20Congreso Nacional 
Sociedad Mexicana de Ciencia de Superficies y Vacío A.C.

Ohmic Electrical Contacts to Gallium Nitride

Paul H. Holloway
Department of Materials Science and Engineering
University of Florida
Gainesville, FL 32611-6400

Compound semiconductors offer the possibilities and reality of exciting new devices, such as HEMTs, light emitting diodes (LEDs) and diode lasers (DLs) from III-V (InGaAlAsP and InAlGaN) and II-VI semiconductors. While the bandgap of GaAs is greater than Si, it is not large enough to make blue LEDs or DLs, nor is it large enough to allow devices to operate at elevated temperatures (e.g. at 500oC). Therefore considerable effort has been expended to develop materials and processing for devices from GaN. With a bandgap of 3.4 eV, GaN can be used at high temperature for microelectronic devices. Since GaN is a direct bandgap semiconductor, it can be used for emission of green to blue to ultraviolet photons from LEDs and DLs. While tremendous progress has been made, formation of low resistance ohmic contacts to p-type GaN remains a problem. This results from misalignment of the quantum energy levels of the contact metals and the semiconductor. The top of the valence band of the semiconductor has an energy equal to the sum of the electron affinity plus the bandgap, which for GaN equals 7.5 eV. In principle, a metal with a work function of >7.5 eV should result in an ohmic contact to p-GaN, however such a metal does not exist. Therefore interfacial treatments and reactions must be used to develop lower resistance contacts. The effects of peroxide surface treatments upon carrier concentrations will be presented. The effects of interfacial reaction between metals and GaN will also be discussed. Disruption of the GaN surface and formation of interfacial compounds will be shown to result in ohmic contacts.

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