Simple ohmic contact formation in HEMT structures: application to AlGaN/GaN

TLDR: In this article, the authors investigated contacts with the metal directly contacting the 2DEG in the GaN, about 20 nm below the top surface, using a 10mm × 10mm sample composed of 3nm-GaN/16-nm-Al0.27Ga0.73N/1-nm aln/1.8-μm-aln (Fe-doped).

Abstract: Low-resistance ohmic contacts on AlGaN/GaN HEMT devices presently require annealing at temperatures up to 850°C, which can adversely affect material properties. Here we investigate contacts with the metal directly contacting the 2DEG in the GaN, about 20 nm below the top surface. For convenience, we employed a 10-mm × 10-mm sample composed of 3-nm-GaN/16-nm-Al0.27Ga0.73N/1-nm-AlN/1.8-μm-GaN (Fe-doped). Four, 2-mm-long, 2-μm deep lines were scribed near the corners of the sample and filled with indium metal from a soldering iron. Hall measurements were then performed from 10 to 320 K at a current of 1 mA and with a magnetic field strength of 10 kG. At 10K (300K) the mobility was 1.96 × 104 (1.88 × 103) cm2·V-1·s-1; the sheet concentration, 9.39 × 1012 (9.35 × 1012) cm-2, and the sheet resistance, 33.9 (353) Ω/sq = rs. The contact resistance Rc was calculated from the average total resistance Rtot across each pair of contacts: Rtot = 2Rc + rs. At 10 K (300 K), Rc ≈ 1 (2) kΩ. Also, Rc has a much smaller temperature dependence than rs, implying tunneling, rather than thermionic current. From a Schrodinger-Poisson calculation, the peak volume carrier concentration in the 2DEG is n ≈ 3.7 × 1019 cm-3. The tunneling probability is P = exp[e(V – Vbi)/e00] and for e = 9.9evac and m* = 0.22m0, e00 = 0.077 eV = 894 K; thus, e00 < kT, further suggesting the dominance of tunneling current. This technique is immediately applicable to any HEMT-type structure, including AlScN/GaN.