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## Creator

[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)

## Rights

J. Liu, T. Teraji, B. Da and Y. Koide, "Suppression of High Threshold Voltage for Boron-Doped Diamond MOSFETs," in IEEE Transactions on Electron Devices, vol. 71, no. 3, pp. 1764-1768, March 2024, doi: 10.1109/TED.2024.3356468.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

## Other metadata

[Suppression of High Threshold Voltage for Boron-Doped Diamond MOSFETs](https://mdr.nims.go.jp/datasets/141025ee-c9fe-4e69-beba-3e548dd26942)

## Fulltext

LIU et al.: SUPPRESSION OF HIGH THRESHOLD VOLTAGE FOR B-DIAMOND MOSFETS 1  Abstract—Suppression of high threshold voltage (VTH) for the boron-doped diamond (B-diamond) metal-oxide-semiconductor field-effect transistors (MOSFETs) plays a key role to design the diamond complementary MOS circuits with low gate drive sources. The VTH can be further suppressed by adjusting B-diamond epitaxial layer thickness, boron doping concentration, and gate oxide thickness. Three MOSFETs with different device structures are fabricated on the same oxygen-terminated B-diamond channel. Thickness and acceptor concentration for the B-diamond epitaxial layer are approximately 800 nm and 1.36 × 1016 cm–3, respectively. A 45 nm-thick Al2O3 is deposited as the gate oxide by an atomic layer deposition technique. Maximum drain currents and on/off ratios for the B-diamond MOSFETs are in the range of −2.4 ~ −4.3 μA/mm and greater than 106, respectively. Their VTH values are lower than 3.4 V with the lowest one of 0.8 V.   Index Terms—Diamond, Boron-doped, MOSFET, Threshold voltage I. INTRODUCTION ver the past two decades, diamond-based metal–oxide–semiconductor field-effect transistors (MOSFETs) have undergone significant development to meet requirements of high-power, high-frequency, and high-temperature applications [1-4]. This is attributed to the excellent intrinsic properties of diamond, such as its wide bandgap energy, high breakdown field, high thermal conductivity, and large carrier mobility [5, 6].  Two types of p-type diamond channels were employed for the fabrication of MOSFETs. Owing to the transfer of electrons from hydrogen-terminated diamond (H-diamond) to the negatively charged acceptors on the surface, two-dimensional hole gas accumulates on the H-diamond with a sheet hole density as large as ~1014 cm–2 [7]. To date, most diamond MOSFETs and MOSFET logic circuits have been fabricated on the H-diamond channels with outstanding electrical properties [1-4, 8]. Unfortunately, their poor thermal stability hinders their use in high-temperature field [9].  Recently, significant efforts have been made toward the development of diamond MOSFETs on p-type boron-doped diamond (B-diamond) channels with oxygen-terminated surface [9-13]. However, because the activation energy of the boron dopant (370 meV) far exceeds the thermal energy at room  This work is supported by the JSPS KAKENHI Projects (JP23K03966 and JP20H00313) and the Nanotechnology Platform Program, 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.   temperature (26 meV) [14], B-diamond MOSFETs operate at low output currents. A relatively flat (surface roughness of 0.15 nm) and highly doped B-diamond (1017 ~ 1018 cm−3) was used to fabricate the MOSFETs [13], which increased their output currents to 490 μA/mm; this value is higher than that of MOSFETs previously studied (120 μA/mm) [10-12].  