# Fileset [shrine20230224-1595-gqy7mo.pdf](https://mdr.nims.go.jp/filesets/5b3837fb-9817-4bb2-a7fe-1d03fe9d08ad/download) ## Creator [LIU, Jiangwei](https://orcid.org/0000-0003-2580-7401), OOSATO, Hirotaka, [DA, Bo](https://orcid.org/0000-0002-0785-8662), KOIDE, Yasuo ## Rights In Copyright[In Copyright](http://rightsstatements.org/vocab/InC/1.0/) ## Other metadata [High Current Output Hydrogenated Diamond Triple-Gate MOSFETs](https://mdr.nims.go.jp/datasets/eeb521b0-1e71-4e2c-94ca-2326d729526a) ## Fulltext High Current Output Hydrogenated Diamond Triple-Gate MOSFETsReceived 7 March 2019; revised 18 April 2019; accepted 2 May 2019. Date of publication 7 May 2019; date of current version 24 May 2019.The review of this paper was arranged by Editor N. Collaert.Digital Object Identifier 10.1109/JEDS.2019.2915250High Current Output HydrogenatedDiamond Triple-Gate MOSFETsJIANGWEI LIU 1, HIROTAKA OHSATO2, BO DA3, AND YASUO KOIDE41 Research Center for Functional Materials, National Institute for Materials Science, Ibaraki 305-0044, Japan2 Nanofabrication Platform, National Institute for Materials Science, Ibaraki 305-0047, Japan3 Research and Services Division of Materials Data and Integrated System, National Institute for Materials Science, Ibaraki 305-0047, Japan4 Research Network and Facility Services Division, National Institute for Materials Science, Ibaraki 305-0047, JapanCORRESPONDING AUTHOR: J. LIU (e-mail: liu.jiangwei@nims.go.jp)This work was supported in part by the KAKENHI Project under Grant 18K13806 and Grant 16H06419, in part by the Leading Initiative for Excellent YoungResearchers Program Project, and in part by the National Institute for Materials Science Nanofabrication Platform of the Nanotechnology PlatformProject sponsored by the Ministry of Education, Culture, Sports, and Technology, Japan.ABSTRACT Planar-type and novel triple-gate fin-type hydrogenated diamond (H-diamond) metal–oxide–semiconductor field-effect transistors (MOSFETs) were fabricated on a single-crystalline diamondsubstrate. The ratio between the height of the lateral side and the width of planar side for each finof the triple-gate MOSFETs was as high as 1.45. The leakage current densities at an electrical fieldstrength of −1.5 MV cm−1 for both the planar-type and triple-gate fin-type MOSFETs were around10−6 A cm−2. Both MOSFETs operated well with on/off ratios as high as 1010. The current outputmaximum normalized by the gate width of the triple-gate H-diamond MOSFET was −271.3 mA mm−1,almost double that of the planar-type MOSFET. The results of this paper are expected to pave the waytowards the fabrication of high current out and downscaled H-diamond MOSFETs.INDEX TERMS Diamond, MOSFET, triple-gate.I. INTRODUCTIONThanks to its extraordinary intrinsic properties, wide bandgapsemiconductor diamond is a promising material for the fabri-cation of electronic devices for low power loss, high-power,high-frequency, and high-temperature applications [1]–[3].Recently, significant progress has been made on the develop-ment of p-type boron-doped bulk diamond and hydrogenateddiamond (H-diamond) surface-channel-based metal-oxide-semiconductor field-effect transistors (MOSFETs) [4]–[10].Excellent operation characteristics have been demonstratedfor all of these MOSFETs. However, since the high activa-tion energy of the boron dopant in the bulk diamond resultsin a relatively low sheet hole density at room temperature,the current outputs of boron-doped diamond MOSFETs wereusually lower than 1.0 mA mm−1 [4], [5]. Conversely, thesurface sheet hole density of a H-diamond channel layeris in the range of 1012 − 1014 cm−2. The current outputsof the single-crystalline and polycrystalline H-diamond-based MOSFETs were over 200 and 1300 mA mm−1,respectively [6], [7]. Additionally, the operation temperatureand breakdown voltage of the single-crystalline H-diamondMOSFETs were reported to be as high as 400 ◦C and 1700 V,respectively [8], [9].Although the single-crystalline H-diamond-basedMOSFETs show excellent electrical properties, a lackof commercially available large-area wafers is currentlypreventing them from being used in practical applications.To resolve this issue, some researchers are engaged ingrowing wafers with diameters of over 2 inches via chem-ical vapour deposition techniques [11], [12]. Instead, wehave tried to enhance the electrical properties of theH-diamond MOSFETs on the small area wafers. Recently,a triple-gate fin-type H-diamond MOSFET was fabricatedsuccessfully [13]. Since holes in the fin channel cantravel along conducting channels on three sides (underthe gate and at both lateral sides), the current output andextrinsic transconductance of fin-type MOSFETs are muchhigher than those of planar-type MOSFETs with the samedevice area. This conclusion was also confirmed by theoxygen-terminated diamond-based fin-type MOSFETs [14].2168-6734 c© 2019 IEEE. Translations and content mining are permitted for academic research only.Personal use is also permitted, but republication/redistribution requires IEEE permission.VOLUME 7, 2019 See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 561https://orcid.org/0000-0003-2580-7401LIU et al.: HIGH CURRENT OUTPUT H-DIAMOND TRIPLE-GATE MOSFETs(a) (b) (c) (d) (e)FIGURE 1. Formation process for the fin-type H-diamond epitaxial layer. (a) Fin model formation using a laser lithography system, (b) MgO formationusing an evaporator, (c) diamond dry etching using an inductively-coupled plasma reactive ion etching system, (d) acid treatment for the MgO anddiamond surface, and (e) H-diamond growth using a microwave plasma-enhanced chemical vapour deposition technique.However, triple-gate fin-type H-diamond MOSFETs stillhave some issues that need to be addressed. Their fabri-cation process is relatively complicated, leading to botha degradation of their electrical properties and issues relatedto the repeatability of researchers’ experiments by otherresearchers. Additionally, the best previously reported ratiobetween the height of the lateral side and the width of theplanar side for each fin was only 0.57, which means thatthe fin channels’ advantages are not being fully utilized.In this study, we have simplified the fabrication process fortriple-gate fin-type H-diamond MOSFETs and increased theratio between the height of the lateral side and the width ofthe planar side for each fin to be 1.45 to further enhance thedevice current output while keeping the device area the same.II. EXPERIMENTALPlanar-type and triple-gate fin-type Al2O3/H-diamondMOSFETs were fabricated on the same Ib-type single-crystalline diamond (100) substrate. The fabrication involvedseveral steps consisting of crossed key-patterns and fins for-mation, H-diamond epitaxial layer growth, mesa-structureformation, ohmic contact metals formation, Al2O3 deposi-tion, and gate cover metals formation. The formation processfor the fin-type H-diamond epitaxial layer was shown inFig. 1. The diamond substrate was coated with LOR 5A andAZ 5214E/PGMEA (a volume ratio of 1:1) positive photore-sists sequentially. The spin-coater’s rotation rate and timewere 7000 rpm and 1 s, respectively. The baking tempera-ture and time for the LOR 5A photoresist were 180 ◦C and5 min, respectively. Those for the AZ 5214E/PGMEA pho-toresist were 110 ◦C and 2 min, respectively. After exposingvia a laser lithography system, the sample was developed ina TMAH solution (2.38%) for 2 min to achieve the fin modelformation [Fig. 1(a)]. A 150-nm-thick MgO film was formedusing an