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Article Reliable Ohmic Contact Properties for Ni/Hydrogen-Terminated at Annealing Temperature up to 900 ◦C

Xiaolu Yuan 1,2 , Jiangwei Liu 2,* , Jinlong Liu 1, Junjun Wei 1, Bo Da 3, Chengming Li 1,* and Yasuo Koide 2

1 Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (X.Y.); [email protected] (J.L.); [email protected] (J.W.) 2 Research Center for Functional Materials, National Institute for (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan; [email protected] 3 Research and Services Division of Materials Data and Integrated System, NIMS, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan; [email protected] * Correspondence: [email protected] (J.L.); [email protected] (C.L.)

Abstract: Ohmic contact with high thermal stability is essential to promote hydrogen-terminated diamond (H-diamond) electronic devices for high-temperature applications. Here, the ohmic contact characteristics of Ni/H-diamond at annealing temperatures up to 900 ◦C are investigated. The

measured current–voltage curves and deduced specific (ρC) are used to evaluate the quality of the contact properties. Schottky contacts are formed for the as-received and 300 ◦C- annealed Ni/H-. When the annealing temperature is increased to 500 ◦C, the ohmic contact −3 2 properties are formed with the ρC of 1.5 × 10 Ω·cm for the Ni/H-diamond. As the annealing ◦ −5 2 temperature rises to 900 C, the ρC is determined to be as low as 6.0 × 10 Ω·cm . It is believed that   the formation of Ni-related carbides at the Ni/H-diamond interface promotes the decrease in ρC. The Ni metal is extremely promising to be used as the ohmic contact electrode for the H-diamond-based Citation: Yuan, X.; Liu, J.; Liu, J.; Wei, electronic devices at temperature up to 900 ◦C. J.; Da, B.; Li, C.; Koide, Y. Reliable Ohmic Contact Properties for Ni/Hydrogen-Terminated Diamond Keywords: hydrogen-terminated diamond (H-diamond); ohmic contact; Ni; specific contact resis- at Annealing Temperature up to tance; high-temperature 900 ◦C. Coatings 2021, 11, 470. https://doi.org/10.3390/coatings 11040470 1. Introduction Academic Editor: Aomar Hadjadj Diamond, with many remarkable intrinsic properties, possesses vast prospect applica- tions for high-power, high-frequency, and high-temperature electronics [1–3]. It exhibits an Received: 23 March 2021 ultrawide energy bandgap (5.5 eV), high carrier mobilities (4500 and 3800 cm2·V−1·s−1 for Accepted: 13 April 2021 electrons and holes, respectively), large breakdown field strength (10 MV·cm−1), and the Published: 17 April 2021 highest 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- Publisher’s Note: MDPI stays neutral ity with a hole carrier concentration up to ~1014 cm−2 [5,6]. Recently, H-diamond-based with regard to jurisdictional claims in field-effect transistors have achieved excellent device performances, such as a high break- published maps and institutional affil- down voltage (2000 V), a high-output current density (1.3 A·mm−1), and a high-output iations. power density (3.8 W·mm−1)[1,7,8]. Meanwhile, the passivation layer protection for the H-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 enables the H-diamond surface damaged during annealing to regain good conductivity [12]. Copyright: © 2021 by the authors. In order to further promote H-diamond-based electronic devices to be operated well Licensee MDPI, Basel, Switzerland. at high temperatures, thermal-stable ohmic contact is essential. Until now, different kinds This article is an open access article of metals are used for ohmic contacts on the H-diamond, such as, Au, Pd, Pt, W, Ti/Au, distributed under the terms and Pt/Au, Pd/Ti/Au, Ti/Ni/Au, etc. [12–15] They show good ohmic contact properties and conditions of the Creative Commons high thermal stability, with annealing temperatures up to 700 ◦C. On the other hand, the Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Ni metal tends to form Schottky contact with the H-diamond at an annealing temperature ◦ 4.0/). lower than 100 C[16]. However, Ni, as a carbophilic element, is prone to react with carbon

Coatings 2021, 11, 470. https://doi.org/10.3390/coatings11040470 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 470 2 of 8

at an elevated temperature [17]. The solid-solution reaction makes the Ni-related carbides formed at the Ni/H-diamond interface, which would possibly contribute to the formation of ohmic contact. Here, ohmic contact characteristics of Ni/H-diamond at annealing temperatures up to 900 ◦C are investigated. The measured current–voltage curves and deduced specific contact resistance (ρC) are used to evaluate the quality of the contact properties.

