Low-resistance Pt/Ni/Au ohmic contacts to p-type GaN

APPLIED PHYSICS LETTERS

VOLUME 74, NUMBER 1

4 JANUARY 1999

Low-resistance Pt/Ni/Au ohmic contacts to p-type GaN Ja-Soon Jang, In-Sik Chang, Han-Ki Kim, Tae-Yeon Seong,a) Seonghoon Lee, and Seong-Ju Park Department of Materials Science and Engineering, and Centre for Electronic Materials Research, Kwangju Institute of Science and Technology (KJIST), Kwangju 506-712, Korea

~Received 24 September 1998; accepted for publication 29 October 1998! We report on a Pt ~20 nm! Ni ~30 nm!/Au ~80 nm! metallization scheme for low-resistance ohmic contacts to the moderately doped p-type GaN:Mg (331017 cm23). Both as-deposited and annealed Pt/Ni/Au contacts on p-GaN exhibit linear current–voltage characteristics, showing that a high-quality ohmic contact is formed. The Pt/Ni/Au scheme shows the specific contact resistance of 5.131024 V cm2 when annealed at 350 °C for 1 min in a flowing N2 atmosphere. © 1999 American Institute of Physics. @S0003-6951~99!03101-0#

termined using a circular-transmission line method (c-TLM) 14 which obviates the need for the fabrication of mesa structures by implantation or etching processes. Preliminary results of this observation have been published elsewhere.15 Metalorganic chemical vapor deposition ~Emcore DGaN125™! was used to grow a 50 nm thick GaN buffer layer on a ~0001! sapphire substrate, on which an unintentionally doped 2.5 mm thick GaN layer was grown. This was followed by a 1 mm thick p-GaN:Mg (331017 cm23). Prior to lithography, the samples were ultrasonically degreased with trichloroethylene, acetone, and methanol for 5 min in each step, and then rinsed with de-ionized ~DI! water. They were then patterned by a standard photolithographic technique. The inner dot radius was 200 mm, and the spacings between the inner and the outer radii were in the range of 5–50 mm. Prior to metal film deposition, buffered hydrofluoric ~HF! acid was used to remove the native oxide layer on p-GaN. The Pt ~20 nm!/Ni ~30 nm!/Au ~80 nm! films were deposited on p-GaN by electron beam evaporation ~PLS 500 model!. Current–Voltage (I – V) data were measured using parameter analyzer ~HP 4155A! and Auger depth profiles using Auger electron spectroscopy ~PHI 670 model!.

Due to the success in the development of gallium nitride based devices such as blue light emitting diodes ~LEDs!, metal–semiconductor field effect transistors ~MESFETs!, high electron mobility transistors ~HEMTs!, and laser diodes ~LDs!,1–5 the fabrication of high quality ohmic contacts with low resistance and excellent reliability is of great technological importance. In fact, the high contact resistance of p-GaN is one of the major problems in the realization of longlifetime continuous wave ~cw! operation of GaN-based optical devices. It is therefore crucial to develop high-quality ohmic contacts on p-GaN to enhance device performance. As for ohmic contacts to n-GaN, Ti- or Al-based metallization schemes ~e.g., Al, Ti, Ti/Al, Ti/Au, Ti/Al/Ni/Au, and Pd/Al! have been widely investigated. In such metallization schemes, low contact resistances ranging from ;1025 to ;1028 V cm2 have been reported, which are good enough for the operation of the optical and electronic devices.6–10 For the ohmic contacts to p-GaN, however, there are two main obstacles making it difficult to develop device quality ohmic contacts on p-type GaN. The first arises from a difficulty in growing a heavily doped p-GaN (.1018 cm23). The second results from the absence of appropriate metals having work functions larger than that of p-type GaN ~;7.5 eV!.11 These problems have led to contact resistances of >1022 V cm2. Trexler et al.11 investigated Ni/ Au, Cr/Au, and Pd/Au metallization schemes for p-GaN (9.831016 cm23) ohmic contacts and showed that only the Cr/Au contact was ohmic with a specific contact resistance of 4.131021 V cm2, when annealed at 900 °C for 15 s. Jang et al.12 investigating ohmic contacts on p-GaN using a Ni/ Pt/Au metallization scheme showed that the metal contact was ohmic with a contact resistance of 2.131022 V cm2, when annealed at 500 °C for 30 s in a flowing Ar atmosphere. Mori et al.13 investigating electrical properties of ohmic contacts on p-GaN using Pt, Ni, Au, and Ti single layers reported a specific contact resistance of 1.3 31022 V cm2 for the as-deposited Pt/p-GaN contacts. In this letter we report low resistance Pt/Ni/Au ohmic contacts on the moderately doped p-GaN:Mg. Electrical characteristics such as specific contact resistances were de-

FIG. 1. The I – V characteristics of the Pt/Ni/Au contacts on p-GaN. Both as-deposited and annealed contacts reveal linear I – V ohmic behavior with the specific contact resistances of 331023 – 5.131024 V cm2.

a!

