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Xiaolu Yuan, Jiangwei Liu, [Jinlong Liu](https://orcid.org/0000-0003-2580-7401), 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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[Reliable Ohmic Contact Properties for Ni/Hydrogen-Terminated Diamond at Annealing Temperature up to 900 °C](https://mdr.nims.go.jp/datasets/98cac5be-9527-4493-8046-67d4a984aec2)

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Reliable Ohmic Contact Properties for Ni/Hydrogen-Terminated Diamond at Annealing Temperature up to 900 CcoatingsArticleReliable Ohmic Contact Properties for Ni/Hydrogen-TerminatedDiamond at Annealing Temperature up to 900 ◦CXiaolu Yuan 1,2 , Jiangwei Liu 2,* , Jinlong Liu 1, Junjun Wei 1, Bo Da 3, Chengming Li 1,* and Yasuo Koide 2�����������������Citation: Yuan, X.; Liu, J.; Liu, J.; Wei,J.; Da, B.; Li, C.; Koide, Y. ReliableOhmic Contact Properties forNi/Hydrogen-Terminated Diamondat Annealing Temperature up to900 ◦C. Coatings 2021, 11, 470.https://doi.org/10.3390/coatings11040470Academic Editor: Aomar HadjadjReceived: 23 March 2021Accepted: 13 April 2021Published: 17 April 2021Publisher’s Note: MDPI stays neutralwith regard to jurisdictional claims inpublished maps and institutional affil-iations.Copyright: © 2021 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).1 Institute for Advanced Materials and Technology, University of Science and Technology Beijing,Beijing 100083, China; luzi@semi.ac.cn (X.Y.); liujinlong@ustb.edu.cn (J.L.); weijj@ustb.edu.cn (J.W.)2 Research Center for Functional Materials, National Institute for Materials Science (NIMS), 1-1 Namiki,Tsukuba, Ibaraki 305-0044, Japan; koide.yasuo@nims.go.jp3 Research and Services Division of Materials Data and Integrated System, NIMS, 1-1 Namiki, Tsukuba,Ibaraki 305-0044, Japan; DA.Bo@nims.go.jp* Correspondence: liu.jiangwei@nims.go.jp (J.L.); chengmli@mater.ustb.edu.cn (C.L.)Abstract: Ohmic contact with high thermal stability is essential to promote hydrogen-terminateddiamond (H-diamond) electronic devices for high-temperature applications. Here, the ohmic contactcharacteristics of Ni/H-diamond at annealing temperatures up to 900 ◦C are investigated. Themeasured current–voltage curves and deduced specific contact resistance (ρC) are used to evaluatethe quality of the contact properties. Schottky contacts are formed for the as-received and 300 ◦C-annealed Ni/H-diamonds. When the annealing temperature is increased to 500 ◦C, the ohmic contactproperties are formed with the ρC of 1.5 × 10−3 Ω·cm2 for the Ni/H-diamond. As the annealingtemperature rises to 900 ◦C, the ρC is determined to be as low as 6.0 × 10−5 Ω·cm2. It is believed thatthe formation of Ni-related carbides at the Ni/H-diamond interface promotes the decrease in ρC. TheNi metal is extremely promising to be used as the ohmic contact electrode for the H-diamond-basedelectronic devices at temperature up to 900 ◦C.Keywords: hydrogen-terminated diamond (H-diamond); ohmic contact; Ni; specific contact resis-tance; high-temperature1. IntroductionDiamond, with many remarkable intrinsic properties, possesses vast prospect applica-tions for high-power, high-frequency, and high-temperature electronics [1–3]. It exhibits anultrawide energy bandgap (5.5 eV), high carrier mobilities (4500 and 3800 cm2·V−1·s−1 forelectrons and holes, respectively), large breakdown field strength (10 MV·cm−1), and thehighest thermal conductivity (22 W cm−1·K−1) [4]. Compared with boron-doped diamond,hydrogen-terminated diamond (H-diamond) shows outstanding p-type surface conductiv-ity with a hole carrier concentration up to ~1014 cm−2 [5,6]. Recently, H-diamond-basedfield-effect transistors have achieved excellent device performances, such as a high break-down voltage (2000 V), a high-output current density (1.3 A·mm−1), and a high-outputpower density (3.8 W·mm−1) [1,7,8]. Meanwhile, the passivation layer protection for theH-diamond surface improves the conductive stability of