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## Creator

[Yosuke Sasama](https://orcid.org/0000-0002-8358-6101), [Takuya Iwasaki](https://orcid.org/0000-0002-1103-2433), [Mohammad Monish](https://orcid.org/0000-0002-2352-2640), [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Yamaguchi Takahide](https://orcid.org/0000-0003-0208-7317)

## Rights

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Yosuke Sasama, Takuya Iwasaki, Mohammad Monish, Kenji Watanabe, Takashi Taniguchi, Yamaguchi Takahide; Self-aligned gate electrode for hydrogen-terminated diamond field-effect transistors with a hexagonal boron nitride gate insulator. Appl. Phys. Lett. 26 August 2024; 125 (9): 092103 and may be found at https://doi.org/10.1063/5.0224192[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

## Other metadata

[Self-aligned gate electrode for hydrogen-terminated diamond field-effect transistors with a hexagonal boron nitride gate insulator](https://mdr.nims.go.jp/datasets/b114a103-8971-41c9-bc64-2d18b162ca42)

## Fulltext

Self-aligned gate electrode for hydrogen-terminated diamond field-effect transistors withahexagonal boron nitride gate insulatorSelf-aligned gate electrode for hydrogen-terminated diamond field-effect transistors1with a hexagonal boron nitride gate insulator2Yosuke Sasama,1, a) Takuya Iwasaki,2 Mohammad Monish,2 Kenji Watanabe,33Takashi Taniguchi,2 and Yamaguchi Takahide2, 441)International Center for Young Scientists, National Institute for Materials Science,5Tsukuba 305-0044, Japan62)Research Center for Materials Nanoarchitectonics, National Institute for Materials7Science, Tsukuba 305-0044, Japan83)Research Center for Electronic and Optical Materials,9National Institute for Materials Science, Tsukuba 305-0044,10Japan114)University of Tsukuba, Tsukuba 305-8571, Japan12Diamond electronic devices have garnered significant interest owing to their excellent13semiconducting properties. We recently demonstrated that excluding surface-transfer14doping results in enhanced carrier mobility and a normally off behavior in diamond15field-effect transistors (FETs) with a hexagonal boron nitride (h-BN) gate insula-16tor. In our previous study, the gate electrode was overlapped onto the source/drain17electrodes to prevent the increase in access resistance caused by the exclusion of the18surface-transfer doping. However, it is known that gate overlap increases parasitic19capacitance and gate leak current. In this study, we developed a technique for self-20aligning the gate electrode with the edge of h-BN using oblique-angle deposition. The21diamond FET with self-aligned gate electrode exhibits optimal FET characteristics,22including high mobility of ≈400 cm2V−1s−1, low sheet resistance of 2.4 kΩ, and out-23put characteristics demonstrating pinch-off behavior. Furthermore, the capacitance-24voltage characteristics clearly indicate distinct ON and OFF states, validating the25efficacy of this technique. This method enables the fabrication of diamond/h-BN26FETs with no gate overlap and without increasing the access resistance, making it27promising approach for developing high-speed, low-loss diamond FETs with a wide28application scope.29a)SASAMA.Yosuke@nims.go.jp1This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192Diamond, a wide-gap semiconductor with excellent properties such as a high mobility,30high breakdown electric field, and high thermal conductivity, is a promising electronic ma-31terial. Recently, research on diamond electric devices such as field-effect transistors (FETs)32has gained momentum. Diamond FETs are primarily fabricated on hydrogen-terminated33diamond surfaces, because the hydrogen-termination-mediated upward shift in the valence34band (VB) of diamond allows the incorporation of hole carriers1. Owing to this upward35shifted VB, atmospheric adsorbates2, acidic gases3 (such as NO2), and solid insulators436with a high electron affinity (such as V2O5) can be used as surface acceptors for hydrogen-37terminated diamonds. Electrons in the VB of a hydrogen-terminated diamond migrate to38these surface acceptors, resulting in the formation of a two-dimensional hole gas, and this39process is called surface-transfer doping. Hydrogen-terminated diamond FETs fabricated40by surface-transfer doping exhibit high-temperature operations5 at 400◦C , high breakdown41source–drain voltage6 exceeding 2000 V, high drain current density7 of 1.3 Amm−1, high-42frequency