edfas.org 33 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 4 electric field pointing into the gate electrode, as would be expected from the Schottky barrier at the gate/AlGaAs interface. Electric field from the two-dimensional electron gas is expected to have the opposite sign (dark contrast), which is consistent with the darker contrast just below the Schottky barrier signal. These electronic structures, critical to HEMT function, have been preserved during sample preparation. More detailed discussion of this data can be found in Hubbard.[17] An overlay of ADF STEM and STEM EBIC signals from the Fig. 4b region is shown in Fig. 5 along with a similar overlay from a silicon photodiode sample (the same device as in Hubbard[2]). The two images are scaled to directly compare distance between them (i.e., they share a scale bar). For the photodiode, the depletion region is more than 100 nm wide, and significant EBIC signal persists 100s of nm away from it. For the HEMT sample, on the other hand, signal from the Schottky barrier, for instance, is con- fined to a ~20 nm-thick region and attenuates sharply within a few nanometers outside of it. Below the AlGaAs and InGaAs layers is the buffer layer, which consists of alternating layers of AlGaAs and GaAs engineered to prevent carriers from tunneling into the GaAs substrate below it. In Fig. 5, the EBIC signal attenuates significantly in the buffer layer, as carrier mobility is limited causing EHPs to recombine, but returns once the beam is on the substrate. While there are other electric field-sensitive imaging techniques for the TEM, such as holography and differential phase contrast, STEM EBIC is unique in that its signal is determined by electric field and carrier mobility. Many critical features of this semiconductor heterostructure, such as decreased carrier mobility in the buffer layer, cannot be visualized by imaging that is strictly field-sensitive. CONCLUSION The results described here provide a practical route to more routine inspection of electronic device features and bias-induced changes in samples extracted from microelectronic components. Samples extracted from pristine devices could be used to assess processing quality, or they may be stressed in situ (e.g. biasing, heating, or irradiation) to observe formation of electronic defects live. (a) (b) Fig. 4 ADF STEM and STEM EBIC images of the Fig. 3 device. The rightmost image in (a) shows the two EBIC images overlaid with the gate signal in red and drain signal in green. The images in (a) are acquired in the yellow box of Fig. 3 and the images in (b) are acquired in the region below the gate electrode. See also Hubbard.[17] Fig. 5 ADF STEM (grayscale) and STEM EBIC (red) overlay images of a Si photodiode (upper) and the Figs. 3-4 GaAs HEMT (lower). The scale bar applies for both images.
RkJQdWJsaXNoZXIy MTYyMzk3NQ==