(a) SEM image of a typical AlGaN/GaN HEMT in the pristine (unbiased) state, inset illustrates schematic of the cross section. (b) Cross-section view of the lift-out indicates the features of the device as well as the Pt protective layers. (c) and (d) represent weak-beam dark-field TEM images of drain-gate region at two different diffraction conditions of and , respectively. Scale bars are 200 nm and the threading dislocations are labeled as 1–5. Pure edge variants are labeled as 1 in Figs. 1(c) and 1(d) , pure screw are labeled as 2 and 4 in Figs. 1(c) and 1(d) , and mixed are labeled as 3 and 5 in Figs. 1(c) and 1(d) .
HRTEM images and corresponding in-plane strain maps of pristine and biased HEMT devices. (a) The HRTEM image of the pristine device shows the interface between the AlGaN and GaN layers (dashed line) under the source side of the gate. The inset shows the diffraction pattern from the zone axis. Maps of strain parallel (b) and perpendicular (c) to the interface reveal the presence of tensile and compressive strains, respectively, in the AlGaN layer relative to the GaN substrate. (d) The HRTEM image of device #2 from the drain side of the gate reveals defects (white arrows highlight representative regions) formed near the interface and in GaN bulk as a result of early stage biasing. (e) and (f) Strain maps from the area in (d) show a slight change from the ones in (b) and (c). 22 (g) The HRTEM image of device #3 shows defects at the interface and in the GaN bulk (indicated by arrows). (h) and (i) Strain maps from the area in (g) show an increase in the strain parallel to the interface. (j) The HRTEM image of device #4 illustrates defects in both AlGaN and GaN bulk (white arrows). (k) and (l) Strain maps from the area in (j) show an relaxation of the strain parallel to the interface.
WBDF images of the drain and source sides of the gate in device #4. (a) and (b) represent the WBDF-TEM images with similar diffraction conditions of a biased device from the edge of the gate on source and drain sides, respectively. (c) and (d) show high magnification WBDF-TEM images of the regions under the source and drain edge of the gate, respectively. White arrows in (d) indicate the formation of defects, which cannot be observed in (c), on the source side of the gate. (e) Characteristic HRTEM image illustrates the high density of defects on the drain edge of the gate in a biased device. (e) and (g) Fourier-filtered HRTEM images (inverse fast Fourier transform) of localized strain regions at the heterointerface and GaN bulk, respectively, as shown with dotted boxes in the device #4 (f). (h) Electron energy loss spectra measured at three locations in a stressed device. (i) The three corresponding locations indicated on the cross sectional TEM image of the stress device. There is an apparent chemical shift on the defected location (3) compared to its adjacent regions (1) and (2).
(a) Plot of sheet polarization charge density and perpendicular ( )and parallel ( ) strain components as a function of bias duration. (b) Representative drain current vs. drain voltage for device #2 showing device degradation.
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