operation8 with high cut-off frequency of 70 GHz, and operation under radiation9.43However, ionized surface acceptors limit the mobility of diamond FETs10.44In our previous studies, we fabricated hydrogen-terminated diamond FETs with a reduced45surface acceptor density to achieve mobility enhancement. First, we fabricated the FETs46using single-crystalline hexagonal boron nitride (h-BN) as the gate insulator to reduce the47density of insulator defects, which could act as acceptor states. The FET showed a relatively48high channel mobility of approximately 300 cm2V−1s−1.11,12 Next, FETs were fabricated49using an air-free process, wherein the hydrogen-terminated surface was not exposed to air, to50reduce the density of surface acceptors originating from air adsorbates, and a high channel51mobility of 680 cm2V−1s−1 was observed, which resulted in the lowest minimum channel52sheet resistance of 1.4 kΩ reported to date for hydrogen-terminated diamond FETs. Our53results show that surface-transfer doping is not necessary for the operation of hydrogen-54terminated diamond FETs and that reducing the density of surface acceptors can improve55the performance of diamond FETs.1356In diamond FETs with a h-BN gate insulator13, the gate electrode overlaps with57source/drain electrodes (Fig. 1a) to eliminate the access region. We fabricated the FETs58on IIa-type hydrogen-terminated diamonds with a low surface-acceptor density. If the gate59electrode does not overlap with the source/drain electrodes, as shown in Fig. 1b, then60the on-resistance of the FET is high owing to the high access resistance. To prevent this612This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192phenomenon, we used FET structures with gate overlap. However, such structures exhibit62certain shortcomings: 1) A large parasitic capacitance that renders the FET unsuitable for63high-speed operations; 2) a large gate leak current that limits carrier density; 3) the h-BN64layer may detach from the diamond surface near the source and drain electrodes, thereby65reducing the gate-bias effect near the electrodes; and 4) h-BN lattice distortions that occur66near the electrodes reduce the breakdown gate voltage.67Figure 1c shows the desirable FET structures for mitigating these limitations: 1) The68gate electrode should extend to the edge of h-BN, and 2) the surface of the access region69should be hydrogen-terminated to allow surface-transfer doping. However, fabricating such70a structure using standard lithography techniques is challenging because of the inevitable71electrode misalignment caused by the lithographic alignment accuracy. Although h-BN may72be dry-etched using the gate electrode as a mask to avoid misalignment of h-BN and the gate73electrode, dry etching may damage the edges of h-BN and the hydrogen-terminated surface74of the access region, potentially causing a high gate leakage current and access resistance.75In this study, we developed a method to induce self-alignment of the gate electrode76with the source/drain electrodes for fabricating a desirable device. A hydrogen-terminated77diamond was laminated with an h-BN thin film, onto which Pd was deposited at an angle of7845◦ (Fig. 2a). Here, the tilt angle is defined as an angle between the direction of evaporated79Pd and the normal direction to the diamond substrate. Because h-BN functioned as a80mask during the deposition, the deposited Pd film was disconnected near the edge of h-BN81(Fig. 2a). The Pd films deposited on h-BN and the diamond were used as the gate and82source/drain electrodes, respectively. We leveraged the surface conductivity induced by the83surface-transfer doping in the access region, and Pd was used to form good ohmic contacts84with the hydrogen-terminated diamond14–16.85Before fabricating the FET structure, we confirmed the occurrence of disconnection of86the Pd film near the h-BN edges owing to the oblique-angle deposition. We used an oxygen-87terminated IIa diamond substrate that did not exhibit electrical conduction. After trans-88ferring a 35-nm-thick h-BN layer, a 20-nm-thick Pd layer was deposited at an angle of 45◦.89Figure 2b shows the scanning electron microscopy (SEM) image of the deposited Pd film;90h-BN is shown in the left half of the SEM image, and a 67-nm gap is visible between the91deposited Pd films. This gap is wider than that expected from the deposited thicknesses92of the h-BN and Pd films. Although the underlying