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Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society

this study, we investigated the electrical property and interfacial reactions of the Si/Ti based ohmic contact to n-GaN grown by metal organic chemical vapor deposition (MOCVD) and the electrical property was related with the material reactions. Table 1.The work function of Ti-Si binary compounds[8] Phase Ti 5Si3 TiSi Work function 3.69 - 3.73eV 3.94 - 3.99eV

TiSi 2 3.95 -

4.18eV

EXPERIMENT GaN was grown epitaxially on sapphire substrate (0001) by MOCVD and Si-doped with 1.5 10 18cm3 and electron mobility was 210cm 2/Vs. All the metal layers were deposited by means of an electron beam evaporator without breaking the vacuum and base pressure was below 1 X 10- 6Torr. Prior to metallization, the samples were etched in a HCI:DI solution for 1 min followed by DI water rinse to remove the possible native oxides. In order to evaluate the ohmic characteristic, I-V measurements were carried out between two front surface square contacts and all square contacts were patterned during deposition using a stainless steel shadow mask. The contact resistance was evaluated using TLM(Transmission Line Method) pattern and the interspacings between the metal pads were 5lam, I0Opm, 15lm, 20pm, 25pm, 3011m, 40Pm and 50htm. The rectangular metal pad was 2 00 gm wide and 100 [tm long and contact patterns were defined by a standard lift-off process. The samples were patterned and then etched to make mesa areas for TLM measurements and mesas were isolated by chemically-assisted ion beam etching(CAIBE) using C12. All the samples were annealed in N 2 ambient at the temperature range from 400 'C to 900 'C for 3min. Following each heat treatment, I-V characteristic was measured over a range of -4V to +4V using HP 4145B semiconductor parameter analyzer. The interfacial reactions were characterized by x-ray diffraction(XRD), Auger electron spectroscopy (AES) with Ar' ion sputtering, cross-section transmission electron microscopy(XTEM) and energy dispersive spectroscopy(EDS). X

RESULTS Electrical Properties In order to examine the ohmic characteristics of Si/Ti based ohmic contacts, the I-V measurements were carried out for Au(1000 A)/Ti(400 A)/Si(1460 A)/Ti(150 A) contacts before and after annealing at 400 - 900 °C for 3min. Au/Ti/Si(l 460 A)/Ti contacts were non-ohmic in the as-deposited condition and it might be related to the native oxide of GaN, which was not removed clearly by HCI solution treatment. It showed the nearly ohmic behavior after annealing at 500 'C for 3min. As the annealing temperature increased up to 500'C, the native oxide of GaN was removed by Ti and then Ti with m-4,33eV was expected to make ohmic contact to nGaN(X,=4.3eV) due to low barrier height between Ti/GaN interface. Au/Ti/Si(1460 A)/Ti contacts showed perfect ohmic behavior after annealing at 700 'C and the total resistance of contacts, which was inversely proportional to the slope of I-V curve, was remained until annealing temperature was increased up to 900 'C.

1084

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