Article Normally Off MOSFET Using Dual-Metal-Gate Structure for the Improvement in Current Drivability

Young Jun Yoon 1 , Jae Sang Lee 1, Dong-Seok Kim 1, Jung-Hee Lee 2 and In Man Kang 2,*

1 Korea Multi-purpose Accelerator Complex, Korea Atomic Energy Research Institute, Gyeongju 38180, Korea; [email protected] (Y.J.Y.); [email protected] (J.S.L.); [email protected] (D.-S.K.) 2 School of Electronics Engineering, Kyungpook National University, Daegu 41566, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-053-950-5513

 Received: 3 August 2020; Accepted: 28 August 2020; Published: 30 August 2020 

Abstract: A gallium nitride (GaN)-based normally off metal–oxide– field-effect (MOSFET) using a dual-metal-gate (DMG) structure was proposed and fabricated to improve current drivability. Normally off operation with a high Vth of 2.3 V was obtained using a Cl2/BCl3-based recess etching process. The DMG structure was employed to improve current characteristics, which can be degraded by recess etching. The ID and gm of a DMG-based device with nickel (Ni)-aluminum (Al) were improved by 42.1% and 30.9%, respectively, in comparison to the performances of a single-metal-gate-based device with Ni because the DMG structure increased electron velocity in the channel region. This demonstrates that the DMG structure with a large work-function difference significantly improves the carrier transport efficiency. GaN-based recessed-gate based on the DMG structure hold promising potentials for high-efficiency power devices.

Keywords: GaN; recessed-gate; normally-off; dual-metal-gate structure

1. Introduction Gallium nitride (GaN)-based electronics have huge potentials for high-efficiency power electronic applications [1,2] because a two-dimensional electron gas (2DEG) with high electron density and high obtained by AlGaN/GaN heterostructures achieves low switching and conduction losses [3,4]. Additionally, GaN-based electronic devices can obtain a higher breakdown voltage than (Si)-based devices because GaN has a higher critical electric field than Si [5]. GaN-based high-electron-mobility (HEMTs) or metal–insulator–semiconductor HEMTs (MIS-HEMTs) show a normally on operation with a negative threshold voltage (Vth) due to the high electron density of 2DEGs. A normally off operation with a high positive Vth is needed to guarantee safe operation in power switching applications [6]. Various structures and fabrication processes such as the recessed-gate [7–11], p-GaN gate [12,13], cascade configuration [14], and fluorine plasma treatment [15,16] have been presented to realize the normally off operation of GaN-based electronic devices. Among the above-stated structures, the recessed-gate devices can achieve a high Vth and wide range of gate drive voltages (VG), as well as low gate leakage current. However, the recess etching, which partially or fully removes the aluminum GaN (AlGaN) layer for normally off operation, significantly degrades current performances due to the reduction in field-effect mobility [8,17]. Although several processes such as tetramethylammonium hydroxide (TMAH) treatment [18] and O2-BCl3 digital etching processes [19] have been employed to minimize etching damage, achieving simultaneously excellent current characteristics, as well as a high Vth, remains a difficult problem. There is a trade-off between the on-state current and Vth because these characteristics are both affected by recess depth [20].

Electronics 2020, 9, 1402; doi:10.3390/electronics9091402 www.mdpi.com/journal/electronics Electronics 2020, 9, 1402 2 of 10

Recently, recessed-gate metal–oxide–semiconductor field-effect transistors (MOSFETs) combined with novel structures such as a regrown AlGaN barrier [11] and double AlGaN barrier [21] were fabricated using epitaxial technology to satisfy both current performance and Vth. However, performances of the devices depended on sensitive epitaxial technology and needed epitaxial equipment. In this work, we applied a dual-metal-gate (DMG) structure in a recessed-gate GaN-based MOSFET to achieve improved current performances and high Vth. The DMG structure has been proposed in Si MOSFETs and can enhance electron velocity due to the distribution impact of the electric field [22]. In addition, we confirmed that the DMG structure enhanced performances of GaN-based MIS-HEMTs [23,24] as previous works. The present work shows the effects of the DMG structure on performances of the recessed-gate MOSFETs. We fabricated a recessed-gate MOSFET with a DMG structure to demonstrate the reduction in channel resistance increased by recess etching. The impacts of the DMG structure on electron density, energy band diagram, electric field, and electron velocity under the recessed-gate region were analyzed using a technology -aided design (TCAD) simulator. The current characteristics of fabricated DMG-based devices were compared to those of single-metal-gate (SMG)-based devices to demonstrate the effects of the DMG structure. Furthermore, the effect of the work-function difference in the DMG structure on current characteristics was investigated.