Another significant issue with B-diamond MOSFETs is their high threshold voltage (VTH) values. When the B-diamond epitaxial layer thickness and acceptor concentration (NA) were 700 nm and 2.92 × 1016 cm−3, respectively, VTH for the as-fabricated B-diamond MOSFET was 63.2 V [13]. In addition, when these parameters were 2000 nm and 6.02 × 1015 cm−3, respectively, VTH was as high as 58.8 V [15]. It was reported that the VTH decreased to 7.0 V by reducing the thickness to 230 nm for the B-diamond with NA of 2 × 1017 cm−3 [11, 16]. Therefore, the choice of B-diamond active layer thickness and boron concentration are important for reduction of VTH for the B-diamond MOSFETs. On the other hand, it was proved that there were negative flat band voltage (VFB) shift for capacitance-voltage (C-V) curves of the Al2O3/B-diamond MOS capacitors [13, 15]. Therefore, positive fixed charges exist in the Al2O3 gate oxides. When the gate–source voltage (VGS) is positive, electric field across the Al2O3 would be enhanced due to the existence of positive charges. Increase of the thickness for the Al2O3 would increase positive charges and the absolute VFB (|VFB|) value, leading to the decrease of the VTH for the MOSFETs. Since the low VTH for the B-diamond MOSFETs plays a key role to reduce system complexity during designing complementary MOS circuits, in this study, we would make effects to further decrease the VTH of B-diamond MOSFETs by adjusting the thickness of the B-diamond epitaxial layer, boron doping concentration, and thickness of the Al2O3 gate oxide. II. EXPERIMENTAL A microwave plasma-assisted chemical vapor deposition technique was employed to grow a B-diamond epitaxial layer on a well-polished (roughness: 0.1 nm) and acid-cleaned (H2SO4 + HNO3 at 300 °C for 3 h) Ib-type (100) diamond substrate [17]. Thickness and boron concentration of the B-diamond channel were confirmed using a secondary-ion mass spectroscopy (SIMS) technique to be approximately 800 nm and 1016 ~ 1017 cm−3, respectively [Fig. 1(a)]. The NA for the B-diamond is deduced by the C-V measurement in the following section to be 1.36 × 1016 cm−3. Surface root mean square roughness is lower than 0.2 nm [13, 15]. Comparing to the B-diamond in Ref. 13 (VTH = 56.1 V), the channel thickness is similar, but the boron concentration decreases. While the doping level is higher than that of the Ref. 15 (VTH = 58.8 V), the B-diamond thickness is reduced from 2000 nm to 800 nm. Jiangwei Liu, Tokuyuki Teraji, Bo Da, and Yasuo Koide  Suppression of High Threshold Voltage for Boron-doped Diamond MOSFETs O https://samurai.nims.go.jp/profiles?unit=kj000https://samurai.nims.go.jp/profiles?unit=kj000LIU et al.: SUPPRESSION OF HIGH THRESHOLD VOLTAGE FOR B-DIAMOND MOSFETS 2 After modifying the hydrogen- to oxygen-terminated surface for the B-diamond channel in the mixture acid solution (H2SO4 + HNO3), the fabrication of MOSFETs was performed. The B-diamond was coated using a spin-coater with photoresists of LOR 5A and AZ 5214E sequentially. The rotation rate and time for both photoresists are 7000 rpm and 1 second, respectively. The baking temperature and time for the LOR 5A are 180 °C and 5 minute, respectively. Those for the AZ 5214E are 110 °C and 2 minute, respectively. The sample was exposed and developed via a scanning maskless lithography system and a Tetramethyl Ammonium Hydroxide (concentration: 2.38%) solution, respectively. The developing time is 2.0 min.  After evaporating the source/drain Ti/Au electrodes in a high-vacuum evaporation chamber with thicknesses of 10/150 nm, the sample was lifted-off in an N-Methyl-2-Pyrrolidone solution at 80 °C for 40 min and annealed at 550 °C for 20 min to form Ohmic contact using a rapid thermal annealing system. The 45 nm-thick Al2O3 film was deposited using an atomic layer deposition technique at 200 °C with Al(CH3)3 and water vapor precursors to cover all the surface of the sample. The thickness of the Al2O3 film is larger than that of the previous reports [13, 15].  Then, the Ti/Au gate electrodes with thicknesses of 10/150 nm was formed. Lastly, the Al2O3 film covering on the source/drain electrodes was etched by a capacitively coupled plasma reactive-ion etching system in the CHF3+Ar atmosphere to open the contact windows. The plasma power, CHF3 flow rate, and Ar flow rate were 100 W, 10 sccm, and 40 sccm, respectively. A schematic of the Al2O3/B-diamond MOSFET is shown in Fig. 1(b). Three types of B-diamond MOSFETs were fabricated by varying the gate length (LG) and interspace length between the source/drain and gate electrodes (LS-G and LD-G). The electrical properties for the B-diamond MOSFETs were measured at room temperature using an MX-200/B prober and a B1500A parameter analyzer. III.  