Differential Potential Distribution Observation in Transmission Electron Microscope with Conventional Thermal Electron Gun

The potential distribution in a material, particularly in semiconductors, has been imaged and measured by using electron holography [1], Lorentz microscopy [2] in a transmission electron microscope and DPC method in a scanning transmission electron microscope [3]. However, all these methods require the dedicated specially designed microscope or limited imaging condition. We have been developing a method to visualize the electric filed in a conventional electron microscope with a thermal electron gun [4,5]. We have modified our method applicable for built in potential in a material. The specimen used in this work is a pn junction in GaAs with the dopant concentration of 10 -19 [6]. The cross-sectional transmission electron microscopy (TEM) specimen was prepared using focused ion beam (FIB) apparatus SIINT SMI3050TB and the FIB damaged layers were well removed using Ar ion beam [7]. TEM observation was performed in JEM-2010 with the accelerating voltage of 200 kV. The specimen was observed in the imaging mode in which the objective lens was turned off and imaged by the objective mini (OM) lens. The largest size of selected area diffraction (SAD) aperture was used to obtain pseudo liner shape of the shadow aperture [5]. The specimen was carefully tilted to avoid the diffraction contrast as in the case of electron holography. In the imaging mode named “Low Mag” used in this work, the SAD aperture i.e., the shadow aperture, is located between the OM lens and the image plane of the lens as shown in Fig. 1. When the electron beam has certain divergency, the shadow of the aperture edge has diffused contrast, however, image of the specimen does not be affected. The intensity of electron beam focused on the image plane