HfAlOx/Al2O3 Bilayer Dielectrics for a Field Effect Transistor on a Hydrogen-Terminated Diamond

In this work, a hydrogen-terminated (H-terminated) diamond field effect transistor (FET) with HfAlOx/Al2O3 bilayer dielectrics is fabricated and characterized. The HfAlOx/Al2O3 bilayer dielectrics are deposited by the atomic layer deposition (ALD) technique, which can protect the H-terminated diamond two-dimensional hole gas (2DHG) channel. The device demonstrates normally-on characteristics, whose threshold voltage (VTH) is 8.3 V. The maximum drain source current density (IDSmax), transconductance (Gm), capacitance (COX) and carrier density (ρ) are −6.3 mA/mm, 0.73 mS/mm, 0.22 μF/cm2 and 1.53 × 1013 cm−2, respectively.

Since a H-terminated diamond is thermally and chemically instable, it is necessary to stabilize the hole carriers for a H-terminated diamond FET with a dielectric layer [8]. Furthermore, the dielectric material with high dielectric constant can control large charge responses at a small bias effectively [14]. To date, many high dielectric constant materials have been employed for the fabrication of a H-terminated diamond FET [10,15]. However, there are few reports on using HfAlO X as dielectric with a high dielectric constant, high crystallization temperature and large band gap (5.8-6.2 eV). [22].
In this work, we study a H-terminated diamond FET with HfAlO x /Al 2 O 3 bilayer dielectrics, and its electrical properties were evaluated by semiconductor analyzer.

Materials and Methods
The fabrication process of the H-terminated diamond FET with HfAlO x /Al 2 O 3 bilayer dielectrics is displayed in Figure 1. A high temperature and high pressure (HPHT) single crystal diamond substrate was cleaned by various solutions before growth [9]. Then, a 200 nm homoepitaxy layer was grown on the substrate with the dimensions of 3 × 3 × 0.5 mm 3 by the microwave plasma chemical vapor deposition (MPCVD) technique. The growth conditions were declared in our previous work [9]. Afterwards, 150 nm Au electrodes with 20 µm source drain gap (L SD ) were realized by photolithography, electron beam evaporation (EB) and the lift-off technique. Next, isolation was carried out with 20 min UV/ozone treatment. After that, a 4 nm Al 2 O 3 film was deposited to protect the H-terminated channel, and a 30 nm HfAlO x film was deposited by the ALD technique sequentially. The atomic percentage of HfAlO x is Hf:Al:O = 2:23:75, evaluated by the energy dispersive X-ray spectroscopy (EDS) technique. Finally, 150 nm Al gate electrode was deposited on the gate region with 4 µm gate length (L G ) and 100 µm gate width (W G ). The electrical properties of this device were characterized by Agilent B1505A. Figure 2 demonstrates the schematic diagram of the H-terminated diamond FET with HfAlO x /Al 2 O 3 bilayer dielectrics. The electrical contacts for the source, drain and gate electrodes are exhibited, and the hole carriers of the channel are illustrated.

Materials and Methods
The fabrication process of the H-terminated diamond FET with HfAlOx/Al2O3 bilayer dielectrics is displayed in Figure 1. A high temperature and high pressure (HPHT) single crystal diamond substrate was cleaned by various solutions before growth [9]. Then, a 200 nm homoepitaxy layer was grown on the substrate with the dimensions of 3 × 3 × 0.5 mm 3 by the microwave plasma chemical vapor deposition (MPCVD) technique. The growth conditions were declared in our previous work [9]. Afterwards, 150 nm Au electrodes with 20 μm source drain gap (LSD) were realized by photolithography, electron beam evaporation (EB) and the lift-off technique. Next, isolation was carried out with 20 min UV/ozone treatment. After that, a 4 nm Al2O3 film was deposited to protect the H-terminated channel, and a 30 nm HfAlOx film was deposited by the ALD technique sequentially. The atomic percentage of HfAlOx is Hf:Al:O = 2:23:75, evaluated by the energy dispersive X-ray spectroscopy (EDS) technique. Finally, 150 nm Al gate electrode was deposited on the gate region with 4 μm gate length (LG) and 100 μm gate width (WG). The electrical properties of this device were characterized by Agilent B1505A. Figure 2 demonstrates the schematic diagram of the H-terminated diamond FET with HfAlOx/Al2O3 bilayer dielectrics. The electrical contacts for the source, drain and gate electrodes are exhibited, and the hole carriers of the channel are illustrated.