H-diamond-based electronic de-vices, even at temperatures as high as 500 ◦C [9–11]. The re-hydrogenation process enablesthe H-diamond surface damaged during annealing to regain good conductivity [12].In order to further promote H-diamond-based electronic devices to be operated wellat high temperatures, thermal-stable ohmic contact is essential. Until now, different kindsof metals are used for ohmic contacts on the H-diamond, such as, Au, Pd, Pt, W, Ti/Au,Pt/Au, Pd/Ti/Au, Ti/Ni/Au, etc. [12–15] They show good ohmic contact properties andhigh thermal stability, with annealing temperatures up to 700 ◦C. On the other hand, theNi metal tends to form Schottky contact with the H-diamond at an annealing temperaturelower than 100 ◦C [16]. However, Ni, as a carbophilic element, is prone to react with carbonCoatings 2021, 11, 470. https://doi.org/10.3390/coatings11040470 https://www.mdpi.com/journal/coatingshttps://www.mdpi.com/journal/coatingshttps://www.mdpi.comhttps://orcid.org/0000-0002-8755-1539https://orcid.org/0000-0003-2580-7401https://doi.org/10.3390/coatings11040470https://doi.org/10.3390/coatings11040470https://doi.org/10.3390/coatings11040470https://creativecommons.org/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://doi.org/10.3390/coatings11040470https://www.mdpi.com/journal/coatingshttps://www.mdpi.com/article/10.3390/coatings11040470?type=check_update&version=1Coatings 2021, 11, 470 2 of 8at an elevated temperature [17]. The solid-solution reaction makes the Ni-related carbidesformed at the Ni/H-diamond interface, which would possibly contribute to the formationof ohmic contact.Here, ohmic contact characteristics of Ni/H-diamond at annealing temperatures upto 900 ◦C are investigated. The measured current–voltage curves and deduced specificcontact resistance (ρC) are used to evaluate the quality of the contact properties.2. Experimental2.1. Preparation of H-Diamond Epitaxial LayerAn Ib-type (100) facet single-crystalline diamond was boiled in mixed H2SO4 andHNO3 solutions at 300 ◦C for 3 h. Then, it was ultrasonically cleaned using acetone, ethanol,and deionized water sequentially. A 150-nm-thick H-diamond epitaxial layer was grownusing a microwave plasma-enhanced chemical vapor deposition system. The CH4 flowrate, H2 flow rate, chamber pressure, and deposition temperature were 0.5 sccm, 500 sccm,80 Torr, and 900–940 ◦C, respectively.2.2. Formation of Transmission Line Model (TLM) Patterns for Ni on the H-DiamondThe H-diamond was sequentially coated with LOR5A and AZ5214E positive photore-sists using a spin-coater with a rotation rate and time of 7000 rpm and 1 s, respectively.After exposing using a mask-less lithography system with a dose energy of 250 mJ·cm−2,the sample was developed in a 2.38% tetramethylammonium hydroxide solution for 90 s.The Ti metal used as key-patterns was deposited on the H-diamond by a J-sputter systemin an Ar atmosphere in order to align the positions of the mesa structure and contact metals.The mesa structure was formed using a capacitively coupled plasma reactive-ion etchingsystem. The plasma power, O2 flow rate, and etching time were 50 W, 100 sccm, and 90 s,respectively. The five-group TLM electrode patterns were completed using the lithographyprocedures. The Ni, with a thickness of 100 nm, was grown on the H-diamond for ohmiccontact via an e-beam evaporation system under a ~10−5 Pa vacuum condition.2.3. Annealing Process and Current–Voltage MeasurementsThe annealing process was performed using a rapid thermal annealing system inan Ar atmosphere. The annealing temperatures were 300, 500, 700, and 900 ◦C with anannealing time of 10 min for each temperature. After annealing, the sample was exposedto air for more than 10 h in order to promote the formation of a negatively adsorbed layeron the surface of the H-diamond, and to regain good surface conductivity. The calculationsof ρC for the Ni/H-diamond with the TLM patterns can be referred to in the previousreports [18,19]. The electrical properties