Hydrogen-terminated diamond MOSFETs on (0 0 1) single crystal diamond with state of the art high RF power density

Diamond field-effect transistor (FET) has great application potential for high frequency and high power electronic devices. In this work, diamond FETs were fabricated on (0 0 1) single crystal diamond with homoepitaxial layer. The nitrogen impurity content in the homoepitaxial layer is greatly decreased as measured by the Raman and photoluminescence spectra. The diamond field effect transistor with 100 nm Al 2 O 3 as gate dielectric shows ohomic contact resistance of 35 Ω . mm, maximum drain saturation current density of 500 mA/mm, and maximum transconductance of 20.1 mS/mm. Due to the high quality of Al 2 O 3 gate dielectric and single crystal diamond substrate, the drain work voltage of −58 V is achieved for the diamond FETs. A continuous wave output power density of 4.2 W/mm at 2 GHz is obtained. The output power densities at 4 and 10 GHz are also improved and achieve 3.1 and 1.7 W/mm, respectively. This work shows the application potential of single crystal diamond for high frequency and high power electronic devices.


Introduction
Diamond is thought to be the most promising semiconductor material for high power and high frequency electronic devices due to its good electrical and thermal properties, including high critical breakdown electric field of 10 MV/cm, high carrier saturation velocity of 2.7 × 10 7 cm/s, and high thermal conductivity of 22 W/cm . K [1]. Diamond has a wide band gap of 5.45 eV, which makes it very difficult to be doped [2]. By now, most of the diamond field-effect transistors (FETs) for high frequency and high power applications were fabricated on hydrogen-terminated diamond (H-diamond) [3][4][5]. A two-dimensional hole gas (2DHG) surface channel forms on H-diamond by transfer doping from adsorbates/dielectrics in contact with H-diamond surface [6][7][8].
Some promising results have realized on H-diamond FETs [9][10][11][12]. Jingu et al. developed a remote plasma processing method [13] to get H-diamond surface after source and drain electrodes lithography, and Imanishi et al. got a maximum output power density of 3.8 W/mm at 1 GHz for the diamond metal-oxide-semiconductor FETs (MOSFETs) using this technique on a (1 1 0) preferential polycrystalline diamond [9]. Kudara et al. also obtained output power densities of 2.5 W/mm at 1 GHz, and 1.5 W/mm at 3.6 GHz for the diamond MOSFETs with 200 nm Al 2 O 3 gate dielectric on a (1 1 0) preferential polycrystalline diamond [14]. Kudara et al. fabricated diamond MOSFETs with regrown P ++ -diamond layers as the source and drain regions on the chemical vapor deposition (CVD) IIa (1 1 1) single crystal diamond substrate [15]. A current density of 1 A/mm and an output power density of 3.6 W/mm at 1 GHz were obtained.
For high frequency and high power electronics, wafer scale single crystal semiconductor materials with low defect density are basically demanded. The (0 0 1) oriented single crystal diamond shows the most promising for extending the crystal size to wafer scale by a CVD process [16][17][18]. Developing of high performance diamond FETs on (0 0 1) single crystal diamond is crucial demand. In this work, H-diamond MOSFETs were fabricated on a (0 0 1) oriented single crystal diamond substrate with a high quality homoepitaxial layer on it. Combined with the atomic layer deposition (ALD) Al 2 O 3 as gate dielectric, the diamond FETs show high maximum drain saturation current density, high breakdown voltage, and acceptable maximum oscillation frequency. As a result, improved output power density of 4.2 W/mm at 2 GHz, 3.1 W/mm at 4 GHz, and 1.7 W/ mm at 10 GHz were obtained.

