Low Energy Inverse Photoelectron Spectroscopy

Low Energy Inverse Photoelectron Spectroscopy

LEIPS NEW

KEY FEATURES of Low Energy Inverse Photoelectron Spectroscopy (LEIPS)*

  • Obtain information on the unoccupied electronic energy levels

  • Measurement of electron affinity of the sample

  • Low damage analysis of organic materials

  • Measurement at same location of XPS and UPS

 


Principle of LEIPS

An energy diagram of a semiconductor sample measurement by LEIPS and UPS is shown in Figure 1. UPS provides information about the occupied energy levels of the valence band by analyzing the photoelectrons (e-) emitted from the sample during irradiation by ultraviolet light (hν). LEIPS reverses the role of light and electrons as it measures the near-ultraviolet light (hν) emitted from a sample during electron (e-) bombardment. LEIPS provides information about the unoccupied levels of the conduction band and enables direct measurement of the electron affinity of the sample.

Figure 1. UPS and LEIPS energy diagram

Figure 1. UPS and LEIPS energy diagram

 

Features of ULVAC-PHI LEIPS

Inverse photoelectron spectroscopy (IPES) measures light generated by irradiating a sample with electrons. LEIPS is very different from conventional IPES and measures near ultraviolet light using low energy electrons of 5 eV or less.  For this reason, electron beam damage to organic samples is dramatically reduced.  On the PHI 5000 VersaProbe III, the low energy electron beam is set to a fixed energy and the sample bias is adjusted to sweep the net energy of the bombarding electrons.  Generated ultraviolet light of 5 eV or less is selected by a band pass filter on the atmosphere side of the belljar and detected with a photomultiplier tube.  A plot is then generated of the photon signal intensity (at a fixed energy band pass filter on the detector) vs. net kinetic energy of the bombarding electrons.  Since the photodetector is located outside the belljar, it is easy to select the light energy by replacing the band pass filter.  The low-energy electron gun can also be used as a neutralization gun on the VersaProbe III. 

Figure 2. Schematic of LEIPS analysis setup

Figure 2. Schematic of LEIPS analysis setup 

 

Measurement of electron affinity by LEIPS

In LEIPS, the energy (hν) of the detected photons can be changed by changing the band pass filter of the photodetector. When plotting the rising (onset) energy against the band pass energy for each filter, there is a linear correlation, and extrapolation of the line to zero kinetic energy of the bombarding electron yields the electron affinity, as shown in Figure 3. Obtaining the electron affinity is difficult with conventional IPES.

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Figure 3. (right) Plot of the rising (onset) energy of the LEIPS spectrum vs. the band pass filter energy.  Extrapolating to zero onset energy yields the electron affinity.

Figure 3. (left) LEIPS spectrum of copper phthalocyanine thin film sample (10 nm film on ITO) measured with 3 different band pass filters (BPF)
(right) Plot of the rising (onset) energy of the LEIPS spectrum vs. the band pass filter energy.  Extrapolating to zero onset energy yields the electron affinity.

Sample provided by: Organic Optoelectronics Practical Development Center(i3-opera)

 

Low damage analysis of organic materials

A comparison of the degree of electron beam damage on a thin film C60 sample using LEIPS electron energies and conventional IPES energies is shown in Figure 4. When the sample was irradiated with 10 eV electrons, equivalent to conventional IPES, the spectral shape changed, indicating that electronic damage had occurred. On the other hand, in the LEIPS measurement using 5 eV of electron energy, there is no change in the spectrum after extensive measurement time, suggesting that no sample damage has occurred.

 

LEIPSFigure 4. LEIPS spectrum of a C60 thin film sample measured at an electron energy of: (left) less than 5 eV with LEIPS.

Conventional IPESFigure 4. LEIPS spectrum of a C60 thin film sample measured at an electron energy of: (right) 10 eV, similar to conventional IPES.

Figure 4. LEIPS spectrum of a C60 thin film sample measured at an electron energy of: (left) less than 5 eV with LEIPS, (right) 10 eV, similar to conventional IPES

Sample provided by: Organic Optoelectronics Practical Development Center(i3-opera)

 

Analysis example

Measurement of both the occupied and unoccupied electronic energy levels of a sample

From the energy of the occupied (valence) level obtained by UPS and the energy of the unoccupied (conduction) level obtained by LEIPS, the whole picture of the band structure can be understood (Figure 5).  By combining these two complementary techniques, LEIPS and UPS, it is possible to know the level of both electrons and vacancies in a semiconductor sample. In addition, the ionization potential can be obtained from the highest occupied molecular orbital (HOMO) of the UPS measurement, and electron affinity can be obtained from the lowest unoccupied molecular orbital (LUMO) of the LEIPS measurement. From the difference in those two values, the semiconductor band gap energy can be calculated.

Figure 5. LEIPS and UPS spectra of copper phthalocyanine thin film sample

Figure 5. LEIPS and UPS spectra of copper phthalocyanine thin film sample

 

Measurement at the same location for LEIPS, XPS, and UPS

In the VersaProbe III, LEIPS, UPS, Argon and GCIB ion beams, and neutralization beams are all aligned to the XPS measurement position.  This allows for comprehensive evaluation of organic semiconductor materials.

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Figure 6. Schematic diagram around sample

Comparison Table

Item ULVAC-PHI LEIPS Conventional inverse photoemission spectroscopy
Measurement position of sample Same location as XPS, UPS

Ar gun, GCIB gun can also irradiate the same position
Different position from XPS, UPS

Sample transport required before and after measurement
Energy of electrons 5 eV or less

Low damage to organic materials
~ 10 eV

Increased damage to organic materials
Energy selection Can exchange band pass filter outside the belljar Band pass filter is in ultra-high vacuum chamber and difficult to replace
Energy resolution 0.45 eV or less ~ 0.6 eV