High-mobility diamond field effect transistor with a monocrystalline h-BN gate dielectric

Diamond has a number of electronic properties that are well suited to power electronics and high-power radio-frequency applications.^1–4 For instance, it has a wide bandgap and high breakdown electric field, and it has been used to make field effect transistors (FETs) that operate at high temperatures (400 °C)⁵ and high breakdown source-drain voltages (2000 V).⁶ It also has high intrinsic carrier mobility, which makes it potentially suitable for high-performance devices. In particular, high carrier mobility enables the on-resistance and the switching time to be reduced, which in turn reduces the conduction and switching losses of high-power switching devices. High carrier mobility also benefits the operation of high-power radio-frequency devices. Here, the generally accepted intrinsic mobilities of diamond are as high as 4500 cm² V⁻¹ s⁻¹ for electrons and 3800 cm² V⁻¹ s⁻¹ for holes at room temperature.⁷ Recent cyclotron resonance experiments on ultrapure CVD diamonds have indicated even higher mobilities: 7300 cm² V⁻¹ s⁻¹ for electrons and 5300 cm² V⁻¹ s⁻¹ for holes.⁸ These values are much larger than those of silicon, germanium,⁹ silicon carbide,¹ and gallium nitride¹⁰ (HEMT¹¹). The high intrinsic mobility of diamond is due to its high-velocity acoustic phonons, which only weakly scatter carriers.

However, high carrier mobility has been difficult to realize in diamond FETs, especially when the carrier density is high. The reported mobilities are mostly lower than 200 cm² V⁻¹ s⁻¹.^12–25 Although higher mobilities have been indicated in a few reports,^26–29 the carrier density was less than 1 × 10¹² cm⁻² and the sheet resistance was high (>10 kΩ). The low mobility and low carrier density were attributed to defects in the amorphous gate dielectric, such as Al₂O₃, used in the FETs. Here, fixed and trapped charges in the gate dielectric and at the dielectric/diamond interface cause Coulomb scattering of carriers, which reduces mobility.³⁰ Furthermore, high gate leak current,^{17,19,23–25} presumably caused by trap-assisted tunneling,³¹ limits the applicable gate electric field and attainable carrier density.

Single-crystalline hexagonal boron nitride (h-BN) is promising as an alternative gate dielectric. Single-crystalline h-BN has few charged impurities, and a flat surface with no dangling bonds can easily be obtained by cleaving it. Indeed, the carrier mobility of graphene on an h-BN substrate has been shown to be much higher than that of graphene on a SiO₂ substrate.³² As it is a flat and charge-free substrate, h-BN is considered indispensable for the study of graphene physics.^33–37 Furthermore, the breakdown electric field of h-BN is as high as 12 MV cm⁻¹ for the direction perpendicular to the layers.³⁸ These properties make h-BN suitable for use as a gate dielectric in FETs with very high mobility and carrier density.

In this study, we fabricated diamond FETs with a monocrystalline h-BN as a gate dielectric. The device structure is shown in Fig. 1(a). The diamond surface is (111)-oriented and hydrogen-terminated [Fig. 1(b)]. The hydrogen termination reduces the density of surface states and also makes the hole accumulation more favorable because of the resulting upward shift of the energy bands.^39,40 A thin monocrystalline h-BN flake [Fig. 1(c)] is laminated on the diamond surface. To fabricate this h-BN/diamond structure, we used the Scotch tape exfoliation technique developed for van der Waals heterostructures of graphene and other two-dimensional materials^32,35 (see “Device fabrication” and Fig. S1 of the supplementary material for details of the device fabrication). The longest edge of the h-BN flake was aligned to the $[1¯10]$ direction of diamond. Ohmic contacts for the source and drain electrodes were produced by deposition of Ti/Pt and subsequent annealing to form TiC. A Hall bar structure was used for the four-probe and Hall measurements, from which the carrier density and mobility were evaluated. An optical microscope image of a fabricated device is shown in Fig. 1(d).

The quality of the interface between h-BN and diamond was examined by transmission electron microscopy (TEM) [Fig. 1(f)]. The left end of the TEM image shows that the distance between the first h-BN layer, and the diamond surface is approximately the same as the spacing between the layers in the h-BN crystal. This indicates that the interface was clean even at an atomic level. The image was taken by adjusting the $[1¯10]$ crystallographic direction of diamond to the electron beam, but the $[1¯1¯20]$ direction of h-BN was also found to be closely aligned, as indicated by the appearance of spots in the h-BN part of the TEM image and the lateral distance between spots in the Fourier transform images [Fig. 1(g)]. This demonstrates that excellent crystal alignment is possible with the lamination technique. The Fourier transform images for the top and bottom halves of Fig. 1(f) show that the difference in lattice spacing between $[11¯00]$ of h-BN and $[1¯1¯2]$ of diamond is 0.61% ± 0.06% (Fig. S2 of the supplementary material). This is in good agreement with the difference (0.71%) in lattice spacing reported in the literature^41,42 [Fig. 1(e)]. Note that the lattice mismatch is even smaller than the one (1.8%) between h-BN and graphene.³³ 

