MTL Annual Report

Figure 1: Square resistance and in-plane biaxial strain as a function of NWs’ diameter. The inset shows a scanning electron microscopy image of NWs with d~70 nm.

The frequency performance of
Al_xGa_1–xN/GaN high-electron-mobility transistors (HEMTs) has rapidly increased in recent years. Transistors with current gain cut-off frequencies (f_T) above 160 GHz and power gain cut-off frequencies (f_max) of more than 300 GHz have been reported ^[1] [2]. In spite of these excellent results, the frequency performance of these devices is still far from their theoretical limit. In this project, we are developing nanowire-based nitride HEMTs to overcome these limitations and to explore the maximum frequency of nitride devices ^[3].

Figure 2: Current measured in TLM devices as a function of SixNy thickness deposited by CVD. The black solid point is the expected value for the current when taking into account the AlGaN/GaN material removed during the fabrication of the NWs (d~70 nm and pitch ~ 140 nm).

Top-down fabrication of horizontal AlGaN/GaN nanowires (NWs) has been performed on planar AlGaN/GaN samples grown on a silicon substrate. E-beam lithography and Cl₂-based dry-etching were used to define nanowires with diameters (d) in the 30-200-nm range. The transmission line method (TLM) and visible Raman spectroscopy were used to characterize the NWs as well as planar devices. Figure 1 shows the square resistance (R_sq) and in-plane biaxial strain (e_xx) in the NWs as a function of diameter ^[4]. The R_sq increases exponentially when the NWs’ diameter is reduced. This behavior can be explained by the tensile strain relaxation occurred during the NWs’ fabrication ^[5]. To improve the NWs’ conductivity (d<100 nm), silicon nitride (Si_xN_y) was deposited by chemical vapor deposition (CVD) on NWs. Figure 2 shows the current measured in TLM devices as a function of Si_xN_y thickness. A linear increase of the current occurs when the Si_xN_y thickness increases. For an optimal Si_xN_y thickness of 45 nm, the current measured through the horizontal assembly of NWs is 8% higher than in planar devices. The characterization of the high-frequency performance of these nanowires is currently underway. It is expected that the large carrier confinement of these devices will allow higher operating frequencies as well as linearity.

1. T. Palacios, A. Chakraborty, S. Heikman, S. Keller, S. P. DenBaars, and U. K. Mishra, “AlGaN/GaN high electron mobility transistors with InGaN back-barriers,” IEEE Electron Device Lett., vol. 27, no. 1, pp. 13-15, Jan. 2006. [↩]
2. J. W. Chung, W. E. Hoke, E. M. Chumbes, and T. Palacios, “AlGaN/GaN HEMT with 300-GHz f_max,” IEEE Electron Device Lett., vol. 31, no. 3, pp. 195-197, Mar. 2010. [↩]
3. Y. Li, J. Xiang, F. Qian, S. Gradecak, Y. Wu, H. Yan, D. A. Blom, and C. M. Lieber, “Dopant-Free GaN/AlN/AlGaN radial nanowire heterostructures as high electron mobility transistors,” Nano Lett., vol. 6, pp 1468–1473, 2006. [↩]
4. S. Tripathy, S. J. Chua, P. Chen and Z. L. Miao, J. Appl. Phys. 92, 3503 (2002). [↩]
5. Y. Zhang, I. P. Smorchkova, C.R. Elsass, S. Keller, J. P. Ibbetson, S. Denbaars, U. Mishra, and J. Singh, J. Appl. Phys. 87, 7981 (2000). [↩]