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[Jiangwei Liu](https://orcid.org/0000-0003-2580-7401), [Tokuyuki Teraji](https://orcid.org/0000-0002-7731-0547), [Bo Da](https://orcid.org/0000-0002-0785-8662), [Yasuo Koide](https://orcid.org/0000-0001-8321-9822)

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[Electrical Properties of Boron-Doped Diamond MOSFETs With Ozone as Oxygen Precursor for Al2O3 Deposition](https://mdr.nims.go.jp/datasets/ee53d67e-c59b-4bfc-afef-79e00e66df4b)

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LIU et al.: ELECTRICAL PROPERTIES OF BORON-DOPED DIMOAND MOSFETs 1 Abstract—Boron-doped diamond (B-diamond) metal-oxide-semiconductor (MOS) capacitors and MOS field-effect transistors (MOSFETs) are fabricated and characterized. The Al2O3 gate insulator is deposited by an atomic layer deposition technique with ozone as oxygen precursor. Leakage current density for the Al2O3 (ozone)/B-diamond MOS capacitor is 2.7 × 10–5 A/cm2 at –9.0 V. Comparing to the capacitance-voltage curve of the Al2O3 (water)/B-diamond MOS capacitor, there are no residual capacitance and improved negative flat band voltage shift for the Al2O3 (ozone)/B-diamond MOS capacitor. The Al2O3 (ozone)/B-diamond MOSFET operates well with the on/off ratio of around 108, which is much higher than that of the previous Al2O3 (water)/B-diamond MOSFET. After annealing at 500 ℃ for as long as 10 hours, the Al2O3 (ozone)/B-diamond MOSFET can still operate well with the on/off ratio larger than 106.   Index Terms—Diamond, Boron-doped, MOS capacitor, MOSFET. I. INTRODUCTION ver the last couple of years, wide bandgap semiconductors of SiC, GaN, Ga2O3, and diamond are developed to meet requirements of next-generation complementary metal-oxide-semiconductor (CMOS) electronic devices [1-4]. Among them, the diamond has the most excellent intrinsic properties. It has wider bandgap energy (5.47 eV), higher breakdown field (10 MV/cm), larger thermal conductivity (22 W/cm·k), and higher carrier mobilities (4500 cm2/V·s and 3800 cm2/V·s for electrons and holes, respectively) [5, 6]. Johnson, Keyes, Baliga, and Baliga high-frequency figure-of-merits of diamond are also much larger than those of other wide bandgap semiconductors [7, 8]. Diamond-based electronic devices are promising for future applications in the fields of high-power, high-frequency, high-temperature, and low power-loss. In order to push forward the development of diamond-based CMOS electronic devices, the diamond MOS capacitors [9, 10], MOS field-effect transistors (MOSFETs) [11-16], and MOSFET logic circuits [17, 18] were fabricated and improved. By far, most of them were fabricated on p-type  This work is supported by the Nanotechnology Platform Program and the KAKENHI Projects (Grant JP20H00313 and Grant JP20H02187), the Ministry of Education, Culture, Sports, and Technology, Japan. (Corresponding author: Jiangwei Liu) J. Liu, T. Teraji, and Y. Koide are with the Research Center for Functional Materials, National Institute for Materials Science (NIMS), Ibaraki 305-0044, Japan (e-mail: liu.jiangwei@nims.go.jp). B. Da is with the Research and Services Division of Materials Data and Integrated System, NIMS, Ibaraki 305-0047, Japan.   hydrogen-terminated diamond (H-diamond) channel layers. Two-dimensional hole gases are formed on the surface of the H-diamond due to the existence of carbon-hydrogen bonds and negatively charged acceptors [19].  Drain current (ID), extrinsic transconductance (gm), cut-off frequency, and breakdown voltage of the H-diamond-based MOSFETs were reported to be as high as 1.35 A/mm, 206 mS/mm, 70 GHz, and 3326 V, respectively [12-14]. Unfortunately, thermal stability of them is still a big issue and very difficult to be resolved [20]. Recently, novel carbon-silicon bonded diamond MOSFETs were fabricated successfully [21].  They have good thermal stability at the measurement temperature of 400 ℃. On the other hand, some researchers made efforts to fabricate n-type phosphorus-doped and p-type boron-doped diamond (B-diamond) MOSFETs [22-25]. Although their maximum ID and gm were quite lower than those of the H-diamond-based ones due to the high activation energies of dopants, they could operate stably at high annealing temperatures (>500 ℃) [25].  