evaporator system [Fig. 1(b)]. The chamber pres-sure and growth rate were 6.0 × 10−5 Pa and 0.15 nm/sec,respectively. After lifting-off the photoresists, the crossedkey-patterns and fins were formed simultaneously usinga high-speed inductively-coupled plasma reactive ion etchingsystem in an O2 gas ambient [Fig. 1(c)]. The etching power,the O2 flow rate, the chamber pressure, and the etching timewere 500 W, 100 sccm, 0.5 Pa, and 10 min, respectively. Thecrossed key-patterns were used for fin height measurementand position calibration for the subsequent fabrication of theMOSFETs. The MgO and diamond substrate were cleanedin a mixed acid solution (H2SO4 and HNO3 with a volumeratio of 1:1) at 300◦C for 3 h [Fig. 1(d)]. A 50-nm-thickH-diamond epitaxial layer was grown using a microwaveplasma-enhanced chemical vapour deposition technique onthe fin-type diamond substrate [Fig. 1(e)]. The growth tem-perature, chamber pressure, H2 flow rate, and CH4 flow ratewere 900–940 ◦C, 80 Torr, 500 sccm, and 0.5 sccm, respec-tively. By comparing the previous formation process of thefin-type H-diamond epitaxial layer [13], the new processeliminated the evaporation and etching steps for tungstenmetal to make it simpler.After the H-diamond growth, the fin height was con-firmed by measuring the crossed key-pattern using a three-dimensional-measurement laser microscopy system. Mesa-structures for the H-diamond and source/drain electrodesmade of Pd/Ti/Au metals for the MOSFETs were formedusing a capacitively coupled plasma reactive ion etch-ing system and an evaporation technique, respectively.A 34.5-nm-thick Al2O3 gate oxide film was deposited usingatomic layer deposition with Ti/Au cover metals. The elec-trical properties of the planar-type and triple-gate fin-typeH-diamond MOSFETs were measured using a MX-200/Bprober and a B1500A parameter analyser.(a) (b)FIGURE 2. (a) Scanning electron microscopy image of the crossedkey-pattern and (b) height of the lateral side for the fin-channel asconfirmed by measuring the crossed key-pattern usinga three-dimensional-measurement laser microscopy system.III. RESULT AND DISCUSSIONIn Figure 2, panels (a) and (b) show scanning electronmicroscopy image of the crossed key-pattern and height ofthe lateral side for the fin-channel via measurements of thecrossed key-pattern using a three-dimensional-measurementlaser microscopy system. The height of the lateral side562 VOLUME 7, 2019LIU et al.: HIGH CURRENT OUTPUT H-DIAMOND TRIPLE-GATE MOSFETs(a) (b) (c)(d) (e) (f)FIGURE 3. (a) and (d) Surface morphologies of the planar-type and triple-gate fin-type H-diamond MOSFETs, respectively. The inset in Fig. 3(d) showsa magnified view of the fin part for the triple-gate MOSFET. (b) and (e) Schematic diagrams of the planar-type and fin part of the triple-gate H-diamondMOSFETs, respectively. (c) and (f) Cross sectional diagrams of the planar-type and the triple-gate H-diamond MOSFETs, respectively.for the fin-channel is 2.9 µm. The ratio between theheight of the lateral side and width of the planar sidefor each fin were calculated to be 1.45, which is muchhigher than the previous report of 0.57 [13]. The gate elec-trode area and average LG for the triple-gate MOSFETwas calculated to be 1.4 × 10−6 cm2 and 3.3 µm,respectively.Panels (a) and (d) in Fig. 3 show the surface morpholo-gies of the planar-type and triple-gate fin-type H-diamondMOSFETs, respectively. The inset in Fig. 3(d) shows themagnified fin part for the triple-gate MOSFET. Schematicdiagrams for the planar-type and fin part of the triple-gate H-diamond MOSFETs are shown in Fig. 3(b) and (e),respectively. Cross sectional diagrams of them are shown inFig. 