In order to examine the possibility of preparing a nonreacted (nonalloyed) Ohmic contact to p-GaN, the effects of GaN surface treatments and work functions of the contact metals on the electrical properties between the metal contacts and p-GaN were investigated. A contamination layer consisting of GaOx and adsorbed carbons was found on the GaN substrate grown by metalorganic chemical vapor deposition. The contamination layer was not completely removed by sputtering the GaN surface with Ar and N ions where the ion densities were ∼10−2 μA/cm2. Although the contamination layer was partially removed by immersing in a buffered HF solution, little improvement of the electrical properties of the GaN/metal interfaces was obtained. Most of the contamination layer was removed by annealing the Ni and Ta contacts at temperatures close to 500 °C. These annealed contacts exhibited slightly enhanced current injection from the contact metal to the GaN. The present surface treatment study indicated that removal of the contamination layer did not significantly reduce the contact resistance. On the other hand, the resistance decreased exponentially with increasing the metal work functions, where Pt, Ni, Pd, Au, Cu, Ti, Al, Ta, and Ni/Au were deposited on the GaN. This result suggests that the Schottky barrier height at the p-GaN/metal interface might not be pinned at the GaN surface. The present study concluded that a contact metal with a large work function is desirable for nonreacted Ohmic contacts to p-GaN. However, these contacts did not provide the low contact resistance required for blue laser diodes.

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Articles you may be interested inElectrical and microstructure analysis of ohmic contacts to p-and n-type GaSb, grown by molecular beam epitaxy J.As a first step to achieve our goal to prepare Ohmic contacts to p-type ZnSe using a conventional deposition and annealing method, the microstructure and chemical composition of the ZnSe surface were analyzed by x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and reflection electron microscopy. A thin native oxide layer consisting of ZnO and SeO x and adsorbed carbons were observed on the ZnSe surface which was grown by the molecular beam epitaxy technique. Most of the oxide layer was removed by saturated bromine water solution. A strong correlation was observed between the surface composition and the electrical properties measured by a current-voltage method for Ni contacting to the N-doped p-type ZnSe substrates. The turn-on voltage ͑which corresponds to a breakdown voltage for back-to-back Schottky contacts͒ was significantly reduced by removing the oxide layer using saturated bromine water solution. The present experiment suggested that the interfacial oxide layer is an effective barrier for the carrier transport through the ZnSe/metal interface.