2. Structure and Fabrication Figure1a,b shows the schematic cross-sections of the SMG- and DMG-based devices, respectively. The SMG-based device is the conventional recessed-gate device with a nickel (Ni) metal gate. The DMG-based device consists of Ni at the source-side gate (M1) and aluminum (Al) or titanium (Ti) at the drain-side gate (M2). The Ni with a high work function was designed on the source-side to obtain a high Vth because the potential barrier height formed by the source-side gate determined V [25]. To form the potential difference in the channel region, Al or Ti with a low work function was Electronicsth 2020, 9, x FOR PEER REVIEW 3 of 10 employed in the M2 region.

(a) (b) (c)

FigureFigure 1. 1. SchematicSchematic cross-sections cross-sections of of the the recessed-gate recessed-gate GaN GaN metal–oxide–semiconductor metal–oxide–semiconductor field-effect field-effect transistortransistor (MOSFET) (MOSFET) with with (a (a) )the the single-metal-gate single-metal-gate (SMG) (SMG) and and (b (b) )dual-metal-gate dual-metal-gate (DMG) (DMG) structures. structures. ((cc)) Optical Optical microscope microscope image image of of the the fabricated fabricated DMG-based DMG-based device. device.

3. ResultsThe SMG-and Discussion and DMG-based devices were fabricated on an identical AlGaN/GaN heterostructure, which was grown by the metal–organic chemical vapor deposition (MOCVD) technique on a c-plane Figure 2a shows the transfer characteristics of the SMG and DMG-based devices. The DMG- substrate. The AlGaN/GaN heterojunction formed a 2DEG with an electron density of based device with Ni-Ti metal exhibited a higher ID and gm than the SMG-based device with Ni metal. 1.0 1013 cm–2 and electron mobility of 1600 cm2/V s, which were estimated using Hall measurement This× result indicates that the DMG structure reduced· the channel resistance because of the improved at room temperature. The epitaxial layers consisted of a 2 µm-thick GaN buffer layer, 100 nm-thick electron velocity. In addition, the DMG structure can improve the channel mobility, which was undoped GaN channel layer, and 17 nm-thick AlGaN layer. The Al composition in the AlGaN layer affected by traps in the Al2O3 gate dielectric layer. The electron beam evaporation of Ni in the SMG was 0.25. The fabrication process started with mesa etching using Cl2/BCl3 plasma-based inductively structure needs higher beam energy compared to Ti, resulting in the high damage in the Al2O3 gate coupled plasma (ICP) to isolate the devices. After the mesa etching, an 8 nm-thick Al O layer as dielectric layer. The damaged gate dielectric layer induced traps [27,28], which degenerated2 3 the channel mobility [29]. In terms of fabrications, as the DMG structure received relatively small damage, the DMG structure had a positive impact on channel mobility. The Vth of the SMG- and DMG (Ni-Ti)-based devices was 2.66 V and 2.3 V, respectively. Although the Vth of the DMG-based device was slightly decreased, the normally off operation was obtained by the potential energy barrier, which was formed by the M1 of the DMG structure. In terms of the off-state leakage current, the leakage current of the DMG-based device was slightly higher than that of the SMG-based device, as shown in Figure 2b. This is due to the potential energy barrier suppressing the leakage current. As shown in Figure 2c, the DMG structure also reduced static on-resistance (Ron) by increasing the maximum ID (ID,max). This result showed that the DMG structure significantly improved Ron and ID,max.

