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Xiaolu Yuan, [Jiangwei Liu](https://orcid.org/0000-0003-2580-7401), Siwu Shao, Jinlong Liu, Junjun Wei, [Bo Da](https://orcid.org/0000-0002-0785-8662), Chengming Li, [Yasuo Koide](https://orcid.org/0000-0001-8321-9822)

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[Thermal stability investigation for Ohmic contact properties of Pt, Au, and Pd electrodes on the same hydrogen-terminated diamond](https://mdr.nims.go.jp/datasets/6f193aa8-8069-4265-a9cb-59b0259da1db)

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Thermal stability investigation for Ohmic contact properties of Pt, Au, and Pd electrodes on the same hydrogen-terminated diamondAIP Advances 10, 055114 (2020); https://doi.org/10.1063/5.0008167 10, 055114© 2020 Author(s).Thermal stability investigation for Ohmiccontact properties of Pt, Au, and Pdelectrodes on the same hydrogen-terminated diamondCite as: AIP Advances 10, 055114 (2020); https://doi.org/10.1063/5.0008167Submitted: 23 March 2020 . Accepted: 02 May 2020 . Published Online: 13 May 2020Xiaolu Yuan, Jiangwei Liu , Siwu Shao, Jinlong Liu , Junjun Wei, Bo Da , Chengming Li, andYasuo Koide https://images.scitation.org/redirect.spark?MID=176720&plid=1167510&setID=378289&channelID=0&CID=390544&banID=519902572&PID=0&textadID=0&tc=1&type=tclick&mt=1&hc=7aaa55a2163a7405f47dd653615c362b4d422573&location=https://doi.org/10.1063/5.0008167https://doi.org/10.1063/5.0008167https://aip.scitation.org/author/Yuan%2C+Xiaoluhttps://aip.scitation.org/author/Liu%2C+Jiangweihttp://orcid.org/0000-0003-2580-7401https://aip.scitation.org/author/Shao%2C+Siwuhttps://aip.scitation.org/author/Liu%2C+Jinlonghttp://orcid.org/0000-0002-8894-9928https://aip.scitation.org/author/Wei%2C+Junjunhttps://aip.scitation.org/author/da%2C+Bohttp://orcid.org/0000-0002-0785-8662https://aip.scitation.org/author/Li%2C+Chengminghttps://aip.scitation.org/author/Koide%2C+Yasuohttp://orcid.org/0000-0001-8321-9822https://doi.org/10.1063/5.0008167https://aip.scitation.org/action/showCitFormats?type=show&doi=10.1063/5.0008167http://crossmark.crossref.org/dialog/?doi=10.1063%2F5.0008167&domain=aip.scitation.org&date_stamp=2020-05-13AIP Advances ARTICLE scitation.org/journal/advThermal stability investigation for Ohmiccontact properties of Pt, Au, and Pd electrodeson the same hydrogen-terminated diamondCite as: AIP Advances 10, 055114 (2020); doi: 10.1063/5.0008167Submitted: 23 March 2020 • Accepted: 2 May 2020 •Published Online: 13 May 2020Xiaolu Yuan,1,2 Jiangwei Liu,2,a) Siwu Shao,1 Jinlong Liu,1 Junjun Wei,1 Bo Da,3 Chengming Li,1,a)and Yasuo Koide2AFFILIATIONS1 Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China2Research Center for Functional Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba,Ibaraki 305-0044, Japan3Research and Services Division of Materials Data and Integrated System, NIMS, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japana)Authors to whom correspondence should be addressed: Liu.jiangwei@nims.go.jp and Chengmli@mater.ustb.edu.cnABSTRACTHere, thermal stabilities for Ohmic contact properties of Pt, Au, and Pd on the same hydrogen-terminated diamond (H-diamond) epitaxiallayer are investigated. A long-term annealing process is performed with an annealing temperature and time of 400 ○C and 8 h, respectively.Before annealing, good Ohmic contact properties are observed for only two contacts of the Pt/H-diamond and Pd/H-diamond with specificcontact resistivity (ρC) values of 2.7 × 10−3 Ω cm2 and 2.6 × 10−4 Ω cm2, respectively. After long-term annealing, all three contacts onthe H-diamond show good Ohmic contact properties. The ρC values for the Pt/H-diamond and Au/H-diamond are 3.1 × 10−2 Ω cm2 and4.2 × 10−4 Ω cm2, respectively. They are higher than that of the Pd/H-diamond (1.1 × 10−4 Ω cm2). Therefore, low ρC and good thermalstability for the Pd/H-diamond are achieved. This is meaningful for pushing forward the development of H-diamond-based electronic devicesfor high-temperature applications.