3(c) and 3(f), respectively. The gate width (WG) of boththe planar-type and triple-gate MOSFETs was the same at25.8 µm. The gate length (LG), interspace for the gate-to-source, and interspace for the gate-to-drain were 4.2, 1.7,and 3.3 µm, respectively, for the planar-type MOSFET. Thegate electrode area of the device was calculated to be1.1 × 10−6 cm2. There are three fins for the triple-gateMOSFET. Each fin had a width and length of 2.0 and20.2 µm, respectively. The interspace between two fins was4.9 µm. Because of the cover-area non-uniformity of thephotoresist layer used during the formation of the gate oxideand cover metals, the LG, interspace for the gate-to-source,and the interspace for the gate-to-drain were different for thetriple-gate MOSFET in the planar and the fin parts. For theplanar part, they were the same as those of the planar-typeH-diamond MOSFET. For the fin part, they were 2.5, 2.4,and 3.7 µm, respectively.Figure 4 shows the leakage current density (J) versus elec-trical field (E) characteristics for the planar-type (red line)and triple-gate fin-type (blue line) H-diamond MOSFETs. Jand E were calculated via the gate-to-source leakage currentdivided by the area of the gate electrode and via the gate-to-source voltage (VGS) divided by the gate oxide thicknessFIGURE 4. The J–E characteristics for the planar-type and triple-gatefin-type H-diamond MOSFETs.(34.5 nm), respectively. At E = 4.0 MV cm−1, there werealmost no holes in the H-diamond channels under the Al2O3,and both MOSFETs had leakage current densities below10−6 A cm−2. At E = −6.0 MV cm−1, hole accumu-lation in the H-diamond channel layer occurred, and theleakage current density for the triple-gate MOSFET was1.5×10−3 A cm−2, which is higher than that of the planar-type device (4.4 × 10−4 A cm−2). This may potentially beattributed to the existence of fins in the H-diamond chan-nel. The leakage current densities for both MOSFETs werearound 10−6 A cm−2 at E = −1.5 MV cm−1, which isaround the same order of magnitude as in our previous reporton an Al2O3/H-diamond MOS capacitor [15]. A low J ispromising for achieving good operations with the H-diamondMOSFETs.In Figure 5, panels (a) and (b) show the drain-to-sourcecurrent versus voltage (IDS − VDS) characteristics for theplanar-type and triple-gate fin-type H-diamond MOSFETs,respectively. The VGS for both MOSFETs was varied from−15.0 to 15.0 V in steps of +2.0 V. Both MOSFETs showdistinct pinch-off and p-type channel characteristics. Thelinear relationships between IDS and VDS at low voltages indi-cate good ohmic contacts between the source/drain electrodesVOLUME 7, 2019 563LIU et al.: HIGH CURRENT OUTPUT H-DIAMOND TRIPLE-GATE MOSFETs(a) (b)(d)(c)FIGURE 5. (a) and (b) The IDS − VDS characteristics for the planar-type andtriple-gate fin-type H-diamond MOSFETs, respectively. (c) and (d) The−√|IDS| − VGS and log |IDS| − VGS characteristics for the H-diamondMOSFETs, respectively.and the H-diamond channels. The IDS at VGS = −15.0 V forthe triple-gate H-diamond MOSFET was −7.0 mA, whichis almost double the value of −3.6 mA for the planar-typeH-diamond MOSFET. Holes can travel at the both planarand lateral sides in the H-diamond fin channel, leading toa high current output for the triple-gate MOSFET. In order tocompare the current outputs for both MOSFETs at the samedevice area, their IDS maxima (IDSmax) are normalized by theWG of 25.8 µm (not effective WG for triple-gate MOSFET).They are −271.3 and −139.5 mA mm−1 for the triple-gatefin-type and planar-type H-diamond MOSFETs, respectively.In Fig. 5, panels (c) and (d) show the −√|IDS| −VGS andlog |IDS|−VGS characteristics for the H-diamond MOSFETs,respectively. The threshold voltages for the planar-type andtriple-gate fin-type H-diamond MOSFETs were 0.7 ±0.1and 15.0 ±0.1 V, respectively. Both MOSFETs operatedwith normally-on