of Ni/H-diamond contacts were characterized bya room-temperature probe system.3. Results and DiscussionFigure 1 shows the surface morphology of five-group TLM patterns of Ni on theH-diamond epitaxial layer before annealing. The length and width of each electrode arethe same as 100 µm. All the Ni metals were stable to be formed on the H-diamond. Thefive groups of TLM patterns were used to characterize current–voltage curves for theNi/H-diamonds of as-received, 300 ◦C-annealed, 500 ◦C-annealed, 700 ◦C-annealed, and900 ◦C-annealed, respectively. The interspace (d) values from left to right in Figure 1,between the two adjacent electrodes for the five-group TLM patterns, are in the ranges of7.9–9.1 µm, 13.5–13.9 µm, 18.0–18.8 µm, 23.1–23.9 µm and 28.2–29.1 µm, respectively.Figure 2 shows current–voltage characteristics of (a) the as-received and (b) the 300 ◦C-annealed Ni/H-diamond contacts, respectively. The applied voltage is in the range of–1.0–1.0 V. For the as-received Ni/H-diamond contact, the output currents are in the order of10−7 A (Figure 2a). All the curves show non-linear characteristics, indicating the Schottkycontacts. After annealing at 300 ◦C for 10 min, as shown in Figure 2b, the output currentsof the Ni/H-diamond increase to the order of 10−4 A. The annealing process improvesCoatings 2021, 11, 470 3 of 8the contact properties of the Ni/H-diamond. However, all the curves still show non-linear characteristics. Therefore, even after annealing at 300 ◦C, the Ni/H-diamond stilloperates with Schottky contacts, which is consistent with the results obtained from theother report [20]. The presence of a chemisorbed species on the H-diamond surface possiblyresults in the formation of Schottky contact. However, the annealing process can improvethe contact interface of the Ni/H-diamond, thereby promoting the current flow.Coatings 2021, 11, x FOR PEER REVIEW 3 of 8    Figure 1. Surface morphology of the five-group transmission line model (TLM) patterns of Ni on the hydrogen-terminated diamond (H-diamond) epitaxial layer before annealing. They were used to characterize current–voltage curves for the Ni/H-diamonds of as-received, 300 °C-annealed, 500 °C-annealed, 700 °C-annealed, and 900 °C-annealed, respectively. Figure 2 shows current–voltage characteristics of (a) the as-received and (b) the 300 °C -annealed Ni/H-diamond contacts, respectively. The applied voltage is in the range of –1.0–1.0 V. For the as-received Ni/H-diamond contact, the output currents are in the order of 10−7 A (Figure 2a). All the curves show non-linear characteristics, indicating the Schottky contacts. After annealing at 300 °C for 10 min, as shown in Figure 2b, the output currents of the Ni/H-diamond increase to the order of 10−4 A. The annealing process im-proves the contact properties of the Ni/H-diamond. However, all the curves still show non-linear characteristics. Therefore, even after annealing at 300 °C, the Ni/H-diamond still operates with Schottky contacts, which is consistent with the results obtained from the other report [20]. The presence of a chemisorbed species on the H-diamond surface possibly results in the formation of Schottky contact. However, the annealing process can improve the contact interface of the Ni/H-diamond, thereby promoting the current flow.  Figure 2. Current–voltage characteristics of (a) the as-received and (b) the 300 °C-annealed Ni/H-diamond contacts, respectively. Figure 3a shows the current–voltage characteristics of the Ni/H-diamond after an-nealing at 500 °C. All the current–voltage curves have linear characteristics, which implies good ohmic contacts for the Ni/H-diamond. For two adjacent electrodes in which d = 8.7 μm, the output currents are 2.0 × 10−4 A at ±1.0 V. The total resistance (RT) for the 500 °C-annealed Ni/H-diamond can be calculated to be 4.9 × 103 Ω (d = 8.7 μm). Based on the -0.8 -0.4 0.0 0.4 0.8-8x10-7-4x10-704x10-78x10-7      8.5 µm13.9 µm18.2 µm23.9 µm28.9 µm-0.8 -0.4 0.0 0.4 0.8-1x10-4-5x10-505x10-51x10-4      