GaN-based high electron mobility transistors (HEMTs) are an important platform for the realization of high-power, high-frequency devices.  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] .  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 Al₂O₃ in stress generation.

NR structures were fabricated using top-down techniques and passivated with varying thicknesses of Al₂O₃.  TEM samples were obtained from the device structures by using focused ion beam techniques.  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.  Splitting profiles were also generated from FEA models of the HEMT structure for comparison.  Finally, device-sized structures were simulated to investigate the stress state of the active HEMT layers as a function of the oxide thickness.

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.  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 Al₂O₃ is applied, as shown in Figure 2.  The magnitude of the tensile stress approaches that of the planar structure as the thickness of the oxide increases.  This data correlates well with earlier published ^[1] electrical characterization of these structures considering the decrease in carrier concentration observed for a compressive strain ^[2] .

1. M. Azize, A. L. Hsu, O. I. Saadat, M. Smith, X. Gao, S. P. Guo, S. Gradečak, and T. Palacios, “High-electron-mobility transistors based on InAlN/GaN nanoribbons,” IEEE Electron Device Letters, vol. 32, pp. 1680-1682, Dec 2011.
2. J. Kuzmik, “InAlN/(In)GaN high electron mobility transistors: Some aspects of the quantum well heterostructure proposal,” Semiconductor Science and Technology, vol. 17, pp. 540-544, June 2002.

Nitride transistors with current gain cut-off frequencies (f_T) of 300 GHz and power gain cut-off frequencies (f_max) of 394 GHz have been reported ^{[1] [2]} . However, the frequency performance of these devices is still far from the theoretical limit due to poor gate modulation efficiency, short channel effects, high access resistances (R_a), and specific contact resistance (r_c). Nanoribbon (NR)-based nitride HEMTs could overcome many of these limitations by improving the electron confinement thanks to the excellent electrostatics of wrap-around gates ^[3] . Moreover, strong piezoelectric-induced doping can be generated in NR nitride-based semiconductors and increase the maximum operating frequency of nitride devices by reducing the parasitic resistances (Ra and r_c). In this project, we study the use of Ti-induced strain in InAlN/GaN NR HEMTs.

 NR and planar devices are fabricated on the same chip. A Ti/Al/Ni/Au metal stack is then deposited for ohmic contact formation. Electron beam lithography and dry etching are performed in some devices to define NR structures between the ohmic contacts with widths (w) in the w~20-90 ± 10 nm range and a period (p) of p~135 ± 10 nm. An additional Ti strip was deposited between the ohmic contacts (cf. inset of Figure 2) with a width of ~100 mm and a length varying from ~2 to 16 mm and annealed at 870 ^◦C for 30 s in N₂ environment. Figure 1 shows the total resistance R_T in NR and planar devicesas a function of the area coverage of the Ti stripes. A quasi-linear R_T decrease is observed when the Ti surface area increases in the InAlN/GaN NRs samples, unlike in the planar device. The mechanical stress introduced by Ti stripes has a strong effect on the transport properties of InAlN/GaN NRs. Figure 2 shows the decrease of the sheet resistance (R_shTi) underneath the Ti stripe in InAlN/GaN NRs as a function of NR widths. The R_shTi in the NR and planar devices is decreased by ~50-75 % and ~10%, respectively.

1. D. S. Lee, X. Gao, S. Guo, D. Kopp, P. Fay, and T. Palacios, “300-GHz InAlN/GaN HEMTs with InGaN back barrier, IEEE Electron Device Lett., to be published.
2. K. Shinohara, D. Regan, I. Milosavljevic, A. L. Corrion, D. F. Brown, P. J. Willadsen, C. Butler, A. Schmitz, S. Kim, V. Lee, A. Ohoka, P. M. Asbeck, and M. Micovic, “Electron velocity enhancement in laterally scaled GaN DH-HEMTs with f_T of 260 GHz,” IEEE Electron Device Lett., vol. 32, no. 8, pp. 1074-10176, Aug. 2011.
3. W. Lu, P. Xie, and C. M. Lieber, “Nanowire transistor performance,” IEEE Trans. Electron Dev., vol. 55, no 11, pp 2859-2876, Nov. 2008.