mechanism for this wide gap remains933This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192obscure, it may be attributed to the ability of the Pd film deposited on h-BN to also function94as a mask. The measured electrical resistance between the Pd film on h-BN (left side of the95SEM image) and that on the diamond substrate (right side of the SEM image) exceeded 196GΩ, confirming the disconnected state of the Pd film.97Hydrogen-terminated diamond FETs with self-aligned gate electrodes were fabricated98using an oblique-angle deposition technique. The fabrication process is as follows: We used99IIa-type (111) single-crystalline diamond substrate (purchased from Technological Institute100for Superhard and Novel Carbon Materials (TISNCM)), same as our previous studies11–13.101First, the diamond substrate was cleaned using an organic solvent (isopropyl alcohol and102acetone) and a hot mixed acid (nitric acid:sulfuric acid = 1:3, 200◦C). The diamond surface103was terminated with hydrogen via hydrogen annealing and plasma treatment in a chemical104vapor deposition (CVD) chamber. Hydrogen annealing were performed at an H2 gas flow105rate of 500 sccm, a pressure of 80 Torr, and a temperature of 650◦C for 35 min. The hydrogen106plasma treatment was conducted at a microwave power of 300 W, an H2 gas flow rate of 500107sccm, a pressure of 30 Torr, and a stage temperature of 600◦C for 10 min. The hydrogen-108terminated diamond substrate was then transferred from the CVD chamber to an Ar-gas-109filled glove box under vacuum using a vacuum suitcase to avoid air exposure. Subsequently,110the hydrogen-terminated diamond surface was laminated with a single-crystalline h-BN thin111film inside the glove box (Fig. 3a). The h-BN film fabricated in this study had a thickness112of 16 nm, with 32-nm-thick folded region near the edge, and functioned as a gate insulator.113To transfer the h-BN film, first, single-crystalline h-BN was cleaved using a Scotch tape114and transferred onto a Si/SiO2 substrate; then, the cleaved h-BN was picked-up from the115substrate using a viscoelastic polymer stamp and transferred onto a hydrogen-terminated116diamond surface using a process similar to that used in previous studies17,18. Unlike the117gel-sheet-based dry transfer method, which was used in our previous study13, the pick-118up method19 is useful for avoiding the formation of hydrocarbon-containing contamination119bubbles at the interface. Huang et al. recently fabricated diamond/h-BN heterostructures120using this pick-up method20. In our previous study, diamond was laminated with h-BN121at room temperature13, whereas in this study, this process was conducted at 110◦C. We122heated the diamond substrate during the lamination process to desorb impurities and thus123reduce the density of impurities at the diamond/h-BN interface. However, heating the124diamond may have heated the polymer holding h-BN, and impurities desorbed from the1254This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192heated polymer may have contaminated the diamond/h-BN interface. After the transfer126of h-BN, the sample was annealed at 200◦C for 3 h in a glove box. The source/drain and127self-aligned gate electrodes were simultaneously fabricated by depositing a 20-nm-thick Pd128layer at an angle of 45◦ with electron-beam lithography and lift-off process. (Fig. 3b) Then,129h-BN was etched into a Hall bar shape. h-BN etching was performed using the capacitively130coupled plasma reactive ion etching method at a radiofrequency power of 35 W, a pressure131of 10 Pa, and N2, CHF3, and O2 gas flow rates of 96, 2, and 2 sccm, respectively. The132diamond surface underwent oxygen termination, except the region under h-BN―the access133region―and the electrodes, for device isolation (Fig. 3c). Subsequently, 30-nm-thick Al2O3134was deposited by atomic layer deposition at 120◦C to passivate the device and was wet-135etched using diluted tetramethylammonium hydroxide to form through-holes for electrical136measurements. Finally, Ti(10 nm)/Au(100 nm) was deposited as a gate contact. (Fig.1373d) Through these processes, a hydrogen-terminated diamond FET with a self-aligned gate138electrode was fabricated, as shown in Fig. 3e. We used a gated Hall bar structure to evaluate139the mobility and carrier density accurately through Hall effect measurements.140Figure 4a shows the output characteristics of an FET with a self-aligned gate electrode.141The