Synchrotron Radiation Center

The MAXIMUM scanning X-ray photoemission microscope at Aladdin has achieved a major breakthrough in the study of semiconductor electronic structure. The band-offset in a semiconductor multilayer has been directly imaged using spatially resolved core-level microscopy, applied to a sample prepared in cross-section.

Photoemission spectroscopy is widely used to study the effects of energy band offsets at the interface between two materials. In the past, interface electronic structure has been studied by depositing a film on top of a substrate, and attempting to view the interface through the intervening overlayer material. Due to the high sensitivity of the photoemission process, the electronic properties of the interface become "buried" in the spectrum from the overlayer. The latest results from the MAXIMUM project show that cross-sectional photoelectron microscopy can overcome this limitation. This opens up the possibility of studying interfaces in practical electronic device structures, as well as more basic studies of interfaces between bulk or thick-film materials.

MAXIMUM, which stands for "Multiple-application X-ray Imaging Microscope and Undulator Monochromator," is the first spectro-microscope to use a multilayer coated Schwartzchild objective to focus the photon beam to a spot smaller than 0.1 micron. It is the result of a collaboration between the Center for X-ray Lithography, LBL, University of Minnesota, Xerox, and SRC. The recent cross-sectional microscopy experiments were performed by a team including Waiman Ng (a graduate student at the University of Wisconsin), working with researchers from the University of Minnesota.

In these experiments, samples of doped GaAs superlattices were grown by molecular beam epitaxy on n-type GaAs substrates. The final sample consisted of a GaAs wafer, 12 layers of alternating p-n junctions of doped GaAs, and a capping layer. The wafer is cleaved in vacuum, and then viewed along a cleaved edge, which exposes the buried p-n junction layers.

 

At the junction between the p and n-type GaAs, the Fermi levels are aligned, which forces an offset of the valence bands on the two sides of the junction. This band offset creates the p-n junction. At the same time, the band offset produces a rigid shift in the binding energy of the Ga 3d levels, so that the energy position of these core levels can be used to image the band offset.

In a typical experiment, the electron spectrometer is set to collect electrons at 70.3 eV, corresponding to the n-type Ga core level. Scanning the X-ray beam over the cross-sectioned edge of the sample, the intensity of Ga electrons maps out the regions of n-type material. The contrast in the image can be reversed by selecting 71.8 electrons, corresponding to p-type Ga core levels.

Describing the impact of this work, Waiman Ng pointed out that "this is the first time that a p-n junction has been imaged by photoemission. In a microelectronic device, the interfaces are often several microns below the surface, with feature sizes at and below 1 micron. We can now begin to study such structures."