Experiments
A commercial 10 mm × 10 mm (0 0 1) oriented single crystal diamond with high nitrogen concentration of ∼2 ppm was used as the substrate. Homoepitaxial single crystal diamond layer with low impurity concentration was grown on it by the microwave plasma chemical vapor deposition (MPCVD) technique. After growth, the sample was polished, and treated by the MPCVD technique in H 2 plasma to form H-diamond [19]. The electrical properties of the H-diamond were measured by a Van-der Pauw Hall method at room temperature (RT) before device fabrication. The sample shows a sheet resistance of R sq = 8596 Ω/sq, hole mobility of μ = 101 cm 2 /V s, and sheet density of Ns = 7.16 × 10 12 /cm 2 .
The fabrication process of the H-diamond MOSFETs was as follow. A 50 nm Au film was deposited by e-beam evaporation on the H-diamond surface. This Au layer was used as a sacrificial layer to protect the H-diamond away from resist and possible contamination or damage during the device fabrication process. The channel regions were defined by optical lithography. The Au sacrificial layer outside the channel regions was etched away by a KI/I 2 solution. Then, device isolation was realized by oxygen plasma treatment. The source electrodes, drain electrodes, and gate pad electrodes were formed by lithography, Au metal evaporation, and lift-off. The source-drain spacing was defined by lithography, and the sacrificial layer of gold film on these regions was removed by KI:I 2 solution wet etching. Al 2 O 3 dielectric with thickness of 100 nm was deposited by an ALD process at 400 °C. After etching of Al 2 O 3 dielectric on the source, drain, and gate pad electrodes, the gates were patterned by lithography. Al was deposited to form gate electrode. Figure 1 shows the cross-sectional schematic and scanning electron microscopy (SEM) image of the H-diamond MOSFET. The gate length of the fabricated H-diamond MOSFET is 0.9 μm. The source and drain space is 3 μm, with source-gate space L SG = 0.6 μm, and drain-gate space L DG = 1.5 μm.
The direct current (DC) characteristics of the H-diamond MOSFETs were test by a semiconductor parameter analyzer. Small signal scattering (S) parameters measurement was carried out by a vector network analyzer with standard ground signal ground (GSG) probes. Continuous wave (CW) microwave power characteristics were measured on-wafer using an active loadpull system. Figure 2 shows the Raman and photoluminescence (PL) spectra of the single crystal diamond substrate and that after homoepitaxial growth. As shown in Figure 2(a), the Raman spectrum of the single crystal diamond substrate shows a sharp diamond Raman peak with the peak position of 1332.4 cm −1 , and full width of half maximum (FWHM) of 3.9 cm −1 . After homoepitaxial growth, the diamond Raman peak position is still 1332.4 cm −1 , and the FWHM decreases to 2.7 cm −1 , indicating the crystal quality of the single diamond is improved. For the single crystal diamond substrate, fluorescence effect induced by nitrogen impurities is obvious, and the nitrogenvacancy (NV) center peaks are dramatic (Figure 2(b)) due to the high nitrogen content of the diamond substrate. For homoepitaxial layer, the nitrogen impurity content decreases greatly, as indicated by the decrease of fluorescence effect. The nitrogen content is below 1 × 10 16 /cm 3 by secondary ion mass spectroscopy (SIMS) measurement. Atomic force microscopy (AFM) measurement of the single crystal diamond after H 2 plasma treated by the MPCVD shows a very smooth surface with surface roughness (RMS) of 0.43 nm in a region of 10 μm × 10 μm. Figure 3 shows the current-voltage characteristics and transfer characteristics of the H-diamond MOSFET.

