High temperature stability of H-diamond high frequency MOSFET with 300°C grown Al2O3 dielectric

Abstract The high frequency H-diamond metal-oxide-semiconductor field effect transistors (MOSFETs) were fabricated on single diamond substrate using 300°C ALD grown Al2O3 as gate dielectric and passivation layer. The devices gate length, gate/drain spacing and dielectric thickness are 100 nm, 2 μm, and 10 nm, respectively. The direct-current and frequency characteristics were investigated. The device shows a maximum saturation drain current of −492.6 mA/mm and gm of 135.2 mS/mm. The device shows good high temperature working performance, and the maximum saturation drain current only has a little decreasing of 7.6%. at 200°C. In addition, the device exhibits a maximum cut-off frequency of 36.2 GHz and maximum oscillation frequency of 70.5 GHz. The transient drain current response measurement indicates that the drain current can follow the changing of gate voltage at the frequency of 1 MHz. These results indicate that the Al2O3 dielectric is suitable for using in high frequency or the high-speed switching devices.


Introduction
Due to its excellent properties, such as wide-bandgap (5.47 eV), high carrier mobility (an electron of 4500 cm 2 V −1 s −1 and a hole of 3800 cm 2 V −1 s −1 ), high thermal conductivity (2200 W/mK), high breakdown electric field (>10 MV cm −1 ), etc. [1][2][3], diamond is regarded as "ultimate semiconductor" and has great potential in the using of high temperature, high frequency and high power electric devices.However, the high activation energy of diamond dopants hinders the development of diamond devices [4,5].By treating the diamond in hydrogen atmosphere in high temperature, a two-dimensional hole gas (2DHG) layer with high carrier density can be formed in the sample surface [6][7][8][9][10].This kind of conductivity has been widely used in the fabrication of diamond field effect transistors (FETs).
In the past decades, great efforts have been made to improve the performance of hydrogen terminated diamond (H-diamond) FETs.Various gate dielectrics layers have been used in H-diamond MOSFETs to improve the device current density and stability, such as Al 2 O 3 [11][12][13][14], MoO 3 [15][16][17], HfO 2 [18], V 2 O 5 [19], WO 3 [20], Y 2 O 3 [21], and so on.Among them, the Al 2 O 3 dielectric shows greatest application potential.By using Al 2 O 3 as gate dielectric or passivation layer, the H-diamond MOSFETs have achieved the maximum drain current density, breakdown voltage, cutoff frequency, oscillation frequency and the output power density of 1.3 A/mm [7], over 2 kV [22], 70 GHz [23], 120 GHz [24], 4.2 W/mm (2 GHz) [25], respectively.The drain current density over 1.1 A/mm with regrown p-type heavily doped diamond ohmic contacts also has been achieved [26].In addition, by passivation the H-diamond surface using high temperature (450°C) [27] atomic layer deposition (ALD) grown Al 2 O 3 , the 2DHG conductivity can work stably at high temperature.As pointed by H. Kawarada et al. that some unoccupied energy levels exist in the bandgap of the ALD grown Al 2 O 3 below the valence band edge of diamond, and could act as negatively charged to attract holes in diamond near the interface [28].This possibly induces the generation of new 2DHG at H-diamond/ALD-Al 2 O 3 interface.
In our previous investigations, we also have achieved low on resistance and high working stability based on the 300°C ALD grown Al 2 O 3 dielectric [29,30], and the 2DHG formation mechanism in the Al 2 O 3 /H-diamond interface also have been investigated by the XPS measurement [14].Recently, we future achieved the high out current density about 1 A/mm, high transconductance of 284 mS/mm and high f T and f max of 41.3/80.6GHz [31].These results indicate that the 300°C ALD grown Al 2 O 3 dielectric also has great potential to be used in the high power and high frequency H-diamond devices.Thus, we plan to future demonstrated the high temperature stability, switching on/off and breakdown characteristics of the H-diamond MOSFET with 300°C ALD grown Al 2 O 3 .
In this work, we fabricated 100-nm gate length H-diamond MOSFETs with 300°C ALD-Al 2 O 3 dielectric layer on the single crystalline diamond substrate.The DC and frequency characteristics were investigated.In addition, we demonstrated the switching and 200°C working performance of this H-diamond device.These results indicate that 300°C ALD grown Al 2 O 3 has great potential to be used in high frequency and high stability H-diamond MOSFETs.

