Precise measurement and characterization of electrostatic potential structures and the concomitant electric fields at nanodimensions are essential to understand and control the properties of modern materials and devices. However, directly observing and measuring such local electric field information is still a major challenge in microscopy. Here, differential phase contrast imaging in scanning transmission electron microscopy with segmented type detector is used to image a p-n junction in a GaAs compound semiconductor. Differential phase contrast imaging is able to both clearly visualize and quantify the projected, built-in electric field in the p-n junction. The technique is further shown capable of sensitively detecting the electric field variations due to dopant concentration steps within both p-type and n-type regions. Through live differential phase contrast imaging, this technique can potentially be used to image the electromagnetic field structure of new materials and devices even under working conditions.
Phase-shifting electron holography and Lorentz microscopy were used to map dopant distributions in GaAs compound semiconductors with step-like dopant concentration. Transmission electron microscope specimens were prepared using a triple beam focused ion beam (FIB) system, which combines a Ga ion beam, a scanning electron microscope, and an Ar ion beam to remove the FIB damaged layers. The p-n junctions were clearly observed in both under-focused and over-focused Lorentz microscopy images. A phase image was obtained by using a phase-shifting reconstruction method to simultaneously achieve high sensitivity and high spatial resolution. Differences in dopant concentrations between 1 × 10(19) cm(-3) and 1 × 10(18) cm(-3) regions were clearly observed by using phase-shifting electron holography. We also interpreted phase profiles quantitatively by considering inactive layers induced by ion implantation during the FIB process. The thickness of an inactive layer at different dopant concentration area can be measured from the phase image.
A novel method with quaternary ammonium salts as internal additives has been applied to the mass-scale calibration of timeof-flight secondary ion mass spectrometry (TOF-SIMS). Five kinds of quaternary ammonium salts, octyltrimethylammonium bromide (C8TMA), tetradecyltrimethylammonium chloride (C14TMA), octadecyltrimethylammonium chloride (C18TMA), cetylpyridinium chloride (CPC) and benzyldimethyltetradecylammonium chloride (Bzl) were diluted with NH 3 aqueous solution and were mixed in equal quantities. The mixed solution was covered on a Si wafer and was measured by TOF-SIMS. When molecular ions of C8TMA, C14TMA, and C18TMA were used for the mass-scale calibration, the relative mass accuracy of lowmass C X H Y fragment ions indicated negative values and depended on the carbon chain length and the degree of unsaturation. This result means that the conventional mass-scale calibration with frequently employed low-mass C X H Y fragment ions leads to degradation of the mass accuracy of high-mass molecular ions. To improve the mass accuracy of molecular ions of CPC and Bzl, ions should be selected for the mass-scale calibration in the following two ways: One way is for ions to consist only of molecular ions of C8TMA, C14TMA, and C18TMA, and the other way is for ions to include one of the molecular ions: C8TMA, C14TMA, and C18TMA, along with the C X H Y fragment ions. This novel mass-scale calibration method is promising for realizing the correct identification of unknown high-mass molecular ions. Those quaternary ammonium salts are powerful candidates of internal additives.
To evaluate the present errors in the calibration of the mass scale of time-of-flight secondary ion mass spectrometry (ToF-SIMS) in the practical field of Japanese industries, ToF-SIMS Working Group (WG) of Surface Analysis Society of Japan (SASJ) conducted the inter-laboratory studies among 14 instruments in 2007, 2008 and 2009. There was significant scatter over 200 ppm in the relative mass accuracy at the first inter-laboratory test in 2007. Clearly, this is poor compared with the requirement for identification. However, the great deviation in the mass accuracy has been dramatically decreased at the second and third inter-laboratory tests by the accumulating knowledge through the discussion among the WG. Based on the results obtained, a practical guide to analysts for mass scale calibration will be provided in near future.
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