© 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0008167., sSemiconductor diamonds have many remarkable intrinsicproperties, such as wide bandgap energy (5.47 eV), highest thermalconductivity (22 W/cm K), high carrier mobilities (4500 cm2/V sand 3800 cm2/V s for electrons and holes, respectively), andhigh breakdown electric field (10 MV/cm).1–4 Therefore, diamondappears promising for producing next-generation high-power, high-frequency, and high-temperature electronic devices.5–7 Currently,the hydrogen-terminated diamond (H-diamond) and bulk-dopeddiamond (such as boron-doped and phosphorous-doped) are usedfor the fabrication of metal–oxide–semiconductor field-effect tran-sistors (MOSFETs).8–13 However, current outputs of bulk-dopeddiamond-based MOSFETs are quite low (<1.6 mA/mm) due tothe high activation energies of dopants.8,9 In contrast, H-diamond-based MOSFETs have much higher current densities, thanks totheir good surface conductivity with a sheet hole density of∼1014/cm2.10 The current output maximum for H-diamond-basedMOSFETs was as high as 1350 mA/mm.10 Furthermore, they couldoperate well with a cutoff frequency and breakdown voltage of70 GHz and 2000 V, respectively.11,12 These electrical propertiesmake H-diamond-based MOSFETs promising for high-power andhigh-frequency applications.For H-diamond-based MOSFETs, another important consid-eration is high-temperature applications. Recently, it was reportedthat H-diamond-based MOSFETs could work well at temperaturesas high as 400 ○C.13 In order to further improve the performanceof H-diamond-based MOSFETs at high-temperature, it is impor-tant to find a suitable metal electrode on the H-diamond to formOhmic contacts with a low specific contact resistivity (ρC) and goodthermal stability. In order to obtain Ohmic contacts on the p-typeH-diamond, a metal electrode with a work function (ΦM) larger thanAIP Advances 10, 055114 (2020); doi: 10.1063/5.0008167 10, 055114-1© Author(s) 2020https://scitation.org/journal/advhttps://doi.org/10.1063/5.0008167https://www.scitation.org/action/showCitFormats?type=show&doi=10.1063/5.0008167https://crossmark.crossref.org/dialog/?doi=10.1063/5.0008167&domain=pdf&date_stamp=2020-May-13https://doi.org/10.1063/5.0008167https://orcid.org/0000-0003-2580-7401https://orcid.org/0000-0002-8894-9928https://orcid.org/0000-0002-0785-8662https://orcid.org/0000-0001-8321-9822mailto:Liu.jiangwei@nims.go.jpmailto:Chengmli@mater.ustb.edu.cnhttp://creativecommons.org/licenses/by/4.0/https://doi.org/10.1063/5.0008167AIP Advances ARTICLE scitation.org/journal/advthat of the H-diamond (ΦM: 4.9 eV) is required.14 Until now, severalmetals such as Au (ΦM: 5.10 eV), Pt (ΦM: 5.65 eV), Pd (ΦM: 5.12 eV),and 500 ○C-annealed Ti/Au (ΦM: 4.33/5.10 eV) have been used asOhmic contacts on the H-diamond.15–20 However, their thermal sta-bility has been rarely reported. On the other hand, Ohmic contactson the H-diamond are affected by the quality of the H-diamondepitaxial layer, which depends on the growth conditions. The con-tact properties of metal electrodes on different H-diamond epitaxiallayers are therefore difficult to be compared with each other.In this study, Pt, Au, and Pd electrodes are formed on the sameH-diamond