2. Experimental 2.1. Preparation of H-Diamond Epitaxial Layer

An Ib-type (100) facet single-crystalline diamond was boiled in mixed H2SO4 and ◦ HNO3 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 grown using a microwave plasma-enhanced chemical vapor deposition system. The CH4 flow rate, 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-Diamond The 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 system in 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 etching system. 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 lithography procedures. The Ni, with a thickness of 100 nm, was grown on the H-diamond for ohmic contact via an e-beam system under a ~10−5 Pa vacuum condition.

2.3. Annealing Process and Current–Voltage Measurements The annealing process was performed using a rapid thermal annealing system in an Ar atmosphere. The annealing temperatures were 300, 500, 700, and 900 ◦C with an annealing time of 10 min for each temperature. After annealing, the sample was exposed to air for more than 10 h in order to promote the formation of a negatively adsorbed layer on the surface of the H-diamond, and to regain good surface conductivity. The calculations of ρC for the Ni/H-diamond with the TLM patterns can be referred to in the previous reports [18,19]. The electrical properties of Ni/H-diamond contacts were characterized by a room-temperature probe system.

3. Results and Discussion Figure1 shows the surface morphology of five-group TLM patterns of Ni on the H-diamond epitaxial layer before annealing. The length and width of each electrode are the same as 100 µm. All the Ni metals were stable to be formed on the H-diamond. The five groups of TLM patterns 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. The interspace (d) values from left to right in Figure1, between the two adjacent electrodes for the five-group TLM patterns, are in the ranges of 7.9–9.1 µm, 13.5–13.9 µm, 18.0–18.8 µm, 23.1–23.9 µm and 28.2–29.1 µm, respectively. Figure2 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 (Figure2a). All the curves show non-linear characteristics, indicating the Schottky contacts. After annealing at 300 ◦C for 10 min, as shown in Figure2b, the output currents of the Ni/H-diamond increase to the order of 10−4 A. The annealing process improves Coatings 2021, 11, x FOR PEER REVIEW 3 of 8

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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 Coatings 2021, 11, x FOR PEER REVIEW 3 of 8 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 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].FigureFigure The 1. 1.Surfacepresence Surface morphology morphology of a ch ofemisorbed the of five-group the five-group species transmission transmion the line ssionH-diamond model line (TLM) model patterns surface (TLM) of Ni patterns on of Ni on the possibly results in thethehydrogen-terminated hydrogen-terminatedformation of Schottky diamonddiamond contact. (H-diamond) (H-diamond) However, epitaxial epitaxial layerthe annealing beforelayer annealing.before process annealing. They werecan They used were used to tocharacterize characterize current–voltage curves curves for for the the Ni/H-diamonds Ni/H-diamonds of as-received, of as-received, 300 ◦C-annealed, 300 °C-annealed, 500 °C- improve the contact interface of the Ni/H-diamond, thereby promoting the current flow. 500annealed,◦C-annealed, 700 °C-annealed, 700 ◦C-annealed, and and 900 900 °C-annealed,◦C-annealed, respectively. respectively.