Materials and Methods
The fabrication process of the H-terminated diamond FET with HfAlOx/Al2O3 bilayer dielectrics is displayed in Figure 1. A high temperature and high pressure (HPHT) single crystal diamond substrate was cleaned by various solutions before growth [9]. Then, a 200 nm homoepitaxy layer was grown on the substrate with the dimensions of 3 × 3 × 0.5 mm 3 by the microwave plasma chemical vapor deposition (MPCVD) technique. The growth conditions were declared in our previous work [9]. Afterwards, 150 nm Au electrodes with 20 μm source drain gap (LSD) were realized by photolithography, electron beam evaporation (EB) and the lift-off technique. Next, isolation was carried out with 20 min UV/ozone treatment. After that, a 4 nm Al2O3 film was deposited to protect the H-terminated channel, and a 30 nm HfAlOx film was deposited by the ALD technique sequentially. The atomic percentage of HfAlOx is Hf:Al:O = 2:23:75, evaluated by the energy dispersive X-ray spectroscopy (EDS) technique. Finally, 150 nm Al gate electrode was deposited on the gate region with 4 μm gate length (LG) and 100 μm gate width (WG). The electrical properties of this device were characterized by Agilent B1505A. Figure 2 demonstrates the schematic diagram of the H-terminated diamond FET with HfAlOx/Al2O3 bilayer dielectrics. The electrical contacts for the source, drain and gate electrodes are exhibited, and the hole carriers of the channel are illustrated.     (1 where JTFE means the IGS caused by TFE model; JS represents the saturation current; and P is a parameter associated with the carrier tunneling probability and temperature [9]. In Figure 4b, the lnJTFE/JS(−exp(qV/kT)) and VGS exhibit a linear relationship under the TFE model.   In Figure 3b, the transfer characteristic of the H-terminated diamond FET with HfAlO x /Al 2 O 3 bilayer dielectrics is presented. The threshold voltage (V TH ) is extrapolated to be 8.3 V at a V DS of −20 V based on the relationship between |I DS | 1/2 and V GS , demonstrating normally-on characteristics [14]. The maximum transconductance (G m ) is 0.73 mS/mm.