epitaxial layer. Their specific contact resistivities andthermal stabilities at the maximum operation temperature (400 ○C)of H-diamond MOSFETs will be investigated and discussed.Three electrodes were formed separately on the H-diamond.Figure 1 shows the formation routines of the Pt contact on theH-diamond epitaxial layer. The Ib-type single-crystalline diamond(100) substrate with dimensions of 3.0 × 3.0 × 1.0 mm3 was cleanedin a mixed acid (H2SO4 and HNO3 with a volume ratio of 1:1)at 300 ○C for 3 h [Fig. 1(a)]. The H-diamond was homoepitaxi-ally grown with a thickness of 150 nm using a microwave plasma-enhanced chemical vapor deposition system (No. AX5200S, SekiTechnotron Corp., Tokyo, Japan) [Fig. 1(b)]. The CH4 flow rate, H2flow rate, chamber pressure, growth temperature, and growth timewere 0.5 SCCM, 500 SCCM, 80 Torr, 900–940 ○C, and 1.5 h, respec-tively. Surface roughness of the H-diamond epitaxial layer was previ-ously reported to be around 1.2 nm via an atomic force microscopytechnique.21 Its sheet hole density and mobility were confirmed tobe around ∼1013/cm2 and 90 cm2/V s, respectively, using the Hallmeasurement system. For each following step of the key-patternformation [Fig. 1(c)], mesa-structure formation [Fig. 1(d)], con-tact electrode formation [Fig. 1(e)], and coating, exposing, devel-oping, and lifting-off of photoresists were performed. The samplewas sequentially coated with LOR5A and AZ5214E positive pho-toresists using a spin-coater with a rotation rate and time of 7000rpm and 1 s, respectively. The baking temperature and time forthe LOR5A/AZ5214E photoresists were 180/110 ○C and 5/2 min,respectively. After being exposed to a mask-less photolithographysystem (No. DL-1000/NC2P, Nanosystem Solutions, Inc. Tokyo,Japan) with a dose energy of 250 mJ/cm2, the sample was developedin a 2.38% tetramethyl ammonium hydroxide (TMAH) solutionfor 90 s.Cross-shaped key-patterns [Fig. 1(c)] were formed using a J-sputter system (No. CFS-4EP-LL, Shibaura Mechatronics Corp.,Kanagawa, Japan) at an Ar atmosphere for calibrating the positionsof the mesa-structure [Fig. 1(d)] and Ohmic contacts [Fig. 1(e)].The mesa-structure for the H-diamond was formed using a capac-itively coupled-plasma reactive ion etching system (No. RIE-200NL,Samco Inc., Kyodo, Japan) [Fig. 1(d)]. The plasma power, O2 flowrate, and etching time were 50 W, 100 SCCM, and 90 s, respec-tively. The Pt metal covered by the Ti/Au bilayer on the H-diamondwas formed via an e-beam evaporation system (No. RDEB-1206K,R-DEC. Co., Ltd., Ibaraki, Japan) under a vacuum condition of∼10−5 Pa [Fig. 1(e)]. After lifting-off of the photoresists in an n-methylpryolidone (NMP) solution for 3 h at room temperature, theAu and Pd (covered by the Ti/Au) electrodes were formed withthe same evaporation system separately. The Ti/Au cover layers areimportant to prevent oxidization of Pt and Pd in air. Thicknessesof the Pt/Ti/Au, Au, and Pd/Ti/Au metals were 10/20/100, 100, and10/20/100 nm, respectively. After finishing the formation process,the sample was annealed at 400 ○C for as long as 8 h under a lowvacuum condition (∼5.0 Pa) via a rapid thermal annealing system(No. QHC-P410, Advance Riko, Inc., Kanagawa, Japan). Then, thesample was exposed to atmosphere for more than 24 h to saturatethe conductivity of the H-diamond surface.22 Electrical propertiesof the three contacts on the H-diamond were characterized using afour-probe system (Vector Semiconductor Co., Ltd., Tokyo, Japan)at room temperature.Figure 2(a) shows the surface morphology of the three contactson the H-diamond. The length and width for each electrode are thesame, 