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FIG. 3. Auger spectral data of ~a! the as-deposited and ~b! annealed Pt/ Ni/Au contacts. The spectral result @Fig. 3~b!# was obtained from the Pt/ Ni/Au scheme after sputtering for 12.5 min.

FIG. 2. Auger depth profiles of the Pt/Ni/Au contacts on p-GaN: ~a! As for the as-deposited sample, there is no obvious evidence for interdiffusion between the metal layers and the GaN. However, ~b! for the annealed sample, Ni diffused to the surface of the sample through Au layer and formed an oxide at the surface.

Figure 1 shows the I – V characteristics of the Pt/Ni/Au contacts on p-GaN. As-deposited contact reveals linear I – V ohmic behavior with the specific contact resistance of ;3 31023 V cm2. The contact resistance is better than that reported by Mori et al.13 Such a low resistance for the asdeposited contacts on p-GaN illustrates the possibility of nonalloyed ohmic contacts to p-GaN. Postdeposition heat treatments can cause a small amount of semiconductor material to be consumed during the interfacial reaction. The extent of the reaction can be detrimental to device performance if the active regions of the device were located near the contact. Thus, from both the viewpoints of interfacial reactions and a manufacturing process, nonalloyed contact is highly desirable. As shown in Fig. 1, annealing at 350 °C for 1 min results in further improvement in the ohmic behavior of the Pt/Ni/Au contact. The specific contact resistance was measured to be 5.131024 V cm2. To the best of our knowledge, this is the lowest contact resistance reported hitherto for the contacts on p-GaN. Foresi and Moustakas6 investigating Al and Au schemes for contacts on GaN showed that ohmic behavior could be predicted by considering the work functions of contact metals, since GaN did not suffer from Fermi level pinning.16–18 Ishikawa et al.16 investigating ohmic contacts on p-GaN using a wide variety of metal schemes showed that the contact resistance to p-GaN decreased exponentially as the work function increased, showing the lack of Fermi level pinning

in GaN. Therefore, the formation of the low specific contact resistance observed in the present work can be attributed to the high work function of the Pt in contact with the p-GaN. 12 Further improvement in the contact resistance of the annealed sample may be related to an increase in the contact areas between the metal scheme and the GaN, since the anneal may lead to the roughening of the interface due to interfacial reactions. Figure 2 shows Auger depth profiles of the Pt/Ni/Au contacts on p-GaN. As for the as-deposited sample, there is no obvious evidence for interdiffusion between the metal layers and the GaN @Fig. 2~a!#. However, for the annealed sample @Fig. 2~b!#, Ni diffused to the surface of the sample through the Au layer and formed an oxide at the surface. This is evident from the Auger spectral data @Fig. 3~a!#; the Ni Auger electron peak is detected on the surface of the annealed Pt/Ni/Au contact. It is also shown that the Pt diffused into the GaN @Fig. 2~b!#, as is evident from the Auger spectral result @Fig. 3~b!# which was obtained from the Pt/ Ni/Au scheme after sputtering for 12.5 min. It is certain that a small amount of both the Ni and the Pt is present in the GaN matrix. However, there is no evidence for the outdiffusion of nitrogen into the metal layers. This indicates that the Pt in the GaN might prevent the formation of nitrogen vacancies which are detrimental to p-type ohmic contact performance.11,12 As for ohmic contacts, it is important to be able to control surface morphology. Atomic force microscope ~AFM! examination showed that the surface morphology of both the as-deposited and annealed samples was remarkably smooth, namely, the rms roughness was 0.8 and 1.45 nm for the as-deposited and annealed samples, respectively. For the annealed sample, the smooth surface may be attributed to the formation of the nickel oxide @Fig. 2~b!#. The oxide layer may prevent the degradation of the surface due to a ‘‘balling up’’ effect which has been widely observed in a metallization process. ~Our previous work showed that as annealing temperature increased from 350 to 700 °C, the surface morphology of Ni/Pt/Au metal contacts was gradually degraded and ended up with island structures. In this case, no evidence for the outdiffusion of Ni into the sample surface was observed throughout the temperature range.!19 Detailed work on the interfacial reactions between the Pt/Ni/Au and the


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GaN as a function of annealing temperature and time will be published elsewhere.20 To summarize, we report a promising Pt/Ni/Au contact scheme on the moderately doped p-GaN:Mg (331017 cm23). Both the as-deposited and annealed Pt/Ni/Au schemes led to high quality ohmic contacts on p-GaN. In particular, upon annealing at 350 °C for 1 min, the contact had the lowest specific contact resistance of 5.1 31024 V cm2 reported so far for the p-type GaN contact. The authors would like to thank the Korea Ministry of Information and Communications ~MIC! and the Korea Science and Engineering Foundation ~KOSEF! for financial support. 1

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