An Overview of High-k Oxides on Hydrogenated-Diamond for Metal-Oxide-Semiconductor Capacitors and Field-Effect Transistors

Thanks to its excellent intrinsic properties, diamond is promising for applications of high-power electronic devices, ultraviolet detectors, biosensors, high-temperature tolerant gas sensors, etc. Here, an overview of high-k oxides on hydrogenated-diamond (H-diamond) for metal-oxide-semiconductor (MOS) capacitors and MOS field-effect transistors (MOSFETs) is demonstrated. Fabrication routines for the H-diamond MOS capacitors and MOSFETs, band configurations of oxide/H-diamond heterointerfaces, and electrical properties of the MOS and MOSFETs are summarized and discussed. High-k oxide insulators are deposited using atomic layer deposition (ALD) and sputtering deposition (SD) techniques. Electrical properties of the H-diamond MOS capacitors with high-k oxides of ALD-Al2O3, ALD-HfO2, ALD-HfO2/ALD-Al2O3 multilayer, SD-HfO2/ALD-HfO2 bilayer, SD-TiO2/ALD-Al2O3 bilayer, and ALD-TiO2/ALD-Al2O3 bilayer are discussed. Analyses for capacitance-voltage characteristics of them show that there are low fixed and trapped charge densities for the ALD-Al2O3/H-diamond and SD-HfO2/ALD-HfO2/H-diamond MOS capacitors. The k value of 27.2 for the ALD-TiO2/ALD-Al2O3 bilayer is larger than those of the other oxide insulators. Drain-source current versus voltage curves show distinct pitch-off and p-type channel characteristics for the ALD-Al2O3/H-diamond, SD-HfO2/ALD-HfO2/H-diamond, and ALD-TiO2/ALD-Al2O3/H-diamond MOSFETs. Understanding of fabrication routines and electrical properties for the high-k oxide/H-diamond MOS electronic devices is meaningful for the fabrication of high-performance H-diamond MOS capacitor and MOSFET gas sensors.


Introduction
Due to limitation of its bandgap energy, thermal conductivity, and electron saturation velocity, Si-based electronic devices cannot meet future demands in fields of high-power, high-temperature, high-frequency, and low power loss. Wide bandgap semiconductors such as SiC, GaN, and diamond are developed to partly replace Si for next-generation power electronic devices [1][2][3]. Table 1 summarizes basic physical properties of Si, 4H-SiC, GaN, and diamond [4][5][6]. Comparing with other semiconductors in this table, diamond has the widest bandgap energy, the highest breakdown field, the largest thermal conductivity, and the largest carrier mobility. Therefore, diamond-based electronic devices are promising for the future applications in fields of high-power handling, high-temperature operation, and high-frequency switching. Meanwhile, since diamond has good chemical inertness, good biocompatibility, and a large electrochemical window, it is also a suitable candidate for Table 1. Material properties of Si, 4H-SiC, GaN, and diamond at room temperature [4][5][6].

Properties
Si

4H-SiC GaN Diamond
Bandgap energy (eV) 1. Although semiconductor diamond-based devices have many potential applications, lack of shallow dopants hinders the development of them. Activation energies of boron and phosphorus doped p-type and n-type diamond are as large as 370 and 570 meV at room temperature (RT), respectively. They are much higher than the RT thermal energy of around 26 meV. Although thin heavily boron-doped diamond channel layer is promising to resolve this issue [15,16], its hole mobility is not high and growth technique still needs to be improved. A p-type hydrogenated diamond (H-diamond) channel layer is considered as another candidate to resolve this issue [17,18]. Two-dimensional hole gases are accumulated on the surface of the H-diamond caused by transfer of electrons from H-diamond to negatively surface adsorbate layer [19]. Its sheet hole density is around 10 13 cm −2 . After exposing H-diamond in NO 2 ambient or annealing treatment in NH 3 + H 2 ambient, its hole density can be enhanced to be as high as~10 14 cm −2 [20,21].
Si-, GaAs-and SiC-based metal-oxide-semiconductor (MOS) capacitor and MOS field-effect transistor (MOSFET) gas sensors have been developed greatly [22][23][24][25][26]. Gate metals (such as Pd, Pt, and Ir) have catalytic properties. They can adsorb hydrogen, ammonia, and carbon monoxide gases, leading to the generation of charges at metal/oxide interfaces, which make capacitance-voltage (C-V) curve shift in the depletion region for MOS capacitors and threshold voltage (V TH ) shift for the MOSFETs [27]. Although diamond has superior properties over other semiconductors, there are rarely reports for the diamond-based MOS capacitor and MOSFET gas sensors. In order to fabricate them successfully, it is important to know the fabrication routines and electrical properties of diamond-based MOS capacitors and MOSFETs.
