Accurate determination of the voltage of a transmission electron microscope (TEM) by 〈0 1 2〉 CBED-HOLZ analyses using GaAs crystal

Abstract

Convergent beam electron diffraction (CBED) is a powerful technique to estimate lattice distortion and lattice strain in crystals. The positions of the higher-order Laue-zone (HOLZ) lines in the transmitted disc of CBED patterns are very sensitive to the lattice parameter, and can therefore be used to estimate changes in the lattice parameter. This offers the possibility to calculate lattice misfit and lattice strain. The positions of the HOLZ lines depend not only on the lattice parameter, but also on the operating voltage of the microscope. It is essential to know the actual voltage of the microscope. In the present work, (1 0 0) GaAs crystal has been used as a standard. Cross-sectional TEM specimens were prepared by argon ion beam thinning technique using a liquid nitrogen cold stage. 〈0 1 2〉 on-zone CBED technique has been used to estimate the actual voltage of the transmission electron microscope (Philips EM430T TEM), when the voltage was set at 250 kV. CBED-HOLZ simulation and analyses have been done, using JEMS software, to correlate with the experimental data. The methodologies adopted for estimating the actual voltage of TEM are discussed in this paper. The studies have also been cross-checked using 〈0 1 2〉 and 〈2 3 3〉 zone axes using (1 0 0) silicon standard. The techniques established are found to be suitable for TEMs operating at a setting voltage of about 250 kV. For the TEM studies, a regular double-tilt specimen holder is required in order to be able to get to the desired zone axes. When the experiments were repeated using a cryogenic double-tilt holder, an improvement in the sharpness of HOLZ lines was observed. Wherever possible, the use of the cryogenic double-tilt holder is recommended. Care must, however, be taken to ensure that effects such as lattice parameter changes (due to temperature changes), phase transformations etc can be properly accounted for.