(a) As-received (b) 300 ℃-annealed 8x10-7 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 1x10-4 -7 –1.0–1.0 V. For the as-received Ni/H-diamond contact, the output currents are in the order 4x10 of 10−7 A (Figure 2a). All5x10 the-5 curves show non-linear characteristics, indicating the Schottky contacts. After annealing at 300 °C for 10 min, as shown in Figure 2b, the output 0 0 currents of the8.5 µm Ni/H-diamond increase to the order of 10−4 A. The annealing process im- 8.7 µm 13.9 µm -5 Current Current (A) Current Current (A) proves the contact properties-5x10 of the Ni/H-diamond. However, 13.5 µm all the curves still show -7 18.2 µm 18.8 µm -4x10 23.9 µm non-linear characteristics. Therefore,-4 even after annealing 23.4 µm at 300 °C, the Ni/H-diamond 28.9 µm still operates with Schottky-1x10 contacts, which is consistent 28.7 µmwith the results obtained from -8x10-7 -0.8 -0.4the other 0.0 report 0.4 [20]. 0.8 The presence-0.8 of a ch -0.4emisorbed 0.0 species 0.4 0.8on the H-diamond surface possiblyVoltage (V)results in the formation of Schottky Voltagecontact. (V) However, the annealing process can

improve the contact interface of the Ni/H-d◦ iamond, thereby promoting the current flow. FigureFigure 2. Current–voltage2. Current–voltage characteristics characteristics of (a) the of as-received (a) the as-received and (b) the and 300 (C-annealedb) the 300 Ni/H-diamond°C-annealed Ni/H- contacts, respectively. diamond contacts, respectively. (a) As-received ℃ -7 (b) 300 -annealed Figure8x10 3a shows the current–voltage characteristics of the Ni/H-diamond after anneal- ◦ Figure 3a showsing atthe 500 current–voltageC. All the current–voltage characteristics curves have of linear the Ni/H-diamond characteristics,1x10-4 which after implies an- good ohmic contacts-7 for the Ni/H-diamond. For two adjacent electrodes in which d = 8.7 µm, nealing at 500 °C. All the4x10 current–voltage curves have linear characteristics, which implies the output currents are 2.0 × 10−4 A at ±1.0 V. The total resistance-5 (R ) for the 500 ◦C- good ohmic contacts for the Ni/H-diamond. For two adjacent electrodes5x10 in whichT d = 8.7 μm, the output currents are0 2.0 × 10−4 A at ±1.0 V. The total resistance (0RT) for the 500 °C- 8.5 µm 3 annealed Ni/H-diamond can be calculated to be 4.9 × 10 Ω (d = 8.7 μm). Based on the 8.7 µm 13.9 µm -5 Current Current (A) Current Current (A) -5x10 13.5 µm -7 18.2 µm 18.8 µm -4x10 23.9 µm -4 23.4 µm 28.9 µm -1x10 28.7 µm -7 -8x10 -0.8 -0.4 0.0 0.4 0.8 -0.8 -0.4 0.0 0.4 0.8 Voltage (V) 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 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

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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]: Coatings 2021, 11, 470 4 of 8 R R =+2Rds (1) TCW × 3 µ annealed Ni/H-diamond=×× can be calculated to be 4.9 10 Ω (d = 8.7 m). Based on the current–voltage characteristicsρCCTR forLW other adjacent electrodes (Figure3a), the RT for(2) the Ni/H-diamond with other d values is also calculated and summarized in Figure3b. There By fitting the spotsare the in following Figure relationships 3b, the RS/ forW the(theRT slopeand ρC ofwith fitting the Ni/H-diamond line) are determined contact resistance to (RC), H-diamond surface sheet resistance (RS), electrode transfer length (LT), and electrode be 4.0 × 102 Ω·μm−1 with an RS of 40 kΩ. The 2RC (the intercept of the y-axis) and 2LT (the width (W)[18]: 3 intercept of the x-axis) are deduced to be 1.5 × 10 Ω and 3.9Rs μm, respectively. Based on RT = 2RC + d (1) Equation (2), the ρC for the 500 °C-annealed Ni/H-diamondW can be calculated to be 1.5 × 10−3 Ω·cm2. ρC = RC × LT × W (2)

