Ohmic Contacts with low contact resistance for GaN HEMTs

Abstract

GaN-based high electron mobility transistor (HEMT) is a promising candidate for high-frequency and high-power applications due to its outstanding material properties, such as high electric breakdown field and high peak electron drift velocity. A low contact resistance (Rc) contact is essential for the device performance including output power, power efficiency, frequency response and noise performances. To obtain low contact resistances, several studies using different metallization schemes have been demonstrated. A standard Ti/Al/Ni/Au metal stack is a conventional ohmic contact of GaN HEMTs. Ti reacts with AlGaN to form TiN, which results in the creation of nitrogen vacancies which act as donors in AlGaN layers. The resultant N-type doped AlGaN and the conductive TiN facilitate tunneling mechanism of carriers at the interface.We have demonstrated high-frequency performances for AlGaN/GaN HEMTs processed with ohmic recess technique. The contact resistances were as low as 0.25 •·mm after annealing. We also optimized the gate structure with larger gate head to enhance the OFF-state breakdown voltage for optimizing the power performance at Ka band. The device exhibited superior electrical performances, including a maximum drain current density (IDS,max) of 1.59 A/mm, a peak extrinsic transconductance (gm.ext) of 480 mS/mm, a high current-gain cutoff frequency (fT)/ maximum frequency of oscillation (fMAX) of 71/123 GHz, and a minimum noise figure of 1.91 dB with an associated gain of 6.13 dB at 40 GHz. The device demonstrated a maximum output power density of 4.6 W/mm, with a power-added efficiency of 19.5 %, and a linear gain of 8.2 dB with a larger gate head biased at Vds = 20 V at 38 GHz.High-performance GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs) using ohmic contact with N-type dopants and Al2O3 gate dielectric deposited by atomic layer deposition (ALD) for millimeter-wave power applications is demonstrated. An alloyed Si/Ge/Ti/Al/Ni/Au contacts was used to reduce the ohmic contact resistance over conventional Ti/Al/Ni/Au ohmic contacts. The improvement of the contact resistance is because of the enhanced N-type doped AlGaN by Si and Ge was added to further enhance the doping concentration of the ohmic contact regions. The MOSHEMT device fabricated exhibits IDS,max of 1.65 A/mm and high gm.ext of 653 mS/mm. The MOSHEMT device also demonstrates excellent RF performances including fT/fMAX = 183/191 GHz measured at Vds=5 V. The fMAX of the device was larger than 200GHz when Vds was biased at 20V.The Ti-based ohmic contacts commonly have a rough surface morphology and reduced edge acuity that affect accurate alignment of the gate electrode for downscaled GaN-based HEMTs. Therefore, a gold-free and Titanium-free Ta-based metal stack was utilized for ohmic contact. Low-resistance ohmic contacts formed by sidewall contacts with ohmic recess for GaN HEMTs were demonstrated. To develop a general and viable method for low contact resistances without the need of exact control of the recess depth, an ohmic recess beyond the 2DEG channel was investigated. The impact of tilt during evaporation to provide better ohmic metal coverage on the 2DEG was discussed. A tilt angle of 10° resulted in better metal coverage as well as better ohmic contact resistance. The influence of the sidewall angle at 2DEG channel on the contact resistance was also studied. The sidewall angle can be controlled by reversal baking temperature or exposure intensity of the process. A steeper sidewall of the developed photoresist is transferred to a larger sidewall angle of the recess at the channel. The lowest contact resistances were as low as 0.24 ••mm after annealing at low temperature of 575 °C with a bottom Ta-layer thickness of 20 nm. Finally, the proposed ohmic contact method was applied on different epitaxial heterostructures with different Schottky barrier thickness as well as with and without AlN spacer layer. All the samples exhibited excellent contact resistances, demonstrating the wide process window for low contact resistances using sidewall contacts. These results attest the great potential of sidewall contacts for high-frequency applications.A gold-free ohmic contact of Ti/Al/Ni/Cu with AlGaN/GaN has been fabricated with low contact resistance, smooth surface morphology, and excellent edge acuity. Besides the cost issue, Cu also has lower resistivity and higher thermal conductivity as compared with Au. Therefore, the Cu has been widely used for multi-level interconnects in the silicon VLSI technology. The Ti/Al/Ni/Cu ohmic contact shows similar contact resistance compared to Ti/Al/Ni/Au ohmic contact. Without Al-Au (purple plague) alloy formation, the surface roughness of copper-Ohmic structure becomes much smoother than Au-contact. From the depth profiling Auger Electron Spectroscopy (AES) analysis, the result shows that Cu diffusion was seen stopped at a depth corresponding to the initial Ti thickness. The results suggested that no Cu diffused into the semiconductor layers. Besides, we also demonstrated ohmic contact with gold-free Tungsten metallization. Tungsten is a metal which has the highest melting point and high density. Its chemical stability is great so that diffusion barrier layer is not needed. The Ti/Al/W metallization also shows a much smoother morphology than Au-contact. The Ti/Al/W ohmic contact shows similar contact resistance compared to standard Ti/Al/Ni/Au ohmic contact. As a result, copper and tungsten ohmic contacts for GaN HEMTs was successfully developed using a CMOS-compatible gold-free process.