The Al2O3 films were employed as the gate insulators for the B-diamond MOSFETs thanks to its high critical breakdown field, high thermal stablility, and large band offsets with diamond [26, 27]. In the previous studies, they were deposited by an atomic layer deposition (ALD) technique with the water vapor as the precursor [23-25, 28]. Good operations for the Al2O3 (water)/B-diamond MOS capacitors and MOSFETs were confirmed. However, there were residual capacitance and large negative flat band voltage shift for the capacitance-voltage (C-V) curves of the MOS capacitors [23, 25]. Meanwhile, the maximum on/off ratio for the B-diamond MOSFETs was only around 5 × 105 [23, 29, 30]. These issues are possibly attributed to the surface defects on the B-diamond and poor interfacial quality for the Al2O3 (water)/B-diamond, which would hinder the devices for high-performance power switches and other CMOS digital applications. Since the ozone precursor has more oxidizing ability than the water vapor [31], it could possible improve oxygen surface of the B-diamond and the Al2O3/B-diamond interfacial quality.  In this study, the B-diamond MOS capacitors and MOSFETs are fabricated and characterized by employing the ozone precursor for the ALD-Al2O3 deposition. Because high-temperature operation for the diamond electronic devices is an important application field, electrical properties of the Al2O3 (ozone)/B-diamond MOSFETs after annealing at 500 ℃ for as long as 10 hours will also be confirmed.     Jiangwei Liu, Tokuyuki Teraji, Bo Da, and Yasuo Koide  Electrical Properties of Boron-doped Diamond MOSFETs with Ozone as Oxygen Precursor for Al2O3 Deposition O https://samurai.nims.go.jp/profiles?unit=kj000https://samurai.nims.go.jp/profiles?unit=kj000LIU et al.: ELECTRICAL PROPERTIES OF BORON-DOPED DIMOAND MOSFETs 2  II. EXPERIMENTAL Figure 1 shows the fabrication routines for the circular-type B-diamond MOSFET. The Ib-type (100) diamond substrate was well-polished with root mean square roughness of ~0.1 nm and cleaned in a mixture solution of H2SO4 and HNO3 at 300 ℃ for 3 hours [Fig. 1(a)]. The B-diamond epitaxial layer was grown using a microwave plasma-assisted chemical vapor deposition (MPCVD) technique [Fig. 1(b)]. The microwave power, substrate temperature, and chamber pressure were kept at 1.4 kW, ~1000°C, and 18.6 kPa, respectively. The boron source was not intentionally fed during the diamond growth. Flow rates for the source gases of H2 and CH4 were 49 sccm and 1 sccm, respectively [32]. After growth, the B-diamond was treated in the mixture acid solution again to change the hydrogen surface to oxygen. For the circular-type B-diamond MOSFET, the device isolation is not required. After coating the B-diamond using a spin-coater with a positive photoresist of LOR5A and an image reversal photoresist of AZ5214E sequentially, it was exposed and developed via a scanning maskless lithography system and a TMAH (concentration: 2.38%) solution, respectively. The Ti/Au bilayer (10/150 nm) was evaporated on the B-diamond and annealed at 550 ℃ for 20 minutes to form Ohmic contact via an electron-gun evaporator and a rapid thermal annealing system, respectively [Fig. 1(c)].  The Al2O3 gate oxide was deposited using the ALD technique at 200 ºC with thickness around 24 nm [Fig. 1(d)]. The precursors were Al(CH3)3 and ozone. The gate metals for the B-diamond MOS capacitors and MOSFETs were Ti/Au bilayer with thicknesses of 10/150 nm [Fig. 1(e)]. The windows of Ohmic contact electrodes were opened by the capacitively coupled plasma reactive-ion etching system in CHF3+Ar atmosphere [Fig. 1(f)]. Electrical properties of them were measured using a MX-200/B prober and a B1500A parameter analyzer at room temperature.  