Recently, fabrication techniques for the H-diamond-based MOS capacitors and MOSFETs have been developed greatly. The electrical properties of them have also been improved. The H-diamond MOS capacitors with low leakage current and trapped charge densities were fabricated [28][29][30][31][32][33][34]. By improving the device structures, T-type and triple-gate fin-type H-diamond MOSFETs were fabricated successfully with current outputs more than 200 mA·mm −1 [32][33][34]. The NO 2 -treated H-diamond channel layer based MOSFETs could operate well with a current output as high as 1.35 A·mm −1 [35]. Improvement of deposition conditions for the oxide insulators enhanced H-diamond MOSFET's operation temperature and breakdown voltage to be more than 400 • C and 1000 V, respectively [36]. Additionally, enhancement-mode H-diamond MOSFETs were developed for low power consumption [37][38][39].
Previously, we have focused on fabrication of high-k oxide/H-diamond MOS electronic devices [28][29][30][31][32][33]. Figure 1 shows polarization charge and sheet hole density in the H-diamond as functions of electric field [40,41], which is applied to gate oxide insulators of SiO 2 , Al 2 O 3 , HfO 2 [42], and TiO 2 [43]. At the same electric field, the oxide insulator with a higher-k value can response a larger sheet hole density for the MOS electronic devices. A gate oxide insulator with k value around 100 based on TiO 2 is essential to attain hole densities around 10 14 cm −2 for the H-diamond channel layer. Here, we summarize our previous reports for

Materials and Methods
Figure 2a,b show fabrication routines for the H-diamond MOS capacitor and MOSFET, respectively. Ib-type single crystalline diamond (001) substrate is boiled using a hotplate in a H2SO4 and HNO3 mixture solution at 300 °C for 3 h to clean surface contaminations (Figure 2ai,bI). The Hdiamond channel layer is epitaxially grown on the cleaned substrate using microwave plasmaenhanced chemical vapor deposition technique (Figure 2aii,bII). Growth gases are H2 and CH4 with flow rates of 500 and 0.5 sccm, respectively. Chamber pressure is fixed at 80 Torr. Microwave power and growth temperature are in the range of 880~960 W and 900~940 °C, respectively. Thicknesses of the H-diamond epitaxial layers are in the range of 150~200 nm. Surface roughness confirmed by atomic force microscope is around 1.2 nm [45]. The sheet hole density and mobility investigated by Hall measurement are around 10 13 cm −2 and 90 cm 2 ·V −1 ·s −1 , respectively. After growing the H-diamond channel layer, mesa-structure is formed for the H-diamond MOSFETs (Figure 2bIII) using capacitive coupled plasma dry-etching system. The chamber pressure and etching time are 10 Pa and 90 s, respectively. Etching gas (O2) flow rate and plasma power are 100 sccm and 50 W, respectively. It should be noted that there is no mesa-structure formation step for the fabrication of oxide/H-diamond MOS capacitor. High-k oxides of the ALD-Al2O3, ALD-HfO2, ALD-HfO2/ALD-Al2O3 multilayer, SD-HfO2/ALD-HfO2 bilayer, SD-TiO2/ALD-Al2O3 bilayer, and ALD-TiO2/ALD-Al2O3 bilayer are deposited as high-k oxides for the MOS capacitors and MOSFETs (Figure 2aiii,bIV). For the SD-HfO2/ALD-HfO2 and SD-TiO2/ALD-Al2O3 bilayers, the ALD-HfO2 and ALD-Al2O3 impact as buffer layers with thicknesses of around 4.0 nm to protect the hydrogen surface from being damaged by SD plasma discharge during the SD-HfO2 and SD-TiO2 depositions, respectively. For the ALD-TiO2/ALD-Al2O3 bilayer, the ALD-Al2O3 with thicknesses of 0~4.0 nm impacts as a buffer layer to suppress high leakage current density (J) of ALD-TiO2/H-diamond MOS capacitor due to a low valence band offset (ΔEV) between TiO2 and H-diamond [42]. The ALD-Al2O3, ALD-HfO2, and ALD-TiO2 are deposited using precursors of trimethylaluminum, tetrakis (ethylmethylamino) hafnium, and tetrakis(dimethylamino)titanium with water vapor, respectively. Although the high deposition temperature for the ALD-oxides can increase their properties possibly, we have deposited them at 120 °C. There are two reasons for the low deposition temperature. Since the hydrogen surface of the H-diamond is thermal sensitivity, the low deposition temperature can  Ib-type single crystalline diamond (001) substrate is boiled using a hotplate in a H 2 SO 4 and HNO 3 mixture solution at 300 • C for 3 h to clean surface contaminations (Figure 2ai,bI). The H-diamond channel layer is epitaxially grown on the cleaned substrate using microwave plasma-enhanced chemical vapor deposition technique (Figure 2aii,bII). Growth gases are H 2 and CH 4 with flow rates of 500 and 0.5 sccm, respectively. Chamber pressure is fixed at 80 Torr. Microwave power and