Effects of Plasma Surface treatment on Ohmic Contact to n-GaN
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Effects of Plasma Surface treatment on Ohmic Contact to n-GaN E.F. Chor and X. J. Kang Center for Optoelectronics, Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576. Abstract: We have investigated the effects of plasma surface treatment on ohmic contacts to n-type GaN. Prior to metal evaporation, n-GaN surface was etched either chemically using aqua-regia (control sample) or by means of inductively coupled Cl2/BCl3, N2 or Ar plasmas. The metal system adopted in our investigation was Ti/Al/Pd/Au. The as-deposited Ti/Al/Pd/Au contact on aqua-regia treated n-GaN exhibits an ohmic behavior with a specific contact resistance (ρc) of 2×10-4 FP2. After annealing at 500Û&IRUPLQρc is reduced by approximately 2 orders of magnitude to 5×10-6 FP2. For Cl2/BCl3 and N2 plasma treated n-GaN surface, the as-deposited Ti/Al/Pd/Au contacts also yield ohmic behavior and ρc’s are about 2×10-4 and 5×10-4 FP2 respectively. After annealing at 500Û&IRUPLQρc’s are reduced by about 3 orders of magnitude to around 5×10-7 and 8×10-7 FP2 respectively. On the other hand, the as-deposited Ti/Al/Pd/Au contact on Ar plasma treated n-GaN exhibits a non-ohmic behavior. After annealing at 500Û&IRU 5min, ρc is about 5×10-7 FP2 and the lowest ρc of 7×10-8 FP2 is obtained after annealing at 700Û&IRUPLQ7KHFRQWDFWVRQRWKHUVXUIDFHWUHDWHGQ-GaN are found to degrade at 700Û& annealing. By comparing the values of ρc for various surface treatments, particularly those after annealing, it is observed that plasma etching can be used beneficially in the formation of ohmic contacts – plasma surface treatment using Cl2/BCl3, N2 or Ar has resulted in an order of magnitude reduction in ρc compared to chemical surface treatment by aqua-regia. I. INTRODUCTION GaN-based materials and devices have been extensively investigated in recent years and field-effect transistors (FETs), high-electron-mobility transistors (HEMTs), visible and ultraviolet (UV) lasers, LEDs, etc have been demonstrated[1]. These exciting applications present challenges and necessities in making high-performance ohmic contact to GaN-based materials. III-V nitride semiconductors are chemically stable and insoluble in most common chemical etchants at room temperature. Therefore, dry etching has become an important technology in IIIV nitrides device fabrication and is used to transfer patterns in device fabrication. Many investigations of dry etching of GaN and related materials using various plasma chemistries have been realized in three primary dry etching techniques: reactive ion etching (RIE), electron cyclotron resonance (ECR) plasma etching and inductively coupled plasma (ICP) etching. Owing to the higher plasma density of ECR than RIE, the use of ECR plasma etching has been demonstrated to have much faster etching rates and less surface damage than RIE etching for IIInitrides, and ICP offers an alternative high-density plasma technique to etch III-V nitrides [2], [3]. One of the problems for dry etc
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