Microstructure of Ti/Al ohmic contacts for n-AlGaN

Ruvimov, S.; Liliental‐Weber, Z.; Washburn, J.; Qiao, D.; Lau, S. S.; Chu, Paul K.

doi:10.1063/1.122512

Cited by 129 publications

(79 citation statements)

References 12 publications

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“…Physically this process is triggered by the negative heat of formation of HfN (-88.2 kcal/mol). 27 In addition, the out-diffusion of N could leave donor-like N-vacancies in In 0.18 Al 0.82 N/GaN, similar to the Ti-based ohmic contact on GaN and related materials, 10,28,29 thus forming a thin heavily doped layer at the interface of Hf-based ohmic contact and In 0.18 Al 0.82 N/GaN substrate to reduce the barrier width, which is essential for efficient electron tunneling. Additionally, Ga out-diffusion is observed, which could be due to the breaking of Ga-N bonds, as the onset of thermal dissociation of GaN around 600…”

Section: Ecs Journal Of Solid State Science and Technology 4 (2) P30mentioning

confidence: 99%

Mechanisms of Ohmic Contact Formation and Carrier Transport of Low Temperature Annealed Hf/Al/Ta on In_0.18Al_0.82N/GaN-on-Si

Liu

Singh

Kyaw

et al. 2014

ECS J. Solid State Sci. Technol.

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The mechanisms of ohmic contact formation and carrier transport of low temperature (600 GaN based high mobility electron transistors (HEMTs) are of interest for applications in high-temperature, high-speed and high-power devices owing to the wide bandgap, high saturation velocity and high critical breakdown field of GaN.1,2 To realize high performance in AlGaN/GaN and InAlN/GaN HEMTs, good ohmic contacts with low resistivity are critical. Although very low contact resistance can be achieved by means of regrowth technique, 3 alloyed ohmic contacts are still most widely used due to their fabrication simplicity and low cost. The typical ohmic metal stacks are Ti/Al based (e.g., Ti/Al/X/Au), where Ti is the bottom layer, followed by Al, a diffusion barrier layer X (where X = Ni, Mo and Ti, etc.) and a Au capping layer. [4][5][6][7][8][9][10][11] In order to realize a low contact resistivity for the aforementioned ohmic contact scheme, the metal stack is usually annealed at high temperature (∼800• C), which leads to the presence of randomly distributed TiN inclusions, so-called contact inclusions or contact spikes, formed at the metal-semiconductor interface. 8,12 The presence of contact inclusions has shown to be vital to achieving good ohmic properties by Kim et al. 13 for InAlN/GaN HEMTs, as contact inclusions provide direct electron transport paths through the InAlN barrier layer to the 2DEG channel. On the other hand, these contact inclusions can also lead to lower breakdown voltage for GaN based HEMTs. 14,15 In our recent study, 16 we have proposed a low thermal budget (≤600• C) Hf/Al/Ta ohmic contact scheme on unintentionally doped In 0.18 Al 0.82 N/GaN grown on Si substrate, which has demonstrated low contact resistivity, smooth surface morphology and metalsemiconductor interface. In addition, the In 0.18 Al 0.82 N/GaN HEMTs fabricated with Hf/Al/Ta ohmic contacts have shown significantly improved breakdown voltage, compared to devices with traditional Ti/Al/Ni/Au ohmic contacts, owing to the source-carrier-injection mechanism suppressed by the smooth metal-semiconductor interface in the former. Our low thermal budget Hf/Al/Ta ohmic contacts show that even with the absence of contact inclusions, good ohmic contact properties can be achieved. This is in contrast to the result of Kim et al., which has shown the necessity of contact inclusions to ohmic contact formation. We noted that our InAlN layer is much thinner than that of Kim et al., hence may not need the presence of contact inclusions for good ohmic contact formation. In addition, it is noted that although analytically studies on the transport mecha- * Electrochemical Society Active Member.z E-mail: a0068100@nus.edu.sg; elecef@nus.edu.sg nism of ohmic contacts with spike structures on AlGaN/GaN 17,18 and InAlN/GaN 13,19 heterostructures have been reported, similar studies for ohmic contacts with a smooth metal-semiconductor interface on InAlN/GaN have not been reported. Hence, the Hf/Al/Ta contacts can be used to understand the carrier transport mechani...

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Section: Ecs Journal Of Solid State Science and Technology 4 (2) P30mentioning

confidence: 99%

Mechanisms of Ohmic Contact Formation and Carrier Transport of Low Temperature Annealed Hf/Al/Ta on In_0.18Al_0.82N/GaN-on-Si

Liu

Singh

Kyaw

et al. 2014

ECS J. Solid State Sci. Technol.

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“…6,8 Work on epitaxial films was only published for Ti/ Al bilayers deposited onto AlGaN / GaN heterostructures, showing interfacial Ti 2 AlN and Ti 3 Al formation after annealing at 950°C for 80 min. 9 Here, we present results of in situ x-ray studies on epitaxial and phase-pure Ti 2 AlN formed by solid-state reaction between epitaxial AlN and Ti at annealing temperatures as low as 500°C in only 5 min.…”

mentioning

confidence: 99%

Topotaxial growth of Ti2AlN by solid state reaction in AlN∕Ti(0001) multilayer thin films

Höglund

Beckers

Schell

et al. 2007

Applied Physics Letters

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The formation of Ti 2 AlN by solid state reaction between layers of wurtzite-AlN and ␣-Ti was characterized by in situ x-ray scattering. The sequential deposition of these layers by dual magnetron sputtering onto Al 2 O 3 ͑0001͒ at 200°C yielded smooth, heteroepitaxial ͑0001͒ oriented films, with abrupt AlN / Ti interfaces as shown by x-ray reflectivity and Rutherford backscattering spectroscopy. Annealing at 400°C led to AlN decomposition and diffusion of released Al and N into the Ti layers, with formation of Ti 3 AlN. Further annealing at 500°C resulted in a phase transformation into Ti 2 AlN͑0001͒ after only 5 min.