8.7 µm13.5 µm18.8 µm23.4 µm28.7 µm(b) 300 ℃-annealed(a) As-receivedCurrent  (A)Voltage (V)Current  (A)Voltage (V)Figure 1. Surface morphology of the five-group transmission line model (TLM) patterns of Ni onthe hydrogen-terminated diamond (H-diamond) epitaxial layer before annealing. They were usedto characterize current–voltage curves for the Ni/H-diamonds of as-received, 300 ◦C-annealed,500 ◦C-annealed, 700 ◦C-annealed, and 900 ◦C-annealed, respectively.Coatings 2021, 11, x FOR PEER REVIEW 3 of 8    Figure 1. Surface morphology of the five-group transmission line model (TLM) patterns of Ni on the hydrogen-terminated diamond (H-diamond) epitaxial layer before annealing. They were used to characterize current–voltage curves for the Ni/H-diamonds of as-received, 300 °C-annealed, 500 °C-annealed, 700 °C-annealed, and 900 °C-annealed, respectively. Figure 2 shows current–voltage characteristics of (a) the as-received and (b) the 300 °C -annealed Ni/H-diamond contacts, respectively. The applied voltage is in the range of –1.0–1.0 V. For the as-received Ni/H-diamond contact, the output currents are in the order of 10−7 A (Figure 2a). All the curves show non-linear characteristics, indicating the Schottky contacts. After annealing at 300 °C for 10 min, as shown in Figure 2b, the output currents of the Ni/H-diamond increase to the order of 10−4 A. The annealing process im-proves the contact properties of the Ni/H-diamond. However, all the curves still show non-linear characteristics. Therefore, even after annealing at 300 °C, the Ni/H-diamond still operates with Schottky contacts, which is consistent with the results obtained from the other report [20]. The presence of a chemisorbed species on the H-diamond surface possibly results in the formation of Schottky contact. However, the annealing process can improve the contact interface of the Ni/H-diamond, thereby promoting the current flow.  Figure 2. Current–voltage characteristics of (a) the as-received and (b) the 300 °C-annealed Ni/H-diamond contacts, respectively. Figure 3a shows the current–voltage characteristics of the Ni/H-diamond after an-nealing at 500 °C. All the current–voltage curves have linear characteristics, which implies good ohmic contacts for the Ni/H-diamond. For two adjacent electrodes in which d = 8.7 μm, the output currents are 2.0 × 10−4 A at ±1.0 V. The total resistance (RT) for the 500 °C-annealed Ni/H-diamond can be calculated to be 4.9 × 103 Ω (d = 8.7 μm). Based on the -0.8 -0.4 0.0 0.4 0.8-8x10-7-4x10-704x10-78x10-7      8.5 µm13.9 µm18.2 µm23.9 µm28.9 µm-0.8 -0.4 0.0 0.4 0.8-1x10-4-5x10-505x10-51x10-4      8.7 µm13.5 µm18.8 µm23.4 µm28.7 µm(b) 300 ℃-annealed(a) As-receivedCurrent  (A)Voltage (V)Current  (A)Voltage (V)Figure 2. Current–voltage characteristics of (a) the as-received and (b) the 300 ◦C-annealed Ni/H-diamond contacts,respectively.Figure 3a shows the current–voltage characteristics of the Ni/H-diamond after anneal-ing at 500 ◦C. All the current–voltage curves have linear characteristics, which implies goodohmic contacts for the Ni/H-diamond. For two adjacent electrodes in which d = 8.7 µm,the output currents are 2.0 × 10−4 A at ±1.0 V. The total resistance (RT) for the 500 ◦C-Coatings 2021, 11, 470 4 of 8annealed Ni/H-diamond can be calculated to be 4.9 × 103 Ω (d = 8.7 µm). Based on thecurrent–voltage characteristics for other adjacent electrodes (Figure 3a), the RT for theNi/H-diamond with other d values is also calculated and summarized in Figure 3b. Thereare the following relationships for the RT and ρC with the Ni/H-diamond contact resistance(RC), H-diamond surface sheet resistance (RS), electrode transfer length (LT), and electrodewidth (W) [18]:RT = 2RC +RsWd (1)ρC = RC × LT × W (2)Coatings 2021, 11, x FOR PEER REVIEW 4 of 8   current–voltage characteristics for other adjacent electrodes (Figure 3a), the RT for the Ni/H-diamond with other d values is also calculated and summarized in Figure 3b. There are the following relationships for the RT and ρC with the Ni/H-diamond contact resistance (RC), H-diamond surface sheet resistance (RS), electrode transfer length (LT), and electrode width (W) [18]: sT C2 RR R dW= +  (1)C C Tρ R L W= × ×  (2)By fitting the spots in Figure 3b, the RS/W (the slope of fitting line) are determined to be 4.0 × 102 Ω·μm−1 with an RS of 40 kΩ. The 2RC (the intercept of the y-axis) and 2LT (the intercept of the x-axis) are deduced to be 1.5 × 103 Ω and 3.9 μm, respectively. Based on Equation (2), the ρC for the 500 °C-annealed Ni/H-diamond can be calculated to be 1.5 × 10−3 Ω·cm2.  