Vertical-structured Ni/n-GaN schottky diode with electroplating nickel substrate

Shui Jinn Wang, Shu Cheng Chang, Kai Ming Uang, Bor Wen Liou

Research output: Chapter in Book/Report/Conference proceedingConference contribution

2 Citations (Scopus)


For the past decade, most of vertical device applications of GaN were processed by either wafer bonding or direct epitaxy on SiC or GaN wafer. However, the former usually suffers from large strain between GaN and bonding wafer because the high temperature and high pressure bonding process, and the later is not cost effective because of using SiC or GaN substrate. In this work, a novel vertical structure of Ni/n-GaN Schottky barrier diode (SBD) with metallic substrate employing nickel electroplating and laser lift-off (LLO) processes is proposed and its electrical characteristics is reported. Figure 1 shows the processes flow of device fabrication. The GaN epi-layer consists of a 25-nm-thick GaN buffer layer, a 2-μm-thick undoped GaN and a 1.5-μm-thick GaN films with doping concentration of around 5×1018 cm -3, which were grown subsequently on (0001) sapphire substrates by metal organic chemical vapor deposition. The Ti(15 nm)/Al(600 nm)/Ti(15 nm)/Au(100 nm) metal system were used as the ohmic contact which were deposited in sequence on n+ GaN using e-beam evaporator under a background pressure of around 1×10-6 torr. After that, the samples were subjected to rapid thermal annealing at 800°C in Ar for 30 sec to further improve the ohmic behavior. Note that the ohmic metal not only ensures good contact properties to both n+ GaN layer and the upper nickel layer, but also provids a good current conductive path for nickel electroplating. The electroplating process was conducted under a current of around 1.7 A for 30 min with the plating solution kept at around 55°C and a nickel layer with a thickness of around 50 μm was achieved. To remove the sapphire substrate, a 248 nm KrF Excimer laser with 800 mJ/cm2 laser power was used to cause local heating at the GaN/sapphire interface which in turn lead to the decomposition of the GaN. After the LLO process, the sample was then heated to 40°C and the electroplated-Ni adhered GaN epilayer was then separated from the sapphire substrate. It is noted that during the LLO process, as shown schematically in Fig. 1(d), different device sizes can be readily obtained by adjusting the spot size of the laser beam via the copper mask. Fig. 1(e) sketches schematically the device area after LLO and sapphire substrate removal. To reduce the possible Ga residues on the surface of the GaN layer, the sample was chemically etched in a 24%-KOH solution at 60°C for 1-3 min. Figure 2 shows the top view of the sample after 1-min KOH etching obtained form optical microscope (OM) photography. The device area is 300×300 μm2. Finally, as shown in Fig. 1(f), the Schottky metal, Ni, was deposited using e-beam evaporation and patterned by lift-off process. Etch SBD has a 200-μm diameter. Figure 3 shows the forward current voltage characteristics of samples subjected to different KOH etching times. Note that SBDs without KOH etching were also fabricated for comparison. For the 3-min KOH etched sample, it is seen that the forward I-V characteristics becomes ohmic, it might be due to the lengthy KOH etching has resulted in a significant surface roughness on the N-face GaN [1]. The ideality factor, η, series resistance, Rs, and Schottky barrier height, ΦB, of the fabricated SBDs as listed in table I were extracted by modified Norde method [2]. It is seen that the 1-min KOH etched sample has the best ideality factor of 1.06 and a Schottky barrier height of 0.78 eV. The series resistances of all samples are all within 1-10 mΩ, which shows the good quality of both the Ni substrate and the Ti/Al/Ti/Au ohmic contact. Comparison of the measured I-V characteristics for various SBDs with and without KOH etching is shown in Fig. 4. It is evident that the sample without KOH etching has a relatively poor reverse characteristic, while the 1- and 3-min etched samples exhibit acceptable reverse characteristics with breakdown voltage of 21.4 and 19.2 V, respectively. In conclusion, a novel vertical-structured Ni/n-GaN SBD with metallic substrate employing nickel electroplating and LLO processes has been successfully fabricated. The influence of KOH etching for the GaN surface has also studied. Experimental results show that the 3-min etched samples has no Schottky behavior because a significant surface roughness on the N-face GaN layer has been formed after lengthy KOH etching. On the contrary, the 1-min etched sample has the best ideality factor of 1.06, a Schottky barrier height of 0.78 eV and a breakdown voltage 21.4V.

Original languageEnglish
Title of host publication63rd Device Research Conference Digest, DRC'05
Number of pages2
Publication statusPublished - 2005 Dec 1
Event63rd Device Research Conference, DRC'05 - Santa Clara, CA, United States
Duration: 2005 Jun 202005 Jun 22

Publication series

NameDevice Research Conference - Conference Digest, DRC
ISSN (Print)1548-3770


Other63rd Device Research Conference, DRC'05
CountryUnited States
CitySanta Clara, CA

All Science Journal Classification (ASJC) codes

  • Engineering(all)

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    Wang, S. J., Chang, S. C., Uang, K. M., & Liou, B. W. (2005). Vertical-structured Ni/n-GaN schottky diode with electroplating nickel substrate. In 63rd Device Research Conference Digest, DRC'05 (pp. 59-60). [1553054] (Device Research Conference - Conference Digest, DRC; Vol. 2005).