100 μm. No electrodes peeled-off after the formation process.There are eight different interspace (d) values between two adjacentelectrodes for each contact. Interspace distances between them areconfirmed via a scanning electron microscope technique after elec-trical property measurement [Fig. 2(b)]. The d values are in the rangeof 5.0 μm–18.5 μm, 4.8 μm–18.8 μm, and 4.9 μm–19.0 μm for the Pt,Au, and Pd contacts, respectively.FIG. 1. Formation routines of the Pt contact on the H-diamond epitaxial layer: (a) the diamond (100) substrate, (b) H-diamond epitaxial layer growth, (c) key-pattern formation,(d) mesa-structure formation, and (e) the Pt contact covered by Ti/Au.AIP Advances 10, 055114 (2020); doi: 10.1063/5.0008167 10, 055114-2© Author(s) 2020https://scitation.org/journal/advhttps://advance-riko.com/en/AIP Advances ARTICLE scitation.org/journal/advFIG. 2. (a) Surface morphologies of three contacts on the H-diamond beforeannealing and (b) its scanning electron microscope images after annealing andelectrical property measurement.Figures 3(a) and 3(c) show the current–voltage characteris-tics of the Pt/H-diamond before and after annealing at 400 ○C for8 h, respectively. The voltage was changed from −0.5 V to 0.5 V.The current was normalized by an electrode width of 100 μm. Allcurrent–voltage curves have linear characteristics, implying goodOhmic contact properties for the Pt/H-diamond before and afterannealing. When d is 5.0 μm, the current is 6.7 × 10−4 A/mm ata voltage of 0.5 V before annealing, which is higher than that (1.7× 10−4 A/mm) after annealing. Total resistance (RT) values for thePt/H-diamond at d = 5.0 μm before and after annealing are calcu-lated to be 7.5 × 102 Ω mm and 2.9 × 103 Ω mm, respectively. Theincrease in RT after annealing is possibly due to the deterioration ofsurface conductivity of the H-diamond.22 Figures 3(b) and 3(d) showRT as functions of d for the Pt/H-diamond before and after anneal-ing, respectively. The black triangle and red square spots representRT at voltages of −0.5 V and 0.5 V, respectively. With the change ind from 5.0 μm to 18.5 μm, RT increases. RT between two adjacentOhmic electrodes is twice the contact resistance (RC) and surfaceresistance (RS), as shown in the following equation:23,24RT = 2RC + RS, (1)RS = Rsheet × d, (2)where Rsheet is the surface sheet resistance. If 2RC for the two adja-cent Ohmic electrodes are stable, the increase in RT with a changein d is attributed to RS. By fitting the spots in Figs. 3(b) and 3(d),the 2RC values (the intercepts of the y-axis for the fitting lines) forthe Pt/H-diamond before and after annealing are determined to be3.5 × 102 Ω mm and 1.9 × 103 Ω mm, respectively. Their Rsheet val-ues extracted from the slopes of the fitting lines are determined to be1.1 × 105 Ω/◽ and 2.9 × 105 Ω/◽, respectively. Therefore, Rsheet forthe H-diamond increases after long-term annealing, which can beascribed to the damage of the surface adsorbates and C–H bonds onthe H-diamond.22 Their twice the transfer length (2LT) (the inter-cept of the x-axis for the fitting lines) is calculated to be 3.1 μm and6.6 μm, respectively. Therefore, the ρC values for the Pt/H-diamondbefore and after annealing are calculated to be 2.7 × 10−3 Ω cm2 and3.1 × 10−2 Ω cm2, respectively, based on the following equation:23,24ρC = RC ⋅ LT . (3)Figures 4(a) and 4(b) show the current–voltage characteris-tics of the Au/H-diamond before and after annealing, respectively.Before annealing, there were not good linear relationships for all thecurves. Meanwhile, the current at a voltage of 0.5 V is disorderedwith the increase in d from 