Electrical characterization and nanoscale surface morphology of optimized Ti/Al/Ta/Au ohmic contact for AlGaN/GaN HEMT
© Wang and Kim; licensee Springer. 2012
Received: 22 July 2011
Accepted: 7 February 2012
Published: 7 February 2012
Good ohmic contacts with low contact resistance, smooth surface morphology, and a well-defined edge profile are essential to ensure optimal device performances for the AlGaN/GaN high electron mobility transistors [HEMTs]. A tantalum [Ta] metal layer and an SiNx thin film were used for the first time as an effective diffusion barrier and encapsulation layer in the standard Ti/Al/metal/Au ohmic metallization scheme in order to obtain high quality ohmic contacts with a focus on the thickness of Ta and SiNx. It is found that the Ta thickness is the dominant factor affecting the contact resistance, while the SiNx thickness affects the surface morphology significantly. An optimized Ti/Al/Ta/Au ohmic contact including a 40-nm thick Ta barrier layer and a 50-nm thick SiNx encapsulation layer is preferred when compared with the other conventional ohmic contact stacks as it produces a low contact resistance of around 7.27 × 10-7 Ω·cm2 and an ultra-low nanoscale surface morphology with a root mean square deviation of around 10 nm. Results from the proposed study play an important role in obtaining excellent ohmic contact formation in the fabrication of AlGaN/GaN HEMTs.
Keywordsohmic contact contact resistance surface morphology edge line definition high electron mobility transistor
AlGaN/GaN high electron mobility transistors [HEMTs] are a promising technology for high-frequency, high-temperature, and high-power electronic devices due to the fact that AlGaN/GaN HEMTs have high breakdown voltage, high sheet carrier density, and high saturation current [1–4]. The formation of low-resistance ohmic contacts is a key issue in the fabrication process of AlGaN/GaN HEMTs, which are also required to have a smooth surface morphology and well-defined edge acuity [5–7]. The conventional ohmic contact metal uses a Ti/Al-based Ti/Al/Ti/Au or Ti/Al/Ni/Au structure [8–11]. Ti is essential because it participates in the reaction at the interface with nitrides to form TiN or AlTi2N layers by high-temperature rapid thermal annealing [RTA], and nitride vacancies are simultaneously formed at the AlGaN/GaN surface. At the same time, the diffused Ti and Al reduce the native gallium oxide on the AlGaN/GaN surface . Au is applied as an outer layer to prevent the oxidation of the Ti/Al metals during the RTA process. In addition, a diffusion barrier layer (Ti or Ni) is applied to prevent or minimize the Au upper layer from diffusing downward. However, this scheme often exhibits a very bumpy surface morphology and a significant lateral overflow during alloying at high temperature due to the intermixing of the Au and Al, which forms a viscous AlAu4 phase at high annealing temperatures. In order to minimize this phenomenon, special Au diffusion barrier layers, such as Pt  and Mo , and various studies regarding such factors as Ti/Al metal thickness adjustment, RTA condition changes, ion implantation annealing, recess etching of the ohmic contact, and epi-layer optimization have been carried out [15–17]. Unfortunately, these methods cannot solve all the problems of low contact resistance, smooth surface morphology, and well-defined line edge issues simultaneously.
In this paper, we present a report on an optimized Ti/Al/Ta/Au ohmic contact metallic system with an application of SiNx encapsulation layer before the annealing process, which results in a significantly better ohmic contact behavior for the AlGaN/GaN HEMTs. We present the comparative electrical, morphological, and microstructural properties for different ohmic metallic stacks. It has been found that the proposed ohmic metallization scheme using Ti/Al/Ta/Au with an SiNx encapsulation layer can provide a reliable solution for AlGaN/GaN HEMT applications by supplying superior electrical, morphological, and microstructural properties.
Results and discussion
We posit that the Ti-Al or Ni-Al alloy aggregation in some local areas is the reason for the poor surface formation, and these precipitated Ti-Al or Ni-Al droplets are surrounded by Au-Al alloys. The surface morphology and edge acuity were improved in the Ti/Al/Ta/Au-based ohmic metallization as can be observed in Figure 5c,d. This could be due to the fact that the high melting point of metal Ta is used as a diffusion-impervious layer to suppress the inter-diffusion and reaction between the Au and Al layers in the contact stack which are the main causes of poor surface smoothness and edge acuity.
An optimized Ti/Al/Ta/Au metallization method has been successfully realized that yields improved contact characteristics that are superior to those of the conventional Ti/Al/Ti/Au or Ti/Al/Ni/Au scheme for AlGaN/GaN HEMTs. The effect of the thickness of Ta and encapsulated SiNx film for the proposed Ti/Al/Ta/Au contact has been studied. We have shown that the Ti/Al/Ta/Au metallization scheme encapsulated by a thin SiNx film before the annealing process leads to an excellent ohmic formation that not only obtains a low contact resistance, but also has a smooth morphology and favorable edge line definition. This optimized ohmic contact is most suitable in meeting the requirements of performance improvement for high-power AlGaN/GaN HEMT applications.
This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (no. 2011-0030819). This work was also supported by the Research Grant of Kwangwoon University in 2012. We would like to thank Nano ENS, Inc. for their help in process development and Dr. Won-Sang Lee for the fabrication support of this program.
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