Low-Resistance Electrical Contacts to p-Type GaN by Using In x ia 1-x N Cap Layers
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Low-Resistance Electrical Contacts to p-Type GaN by Using InxGa1-xN Cap Layers Th. Gessmann, Y.-L. Li, J. W. Graff, and E. F. Schubert Department of Electrical and Computer Engineering, Boston University, Boston, MA 02215, U.S.A. J. K. Sheu Optical Science Center, National Central University, Chung-Li 32054, Taiwan, R.O.C. ABSTRACT A novel type of low-resistance ohmic contacts is demonstrated utilizing polarization-induced electric fields in thin p-type InGaN layers on p-type GaN. An increase of the hole tunneling probability through the barrier and a concomitant significant decrease of the specific contact resistance can be attributed to a reduction of the tunneling barrier width in the InGaN capping layers due to the polarization-induced electric fields. The specific contact resistance of Ni (10 nm) / Au (30 nm) contacts deposited on the InGaN capping layers was determined by the transmission line method. Specific contact resistances of 1.2 × 10-2 Ω cm2 and 6 × 10-3 Ω cm2 were obtained for capping layer thicknesses of 20 nm and 2 nm, respectively.
INTRODUCTION To avoid excessive heating resulting in failure of devices fabricated from GaN and related compounds, specific contact resistances in the order of < 10-3 Ω cm2 for light-emitting diodes (LEDs) and < 10-4 Ω cm2 for laser diodes are required [1]. This applies in particular to ohmic contacts on p-type GaN (p-GaN) due to the resistive nature of GaN obtained by standard p-type doping techniques [1,2]. Several methods have been utilized to attain specific contact resistances to p-GaN smaller than 10-3 Ω cm2 such as deposition of high work function metals [3], growth of AlGaN / GaN superlattices [4,5] and tunnel-diode structures on top of p-GaN [6]. In this publication, the use of thin InxGa1-xN capping layers pseudomorphically grown on top of p-type GaN is presented. Strain-induced piezoelectric as well as spontaneous polarization fields in the InGaN capping layer and the lower p-GaN layer cause band bending that leads to the formation of a two-dimensional hole gas (2DHG) [7]. As a result, the concentration of free holes near the surface is greatly increased and the tunneling barrier width of the metal-semiconductor contact is reduced, thereby allowing for a high hole tunneling probability through the barrier.
EXPERIMENTAL AND THEORETICAL DETAILS The samples were grown by metalorganic chemical vapor deposition (MOCVD) along the c-direction on top of single crystalline sapphire substrates. The grown structures consist of a 4 µm thick n-type GaN layer, a 5 pair multi-quantum well LED with barrier / well widths of 7.5 nm / 2.5 nm, respectively, a 20 nm Al0.15Ga0.85N electron blocking layer, a 130 nm thick p-type GaN upper cladding layer and a p-type In0.27Ga0.73N capping layer that is either 2 nm or I11.35.1
20 nm thick. The two p-type layers were Mg-doped to a concentration of about NMg = 3 × 1018 cm-3 as determined from capacitance-voltage measurements. Ni (10 nm) / Au (30 nm) contacts were deposited by electron beam evaporation using lift-off photolithographic
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