, and compare these against molecular orbit calculations.

Fig. 5: Relationship between ZnII-TPP irradiated UV energy and the square root of photoelectron yield [8]

Fig. 6: TPP density of states (DOS) obtained from photoelectron spectra [8]

Fig. 7: Comparison of phthalocyanine density of states obtained from photoelectron spectra (upper) against results calculated using DV-Xa method (lower) [9]

[6] M. Uda, Y. Nakagawa, T. Yamamoto, M. Kawasaki, A. Nakamura, T. Saito, and K. Hirose, J. Electron. Spectrosc. and Related Phenom. 88-91, 767 (1998).
https://doi.org/10.1016/S0368-2048(97)00237-5

[7] Y. Nakajima, T. Ishiji, N. Nakano, and M. Uda, “Serial Measurements of the Work Function of Metals Exposed to Air Using a Contact Potential Difference Method”, Journal of the Surface Finishing Society of Japan, 51, 861 (2000).
https://doi.org/10.4139/sfj.51.861

[8] Y. Nakajima, M. Hoshino, D. Yamashita, and M. Uda “Near Edge Structures of Tetraphenylporphyrins Measured by PESA and Calculated with DV-Xα” Adv. Quantum Chem. 42 (2003) 399.
https://doi.org/10.1016/S0065-3276(03)42063-7< />>

[9] D. Yamashita, Y. Nakajima, A. Ishizaki, and M. Uda, “Photoelectron spectrometer equipped with open counter for electric structures of organic materials”, J. Surf. Anal. 14,433 (2008);
http://www.sasj.jp/JSA/CONTENTS/vol.14_4/Vol.14%20No.4/Vol.14%20No.4%20433-436.pdf

There are numerous techniques for measuring the thickness of thin films. But for thin films with thicknesses on the order of nanometers (nm; 10^-9m), only photoelectron spectroscopy techniques like XPS are effective. However, XPS requires that the sample be placed in a vacuum. Applications in the field of product inspection require a simpler technique in terms of time and effort. Photoemission yield spectroscopy using an open counter meets this need. The low kinetic energy photoelectrons counted using photoemission yield spectroscopy have lower penetrating power compared to light or radiation. The escape depth is shallow—only several tens of nm. The number passing through decreases as the thickness of the film increases. This allows use in measurements of ultra-thin films measuring only several tens of nm in thickness. Paper [2] describes applications in measuring the thickness of a thermal oxidation film on a silicon wafer surface.

Fig. 8: Relationship between count rate and the thickness of a thermal oxidation film on a silicon wafer surface [2]

The horizontal axis represents oxidation film thickness measured using XPS or ellipsometry, and the vertical axis represents the log of the count rate, giving a linear graph. This graph can be used as a calibration curve to allow fast, easy measurements of the thickness of oxidation films, even on the order of nanometers, on a silicon wafer surface. This technique can also be used for measuring the thickness of surface lubricating film on video tape.