III. RESULT AND DISCUSSION A. Thickness and Surface Morphology for B-diamond Figure 2(a) shows depth profile for concentration of boron atoms in the B-diamond epitaxial layer measured by secondary ion mass spectroscopy (SIMS). Thickness for the B-diamond epitaxial layer is 2000 nm. At the beginning state of its growth, the concentration of boron atoms is as high as 1017 cm3. With the thickness increase, the concentration decreases to be around 4 × 1015 cm3, which is close to the value in the diamond substrate. At the surface region (~340 nm) of the B-diamond epitaxial layer, the concentration is around 1016 cm3. Because the boron source was not intentionally fed, we can not control the doping concentration very well. Further efforts for the growth of B-diamond epitaxial layer with a stable doping concentration will be performed. The acceptor concentration for the B-diamond epitaxial layer will be deduced by the following capacitance-voltage (C-V) measurement.  The atomic force microscopy (AFM) image for the B-diamond epitaxial layer was measured and shown in Fig. 2(b). The root mean square (RMS) value for the 2000 nm-thick B-diamond epitaxial layer is 0.18 nm. It is higher than that (0.15 nm) of the 700 nm-thick B-diamond [33] and lower than that (0.3 nm) of the 30 μm-thick B-diamond epitaxial layer [32]. B. Surface Morphology and Schematic Diagram for B-diamond MOSFETs Figure 3(a) shows the scanning electron microscopy image of the entire sample surface. The B-diamond MOS capacitors and MOSFETs are fabricated on the same substrate. The diameters for the gate electrodes of the MOS capacitors and drain electrodes of the MOSFETs are the same as 398.2 μm. Figs. 3(b) and 3(c) show the image and schematic diagram of the B-diamond MOSFET marked with the red box in Fig. 1(a), respectively. The gate length is 5.6 μm. Interspace distances for Fig. 1. Fabrication routines for the B-diamond MOSFET: (a) Cleaned diamond, (b) B-diamond growth, (c) Ti/Au Ohmic formation, (d) Al2O3 deposition, (e) Ti/Au gate metal formation, and (f) opening windows for Ohmic contact.  Diamond B-diamond(a) (b) (c)(f) (e) (d)Ti/AuAl2O3Ti/AuFig. 3. Scanning electron microscopy image of the entire sample surface. (b) and (c) Image and schematic diagram of the B-diamond MOSFET marked with the red box in Fig. 1(a), respectively.   1 mm Diamond (100)Source5.6 μm9.8 μmGate15.3 μmAl2O3 (24 nm)GateSourceDrain200 μ m(a) (b)(c)DrainBoron-doped diamondMOSMOSFETFig. 2. (a) Depth profile of the SIMS measurement for concentration of boron atoms and (b) AFM image for the B-diamond epitaxial layer.  0 500 1000 1500 2000101510161017    500 nmRMS: 0.18 nm(a) (b)Depth (nm)Concentration (Atom cm‒3)1660 nmLIU et al.: ELECTRICAL PROPERTIES OF BORON-DOPED DIMOAND MOSFETs 3 gate-to-source and gate-to-drain electrodes are 9.8 and 15.3 μm, respectively. According to the diameter of the gate electrode (398.2 μm), the gate width for the B-diamond MOSFET can be computed to be 1.25 mm. C. Electrical Properties for B-diamond MOS Capacitor Figure 4(a) shows leakage current density (J) properties for the Al2O3 (ozone)/B-diamond MOS capacitor. The variation state of the J for the Al2O3 (ozone)/B-diamond MOS capacitor is similar with that of the Al2O3 (water)/B-diamond MOS capacitor [25]. When the voltage is changing from –1.5 V to –2.0 V, the J has a linear relationship with the voltage, which indicates that its conduction mechanism is hopping conduction. With measurement voltage changing from –2.0 V to –9.0 V, the conduction mechanism varies to the Fowler-Nordheim tunneling model [25, 34].  The J is 2.7 × 10–5 A/cm2 at –9.0 V, which is larger than that (<10–7 A/cm2) of the Al2O3 (water)/B-diamond MOS capacitor [25]. We have fabricated the Au/Ti/Al2O3 (ozone)/Pt/Ti metal-insulator-metal structure to confirm the dielectric constant for the Al2O3 (ozone) to be 7.8 at deposition temperature of 200 ºC, which is lower than that of the Al2O3 (water) of 8.1 [35]. The dielectric quality of the Al2O3 (ozone) is possibly poorer than the Al2O3 (water), which leads to the increase of the J value. This is possibly attributed to the inadequate reaction between the Al(CH3)3 and ozone at 200 ºC. Quality of the Al2O3 (ozone) film would be further improved by optimizing the deposition conditions in the following study. Figure 4(b) shows the C-V characteristic for the Al2O3 (ozone)/B-diamond MOS capacitor. The red and black lines represent the voltage swept from negative to positive and from positive to negative, respectively. The capacitance decreases