4.8 μm to 18.8 μm. The poor Ohmic con-tact properties for the Au/H-diamond before annealing are possiblyattributed to the adhesion issue of Au on the H-diamond.25 Afterannealing at 400 ○C for 8 h, there are good linear relationships forthe current–voltage curves, as shown in Fig. 4(c). The current at d= 4.8 μm is 4.2 × 10−4 A/mm, which is one order larger than thatbefore annealing. Figures 4(b) and 4(d) show RT as functions of d forthe Au/H-diamond before and after annealing, respectively. Beforeannealing, the black triangle (the RT calculated at −0.5 V) and redsquare (the RT calculated at −0.5 V) spots are quite different fromeach other at the same d value. It is difficult to deduce the 2RC and2LT values. After annealing, the black triangle and red square spotsare in good agreement with each other. Therefore, 2RC, Rsheet , and2LT for the Au/H-diamond after annealing are calculated to be 2.1× 102 Ω mm, 2.7 × 105 Ω/◽, and 0.8 μm, respectively. Its ρC can becalculated, based on Eq. (2), to be 4.2 × 10−4 Ω cm2, which is lowerthan those of the Pt/H-diamond before (2.7 × 10−3 Ω cm2) and afterannealing (3.1 × 10−2 Ω cm2). However, it is larger than that of the600 ○C-annealed Au/H-diamond (4.3 × 10−5 Ω cm2).26 Therefore, ahigher annealing temperature for the Au/H-diamond contact wouldfurther decrease its ρC.Figures 5(a) and 5(c) show the current–voltage characteristicsof the Pd/H-diamond before and after annealing, respectively. For allcurrent–voltage curves, there are linear characteristics. Good Ohmiccontacts for the Pd/H-diamond are obtained. Before annealing, thecurrent at a voltage of 0.5 V is 1.2 × 10−3 A/mm at d = 4.9 μm.After annealing, it decreases to 3.6 × 10−4 A/mm. RT as functionsof d for the Pd/H-diamond before and after annealing are shownin Figs. 5(b) and 5(d), respectively. The black triangle (the RT cal-culated at −0.5 V) and red square (the RT calculated at −0.5 V)spots are in good agreement with each other. By fitting them, 2RCfor the Pd/H-diamond before and after annealing is determinedAIP Advances 10, 055114 (2020); doi: 10.1063/5.0008167 10, 055114-3© Author(s) 2020https://scitation.org/journal/advAIP Advances ARTICLE scitation.org/journal/advFIG. 3. (a) and (c) Current–voltage characteristics of the Pt/H-diamond before annealing and after annealing, respectively and (b) and (d) RT as functions of d before andafter annealing, respectively. Black triangle and red square spots represent the RT values calculated at voltages of −0.5 V and 0.5 V, respectively.FIG. 4. (a) and (c) Current–voltage characteristics of the Au/H-diamond before annealing and after annealing, respectively and (b) and (d) RT as functions of d before andafter annealing, respectively. Black triangle and red square spots represent the RT values calculated at voltages of −0.5 V and 0.5 V, respectively.AIP Advances 10, 055114 (2020); doi: 10.1063/5.0008167 10, 055114-4© Author(s) 2020https://scitation.org/journal/advAIP Advances ARTICLE scitation.org/journal/advFIG. 5. (a) and (c) Current–voltage characteristics of the Pd/H-diamond before annealing and after annealing, respectively and (b) and (d) RT as functions of d before andafter annealing, respectively. Black triangle and red square spots represent the RT values calculated at voltages of −0.5 V and 0.5 V, respectively.to be 86.7 Ω mm and 110.0 Ω mm, respectively. Rsheet for the H-diamond surfaces is 7.5 × 104 Ω/◽ and 2.6 × 105 Ω/◽, respectively.The 2LT values are deduced to be 1.2 μm and 0.4 μm, respectively.The ρC values for the Pd/H-diamond before and after annealingare calculated, based on Eq. (3), to be 2.6 × 10−4 Ω cm2 and 1.1× 