growth temperature are in the range of 880~960 W and 900~940 • C, respectively. Thicknesses of the H-diamond epitaxial layers are in the range of 150~200 nm. Surface roughness confirmed by atomic force microscope is around 1.2 nm [45]. The sheet hole density and mobility investigated by Hall measurement are around 10 13 cm −2 and 90 cm 2 ·V −1 ·s −1 , respectively. After growing the H-diamond channel layer, mesa-structure is formed for the H-diamond MOSFETs (Figure 2bIII) using capacitive coupled plasma dry-etching system. The chamber pressure and etching time are 10 Pa and 90 s, respectively. Etching gas (O 2 ) flow rate and plasma power are 100 sccm and 50 W, respectively. It should be noted that there is no mesa-structure formation step for the fabrication of oxide/H-diamond MOS capacitor. High-k oxides of the ALD-Al 2 O 3 , ALD-HfO 2 , ALD-HfO 2 /ALD-Al 2 O 3 multilayer, SD-HfO 2 /ALD-HfO 2 bilayer, SD-TiO 2 /ALD-Al 2 O 3 bilayer, and ALD-TiO 2 /ALD-Al 2 O 3 bilayer are deposited as high-k oxides for the MOS capacitors and MOSFETs (Figure 2aiii,bIV). For the SD-HfO 2 /ALD-HfO 2 and SD-TiO 2 /ALD-Al 2 O 3 bilayers, the ALD-HfO 2 and ALD-Al 2 O 3 impact as buffer layers with thicknesses of around 4.0 nm to protect the hydrogen surface from being damaged by SD plasma discharge during the SD-HfO 2 and SD-TiO 2 depositions, respectively. For the ALD-TiO 2 /ALD-Al 2 O 3 bilayer, the ALD-Al 2 O 3 with thicknesses of 0~4.0 nm impacts as a buffer layer to suppress high leakage current density (J) of ALD-TiO 2 /H-diamond MOS capacitor due to a low valence band offset (∆E V ) between TiO 2 and H-diamond [42]. The ALD-Al 2 O 3 , ALD-HfO 2 , and ALD-TiO 2 are deposited using precursors of trimethylaluminum, tetrakis (ethylmethylamino) hafnium, and tetrakis(dimethylamino)titanium with water vapor, respectively. Although the high deposition temperature for the ALD-oxides can increase their properties possibly, we have deposited them at 120 • C. There are two reasons for the low deposition temperature. Since the hydrogen surface of the H-diamond is thermal sensitivity, the low deposition temperature can protect the surface from being damaged by the heating at high temperature. Additionally, since deposition areas of oxide insulators are patterned using LOR 5A/AZ 5214E bilayer photoresists and the temperature limitation of them is 150 • C, we set the temperature of 120 • C to deposit the ALD-oxides. The SD-HfO 2 layer is deposited on the ALD-HfO 2 /H-diamond in a pure Ar ambient at RT. The radio-frequency (RF) power, gas flow rate, and chamber pressure are 30 W, 2.0 sccm, and 1 Pa, respectively. The SD-TiO 2 layer is deposited on the ALD-Al 2 O 3 /H-diamond in an Ar+O 2 ambient at RT. Oxygen content in the SD chamber is in the range of 0~20%. The RF power, total gas flow rate, and chamber pressure are 40 W, 2.0 sccm, and 1 Pa, respectively. Total thickness of oxide insulators for each MOS capacitor is in the range of 18.9~34.1 nm. After depositing the oxide insulators, gate cover metals of Au/Ti or Au/Ti/Pd are formed using evaporator system. Lastly, Au/Ti/Pd triple-layer metals are evaporated on the H-diamond for the source/drain ohmic contacts (Figure 2aiv,bV). protect the surface from being damaged by the heating at high temperature. Additionally, since deposition areas of oxide insulators are patterned using LOR 5A/AZ 5214E bilayer photoresists and the temperature limitation of them is 150 °C, we set the temperature of 120 °C to deposit the ALDoxides. The SD-HfO2 layer is deposited on the ALD-HfO2/H-diamond in a pure Ar ambient at RT. The radio-frequency (RF) power, gas flow rate, and chamber pressure are 30 W, 2.0 sccm, and 1 Pa, respectively. The SD-TiO2 layer is deposited on the ALD-Al2O3/H-diamond in an Ar+O2 ambient at RT. Oxygen content in the SD chamber is in the range of 0~20%. The RF power, total gas flow rate, and chamber pressure are 40 W, 2.0 sccm, and 1 Pa, respectively. Total thickness of oxide insulators for each MOS capacitor is in the range of 18.9~34.1 nm. After depositing the oxide insulators, gate cover metals of Au/Ti or Au/Ti/Pd are formed using evaporator system. Lastly, Au/Ti/Pd triple-layer metals are evaporated on the H-diamond for the source/drain ohmic contacts [ Figure 2aiv,bV]. Band configurations of oxide/H-diamond heterointerfaces are determined using X-ray photoelectron spectroscopy (XPS) technique, which is performed with a monochromated Al Kα X-ray source (hv = 1486.6 eV). All core level spectra are recorded with a 0.05 eV step and a 55 eV pass energy. Electrical properties of the MOS capacitors and MOSFETs are measured under a dark condition using MX-200/B prober and B1500A parameter analyzer at RT.