Figure 3. (a,c) Current–voltage characteristics of the 500 °C-annealed and 700 °C-annealed Ni/H-diamonds, respectively. (b,d) RT as functions of d for the 500 °C-annealed and 700 °C-annealed Ni/H-diamonds, respectively. When the annealing temperature is increased to 700 °C, the output current maxima are the same level as those of the 500 °C-annealed Ni/H-diamond (Figure 3c). The linear characteristics for all the current–voltage curves are observed, indicating the good ohmic properties for the 700 °C-annealed Ni/H-diamond. Figure 3d shows the RT as a function of d for the Ni/H-diamond after annealing at 700 °C. By fitting the spots, the RS/W, 2RC, and 2LT are obtained to be 4.0 × 102 Ω·μm−1, 1.9 × 103 Ω, and 4.8 μm, respectively. The RS and ρC for the Ni/H-diamond after annealing at 700 °C are calculated to be 40 kΩ and 2.3 × 10−3 Ω·cm2, respectively, which are close to those of the 500 °C-annealed Ni/H-diamond. Figure 4a shows the current-voltage curves of the Ni/H-diamond after annealing at 900 °C. All the curves have good linear relationships. Therefore, the ohmic contacts of the -5 0 5 10 15 20 25 3004x1038x1031x104  d (µm)R T(Ω)2LT2RC-5 0 5 10 15 20 25 3004x1038x1031x104  -0.8 -0.4 0.0 0.4 0.8-2x10-4-1x10-401x10-42x10-4     8.7 µm13.5 µm18.8 µm23.4 µm28.7 µmCurrent  (A)Voltage (V)2LT 2RCd (µm)R T(Ω)(b) 500 ℃-annealed(a) 500 ℃-annealed-0.8 -0.4 0.0 0.4 0.8-2x10-4-1x10-401x10-42x10-4     7.9 µm13.5 µm18.0 µm23.8 µm28.2 µmCurrent  (A)Voltage (V)(d) 700 ℃-annealed(c) 700 ℃-annealedFigure 3. (a,c) Current–voltage characteristics of the 500 ◦C-annealed and 700 ◦C-annealed Ni/H-diamonds, respectively.(b,d) RT as functions of d for the 500 ◦C-annealed and 700 ◦C-annealed Ni/H-diamonds, respectively.By fitting the spots in Figure 3b, the RS/W (the slope of fitting line) are determined tobe 4.0 × 102 Ω·µm−1 with an RS of 40 kΩ. The 2RC (the intercept of the y-axis) and 2LT(the intercept of the x-axis) are deduced to be 1.5 × 103 Ω and 3.9 µm, respectively. Basedon Equation (2), the ρC for the 500 ◦C-annealed Ni/H-diamond can be calculated to be1.5 × 10−3 Ω·cm2.When the annealing temperature is increased to 700 ◦C, the output current maximaare the same level as those of the 500 ◦C-annealed Ni/H-diamond (Figure 3c). The linearcharacteristics for all the current–voltage curves are observed, indicating the good ohmicproperties for the 700 ◦C-annealed Ni/H-diamond. Figure 3d shows the RT as a functionof d for the Ni/H-diamond after annealing at 700 ◦C. By fitting the spots, the RS/W, 2RC,and 2LT are obtained to be 4.0 × 102 Ω·µm−1, 1.9 × 103 Ω, and 4.8 µm, respectively.The RS and ρC for the Ni/H-diamond after annealing at 700 ◦C are calculated to beCoatings 2021, 11, 470 5 of 840 kΩ and 2.3 × 10−3 Ω·cm2, respectively, which are close to those of the 500 ◦C-annealedNi/H-diamond.Figure 4a shows the current-voltage curves of the Ni/H-diamond after annealing at900 ◦C. All the curves have good linear relationships. Therefore, the ohmic contacts of theNi/H-diamond possess good thermal stability even after annealing at temperatures ashigh as 900 ◦C for 10 min, which is comparable with those of the Ni/SiC contacts [21,22].Coatings 2021, 11, x FOR PEER REVIEW 5 of 8   Ni/H-diamond possess good thermal stability even after annealing at temperatures as high as 900 °C for 10 min, which is comparable with those of the Ni/SiC contacts [21,22].  