A C Twitchett

Improvement in electron holographic phase images of focused-ion-beam-milled GaAs and Si p-n junctions by in situ annealing

Low temperature (200–600°C) annealing in the transmission electron microscope (TEM) is used to provide a significant noise reduction in phase images of focused-ion-beam-milled GaAs and Si p-n junctions recorded using off-axis electron holography, as well as increasing the measured phase shifts across the junctions. Our results suggest that annealing removes defects resulting from Ga+ implantation and reactivates dopant atoms in the thin TEM specimens. In GaAs, electrically inactive surface layer thicknesses are reduced from 80to17nm on each specimen surface after annealing at 500°C. In Si the improvement is from 25to5nm.

Off-axis electron holography of electrostatic potentials in unbiased and reverse biased focused ion beam milled semiconductor devices

Off‐axis electron holography in the transmission electron microscope (TEM) is used to measure two‐dimensional electrostatic potentials in both unbiased and reverse biased silicon specimens that each contain a single p–n junction. All the specimens are prepared for examination in the TEM using focused ion beam (FIB) milling. The in situ electrical biasing experiments make use of a novel specimen geometry, which is based on a combination of cleaving and FIB milling. The design and construction of an electrical biasing holder are described, and the effects of TEM specimen preparation on the electrostatic potential in the specimen, as well as on fringing fields beyond the specimen surface, are assessed.

Off-axis electron holography of unbiased and reverse-biased focused ion beam milled Si p-n junctions.

Off-axis electron holography is used to measure electrostatic potential profiles across a silicon p-n junction, which has been prepared for examination in the transmission electron microscope (TEM) in two different specimen geometries using focused ion beam (FIB) milling. Results are obtained both from a conventional unbiased FIB-milled sample and using a novel sample geometry that allows a reverse bias to be applied to an FIB-milled sample in situ in the TEM. Computer simulations are fitted to the results to assess the effect of TEM specimen preparation on the charge density and the electrostatic potential in the thin sample.

Site-specific dopant profiling in a scanning electron microscope using focused ion beam prepared specimens

Site-specific dopant profiling across a silicon p-n junction has been performed in the scanning electron microscope (SEM) on samples prepared using focused ion beam (FIB) milling. The results are compared with the dopant contrast obtained after using non-site-specific cleaving and tripod polishing of specimens. FIB milling generates a damaged surface layer which reduces the dopant contrast observed in the SEM using secondary electron imaging. These results show that two dimensional dopant contrast can be observed from FIB-prepared membranes, thereby extending the application of SEM dopant profiling to the examination of complex nanometer-scale device structures for which site selection is essential.

Comparison of off-axis and in-line electron holography as quantitative dopant-profiling techniques

Many different dopant-profiling techniques are available for semiconductor device characterization. However, with length scales shrinking rapidly, only transmission electron microscopy (TEM) techniques promise to fulfil the spatial resolution required for the characterization of future device generations. Here, we use three advanced TEM techniques, off-axis electron holography, Fresnel imaging (in-line electron holography) and Foucault imaging, to examine a focused ion beam-prepared silicon p–n junction device. Experiments are carried out on electrically unbiased samples and with an electrical bias applied in situ in the TEM. Simulations are matched to experimental data to allow quantitative conclusions to be drawn about the underlying electrostatic potential distributions. The off-axis electron holography and Fresnel results are compared to assess whether the techniques are consistent, and whether they can be used to provide complementary information about dopant potentials in semiconductor devices.

Electron holography of doped semiconductors: when does it work and is it quantitative?

The application of off-axis electron holography to the measurement and interpretation of variations in electrostatic potential in doped semiconductors is described and reviewed. Both experimental measurements and computer simulations of electrostatic potentials in thin specimens are presented, and the degree to which parameters such as the built-in voltage and the depletion width across a p-n junction can be measured reliably is discussed.

Towards quantitative electron holography of electrostatic potentials in doped semiconductors

Simulations of the electrostatic potential within thin Si samples containing an abrupt p-n junction have been compared with experimental measurements obtained using off-axis electron holography from samples prepared using focused ion beam milling. In order to obtain agreement between the simulated and experimental potential profiles, a layer of altered dopant concentration is introduced at the specimen surface.

Off-axis electron holography of focused ion beam milled GaAs and Si p-n junctions

Si and GaAs p-n junctions have been characterised in the transmission electron microscope using off-axis electron holography. Focused ion beam milling was used to prepare parallel-sided membranes with thicknesses of 200–500 nm. Off-axis electron holograms were acquired at 200kV in order to assess the effect of specimen preparation on the electrostatic potentials measured across the junctions.

Three-dimensional analysis of the dopant potential of a silicon p-n junction by holographic tomography

Off-axis electron holography and tomography have been combined to examine the 3-D electrostatic potential associated with a Si p-n junction. The device was prepared in a novel specimen geometry using focused ion beam milling and a series of holograms was acquired over a tilt range of −70° to +70°. Simultaneous iterative reconstruction was used to reconstruct the 3-D electrostatic potential in the specimen. The experimental results were compared to simulations of the potential variation. Quantitative results from the central, ‘bulk’ semiconducting regions and from the surface layers were extracted from the 3-D reconstruction.

Simulations of the electrostatic potential in a thin silicon specimen containing a p-n junction

The measurement of potentials associated with dopant atoms in semiconductors at nanometer spatial resolution using off-axis electron holography is known to be affected by the presence of the surfaces of thin specimens. In particular, the potential across a p-n junction is often found to be lower than would be expected from predicted properties of bulk devices. Here we present simulations of two-dimensional potential profiles within a thin ( p-n junction. We find that the potential across the p-n junction is always smaller, when projected through the specimen, than would be expected from the properties of the bulk material. Crucially, the step in potential across the junction is independent of the value of the potential on the surface of the specimen for high dopant concentrations (>10 17 cm −3 ). The simulations are compared with experimental data. Although they can account for some of the reduction in the observed potential, they do not fully explain the experimental results.