RESULTS AND DISCUSSION Figures 2(a), (c), and (e) show optical microscope images of B-diamond MOSFET-I, -II, and -III, respectively. The diameter of the drain electrodes of the three MOSFETs was maintained as 398.2 μm with a gate width of 1.25 mm. Figures 2(b), (d), and (f) show scanning electron microscopy (SEM) images of the B-diamond MOSFET-I, -II, and -III marked with blue boxes in Figs. 2(a), (c), and (e), respectively. The LG, LS-G, and LD-G values for MOSFET-I are 3.7, 7.3, and 8.9 μm, respectively, and 5.4/11.8/13.9 and 10.3/9.9/13.6 μm for MOSFET-II and -III, respectively. Figure 3(a) shows the leakage current density (J) as a function of the voltage for the B-diamond MOS capacitor. The J is in the region of 10–8 A/cm2 at −19.0 ~ 5.0 V. The low J value indicates a good quality Al2O3 gate insulator and high band offsets for the Al2O3/B-diamond heterojunction. As the voltage changes from −19.0 to −25.0 V, the J increases to 4.1 × 10–4 A/cm2. Based on the J–V curve, the possible conduction mechanism at high voltage for the Al2O3/B-diamond MOS capacitor was analyzed.  Figure 3(b) shows the ln(J/E2)-1/E characteristic. The E is the electrical field of Al2O3, which was obtained by dividing the voltage by the thickness (45 nm) of the Al2O3 film. There is a linear dependence of the ln(J/E2)-1/E characteristic on the 1/E that ranges from −0.19 V to −0.22 cm/MV. Therefore, the conduction mechanism at high voltage of the Al2O3/B-diamond MOSFET is consistent with the Fowler–Nordheim tunneling model, according to the following equation [18]: 32 4 2 1exp( )3mJ AEq E  ,                      (1) 0 200 400 600 800 1000101510161017    Depth (nm)Concentration (Atom cm‒3)Diamond (100)SourceGateAl2O3 (45 nm) DrainBoron-doped diamondLGLS-G LD-G(a) (b)Fig. 1. (a) Depth profile of the SIMS measurement for concentration of boron atoms in the B-diamond epitaxial layer, and (b) schematic diagram of the Al2O3/B-diamond MOSFET. Fig. 2. (a), (c), and (e) Microscope images of B-diamond MOSFET-I, -II, and III, respectively. (b), (d), and (f) SEM images of B-diamond MOSFET-I, -II, and -III marked with blue boxes in Figs. 2(a), (c), and (e), respectively. 20 μm20 μm20 μm(b) MOSFET-I(d) MOSFET-II(f) MOSFET-IIISourceGateDrainSourceGateDrainSourceGateDrain(a) MOSFET-I(c) MOSFET-II(e) MOSFET-IIIGateSourceDrainGateSourceDrainGateSourceDrain200 μm200 μm200 μm-0.19 -0.20 -0.21 -0.22 -0.23-20-18-16-14-12    -25 -20 -15 -10 -5 0 510-1110-910-710-510-3   Voltage (V)J(A/cm2)1/E (cm/MV)ln(J/E2) (A/MV2)(a) (b)–19.0 VFig. 3. (a) and (b) J-V and ln(J/E2)-1/E characteristics of B-diamond MOSFET.  LIU et al.: SUPPRESSION OF HIGH THRESHOLD VOLTAGE FOR B-DIAMOND MOSFETS 3 where m*, Φ, h, and q are the tunneling effective mass of a hole in Al2O3 (0.36m0, where m0 is the free electron mass) [19], valence band offset of the Al2O3/H-diamond heterojunction, Planck’s constant, and elementary charge, respectively. A is a parameter that depends on m*, Φ, and temperature. By fitting the linear portions of ln(J/E2)-1/E characteristics using Eq. (1), the valence band offset of the Al2O3/B-diamond heterojunction was calculated to be 2.9 eV. According to the bandgap energies of diamond (5.5 eV) and Al2O3 (7.2 eV) [20], the conduction band offset for the Al2O3/B-diamond heterojunction was deduced to be 1.2 eV. Both the valence and conduction band offsets are larger than 1.0 eV, which explains the low J of the B-diamond MOSFET [21]. Further, the band configuration of the Al2O3/B-diamond heterojunction exhibited a type II staggered structure, as shown in Fig. 4.  