as the measurement voltage is changing from –8.0 V to –9.0 V. This phenomenon can be ascribed to the increase of the J. The maximum capacitance (Cmax) and hysteresis voltage are 0.142 μF/cm2 and 0.4 V, respectively. Based on the dielectric constant for the Al2O3 (ozone) of 7.8, the capacitance for our 24 nm-thick Al2O3 was calculated to be 0.288 μF/cm2, which is around two times higher than that of the Cmax. The low Cmax is possibly attributed to the parallel capacitances between Al2O3 insulator and B-diamond semiconductor or the existence of Fermi level pinning effect [36]. Figure 4(c) compared the C/Cmax-V characteristics for the current Al2O3 (ozone)/B-diamond (red circle line) and previous studied Al2O3 (water)/B-diamond (blue circle line) MOS capacitors [25]. The measurement voltage is swept from positive to negative. For the Al2O3 (water)/B-diamond MOS capacitor, large negative flat band voltage shift in the depletion region and residual capacitance at voltage around –12.0 V are observed. These indicate the high positive fixed charge density in the Al2O3 (water)/B-diamond MOS capacitor and poor Al2O3 (water)/B-diamond interfacial quality. Since there are no above phenomena for the MOS capacitor with the ALD-Al2O3 (water) deposited on the H-diamond at 200 ℃ [35], it is natural to believe that the quality of the ALD-Al2O3 (water) is good enough for fabricating high-performance B-diamond MOS capacitor. Therefore, the poor C/Cmax-V characteristic for the Al2O3 (water)/B-diamond MOS capacitor is mainly attributed to the existence of surface defects on the B-diamond.  On the other hand, no residual capacitance is observed for the C-V curve of the Al2O3 (ozone)/B-diamond MOS capacitor. Its flat band voltage shift is also much smaller than that for the Al2O3 (water)/B-diamond one with the difference between them of 7.1 V. Based on the acceptor density (6.02 × 1015 cm–3) in the B-diamond deduced by the C–2-V characteristic [Fig. 4(d)], the Debye length for the B-diamond epitaxial layer can be calculated as 36.7 nm [25]. The flat band capacitance and voltage can be determined to be 0.09 μF/cm2 and –6.2 V, respectively. Then, the fixed charge density of the MOS capacitor is computed to be 9.2 × 1012 cm–2. It is lower than the value (1.8 × 1013 cm–2) for the Al2O3 (water)/B-diamond MOS capacitor [25].  After treating the B-diamond epitaxial layer in the mixture acid solution (H2SO4+HNO3), the surface carbon-hydrogen bonds are modified to carbon-oxygen bonds. However, the coverage of oxygen is only 0.58 monolayer [37] and the carbon dangling bond defects exist on the surface of B-diamond [38, 39]. Since the oxidizing ability of ozone precursor is better than the water vapor [31], it could saturate the carbon dangling bonds and improve the Al2O3 (ozone)/B-diamond interfacial quality. The modification of surface defects for the B-diamond with ozone precursor is the possible reasons to make the disappearance of the residual capacitance and the decrease of fixed charge density for the Al2O3 (ozone)/B-diamond MOS capacitor.  In this study, the ozone precursor is first supplied in the ALD chamber to oxidize the B-diamond surface. If the B-diamond surface oxidizes first with ozone, then deposits Al2O3 film with water precursor, the interfacial quality would be possibly similar with that of the Al2O3(ozone)/B-diamond. The residual capacitance for the C-V curve of the MOS capacitor would also be improved.  D. Electrical Properties for B-diamond MOSFETs Figures 5(a) and 5(b) show the ID–VD characteristics for the as-fabricated and 500 ºC-annealed B-diamond MOSFETs, respectively. Gate-source voltage (VGS) for both MOSFETs varies from ‒6.0 to 92.0 V in steps of +2.0 V. Both of them show distinct saturation and pinch-off characteristics. The Fig. 4. (a) Leakage current density, (b) C-V, (c) C/Cmax-V, and (d) C–2-V characteristics for the B-diamond MOS capacitors -14 -12 -10 -8 -6 -4 -2 00.00.20.40.60.81.0      -9 -8 -7 -6 -5 -4 -3 -2 -1 00.000.050.100.15    -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 210-1110-910-710-510-3   Voltage (V)J(A/cm2)2.7 ×10–5 A/cm2Voltage (V)C(μF/cm2)-4 -2 0 2 4 6 8 1001x1042x1043x1044x1045x1046x104    