Diamond Based Field-Effect Transistors of Zr Gate with SiN x Dielectric Layers

Investigation of Zr-gate diamond field-effect transistor with SiN x dielectric layers (SD-FET) has been carried out. SD-FET works in normally on depletionmode with p-type channel, whose sheet carrier density and hole mobility are evaluated to be 2.17 × 10 cm and 24.4 cm⋅V⋅s, respectively. The output and transfer properties indicate the preservation of conduction channel because of the SiN x dielectric layer, which may be explained by the interface bond of C-N. High voltage up to −200V is applied to the device, and no breakdown is observed. For comparison, another traditional surface channel FET (SC-FET) is also fabricated.


Introduction
Diamond appears promising for high-power and highfrequency devices, since it has remarkable properties, such as wide band gap (  ) of 5.47 eV, the highest thermal conductivity (22 W/cm⋅K), high breakdown field (10 MV/cm), high carrier mobility of electron 4500 cm 2 ⋅V −1 ⋅s −1 , hole 3800 cm 2 ⋅V −1 ⋅s −1 , and high carrier velocity (10 7 cm/s) [1][2][3][4].However, due to the large activation energies of dopants (boron, phosphorus, and nitrogen) for diamond at room temperature, carrier concentrations in the diamond are very low [1,5], which limit the development of the diamondbased semiconductor devices.Recently, most FETs based on diamond focus on the hydrogen-terminated surface [6][7][8][9], which can introduce a p-type conduction layer with a sheet hole density and hole mobility of 10 13 cm −2 and 50-150 cm 2 ⋅V −1 ⋅s −1 [10,11], respectively.In this case, a twodimensional hole gas (2DHG) exists near the surface of the diamond, associated with the electrochemical potential of adsorbates on a humidity saturated surface [12].Thus, the conduction surface is thermally and chemically instable and vulnerable to the ambient environment.The oxidation of diamond surface or higher annealing even in vacuum could lead to the missing of 2DHG and degrade the performance of FET device.This problem can be solved by applying dielectric layers to the diamond surface to maintain the conduction channel for stabilizing or replacing the adsorbates.Various dielectric layers of the H-terminated diamond surface, such as SiO 2 [13], Al 2 O 3 [14], AlN [15], HfO 2 [16], LaAlO 3 [17], Ta 2 O 5 [18], and ZrO 2 [19], have been mainly used as the insulators by the method of thermal evaporation, atomic layer deposition (ALD), or metal organic chemical vapor deposition (MOCVD).However, there are few reports on using dielectric of SiN  .
In this paper, the single crystal diamond FET with SiN  layers was proposed, and its properties were studied.For comparison, traditional H-termination surface channel FET was also fabricated.

Device Fabrication
Undoped homoepitaxial diamond films were grown on hightemperature high-pressure (HTHP) synthesized (001) IIa single crystal diamond substrates with the dimension of 3 × 3 × 0.5 mm 3 by a microwave plasma CVD system (AX5200S Seki Technotron Corp.).Before growth, the substrates were dipped into a mixed acid of HNO 3 and H 2 SO 4 (1 : 1) at 250 ∘ C for 1 hour to remove nondiamond phase.During growth, the total flow rate of the reaction gas was 500 sccm, and the ratio of CH 4 /H 2 was 1%.The process pressure, growth temperature, and microwave power were 100 Torr, 900 ∘ C, and 1 kW, respectively.After growth, the samples were kept in hydrogen plasma for 1 minute at 900 ∘ C to generate a hydrogen-terminated surface.
The fabrication began with a standard lithography to pattern the drain and source electrodes before the metal evaporation.A carbon soluble metal palladium (Pd) was selected as the electrodes because of its good adhesion and relatively low specific contact resistance with the diamond surface [20].Next, mesa isolation, protected by the photoresist, was performed on the grown diamond film by irradiation of O 3 and UV rays for 15 mins.The SiN  dielectric layer was then deposited onto the surface at the temperature of 250 ∘ C and etched using BOE (6 : 1) solution for the patterns.Some of the device's channels were not covered with SiN  for comparison.Zirconium (Zr) as the gate metal was finally sputtered onto the channels.The total fabrication process procedure is shown in Figure 1.The gate length (  ) and width (  ) for both of the devices were 3 m and 90 m, respectively.The electrical properties of the device with and without SiN  layers were then investigated.