Band Configurations of High-k Oxide/H-Diamond Heterointerfaces
Band configurations of heterointerfaces are considered as the most fundamental properties in material physics. Understanding of them for high-k oxide/H-diamond heterojunctions is very important for the development of high-performance H-diamond-based MOS electronic devices. Band configurations of ALD-Al2O3/H-diamond [46], ALD-HfO2/H-diamond [46], and ALD-TiO2/ALD-Al2O3/H-diamond [31]   Band configurations of oxide/H-diamond heterointerfaces are determined using X-ray photoelectron spectroscopy (XPS) technique, which is performed with a monochromated Al Kα X-ray source (hv = 1486.6 eV). All core level spectra are recorded with a 0.05 eV step and a 55 eV pass energy. Electrical properties of the MOS capacitors and MOSFETs are measured under a dark condition using MX-200/B prober and B1500A parameter analyzer at RT.

Band Configurations of High-k Oxide/H-Diamond Heterointerfaces
Band configurations of heterointerfaces are considered as the most fundamental properties in material physics. Understanding of them for high-k oxide/H-diamond heterojunctions is very important for the development of high-performance H-diamond-based MOS electronic devices. Band configurations of ALD-Al 2 O 3 /H-diamond [46], ALD-HfO 2 /H-diamond [46], and ALD-TiO 2 /ALD-Al 2 O 3 /H-diamond [31] are investigated. The ∆E V values of ALD-Al 2 O 3 /H-diamond and ALD-HfO 2 /H-diamond heterojunctions are calculated using the equation below, is the difference in binding energies between C 1s and Al 2p 3/2 for the ALD-Al 2 O 3 (4 nm)/H-diamond sample or between C 1s and Hf 4f 7/2 for the ALD-HfO 2 (4 nm)/H-diamond sample. The ∆E V for the ALD-TiO 2 /ALD-Al 2 O 3 heterojunction can be calculated using the equation below, is the difference in binding energies between Ti 2p 3/2 and VBM for the is the difference in binding energies between Al 2p 3/2 and VBM for the 20 nm thick Al 2 O 3 film [46]. The  ] is fitted with three components of C-C, CH x , and C-OH [47]. Binding energy difference between C-C and CH x is 0.6 eV. That between C-C and C-OH is 1.2 eV [48]. According to angle-resolved XPS measurement result [49,50], the CH x and C-OH are attributed to surface contaminations. The Al-O ( Figure 3c) and Hf-O [ Figure 3e] are used to fit the Al 2p and Hf 4f spectra, respectively. The VBM values for the H-diamond, Al 2 O 3 (20 nm), HfO 2 (20 nm), and ALD-TiO 2 (25 nm)/ALD-Al 2 O 3 samples are determined to be 1.2 ± 0.2, 5.4 ± 0.2, 4.3 ± 0.2 eV, and 3.4 ± 0.2 eV, respectively. Figure 4a,b show C 1s and Al 2p spectra for the Al 2 O 3 (4 nm)/H-diamond sample, respectively. Figure 4c,d show C 1s and Hf 4f for the HfO 2 (4 nm)/H-diamond sample, respectively. Figure 4e,f show Al 2p and Ti 2p spectra for TiO 2 (3 nm)/Al 2 O 3 (4 nm)/H-diamond sample, respectively. Three components of C-C, CH x , and C-OH are used to fit each C 1s spectrum, which is similar to those of the H-diamond substrate. The Al-O, Hf-O, and Ti-O are used to fit the Al 2p, Hf 4f, and Ti 2p spectra, respectively. Table 2 summarizes the CL peak energies and VBM values corresponding to the spectra in Figures 3 and 4. The peak energies for the C 1s are relative to the C-C bonds. Binding energy error for each peak is ±0.2 V. By inserting CL binding energies and the VBM values into the Equations (1) and (2), the ∆E V values for the ALD-Al 2 O 3 /H-diamond, ALD-HfO 2 /H-diamond, and ALD-TiO 2 /ALD-Al 2 O 3 heterojunctions are calculated to be 2.9 ± 0.2, 2.6 ± 0.2, and −0.6 ± 0.2 eV, respectively. Based on the bandgap energies of ALD-Al 2 O 3 (7.2 eV) [46], ALD-HfO 2 (5.4 eV) [46], and ALD-TiO 2 (3.4 eV) [51], conduction band offset (∆E C ) values for them can be deduced to be 1.2 ± 0.2, 2.7 ± 0.2, and 3.2 ± 0.2 eV, respectively. thick oxide insulators are determined by extrapolating linear fitting to leading edges of the VB spectra to intersect with the baselines. The C 1s spectrum of H-diamond [ Figure 3a] is fitted with three components of C-C, CHx, and C-OH [47]. Binding energy difference between C-C and CHx is 0.6 eV. That between C-C and C-OH is 1.2 eV [48]. According to angle-resolved XPS measurement result [49,50], the CHx and C-OH are attributed to surface contaminations. The Al-O ( Figure 3c) and Hf-O [ Figure 3e] are used to fit the Al 2p and Hf 4f spectra, respectively. The VBM values for the H-diamond, Al2O3 (20 nm), HfO2 (20 nm), and ALD-TiO2 (25 nm)/ALD-Al2O3 samples are determined to be 1.2 ± 0.2, 5.4 ± 0.2, 4.3 ± 0.2 eV, and 3.4 ± 0.2 eV, respectively.  