FET exhibits p-type operation; the drain current density increases under negative142gate voltages. Although the device has a long gate length of 25 µm, a relatively high143maximum drain current density of ≈30 mAmm−1 is obtained. Figure 4b shows the transfer144characteristics. The sheet conductance was measured using the four-probe method, in which145current flows between the source and drain electrodes (electrodes Nos. 2 and 5 in Fig. 3e),146and the voltage drop between the voltage probes (electrodes Nos. 3 and 4 in Fig. 3e)147was measured. The measurement was carried out with a drain current of less than 10 nA148(by applying a voltage of 100 mV through a 10 MΩ series resistor). The maximum sheet149conductance was 4.1×10−4 S. This corresponds to the sheet resistance of 2.4 kΩ. This150minimum sheet resistance is lower than those of most diamond FETs reported to date.151The resistances measured using the four- and two-probe configurations were used to152evaluate the contact resistances (RcS and RcD) at the source and drain electrodes normalized153by the channel width (WG):154(RcS +RcD)WG = [R2p − (LG/Lp)R4p]WG; (1)here, R4p and R2p are the resistances measured using the four- and two-probe configurations,1555This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192respectively; LG is the channel length; and Lp is the distance between the voltage probes.156The normalized contact resistance ((RcS +RcD)WG) of the FET is evaluated to be 50 Ωmm157with an applied gate voltage of -6 V. This contact resistance includes the access resistance.158This value was larger than 5∼15 Ωmm of the contact resistance of the TiC electrodes formed159by Ti/Pt deposition and subsequent annealing in our previous FETs13, and 1.74 Ωmm for Au160electrode21. The larger contact resistance of Pd was also observed in previous reports15,16 and161may be caused by physiosorbed molecules16 which reduce the work function of the deposited162Pd film. Depositing Pd in ultra-high vacuum could suppress the reduction of work function163and decrease the contact resistance. The contact resistance increases at lower temperatures,164but is still 440 Ωmm at 4.4 K with applied gate voltage of -6 V. This indicates that the Pd165electrodes functioned even at low temperatures. In this study, carriers in the access region166were introduced using air adsorbates. Access resistance may be reduced by increasing the167carrier density in the access region through NO2 gas exposure or V2O5 deposition22.168Figure 4c shows the capacitance–voltage (C–V) characteristics measured with an AC169amplitude of 50 mV across various gate voltages and frequencies. During the C-V measure-170ments, the five electrodes of the Hall bar were shorted to avoid the estimate of the insulator171capacitance being affected by parasitic capacitances, as discussed below. Initially, the ca-172pacitance between the five electrodes and gate was measured at frequencies ranging from1731 kHz to 1 MHz with VGS set to 0 V. Subsequently, similar measurements were performed174with an applied gate voltage ranging from -6 V to 6 V in steps of 0.5 to 1 V. The capacitance175was normalized to the area of the gate electrode (117 µm2). In the ON state regime, the176measured capacitance at VGS = -6 V is 0.24 µFcm−2 and remains nearly constant upto VGS177= 0 V. However, during the transition from the ON to OFF state, the capacitance decreases178and becomes ≈0.07 µFcm−2 at VGS = 2 V, which stays nearly unchanged thereafter. It179may be mentioned here that the OFF-state capacitance is the parasitic capacitance between180the five electrodes and gate, while the ON-state capacitance is the sum of the parasitic and181insulator (h-BN) capacitances. Therefore, the capacitance of h-BN can be calculated by182subtracting the OFF-state capacitance from the ON-state capacitance, which is found to183be 0.17 µFcm−2. Since the thickness of h-BN is 16 nm, the dielectric constant of h-BN184is calculated to be 3.2ϵ0 (ϵ0 is the permittivity in vacuum), which is in good agreement185with those reported in previous studies23. Here, the capacitance in the two-folded regions186(13 µm2) is considered to be half of that in the non-folded region. It is noteworthy that1876This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192obtaining an accurate estimate of h-BN capacitance is challenging if the five electrodes are188not shorted and the capacitance is measured between gate and only one of the