The electrical measurements showed that our diamond FETs with h-BN gate dielectric had excellent transport characteristics that were superior to those reported so far. We measured three devices. The transfer characteristics of device D1 at 300 K are shown in Fig. 2(a). The sheet conductance σ increases approximately linearly with the negative gate voltage. The maximum sheet conductance is 3.8 × 10⁻⁴ Ω⁻¹, which corresponds to a sheet resistance ρ of 2.6 kΩ. This sheet resistance is smaller than most of those reported for diamond surfaces, except for NO₂-exposed and V₂O₅-deposited surfaces on which carriers with a very high density (≈10¹⁴ cm⁻²) were generated.^4,43 The output characteristics of the h-BN/diamond FET at 300 K are shown in Fig. 2(b). They show a p-type behavior and current saturation. The maximum drain current density is as high as 33 mA mm⁻¹, despite that the gate length L_G, which is the distance between the source and drain in our FETs, is relatively large (=21.0 μm). The maximum drain current density is comparable to those reported for diamond FETs with a much shorter L_G (=4 μm).^17,24

The low on-resistance [Fig. 2(a)] and high maximum current density [Fig. 2(b)] result from the high carrier mobility and high carrier density of our device. Figures 2(c) and 2(d) show the gate voltage dependence of the sheet carrier density and carrier mobility obtained from the Hall-effect measurements. The carrier density increases linearly with the gate voltage, as expected. The carrier density at V_GS = −4 V is 7 × 10¹² cm⁻². The mobility is ≈320 cm² V⁻¹ s⁻¹ for V_GS≤ − 2 V, and its dependence on the gate voltage is weak. For an independent and complementary evaluation of the mobility, we also calculated the effective mobility using the equation, $μeff=th−BNϵh−BN∂σ∂VGS$⁠, where ϵ_h−BN is the dielectric constant and t_h−BN is the thickness of h-BN. Here, we used ϵ_h−BN = 3ϵ₀⁴⁴ and t_h−BN = 7 nm, the latter of which was obtained from an atomic force microscopy (AFM) measurement. The effective mobility was evaluated to be ≈300 cm² V⁻¹ s⁻¹ at V_GS = −4 V. This is in good agreement with the Hall mobility within the uncertainty of the gate capacitance. The relation between the Hall mobility and carrier density for the three FETs D1-3 is shown in Fig. 3. The figure also shows data for diamond FETs taken from the literature^{12–22,24–28} and for the surface conductivity of hydrogen-terminated diamond exposed to air^2,15,45,46 and NO₂.⁴ This figure shows that high mobility (>300 cm² V⁻¹ s⁻¹) is realized for moderately high carrier densities (>5 × 10¹² cm⁻²) in our FETs. Consequently, the on-sheet resistance of our FETs is exceptionally low (<3 kΩ).

The high mobility and high carrier density of our device are ascribed to a low density of defects in monocrystalline h-BN. This is evidenced by the gate voltage dependence of the leak current [Fig. 2(e)]. The leakage current remains below the detection limit up to a high gate electric field V_GS/t_h−BN = 5.7 MV cm⁻¹. The electric field at which the leakage current density exceeds 10⁻⁵ A cm⁻² is V_GS/t_h−BN = 6.6 MV cm⁻¹ and is significantly higher than those reported so far for diamond FETs and MOS capacitors with gate dielectrics, Al₂O₃ (0.9 MV cm⁻¹),³¹ HfO₂ (1.8 MV cm⁻¹),¹⁷ MoO₃ (1 MV cm⁻¹),²⁴ and Y₂O₃ (0.7 MV cm⁻¹).²⁵ The gate dielectrics in these previous studies were amorphous (or polycrystalline at best), and hence, their trap density would have been relatively large. Indeed, trap-assisted tunneling was considered a major cause of the leakage.³¹ By contrast, the leakage current in Fig. 2(e) is attributed to hole tunneling through a triangular barrier produced in h-BN in the presence of an electric field (see the supplementary material for a detailed analysis). This indicates a very low density of charge traps in h-BN. This feature leads to both high carrier mobility and a high applicable gate electric field.

The temperature dependence of the channel resistance [Fig. 4(a)] also indicates the high quality of our FETs. At low gate voltage (V_GS = −1 and −2 V), the resistance increases with decreasing temperature, indicating the insulating phase. However, at V_GS = −3 and −4 V, the resistance exhibits almost no temperature dependence. (The resistance at V_GS = −4 V even shows a slight decrease with decreasing temperature at low temperature.) This indicates that the system at V_GS = −3 and −4 V is in the metallic phase. The sign of the temperature derivative of the mobility at low temperature also changes in going between V_GS = −2 and −3 V [Fig. 4(b)]. These results suggest an insulator-metal transition between V_GS = −2 and −3 V at a carrier density of ≈4 × 10¹² cm⁻² [Figs. 4(a)–4(c)]. So far, an electric field-induced insulator-metal transition in diamond has been observed at much higher carrier density n ≈ 3 × 10¹³ cm⁻²; this required the use of the ionic liquid gating technique.^47,48 Although the origin of insulator-metal transitions in two dimensions has been the subject of debate, the strength of disorder in the system has been shown to play a decisive role in determining the critical carrier density.⁴⁹ The one-order-of-magnitude smaller carrier density for the transition suggests a significantly smaller degree of disorder in our h-BN/diamond FETs. Note that Shubnikov-de Haas oscillations were also observed in the h-BN/diamond FETs at low temperature, which will be reported elsewhere.