(a)

ElectronicsElectronics2020 2020, 9,, 14029, x FOR PEER REVIEW 3 of3 10 of 10 a protection layer for thermal annealing was deposited using atomic layer deposition (ALD). Then, Au/Ni/Al/Ti/Si metal layers were deposited and annealed at 800 ◦C for 30 s in a dinitrogen (N2) ambient to form in the source and drain regions. After the ohmic process, a 300 nm-thick SiN layer was additionally deposited using plasma-enhanced chemical vapor deposition (PECVD) at 370 ◦C to form a field-plate structure. The SiN, Al2O3, and AlGaN layers were sequentially etched using a buffered oxide etch (BOE) solution and Cl2/BCl3 plasma-based ICP for the normally off operation. Here, the channel resistance can be increased by the surface damage, which was obtained by etching the AlGaN layer. A 30 nm-thick Al2O3 layer as a gate dielectric was deposited using ALD. The gate metal was formed using an electron beam evaporator. The gate metal of the SMG structure consisted of the Ni/Al/Ni metal layer. For the DMG structure, the Ni and Ti/Ni or Al/Ni metal layers were deposited as the M1 and M2, respectively.(a) The DMG structure was fabricated(b) through two photolithography(c) and two metalFigure deposition 1. Schematic processes. cross-sections Finally, of the the recessed-gate post metallization GaN metal–oxide–semiconductor annealing process was field-effect performed to improvetransistor the quality (MOSFET) of the with metal (a) the and single-metal-gate Al2O3 interface (SMG) [26]. and (b) dual-metal-gate (DMG) structures. (c) Optical microscope image of the fabricated DMG-based device. 3. Results and Discussion 3. Results and Discussion Figure2a shows the transfer characteristics of the SMG and DMG-based devices. The DMG-based device withFigure Ni-Ti 2a shows metal exhibitedthe transfer a highercharacteristicsID and gofm thethan SMG the SMG-basedand DMG-based device devices. with Ni The metal. DMG- This resultbased indicates device thatwith the Ni-Ti DMG metal structure exhibited reduced a higher the ID channel and gm than resistance the SMG-based because of device the improved with Ni metal. electron velocity.This result In addition, indicates the that DMG the DMG structure structure can improve reduced the the channel channel mobility, resistance which because was of a theffected improved by traps electron velocity. In addition, the DMG structure can improve the channel mobility, which was in the Al2O3 gate dielectric layer. The electron beam evaporation of Ni in the SMG structure needs affected by traps in the Al2O3 gate dielectric layer. The electron beam evaporation of Ni in the SMG higher beam energy compared to Ti, resulting in the high damage in the Al2O3 gate dielectric layer. Thestructure damaged needs gate higher dielectric beam layer energy induced compared traps to [27 Ti,,28 resulting], which degeneratedin the high damage the channel in the mobilityAl2O3 gate [29 ]. In termsdielectric of fabrications,layer. The damaged as the DMG gate structuredielectric receivedlayer induced relatively traps small [27,28], damage, which thedegenerated DMG structure the channel mobility [29]. In terms of fabrications, as the DMG structure received relatively small had a positive impact on channel mobility. The Vth of the SMG- and DMG (Ni-Ti)-based devices was damage, the DMG structure had a positive impact on channel mobility. The Vth of the SMG- and DMG 2.66 V and 2.3 V, respectively. Although the Vth of the DMG-based device was slightly decreased, the (Ni-Ti)-based devices was 2.66 V and 2.3 V, respectively. Although the Vth of the DMG-based device normally off operation was obtained by the potential energy barrier, which was formed by the M1 of was slightly decreased, the normally off operation was obtained by the potential energy barrier, the DMG structure. In terms of the off-state leakage current, the leakage current of the DMG-based which was formed by the M1 of the DMG structure. In terms of the off-state leakage current, the deviceleakage was current slightly of higher the DMG-based than that of device the SMG-based was slightly device, higher as than shown that inof Figurethe SMG-based2b. This is device, due to as the potentialshown energyin Figure barrier 2b. This suppressing is due to the the potential leakage current.energy barrier As shown suppressing in Figure the2c, leakage the DMG current. structure As alsoshown reduced in Figure static on-resistance 2c, the DMG ( Rstructureon) by increasing also reduced the maximum static on-resistanceID (ID,max ).(R Thison) by result increasing showed the that themaximum DMG structure ID (ID,max significantly). This result improvedshowed thatRon theand DMGID,max structure. significantly improved Ron and ID,max.