Figure 3b,d,f,h, respectively (Reprinted from references [31,46]).    Table 2 summarizes the CL peak energies and VBM values corresponding to the spectra in Figures 3 and 4. The peak energies for the C 1s are relative to the C-C bonds. Binding energy error for each peak is ±0.2 V. By inserting CL binding energies and the VBM values into the Equations (1) and (2), the ΔEV values for the ALD-Al2O3/H-diamond, ALD-HfO2/H-diamond, and ALD-TiO2/ALD-Al2O3 heterojunctions are calculated to be 2.9 ± 0.2, 2.6 ± 0.2, and −0.6 ± 0.2 eV, respectively. Based on the bandgap energies of ALD-Al2O3 (7.2 eV) [46], ALD-HfO2 (5.4 eV) [46], and ALD-TiO2 (3.4 eV) [51], conduction band offset (ΔEC) values for them can be deduced to be 1.2 ± 0.2, 2.7 ± 0.2, and 3.2 ± 0.2 eV, respectively.

ALD-Al 2 O 3 and ALD-HfO 2 Single Layers
Since Al 2 O 3 and HfO 2 are two common high-k oxide insulators, electrical properties of ALD-Al 2 O 3 /H-diamond and ALD-HfO 2 /H-diamond MOS capacitors have been investigated firstly [28,29]. Figure 6a,b show J-V and C-V characteristics for ALD-Al 2 O 3 /H-diamond MOS capacitor, respectively. The J is deduced using leakage current divided by the gate electrode area. It is lower than  Figure 6b represent the C-V curves with the gate voltage sweeping directions from negative to positive and from positive to negative, respectively. Distinct accumulation region and sharp dependence at depletion region are observed in the C-V curves. Interfacial trapped charge density for the ALD-Al 2 O 3 /H-diamond is thus quite low [52]. There is a quite low voltage shift relative to 0 V in the depletion region for the C-V curves, indicating the low fixed charge density in the ALD-Al 2 O 3 [53]. Hysteresis loop voltage for the C-V curves with the change of sweeping directions is 0 V, which suggests that there is low trapped charge density in the ALD-Al 2 O 3 single layer. High quality Al 2 O 3 film is thus deposited by the ALD technique at 120 • C. Maximum capacitance (C max ) for the MOS capacitor is 0.187 µF·cm −2 . By considering the ALD-Al 2 O 3 thickness of 25.4 nm, the k value of the single ALD-Al 2 O 3 layer can be calculated to be 5.4, which is much lower than that for the ideal Al 2 O 3 of 8.5~9. This is possibly attributed to the low deposition temperature (120 • C) for our ALD-Al 2 O 3 film. from negative to positive and from positive to negative, respectively. Distinct accumulation region and sharp dependence at depletion region are observed in the C-V curves. Interfacial trapped charge density for the ALD-Al2O3/H-diamond is thus quite low [52]. There is a quite low voltage shift relative to 0 V in the depletion region for the C-V curves, indicating the low fixed charge density in the ALD-Al2O3 [53]. Hysteresis loop voltage for the C-V curves with the change of sweeping directions is 0 V, which suggests that there is low trapped charge density in the ALD-Al2O3 single layer. High quality Al2O3 film is thus deposited by the ALD technique at 120 °C. Maximum capacitance (Cmax) for the MOS capacitor is 0.187 μF·cm −2 . By considering the ALD-Al2O3 thickness of 25.4 nm, the k value of the single ALD-Al2O3 layer can be calculated to be 5.4, which is much lower than that for the ideal Al2O3 of 8.5~9. This is possibly attributed to the low deposition temperature (120 °C) for our ALD-Al2O3 film.   [29]); (d,e) C-V characteristics of the ALD-HfO 2 /H-diamond MOS capacitors before and after annealing at 300 • C, respectively (Reprinted from reference [29]). Figure 6c shows annealing effect on the J-V characteristics of the ALD-HfO 2 /H-diamond MOS capacitors. For gate voltage higher than 0 V, the leakage current densities of the MOS capacitors before and after annealing at 300 • C are lower than 10 −8 A·cm −2 . For gate voltage more negative than 0 V, the J of MOS capacitor before annealing increases from 10 −8 A·cm −2 at −3.4 V to 10 −2 A·cm −2 at −5.0 V. However, the J after annealing at 300 • C is still lower than 1.6 × 10 −8 A·cm −2 . Therefore, after annealing at 300 • C, the qualities of the ALD-HfO 2 film and the ALD-HfO 2 /H-diamond interface are improved greatly. When the annealing temperature increases to 500 • C, the large J is observed at gate voltage from −5.0 to 4.0 V, which is probably attributed to the formation of polycrystalline HfO 2 film at this high temperature [54]. Figure 6d,e show C-V characteristics of ALD-HfO 2 /H-diamond MOS capacitors before and after annealing at 300 • C, respectively. Both C-V curves shift to the left hand sides greatly relative to 0 V, thus positive fixed charges with high densities exist in the bulk HfO 2 film or close to the HfO 2 /H-diamond interface [53]. Both C-V curves show hysteresis loops with voltages of 0.4 and 0.5 V, respectively, which implies that there are higher trapped charge density in the bulk HfO 2 film than that in the ALD-Al 2 O 3 film, which is possibly ascribed to the oxygen vacancies in the ALD-HfO 2 film. Noted that the C-V curves in the depletion regions for the ALD-HfO 2 /H-diamond MOS capacitor after annealing are sharper than those of before annealing and there is no residual capacitance from −1.2 V to −3.8 V in Figure 6e. Therefore, there is a lower interface trapped charge density for the ALD-HfO 2 /H-diamond interface after annealing than that before annealing [52]. The C max values for both MOS capacitors are 0.393 and 0.359 µF·cm −2 , respectively. The difference is possibly ascribed to a little variation of gate electrode area during the fabrication. Based on the C max values and the HfO 2 thickness (27.3 nm), the dielectric constants of the HfO 2 films before and after annealing are calculated to be 12.1 and 11.2, respectively, which are in good agreement with the reported values (11.7~14) of ALD-HfO 2 on Si and GaN substrates [54,55]. Low dielectric constants comparing with the ideal value of around 24 are believed to be the intrinsic property for the amorphous ALD-HfO 2 deposited at low temperature (120 • C).

ALD-HfO 2 /ALD-Al 2 O 3 Multilayer and SD-HfO 2 /ALD-HfO 2 Bilayer
Since k value of the ALD-Al 2 O 3 single layer on the H-diamond is not high and there are high positive fixed charge densities in the ALD-HfO 2 , we investigate electrical properties of ALD-HfO 2 /ALD-Al 2 O 3 multilayer and SD-HfO 2 /ALD-HfO 2 bilayer on the H-diamond for MOS capacitors [28,30]. Figure 7a,b show J-V and C-V characteristics for the H-diamond MOS capacitor with the ALD-Al 2 O 3 /ALD-HfO 2 multilayer as the oxide insulator, respectively. The ALD-Al 2 O 3 and ALD-HfO 2 are the first and top layers in contact with the H-diamond surface and gate cover metal, respectively. Each monolayer thickness for ALD-Al 2 O 3 and ALD-HfO 2 is 1.0 nm with total thickness for the multilayer of 32.0 nm. The J for the MOS capacitor in Figure 7a at −4.0 V is 2.7 × 10 −8 A·cm −2 , which is lower than those of ALD-Al 2 O 3 /H-diamond and as-fabricated ALD-HfO 2 /H-diamond MOS capacitors. The C-V curve of the MOS capacitor in Figure 7b shows decrease of stretch-out in the depletion region compared to that for the as-fabricated ALD-HfO 2 /H-diamond MOS capacitor. Therefore, the interfacial trapped charge densities for the MOS capacitor with the ALD-HfO 2 /ALD-Al 2 O 3 multilayer as the oxide insulator are lower than that for the as-fabricated ALD-HfO 2 /H-diamond MOS capacitor [53]. However, there is a large hysteresis loop voltage of around 1.0 V for the C-V curves in Figure 7b, indicating the high trapped charge density in the ALD-HfO 2 /ALD-Al 2 O 3 multilayer. The C max for the C-V curves of the MOS capacitor is 0.216 µF·cm −2 and the k value for the HfO 2 /Al 2 O 3 multilayer can be calculated to be 7.8, which is lower than that for the ALD-HfO 2 single layer of 12.1 and larger than that for the ALD-Al 2 O 3 single layer of 5.4.   