Hall bar189electrodes. This difficulty arises because, in the OFF-state, the measured capacitance pri-190marily consists of the parasitic capacitance between gate and the Hall-bar electrode, while191in the ON-state, capacitance includes the parasitic capacitances between gate and the other192Hall-bar electrodes24.193The carrier density of the FET was evaluated through Hall-effect measurements with194magnetic field sweeping between -1 and 1 T. Here, current flows between electrodes Nos. 2195and 5 in Fig. 3e, and the Hall voltage between electrodes Nos. 1 and 3 and the longitudinal196voltage between electrodes Nos. 3 and 4 are measured. Figure 4d shows the gate voltage197dependence of the carrier density obtained from the Hall effect measurements. The carrier198density increased linearly with the gate voltage, and the maximum sheet carrier density199reached 6.5×1012 cm−2. Linear fitting of this curve reveals the threshold voltage as ≈0.3 V.200Unlike our previous study13, the FET operated in the normally on mode. This normally on201operation can be ascribed to two primary mechanisms. The first reason is that the material202of gate electrode differs from our previous studies13. Pd was used for gate electrode in this203study, while graphite was used for gate electrode in our previous study13. The work function204of Pd (5.1 eV)25 is higher than that of graphite (4.7 eV)26, which induces a positive shift in205the threshold voltage. Second, the density of the charged impurities at the diamond/h-BN206interface was higher than that observed in our previous study. According to the equation27207Vth = −e(ndepl − nic)thBNϵhBN+ ψs(p2D → 0) + ϕms, (2)the measured threshold voltage (Vth) corresponds to a net sheet density of 1×1012 cm−2 for208the negative charges at the interface. Here, ndepl is the sheet density of the fixed charge in the209depletion layer; nic is the net sheet density of the interface negative charge; ψs(p2D → 0) (<0)210is the surface potential at the limit of the low-hole sheet density (p2D); eϕms = eϕm − eϕs(≈2114.8 eV) is the difference between the work function (eϕm ≈ 5.1 eV)25 of the Pd gate and212that (eϕs ≈ 0.3 eV)13,28 of the hydrogen-terminated surface; and ndepl and ψs are calculated213using the Schrödinger and Poisson equations for a given p2D with ND (nitrogen) = 500 ppb214and NA (boron) = 7 ppb. The negative charge density of 1×1012 cm−2 is higher than that215in our previous study13 (4×1011 cm−2). We believe that the charged impurities originate216from those desorbed from the polymer holding h-BN; as mentioned before, this polymer is2177This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192heated during the transfer of h-BN. Notably, the normally on mode cannot be attributed to218the unintentional contamination of boron in diamond. In the transfer characteristics shown219in Fig. 4b, the conductance decreases to almost zero at a positive gate voltage, whereas220boron-contamination-induced conductance should have a finite value.221Figure 4e shows the gate voltage dependence of the mobility. Evidently, the Hall mobility,222effective mobility(µeff = thBNϵhBNσ|VGS−Vth|), and field-effect mobility(µFE = thBNϵhBN∣∣∣∂σ∂VGS∣∣∣) are almost223consistent; here thBN and ϵhBN are the thickness and dielectric constant of h-BN, respectively;224ϵhBN is 3.2ϵ0 as mentioned before; VGS and Vth denote the gate and threshold voltages,225respectively; and σ is the sheet conductance (Fig. 4b). The maximum mobility (≈400226cm2V−1s−1) is relatively high (Fig. 4e). However, this value was slightly lower than that227in our previous study13 possibly owing to the slightly high density of charged impurities at228the diamond/h-BN interface. We believe that this discrepancy can be mitigated by further229improving the h-BN transfer technique; for instance, the impurities in the polymer holding230h-BN may be desorbed by annealing the polymer in a glove box before use.231In this study, we developed a method for fabricating gate electrodes that self-align with232the source/drain electrodes by depositing Pd at an angle of 45◦. During the oblique-angle233deposition, the h-BN gate insulator acted as a mask, allowing simultaneous fabrication of234source/drain electrodes and a self-aligned gate electrode. We fabricated a diamond FET235with a self-aligned gate electrode and obtained good FET characteristics. We envision that236increase in the h-BN thickness and deposition from a larger angle