Results and discussion
The H-diamond MOSFET shows p type doping with the maximum drain saturation current of 500 mA/mm at the gate-source voltage V GS of −10 V, and the channel can be completely turned off at gate-source voltage V GS of 30 V. The on-resistance R on is 35 Ω . mm for the H-diamond MOSFET. The source/drain ohmic contact resistance R c measured by a transmission line method (TLM) is ∼3.2 Ω · mm, which indicates that a good ohmic contact is achieved. The high on-resistance R on is mainly due to the high sheet resistance of the H-diamond material. The low carrier mobility is the limited factor for such high sheet resistance [16]. Improvement of carrier mobility and the conductivity of the diamond channel has been the critical mission for developing of high The calculated negative fixed charge density in the interface is 3.7 × 10 12 /cm 2 . The carrier density N s of the 2DHG of H-diamond was calculated by ∫C GS dV GS , and the maximum carrier density N s is 1.03 × 10 13 /cm 2 at V GS = −10 V, as shown in Figure 4(b). The linear tendency can be observed in the N s -V GS curve, suggesting a relatively uniform carrier distribution in the 2DHG conduction channel of H-diamond.
Small hysteresis phenomenon was observed in the C-V characteristics of the H-diamond diode. The hysteresis value was 0.2 V. The presence of the hysteresis should be due to the injection of hole carriers into the ALD Al 2 O 3 dielectric. The trapped charge density (N tra ) can be calculated by Equation (2) The calculated trapped charge density in the ALD Al 2 O 3 dielectric is 9 × 10 10 /cm 2 . The C-V characteristics show that the Al 2 O 3 dielectric has a relative high quality [21]. Figure 5(a) shows the high-frequency characteristics of a dual gate H-diamond MOSFET with gate length of 0.9 μm and gate width of 2 × 100 μm measured at  V DS = −30 V, V GS = 9 V. The H-diamond MOSFET exhibits an extrinsic current gain cut-off frequency f T of 6.2 GHz and maximum oscillation frequency f max of 17 GHz, respectively. The f T and f max are reasonable values for the H-diamond MOSFETs with gate length of 0.9 μm [22].
The RF power output characteristic of the H-diamond MOSFET was measured under a CW signal at 2, 4, and 10 GHz, as shown in Figure 5(b)-(d). As shown in Figure 5(b), benefit from the high quality Al 2 O 3 dielectric, the drain work voltage reaches −58 V for the diamond MOSFET at 2 GHz. With V GS = 8 V, the maximum power gain, maximum CW output power density (P out ), and maximum power added efficiency (PAE) are 12.2 dB, 4.2 W/mm, and 21.5%, respectively. As shown in Figure 5(c), at 4 GHz, with V DS = −55 V, the maximum  power gain, maximum CW P out , and maximum PAE are 6.8 dB, 3.1 W/mm, and 10.3%, respectively. As shown in Figure 5(d), at 10 GHz, with V DS = −40 V, the maximum power gain, maximum CW P out , and maximum PAE are 3.7 dB, 1.7 W/mm, and 5.7%, respectively.
As shown in Figure 6, the maximum output power density of 4.2 W/mm at 2 GHz, 3.1 W/mm at 4 GHz, and 1.7 W/mm at 10 GHz are all the highest output power densities obtained for H-diamond FETs [14,[23][24][25][26][27][28][29]. The P out can be estimated by where I DS-max is the maximum drain current density, V work is the drain voltage for the measurement of P out , and V knee is the knee voltage. We think there are two main reasons for the good performances of the H-diamond MOSFET in this work. The first one is that the diamond is single crystal with high material quality and low surface roughness. Second, the diamond channel is passivated by the high quality ALD Al 2 O 3 with low trapped charge density and negative fixed charge density in interface of the Al 2 O 3 dielectric and H-diamond. As studied by Chen et al. [30], trap effects are crucial for improving the performance of H-diamond MOSFETs.
The drain current will be degraded by traps in both Al 2 O 3 dielectric and diamond substrate. For the transistors working at high frequency and large power conditions, the trap effects would be more serious. The improvements of diamond substrate quality and the Al 2 O 3 dielectric in this work are both beneficial for the improvement of output power characteristics of the H-diamond MOSFET. Compared with the widely used GaN HEMT [31][32][33], the maximum PAE of the H-diamond MOSFET is low at now. The PAE is expressed as: The H-diamond MOSFET is working as a Class A mode, for which the PAE is limited to 50%. There are some reasons for the relatively low PAE of the H-diamond MOSFET. The most important reason is that the sheet resistance of the H-diamond channel is high and the carrier mobility is low. At the same time, the ohmic contact resistance is high for the diamond FETs. The above reasons make the parasitic resistance is very high for the H-diamond MOSFET. In this work, the parasitic resistance is about 10× higher than that of GaN HEMTs, which was caused by higher sheet resistance and higher ohmic contact resistance. The large parasitic resistance will produce Joule's heat and limit the P out and PAE of the H-diamond MOSFETs. The large sheet resistance and ohmic contact resistance also make the drain saturation current is low and the knee voltage is high, which also limited the P out and PAE of the H-diamond MOSFETs (Equation (3)). The second reason is that the maximum oscillation frequency f max is relatively low, which limites the gain of H-diamond MOSFET. The low gain makes the PAE is low. The maximum oscillation frequency is influenced by many factors, including the parasitic parameters, the gate length, and gate dielectric thickness. The structure of the H-diamond MOSFETs needs to be improved further.
The maximum CW output power density (P out ) and maximum PAE for our H-diamond MOSFET are lower than the GaN HEMT devices mainly due to the imperfect electrical properties of the diamond channel at now. But the large heat dissipation demand and high junction temperature have been the limited factors for the GaN HEMT devices for high frequency and high power application. Diamond possesses perfect thermal conductivity, which make it more suitable for application in high frequency and high power area. Improvements of electrical properties and device structure of the diamond FETs will make them play an important role in the future.

Conclusion
In summary, H-diamond MOSFETs with ALD Al 2 O 3 as gate dielectric were fabricated on (0 0 1) single crystal diamond to optimize the high frequency and high power characteristics. The H-diamond MOSFETs possess good ohmic contact and high quality Al 2 O 3 gate dielectric. The H-diamond MOSFET with gate length of 0.9 μm and Al 2 O 3 gate dielectric of 100 nm shows maximum drain saturation current of 500 mA/mm, maximum transconductance of 20.1 mS/mm. Due to the high quality diamond substrate and gate dielectric, the drain work voltage of the H-diamond MOSFET reaches −58 V, and a maximum CW output power density of 4.2 W/mm at 2 GHz is obtained. The CW output power density of 3.1 W/mm at 4 GHz and 1.7 W/mm at 10 GHz are obtained at the same time. The results show the application promising of H-diamond FETs on (0 0 1) single crystal diamond for high frequency and high power electronics.

Disclosure statement
No potential conflict of interest was reported by the authors.