Results and Discussion
The optical microscope and AFM images of as-deposited single crystal diamond epitaxial films are shown in Figure 2. Figure 2(a) presents the full-scale of the growth film with the dimension of 3 × 3 mm 2 ; no point defect pits and steps are found in the surface, indicating remarkable quality for electronic device fabrication.Figure 2(b) displays the AFM image in the area of 5 × 5 m 2 .The root mean square (RMS) roughness was measured to be 0.446 nm, illustrating an adequate morphology for device fabrication.
The room temperature properties of the Zr-gate diamond based field-effect transistors were investigated.The output characteristics are exhibited in Figure 3, where the drain current (  ) has been normalized with respect to   .The blue solid line and the red dash dotted line indicate the FETs with and without SiN  gate dielectric, respectively.As can be seen in Figure 3, the absolute value of   (|  |) for SD-FET increases with an increasing absolute value of   (|  |), confirming that the channel under SiN  is ptype with hole-carriers.It is the evidence that the channel at the SiN  /diamond interface is preserved, which may be related to the C-N bonds [21].In addition, the device operates in the normally on depletion mode.An obvious typical output characteristic for both of the FETs could be found in the figure .This interesting phenomenon may be in connection with the interface states where the charges are trapped.
As shown in Figure 4, the transfer and transconductance curves of the two kinds of devices are familiar.Based on the   -  relationship, the threshold voltage is determined to be 0.75 V for SD-FET and 0.65 V for SC-FET.The transconductance characteristics presented in Figure 4 were measured at   = −10 V.The maximum   is 1.19 mS/mm at   = −2.5 to −3.5 V and 1.22 mS/mm at   = −3.5 to −5 V for SC-FET and SD-FET, respectively.
The charge carrier profile can be derived from the room temperature capacitance-voltage (CV) measurements.Figure 5 shows the calculated profile of H-terminated diamond surface as a function of the depth from the metal/diamond interface.The 2DHG is clearly identified to be 6.6 nm underneath the surface of diamond, corresponding to a gate-to-channel capacitance of 0.76 F/cm 2 .The sheet charge density was calculated to be 2.03 × 10 13 cm −2 according to the integration of the profile as revealed in Figure 5.Using the sheet resistance of 11.0 kΩ measured by TLM method, the hole mobility was derived to be 28.0 cm 2 ⋅V −1 ⋅s −1 .Figure 6 exhibits the charge carrier profile of SD-FET with a lower gate-to-channel capacitance of 0.17 F/cm 2 , because of the series connection of the intrinsic gate capacitor and the SiN  gate dielectric capacitor.The permittivity of SiN  ( SiN  = 5.0) and diamond for dielectric layer and gate capacitor were used to evaluate the depth profile.The 2DHG was determined to be 26.2 nm under the interface of SiN  /diamond.As the 2DHG of H-terminated diamond located 6.6 nm under the surface, the thickness of SiN  was estimated to be 19.6 nm, in accordance with nominal thickness of 20 nm.Integration of the charge carrier profile yields a sheet charge concentration of  = 2.17 × 10 13 cm −2 .Based on sheet resistance of 11.8 kΩ and the carrier density, the mobility was extracted to be 24.4 cm 2 ⋅V −1 ⋅s −1 , indicating that it is consistent with that of SC-FET.
The plot of the drain current versus the drain-source voltage up to −200 V is investigated in Figure 7.For SD-FET (Figure 7

Conclusion
In this work, SD-FET has been demonstrated with a 20 nm thick SiN  dielectric layer on (001) oriented H-termination diamond film.For comparison, a standard SC-FET was also  fabricated.The transistor output and transfer characteristics of both devices were observed, confirming the presence of the 2DHG at the surface and the interface, respectively.Notably, the maximum drain current of SD-FET was the same compared with SC-FET.The modulated 2DHG sheet charge densities extracted from CV curves were 2.03 × 10 13 cm −2 and 2.17 × 10 13 cm −2 for SC-FET and SD-FET, respectively.The hole mobility was calculated to be 28 cm 2 ⋅V −1 ⋅s −1 and 24.4 cm 2 ⋅V −1 ⋅s −1 for SC-FET and SD-FET, respectively.The cut-off drain current was measured, and no breakdown was presented as the drain-source voltage was applied as high as −200 V.The results indicated that the channels had been preserved after SiN  deposition.The preserving channel may be related to C-N formation at the SiN  /diamond interface.

Figure 1 :
Figure 1: The process flow of the device fabrication.
As the |  | increases, |  | increases linearly in the linear region, and the drain current (|  |) becomes saturated in the saturation region.Compared with the output characteristic of the SC-FET, the |  | of SD-FET is larger when |  | < 5 V and gets close at |  | = 5 V.And the max current density is consistent for both kinds of devices.

Figure 2 :
Figure 2: The optical microscope image (5x) (a) and AFM photo of MPCVD deposited single crystal diamond epitaxial films.

Figure 3 :
Figure 3: FET output characteristics of the Zr-gate diamond based field-effect transistors with and without SiN  dielectric layers.

Figure 4 :
Figure6exhibits the charge carrier profile of SD-FET with a lower gate-to-channel capacitance of 0.17 F/cm 2 , because of the series connection of the intrinsic gate capacitor and the SiN  gate dielectric capacitor.The permittivity of SiN  ( SiN  = 5.0) and diamond for dielectric layer and gate capacitor were used to evaluate the depth profile.The 2DHG was determined to be 26.2 nm under the interface of SiN  /diamond.As the 2DHG of H-terminated diamond located 6.6 nm under the surface, the thickness of SiN  was estimated to be 19.6 nm, in accordance with nominal thickness of 20 nm.Integration of the charge carrier profile yields a sheet charge concentration of  = 2.17 × 10 13 cm −2 .Based on sheet resistance of 11.8 kΩ and the carrier density, the mobility was extracted to be 24.4 cm 2 ⋅V −1 ⋅s −1 , indicating that it is consistent with that of SC-FET.The plot of the drain current versus the drain-source voltage up to −200 V is investigated in Figure7.For SD-FET (Figure7(a)), |  | increases quickly with the voltage increases from 0 V to 50 V and then starts to saturate until the applied voltage gets to −200 V.The max |  | was measured to be about

3 )Figure 5 :
Figure 5: Hole-density depth profile in diamond from the Hterminated diamond according to the CGS-VG curve.

Figure 6 :
Figure 6: Hole-density depth profile in diamond from the SiN  /diamond interface according to the CGS-VG curve.

Figure 7 :
Figure 7: High voltage of   applied on (a) SC-FET and (b) SD-FET at   = 15 V.