(a) 500 ℃-annealed (b) 500 ℃-annealed 2x10-4 1x104

1x10-4

) 8x103 Ω (

0 T R 8.7 µm 13.5 µm 3 Current (A) Current 4x10 -1x10-4 18.8 µm 23.4 µm 2LT 28.7 µm 2RC -2x10-4 0 -0.8 -0.4 0.0 0.4 0.8 -5 0 5 10 15 20 25 30 Voltage (V) d (µm)

(c) 700 ℃-annealed (d) 700 ℃-annealed 2x10-4 1x104 1x10-4 ) Ω

( 3

T 8x10

0 R 7.9 µm

Current (A) Current 13.5 µm 4x103 -1x10-4 18.0 µm 23.8 µm 2LT 28.2 µm 2RC -2x10-4 0 -0.8 -0.4 0.0 0.4 0.8 -5 0 5 10 15 20 25 30 Voltage (V) d (µm)

◦ ◦ FigureFigure 3. (3.a, c()a Current–voltage,c) Current–voltage characteristics characteristics of the 500 C-annealed of the 500 and °C-annealed 700 C-annealed and Ni/H-diamonds, 700 °C-annealed respectively. Ni/H- ◦ ◦ (b,d) RT as functions of d for the 500 C-annealed and 700 C-annealed Ni/H-diamonds, respectively. 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 Figure3b, the RS/W (the slope of fitting line) are determined to 2 −1 be 4.0 × 10 Ω·µm with an RS of 40 kΩ. The 2RC (the intercept of the y-axis) and 2LT When the annealing(the intercept temperature of the x-axis) is areincrease deducedd to be700 1.5 °C,× 10 the3 Ω andoutput 3.9 µ m,current respectively. maxima Based on Equation (2), the ρ for the 500 ◦C-annealed Ni/H-diamond can be calculated to be are the same level as those of the 500C °C-annealed Ni/H-diamond (Figure 3c). The linear 1.5 × 10−3 Ω·cm2. characteristics for all theWhen current–voltage the annealing temperature curves are is increasedobserved, to 700indicating◦C, the output the good current ohmic maxima ◦ properties for the 700are the°C-annealed same level as Ni/H-diamond. those of the 500 C-annealed Figure 3d Ni/H-diamond shows the (FigureRT as 3ac). function The linear characteristics for all the current–voltage curves are observed, indicating the good ohmic of d for the Ni/H-diamond after annealing◦ at 700 °C. By fitting the spots, the RS/W, 2RC, properties for the 700 C-annealed Ni/H-diamond. Figure3d shows the RT as a function and 2LT are obtained to be 4.0 × 102 Ω·μm−1, 1.9 × 103 Ω, and◦ 4.8 μm, respectively. The RS of d for the Ni/H-diamond after annealing at 700 C. By fitting the spots, the RS/W, 2RC, 2 −1 3 and ρC for the Ni/H-diamondand 2LT are obtainedafter annealing to be 4.0 ×at 10700Ω °C·µm are, 1.9calculated× 10 Ω, to and be 4.8 40µ m,kΩ respectively. and 2.3 ◦ × 10−3 Ω·cm2, respectively,The RS andwhichρC forare the close Ni/H-diamond to those of after the 500 annealing °C-annealed at 700 C Ni/H-diamond. are calculated to be 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

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Coatings 2021, 11, x FOR PEER REVIEW −3 2 ◦5 of 8 40 kΩ and 2.3 × 10 Ω·cm , respectively, which are close to those of the 500 C-annealed Ni/H-diamond. Figure4a 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 Ni/H-diamond possessNi/H-diamond good thermal possess stability good thermal even stability after annealing even after annealingat temperatures at temperatures as as high as 900 °C for 10high min, as 900which◦C for is 10comparable min, which iswith comparable those of with the those Ni/SiC of the contacts Ni/SiC contacts[21,22]. [21 ,22].

