High Transparent and Low Resistance Molybdenum-doped Indium Oxide Ohmic Contact to p-GaN

Abstract:
Ohmic contacts between electrodes and semiconductors play a crucial role in performance of light emitting diodes (LEDs). Up to now, very low resistance ohmic contacts on n-type GaN have been consistently preformed. But for p-type GaN ohmic contacts, it has been difficult to achieve low resistance contacts due to both the difficulty of growing heavily doped p-GaN (>1018 cm-3), and the absence of metal with a work function larger than that of p-GaN(∼7.5eV), which limits the operating voltage of the devices. Owing to the shortcomings of metal schemes, transparent conducting oxide (TCO) materials substitute for metal schemes as p-electrodes. The Ni/Au/ITO scheme is widely used on p-GaN in industry. However using such electrode requires two processes during the deposition. Therefore it is essential to find another TCO material that could form an ohmic contact on p-GaN with only one single layer.
In this thesis, we have investigated a promising high transparent and low resistance molybdenum-doped indium oxide (IMO) ohmic contact on p-GaN. We try different surface treatments and different sputtering conditions to find the most suitable process. We find this IMO contact film exhibits ohmic contact with low contact resistance after annealing under nitrogen ambient. For further investigation of the interface between IMO layer and p-GaN, a series of surface analyses such as atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) are performed. Based on these results, we explain the mechanisms of the IMO ohmic contact on p-GaN. Finally, with such high quality ohmic contacts, we fabricate the LEDs with annealed IMO ohmic contacts showing a low forward voltage at injection current of 20mA. And the light transmittance of IMO layer (3500 Å) on quartz is above 80% at 400nm. Therefore IMO contact scheme is a potential candidate for application to the p-electrode in GaN-based LED.