Figure 4. (a) Current–voltage characteristics and (b) total resistance (RT) as functions of d for the 900 °C-annealed Ni/H-diamond. Comparing with the output currents of the 500 °C-annealed and 700 °C-annealed Ni/H-diamonds at ±1.0 V, those for the 900 °C-annealed one decreased slightly. Figure 4b shows the corresponding RT as a function of d for the Ni/H-diamond after annealing at 900 °C. The RS/W, 2RC, and LT are deduced to be 6.1 × 102 Ω·μm−1, 37.9 Ω, and 0.1 μm, respectively. The RS and ρC are calculated to be 60.6 kΩ and 6.0 × 10−5 Ω·cm2. The ρC is comparable with other metals on the H-diamond [23,24]. The thermal-stable Ni/H-dia-mond ohmic contacts exhibit advantages for the high-temperature application of H-dia-mond-based devices. Figure 5a,b compare the RC, RS and ρC of the Ni/H-diamonds after annealing at 500, 700, and 900 °C, respectively. After annealing at 500 and 700 °C, the RC and RS values of the Ni/H-diamonds show no obvious changes. After annealing at 900 °C, however, the RC decreases to 19.0 Ω, while the RS increases to 60.6 kΩ. The increased RS is possibly attributed to the damage of C–H bonds or the desorbed absorption layer on the H-dia-mond surface after multiplied high-temperature treatments [5]. The decrease in RC may be due to the carbon phase transition at the interface between Ni and H-diamond at high temperatures [24,25]. Under the catalysis of nickel, diamond is prone to transform into the graphite phase or form carbide related with Ni, which greatly increases the electrical conductivity, thereby greatly reducing the contact resistance at the interface. In order to confirm the interface reaction between Ni and H-diamond after annealing, transmission electron microscope (TEM) (Figure 6a) and energy dispersive spectrometer (EDS) (Figure 6b) measurements for the Ni/H-diamond after annealing were performed. The interface for the Ni/H-diamond is ambiguous and curved after annealing. The EDS result shows a great number of carbon atoms from diamond dissolved into the Ni lattice. Therefore, the carbides related with Ni for the Ni/H-diamond are formed at the interface, which leads to a lower ρC. -0.8 -0.4 0.0 0.4 0.8-2x10-4-1x10-401x10-42x10-4     9.1 µm13.9 µm18.8 µm23.1 µm29.1 µmCurrent  (A)Voltage (V)-5 0 5 10 15 20 25 3004x1038x1031x1042x1042x104  2LT2RCd (µm)R T(Ω)(b) 900 ℃-annealed(a) 900 ℃-annealedFigure 4. (a) Current–voltage characteristics and (b) total resistance (RT) as functions of d for the 900 ◦C-annealed Ni/H-diamond.Comparing with the output currents of the 500 ◦C-annealed and 700 ◦C-annealedNi/H-diamonds at ±1.0 V, those for the 900 ◦C-annealed one decreased slightly. Figure 4bshows the corresponding RT as a function of d for the Ni/H-diamond after annealingat 900 ◦C. The RS/W, 2RC, and LT are deduced to be 6.1 × 102 Ω·µm−1, 37.9 Ω, and0.1 µm, respectively. The RS and ρC are calculated to be 60.6 kΩ and 6.0 × 10−5 Ω·cm2.The ρC is comparable with other metals on the H-diamond [23,24]. The thermal-stableNi/H-diamond ohmic contacts exhibit advantages for the high-temperature application ofH-diamond-based devices.Figure 5a,b compare the RC, RS and ρC of the Ni/H-diamonds after annealing at 500,700, and 900 ◦C, respectively. After annealing at 500 and 700 ◦C, the RC and RS values ofthe Ni/H-diamonds show no obvious changes. After annealing at 900 ◦C, however, theRC decreases to 19.0 Ω, while the RS increases to 60.6 kΩ. The increased RS is possiblyattributed to the damage of C–H bonds or the desorbed absorption layer on the H-diamondsurface after multiplied high-temperature treatments [5]. The decrease in RC may bedue to the carbon phase transition at the interface between Ni and H-diamond at hightemperatures [24,25]. Under the catalysis of nickel, diamond is prone to transform intothe graphite phase or form carbide related with Ni, which greatly increases the electricalconductivity, thereby greatly reducing the contact resistance at the interface.In order to confirm the interface reaction between Ni and H-diamond after annealing,transmission