Figure 5(a) shows C-V properties for the Al2O3/B-diamond MOS capacitor. The black and red circle lines represent the measurement voltage swept from 0 V to –25.0 V and from –25.0 V to 0 V, respectively. Maximum capacitance is 0.065 μF/cm2, which is much lower than the theoretical value of 0.157 μF/cm2 based on the dielectric constant (8.0) of 45-nm-thick Al2O3 deposited at 200 °C [22]. This is possibly attributed to that the measurement capacitance is the series value between Al2O3 insulator and B-diamond channel layer. The capacitance decreases with the voltage shifting to the left hand side relative to –24.2 V. This is possibly attributed to the increase of the J.  Hysteresis voltage shift is 0.3 V at the capacitance of 0.06 μF/cm2 due to the trapped charges in the Al2O3 film. There are residual capacitances for the C-V curve around –20.0 V. This phenomenon can be suppressed by changing the water to ozone precursor for the Al2O3 deposited by the atomic layer deposition technique [15].  The C-V curves shift to the left hand side relative to the 0 V. Positive fixed charges exist in the Al2O3 film, leading to the VFB shift for the B-diamond MOS capacitor. The C–2–V property is shown in Fig. 5(b). Based on the value of 1969.3 cm4/μF2V for the dC–2/dV, the NA in the B-diamond channel layer can be calculated to be 1.36 × 1016 cm–3. Debye length for the B-diamond is deduced to 244.3 nm by considering the hole density for the uncompensated p-type B-diamond is approximately 1% of the NA [23], Experimental flat band capacitance and VFB shift can be determined to be 0.018 μF/cm2 and –19.5 V, respectively.  The |VFB| is larger than those of our previous reports (13.3 and 4.1 V) [13, 15]. Figures 6(a), (c), and (e) show the drain current–drain voltage (ID–VD) characteristics of MOSFET-I, -II, and -III, respectively. The VGS for all three MOSFETs varies from ‒16.0 V to 40.0 V in steps of +1.0 V. Distinct saturation and pinch-off characteristics are observed. The maximum ID (ID,max) values for MOSFET-I, -II, and -III are −3.0, −4.3, and −2.4 μA/mm, respectively. Although the MOSFET-I has smaller LG, LS-D, and LD-G, its ID,max is lower than that of the MOSFET-II. This is possibly attributed to the nonuniformity of the B-diamond channel layer or the unintentional damage for the MOSFETs during the fabrication process.  Figures 6(b), (d), and (f) show the ID–VGS characteristics for B-diamond MOSFET-I, -II, and -III, respectively. Through linear extrapolation, their respective VTH were determined to be 3.4, 3.2, and as low as 0.8 V, respectively. The on/off ratios for the three MOSFETs were greater than 106, which is reasonable for B-diamond MOSFETs with water vapor as the precursor for Al2O3 deposition [13]. Subthreshold voltage (SS) values were determined as 380, 261, and 260 mV/dec for MOSFET-I, -II, and -III, respectively. The interfacial trapped charge density (Dit) of the Al2O3/B-diamond heterojunction can be calculated using Eq. (2) [24]. Fig. 6. (a), (c), and (e) ID–VD characteristics of B-diamond MOSFET-I, -II, and -III, respectively. (b), (d), (f) ID–VGS characteristics of B-diamond MOSFET-I, -II, and -III, respectively.   20 15 10 5 0 -5 -10 -1510-710-510-310-1101    20 15 10 5 0 -5 -10 -1510-710-510-310-1101    20 15 10 5 0 -5 -10 -1510-710-510-310-1101    0 -2 -4 -6 -8 -10 -12 -14 -160-1-2-3    0 -2 -4 -6 -8 -10 -12 -14 -160-1-2-3    0 -2 -4 -6 -8 -10 -12 -14 -160-1-2-3-4-5    I D  (μA/mm)I D  (μA/mm)I D  (μA/mm)VD (V)(a) MOSFET-I(c) MOSFET-II(e) MOSFET-IIII D  (μA/mm)VGS (V)I D  (μA/mm)I D  (μA/mm)(b) MOSFET-I(d) MOSFET-II(f) MOSFET-IIIOn/off: >106SS: 380 mV/decOn/off: >106SS: 261 mV/decOn/off: >106SS: 260 mV/decVTH = 3.4 VVTH = 3.2 VVTH = 0.8 VFig. 4. Band configuration of Al2O3/B-diamond heterojunction. The CBM and VBM are conductance band minimum and valence band maximum, respectively. CBO: 1.2 eVB-diamondAl2O3CBMVBMVBMCBMholes5.5 eV7.2 eVHole tunnelingVBO: 2.9 eVFig. 