(a)

Figure 2. Cont. Electronics 2020, 9, x FOR PEER REVIEW 4 of 10 Electronics 2020, 9, 1402 4 of 10

(b)

(c)

FigureFigure 2. 2.(a )(a Linear) Linear scale scaleI IDD and ggmm, (,(b)b logarithmic) logarithmic scale scale ID, andID, and(c) output (c) output characteristics characteristics of the SMG of the SMGand and DMG-based DMG-based devices. devices. The The LG, LDGGD, D, GDand, and DGSD ofGS bothof both devices devices were were 3 μ 3m,µ m,10 10μm,µ m,and and 5 μ 5m,µm, respectively.respectively. The The M1 M1 and and M2 M2 in in the the DMG DMG structure structure consist consist of of NiNi andand Ti,Ti, respectively.respectively.

WeWe simulated simulated the the SMG- SMG- and and DMG-based DMG-based devices devices usingusing thethe TCAD simulator [30] [30 ]to to investigate investigate thethe effects effects of DMGof DMG structure structure on on electron electron density, density, energy energy band,band, electricelectric field, field, and and electron electron velocity velocity at at thethe recess recess region. region. The The simulation simulation was was performed performed with with variousvarious physical models models for for GaN GaN and and AlGaN AlGaN materials,materials, including including polarization, polarization, low- low- and and high-field-dependent high-field-dependent mobility, and and Schockley–Read–Hall Schockley–Read–Hall (SRH) recombination models. Simulation results of electron density at a zero bias (VG = 0 V and VD = (SRH) recombination models. Simulation results of electron density at a zero bias (VG = 0 V and 0 V) of the SMG- and DMG-based device are shown in Figure 3a. Both devices achieved a low electron VD = 0 V) of the SMG- and DMG-based device are shown in Figure3a. Both devices achieved a low electron density under the gate region at VG = 0 V because the GaN channel layer was fully depleted by the high work-function of Ni(M1). Figure3b shows the energy band diagrams at the o ff-state Electronics 2020, 9, x FOR PEER REVIEW 5 of 10 Electronics 2020, 9, 1402 5 of 10

density under the gate region at VG = 0 V because the GaN channel layer was fully depleted by the high work-function of Ni(M1). Figure 3b shows the energy band diagrams at the off-state condition condition (VG = 0 V and VD = 10 V) of the SMG- and DMG-based devices. The electron potential (VG = 0 V and VD = 10 V) of the SMG- and DMG-based devices. The electron potential barrier was barrier was maintained under the M1 gate region due to the M1 in the DMG structure, even though a maintained under the M1 gate region due to the M1 in the DMG structure, even though a VD = 10 V V = 10 V was applied. As the effect of the M1 gate region on the electron potential barrier is dominant, Dwas applied. As the effect of the M1 gate region on the electron potential barrier is dominant, the the DMG-based device can obtain normally off operation with a positive Vth. The electron potential DMG-based device can obtain normally off operation with a positive Vth. The electron potential distributiondistribution formed formed by by the the DMG DMG structure structure aaffectedffected thethe electricelectric fieldfield distribution at at the the interface interface of of M1M1/M2/M2 and and the the drain-sidedrain-side gate edge. edge. As As shown shown in Figu in Figurere 3c, 3whilec, while the electric the electric field peak field at peak the drain- at the drain-sideside gate gateedge edgeof the of DMG-based the DMG-based device device was smaller was smaller than that than of thatthe SMG-based of the SMG-based device, the device, electric the electricfield peak field at peak the atinterface the interface of M1/M2 of M1 increased,/M2 increased, resulting resulting in the increase in the increase in electron in electron velocity velocity at the atrecess the recess region region because because electron electron velocity velocity is dependent is dependent on the onelectric the electric field [31,32], field [as31 ,shown32], as in shown Figure in Figure3d. Consequently,3d. Consequently, the current the current characteristics characteristics can be can improved be improved by the byDMG the structure DMG structure because, because, as the aselectron the electron velocity velocity increases, increases, carrie carrierr transport transport efficiency efficiency is enhanced. is enhanced.

