Synthesis of titanium nitride for self-aligned gate AlGaN/GaN heterostructure field-effect transistors
© Li et al.; licensee Springer. 2014
Received: 30 June 2014
Accepted: 16 October 2014
Published: 28 October 2014
In this study, titanium nitride (TiN) is synthesized using reactive sputtering for a self-aligned gate process. The Schottky barrier height of the TiN on n-GaN is around 0.5 to 0.6 eV and remains virtually constant with varying nitrogen ratios. As compared with the conventional Ni electrode, the TiN electrode presents a lower turn-on voltage, while its reverse leakage current is comparable with that of Ni. The results of annealing evaluation at different temperatures and duration times show that the TiN/W/Au gate stack can withstand the ohmic annealing process at 800°C for 1 or 3 min. Finally, the self-aligned TiN-gated AlGaN/GaN heterostructure field-effect transistors are obtained with good pinch-off characteristics.
The AlGaN/GaN heterostructure field-effect transistors (HFETs) are excellent candidates for high-power and high-frequency electronic devices [1, 2]. To achieve a high-temperature performance, it is very desirable to produce a gate contact with a large Schottky barrier height (SBH) and an excellent thermal stability. In high-frequency applications, a self-aligned gate (SAG) process is proposed to minimize the source-to-gate and drain-to-gate distances for smaller access resistance, in which a T-shaped Schottky gate is fabricated first and then used as a mask directly for ohmic metal evaporation. Then, the Schottky gate and the ohmic electrodes are annealed simultaneously to obtain ohmic contacts . An important technology to form the SAG structure is the Schottky gate which can withstand the ohmic annealing process, because the optimized ohmic contact annealing temperature of the Ti-based multilayers on GaN-based materials is usually around 800°C to 850°C [4, 5]. Therefore, the Schottky gate must be able to withstand such a high temperature during the source-drain ohmic contact annealing process.
In previous studies, we have evaluated the electrical performance of Schottky contacts produced using different kinds of refractory metal nitrides such as titanium nitride (TiN), MoN, TaN, MoSiN, WTiN, ZrN, and HfN on GaN by reactive sputtering in an ambient of Ar and N2 mixture sputtering gas [6–8]. Considering the adhesion on GaN, sheet resistivity, reverse leakage current, SBH, and thermal stability of these devices, we regard TiN as the suitable material for the Schottky electrode. It can be obtained easily by reactive sputtering with nitrogen as the reactive gas and shows a relatively smaller resistivity, good adhesion, and less leakage current on the GaN Schottky contact.
Herein, we obtain titanium nitride (TiN) by reactive sputtering using different N2/Ar sputtering gas ratios. The annealing evaluation results demonstrate that the TiN/W/Au gate on AlGaN/GaN HFETs can withstand the 800°C annealing temperature for 1 or 3 min. Finally, the TiN-gated AlGaN/GaN HFETs fabricated with a self-aligned process are obtained.
To evaluate the effect of the precursor composition, we deposited TiN films on Si-doped n-GaN (3 × 1017 cm−3 dopant density, 1-μm thick) using different N2/Ar sputtering gas ratios (nitrogen percentage) of 0:18 sccm (0%), 1:17 sccm (5%), 3:15 sccm (15%), 7:11 sccm (40%), 11:7 sccm (60%), and 15:3 sccm (85%). To maintain a uniform current flow between the ohmic contact and the Schottky contact, a circular Schottky pattern with a diameter of 166 μm was adopted. The ohmic contact was placed on the same side as the Schottky contact with a separating distance of 15 μm to simplify the process. A standard lift-off technology was used to form both the ohmic and the Schottky contacts. Ohmic contact was formed using a Ti/Al/Ti/Au (50/200/40/40 nm) multilayer structure, a fixed structure which is being adopted in our process system, and annealed at 800°C for 1 min. Prior to the TiN deposition process, the sample surfaces were cleaned by O2 plasma ashing and immersion in a diluted HCl (HCl:H2O = 1:1) solution for 5 min to remove any oxide layer that developed after the lithography process. The direct current (DC) sputtering power was fixed at 75 W with a chamber pressure of 0.14 Pa during the reactive sputtering.
To fabricate the TiN-gated devices by gate-first process, we firstly optimized the T-gate fabrication process on a silicon substrate using a three-layer e-beam resist technique . Then, the self-aligned gate HFETs were obtained on AlGaN/GaN wafer with the optimized conditions. For the three-layer e-beam resist technique, the bottom resist layer was a ZEP520A (500 nm) layer, followed by a LOR 5B (800 nm) resist. The upper layer was also a ZEP520A (500 nm) layer. At first, the upper layer was exposed by the electron beam and developed. Then, the LOR layer was developed using a specific developer without exposure. Finally, the bottom layer was exposed by the electron beam and developed again. The stack structure of TiN/W/Au (200/50/200 nm) was then deposited to form a T-gate electrode. The drain and source regions, including the gate and the access regions, were exposed and developed using lithography technology. Ti/Al/Ti/Au multilayers with thickness of 30/120/40/40 nm were formed by lift-off technology for the drain and source contact, where the T-shaped Schottky gate was used as the mask directly. The Schottky gate and the ohmic electrodes were annealed simultaneously at 800°C for 1 min to obtain ohmic contacts.
Results and discussion
Beside, the reverse current leakage of Ni is just about 1 ~ 2 orders of magnitude lower than that of TiN diode. As compared with the value calculated from the thermionic emission theory using the Schottky barrier height of 0.59 and 0.87 eV for TiN and Ni diode, respectively, the measured leakage current of Ni is much higher than the calculated one, while those currents are in the same level for TiN. The obvious increase of leakage current of the Ni diode is attributed to the higher interface states (as compared with TiN) existing at the Ni/GaN interface caused by the interaction, leading to a defect-assisted tunneling effect .
We deposited TiN films with different N2/Ar sputtering gas ratios for self-aligned gate process. All of the samples showed good rectifying properties with an SBH of 0.5 to 0.6 eV. As compared with the conventional Ni electrode, it presented a lower turn-on voltage while the reverse leakage currents are comparable with Ni. The effects of annealing temperature and time on the electrical properties of TiN/W/Au-gate AlGaN/GaN HFETs have been investigated. It is demonstrated that the TiN/W/Au electrode could withstand the ohmic annealing process at 800°C for 1 or 3 min. Then, AlGaN/GaN HFETs with TiN gate were obtained with a self-aligned process, showing an excellent operation with a threshold voltage of about −4 V and a maximum drain current density of 540 mA/mm.
This work is partly supported by the Grant-in-Aid for Scientific Research (No. 22560332) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors would like to thank Mr. Yunbo Liu from SAE magnetic Ltd. for supporting in the measurements and analysis of the atomic force microscope images.
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