Contacts to High Aluminum Fraction p -type Aluminum Gallium Nitride
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Contacts to High Aluminum Fraction p-type Aluminum Gallium Nitride Brett A. Hull1, Suzanne E. Mohney1, Uttiya Chowdhury2, Russell D. Dupuis2, David Gotthold3, Ronald Birkhahn3, and Milan Pophristic3 1 Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802 2 Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas 78712 3 Emcore Corporation, Somerset, NJ 08873 ABSTRACT Gold, palladium, platinum or nickel ohmic contacts on Mg doped p-type AlxGa1-xN with x = 0.4 and x = 0.45 have been examined. The Au contact provided the lowest contact resistivity with ρc = 1.8 (± 1.1) x 10-3 Ωcm2, but only following annealing at 850ºC. For the Pd, Au, and Pt contacts annealed at greater than 700ºC, a rapid degradation in the current-voltage curves was observed upon testing. The degradation was induced by exposure to sub-bandgap light and was reversed with a mild anneal at 500ºC. Possible mechanisms for the degradation are discussed. INTRODUCTION Efficient optoelectronic devices based on GaN and its related III-nitride alloys (InGaN and AlGaN) that emit from the green to the near ultraviolet (UV) have been successfully developed [1]. Interest is growing to extend the range of light emission of these devices deeper into the UV by incorporating more Al-rich AlxGa1-xN compositions. Historically, one limitation of GaN-based light emitters has been the ohmic contact to the p-GaN terminal for hole injection [2]. The most common approach thus far for AlGaN-based UV devices has been to employ a thin capping layer of the lower bandgap p-GaN on the p-AlGaN active layer to facilitate the formation of an ohmic contact. However, little work has been reported on contacts made directly to p-type AlGaN. There have been no reports of ohmic contacts to p-AlxGa1-xN near the compositions studied in this investigation (x = 0.4 and 0.45). There have been reports of ohmic contacts to much lower Al fractions of x = 0.06 [3] and x = 0.15 [4] or contacts to p-GaN/AlxGa1-xN superlattices with x = 0.1 or 0.2 [5,6]. In this investigation, we examine the electrical properties of Au, Pd, Pt, and Ni single layer contacts to high Al fraction Mg doped p-type AlxGa1-xN (x = 0.4 and 0.45). We have employed single layer contacts rather than the more typical bi- or trilayer contacts for two reasons: a) there have been no previous investigations of contacts to this AlxGa1-xN composition for comparison, and b) the severe annealing conditions required (T > 800ºC) would complicate interfacial reaction analysis due to significant interdiffusion between the metals of multilayer contacts. EXPERIMENTAL PROCEDURE Two separate epitaxial layers of Mg doped AlxGa1-xN with x = 0.4 and x = 0.45 were grown by metalorganic chemical vapor deposition on sapphire substrates. The layer thicknesses were 170 nm and 125 nm for the p-Al0.4Ga0.6N and the p-Al0.45Ga0.55N, respectively. The same
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growth process employed for growing the p-Al0.45Ga0.55N epitaxial layer in this investig
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