10−4 Ω cm2, respectively, which are lower than that of the recentlyreported value (8 ± 1 × 10−4 Ω cm2) for the Pd/H-diamond at roomtemperature.20Figures 6(a) and 6(b) summarize the annealing effects on RCand ρC for three contacts on the H-diamond, respectively. Becauseof the poor Ohmic contact for the Au/H-diamond before annealing,its RC and ρC are not obtained. Annealing makes the RC increaseto more than five times that before annealing for the Pt/H-diamond.However, annealing does not affect RC of the Pd/H-diamond greatly.Before annealing, RC for the Pd/H-diamond is lower than that forthe Pt/H-diamond. After annealing, it is still much lower than thoseof the Pt/H-diamond and Au/H-diamond. Annealing makes ρC forthe Pt/H-diamond increase by around one order of magnitude com-pared with that before annealing [Fig. 6(b)]. It is also much largerthan those of the Au/H-diamond and Pd/H-diamond. The ρC forFIG. 6. Summary of (a) RC and (b) ρC of three contacts on the H-diamond before and after annealing.AIP Advances 10, 055114 (2020); doi: 10.1063/5.0008167 10, 055114-5© Author(s) 2020https://scitation.org/journal/advAIP Advances ARTICLE scitation.org/journal/advthe Pd/H-diamond after annealing is a little lower than that beforeannealing and is also the lowest one among the three contacts.Based on the above comparisons for the three contacts shown inFigs. 6(a) and 6(b), it can be concluded that annealing degrades andimproves the contact properties of the Pt/H-diamond and Au/H-diamond, respectively. After the long-term annealing process, thecontact properties of the Pd/H-diamond are still very good and sta-ble. Therefore, Pd is a good choice as an Ohmic contact electrodeto push forward the development of H-diamond-based electronicdevices for high-temperature applications.In conclusion, the contact properties and thermal stabilitiesof Pt, Au, and Pd contacts on the same H-diamond epitaxial layerwere investigated and discussed. Before annealing, only the Pt/H-diamond and Pd/H-diamond showed good Ohmic contact proper-ties, with a ρC of 2.7 × 10−3 Ω cm2 and 2.6 × 10−4 Ω cm2, respec-tively. After annealing at 400 ○C for as long as 8 h, a good Ohmiccontact was observed for all three contacts, with a ρC values of 3.1× 10−2 Ω cm2, 4.2 × 10−4 Ω cm2, and 1.1 × 10−4 Ω cm2, respectively.The long-term annealing process degrades and improves the contactproperties of the Pt/H-diamond and Au/H-diamond, respectively.They were still stable for the Pd/H-diamond and were better thanthose of the other two contacts.This work was supported by the KAKENHI Project, underGrant Nos. JP18K13806 and JP16H06419, the Leading Initiative forExcellent Young Researchers Program Project, the NIMS Nanofabri-cation Platform of the Nanotechnology Platform Project sponsoredby the Ministry of Education, Culture, Sports, and Technology,Japan, and the Murata Science Foundation. It was supported partlyby the National Key Research and Development Program of China(Grant No. 2016YFE0133200).DATA AVAILABILITYThe data that support the findings of this study are availablefrom the corresponding author upon reasonable request.REFERENCES1C. J. H. Wort and R. S. Balmer, Mater. Today 11, 22 (2008).2J. Isberg, J. Hammersberg, E. Johansson, T. Wikström, D. J. Twitchen, A. J.Whitehead, S. E. Coe, and G. A. Scarsbrook, Science 297, 1670 (2002).3M. C. Rossi, S. Salvatori, and F. Galluzzi, J. Vac. Sci. Technol., B 16, 1725(1998).4L. Reggiani, S. Bosi, C. Canali, F. Nava, and S. F. Kozlov, Phys. Rev. B 23, 3050(1981).5H. Umezawa, M. Nagase, Y. Kato, and S.-i. Shikata, Diamond Relat. 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