(a) 900 ℃-annealed 4 (b) 900 ℃-annealed 2x10-4 2x10

2x104 1x10-4

) 4

Ω 1x10 (

0 T R 8x103

Current Current (A) 9.1 µm -4 13.9 µm -1x10 3 18.8 µm 4x10 2LT 23.1 µm 2RC 29.1 µm -2x10-4 0 -0.8 -0.4 0.0 0.4 0.8 -5 0 5 10 15 20 25 30 Voltage (V) d (µm)

◦ Figure 4. (a) Current–voltage characteristics and (b) total resistance (RT) as functions of d for the 900 C-annealed Ni/H- diamond.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 ◦ Comparing withNi/H-diamonds the output atcurrents±1.0 V, those of the for the500 900 °C-annealedC-annealed oneand decreased 700 °C-annealed slightly. Figure 4b shows the corresponding RT as a function of d for the Ni/H-diamond after annealing Ni/H-diamonds at ±1.0 V,◦ those for the 900 °C-annealed one decreased slightly.2 Figure−1 4b at 900 C. The RS/W, 2RC, and LT are deduced to be 6.1 × 10 Ω·µm , 37.9 Ω, and T −5 2 shows the corresponding0.1 µm, R respectively. as a function The R Sofand d ρforC are the calculated Ni/H-diamond to be 60.6 after kΩ and annealing 6.0 × 10 atΩ ·cm . 2 −1 900 °C. The RS/W, 2TheRC, ρandC is comparableLT are deduced with other to be metals 6.1 on× 10 the Ω H-diamond·μm , 37.9 [23 ,Ω24,]. and The 0.1 thermal-stable μm, respectively. The RSNi/H-diamond and ρC are calculated ohmic contacts to exhibitbe 60.6 advantages kΩ and for 6.0 the × high-temperature10−5 Ω·cm2. The application ρC is of comparable with otherH-diamond-based metals on the devices. H-diamond [23,24]. The thermal-stable Ni/H-dia- Figure5a,b compare the RC, RS and ρC of the Ni/H-diamonds after annealing at 500, mond ohmic contacts exhibit advantages◦ for the high-temperature application◦ of H-dia- 700, and 900 C, respectively. After annealing at 500 and 700 C, the RC and RS values of mond-based devices.the Ni/H-diamonds show no obvious changes. After annealing at 900 ◦C, however, the Figure 5a,b compareRC decreases the R toC, 19.0RS andΩ, while ρC of the theR SNi/H-diamondsincreases to 60.6 k Ωafter. The annealing increased RatS is500, possibly attributed to the damage of C–H bonds or the desorbed absorption layer on the H-diamond 700, and 900 °C, respectively. After annealing at 500 and 700 °C, the RC and RS values of surface after multiplied high-temperature treatments [5]. The decrease in RC may be the Ni/H-diamondsdue show to theno carbonobvious phase changes. transition After at the annealing interface betweenat 900 °C, Ni however, and H-diamond the R atC high decreases to 19.0 Ωtemperatures, while the [ 24R,25S ].increases Under the to catalysis 60.6 k ofΩ nickel,. The diamondincreased is proneRS is to possibly transform into attributed to the damagethe graphite of C–H phase bonds or form or carbide the desorbed related with absorption Ni, which greatlylayer increaseson the H-dia- the electrical mond surface after conductivity,multiplied therebyhigh-tempe greatlyrature reducing treatments the contact [5]. resistance The decrease at the interface. in RC may be due to the carbon phaseIn order transition to confirm at thethe interface interface reaction between between Ni Niand and H-diamond H-diamond after at high annealing, transmission electron microscope (TEM) (Figure6a) and energy dispersive spectrometer temperatures [24,25].(EDS) Under (Figure the6 b)catalysis measurements of nickel, for the diamond Ni/H-diamond is prone after to annealingtransform were into performed. the graphite phase or formThe interface carbide for related the Ni/H-diamond with Ni, iswhich ambiguous greatly and increases curved after the annealing. electrical The EDS conductivity, therebyresult greatly shows reducing a great number the contact of carbon resistance atoms from at diamond the interface. dissolved into the Ni lattice. In order to confirmTherefore, the interface the carbides reaction related between with Ni for Ni the and Ni/H-diamond H-diamond are after formed annealing, at the interface, which leads to a lower ρ . transmission electron microscope (TEM)C (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.