(a)

(b)

Figure 3. Cont.

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(c)

(d)

FigureFigure 3. Technology 3. Technology computer-aided computer-aided design design (TCAD) (TCAD) simulated simulated results results of of the the devices devices with with the the SMG SMG (Ni) and(Ni) DMGand DMG (Ni-Ti) (Ni-Ti) structures. structures. (a) Electron(a) Electron density density at zeroat zero bias, bias, (b )(b energy) energy band band diagrams diagrams at at the the

off-state,off-state, (c) electric (c) electric field field at the at otheff-state, off-state, and and (d) electron(d) electron velocity velocity at theat the on-state. on-state. The The total totalLG LofG of the the SMG-SMG- and DMG-basedand DMG-based devices devices was was 3 µm. 3 μ Them. TheLM1 LandM1 andLM2 LM2of of the the DMG-based DMG-based device device are are 2 2µ mμm and and 1 1 µm, respectively.μm, respectively.

FigureFigure4a shows 4a shows the transferthe transfer characteristics characteristics of the of the SMG- SMG- and and DMG-based DMG-based devices devices with with di differentfferent LG atLG a atV Da =VD10 = V.10 WeV. We compared compared the the DMG DMG (Ni (Ni and and Ti) Ti) and and SMG SMG (Ni) (Ni) devices devices to to investigate investigate e ffeffectsects of of the DMGthe DMG with with different differentLG on LG currenton current performance. performance. The The DMG-based DMG-based device device exhibited exhibited a highera higherID IDand and m G M1 gm thang than the SMG-basedthe SMG-based device, device, even even though though the the DMG-based DMG-based device device with withLG L= 4 =µ 4m μm (L M1(L = =2 2µ μmm and and LM2 = 2 μm) has a longer channel length than the SMG-based device. This result demonstrates that LM2 = 2 µm) has a longer channel length than the SMG-based device. This result demonstrates that the the effect of the DMG structure significantly increases carrier transport efficiency. The ID of the DMG- effect of the DMG structure significantly increases carrier transport efficiency. The ID of the DMG-based based device gradually decreased as LM2 increased due to the increase in recess resistance. Thus, the device gradually decreased as LM2 increased due to the increase in recess resistance. Thus, the DMG DMG effect significantly reduced when LG largely increased over 4 μm although the LM1/LM2 ratio was effect significantly reduced when LG largely increased over 4 µm although the LM1/LM2 ratio was small. small. In addition, the LM1/LM2 ratio must be optimized to maximize the DMG effect because a short In addition, the LM1/LM2 ratio must be optimized to maximize the DMG effect because a short LM1 LM1 can be affected by short channel effects. The ID,max of the DMG-based device with LG = 4 μm was can be affected by short channel effects. The ID,max of the DMG-based device with LG = 4 µm was also higher than that of the SMG-based device, as shown Figure4b. The DMG structure considerably µ improved the channel resistance of the DMG-based device with LG = 4 m. Electronics 2020, 9, x FOR PEER REVIEW 7 of 10

also higher than that of the SMG-based device, as shown Figure 4b. The DMG structure considerably improved the channel resistance of the DMG-based device with LG = 4 μm. Electronics 2020, 9, 1402 7 of 10

(a)

(b)

FigureFigure 4. (4.a) Transfer(a) Transfer and (andb) output (b) output characteristics characteristics of the SMG- of the and SMG- DMG-based and DMG-based devices with devices different with

LGdifferent. The DGD LGand. TheD GSDGDof and both D devicesGS of both were devices 10 µm were and 10 5 µ μm,m respectively.and 5 μm, respectively. The M1 and The M2 M1 in the and DMG M2 in structurethe DMG consist structure of Ni consist and Ti, of respectively. Ni and Ti, respectively.