Ozone Water7.1 VC/CmaxVoltage (V)(c)Voltage (V)C–2 (cm4/μF2)(d)                      (a) (b)0.4 VLIU et al.: ELECTRICAL PROPERTIES OF BORON-DOPED DIMOAND MOSFETs 4 maximum ID values are ‒109.7 and ‒344.1 μA/mm, respectively. They are lower than the previous reported value (600 μA/mm) in Ref. 25 due to the lower doping concentration for the B-diamond epitaxial layer in this study. There are better linear relationships between the ID and the low VD than other previous reports especially as the boron doping concentration lower than 1017 cm3 [23, 24]. This indicates that good Ohmic contacts for our B-diamond-based MOSFETs are formed. Their on-resistance values for the as-fabricated and 500 ºC-annealed B-diamond MOSFETs are determined to be 2.2 × 105 and 2.2 × 104 Ω mm, respectively. Annealing at 500 ºC for as long as 10 hours can possible improve the Ohmic contact properties for the Au/Ti and activate the boron dopants for the B-diamond, which would lead to the increase of current output and the decrease of on-resistance for the B-diamond MOSFET. Figures 6(a) and 6(b) shows the ID and gm as functions of VGS, respectively. By a linear extrapolation method, threshold voltage (VTH) values for the as-fabricated and 500 ºC-annealed B-diamond MOSFETs are determined to be 58.8 ±0.1 and 32.0 ±0.1 V, respectively. They are still very large and similar with the previous reports [23-25]. This issue is another important research topic and should be resolved. After annealing, the  VTH for the B-diamond MOSFET decreases greatly. This is possibly attributed to that the annealing leads to variation of charges in the Al2O3 film and at the Al2O3/B-diamond interface.  According to ID-VGS characteristics, the subthreshold voltage (SS) values for the B-diamond MOSFETs before and after annealing were determined to be 414 and 3320 mV/dec, respectively. The interfacial trapped charge density (Dit) of the Al2O3/B-diamond can be calculated based on the following equation (1). ln(10) 1 itOXqDkTSSq C     ,                 (1) where the k and T are Boltzmann’s constant and room temperature, respectively. COX is the oxide capacitance of Al2O3 film (0.288 μF/cm2). The Dit values were computed as 1.1 × 1013 and 9.9 × 1013 eV–1 cm–2 for the Al2O3/B-diamond interfaces before and after annealing, respectively. Thus, annealing process leads to the degradation of Al2O3/B-diamond interfaces. The Dit for the Al2O3/B-diamond are greater than those for Al2O3/H-diamond (6.2 × 1011 eV–1 cm–2) [41]. This is attributed to the existence of more defects at the Al2O3/B-diamond interface. After annealing at 500 ℃ for as long as 10 hours, the leakage current level at VGS = 90 V increases from 10-6 μA/mm to 10-4 μA/mm for the Al2O3/B-diamond MOSFET. This is possibly ascribed to that the long-term annealing process leads to the quality degradation for the ALD-Al2O3 film  or the diffusion of gate Ti metal into the Al2O3 film.  On/off ratio for the as-fabricated B-diamond MOSFET is around 108. It is much larger than those of the Al2O3 (water)/B-diamond MOSFETs [23, 29, 30]. This once again approve the improvement of the Al2O3/B-diamond interfacial quality by depositing the ALD-Al2O3 with the ozone precursor. The high on/off ratio for the Al2O3 (ozone)/B-diamond MOSFET makes it suitable for the power switches and other CMOS digital applications. After annealing at 500 ºC for 10 hours, the B-diamond MOSFET still operates stably with the on/off ratio larger than 106. This performance is superior to the H-diamond-based MOSFETs [20]. The maximum gm values are obtained as 4.1 and 10.9 μS/mm for the MOSFETs before and after annealing, respectively, which are the same level with the previous studies [23, 25]. IV. CONCLUSIONS The ALD-Al2O3 gate insulator was deposited with the ozone precursor for the B-diamond MOS capacitors and MOSFETs. Comparing to the C-V curve of the Al2O3 (water)/B-diamond MOS capacitor, there were no residual capacitance and lower negative flat band voltage shift for the Al2O3 (ozone)/B-diamond MOS capacitor. There were good operations for the B-diamond MOSFETs even after annealing at 500 ℃ for as long as 10 hours. 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