Reduced Contact Resistance and Improved Surface Morphology of Ohmic Contacts on GaN Employing KrF Laser Irradiation

Abstract

We employ excimer laser annealing for ohmic contact formation to n- and p-type GaN layers grown on sapphire substrates. The laser irradiation of the n-GaN layers led to increased nitrogen vacancies at the nitride surface, which promoted tunneling currents with a less resistive n-contact. For p-GaN layer, the laser irradiation increased the effective hole concentration that resulted in a reduced contact resistivity. The lowest specific contact resistance measured using the transmission line method was about 2.4 ×10-7 and 3.2 ×10-4 Ω cm2 for n- and p-contacts, respectively. Laser irradiation also resulted in a comparatively good surface morphology as compared to rapid thermal annealing, which in turn improved the transmittance of contacts for light extraction from active layers. It was found out that both the electrical and optical characteristics of the p-GaN contacts exhibited a good thermal stability and an improved transmittance in the blue–green spectral range. An increased forward current with a reduced ohmic contact resistance in such high thermal stable contacts enable the fabrication of GaN light emitting diodes.

Role of Al and Ti for ohmic contact formation in AlGaN/GaN heterostructures

“…So far, Ti/Al based metals, such as Ti/Al/Ni/Au and Ti/Al/Mo/Au, have been commonly used for ohmic contact to AlGaN/GaN materials. There have been three models proposed as a mechanism for ohmic contact formation using Ti/Al on AlGaN/GaN heterostructures: 7,8 (i) Ti reacts with N and forms Ti-N complex and/or N vacancies in AlGaN, acting as high density donor layer to form low resistive ohmic contact, 7,[9][10][11] (ii) barrier height at Ti and AlGaN interface is lowered by formation of Ti-N, 12,13 and (iii) alloyed Ti/Al layer makes a spike, which penetrates into AlGaN barrier layer and touches two dimensional electron gas (2DEG) at AlGaN/GaN interface. 7,8,14,15 However, there are several exceptions, for example, a low resistive ohmic contact is obtained by V (vanadium) instead of Ti 16 and a spike is not always observed in the ohmic contact.…”

A Prediction Model of Electrical Constriction Resistance (ECR) between Dry-Friction Rough Surfaces Based on Three-Dimensional (3-D) Fractal Theory

“…Its operational lifetime and reliability are directly related to the energy supply of spacecraft, that is, the collector ring is the life line of the spacecraft [6]. Hence, the ECR is an essential parameter, which greatly affects the electrical transmission efficiency, long-lifetime operation and reliability of electrical equipment [7][8][9]. It is significantly vital developing a more accurate model to predict [10][11] and estimate the ECR during the design of the such electrical equipment.…”