1 nm), the k of the SD-HfO2/ALD-HfO2 bilayer is calculated to be 9.4, which is smaller than the value (12.1) of the ALD-HfO2 single layer. This is probably attributed to the low deposition temperature (at RT) for the SD-HfO2. The voltage shift relative to 0 V of the C-V curves for the SD-HfO2/ALD-HfO2/Hdiamond MOS capacitor is one order magnitude lower than that for the ALD-HfO2/H-diamond MOS capacitor. Thus, the issue of high fixed charge density for the ALD-HfO2/H-diamond MOS capacitor is resolved. Additionally, the C-V curve in the depletion region shows sharp dependence and small hysteresis voltage loop of 0.1 V for the SD-HfO2/ALD-HfO2/H-diamond MOS capacitor, indicating its low trapped charge densities in the SD-HfO2/ALD-HfO2 bilayer and at the SD-HfO2/ALD-HfO2/Hdiamond interfaces. Figure 8a shows J-V characteristics for the SD-TiO2/ALD-Al2O3/H-diamond MOS capacitors. Thicknesses of SD-TiO2 films are 24.4, 18.9, and 23.3 nm with the change of oxygen gas content in the SD chamber of 0%, 10%, and 20%, respectively. When the chamber gas during the SD-TiO2 deposition is only Ar (O2: 0%), the J of the received MOS capacitor is lower than 10 −7 A·cm −2 with the gate voltage changing from −2.0 to 4.0 V. As the gate voltage sweeps from −2.0 to −4.0 V, the J of the MOS capacitor increases from 10 −7 to as large as 10 −2 A·cm −2 . As the oxygen gas content in the SD chamber increases to 10% and 20%, there are very high leakage current densities for the MOS capacitors. Figure 8b shows the C-V characteristic of the SD-TiO2/ALD-Al2O3/H-diamond MOS capacitor with 0% O2 content in the SD chamber during the SD-TiO2 deposition. Depletion regions of the C-V curves locate at the left hand side relative to 0 V. Thus, positive charges exist at the ALD-Al2O3/H-diamond interface or in   1 nm), the k of the SD-HfO 2 /ALD-HfO 2 bilayer is calculated to be 9.4, which is smaller than the value (12.1) of the ALD-HfO 2 single layer. This is probably attributed to the low deposition temperature (at RT) for the SD-HfO 2 . The voltage shift relative to 0 V of the C-V curves for the SD-HfO 2 /ALD-HfO 2 /H-diamond MOS capacitor is one order magnitude lower than that for the ALD-HfO 2 /H-diamond MOS capacitor. Thus, the issue of high fixed charge density for the ALD-HfO 2 /H-diamond MOS capacitor is resolved. Additionally, the C-V curve in the depletion region shows sharp dependence and small hysteresis voltage loop of 0.1 V for the SD-HfO 2 /ALD-HfO 2 /H-diamond MOS capacitor, indicating its low trapped charge densities in the SD-HfO 2 /ALD-HfO 2 bilayer and at the SD-HfO 2 /ALD-HfO 2 /H-diamond interfaces. Figure 8a shows J-V characteristics for the SD-TiO 2 /ALD-Al 2 O 3 /H-diamond MOS capacitors. Thicknesses of SD-TiO 2 films are 24.4, 18.9, and 23.3 nm with the change of oxygen gas content in the SD chamber of 0%, 10%, and 20%, respectively. When the chamber gas during the SD-TiO 2 deposition is only Ar (O 2 : 0%), the J of the received MOS capacitor is lower than 10 −7 A·cm −2 with the gate voltage changing from −2.0 to 4.0 V. As the gate voltage sweeps from −2.0 to −4.0 V, the J of the MOS capacitor increases from 10 −7 to as large as 10 −2 A·cm −2 . As the oxygen gas content in the SD chamber increases to 10% and 20%, there are very high leakage current densities for the MOS capacitors. Figure 8b   For the MOS capacitor gas sensors, reactions between gases and catalytic metals make charges accumulation at the metal/oxide interfaces, leading to the shift of C-V curve in the deletion region. Therefore, the best oxide insulators for the MOS capacitor gas sensors are those having low fixed and trapped charge densities. Based on the electrical properties of MOS capacitors, the ALD-Al 2 O 3 and SD-HfO 2 /ALD-HfO 2 bilayer are possibly two good choices for the applications of the H-diamond MOS capacitor gas sensors. Table 3. Electrical properties of high-k oxide/H-diamond MOS capacitors.  [56,57]. Since the carbon-hydrogen bonds are very stable at the temperature lower than 250 • C, the enhancement-mode characteristics for the MOSFETs result from the disappearance of negatively adsorbed layer or the formation of positive charges at the SD-HfO 2 /ALD-HfO 2 /H-diamond and ALD-TiO 2 /ALD-Al 2 O 3 /H-diamond interfaces [58]. On/off ratios for all MOSFETs are higher than 10 8 . Subthreshold swings are 138, 195, and 79 mV·dec −1 for the ALD-Al 2 O 3 /H-diamond, SD-HfO 2 /ALD-HfO 2 /H-diamond, and ALD-TiO 2 /ALD-Al 2 O 3 /H-diamond MOSFETs, respectively [31,39]. diamond channel layer for the SD-HfO2/ALD-HfO2/H-diamond MOSFET [30], which is around 38.7 ± 0.5 cm 2 ·V −1 ·s −1 .

