Low Resistance Ohmic Contacts And The Mechanism Of Current Transport Through Au/Ni/p-GaN And Au/Ni/p- AlxGa1-xN

The gallium nitride (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). 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 .In order to realize the high performance and reliability of these devices, the formation of low resistance ohmic contacts to both p and n-type GaN and aluminum gallium nitride (AlGaN) is necessary. Specific contact resistances (ρc) in the range of 10-6 to10-8 Ω cm2 for n-GaN and in the range of 10-5 to 10-8 Ω cm2 for n-AlGaN have been achieved and reported. However, low resistance ohmic contacts to p-GaN and p-AlGaN alloys are difficult to realize 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. This dissertation documents the development of low resistance ohmic contacts to p-type GaN-AlGaN material system. The quality of ohmic contacts to p-GaN 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). The primary goal of this work is the development of processes with precise control over the above mentioned factors. A detailed study on the dependence of specific contact resistance on various surface treatment conditions, contact metal thicknesses and annealing conditions is presented. These processes are extended to p-type AlGaN and also to p-type AlGaN/AlN, GaN/AlGaN superlattices and optimized. The lowest specific contact resistance in the order of 10-6 Ω cm2 is reported. The mechanism of current transport through the metal-semiconductor interfaces has also been investigated. Thermionic and thermal field emissions are studied in detail as the dominant transport mechanisms.