can widen the gap between237the gate and source/drain electrode, thereby allowing the application of high source-drain238voltage and reduction in parasitic capacitance. Although the same material was used for239the source/drain and gate electrodes in this study, the materials of the source/drain and240gate electrodes could be modified by depositing two different metals from different angles.241For example, if a low-work-function metal (such as Al) is deposited at an angle of 70◦,242followed by a high-work-function metal (e.g., Pd, Au) deposited at an angle of 45◦, then243we can fabricate an FET with both good ohmic contacts and a normally off operation. We244believe that the method developed in this study for fabricating self-aligned gate electrodes245can contribute to the construction of diamond FETs incorporated with h-BN gate insulators246that can operate at remarkably high speeds.2478This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192Data availability248The data supporting the findings of this study are available from the corresponding author249upon reasonable request.250REFERENCES1S. J. Sque, R. Jones, and P. R. Briddon, “Structure, electronics, and interaction of hy-drogen and oxygen on diamond surfaces,” Physical Review B 73, 085313 (2006).2F. Maier, M. Riedel, B. Mantel, J. Ristein, and L. Ley, “Origin of surface conductivity indiamond,” Physical Review Letters 85, 3472–3475 (2000).3M. Kasu, “Diamond field-effect transistors for RF power electronics: Novel NO2 holedoping and low-temperature deposited Al2O3 passivation,” Japanese Journal of AppliedPhysics 56, 01AA01 (2017).4C. Verona, F. Arciprete, M. Foffi, E. Limiti, M. Marinelli, E. Placidi, G. Prestopino, andG. 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Ng, Physics of Semiconductor Devices, 3rd ed. (John Wiley & Sons,Inc., 2007).28J. Cui, J. Ristein, and L. Ley, “Electron affinity of the bare and hydrogen covered singlecrystal diamond (111) surface,” Physical Review Letters 81, 429 (1998).Acknowledgments251We thank H. Osato, E. Watanabe, D. Tsuya, N. Ikeda, and A. Ohi for technical sup-252port. We thank T. Ando, S. Koizumi, and T. Teraji for their assistance with this study.25311This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192This study was financially supported by the NEDO Uncharted Territory Challenge 2050254(Project No. JPNP14004), JSPS KAKENHI (Grant No. JP22H01962), and Advanced Re-255search Infrastructure for Materials and Nanotechnology in Japan (ARIM) (Proposal No.256JPMXP1223NM5196).257Competing Interests258The authors declare that they have no competing financial interests.25912This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192FIG. 1. Schematic of diamond FETs with h-BN gate insulators. (a) Structure fabricated in ourprevious studies, (b) an undesirable structure, and (c) a preferred structure.13This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192FIG. 2. (a) Schematic of the oblique-angle deposition of Pd. The resist is not shown here forclarity. (b) SEM image of the Pd film deposited at an angle of 45◦ on diamond/h-BN. Althoughh-BN is not visible in the SEM image, it is present in the left of the vertical dashed line. Thethicknesses of the h-BN and Pd films are 35 and 20 nm, respectively. A charge dissipation agent(Espacer 300Z) was coated to prevent charge accumulation during the SEM observation.14This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192FIG. 3. (a–d) Schematic of the FET fabrication process. (a) h-BN is transferred to hydrogen-terminated diamond. (b) Pd is deposited at an angle of 45◦. The resist with the electrode patternsis not shown here for clarity. (c) h-BN is etched into a Hall-bar shape. (d) Al2O3 is deposited, andholes are formed via wet etching of Al2O3. Although Al2O3 is deposited on the entire surface inthe actual device, only a part of the deposited Al2O3 is shown here . Ti/Au is deposited as a lead.(e) Optical microscopic image of the diamond FET fabricated in this study.15This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192FIG. 4. Electrical characteristics of the fabricated diamond FET with a self-aligned gate electrode.(a) Output characteristics, (b) transfer characteristics, and (c) C–V curve. Gate voltage depen-dence of the (d) carrier density and (e) mobility. The measurements were performed at 300 K (a,b, d, and e) or at room temperature (c).16This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0224192