electron microscope (TEM) (Figure 6a) and energy dispersive spectrometer(EDS) (Figure 6b) measurements for the Ni/H-diamond after annealing were performed.The interface for the Ni/H-diamond is ambiguous and curved after annealing. The EDSresult shows a great number of carbon atoms from diamond dissolved into the Ni lattice.Therefore, the carbides related with Ni for the Ni/H-diamond are formed at the interface,which leads to a lower ρC.Coatings 2021, 11, 470 6 of 8Coatings 2021, 11, x FOR PEER REVIEW 6 of 8      Figure 5. Summary of (a) the contact resistance (RC) and surface sheet resistance (RS), and (b) the deduced specific contact resistance (ρC) for the Ni/H-diamond after annealing at 500, 700, and 900 °C, respectively.  Figure 6. (a) Transmission electron microscope (TEM) and (b) energy dispersive spectrometer (EDS) images for the interface of the Ni/H-diamond after annealing at 900 °C, respectively. 4. Conclusions In this study, ohmic contact characteristics of the Ni/H-diamond at annealing tem-peratures up to 900 °C were investigated. Schottky contacts were formed for the as-re-ceived and the 300 °C-annealed Ni/H-diamonds. When the annealing temperatures were increased to 500 °C, ohmic contacts were formed with the ρC of 1.5 × 10−3 Ω·cm2 for the Ni/H-diamond. For the 700 °C-annealed Ni/H-diamond, the ρC was the same level as that of the 500 °C-annealed one. As the annealing temperature rose to 900 °C, the specific con-tact resistance was as low as 6.0 × 10−5 Ω·cm2. It is believed that the formation of Ni-related carbides at the Ni/H-diamond interface promoted the decrease in specific contact re-sistance. Therefore, the thermal-stable Ni/H-diamond ohmic contacts exhibited ad-vantages for the high-temperature application of H-diamond-based devices. Author Contributions: Conceptualization, J.L. (Jiangwei Liu); methodology, X.Y. and J.L. (Jiangwei Liu); validation, X.Y., J.L. (Jinlong Liu), and J.W.; formal analysis, X.Y. and B.D.; investigation, X.Y. and J.L. (Jiangwei Liu); resources, J.L. (Jiangwei Liu), C.L. and Y.K.; data curation, X.Y. and J.L. (Jiangwei Liu); writing—original draft preparation, X.Y.; writing—review and editing, J.L. (Jiangwei Liu); supervision, J.L. (Jiangwei Liu) and C.L.; funding acquisition, J.L. (Jiangwei Liu) Y.K., and C.L. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported partly by the Leading Initiative for Excellent Young Researchers Program Project, the KAKENHI Project under grant numbers of JP20H00313 and JP16H06419, and the NIMS Nanofabrication Platform of the Nanotechnology Platform Project sponsored by the 1010010000204060801001x10-71x10-61x10-51x10-41x10-31x10-2TemperatureRC(Ω )RS(kΩ)500 ℃ 700 ℃ 900 ℃ρ C(Ω·cm2 )(b) (a) RCRSTemperature500 ℃ 700 ℃ 900 ℃Figure 5. Summary of (a) the contact resistance (RC) and surface sheet resistance (RS), and (b) the deduced specific contactresistance (ρC) for the Ni/H-diamond after annealing at 500, 700, and 900 ◦C, respectively.Coatings 2021, 11, x FOR PEER REVIEW 6 of 8      Figure 5. Summary of (a) the contact resistance (RC) and surface sheet resistance (RS), and (b) the deduced specific contact resistance (ρC) for the Ni/H-diamond after annealing at 500, 700, and 900 °C, respectively.  Figure 6. (a) Transmission electron microscope (TEM) and (b) energy dispersive spectrometer (EDS) images for the interface of the Ni/H-diamond after annealing at 900 °C, respectively. 