Conclusions
Here, fabrication routines for the H-diamond MOS capacitor and MOSFET, band configurations of high-k oxide/H-diamond heterointerfaces, and electrical properties of the H-diamond MOS capacitors and MOSFETs were summarized. There were high valence band offsets for ALD-Al2O3/Hdiamond, ALD-HfO2/H-diamond, and ALD-TiO2/ALD-Al2O3/H-diamond heterointerfaces. Electrical properties of the H-diamond MOS capacitors with the ALD-Al2O3, ALD-HfO2, ALD-HfO2/ALD-Al2O3 multilayer, SD-HfO2/ALD-HfO2 bilayer, SD-TiO2/ALD-Al2O3 bilayer, and ALD-TiO2/ALD-Al2O3 bilayer were investigated and discussed. Except for the SD-TiO2/ALD-Al2O3/H-diamond MOS capacitor, the leakage current densities at −4.0 V for the other MOS capacitors were lower than 6.0 × 10 −6 A·cm −2 . The k value of ALD-TiO2/ALD-Al2O3 bilayer was 27.2, which was higher than those of other oxide insulators. There were low fixed and trapped charge densities for the ALD-Al2O3/Hdiamond and SD-HfO2/ALD-HfO2/H-diamond MOS capacitors and good operations for the MOSFETs. These characteristics made them promising for the fabrication of high-performance Hdiamond MOS capacitor and MOSFET gas sensors. There is the following relationship between on-resistance (R ON ) and effective mobility (µ eff ) for the H-diamond channel layer of the MOSFETs with condition of R ON and source/drain-to-gate resistance (2R SD ) much higher than source/drain ohmic contact resistances.
where the R ON normalized by the W G was obtained from fitting the I DS -V DS curve. The C OX is the C max of the oxide insulator. The 2R SD value was determined based on the linear function between the L G and R ON . Since the 2R SD values for the ALD-Al 2 O 3 /H-diamond and ALD-TiO 2 /ALD-Al 2 O 3 /H-diamond MOSFETs were not investigated in our previous studies [31,39], we only calculate the µ eff of H-diamond channel layer for the SD-HfO 2 /ALD-HfO 2 /H-diamond MOSFET [30], which is around 38.7 ± 0.5 cm 2 ·V −1 ·s −1 .

Conclusions
Here, fabrication routines for the H-diamond MOS capacitor and MOSFET, band configurations of high-k oxide/H-diamond heterointerfaces, and electrical properties of the H-diamond MOS capacitors and MOSFETs were summarized. There were high valence band offsets for ALD-Al 2 O 3 /H-diamond, ALD-HfO 2 /H-diamond, and ALD-TiO 2 /ALD-Al 2 O 3 /H-diamond heterointerfaces. Electrical properties of the H-diamond MOS capacitors with the ALD-Al 2 O 3 , ALD-HfO 2 , ALD-HfO 2 /ALD-Al 2 O 3 multilayer, SD-HfO 2 /ALD-HfO 2 bilayer, SD-TiO 2 /ALD-Al 2 O 3 bilayer, and ALD-TiO 2 /ALD-Al 2 O 3 bilayer were investigated and discussed. Except for the SD-TiO 2 /ALD-Al 2 O 3 /H-diamond MOS capacitor, the leakage current densities at −4.0 V for the other MOS capacitors were lower than 6.0 × 10 −6 A·cm −2 . The k value of ALD-TiO 2 /ALD-Al 2 O 3 bilayer was 27.2, which was higher than those of other oxide insulators. There were low fixed and trapped charge densities for the ALD-Al 2 O 3 /H-diamond and SD-HfO 2 /ALD-HfO 2 /H-diamond MOS capacitors and good operations for the MOSFETs. These characteristics made them promising for the fabrication of high-performance H-diamond MOS capacitor and MOSFET gas sensors.