Furthermore,Furthermore, we we investigated investigated the the effects effects of the of work-functionthe work-function difference difference between between the M1 the and M1 M2 and onM2 current on current characteristics. characteristics. Figure Figure5 shows 5 shows the transfer the transfer and output and output characteristics characteristics of the of DMG the DMG (Ni-Al, (Ni- Ni-Ti)-Al, Ni-Ti)- and SMG and SMG (Ni)-based (Ni)-based devices. devices. The The DMG-based DMG-based device device with with the the Ni-Al Ni-Al improved improvedID andID andgm gm byby 42.1% 42.1% and and 30.9%, 30.9%, respectively, respectively, compared compared to to the the SMG-based SMG-based device. device. This This result result indicated indicated that that the the effeffectect of of the the DMG DMG structure structure on on current current characteristics characteristics was was further further increased increased by by a largea large work-function work-function diffdifferenceerence between between the the M1 M1 and and M2. M2. The The Ni-Al Ni-Al combination combination has has a largea large electron electron potential potential di ffdifferenceerence becausebecause the the work-function work-function value value of Alof Al (about (about 4.1 4.1 eV) eV) is loweris lower than than that that of Tiof (aboutTi (about 4.3 4.3 eV). eV). The TheID,max ID,max of the DMG-based device with Ni-Al was increased to 258.4 mA/mm, as shown in Figure5b. The DMG structure with a large work-function difference improved carrier transport efficiency.

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of the DMG-based device with Ni-Al was increased to 258.4 mA/mm, as shown in Figure 5b. The ElectronicsDMG structure2020, 9, 1402 with a large work-function difference improved carrier transport efficiency. 8 of 10

(a)

(b)

FigureFigure 5. 5.E ffEffectect of of work-function work-function diff erencedifference in the in DMG the DM structureG structure on current on characteristics:current characteristics: (a) Transfer (a) andTransfer (b) output and (b characteristics) output characteristics of the devices of the with devices the SMG with (Ni) the andSMG DMG (Ni) (Ni-Aland DMG and (Ni-Al Ni-Ti) structures.and Ni-Ti)

Thestructures.LG, DGD The, and LGD, DGSGDof, and the devicesDGS of the were devices 3 µm, were 10 µ m,3 μ andm, 10 5 μµm, respectively.and 5 μm, respectively.

4.4. ConclusionsConclusions WeWe fabricatedfabricated GaN-based recessed-gate recessed-gate MOSFETs MOSFETs with with the the DMG DMG structure structure and and investigated investigated the theeffects effects of ofDMG DMG structure structure on on the the performances performances of of recessed-gate recessed-gate MOSFETs. The fabricatedfabricated devicedevice improvedimproved current current characteristics characteristics because because the the DMG DMG structure structure enhanced enhanced electron electron velocity velocity under under the gate the region.gate region. The ITheD and ID andgm of gmthe of the DMG-based DMG-based devices devices were were higher higher than than those those of of the the SMG SMG device, device, even even thoughthough the theL LGGof of the the DMG-based DMG-based devices devices was was longer longer than than that that of theof the SMG-based SMG-based device. device. A high A highVth Vofth theof the DMG-based DMG-based devices devices was was maintained maintained by by the the work-function work-function of of the the source-side source-side metal. metal. The The device device withwith the the DMG DMG (Ni-Al) (Ni-Al) structure structure achieved achieved a a higher higherI D,maxID,max of approximately 42%42% becausebecause thethe eeffectsffects ofof the DMG structure on current characteristics were further enhanced by the large difference between work-functions in the DMG structure. As a result, the recessed-gate MOSFET using the DMG structure Electronics 2020, 9, 1402 9 of 10

simultaneously achieved excellent current characteristics and a high Vth without controlling the recess etching process.

Author Contributions: Conceptualization, Y.J.Y.; Investigation, Y.J.Y.; Data analysis, Y.J.Y., J.S.L., D.-S.K., J.-H.L. and I.M.K.; writing—original draft preparation, Y.J.Y.; writing—review and editing, I.M.K. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2020R1A2C1005087) and in part by Samsung Electronics Co. Ltd. The study was supported by the BK21 Plus project funded by the Ministry of Education, Korea (21A20131600011), by the Ministry of Trade, Industry & Energy (MOTIE) (10080513), and Korea Semiconductor Research Consortium (KSRC) support program for developing the future semiconductor devices. This work was also supported by the KOMAC (Korea Multi-purpose Accelerator Complex) operation fund of KAERI (Korea Atomic Energy Research Institute). Conflicts of Interest: The authors declare no conflict of interest.

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