IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 2, FEBRUARY 2013 199 High Electron Velocity Submicrometer AlN/GaN MOS-HEMTs on Freestanding GaN Substrates David J. Meyer, Member, IEEE, David A. Deen, Member, IEEE, David F. Storm, Mario G. Ancona, Member, IEEE, D. Scott Katzer, Member, IEEE, Robert Bass, Jason A. Roussos, Member, IEEE, Brian P. Downey, Steven C. Binari, Member, IEEE, Theodosia Gougousi, Tanya Paskova, Edward A. Preble, and Keith R. Evans Abstract—AlN/GaN heterostructures with 1700-cm 2 /V · s Hall mobility have been grown by molecular beam epitaxy on freestanding GaN substrates. Submicrometer gate-length (L G ) metal–oxide–semiconductor (MOS) high-electron-mobility tran- sistors (HEMTs) fabricated from this material show excellent dc and RF performance. L G = 100 nm devices exhibited a drain current density of 1.5 A/mm, current gain cutoff frequency f T of 165 GHz, a maximum frequency of oscillation f max of 171 GHz, and intrinsic average electron velocity v e of 1.5 × 10 7 cm/s. The 40-GHz load-pull measurements of L G = 140 nm devices showed 1-W/mm output power, with a 4.6-dB gain and 17% power-added efficiency. GaN substrates provide a way of achieving high mobil- ity, high v e , and high RF performance in AlN/GaN transistors. Index Terms—Atomic layer deposition, AlN, GaN, high- electron-mobility transistors (HEMTs), HfO 2 , hydride vapor phase epitaxy (HVPE). I. I NTRODUCTION R ECENT research to increase the frequency performance of GaN high-electron-mobility transistors (HEMTs) has focused on aggressively scaling the device geometry. As GaN HEMT gate lengths L G are reduced below 0.25 μm, the de- mands of electrostatic control have led to the use of novel ultrathin barriers with higher Al mole fractions than conven- tional AlGaN alloys. For Ga-polar heterostructures, AlN is the thinnest pseudomorphic barrier material available that can induce (via polarization and conduction band discontinuity) a 2-D electron gas (2DEG) density suitable for transistor use in GaN [1]. Early reports of RF devices based on the AlN/GaN heterostructure showed the potential of scaling the barrier Manuscript received October 23, 2012; revised November 8, 2012; accepted November 12, 2012. Date of publication January 2, 2013; date of current version January 23, 2013. This work was supported by the Office of Naval Re- search with funding from Dr. P. Maki. The work at the University of Maryland Baltimore County was supported in part by the National Science Foundation under Grant DMR 0846445. The review of this letter was arranged by Editor J. A. del Alamo. D. J. Meyer, D. F. Storm, M. G. Ancona, D. S. Katzer, R. Bass, J. A. Roussos, B. P. Downey, and S. C. Binari are with the U.S. Naval Research Laboratory, Washington, DC 20375 USA (e-mail: david.meyer@nrl.navy.mil). D. A. Deen was with the U.S. Naval Research Laboratory, Washington, DC 20375 USA. He is now with the Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455 USA (e-mail: dadeen@umn.edu). T. Gougousi is with the Department of Physics, University of Maryland- Baltimore County, Baltimore, MD 21250 USA. T. Paskova was with Kyma Technologies, Raleigh, NC 27617 USA. She is now with the Department of Material Science and Engineering, North Carolina State University, Raleigh, NC 27695 USA. E. A. Preble and K. R. Evans are with Kyma Technologies, Raleigh, NC 27617 USA. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2012.2228463 Fig. 1. Cross-sectional schematic of an L G = 100 nm AlN/GaN MOS- HEMT. thickness by demonstrating current gain cutoff frequency f T values in the 50- to 110-GHz range [2], [3]. More recently, dramatic reduction in source–drain spacing L SD down to 100 nm, selective regrowth of n + GaN source and drain regions by molecular beam epitaxy (MBE), and AlGaN back barriers have been used to demonstrate the highest combination of f T = 310 GHz and the maximum frequency of oscillation f max of 364 GHz for depletion-mode GaN HEMTs to date [4]. While device engineering can be used to minimize certain electron delay components such as parasitic and channel charging, the primary constituents of total electron delay (2πf T ) −1 are typically the drain delay and intrinsic gate transit times, which are inversely proportional to the average electron velocity v e in their respective regions [4], [5]. The goal of this study was to determine whether higher v e can be achieved with the use of hydride vapor phase epitaxy (HVPE)-grown freestanding GaN as a substrate for AlN/GaN HEMT device epitaxy. Since epitaxial layers can be grown with a low dislocation density (< 10 7 cm −2 ) on HVPE GaN [6], this substrate offers a lattice- and thermal-expansion- matched platform for heterostructure growth that has potential advantages over previously examined substrates such as Si [7], sapphire [1]–[3], [8], [9], and SiC [4], [5], [9]–[12]. To evaluate the dc and RF electrical performance of this material, we fabricated and tested submicrometer T-gate AlN/GaN MOS-HEMT devices, as schematically shown in Fig. 1. II. EXPERIMENT The HEMT structure was grown by RF-plasma-assisted MBE on a 1 cm × 1 cm freestanding HVPE-grown GaN semi-insulating substrate at 650 ◦ C. Following a 60-s surface nitridation of the GaN substrate, growth began with a 1.5-nm- thick AlN nucleation layer [13]. A 1.3-μm-thick GaN layer was then grown with beryllium doping used to suppress buffer leakage current [14]. Finally, a 200-nm-thick unintentionally doped (UID) GaN buffer layer was grown and capped with a 0741-3106/$31.00 © 2013 IEEE