Results and Discussion
The leakage current density (I GS ) in the log coordinate of the H-terminated diamond FET with HfAlO x /Al 2 O 3 bilayer dielectrics is shown in Figure 4a. The V GS changes from −6 to 8 V, and the absolute value of I GS (|I GS |) is 7.95 × 10 −7 A/cm 2 at a V GS of −6 V, demonstrating a low |I GS |. Table 1 where J TFE means the I GS caused by TFE model; J S represents the saturation current; and P is a parameter associated with the carrier tunneling probability and temperature [9]. In Figure 4b, the lnJ TFE /J S (−exp(qV/kT)) and V GS exhibit a linear relationship under the TFE model.
previous work [13,23], and the reason may be attributed to the undamaged 2DHG duction channel protected by Al2O3. In Figure 3b, the transfer characteristic of the H-terminated diamond FET with H lOx/Al2O3 bilayer dielectrics is presented. The threshold voltage (VTH) is extrapolated 8.3 V at a VDS of −20 V based on the relationship between |IDS| 1/2 and VGS, demonstra normally-on characteristics [14]. The maximum transconductance (Gm) is 0.73 mS/mm The leakage current density (IGS) in the log coordinate of the H-terminated diam FET with HfAlOx/Al2O3 bilayer dielectrics is shown in Figure 4a. The VGS changes fro to 8 V, and the absolute value of IGS (|IGS|) is 7.95 × 10 −7 A/cm 2 at a VGS of −6 V, dem strating a low |IGS|. Table 1 demonstrates the |IGS| comparison with the reported H minated FETs. The |IGS| for the MoO3, LiF/Al2O3, Ta2O5/Al2O3 and ZrO2/Al2O3 H-te nated diamond FET are 3.33 × 10 −4 A/cm 2 , 1 × 10 −6 A/cm 2 , 7.6 × 10 −4 A/cm 2 and 4.8 × A/cm 2 , respectively [21,[24][25][26]. Their values are larger than those of the HfAlOx/Al2O3 As shown in Figure 4b, the relationship between IGS and VGS can be described by the mionic field emission (TFE) model (1) [9]: where JTFE means the IGS caused by TFE model; JS represents the saturation current; a is a parameter associated with the carrier tunneling probability and temperature [9 Figure 4b, the lnJTFE/JS(−exp(qV/kT)) and VGS exhibit a linear relationship under the model.     Figure 5a displays the capacitance-voltage (C-V) curve measured at 1 MHz of the H-terminated diamond FET with HfAlO x /Al 2 O 3 bilayer dielectrics. Evident accumulation and depletion regions can be observed. The maximum capacitance (C OX ) is 0.22 µF/cm 2 at V GS of −2 V. Based on the method d 2 C/d 2 V GS = 0, the flat band voltage (V FB ) is determined to be 8.5 V and 7.6 V in the forward and reverse directions, respectively [18]. The trapped charge density is evaluated to be 1.24 × 10 12 cm −2 based on the hysteresis voltage of 0.9 V [8]. Additionally, the CV curve shifts to the positive direction, indicating the presence of fixed negative charge in the dielectric layer. This increases the hole carriers in the 2DHG channel, thus resulting in a normally-on operation. The fixed negative charge density is deduced to be 1.25 × 10 13 cm −2 on the basis of C OX , V FB , and the work function difference between Al and the H-terminated diamond [8]. In addition, the dielectric constant of HfAlO x /Al 2 O 3 is calculated to be 8.45, suggesting that the quality of HfAlO x /Al 2 O 3 needs to be further improved. As demonstrated in Figure 5b, the carrier density (ρ) is 1.50 × 10 13 cm −2 at the V GS of −2 V evaluated based on CdV GS [18]. The ρ is large, and this can be attributed to the high quality of the H-terminated diamond [8,21].   Figure 5a displays the capacitance-voltage (C-V) curve measured at 1 MHz of th terminated diamond FET with HfAlOx/Al2O3 bilayer dielectrics. Evident accumula and depletion regions can be observed. The maximum capacitance (COX) is 0.22 μF/cm VGS of −2 V. Based on the method d 2 C/d 2 VGS = 0, the flat band voltage (VFB) is determ to be 8.5 V and 7.6 V in the forward and reverse directions, respectively [18]. The trap charge density is evaluated to be 1.24 × 10 12 cm −2 based on the hysteresis voltage of 0 [8]. Additionally, the CV curve shifts to the positive direction, indicating the presenc fixed negative charge in the dielectric layer. This increases the hole carriers in the 2D channel, thus resulting in a normally-on operation. The fixed negative charge densi deduced to be 1.25 × 10 13 cm −2 on the basis of COX, VFB, and the work function differenc tween Al and the H-terminated diamond [8]. In addition, the dielectric constant of H lOx/Al2O3 is calculated to be 8.45, suggesting that the quality of HfAlOx/Al2O3 needs t further improved. As demonstrated in Figure 5b, the carrier density (ρ) is 1.50 × 10 13 at the VGS of −2 V evaluated based on ∫ C V GS [18]. The ρ is large, and this can be attrib to the high quality of the H-terminated diamond [8,21].

Conclusions
In summary, the electrical properties of H-terminated diamond FET with H lOx/Al2O3 bilayer dielectrics were investigated. The output characteristics demonstrat evident p-type channel, and the IDSmax is −6.3 mA/mm obtained at VGS of −6 V. The tran characteristics exhibits the VTH of 8.3 V, indicating normally-on characteristics. The |IGS| is × 10 −7 A/cm 2 at a VGS of −6 V, demonstrating a low |IGS|. In addition, the COX is 0.22 μF based on the C-V curve. Additionally, the ρ is 1.50 × 10 13 cm −2 at a VGS of −2 V. The res are meaningful for the research of a H-terminated diamond FET, and the electrical pe mance of HfAlOx/Al2O3 FET will be further improved by optimizing the fabrication cess in our future work.

Conclusions
In summary, the electrical properties of H-terminated diamond FET with HfAlO x /Al 2 O 3 bilayer dielectrics were investigated. The output characteristics demonstrate an evident p-type channel, and the I DSmax is −6.3 mA/mm obtained at V GS of −6 V. The transfer characteristics exhibits the V TH of 8.3 V, indicating normally-on characteristics. The |I GS | is 7.95 × 10 −7 A/cm 2 at a V GS of −6 V, demonstrating a low |I GS |. In addition, the C OX is 0.22 µF/cm 2 based on the C-V curve. Additionally, the ρ is 1.50 × 10 13 cm −2 at a V GS of −2 V. The results are meaningful for the research of a H-terminated diamond FET, and the electrical performance of HfAlO x /Al 2 O 3 FET will be further improved by optimizing the fabrication process in our future work.