Low Resistance Ohmic Contacts Formation and Mechanism of Current Transport Through p-GaN and p-AlGaN
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1108-A09-30
Low resistance ohmic contacts to p-type GaN and AlGaN I. Chary1, B. Borisov1, V. Kuryatkov1, Yu. Kudryavtsev2, R. Asomoza2, S. Nikishin1 and M. Holtz1 1
Nano Tech Center, Texas Tech University, Lubbock, TX-79409, USA
2
SIMS Laboratory of SEES, CINVESTAV, Mexico D.F.07300, Mexico
ABSTRACT We report the influence of surface treatment, annealing temperature and metal bilayer thickness on the specific contact resistance (ρc) of Au/Ni ohmic contacts to p-GaN and p-AlGaN. Ohmic contact on p-GaN with a hole concentration of 6.5 x 1017 cm-3, shows the lowest ρc of ~9.2 x 10-6 Ω cm2, when GaN was treated in HCl:H2O (3:1) solution before metal deposition and annealed at 500 oC for 10 minutes in 90% N2 and 10% O2 atmosphere. Similar procedure applied on p-AlxGa1-xN (x = 5-7%), with a hole concentration of 2.3 x 1017 cm-3, yields a ρc of 1.8 x 10-4 Ω cm2. An increase is observed in ρc when Mg doping exceeds 4 x 1019 cm-3 in both p-GaN and p-AlGaN. This is attributed to Mg self compensation. This increase is more pronounced in AlGaN which we attribute to the presence of residual native aluminum oxides. INTRODUCTION The GaN material and its alloys have great technological importance owing to their applications in the optoelectronics devices such as short wavelength light emitting diodes (LEDs) and laser diodes (LDs) [1-5]. GaN based high-temperature, high-power devices, ultraviolet detectors, metal semiconductor field effect transistors (MESFETs) and high-electron mobility transistors (HEMTs) are also extensively studied [6, 7]. In order to realize the high performance and reliability of these devices, the formation of low resistance ohmic contacts to both p and ntype GaN and AlGaN is necessary. Specific contact resistances (ρc) in the range of 10-6 to10-8 Ω cm2 for n-GaN [8-10] and in the range of 10-5 to 10-8 Ω cm2 for n-AlGaN have been reported [11, 12]. However, low resistance ohmic contacts to p-GaN and p-AlGaN alloys are difficult due to (a) the high activation energy of Mg (140-200 meV), which makes it difficult to achieve high hole densities and (b) the absence of metals with high work functions comparable to p-GaN and p-AlGaN. The quality of ohmic contacts to p-GaN and p-AlGaN mainly depends on (a) doping levels (b) the choice of contact metals, (c) surface preparation prior to contact metal deposition, and (d) the annealing conditions (temperature, time, and atmosphere) [13]. The presence of surface contaminants and oxides (GaOx) on GaN surface increases the barrier height at the interface, which leads to high contact resistance. Therefore an effective removal of these contaminants and oxides is critical for lowering the contact resistance [13]. A detailed study of surface treatment dependence on contact resistance has been reported elsewhere [14]. Based on our study, we concluded that the diluted HCl solution was effective in removing the surface oxide to achieve the lowest ρc. In this paper, we investigate the influence of surface treatment, annealing temperature and the thickness of the metal bilayer
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