5. (a) C-V and (b) C–2-V properties for the Al2O3/B-diamond MOS capacitor, respectively. -25 -20 -15 -10 -5 00.000.020.040.060.08    Voltage (V)C(μF/cm2)-20 -18 -16 -14 -12 -10 -80500010000150002000025000                          Voltage (V)C–2 (cm4/μF2)(a) (b)LIU et al.: SUPPRESSION OF HIGH THRESHOLD VOLTAGE FOR B-DIAMOND MOSFETS 4 TBALE1 SUMMARY OF ELECTRICAL PROPERTIES FOR THE AS-FABRICATED B-DIAMOND MOSFETS [10, 11, 13, 15]   B-diamond  thickness (nm) Acceptor  concentration (cm–3) Al2O3 thickness (nm) Dielectric constant of Al2O3  LG/LS-G/LD-G  (μm) |VFB| (V) ID,max  (μA/mm) VTH (V) MOSFET [10] 230   1.75 × 1017 40 9.0  3.0/4.0/4.0 - –1.9 7.0 MOSFET [11] 500 2 × 1017 20 -  4.0/3.0/4.0 - –120 >20.0 MOSFET [13] 700   2.92 × 1016 23.3 8.0  7.0/9.9/17.4 13.3 –490 56.1 MOSFET [15] 2000 6.02 × 1015 24 7.8  5.6/9.8/15.3 4.1 –109.7 32.0 MOSFET [this work] 800 1.36 × 1016 45 8.0  3.7/7.3/8.9 19.5 –3.0 3.4  5.4/11.8/13.9 –4.3 3.2  10.3/9.9/13.6 –2.4 0.8 ln(10) 1 itOXqDkTSSq C     ,                  (2) where k and T are Boltzmann’s constant (8.62×10−5 eV/K) and room temperature (298.15 K), respectively. COX is the oxide capacitance of 45-nm-thick Al2O3 film (0.157 μF/cm2) [22]. The Dit values for the three Al2O3/B-diamond interfaces were calculated as 5.32 × 1012, 3.35 × 1012, and 3.33 × 1012 eV–1 cm–2, respectively. These values are greater than those for Al2O3/H-diamond (6.2 × 1011 eV–1 cm–2) [25]. This is possibly attributed to the existence of more defects at the Al2O3/B-diamond interface. The gm-VGS characteristics for the B-diamond MOSFET-I, -II, and -III are shown in Fig. 7. They are determined based on the slopes of the linear portions for the ID-VGS characteristics in Figs. 6(b), (d), and (f). The gm,max values for them are –0.30, –0.44, and –0.30 μS/mm, respectively. Table 1 summarizes electrical properties of the as-fabricated B-diamond MOSFETs reported previously [10, 11, 13, 15] and this work. The absolute ID,max (|ID,max|) values in this study are higher than that (1.9 μA/mm) of the MOSFET on the B-diamond channel with a thickness and the NA of 230 nm and 1.75 × 1017 cm−3, respectively [10]. However, because of the thinner B-diamond layer and lower NA (800 nm and 1.36 × 1016 cm−3) in this work, the |ID,max| values are much lower than those of MOSFETs with B-diamond thicknesses and NA values of 700 nm/2.92 × 1016 cm−3 [13] and 2000 nm/6.02 × 1015 cm−3  [15], respectively. For the B-diamond MOSFETs reported in references [10] and [11], the NA values of the B-diamond channel layers are similar. However, the thinner B-diamond and Al2O3 leads to the decrease of VTH greatly. Although the VTH values for the B-diamond MOSFETs in this work are unstable with different device structures, they are much lower than those reported in previous studies [10, 11, 13, 15]. The B-diamond thickness for the reference [13] and this work is similar. The lower NA for the B-diamond channel layer and the larger |VFB| value lead to the decrease of VTH. On the other hand, even the NA for the B-diamond channel layer in reference [15] is lower than that in this work, its thicker B-diamond layer and lower |VFB| value generate a high VTH. Therefore, larger |VFB| value, thinner channel layer, and lower dopant concentration can decrease VTH for B-diamond MOSFETs greatly. In contrast, this type of B-diamond MOSFETs demonstrate low output currents. Therefore, the balance between low VTH and high output current is important to advance the development of B-diamond MOSFETs for future applications. IV. CONCLUSIONS In this study, we suppressed the high VTH of the B-diamond MOSFETs by adjusting the thickness of the B-diamond epitaxial layer, NA, and thickness of the Al2O3 gate oxide to 800 nm, 1.36 × 1016 cm–3, and 45 nm. The J value for the B-diamond MOS capacitor was approximately 10–8 A/cm2 at voltage in the range of −19.0 ~ 5.0 V. 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