4. Conclusions In this study, ohmic contact characteristics of the Ni/H-diamond at annealing tem-peratures up to 900 °C were investigated. Schottky contacts were formed for the as-re-ceived and the 300 °C-annealed Ni/H-diamonds. When the annealing temperatures were increased to 500 °C, ohmic contacts were formed with the ρC of 1.5 × 10−3 Ω·cm2 for the Ni/H-diamond. For the 700 °C-annealed Ni/H-diamond, the ρC was the same level as that of the 500 °C-annealed one. As the annealing temperature rose to 900 °C, the specific con-tact resistance was as low as 6.0 × 10−5 Ω·cm2. It is believed that the formation of Ni-related carbides at the Ni/H-diamond interface promoted the decrease in specific contact re-sistance. Therefore, the thermal-stable Ni/H-diamond ohmic contacts exhibited ad-vantages for the high-temperature application of H-diamond-based devices. Author Contributions: Conceptualization, J.L. (Jiangwei Liu); methodology, X.Y. and J.L. (Jiangwei Liu); validation, X.Y., J.L. (Jinlong Liu), and J.W.; formal analysis, X.Y. and B.D.; investigation, X.Y. and J.L. (Jiangwei Liu); resources, J.L. (Jiangwei Liu), C.L. and Y.K.; data curation, X.Y. and J.L. (Jiangwei Liu); writing—original draft preparation, X.Y.; writing—review and editing, J.L. (Jiangwei Liu); supervision, J.L. (Jiangwei Liu) and C.L.; funding acquisition, J.L. (Jiangwei Liu) Y.K., and C.L. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported partly by the Leading Initiative for Excellent Young Researchers Program Project, the KAKENHI Project under grant numbers of JP20H00313 and JP16H06419, and the NIMS Nanofabrication Platform of the Nanotechnology Platform Project sponsored by the 1010010000204060801001x10-71x10-61x10-51x10-41x10-31x10-2TemperatureRC(Ω )RS(kΩ)500 ℃ 700 ℃ 900 ℃ρ C(Ω·cm2 )(b) (a) RCRSTemperature500 ℃ 700 ℃ 900 ℃Figure 6. (a) Transmission electron microscope (TEM) and (b) energy dispersive spectrometer (EDS) images for the interfaceof the Ni/H-diamond after annealing at 900 ◦C, respectively.4. ConclusionsIn this study, ohmic contact characteristics of the Ni/H-diamond at annealing temper-atures up to 900 ◦C were investigated. Schottky contacts were formed for the as-receivedand the 300 ◦C-annealed Ni/H-diamonds. When the annealing temperatures were in-creased to 500 ◦C, ohmic contacts were formed with the ρC of 1.5 × 10−3 Ω·cm2 for theNi/H-diamond. For the 700 ◦C-annealed Ni/H-diamond, the ρC was the same level asthat of the 500 ◦C-annealed one. As the annealing temperature rose to 900 ◦C, the spe-cific contact resistance was as low as 6.0 × 10−5 Ω·cm2. It is believed that the formationof Ni-related carbides at the Ni/H-diamond interface promoted the decrease in specificcontact resistance. Therefore, the thermal-stable Ni/H-diamond ohmic contacts exhibitedadvantages for the high-temperature application of H-diamond-based devices.Author Contributions: Conceptualization, J.L. (Jiangwei Liu); methodology, X.Y. and J.L. (Jiangwei Liu);validation, X.Y., J.L. (Jinlong Liu), and J.W.; formal analysis, X.Y. and B.D.; investigation, X.Y. and J.L.(Jiangwei Liu); resources, J.L. (Jiangwei Liu), C.L. and Y.K.; data curation, X.Y. and J.L. (Jiangwei Liu);writing—original draft preparation, X.Y.; writing—review and editing, J.L. (Jiangwei Liu); supervision,J.L. (Jiangwei Liu) and C.L.; funding acquisition, J.L. (Jiangwei Liu) Y.K., and C.L. All authors have readand agreed to the published version of the manuscript.Coatings 2021, 11, 470 7 of 8Funding: This work is supported partly by the Leading Initiative for Excellent Young ResearchersProgram Project, the KAKENHI Project under grant numbers of JP20H00313 and JP16H06419, and theNIMS Nanofabrication Platform of the Nanotechnology Platform Project sponsored by the Ministryof Education, Culture, Sports, and Technology, Japan. It is supported partly by the National KeyResearch and Development Program of China (No. 2016YFE0133200).Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.Data Availability Statement: The data presented in this study are available on request from thecorresponding author.Conflicts of Interest: The authors declare no conflict of interest.References1. 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[CrossRef]http://doi.org/10.1063/5.0008167http://doi.org/10.3390/coatings10090876http://doi.org/10.1016/j.diamond.2017.02.014 Introduction  Experimental  Preparation of H-Diamond Epitaxial Layer  Formation of Transmission Line Model (TLM) Patterns for Ni on the H-Diamond  Annealing Process and Current–Voltage Measurements  Results and Discussion  Conclusions  References