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Coatings 2021, 11, x FOR PEER REVIEW 6 of 8 Coatings 2021, 11, 470 6 of 8 (a) (b) 100 1x10-2 1000 RC (a) -3(b) RS 10080 1x101x10-2 ) 2 )

) RC

1000 Ω

Ω -4

·cm -3 ( R 60 1x10 S 80 (k 1x10 Ω C ) S ( 2 ) R C R ) ρ

100 Ω -5

Ω 40 -4

·cm 1x10 ( 60 1x10 (k Ω C S ( R C R

100 ρ -6 40 20 1x101x10-5

10 20 0 1x10-6 -7 500 ℃ 700 ℃ 900 ℃ 1x10 500 ℃ 700 ℃ 900 ℃ Temperature Temperature 10 0 1x10-7 500 ℃ 700 ℃ 900 ℃ 500 ℃ 700 ℃ 900 ℃ Figure 5. SummaryTemperature of (a) the contact resistance (RC) and surface Temperaturesheet resistance (RS), and (b) the deduced specific contact resistance (ρC) for the Ni/H-diamond after annealing at 500, 700, and 900 Figure 5. Summary of (a) the contact resistance (RC) and surface sheet resistance (RS), and (b) the deduced specific contact Figure°C, respectively. 5. Summary of (a) the contact resistance (RC) and surface◦ sheet resistance (RS), and (b) the resistance (ρC) for the Ni/H-diamond after annealing at 500, 700, and 900 C, respectively. deduced specific contact resistance (ρC) for the Ni/H-diamond after annealing at 500, 700, and 900 °C, respectively.

