In this presentation we will compare differential phase contrast (DPC) [1] and off-axis electron holography [2] for the measurement of electrostatic potentials in semiconductor devices. DPC uses the lateral shifts of a convergent electron beam to determine the field in the sample whereas for electron holography the changes in potential is encoded in interference fringes that are formed using a biprism [1]. To fairly assess the relative sensitivity of the different techniques on the same specimen, a symmetrically doped p-n junction with a dopant concentration of 1 x 10¹⁹ cm^-3 has been measured as a function of reverse bias applied in situ in the TEM.  Figure 1(a) and (b) shows maps acquired by DPC of the p-n junction with 0V and 4V reverse bias applied. At 0V, it is difficult to see the presence of the junction whereas for a reverse bias of 4 V the space charge region is now visible. Profiles acquired from across the junction for various reverse bias voltages can be seen in Figure 1(c).  Electron holograms were acquired and Figure 1(d) and (e) show reconstructed phase images of the junction at zero bias and 4 V reverse bias. Even though a low magnification has been used to obtain a large field of view at the expense of sensitivity, the junction is clearly visible in both of the phase images. Corresponding electric field profiles that have been calculated from the potential maps are shown in Figure 1(f).  These results show that off-axis electron holography has a significantly better sensitivity than DPC. However  the advantages of using DPC is that a large field of view has been obtained and it is not necessary to examine a region close to vacuum.

These techniques have also been applied to an InGaN/GaN system. Figure 2(a) shows a HAADF STEM image of the specimen. Figure 2(b) shows a potential map and (c) profile acquired by off-axis electron holography with a spatial resolution of 5 nm.  When using DPC, sub-nanometer spatial resolution is expected and maps of the electric field in the InGaN layers can be observed in Figures 2(d) and (e). Here the specimen has been tilted onto and away from a zone axis and a large variation in the measured signal is observed which is visible in Figure 2(f). In this presentation will discuss the effects of diffracted beams on measured DPC signal. We will also discuss the advantages and disadvantages of using DPC and electron holography for the measurement of electrostatic fields in a range of different doped and III/V semiconductor specimens and show improvements that have been applied.

Acknowledgements : This work has been funded by the ERC Starting Grant 306365 « Holoview ». The experiments have been performed on the platform nanocharacterisation at Minatec.

References

[1] N.H. Dekkers, Optik, 30 (1974) 452

[2] A. Tonomura, Reviews of Modern Physics, 59 (1987) 1

Figures:

FIG. 1. (a) Maps of DPC intensity for an unbiased and (b) 4 V reverse biased p-n junction. (c) DPC Intensity profiles measured across the junction for a range of reverse bias voltages. (d) Electron holography phase image.

FIG. 2. (a) HAADF STEM image of InGaN/GaN multilayer specimen. (b) Potential Map and (c) profile acquired by electron holography. (d) and (e) Map of the electric field obtained by DPC with specimen aligned on and away from zone axis and (f) electric field profiles.

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