Experimental
The single crystalline diamond substrate of 7 × 7 mm 2 is used in this experiment.Before epitaxy growth, the substrate is firstly polished to obtain a flat surface, and the roughness of the sample is measured using atomic force microscope (AFM) to be 0.085 nm (Figure 1a).Then, the polished substrate is immersed in H 2 SO 4 /HNO 3 (1:1) mixed solution for an hour at 250°C to remove non-diamond phase and contaminant from the substrate surface.After that, the substrates were cleaned in acetone, absolute ethyl alcohol and deionized water for 15 min, respectively, to obtain clean surface.Finally, we use microwave plasma chemical vapor deposition (MPCVD) equipment for epitaxy growth of single diamond layer and produce H-terminated diamond with the 2DHG conduction layer.The thickness of the epitaxy layer is about 200 nm.After finishing the growth, we keeping the sample in hydrogen plasma for 15 min to form H-diamond surface.The roughness of the H-diamond surface is measured to be 1.027 nm (Figure 1b).Since hydrogen plasma has strong etching effect on the diamond surface, the roughness of the sample surface has an obvious increasing.Even though, the value of 1.08 nm is enough for the fabrication of H-diamond FETs.
Then, we fabricated the H-diamond MOSFETs on the H-diamond surface.In order to protect the H-diamond surface, Au mask processes were used.The device fabrication processes were performed by the help of Nanjing Electronic Devices Institute.The devices fabrication details can be found in reference [23].The value of ohmic contact resistance obtained from the TLM structure is 5.5 Ω•mm.The schematic cross-sectional structure and microscope picture of the H-diamond MOSFET are shown in Figure 2. The 10-nm-thick Al 2 O 3 deposited by ALD at 300°C was used as gate dielectric and passivation layer.The T-shaped gates using Ti/Au metal were fabricated.The gate length and width are 100 nm and 2 × 100 μm, respectively.Gate-drain (L GD) gate-source (L GS ) distance are 2 µm and 500 nm, respectively.All of the device performances were measured in the air atmosphere.