Figure 6.Figure(a) Transmission 6. (a) Transmission electron microscope electron (TEM) microscope and (b) energy (TEM) dispersive and ( spectrometerb) energy dispersive (EDS) images spectrometer for the interface of the Ni/H-diamond(EDS) images after for annealingthe interface at 900 of◦C, the respectively. Ni/H-diamond after annealing at 900 °C, respectively. Figure 6. (a) Transmission electron microscope (TEM) and (b) energy dispersive spectrometer 4. Conclusions (EDS)4. Conclusions images for the interface of the Ni/H-diamond after annealing at 900 °C, respectively. In this study, ohmic contact characteristics of the Ni/H-diamond at annealing temper- In this study,atures ohmic up to 900contact◦C were characterist investigated.ics Schottky of the Ni/H-diamond contacts were formed at annealing for the as-received tem- 4. Conclusions peratures up toand 900 the °C 300 were◦C-annealed investigated. Ni/H-diamonds. Schottky Whencontacts the annealingwere formed temperatures for the wereas-re- in- ◦ −3 2 In this study,creased ohmic to contact 500 C, ohmiccharacterist contactsics were of formedthe Ni/H-diamond with the ρC of 1.5at ×annealing10 Ω·cm tem-for the ceived and the 300 °C-annealed Ni/H-diamond◦ s. When the annealing temperatures were Ni/H-diamond. For the 700 C-annealed Ni/H-diamond, the ρC was−3 the same2 level as peraturesincreased up to to 500 900 °C, °C ohmicwere investigated.contacts◦ were Schottky formed contactswith the wereρC of formed1.5 × 10 for Ω the·cm◦ as-re- for the ceived and the 300that °C-annealed of the 500 C-annealed Ni/H-diamond one. Ass. theWhen annealing the annealing temperature temperatures rose to 900 C, were the spe- Ni/H-diamond.cific For contact the 700 resistance °C-annealed was as Ni/H-diamond, low as 6.0 × 10−5 Ω the·cm ρ2.C Itwas is believed the same that level the formation as that −3 2 increasedof the 500 to °C-annealed500 °C,of Ni-relatedohmic one. contacts carbides As the atwereannealing the Ni/H-diamondformed temperature with interfacethe ρ roseC of promoted to1.5 90 ×0 10 °C, the Ωthe decrease·cm specific for in specificthe con- Ni/H-diamond.tact resistance For wascontact the as 700low resistance. °C-annealed as 6.0 Therefore, × 10−5 Ω Ni/H-diamond,·cm the2 thermal-stable. It is believed the Ni/H-diamond thatρC was the the formation ohmicsame contactslevel of Ni-related as exhibited that of thecarbides 500 °C-annealed at theadvantages Ni/H-diamond one. As for thethe high-temperature annealinginterface prom temperature applicationoted the rose ofdecrease H-diamond-based to 900 in°C, specific the devices.specific contact con- re- tact resistance was as low as 6.0 × 10−5 Ω·cm2. It is believed that the formation of Ni-related sistance. Therefore,Author Contributions:the thermal-stableConceptualization, Ni/H-diamond J.L. (Jiangwei Liu); ohmic methodology, contacts X.Y. and exhibitedJ.L. (Jiangwei ad- Liu); carbidesvantages at forthe theNi/H-diamondvalidation, high-temperature X.Y., J.L. interface (Jinlong application Liu), prom and J.W.;oted of formal H-diamond-based the analysis, decrease X.Y. and in B.D.;specific devices. investigation, contact X.Y. re- and J.L. sistance. Therefore,(Jiangwei the Liu);thermal-stable resources, J.L. (Jiangwei Ni/H-diamond Liu), C.L. and Y.K.;ohmic data curation,contacts X.Y. exhibited and J.L. (Jiangwei ad- Liu); vantagesAuthor forContributions: the high-temperaturewriting—original Conceptualization, draft preparation,application J.L. X.Y.;(Jiangwei of writing—review H-diamond-based Liu); methodology, and editing, devices. J.L. X.Y. (Jiangwei and Liu);J.L. (Jiangwei supervision, J.L. (Jiangwei Liu) and C.L.; funding acquisition, J.L. (Jiangwei Liu) Y.K., and C.L. All authors have read Liu); validation, X.Y.,and agreed J.L. (Jinlong to the published Liu), and version J.W.; of the formal manuscript. analysis, X.Y. and B.D.; investigation, X.Y. Authorand J.L.Contributions: (Jiangwei Liu); Conceptualization, resources, J.L. (JiangweiJ.L. (Jiangwei Liu), Liu); C.L. methodology, and Y.K.; data X.Y. curation, and J.L. X.Y. (Jiangwei and J.L. Liu);(Jiangwei validation, Liu); X.Y., writing—original J.L. (Jinlong Liu), draft and preparation, J.W.; formal X.Y.; analysis, writing—review X.Y. and B.D.; and editing,investigation, J.L. (Jiangwei X.Y. andLiu); J.L. supervision,(Jiangwei Liu); J.L. (Jiangweiresources, Liu) J.L. and(Jiangwei C.L.; fund Liu),ing C.L. acquisition, and Y.K.; J.L. data (Jiangwei curation, Liu) X.Y. Y.K., and and J.L. C.L. (JiangweiAll authors Liu); havewriting—original read and agreed draft to preparation, the published X.Y.; version writing—review of the manuscript. 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. Funding: This work is supported partly by the Leading Initiative for Excellent Young Researchers All authors have read and agreed to the published version of the manuscript. Program Project, the KAKENHI Project under grant numbers of JP20H00313 and JP16H06419, and Funding:the NIMS This Nanofabrication work is supported Platform partly byof the LeadingNanotechnology Initiative Platformfor Excellent Project Young sponsored Researchers by the 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

Coatings 2021, 11, 470 7 of 8

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 Ministry of Education, Culture, Sports, and Technology, Japan. It is supported partly by the National Key Research 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 the corresponding author. Conflicts of Interest: The authors declare no conflict of interest.

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