Abstract—The electrical and structural properties of Hf/Al/Ni/Au (20/100/25/50 nm) ohmic contact to n-GaN are reported in this study. Specific contact resistivities of Hf/Al/Ni/Au based contacts have been investigated as a function of annealing temperature and achieve the lowest value of 1.09×10 -6 Ω·cm 2 after annealing at 650 o C in vacuum. A detailed mechanism of ohmic contact formation is discussed. By using different chemical analyses, it is anticipated that the formation of Hf-Al-N alloy might be responsible to form low temperature ohmic contacts for the Hf-based scheme to n-GaN. Keywords—Gallium nitride, ohmic contact, Hafnium I. INTRODUCTION ALLIUM nitride, with many characteristic properties compared to other semiconductor materials, has been attractive to researchers in the areas of electronics and optoelectronics since the last couple of decades. GaN has been successfully used in many applications of lasers [1], including blue and green light emitting diodes [2] by exploiting its direct wide bandgap (3.4 eV) [3]. Besides the wide bandgap, high breakdown field and high saturation velocity of GaN make it very promising for high-power, high-speed and high temperature electronic devices [4, 5]. However, in order to boost up the performance of GaN based devices, the formation of ohmic contact to n-GaN and its related alloys with AlN and AlInN is very essential to reduce power consumption and parasitic delays associated with the access resistances. Among the many potential metal candidates suitable for ohmic contact to n-GaN, Ti based multilayer structures are the most widely used metal scheme followed by a rapid thermal annealing (RTA) process which is usually performed at temperatures higher than 750 °C in order to obtain a low resistance ohmic contact [6-8]. Theoretically, any metal with a work function close to the electron affinity of GaN (~2.9–3.5 eV) could form an ohmic contact to n-GaN [9]. Among few potential candidates, hafnium, Hf is attractive since it has low work function ~3.9 eV close to electron affinity of n-GaN [10]. Therefore, it is expected that Hf could form an ohmic contact to n-GaN. Y. Liu is a PhD candidate of the Department of Electrical and Computer Engineering, National University of Singapore, Singapore 119074, and is also attached to the Institute of Microelectronics, Agency for Science, Technology, and Research (A*STAR), Singapore 117685.(e-mail: a0068100@nus.edu.sg). M. K. Bera, and E. F. Chor are with the Department of Electrical and Computer Engineering, National University of Singapore, Singapore 119074. (e-mail: elecef@nus.edu.sg). L. M. Kyaw is a PhD candidate of the Department of Electrical and Computer Engineering, National University of Singapore, Singapore 119074, G. Q. Lo is with the Institute of Microelectronics, Agency for Science, Technology, and Research (A*STAR), Singapore 117685. (e-mail: logq@ime.a-star.edu.sg). Although, Hf based metallization scheme has been reported recently, however, the detail mechanism of ohmic contact formation has not yet been explored [11-13]. In this work, we report the electrical and structural properties of Hf/Al/Ni/Au based ohmic contacts to n-GaN. Specific contact resistivities of Hf/Al/Ni/Au based contacts are investigated as a function of annealing temperature in the range from 600 to 900 o C and are compared with those for conventional Ti/Al/Ni/Au based ohmic contacts to n-GaN. Besides, a detailed mechanism of ohmic contact formation has also been discussed. II. EXPERIMENTAL In this experiment, we used n-type GaN epi-structure grown on 4 inch Si (111) substrate by means of metal organic chemical vapor deposition (MOCVD), purchased from NTT-AT Corporation, and the detail of our contact structure is schematically shown in Fig. 1. Hall measurements conducted at room temperature exhibit the carrier concentration of ~3.5×10 18 cm -3 , the carrier mobility of ~220 cm 2 /V.s, and a sheet resistance of 84.38 Ω/□. The 4 inch n-GaN-on-Si wafer was cut into several small pieces. Thereafter, a proper surface cleaning procedures were carried out first by degreased in acetone solution followed by 2-propanol in an ultrasonic bath for about 10 mins each before thoroughly rinsed in de-ionized (DI) water. Subsequently, a dilute HCl acid solution (HCl: H 2 O=1:3) was used to remove the remaining native oxides on n-GaN surface followed by rinsed in DI water and blown dry with N 2 . Before patterning, the samples were pre-baked at 110 °C for 10 mins in an oven for dehydration. The circular transmission line model (CTLM) structure was defined using UV photolithography. The detailed configuration of CTLM structure is shown in Fig. 2. It consists of an outer circle of radius 90 µm with different inner circles having gap spacing in the range between 5 to 45 µm. Pre-metallization cleaning was performed by dipping the patterned samples into dilute HCl solution (HCl: H 2 O =1:10) for about 15 sec .The samples were then immediately loaded into an e-beam evaporator chamber. Metal layers of Hf/Al/Ni/Au (20/100/25/50 nm) and Ti/Al/Ni/Au (20/100/25/50 nm) were deposited sequentially by e-beam evaporation. A rapid thermal annealing (RTA) process was carried out to do thermal annealing at various temperatures in the range between 600-900 o C for 1 min in vacuum. Current-voltage (I-V) characteristics were obtained at room temperature using an Agilent B1500A semiconductor parameter analyzer. X-ray diffraction (XRD), time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and cross-sectional transmission electron microscope (TEM) were used in order to investigate the metallurgical reactions and the mechanism behind the ohmic contact formation for Hf/Al/Ni/Au based scheme. Y. Liu, M. K. Bera, L. M. Kyaw, G. Q. Lo, E. F. Chor Low resistivity Hf/Al/Ni/Au Ohmic Contact Scheme to n-Type GaN G World Academy of Science, Engineering and Technology International Journal of Electrical and Computer Engineering Vol:6, No:9, 2012 957 International Scholarly and Scientific Research & Innovation 6(9) 2012 ISNI:0000000091950263 Open Science Index, Electrical and Computer Engineering Vol:6, No:9, 2012 publications.waset.org/6868/pdf