Results and discussions
Figure 3a shows the output characteristics of the Hdiamond MOSFET.The drain-source current (I DS ) depends on the drain-source voltage (V DS ) at different gate voltages (V GS ) changing from 3 to −3 V with a step of −0.5 V. Additionally, I DS reaches their highest value of −492.6 mA/mm at a V GS = −3 V and a V DS = −8 V.The value of on-resistance is calculated to be 14.36 Ω•mm.Though this value is higher than the reported value in reference [23], consideration on the L SD of 2.6 μm, this value is quite small compared with the reported  H-diamond with same L SD [25,32].Also, it can be observed that the device did not turn off well at V GS =3V.This phenomenon often appears with submicron gate length [23,25], this may be induced by the short-channel effect or the isolation process and gate leakage of the device.
In order to investigate the high temperature working stability of the device, we measured the device output characteristics again at 200°C.The results are shown in Figure 3b.The maximum |I DS | decreases from 492.6 to 454.9 mA/mm at 200°C, which has a decreasing of 7.6% compared with the value at room temperature.It also can be observed that with the temperature increasing to 200°C, the on resistance and the I DS at V DS =0V show an obvious increasing.In addition, an obvious gate leakage current also can be found at 200°C.These could be induced by the deterioration of the Ohmic contact and the Al 2 O 3 dielectric.The optimizing of the dielectric quality still need to do in the future to improve the high temperature working stability of the device.
Figure 4 shows the transfer characteristics of Hterminated diamond FET, which was measured at the V DS = −8 V.The device shows a maximum transconductance of 135.2 mS/mm and drain saturation current of 537.16 mA/mm.The maximum current has a slight difference between the output and transfer characteristics, which usually appears in the H-diamond FETs.This could be induced by the traps in the interface of diamond and Al 2 O 3 .The device shows a threshold voltage of 4.06 V, which is in normally-on mode.
The breakdown performance is very importance for high power density H-diamond MOSFET.Thus, we measured the off-state breakdown voltage of this device.The results are shown in Figure 5.This device shows a breakdown voltage of 138 V, which corresponds to the breakdown electric field strength of about 0.74 MV/cm according to the L GD of 2 μm.This breakdown electric field strength is comparable to the reported value of 0.8-1.0MV/cm [22], which is achieved by the thick gate dielectric.The breakdown electric field strength has great potential to be further improved by increasing the Al 2 O 3 thickness in the future.
The RF small signal S-parameters were measured between 100 MHz and 66 GHz.Open and short structures were used to extract and remove the parasitic elements.By extrapolating the current gain |H21| and unilateral transducer power gain (UPG), the values of the cutoff frequency (f T ) and the maximum frequency of oscillation (f max ) can be obtained.Figure 6 shows the high-frequency characteristics of H-terminated diamond FET.The f T of 36.2GHz and f max of 70.5 GHz at   V DS = −10 V V GS = V are obtained, which is relative high among the reported H-diamond MOSFET in consideration of the large L GD of 2 μm.The f T and f max be expressed as follows: It can be observed from above expressions that the Ohmic contact or access resistance have strong influence on the device frequency characteristics.The small Ohmic contact resistance and L GD can reduce the parasitic source and drain resistances and improve transconductance which can contribute to the high f T and f max .The frequency characteristics of this device can be improved in the future by scaling down the L GD or improving ohmic contact performance.The Al 2 O 3 gate dielectric has an obvious advantage to be used in high power and high frequency device.
At last, we characterized the switching performance of this device.The gate switching on/off characteristics of the device were measured at 1 MHz.The results are shown in Figure 7.During measurement, the drain current responses to V GS pulsing between 7 and −5 V.The maximum drain current density at V GS = −5V is similar to the output and transfer characteristics, which means the transient current can follow the changing of gate voltage at the frequency of 1 MHz.These results indicate that the traps existing in the interface and dielectrics have rare effects on the device performance.The diamond MOSFETs could be used in the power switch devices.

Conclusions
The H-diamond MOSFETs were fabricated on the CVD grown 7 × 7 mm 2 single crystalline diamond substrate.The 10-nm-thick ALD-grown Al 2 O 3 was using as gate dielectric layer and passivation layer.The dielectric was grown at 300°C.The device with 100 nm gate length and 2 µm gate-drain distance shows a maximum I DS of −492.6 mA/mm at a V GS of −3 V and V DS of −8 V. Also, the devices exhibit a g m of 135.2 mS/mm.In addition, the high temperature working stability was investigated.The device can working normally at a high temperature of 200°C, which only has a few I DS decreasing of 7.6% compared with the value at room temperature.The breakdown voltage was measured to be 138 V, which corresponds to a breakdown electric field strength about 0.7 MV/cm.The large signal gate switching on/off pulse measurement results demonstrate a switching frequency over 1 MHz.In addition, this device exhibits a f T and f max of 36.2 and 70.5 GHz, respectively.The high frequency and switching characteristics indicate that H-diamond MOSFETs has great potential to be used in high power microwave and switching devices.In the future, we will optimize the Al 2 O 3 quality and increase the dielectric thickness further more to improve the device stability and working voltage, and achieve the improving of the power density and stability of the device.

Figure 1 .
Figure 1.aFm scope of sample (a) before and (b) after hydrogen plasma treatment.

Figure 2 .
Figure 2. (a) Schematic structure, (b) microscope picture and (c) SEm picture of the H-diamond moSFEt device structure.

Figure 4 .
Figure 4. transfer characteristics of H-terminated diamond FEt with 10 nm gate dielectric layer.