Combining X-ray Absorption Spectroscopy and Photoemission Electron Microscopy

Our studies of strained SiNMs with x-ray absorption spectroscopy (XAS) have made new detailed measurements of the Si band structures various valleys shifting and splitting under strain [1]. The technique focuses a monochromatic beam of x rays with a spot size tens of microns on a side onto the sample and measures the number of electrons kicked out of the sample by the incoming beam. Plotting the number of electrons that leave as a function of the x-ray energy coming in shows information about the conduction band structure. The technique is described elsewhere on this site.

Because we also work with local strain (i.e., creating strain differences across nearby regions of the sample) we want to make similar measurements over small length scales. to do this we turn to photoemission electron microscopy, or PEEM. X rays come in from the synchrotron and knock electrons out of the sample. But instead of measuring the electron current with a wire attached to the sample, we use the PEEM to collect the electrons leaving the sample and focus them into an image.

The particular instrument we use is the Spectromicroscope for the Photoelectron Imaging of Nanostructures with X-rays, or SPHINX. More information on the SPHINX is available in Frazer, et al.[2] or on Pupa Gilbert's group webpage.

 

Simple PEEM Schematic
Figure 1. Schematic of the way photoemission electron microscopy works at the SRC.

 

We record the PEEM image for each x-ray energy in our scan range and can plot the intensity of a pixel of set of pixels as a function of energy to see the XAS signal from that particular region of the sample. The graph below shows that the spectra from a strained membrane is shifted as we expect from the bulk Si substrate that it is resting on.

membrane vs. bulk, PEEM
    spectra
Figure 2. Spectra taken from a strained membrane region and a bulk Si region in the same movie.

 

We can even examine the XAS signal from each pixel on the scan and make a map of the conduction band edge position as a function of location on the sample, as shown below for a membrane stretched over a pillar.

 

spatial map
Figure 3. A PEEM movie is analyzed to produce a "map" of the conduction band minimum as a function of position.

 

Author: Clark Ritz

References:
[1] C. Euaruksakul et al., Physical Review Letters 101, 147403 (2008).
[2] B. Frazer, et al., Ultramicroscopy 99, 87-94 (2004)

 

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