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“…Al concentration in the barrier layer also increases reactivity of Ti from the metal contact and N from the barrier layer to form an AlTi 2 N alloy. 15,16 The addition of Al into the metallization scheme is observed in Fig. 3 with HRTEM imaging at 900…”

Section: Ohmic Contact Formationmentioning

confidence: 99%

Low Source/Drain Contact Resistance for AlGaN/GaN HEMTs with High Al Concentration and Si-HP [111] Substrate

Duffy

Benbakhti

Mattalah

et al. 2017

ECS J. Solid State Sci. Technol.

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An optimized fabrication process of ohmic contacts is proposed to reduce the source/drain access resistance (R C ) and enhance DC/RF performance of AlGaN/GaN HEMTs with a high Al concentration. We show that source/drain R C can be considerably lowered by (i) optimally etching into the barrier layer using Ar + ion beam, and by (ii) forming recessed contact metallization using an optimized Ti/Al/Ni/Au (12 nm/200 nm/40 nm/100 nm) multilayers. We found that a low R C of ∼0.3 .mm can be achieved by etching closer to the 2-Dimensional Electron Gas (2DEG) at an optimum etching depth, 75% of the barrier thickness, followed by a rapid thermal annealing at 850 • C. This is due to the very small distance between the alloy and the 2DEG (higher electric field) as shown by 2D AlGaN/GaN High Electron Mobility Transistors (HEMTs) are significantly beneficial to the semiconductor device industry. They have the potential to replace Si in a range of high-power and high-frequency applications due to the wide bandgap, high electron saturation velocity, good thermal conductivity and high piezoelectricity of III-Nitrides materials.1-4 To fully benefit from the aforementioned material properties, reduction of extrinsic source and drain access resistances (R C ) of AlGaN/GaN HEMTs is essential to enhance their DC/RF performance. To achieve this, several methodologies have been previously proposed.5-9 Firstly, implanting Si into the AlGaN barrier is known to reduce ohmic contact, R C . However, annealing contacts at high temperatures, to activate dopants, results in diffusion of the Si dopants away from the contacts. The result is increased high-frequency (HF) charge trapping and gate leakage current.10 Secondly, increasing the Al concentration in the barrier and/or using AlN exclusion layer is known to increase 2-Dimensional Electron Gas (2DEG) confinement and to enhance 2DEG density and mobility, which enhances performance. 8,9 However, it results in increased R C due to low metal diffusion beneath the metal contact. 11,12 In this work, we propose a fabrication process of ohmic contacts to reduce R C of AlGaN/GaN HEMTs with a high Al concentration, while avoiding implantations that cause HF-traps and gate leakage. This is essential for enhancing device DC and RF performance. For this new proposal, the outer edges of an AlGaN/GaN HEMT, where the source and drain ohmic contacts are to be placed, are etched from the surface of the device down into the AlGaN barrier layer. Contact metal is then evaporated onto the etched locations and rapidly annealed under high temperatures. During the annealing process, contact metal diffuses into the AlGaN barrier layer to form an alloy beneath the metal contact. The change in R C at different etching depths at various annealing conditions is investigated to identify the optimal etching depth given by the lowest R C .The epi-structure of the investigated devices is presented in Device structure section of this paper. The source and drain contact metallization multilayer scheme and ohmic contact formatio...

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Microstructure of Ti/Al ohmic contacts for n-AlGaN

Cited by 129 publications

References 12 publications

Mechanisms of Ohmic Contact Formation and Carrier Transport of Low Temperature Annealed Hf/Al/Ta on In_0.18Al_0.82N/GaN-on-Si

Mechanisms of Ohmic Contact Formation and Carrier Transport of Low Temperature Annealed Hf/Al/Ta on In_0.18Al_0.82N/GaN-on-Si

Topotaxial growth of Ti2AlN by solid state reaction in AlN∕Ti(0001) multilayer thin films

Low Source/Drain Contact Resistance for AlGaN/GaN HEMTs with High Al Concentration and Si-HP [111] Substrate

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Microstructure of Ti/Al ohmic contacts for n-AlGaN

Cited by 129 publications

References 12 publications

Mechanisms of Ohmic Contact Formation and Carrier Transport of Low Temperature Annealed Hf/Al/Ta on In0.18Al0.82N/GaN-on-Si

Mechanisms of Ohmic Contact Formation and Carrier Transport of Low Temperature Annealed Hf/Al/Ta on In0.18Al0.82N/GaN-on-Si

Topotaxial growth of Ti2AlN by solid state reaction in AlN∕Ti(0001) multilayer thin films

Low Source/Drain Contact Resistance for AlGaN/GaN HEMTs with High Al Concentration and Si-HP [111] Substrate

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Mechanisms of Ohmic Contact Formation and Carrier Transport of Low Temperature Annealed Hf/Al/Ta on In_0.18Al_0.82N/GaN-on-Si

Mechanisms of Ohmic Contact Formation and Carrier Transport of Low Temperature Annealed Hf/Al/Ta on In_0.18Al_0.82N/GaN-on-Si