Effect of Au distribution in NiO/Au film on the ohmic contact formation to p-type GaN
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S.J. Chang Department of Electrical Engineering, Institute of Microelectronics, National Cheng Kung University, Tainan 70101, Taiwan, Republic of China (Received 16 June 2004; accepted 5 November 2004)
The distribution of Au and NiO in NiO/Au ohmic contact on p-type GaN was investigated in this work. Au (5 nm) films were deposited on p-GaN substrates by magnetron sputtering. Some of the Au films were preheated in N2 ambient to agglomerate into semi-connected structure (abbreviated by agg-Au); others were not preheated and remained the continuous (abbreviated by cont-Au). A NiO film (5 nm) was deposited on both types of samples, and all samples were subsequently annealed in N2 ambient at the temperatures ranging from 100 to 500 °C. The surface morphology, phases, and cross-sectional microstructure were investigated by scanning electron microscopy, glancing incident angle x-ray diffraction, and transmission electron microscopy. I-V measurement on the contacts indicates that only the 400 °C annealed NiO/cont-Au/p-GaN sample exhibits ohmic behavior and its specific contact resistance (c) is 8.93 × 10−3 ⍀ cm2. After annealing, Au and NiO contact to GaN individually in the NiO/agg-Au/p-GaN system while the Au and NiO layers become tangled in the NiO/cont-Au/p-GaN system. As a result, the highly tangled NiO-Au structure shall be the key to achieve the ohmic behavior for NiO/cont-Au/p-GaN system.
I. INTRODUCTION
GaN is a semiconductor that has been extensively investigated for optoelectronic applications,1,2 such as light emitting diodes (LEDs) and laser diodes (LDs), because of its wide-spectrum and short-wavelength light emission. The optoelectronic performance of GaN depends on not only the quality of GaN crystals but also the specific contact resistance of metal electrodes to n- and p-type GaN. However, it is a great challenge to obtain low contact resistance to p-type GaN because the carrier concentration of p-GaN cannot reach the degenerated level and lack of metals with high work function compared to the band gap of GaN. Thus, metallic materials with high work function are expected to alleviate the resistance. Multilayer metallizations with high work function, including Ni/Au,3–7 Pt/Au,8 Pt/Ru,9 and Pd/Ru,10 etc., have been widely fabricated as a contact to p-type GaN. Many authors7,11–14 have reported that Au/Ni based bi-layer structure annealed in oxygen ambient or air is a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0066 456
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 2, Feb 2005 Downloaded: 22 Mar 2015
able to form ohmic contacts to p-GaN with a specific contact resistance in the range of 10−2 to 10−6 ⍀ cm2. The exact mechanism on the formation of ohmic contact to p-type GaN is still unclear and controversial. Ho et al.12 fabricated Au/Ni/p-GaN contact structure and annealed the contacts in air. The final contact structure after annealing is Au/p-NiO/p-GaN. According to the equilibrium energy-band diagram shown in that paper, p-type N
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