characteristics. The triple-gate MOSFEThad a higher threshold voltage than the planar-type one,which implies that it is more difficult to make the H-diamondfin channel “off”. This is possibly attributed to the roughersurface for the fin-type H-diamond channel. According tothe log |IDS| − VGS characteristics in Fig. 5(d), the on/offratios for both MOSFETs were as high as 1010. The superiorline-shape for the triple-gate MOSFET compared with forthe planar-type MOSFET indicates a low interfacial trappedcharge density for the Al2O3 film on the fin-type H-diamondchannel, which is in good agreement with our previousreport [13].Figure 6 summarizes the reported |IDSmax| for theH-diamond MOSFETs on the single-crystalline dia-mond (100) substrates. Even though the gate lengthsfor the planar-type H-diamond MOSFETs are sub-micrometre, the |IDsmax| values are still lower than160.0 mA mm−1 [6], [16]–[24]. Thanks to a high sur-face charge-transfer doping efficiency, the WO3 on theFIGURE 6. Summary of |IDSmax | for the H-diamond MOSFETs onsingle-crystalline diamond (100) substrates.H-diamond was able to significantly increase the chan-nel sheet hole density. However, the |IDsmax| valuesfor the WO3/H-diamond type MOSFETs were below33.0 mA mm−1 [25]–[27]. Possible explanations for thelow |IDsmax| are an incomplete treatment of the surfacefor negatively charged adsorbates and damage of the C–Hbonds on the H-diamond channel during the fabrication ofthe WO3/H-diamond type MOSFETs. Since there are nointerspaces between the source/drain and gate electrodesof the T-type H-diamond MOSFET, its on-resistance wasmuch lower than that of the planar-type MOSFET, lead-ing to a high |IDSmax| of 224.1 mA mm−1 at an LG of4.0 µm [6]. Holes in the vertical-type H-diamond MOSFETcan also transfer at both the lateral and planar sides; its|IDSmax| was 234.0 mA mm−1 [28]. Our previous triple-gateMOSFET showed an |IDSmax| of 242.0 mA mm−1 (nor-malized by the WG) at an LG of 0.5 µm with a ratio of0.57 between the height of the lateral side and the width ofthe planar side for each fin [13]. Here, we have improvedthe fin formation process to increase the ratio to 1.45. The|IDSmax| for the novel triple-gate fin-type MOSFET was271.3 mA mm−1 at an average LG of 3.3 µm. Recently,there have been new developments for the fabrication ofhigh current output H-diamond MOSFETs [29], [30]. It hasbeen reported that when the entire surface of the H-diamondchannel is covered by a V2O5 film, the |IDSmax| values forthe V2O5/H-diamond type MOSFETs are as high as 280.0and 375.0 mA mm−1 at an LG of 2.0 and 0.25 µm, respec-tively [29], [30]. Although the |IDSmax| (271.3 mA mm−1) ofour novel triple-gate MOSFET is lower than those values, weexpect that this value could be further increased by decreas-ing the LG of 3.3 µm to sub-micrometres. Moreover, it isalso expected that covering the fin-type H-diamond channelwith a V2O5 film will generate a much higher |IDSmax| forthe MOSFETs. It should be noted that the |IDSmax| valuesfor the single-crystalline H-diamond (100)-based MOSFETswere much lower than for NO2-treated polycrystalline H-diamond-based devices (1350 mA mm−1) [7]; there aretwo possible explanations for this. (1) The NO2 treatmentmay increase the sheet hole density of the H-diamondchannel to around 1014 cm−2 [31]. (2) Conversely, otherH-diamond planes such as (111) in the polycrystalline dia-mond may have higher sheet hole densities than the (100)plane [32].564 VOLUME 7, 2019LIU et al.: HIGH CURRENT OUTPUT H-DIAMOND TRIPLE-GATE MOSFETsIV. CONCLUSIONIn this study, a novel triple-gate H-diamond MOSFET wasfabricated with a ratio for each fin of 1.45 between the heightof the lateral side and the width of the planar side. 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