{"id":5624,"date":"2012-07-18T22:28:04","date_gmt":"2012-07-18T22:28:04","guid":{"rendered":"https:\/\/mtlsites.mit.edu\/annual_reports\/2012\/?p=5624"},"modified":"2012-07-18T22:28:04","modified_gmt":"2012-07-18T22:28:04","slug":"stress-state-characterization-of-inalngan-nanoribbon-hemt-structures-using-convergent-beam-electron-diffraction","status":"publish","type":"post","link":"https:\/\/mtlsites.mit.edu\/annual_reports\/2012\/stress-state-characterization-of-inalngan-nanoribbon-hemt-structures-using-convergent-beam-electron-diffraction\/","title":{"rendered":"Stress State Characterization of InAlN\/GaN Nanoribbon HEMT Structures using Convergent Beam Electron Diffraction"},"content":{"rendered":"

GaN-based high electron mobility transistors (HEMTs) are an important platform for the realization of high-power, high-frequency devices.\u00a0 Nanoribbon (NR) HEMT structures represent a novel route towards piezodoping by allowing external stresses to be applied in the plane of the active layer and have been shown to enhance carrier transport properties [1<\/a>] <\/sup>.\u00a0 This work uses transmission electron microscopy (TEM) and finite element analysis (FEA) to investigate the stress state of InAlN\/GaN NR HEMT devices and explore the role of Al2<\/sub>O3<\/sub> in stress generation.<\/p>\n

NR structures were fabricated using top-down techniques and passivated with varying thicknesses of Al2<\/sub>O3<\/sub>.\u00a0 TEM samples were obtained from the device structures by using focused ion beam techniques.\u00a0 Using convergent beam electron diffraction, strain relaxation profiles were obtained by analyzing the splitting of higher order Laue zone lines contained in the [5 4 0] zone axis pattern.\u00a0 Splitting profiles were also generated from FEA models of the HEMT structure for comparison.\u00a0 Finally, device-sized structures were simulated to investigate the stress state of the active HEMT layers as a function of the oxide thickness.<\/p>\n

Comparison of the experimental and simulated splitting profiles in Figure 1 shows not only that the FEA model correctly replicates overall splitting behavior and the dependence on sample thickness, but that it also consistently under-estimates the experimental results, suggesting an additional source of stress not present in the current model.\u00a0 Models of device structures showed a compressive stress generated in the active HEMT layer upon the creation of a NR structure that becomes tensile when a layer of Al2<\/sub>O3<\/sub> is applied, as shown in Figure 2.\u00a0 The magnitude of the tensile stress approaches that of the planar structure as the thickness of the oxide increases.\u00a0 This data correlates well with earlier published [1<\/a>] <\/sup> electrical characterization of these structures considering the decrease in carrier concentration observed for a compressive strain [2<\/a>] <\/sup>.<\/p>\n\n\t\t