Now let us examine what limits the mobility of the FETs in the present and previous studies. The data of the previous studies in Fig. 3 show a general trend that the mobility decreases with increasing carrier density. The reason for this trend is clear for air and NO₂-exposed hydrogen-terminated surfaces. In this case, the charges of the accumulated carriers are balanced by the negative charges adsorbed on the diamond surface.³⁹ Therefore, a higher carrier density means a higher density of adsorbed negative charges, which leads to carrier scattering and decreases mobility. Here, adsorption of negative charges is assisted by the electrically polar surface with C⁻–H⁺ dipoles.^39,40 The origin of the similar tendency of mobility for diamond FETs in the previous studies is less clear because carriers can be introduced with the application of a gate voltage and the density of the carriers is independent of the density of the fixed charges at the diamond surface. The mobilities of these FETs are comparable to or smaller than that of the air-exposed surface. This suggests that fixed or trapped charges in the gate dielectrics cause additional carrier scattering.

The weak dependence of the mobility on the gate voltage in our FETs suggests that the mobility is limited by charged impurities not in the bulk gate dielectric but near the surface of the diamond;⁵⁰ that is, increasing the gate voltage increases the carrier density and should enhance the screening of the impurity potential, but this effect is compensated for by the approach of the average position of the carriers to the impurities near the surface. In our study, the transfer of the h-BN film onto the diamond surface was performed in air although we kept the air exposure time as short as possible. Therefore, it is likely that the diamond surface was covered with adsorbates before the h-BN lamination. Such adsorbates include negative charges that decrease mobility. In the case of a graphene/h-BN heterostructure, the adsorbates on each surface form isolated bubbles after lamination and make the rest of the interface atomically clean;⁵¹ this was evidenced by observation of novel quantum phenomena.^33–37 Bubbles were also seen after the lamination of h-BN on diamond in the present study (Fig. S3 of the supplementary material). Therefore, a similar self-cleansing may occur particularly when the adsorbates consist of hydrocarbons. Since the hydrogen-terminated surface of diamond is hydrophobic, this will be advantageous for self-cleansing.⁵² Nevertheless, some of the charged impurities may not form bubbles and remain to be distributed over the surface because of their repulsive interaction. These remaining charged impurities should limit the mobility of our FETs. The influence of remaining adsorbates is suggested by the fact that the mobility (≤170 cm² V⁻¹ s⁻¹) of a test device in which the diamond surface was exposed to air for a long time (≈6 h) before the h-BN lamination was smaller than those of devices D1-3. We expect that reducing the charged adsorbates will further improve the carrier mobility. This would also lower the conductance at V_GS = 0 and may lead to a normally off property, which is suitable for low-loss applications.

In summary, we demonstrated the excellent characteristics of diamond FETs with monocrystalline h-BN integrated as a gate dielectric. TEM images confirmed that a high-quality interface between h-BN and hydrogen-terminated diamond formed. Electrical measurements showed that high mobility (>300 cm² V⁻¹ s⁻¹) could be realized for moderately high carrier densities (>5 × 10¹² cm⁻²). An insulator-metal transition occurred at a carrier density as low as ≈4 × 10¹² cm⁻², also indicating a high-quality two-dimensional hole gas at the diamond surface. In view of the device design, the use of h-BN as a gate dielectric will be suitable for high-power and high-temperature operation because of its high thermal conductivity⁵³ and thermal durability.⁵⁴ We envision that h-BN will be heteroepitaxially grown on diamond for scalable device fabrication. For this purpose, the small lattice mismatch (≈0.7%) between h-BN and the hydrogen-terminated (111) diamond surface will be advantageous. Thus, the heterostructure of monocrystalline h-BN and diamond paves the way for fabrication of high-performance electronic devices that fully utilize the outstanding properties of diamond.

See supplementary material for experimental details and Figs. S1-S3.

We thank H. Osato, E. Watanabe, D. Tsuya, Y. Nishimiya, and F. Uesugi for their technical support. We also thank T. Ando, S. Koizumi, Y. Wakayama, and T. Nakayama for their kind support. This study was supported by Grants-in-Aid for Scientific Research (Grant Nos. 25287093, 26630139, 15H03980, 26220903, and 16H06326) and the “Nanotechnology Platform Project” of MEXT, Japan, and CREST (Grant No. JPMJCR1773) of JST, Japan.