Ohmic and Rectifying Contacts to n and p-type GaN Films
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Ohmic and Rectifying Contacts to n and p-type GaN Films H. P. Hall, M. A. Awaah, A. Kumah, K. Das and F. Semendy1 Tuskegee University, Dept. of Electrical Engineering Tuskegee, AL 36088. 1 Army Research Laboratory Adelphi, MD. ABSTRACT Electrical contacts to both n and p-type GaN films have been investigated using electron-beam evaporated and sputtered films of metals such as Al, Au, Cr, Cu, Ni, Pt, and Ti. Films deposited by electron-beam evaporation for the n-type films with doping levels of 1 x 1018/cm3 and lower showed rectifying characteristics with all the metals studied with the exception of Al. Aluminum contact diodes were ohmic in the as-deposited state. The Pt rectifying contact was near-ideal with an ideality factor close to 1.0. Ideality factors for the other metals were much greater than 1. This deviation from thermionic behavior was interpreted as space charge limited current conduction in the presence of deep-level states. Sputtered films showed very similar characteristics to electron-beam deposited films, with the exception of Ti. The Ti contact was ohmic in the as-deposited state. Non-linear Cu contacts to n-type films became ohmic on annealing. However, for p-type films, Ar ion sputter-cleaning prior to metal deposition by sputtering created ohmic contacts with Cu and Pt. Low resistance ohmic contacts were achieved by ion implantation and anneal of Si in n-type and Mg in p-type films, prior to metallization. The implant parameters and anneal temperatures are currently being optimized. INTRODUCTION Group III nitride based semiconductors have attracted a considerable amount of attention during the last decade [1-2]. Gallium nitride in particular, having a bandgap of 3.39 eV at room temperature, is being used for the fabrication of optoelectronic devices such as light emitting diodes, photodetectors and lasers within the blue and ultraviolet region of the spectrum. Promising operation of GaN MESFETs [3] and AlGaN/GaN HEMTs [4] and HBTs [5] has been reported. Gallium nitride devices are also expected to operate at high temperatures, frequencies and power levels. Almost all these devices rely on rectifying and ohmic metal-semiconductor contacts for their operation. For the continued development of these devices, electrical properties of the metal-semiconductor contacts need to be thoroughly understood so that sufficient degree of process control can be achieved for obtaining optimal device properties. In this present paper, we report on the observed electrical properties of metal-GaN contact diodes formed by e-beam evaporated and sputtered metal films on n-and p-type GaN films. Emphasis was placed on developing an understanding of the current transport process for n-type rectifying contacts and establishing low resistance ohmic contacts for both n- and p-type films. The high ideality factors obtained for some of the rectifying contacts on the n-type films indicated that thermionic emission was probably not the dominant current transport mechanism, but instead it was likely to be space charge
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